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Radio control racer needs an electrical mentor
13

Radio control racer needs an electrical mentor

Radio control racer needs an electrical mentor

(OP)
Hello,

Hey I get the first post!!!!

I race high speed RC cars, nitro for the most part. A racing buddy and I decided to go after the overall record, 161 mph set with a brushless motor and Lipo cells in 2008.

I have built the car (below) but my 'buddy', who had the electrical knowledge, decided to go off on his own. It has taken a year to get the car sorted out so it runs straight and stays on the ground (not that easy to do). I'm to the point that I'm starting to put power to the ground (hit 130 last weekend) and I have data from the speed control (watts, amps, rpm, voltage, etc.) but I don't know what to do with the info.

The ultimate goal is to be the first to 200 mph.

I could really use some help interpreting the data to tell me which way to go as well as making sure I'm not making other wiring errors (I'm sure I am!) Once I get the jist of what does what does what I should be okay and you'll be off the hook unless you want to stay involved.

Thanks in advance,

Gary

RE: Radio control racer needs an electrical mentor

Yes. Actually, at the time you posted, only you, me, and the site management knew this forum existed. Be patient and someone may be here to help you.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Radio control racer needs an electrical mentor

I replied to the original post, but it got lost in the transition. Stuff happens.

First question: Is any of your motor or battery wiring, e.g. intercell connectors, smaller than the 8 gage wire that the controller uses? I.e., did you follow the comprehensive instructions in the controller users manual? What did you do differently?

Second question: What's your gearing like? I'm assuming (correct me if I'm wrong) that you're using a brushless motor, in which case the speed of the vehicle is controlled by the maximum frequency applied by the motor controller, the gearing between the motor and the tires, the tire diameter, the growth in tire diameter with speed, and slip between the tire and the ground. If you're already reaching the controller's maximum rpm speed, then you need to change the gearing or the tires, a lot, in order to reach your goal.

Third question: What data? Post it and we'll take a look. If you're concerned about discussing it publicly, I'm not that hard to find. Look on LinkedIn.



Mike Halloran
Pembroke Pines, FL, USA

RE: Radio control racer needs an electrical mentor

(OP)
Thanks Mike....my original post was pulled ( and likely your original response)because it had to do with hobbies rather than dull, old work but the administrators opened this thread so it's all good.

*Is any of your motor or battery wiring, e.g. intercell connectors, smaller than the 8 gage wire that the controller uses?
- The front 4s pack has 8ga wire but the rest, I believe is 10ga. I tried to get 8ga wires for the rear 2s packs but the battery supllier would only put 10ga on them. The motor leads are 10 ga. Another question, what effect does the length of the wires have? I've been told to keep the wires between the batteries and the esc as short as possible which I have but I have a fairly long (8 inch) 10ga wire connecting the front and rear packs.

*did you follow the comprehensive instructions in the controller users manual?
- They're not all that comprehensive....they just show the basic wiring diagram and arming instructions

*What did you do differently?
- With the exception of trying to go REALLY fast, nothing.

*What's your gearing like?
- The chassis gearing is 1.66:1. The tires are rotating @ 14400 rpm @ 100mph

*I'm assuming (correct me if I'm wrong You're correct) that you're using a brushless motor, in which case the speed of the vehicle is controlled by the maximum frequency applied by the motor controller,...
- These are MY assumptions and correct me if I'm wrong...The esc(electronic speed control) is built for up to 32 volts, which is what I'm putting into it. The motor is 2200kv which, at 32 volts, would rev at 70,400 rpm (but the motor has a 60,000 rpm limit). ASSUMING an 80% efficiency it would top out at 56,000. With the gearing and tire diameter of the car it should hit 180mph @ 43,000 rpm. That gives me 13,000 rpm to give up to aero and parasitic drag. My first goal is the record, then I can play with gearing for 200.

...the growth in tire diameter with speed,...
- There is very little tire growth. Growth is bad because centrifugal(?) force literally tears the foam apart. I turn the tires down so the foam is only about 1/8 - 3/32 of an inch thick, thereby reducing the mass that's trying to tear itself apart. I have spin tested these tires to 25,000 rpm on my router with no issues.

...and slip between the tire and the ground.
- Once the car is moving the shape of the nose and the rear wing put down force on the tires. I have a speedometer that I'm going to install on the rear axle so I can see if the tire speed is significantly different from the timed speed. My feeling is that they're not. During my last run the data showed a 281 amp draw...my intuition tells me that the tires were firmly planted. If they were spinning the amp draw would have been less...yes?

*If you're already reaching the controller's maximum rpm speed, then you need to change the gearing or the tires, a lot, in order to reach your goal.
- I don't believe I'm reaching the rpm limit but do appear to be reaching the maximum amperage limit since it shut down part way through a run. The limit had been set at 250A... I have bumped it up to 275 and have the 300 setting in reserve. Even at that amperage draw the temp of the esc was only 110 degrees. Because the car is driving so well and 'stuck' I have been punching the throttle sooner... I'm thinking that a slower application will keep the amperage draw down and allow a complete run.

*What data? Post it and we'll take a look.
- The esc spits out rpm, ripple, amperage, voltage, temperature, throttle position, time to full throttle and more.... I'll be running this weekend and will post it up. I don't believe my data will do anything for anyone else but me since the few cars trying for this target are so vastly different in weight, size and drive wheels.

I have attached a pic of the other side of the car...the center black gear is on the motor shaft while the other two gears are jack shafts... the rear has a pulley with a belt that runs between the two 2s packs and the front shaft has a pulley and belt visible in the previous photo driving the front wheel. In the configuration shown here, with 6s (24 volts) and a 1500kv motor the car ran through the taps at 116 mph....assuming 80% efficiency, the calculations work out to 120 mph....pretty darned close to reality!

I deeply appreciate your offer to help, Mike....
Gary O




RE: Radio control racer needs an electrical mentor

I've been out of touch with this particular discipline, but the objective is to dump as much power into the motor as quickly as possible. Thus, parasitic resistances are KILLERS. EVERYTHING in the circuit has resistance; batteries, cables, ESC, connectors, even the motor. I remember that some people would have specially conditioned NiCd batteries that were essentially "burned" to reduce its internal resistance; presumably, now, Lithium polymer batteries, but are yours as high a discharge as possible? Some fanatics would use oxygen-free copper or even silver wires to reduce resistance. Motors were wound with low gauge wires, etc. And, not to ignore gold plated connector contacts. Cables need to be as short as physically possible; it looks like you could cut an inch or two off the cables if you really, really tried.

re. ESC, is 32V the maximum supply voltage rating?

