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Slip and Axle Stiffness

Slip and Axle Stiffness

Slip and Axle Stiffness

(OP)
Hello Eng-tips experts. Is there a short answer to why increasing axle stiffness at one axle will create more slip at that axle relative to the other axle? This question is related to how to get more oversteer/understeer ie suspension tuning/roll bar tuning.. Thanks!

RE: Slip and Axle Stiffness

Long - the lateral force supplied by a tire at a given slip angle increases with increasing vertical load, but not by the same proportion. Therefore the total lateral force supplied by an axle reduces as weight is transferred from the inner wheel to the outer is increased.

One gotcha is that if you add more bar at one end, with no other changes, you reduce the roll gain, and if your car relies on roll steer, you might find that adding front end bar alone has little effect on understeer.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

RE: Slip and Axle Stiffness

(OP)
So increasing axle stiffness increases the weight transfer to that axle relative to the other.. and with more vertical force(on that outside tire) it develops more slip, ok?

RE: Slip and Axle Stiffness

(OP)
So best case is: having both outside tires with identical load on them, ie Fz_out_front = Fz_out_rear -> neutral steer and maximizes grip ** assuming same tire F & R (if the rear were a larger tire you may want more Fz there relative to achieve neutral steer)


Lastly, why does the stiffer axle take/get more of the weight transfer? (I assume this is a basic physics/mechanics question)

RE: Slip and Axle Stiffness

Except that having identical loads on outside front & rear tires will still produce an understeering car because of the summation of all 4 tire's Mz moments. Also, your favorite goal of Neutral Steer will produce a car or truck with likely very poor dynamics (transient response) because the system collapses down to slow first order behavior.

RE: Slip and Axle Stiffness

(OP)
Thanks for the twisting tube analogy, I will consider...

So as far as "likely very poor dynamics (transient response) because the system collapses down to slow first order behavior", how else can I understand this? an explanation would be helpful.. thx

RE: Slip and Axle Stiffness

The time it takes to sense a response for a neutral steer vehicle is a long longer than a car with some understeer.

RE: Slip and Axle Stiffness

(OP)
Gotcha..

My initial question has been answered so thank you.. however its created more questions.. so a question to "sensing response" and understeer:

If my main objective is to get a car around a (asphalt)track the fastest, lets say this track 2.5 mi



should I be setting up for under neutral or over steer.. ie what is the norm in race car land for the fastest setup? Thanks again!

RE: Slip and Axle Stiffness

Az. Motorsports Park speeds can get ~200 mph. That means aero balance moves around quite a bit. It all depends on tires and driver skill. You have 4 wheels or 2 ? 4 wheels usually likes a tad of understeer to align the car's frequency response with the drivers. So, digital sound source, incredible amp, great ears and shit speakers means shit sound. They all have to be in "Q". Generally oversteer can be tolerated at "low" speed, as long as the steering gain is still high and the steering effort is light. At high speed, an understeering car can be nervous because of its lower damping coefficient.

Then there is the vernacular: What is "understeering" ? I imply that "understeering" from any level of driver means that the change in front vs. rear axle slip angle gradients is no longer constant, but is up (understeering) or down (oversteering).

Your question also needs clarification between qualifying (I own the line for a few laps), or racing (I have to share the line with a better or worse vehicle + driver combo). Podium for Finishers, not crashers.

RE: Slip and Axle Stiffness

(OP)
Gotta love it.. a tad of understeer it is. Now to find a way to test that (in my stripped down 07 sti)

Thanks!

RE: Slip and Axle Stiffness

There's probably a decent Roundabout nearby to where you live. Warm it up, drive at a constant speed for 1 full lap. Note the steering wheel angle. Try to be exact and not jerking it all around. If possible, time the length of each lap. Run 5 or 10 mph increments until you smell burning rubber or get arrested for "impeding traffic". Now make a plot. A Neutral Steer car will run the entire gamut of speeds at the same steer angle. Google the radius of your Roundabout and compute the lateral acceleration at the speeds you ran at. Now you will have a plot of steer angle as a function of lateral g.

If you get this far, call back in. We can get you the next simple step to get your understeer vs. Ay level function. I presume you have Excel to kick this all around... ?

RE: Slip and Axle Stiffness

(OP)
I like this approach, so we're skid pad testing.. yes can compute no prob. Interesting thing about the sti is im having a hard time driving it off the track, ie oversteer. Since I've done lots to lighten the car, keeping the original suspension actually makes for stiffer rear rate so the understeer is at bay now a bit. As far as I can tell, and it's only been a few track days with it, it looks like the fastest way through a corner with that car, awd & a tad smaller R comp tires is: brake in straight line, quick shift, turn and accelerate hard in turn(weight shift to the rear).. if the car goes a bit too sideways put the clutch in to balance it, then clutch out and hard accel out of the turn. It almost feels a bit like drifting but in an awd youre definitely getting grip.. thats about as neutral as I've felt. I'll try to put some data together for the skid pad.. thx NOTE: the beauty of having the clutch in mid turn is you get some good RPM going and in a turbo car like the sti it REALLY pays to have that RPM when the clutch goes out.. big turbo kick..