TTFN
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RE: Radio control racer needs an electrical mentor

IRStuff, search for Castle XL2 speed control.

Mike Halloran
Pembroke Pines, FL, USA

RE: Radio control racer needs an electrical mentor

2
The '2200 kv' in the context of this hobby apparently means 2200 rpm/Volt. ... which is an odd spec to use for rating a three phase brushless motor, but widely used. I wasn't able to figure out exactly which motors are in play here.

Gary, the 10 gage (vs 8 gage) wire is hurting more than an extra inch of length.
An online calculator says that for copper wire the resistance per 10 feet is
10 milli-ohms for 10 gage wire, and
6 milli-ohms for 8 gage wire, and
4 milli-ohms for 6 gage wire.
Single milli-ohms start making a big difference at a couple hundred amperes.

It might be a worthwhile exercise to map your wiring by length and gage,
put it in a spreadsheet, and compute the resistance and voltage drop at 250A for the whole circuit.

Does anybody use busbars as chassis elements? Just a thought...

Mike Halloran
Pembroke Pines, FL, USA

RE: Radio control racer needs an electrical mentor

2
The efficiency relates to heat loss, not speed loss. 80% efficiency means that 80% of the power input to the motor is doing work and 20% is generating heat.
Voltage drop calculations based on 250 Amps and the DC resistance of the wire show 0.25 Volts per foot for #10 AWG wire and 0.15 Volts per foot for #8 AWG wire.
But; The heating loss in a foot of wire at 250 Amps is 62 Watts for #10 wire and 37.5 Watts for #8 Wire.
Also, you will be running at at least 400 Hz at 100 mph and skin effect will be increasing both the voltage drop and the Watts loss.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Radio control racer needs an electrical mentor

"skin effect will be increasing both the voltage drop and the Watts loss."

All the more reason for fatter and shorter wires. Additionally, since the wires are likely to get warm from all the current, getting them cooled with lots of air flow might help, but that's a trade vs. even fatter wires than you might otherwise need.

TTFN
FAQ731-376: Eng-Tips.com Forum Policies

RE: Radio control racer needs an electrical mentor

(OP)
Wow! Thanks for the responses...lots of good info there and lots to work on.

IRStuff, yes my packs are 150c discharge which is the max on the market. Although the photos show standard plug type connectors to the batteries, they've all been changed over to gold 'bullet' connectors. I can certainly shorten several of the wires and will do so. The esc's maximum rating is '8s' which would be 32V. Whether or not it would live at a higher voltage I may have to find out when the time comes that I need more voltage! Here's a link....
http://www.castlecreations.com/products/mamba_xl2....

Mike, yes the motor rating is rpm per volt. The motor I'm currently using is a Castle 1515... their site has limited data http://www.castlecreations.com/products/neu-castle... . Once I have this motor topped out I'll switch to a Neu motor which is the standard of the hobby http://www.neumotors.com/Site/1500_series.html . I like your idea of mapping the wires...I'll do a before and after calc.

Busbar as a chassis element? Hmmmm...let me think on that one.

Bill, I researched 'skin effect' (what a crazy concept!)... since it is an AC factor that would only effect the wires between the esc and motor, correct?. Not much I can do there...their as short as they can be and the wires can't be changed. The next car iteration could resolve that with a relocated esc.

One thing that may come into play is that full power is applied for about 5-7 seconds with the throttle being 'rolled' on prior to that. Also I will be drilling out the aluminum pieces and replacing the fiberglass parts of the car with 'swiss cheesed' carbom fiber plates to lower the weight.

Thanks so much for the thoughts...I have work to do now!

Gary

RE: Radio control racer needs an electrical mentor

(OP)
Okay, you guys all got me looking.... I can rotate the motor 180 degrees and shorten the three 10ga wires to less than an inch long. The esc has all 8ga and the front pack has 8ga. I can easily change the wire than connects the front and rear packs to 8ga but should I go to 6ga for that wire because of it's length? (six inches) The only 10ga will be the two rear packs and I can shorten them considerably.

See? This is the kind of stuff I wouldn't think was important and looking at one of my rival's cars he doesn't either. However, by 'buddy's' wires are as short as possible....

RE: Radio control racer needs an electrical mentor

If memory serves, doubling up the 10 gage fore/aft wires makes each pair effectively 7 gage. ... if your connectors can deal with it, or you're willing to make custom connectors. That may or may not simplify your packaging issues.



Mike Halloran
Pembroke Pines, FL, USA

RE: Radio control racer needs an electrical mentor

(OP)
Mike, I tallied up the wires and am surprised just how much wire is in there! Turns out there is 32 inches of 10 ga and 34 inches of 8ga. I wouldn't have thought there was more than 36 inches total.

I flipped the motor and shortened the wires, ending up with 10 inches less 10ga and as a side benefit took seven ounces out of the car. Double bonus! Here's the before and after....



RE: Radio control racer needs an electrical mentor

(OP)
Mike, you're a genius! I spent the evening shortening wires...I was able to move a rear pack around and by the time I was done I had removed a full foot of both 10ga and 8ga (including the motor leads above). I put the cutoffs on the scale and they were a pound....a POUND of wires!... not to mention the reduced resistance. I reduced the wire length in the car by 20%. I have raced for years and never looked at wires as being a way to lose weight.

I'll be running the car Sunday at my local test site, an industrial road with no traffic, straight, long and smooth which will give me about 600 feet of run up before going by the radar gun. I should have a bunch of data to share after that. I'll be most interested to see if there was a significant amperage draw drop and voltage increase.

RE: Radio control racer needs an electrical mentor

Next step might be (if possible) more voltage at the source, from the battery pack. If cooling isn't your main problem, then you're not trying hard enough. smile

A million years ago (1970s), we were playing around with the toy RC cars sold by Radio Shack. There was a fairly large "Porsche" that used something like 6 alkaline D-cells for motor supply. I added another batch (4?, 6?) of D-cells to the roof and wired them in series using wires and the clever trick of a piece of double-sided PCB to insert between two of the existing cells. This rather mindless concept worked extremely well and the toy RC car was much more powerful than originally designed.

PS: at the currents you're running, beware the possibility of magnetic (DC) or EMI (AC components) effects. If you see mysterious malfunctions of the electronics then consider EMI as one possibility. Keeping the supply and return paths close together can help self-cancel the emissions.

RE: Radio control racer needs an electrical mentor

(OP)
VE1BLL, I'm a little hesitant to put TOOOOO much voltage to a $250 speed control but once I have topped everything out that'll be the next step. I know that 180 is possible with the present set up...200 may take a couple extra cells.

The electronics are light years ahead of what they were just five years ago. We can get about 3/8 of a mile range from our radios and now the problem is these old eyes! Electronic 'funny stuff' isn't all that common anymore....but if I do, that'll be the first thing to look at!

Thanks!
Gary

RE: Radio control racer needs an electrical mentor

Your $250 speed control will probably have a handful of $5 transistors on the output. You might eventually be forced to make friends with them. smile!!

RE: Radio control racer needs an electrical mentor

(OP)
"I fear what I don't understand..." HAHAHAHAAHAA! The mechanical end of things make perfect sense to me, it's that fuzzy electronics that causes me to inevitably create smoke of varying colors....the last esc I killed put out the most beautiful and flourescent pink smoke.

That's why I'm here..... Like the wire post above, once the concept was given to me I could understand it. Things like that 'skin effect'? Never heard of it. You guys have given me more guidance than I could hope for.

I do have a question.... If my car were to suddenly break traction and the motor start to spin up, my electrical 'buddy' said there would be a big spike up in the amperage..... that doesn't make sense to me. It would seem that the tires breaking loose would cause a decrease in the amperage since the motor wouldn't be pulling as hard as it would with good traction. Yes? No? If I were to punch it at a standing start the amps would go way up, wouldn't they?

Thanks!

RE: Radio control racer needs an electrical mentor

What happens depends a lot on how 'smart' the controller is, and that's not clear.
If the controller were commanding constant speed and the load went away, the current should go down.
If the controller were commanding acceleration and the tires broke traction, the motor would now be accelerating only itself, the gears and the wheels, the controller might immediately go to its programmed current limit. ... or not; I don't know that much about BLDC motor controls in general, and nothing about this one in particular.

In particular, I don't see an encoder on the motor, so the controller must be sensing shaft rpm indirectly, or maybe not at all.

I'm on my second energy saving variable speed air conditioner. They don't have encoders on their three phase motor either, but they manage to sense the load on the air handler fan somehow anyway. I've read one of the patents, and I still don't understand it.

Mike Halloran
Pembroke Pines, FL, USA

RE: Radio control racer needs an electrical mentor

An encoder is only really useful for measuring absolute position, which is a don't care for car motors. Using position to calculate rate is a very noisy proposition, and unnecessary. The controller uses the back EMF of the motor to determine when to commutate, and once the rate is stabilized, the rate of commutation is the rate of the motor as well. see: http://ww1.microchip.com/downloads/en/AppNotes/010... starting at page 5 of the application note. Essentially, the undriven phase has a voltage that is generated by the rotor moving across the phase coil. You can use that information to determine the performance of the motor.

TTFN
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RE: Radio control racer needs an electrical mentor

A current spike when a motor loses its load is not common, however, it may be an idiosyncrasy of the controller. Then again it may be urban legend.
A voltage spike is more to be expected.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Radio control racer needs an electrical mentor

Current in a motor is a direct consequence of load. Under constant speed, the current is at its minimum, since the motor is only loaded by the frictional and other parasitic losses. That said, however, instantaneous changes in load can result in an instantaneous commutation error, which must require some finite time before the controller can determine that the operating conditions have changed.

TTFN
FAQ731-376: Eng-Tips.com Forum Policies

RE: Radio control racer needs an electrical mentor

(OP)
"...I don't see an encoder on the motor,"

There are two types of brushless motors used in RC racing, sensored and unsensored. The motor in my car is the unsensored type. Sensored motors(mostly made by NOVAK) have a harness going between the end of the motor and the speed control. I can only assume that's what the encoder would be.

RE: Radio control racer needs an electrical mentor

Generally, sensored motors do not have encoders; as I've indicated before, encoders are for absolute positioning determination, and encoders would be incredible overkill in one respect and less than useful in another. What sensored motors do generally have are Hall Effect index sensors. Thus, once a revolution, a timing pulse is generated from the Hall sensor is used by the motor controller to calculate rpms.

TTFN
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RE: Radio control racer needs an electrical mentor

By going to a higher voltage controller (I would recommend a 50V max controller) combined with a Neumotor capable of handling the increased voltage you can produce the same power output at a much lower current setting. This will help in several different ways: it will reduce IR losses in the circuitry, and it will also allow the batteries to run much cooler. This will increase their useful life and their efficiency, since LiPo batteries tend to become less efficient as they heat up. End result - you will go faster.

Maui

www.EngineeringMetallurgy.com

RE: Radio control racer needs an electrical mentor

(OP)
MAui..... not really sure there are any higher voltage esc's, are there? Castle makes one that will go to 10s but can't handle the amperage draw. I have room for 2 more cells so I'm willing to try for more!

Size is also a consideration and the one I have in there is about the biggest I can fit (was lucky to squeeze that one in)

Gary O

RE: Radio control racer needs an electrical mentor

You won't need nearly the same amperage draw with a 50 volt system, so you can go to a lower current rated ESC. The Castle HV ESCs that I have for airplanes will handle up to 50 volts.

Maui

www.EngineeringMetallurgy.com

RE: Radio control racer needs an electrical mentor

(OP)
The issue with the airplane esc's is that they don't have brakes, per se, rather a 'prop stop'. I had a Phoenix 200(?) in the car and found it unnerving to not really have control of the car's speed as it slowed, rather having a preprogrammed decelleration. At the speeds we're running we need to be able to throw on the brakes when needed.

Do you think the XL2 would take a couple more cells? one more?

RE: Radio control racer needs an electrical mentor

You sparkies are way over my head with most of this stuff, but wanted to offer a suggestion for determining your vehicle speed.

Rather than using a radar gun you might want to consider a photogate timer.

RE: Radio control racer needs an electrical mentor

(OP)
They're way over my head too, Ron....I'm trying to stay up with them and glean what I can from their posts.

I use a radar gun for testing but at our speed meets we use dual infrared optical timers which have to be within 3% of each other to be an official record. They are amazingly accurate and consistent usually being within 1%.

RE: Radio control racer needs an electrical mentor

(OP)
I wanted to run the car last weekend but had to accompany my mom to Dunkirk, NY to visit her 90 yr old sister this week. It's presently 15 degrees with 40+/-mph winds which makes for a wind chill somewhere around absolute zero.... reminds of why I left Detroit.

As long as the plane is able to get me back to beautiful Cali-for-ny-ay I'll run it Sunday and get some data from the esc to see where it's at now. Anxious to see the difference with shorter wires and a pound lighter.

RE: Radio control racer needs an electrical mentor

(OP)
Finally got to make a run today after making the wiring mods suggested here. I tried to post the graph of the run but am unable but the long and short is that the run was amazing....the stop, not so hot. The car pulled very hard and according to the graph topped out at 35000 rpm which works out to 149 mph in about 850 feet of run up.

The amperage draw was quite a bit lower with a max of 211 amps vs 281 on a previous run. The esc never shutdown and didn't really get hot (113 degrees)

The only down side is that the car hit a bump...I got out of the 'groove' but didn't let up...and it made a slight right turn into the curb and tumbled to a stop....and it took a looooooong time to stop! Unfortunately my radar gun didn't work and the little speedometer I had in the car got pretty banged up. Also the main switch got turned off during the tumble erasing any speed saved. The only damage is the fin sheared off but it's just the aluminum brackets that were damaged.

Some of the other available data....
Voltage ranged from a high of 32.8(static) to 26.9 (a downward spike under load)
Maximum watts was 5800 and the line on the graph pretty much paralleled the amperage.
Ripple had a max of 4.96....what is that? It again mimiced the amperage but then dropped way off as the rpm climbed.
There was also a percentage power out of 85% which again lsee closely followed the amperage line.

Anything wierd here? Despite the clumsy stop, I'm very happy with the reults. Next time I'm at the airstrip it's new record time!!!!!

Thank you all!

RE: Radio control racer needs an electrical mentor

Thank you very much for the report.
We'll be waiting for the new record.
Yours
Bill

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Radio control racer needs an electrical mentor

I've messed with RC cars & airplanes for years. Always thought this would be an interesting project to attempt, but never really began anything.

That said, let me play armchair general and bounce ideas at you. You are totally free to disregard these, as they are entirely off the cuff.

Consider an airplane ESC and a secondary means to stop (parachute? servo actuated brake?) I think you need to throw a lot more voltage at the problem as it would give you much more headroom regarding heating, wire gauge, etc.

I'm not sure that Bonneville streamliners are the right analog for this scale. Given a 1/4 mile to accelerate, you need a constant 1G acceleration to trap @ 200mph. This is more of an acceleration problem than a sustained top speed problem. Top fuel might be better design inspiration?

I'm not positive that weight/mass is your enemy. At low speed, I'd expect with this much power you are limited by traction. Traction is roughly Mass*g, and at low speeds it is fighting against Mass*a. The M's roughly cancel because I don't suspect these tiny tires loose much grip due to increased load.
At high speed traction is roughly Mass*g + aero_downforce. And it is fighting aero_drag. In this situation the added mass may actually help (since acceleration is tapering off, the Mass*a term becomes small). In addition any additional inertia might help dampen things out.
While I'm not suggesting you intentionally add mass, if there are places where you can benefit from a heavier component or stiffer structure, it might be a worthwhile swap.

Thought about active-aero? IE: Steer with a rudder? Or trim tabs controlled via heading-hold gyro? You are effectively building a ground-based airplane here...

Maybe an on-board microcontroller to automate your throttle-ramp via some kind of feedback (like accelerometer data?). This way, the "driver" could just "punch it!" and the onbourd MCU could execute an elaborate acceleration profile.

Did the Team Associated car ever make 200? Their website is kinda dead.



RE: Radio control racer needs an electrical mentor

(OP)
imcjoek,
I'm always open to ideas....anyone who thinks he's solved ALL the problems hasn't really tried to do anything of merit.

That said, let me play armchair general and bounce ideas at you. You are totally free to disregard these, as they are entirely off the cuff.
*I do my best thinking at bizarre times....nothing is too far out of the box.

Consider an airplane ESC and a secondary means to stop (parachute? servo actuated brake?) I think you need to throw a lot more voltage at the problem as it would give you much more headroom regarding heating, wire gauge, etc.
This past weekend I came across a three mile long, freshly paved stretch of road that is free of bumps, cracks, reflectors, manholes, houses and any other defects normally found in a road so braking becomes less of an issue. So far heat hasn't been an issue either....the motor is warm and the esc is only 115 degrees. The rpm graph on my 149 mph run showed a fairly straight line with no signs of flattening out so I think I'm still in the power band and only need another 15 mph for the record. For 200 I'm definitely going to have to look at more voltage. At that point an airplane ESC would be needed...do they have any with 200 amp capacity? I’ll have to check that out.

I'm not sure that Bonneville streamliners are the right analog for this scale. Given a 1/4 mile to accelerate, you need a constant 1G acceleration to trap @ 200mph. This is more of an acceleration problem than a sustained top speed problem. Top fuel might be better design inspiration?
*The guy who was going to be doing the electrical part of this project has gone with a dragster for his attempt. I see two problems, the first being only two wheel drive. It's my feeling that no wheel should touch the ground unless it's helping to move the car. He has been struggling to put his power to the ground since 30% of his weight ( I would guess) is just holding the front of the car down. He is putting a fairly aggressive rear wing on the car to counteract this problem. The second is the center of gravity in relation to the center of pressure.... when he has gotten sideways the car just starts to spin with the front end visibly being blown to the rear.

I'm not positive that weight/mass is your enemy. At low speed, I'd expect with this much power you are limited by traction. Traction is roughly Mass*g, and at low speeds it is fighting against Mass*a. The M's roughly cancel because I don't suspect these tiny tires loose much grip due to increased load.
With all wheel drive and about a six pound weight I am able to put full power to it fairly early. My earlier design(with much narrower rear tires) could spin them easily.

At high speed traction is roughly Mass*g + aero_downforce. And it is fighting aero_drag. In this situation the added mass may actually help (since acceleration is tapering off, the Mass*a term becomes small). In addition any additional inertia might help dampen things out.
While I'm not suggesting you intentionally add mass, if there are places where you can benefit from a heavier component or stiffer structure, it might be a worthwhile swap.

*The 'weight needed for acceleration' vs the 'weight restricting acceleration' balance is the holy grail obviously. Since I can apply full power somewhat quickly (within a couple hundred feet) I'm thinking I could stand to lose a few more ounces...I could always add some if it was too light. The overall restriction is these old eyes ability to see the dad gummed car a quarter mile away. we have a ladder in the bed of a truck which helps tremendously but to get to 200 we need as much run up and as much acceleration as possible.

Thought about active-aero? IE: Steer with a rudder? Or trim tabs controlled via heading-hold gyro? You are effectively building a ground-based airplane here...
You and I think a lot alike. The first version of this car was rudder steered...and it crashed every run. My goal was to make the nose as narrow as possible for less aero drag and didn't think I could fit steering in as well. At slow speed it had no steering and at high speed any input would roll the car. I have posted a pic below. An airplane has the air from the prop to move the tail at low speeds and it doesn't have the tires resisting every change in direction. After the 20th+/- crash I came up with the existing steering system and have no steering related crashes. Another issue that came up with the rudder is that the chassis has to roll perfectly straight with no steering input. With front wheel steering I can adjust the trim and get it rolling straight. While typing this I realized that the front end doesn’t respond well at high speed so maybe a small rudder AND front wheel steering would assist at high speeds.

Maybe an on-board microcontroller to automate your throttle-ramp via some kind of feedback (like accelerometer data?). This way, the "driver" could just "punch it!" and the onbourd MCU could execute an elaborate acceleration profile.
I have a 'delay box' used to slow the landing gear doors on airplanes and combined with a modified acceleration curve in the ESC I could do that but since adding the wider rear tires it hasn't been necessary. I just roll on the throttle and its on its way!


Did the Team Associated car ever make 200? Their website is kinda dead.
Associated was sponsoring the guy (Nic Case) who holds the present speed record to be the first to 200. First off, no....no one has been anywhere near 200 yet. Associated's plan was to go with a shaft drive car rather than belt drive. I used to run shaft drive cars until I had a shaft bend and spin itself right out of the car at about 120, taking several components with it. The differentials have a 2.5:1 reduction so for the tires to be spinning at about 28,000 rpm (200 mph) the shaft would have to be spinning at 70,000 rpm, assuming no tire slippage. After spending an 'alleged' $50,000 on the project it died on the vine. Nic is presently building a 4wd belt car for his own 200 attempt. Rumor has it he has apparently been in the mid 170's but the car blows over each time.

IMHO, the above car is too wide, increasing drag and introducing more top surface to the whims of the wind trying to pick it up. The down force needed to keep it on the ground will also keep it from 200

The 2wd dragster will need too much down force to put all that power to the ground with only 60-70% of the mass on the tires.

I have confidence in my design and will be looking for the little things, like your suggestions and other on this thread, to get it to 200.....

Thanks!

RE: Radio control racer needs an electrical mentor

(OP)
Another question if you’ll indulge me....

I presently have a 27 volt rated 2200 kv (rpm per volt) motor on 32 volts with a 1.66 ratio to the wheels.

If I were to replace that motor with a 1650 kv 36 volt motor (same model and size motor) on 32 volts with a 1.24 ratio to end up with the same wheel speed as above will I have more or less power at the wheels?. What are the advantages or disadvantages to this new set up?

Several of you have suggested higher voltage but I have not been able to find an esc that will take more than 8s at anywhere near the amperage I’m pulling. Any other suggestions or product examples?

Thanks!

RE: Radio control racer needs an electrical mentor

(OP)
I should add that because the runs are short (15seconds +/-) heat hasn't really been a problem. The motor and esc get warm but no where near hot.

RE: Radio control racer needs an electrical mentor

http://www3.towerhobbies.com/cgi-bin/wti0001p?&...
Is supposed to be good for 12S @ 160A. Kind of pricey though, and you are left with the problem of stopping with an airplane ESC.
If you're feeling ambitious, maybe time to use a smaller ESC to drive your own IGBT amplifier. How you handle commutation is a bit tricky there, though.

It's difficult to compare motors based only on the silly hobbyist KV ratings. It only gives you a no-load RPM @ voltage. I would *expect* the lower KV motor to have more winds, and to draw less current at the same voltage (with rotor stalled). Thus you could potentially throw more voltage at it.

RE: Radio control racer needs an electrical mentor

(OP)
Oh, I'm ambitious but the research I did on IGBT amps just lets me know how much MORE I don't know! I had one of those esc's but sold it when I received the XL2's... I may regret having sold it. On a wide open road I think the 'prop stop' braking might actually be okay. Where I test now I have the occasion to have to slam on the brakes (traffic, curbs, etc.)

There is more info on the motors on the Neu site... I just used the Kv since that's what I understand.... here is a link to the page, I'm using 1515 motors because of their length.

http://www.neumotors.com/Site/1500_series.html

RE: Radio control racer needs an electrical mentor

2
Here's a quick-n-dirty spreadsheet comparing the two motors. Since the motors have such low armature resistance, and inductance is ignored, you have to make some WAG as to the resistance of the rest of your system to make the numbers come out sane. IE: 32V shorted across the 0.006ohms of the motor is some absurd amount of amps. Obviously the battery/wire/ESC resistances factor in here. I think for comparison purposes, it's good enough, though.

The basic gist of it: at the same input voltage, the lower Kv motor will have a bit less power, at less rpm, and at less current draw. Upping the input voltage until the current draws are comparable results in better performance from the lower Kv motor, but now your ESC needs to handle that higher voltage.

RE: Radio control racer needs an electrical mentor

(OP)
Wow! Thanks for that spread sheet!! It looks like there's a sweet spot at 34-36000 rpm with the 2200 kv motor whereas it's much lower with the 1650. Could one draw the conclusion that a 2700kv motor have a similar sweet spot at a higher rpm level (42,000+/-rpm?) along with a higher power level?

I feel like I'm on the right track with the voltage available to me, depending on how brave I want to get running the motors at higher voltages than they're rated for. It's only money!

Gary O

RE: Radio control racer needs an electrical mentor

PMDC motors always have peak power at 50% of no-load RPM. It's just how the math works out since their "torque curve" is actually a straight line.
I've never thrown voltage at one of these motors until it failed. Get yourself a cheaper one and see what the actual failure mechanism is (take pictures). I would suspect the magnets could be damaged by overheat, or the coil windings may just burn up from excess current.

RE: Radio control racer needs an electrical mentor

(OP)
"PMDC motors always have peak power at 50% of no-load RPM"....that little nugget right there is worth the price of admission!!! Somewhere I got thr impression that these were AC motors.....

I can pick up Castle motors 'relatively' cheap ($100+/-) and since the present motor isn't overheating I was going to step up the kv and give it a try. You can't go after world records and worry about a few bucks. If it was easy, everybody'd be doing it!

Thanks!

RE: Radio control racer needs an electrical mentor

(OP)
...and as a follow up question, would it then be your suggestion that I gear the car so that the target speed is achieved at the 50% no load rpm? If it revs above that then there's more speed in it....Theoretically, if the car reaches it's fastest speed at that 50% point, gearing it up or down won't make it any faster...correct?

RE: Radio control racer needs an electrical mentor

Quote:

Somewhere I got the impression that these were AC motors.....
Eh, the naming can get a little strange because, I'd guess, of marketing nonsense. Industrial motor naming is even more bizarre.
The motors you are using work the same as a brushed DC motor (Mabuchi, etc), except the commutation is handled electronically, instead of mechanically via comm/brushes. They are usually powered by DC (batteries), but the controller generates the AC synchronous bit. So in some sense, you could call it either.

On the gearing thing: on paper it's a "maximize area under the curve" problem. In reality it gets more convoluted because the power requirements during your run can vary, and you need to watch the current draw and temperatures. You could "try it and see" and hope you can make sense of why things improved or did not. Or cobble together a basic model of your system and attempt to feed data into it to refine it until you can predict what your changes should do.

I've never been down your path, so with any luck I am not leading you entirely astray.



RE: Radio control racer needs an electrical mentor

(OP)
As you stated, there are many factors to consider as I gear the car but at least I have some sort of target and a general idea of whether I'm above, below or right in the power band. I have revamped the wiring and realized some real gains there. I even went through my weekend 'club' cars and shortened all of those wires and hope to see some gains.

According to a friend with 'friends in high places' at Castle they're not making 10-12s airplane/heli esc's anymore so my voltage is limited to 32 for now. I really think that'll be enough to do the job. If not I'll revisit the esc facet of this adventure.

Thanks!
Gary O

RE: Radio control racer needs an electrical mentor

(OP)
Nothing but good news.... Castle has apparently started producing the ICE2 esc again, good for 160+ amps. Once I have topped out this set up I'll be switching over since I have room for two more cells at the back of the car and maybe one more up front.

I picked up a Neu 2700kv motor and a couple pulley combos so I can adjust my way into that sweet spot rpm range.

The best news is that a friend has managed to get permission to run on a beautiful airstrip about an hour and a half from my house. It's 3000 feet long and 75 feet wide......PERFECT!!!!!!

On March 30th I'll be running on our usual track site in Shafter, CA. Its about 2000 feet long but only 30 feet wide and somewhat oddly shaped. It'll be the first time at this site since I started getting suggestions from you guys. I feel very confident the car will live up to it's potential.

I wasn't able to get permission to use the nice long road I found....the airstrip is better anyway.

imcjoek, while running my car on the street I have noticed that I'm lacking sufficient steering input under acceleration and have put a small rudder back on the car to help with that.... maybe the combination of steering methods will give it the stability needed for these speeds.

BTW, one of my competitors bumped the record up to 169.19 a week ago. I felt his car was too wide and had too much down force to reach 200 and fortunately it was....gives me a little breathing room. Here's a video of his runs.....

https://www.youtube.com/watch?v=SLd53jU81as

RE: Radio control racer needs an electrical mentor

It sounds like you're having good success.

I believe imcjoek was referring to not just using a steering fin but using a gyro with the fin. Basically, the gyro would automatically adjust the fin to keep the car going straight.

I believe there are 2 types of gyro controls. One that tries to keep it going the same direction no matter what (heading holding) and one that adjusts for any change in direction not called for by the radio.

The heading holding gyros are often used in RC planes during takeoff and landing to keep the plane going straight down the runway.

So, I'm thinking run a gyro on the steering. Drive long enough to know the car is heading true down the runway and then turn on the heading holding and not steer any more.

If it is required then add a steering fin that works with the steering wheel. I'd investigate putting it towards the front so it assists the steering wheel. At any rate, you'll have to play with fin vs wheel turning to find some stable workable ratio.

RE: Radio control racer needs an electrical mentor

(OP)
What is this magical, mystical heading hold gyro of which you speak???????

Did a bit of searching and voila! What a piece of wonderfulness! This allows me to start the car at a distance beyond my ability to see it! That is a must have! Thanks for the great tip....

Rudder-wise, I reinstalled it in it's original position (rear) mainly because I already had most of the parts made and it pretty much bolted back in. If it's not effective I'll investigate mounting it up front.

Gary O

RE: Radio control racer needs an electrical mentor

Check that center-of-lift on the rudder is not too high. A rear mounted rudder is likely to have roll-coupling problems. IE: Try to yaw (steer) left, also gives you roll to the right. This is added to a car's natural tendency to roll to the right in a left turn... You need your rudder surface area as low as possible. A front mounted rudder would roll-couple the opposite way. Left yaw, left roll. So that might be better.

Heading hold gyros can still drift over time, because they use numerical integration methods to obtain the angle. Still probably better than a squishy human can do at these speeds, though.
There are also Inertial Measurement boards which combine gyro/accel/magnetometers/and sometimes GPS. They're usually used in UAV's and quadcopters, but you could probably tweak the software to give you rock solid steering.

RE: Radio control racer needs an electrical mentor

(OP)
imcjoek, I had thought of the tendency of a high mounted rudder to roll but not the difference of the front vs rear rudders. I kept the majority of the rudder low, as you mentioned. Any additional rudder will also be added lower.

RE: Radio control racer needs an electrical mentor

(OP)
Just an update..... I made carbon fiber parts to replace the fiberglass originals used to develop the car. While I had it apart I drilled out the aluminum bulkheads for more wieght loss. All together, with the re-wiring suggestion y'all made and this revamp the car is a pound and a half lighter than it was the last time it ran. I'm also talking to a Lipo battery company to see if their lower Mah, and subsequently much lighter cells will keep the voltage higher enough during the runs. If they work they'll take another half pound out of the car.... taking two pounds out of a 7-8 pound car is phenomenal!!!

I got my heading hold gyro yesterday....that's going in next to see how it works in a car. I've gotten opinions both ways.

Our next run is April 20th near Bakersfield. It's a 2000+/- foot strip about 30 feet wide. Once I get the timing lights back (assuming the car comes back in one piece!...not guaranteed) I'll make arrangement to go to the 3000 foot long 80 foot wide airstrip we got permission to use to continue the quest for 200.

To say I have high expectations is putting it lightly

RE: Radio control racer needs an electrical mentor

(OP)
I forgot toadd that with the lower weight and bigger motor I added a tooth onto the pinion pulley so that at 1/2 the maximum rpm it'll be travelling 190.....ya know, if it happens to be able to rev higher, something cool might occur.

RE: Radio control racer needs an electrical mentor

(OP)
Got my new cells today..... 8s worth of 3850 Mah 70c/140c burst Thunder Power with 8ga wires. As an assembly they're about 100 grams lighter than the previous packs so I'm still short of my target two pound weight reduction but still WAY better off than I was!

The batteries I was running were 4s 'hand me downs' that had crashed at 170 and were seemingly getting confused and a pair of 2s packs with 10ga wire. Now I have fresh cells with monster wires!

April 20th is the meet at Minter Field near Shafter, CA. Wish me luck!!!!!!

RE: Radio control racer needs an electrical mentor

(OP)
A friend of mine described what I do as similar to a slot machine addict.....I just know that this next pull of the lever (or run of the car) is gonna be a WINNER!!!!! Ooops, maybe the next one.....maybe the next one.

Ran at Shafter last Saturday and had a long, wide airstrip to run on. The car had more radio range than I had sight range. When the car hit about 125 it started porpoising, the rear wing pushing down hard enough to raise the front wheels. When the rear fin hit the ground and picked the rear wheels up, the front came back down and did it over and over again.

This banging broke a wire connector in the reciver pack and I lost radio control just before the traps. The car strayed off the pavement into the gravel and came to a stop in the sage brush. Wasn't any real damage, just real dusty. Also while changing the motors out I must not have tightened the pinion down enough because it departed somewhere along the way.

Here's a vodeo of the run. Although you can't see the up and down motion (I could) you can hear it. There's a puff of dust in the distance at the finish when it come to a stop.

http://www.youtube.com/watch?v=_lUYzMyl3BI

Next run/pull of the lever is May 18th. I have cut the rear wing down, made a new pinion and have rebuilt the reciever pack.

RE: Radio control racer needs an electrical mentor

Can you move the rear wing forward? If it was ahead of the rear wheels the porpoising would not happen.

----------------------------------------

The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.

RE: Radio control racer needs an electrical mentor

(OP)
The fin is way out back to aid in stability. The further forward it goes the closer to the center of pressure it gets and becomes less stable. I should have mentioned that the elevator angle is adjustable and I will reduce it's angle of attack by a couple degrees...it was at 3 degrees on that run. I have also narrowed the wing an inch on each side so it's now four inches wide vs six.

Part of the idea of the wing in the back was to tansfer some of the weight to the rear. The front tire was getting chewed up quickly without the elevator but after the wear was more even with the rears. It's just another adjustment to be made.

RE: Radio control racer needs an electrical mentor

(OP)
A second video of the same run, closer and you egt the feeling of the speed. I keep getting closer to a full length, full power run.

http://www.youtube.com/watch?v=mF6Tc1RxINQ

RE: Radio control racer needs an electrical mentor

I've been wondering the same thing. Hope that there weren't any 'accidents'.

John R. Baker, P.E.
Product 'Evangelist'
Product Engineering Software
Siemens PLM Software Inc.
Industry Sector
Cypress, CA
Siemens PLM:
UG/NX Museum:

To an Engineer, the glass is twice as big as it needs to be.

RE: Radio control racer needs an electrical mentor

(OP)
Sorry for being quiet.....haven't had a chance to run. I was supposed to run the beginning of June at Bakersfield but the date got switched by my 'buddy' to a date that I specified I could not attend..... read what you want into that!

I have secured permission to run at the Willits airport, tentatively Aug 25th. I will post all results here.

Gary O

RE: Radio control racer needs an electrical mentor

(OP)
OY! The airport manager wants to talk with me about running there. At least he didn't say NO!

Thought this was all set!

RE: Radio control racer needs an electrical mentor

Thanks to everyone who helped this guy on his project. As I have learned plenty about my Radio Boat and what to watch out for before I let the smoke out.
Mike Caruso
Chicago, IL

RE: Radio control racer needs an electrical mentor

OP has a video posted in Dec, crashed at about 150 mph. Says he intends to rebuild.
Select either of his other videos, click on his username. That brings up the other vids he's posted.

Jay Maechtlen
http://www.laserpubs.com/techcomm

RE: Radio control racer needs an electrical mentor

Reduce the connections between the motor and the ESC single wire, and the same with the battery to the ESC.

RE: Radio control racer needs an electrical mentor

(OP)
Sorry for the long delay, been a frustrating few months..... yes at the airport I was careless and didn't set the failsafe (which would bring the car to a stop in case of radio signal failure) because I was going to shut it down a couple hundred feet down course. Well there was some SERIOUS radio interference and the car just kept accelerating before plummeting over the cliff at the end of the airstrip!!!!! The area was very overgrown and it took me 15 minutes just to find the car! The one good thing that came out of the day was this cool pic of the car at speed just before flying off into oblivion.


The tail was busted, fin/elevator was missing along with some other damage but the 'core' of the car was intact. I rebuilt again and returned, this time setting the failsafe... to no avail as the car just took off after being set on the ground. It smashed into a rock damaging many of the pieces I had just repaired without making a single run.

Rebuild for the umpteenth time, fortunate that the core had again survived. Traveled down to Shafter airport (near Bakersfield) and ran 146 through the timers only to have the pinion come off so I had no brakes..... it rolls a looooooooong way at that speed and did a few endos at the end of the strip. At least there was no cliff!

Rebuild for the umpteen and one-th time and ran at Shafter again this past weekend. Actually got three runs in, all around 140 - 150 mph but wore out a front tire each time....except the first when the front blew up. Data from the speed control shows an rpm peak that calcs out to 190 mph so the tires are spinning madly. When I came home I threw aero to the wind (pun) and mounted two full size tires on the front. The weather cleared long enough for a quick test and the difference is amazing...it will accelerate to 100 in just a couple hundred feet. I underestimated the power of that motor by a bunch and will be returning to Shafter a few weeks from now to run it with those tires and see what the results are. Here's a comparison of the original single tire and the new set up... five times the rubber on the road.

If it works as well as it seems I can redesign the front half and narrow it up by an inch and a half and still retain the full width tires.

RE: Radio control racer needs an electrical mentor

(OP)
Bluecat46.... there has to be connector between the components to change them out. I understand that a one piece wire would offer less resistance but for maintenance it would be very work intensive. The bullet connectors I use are very efficient and probably the best on the market. Do you have knowledge of a better connector? I'm open to all suggestions!
Thanks,
Gary O

RE: Radio control racer needs an electrical mentor

(OP)
Supposed to be able to run at Shafter again April 5th. I have switched the two really wide tires for three narrower versions and didn't lose much tire width (5/8 inch) while narrowing the car by more than an inch. I made some fairings from foam and duct tape to cover the tires and will re-do the mold for the front if it's successful.

Always optimistic!

RE: Radio control racer needs an electrical mentor

Watch for witness marks from the third tire.
That will give you a good measure of tire growth at speed.

Mike Halloran
Pembroke Pines, FL, USA

RE: Radio control racer needs an electrical mentor

(OP)
Not sure what you mean, Mike. They're all the same diameter after cutting them down on the tire lathe.

RE: Radio control racer needs an electrical mentor

(OP)
Ahhhhh..... these tires don't grow like that. They're foam and securely glued to the rim. I have spun them up to 25000 rpm on my router with no funny business. Because they've been cut down there's less mass trying to pull them apart.

RE: Radio control racer needs an electrical mentor

(OP)
Hey guys! I ran the car this weekend and got three solid, complete runs in the 150's each time. The car ran straight and stable, stayed on the ground and my new tire arrangement worked great with even tire wear all the way around.

The bad thing is it was in the 150's.....

When the car comes off the track all the components are warm, but they're not really hot. VE1BILL posted earlier that "If cooling isn't your main problem, then you're not trying hard enough." It sounds like I need to up the gearing and start getting things hot. If that doesn't get me to my goal I'm going to have to start looking for a speed control and motor that can handle 12 cells, the max I can fit in the car. (I'm at 8 right now)

I have in-car video which I'll post when it downloads.

RE: Radio control racer needs an electrical mentor

Castle Creations HV ESC controllers are designed to handle up to 12 cells (up to 50 volts).

Maui

www.EngineeringMetallurgy.com

RE: Radio control racer needs an electrical mentor

(OP)
Maui.... I'm not crazy about the fact that the heli esc's have prop stops rather than brakes but it's the only option. The amperage rating is lower than the esc I'm using but I have exceeded that with no repercussions. If I understand correctly, as the voltage goes up the amperage draw goes down.

I'll see what changes the gearing makes on the apmperage.

Thanks!

RE: Radio control racer needs an electrical mentor

They are designed with different amperage ratings: 85, 110, and 160 amps continuous. The transient current draw can be MUCH higher than this with these controllers. I own the 85 and 110 amp versions. They work flawlessly.

www.EngineeringMetallurgy.com

RE: Radio control racer needs an electrical mentor

(OP)
Oh, so you gotta throw THAT in my face, eh?????? HA!

I know Nic very well and admire his accomplishments but he SAYS he can't go much faster because he's burning up speed controls. His car weighs 11+ pounds (mine is 6.5), he has considerably more frontal area and down force and less rubber on the ground. On paper my car should be MUCH faster but the race isn't on the paper.

With lower amperage draw I should be able to run lower mAh packs thereby recouping some of the increased weight of 12s vs 8s batteries. Castle has a 12s speed control but I haven't been able to open the tech sheet on their site. I'm going to have to bite the bullet and spend some bucks because I seriously doubt that I can add 50 mph to my speed with only regearing!

RE: Radio control racer needs an electrical mentor

"The car’s body has an integrated virtual CFD wind tunnel for ultimate downforce and low drag."

What the heck is that supposed to mean?

RE: Radio control racer needs an electrical mentor

Quote:

The car’s body has an integrated virtual CFD wind tunnel for ultimate downforce and low drag.

I had a good laugh at that as well. At a guess, I'd say it means that whoever designed the body used a computational fluid dynamics (CFD) program to tweak & optimize the body design, in essence using a 'virtual wind tunnel' for testing. Either the article author, or the person (s)he interviewed was confused by the jargon.

RE: Radio control racer needs an electrical mentor

(OP)
Nic works at Ford Motor Company's design center and has access to lots of specialized computers and gizmos. He has expressed aero opinions that make no sense to me but I'm not an aero engineer.... for instance, he believes that a flat, open rear end on the body has less drag than a pointed rear like, oh I don't know, EVERY SINGLE airplane and streamliner on the planet.

I'm glad you guys don't get it either.

RE: Radio control racer needs an electrical mentor

"EVERY SINGLE airplane and streamliner on the planet."

Up to a point, yes. However, the best performing bullets are boat-tailed, and the noses are not perfectly pointed, but instead, the meplat is small and flat. Of course, much of that may have to do with consistency, rather that outright speed. Competition is more about accuracy, although a faster round is less affected by wind. All designs are compromises, so certain features may be sacrificed to achieve some other result.

TTFN
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RE: Radio control racer needs an electrical mentor

(OP)
More info..... the throttle is topped out at the same time the rpm is max'd out (34k rpm). At the same moment the amperage draw drops telling me it's not accelerating or pulling anymore. the draw decreases until it hits a level to maintain that speed for a short time.

With the voltage and kv of the motor this thing should continue winding until 45-50k.

Any ideas?

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