RE: Slip and Axle Stiffness

So your WRX STI (according to my database), had Bridgestone 225/45R17 Potenza RE070 skins at 36 psi front and 30 psi rear. K&C data indicates a few % roll oversteer in the front and a few % roll understeer in the back. Roll centers near 0 in front and 100 mm in the back. I did find that we did tire tests on those meats on 2 rim widths with (surprize) the tires are 'better' on 7" rims than 8". Not an uncommon finding, but not expected. What would raise my eyebrows is a relatively soggy steering system on-center that stays that way over the course of most cornering levels. So: soggy but relatively linear. All this attempting to get the car some understeer and keep it from going North (so to speak). So, It looks like you could do a couple of things to help your situation:

Find a better rear tire/rim/pressure combo. Get a bigger pump for the steering gear hydraulic supply because that's a high steering compliance influencer/fixer. Still, the weight distribution it came with is quite fwd biased, with roll centers and stiffness designed (or discovered to be) necessary to pull it back. If you have stripped the car, get the rear roll stiffness down. Dampers are (probably) also proportioned to effect a fwd dynamic load transfer, so maybe back off the rears.

Get the Roundabout data and report back. Because of the architecture & tires on this car (presuming yours is near to production intent, there won't be a big increase in steer angle Maybe just a few degrees or so for a speed range of, lets say, 30 to 140 kph. Judging from your comments, it may actually cause you to reduce steer angle during an increasing speed constant radius test.

RE: Slip and Axle Stiffness

BTW: If you can find a decent large diameter roundabout to play on, take a few friends along to block other traffic from entering so that you don't have to run them of the road. You get extra points for running in the opposite direction, best done after midnight in some areas.

Once you have the data, (you also need to know the overall steering ratio), the running derivative of steering wheel angle divided by steer ratio by lateral acceleration is the vehicles understeer with this test procedure. Then all you need to know is how good are you at driving with the control sensitivity that results from all of this.

RE: Slip and Axle Stiffness

(OP)
Great info on the sti. I have little to compare the car since I haven't had a "sports" car in about 15 years. Tires are 215/45/17 Toyo R888R and seem to be holding up ok. After last session I notice the fronts outer edge got burned up a bit as I was perfecting my slide through turn method. The 215s were 2 lbs lighter than the 225s is why I went a bit smaller. I also figured that I'd trade a bit of grip for accel out of turn(only 300hp stock, but 300 ft/lbs torque) and seems to be hanging in that regard with much "faster" cars. Pressure about 32lbs. The strip was mainly interior, gut doors(lexan in back doors), single racing seat, added one of those tiny batteries(anti gravity).. weight down to 2900lbs, so about 450lbs from stock. I'll look into a test area for the skid pad test and report back.. thx!

RE: Slip and Axle Stiffness

I went sniffing in my tire test database and with your loads/wheel of about 450kg front and 300kg rear (just a swag), here are by far the best meats for a car kike yours:

1) 225/45R17 GDY Response Edge 35 psi hot. .400+ cornering coefficient (load normalized 1 degree slip value).
2) 215/45R17 DUN (don't have the construction number/useage, but still near .400 cornering coefficient.
3) 235/45ZR17 HAN (same conditions, slightly less cornering grip).

Keep in mind that the larger load capacity tires probably need a more aggressive load transfer formulation because they give up less as vertical load is stuffed on them. But they hold on longer than a smaller tire. So, it's now back to Einstein's General Theory of Tires: Good, Fast, Cheap. You only get to pick two qualities of these 3.

[Not Albert Einstein, his cousin Fred.}

RE: Slip and Axle Stiffness

(OP)
Interesting.. curious how do the R888R compare? what metrics are you using?

Why not an R-comp tire? car mainly sees track use.. thx

RE: Slip and Axle Stiffness

A word of caution here:

I think most of the folks responding work for an OEM and their data is of OEM tires. That's important because OE tires are different than aftermarket tires in that OE tires are designed to OEM specs, which usually include rolling resistance as an important parameter - unlike aftermarket tires where rolling resistance is barely an issue and sometimes is not an issue at all!

Further, if the data is from an OEM source, it will be individually unique as the OEM specs vary widely and no 2 OE tires are the same - even if the name on the sidewall is the same.

As an example, the tire company I used to work for supplied 5 different tires of the same size and name on the sidewall to 5 different OEM's and the only way to tell the difference was 1) the symbol on the sidewall required by the OEM and 2) the DOT code - and, of course, each of these was different than the non-OE version of the tire.

RE: Slip and Axle Stiffness

Not necessarily true. My OEM tests plenty of performance tires, especially for the 'max' handling Level 3 versionsbeing persued. This is necessary to push the OE TIRE manufacturers to advance the ball, AND because the rules of "Forseeable Misuse" require you to know what a 3 sigma tire does on a car on which it fits. Called "Tire Intermix", the most common issue is a "high performance" tire replacing just the 2 front tires while the worn out rears remain in the rear.

Also, if a 911 GT3 or a Ferrari F430 is brought in for 'fingerprinting' you'd better believe I have tire F&M tests of both front & rear tires off them. The OEM tires, not ones a dealer might substitute on them to sell separately. so, yes I have this information. A few years old because I'm long gone, but not out of date because there are limits to a tir'e sizes capability no matter what compound you slap on it.

My OEM also challenged new engineers to create a suitable SCCA autocross vehicle, requiring plenty of race slick data, usually Hoosiers, in fitments suitable for the weight and performance class selected.

The tires I showed ARE from production competitive cars, meaning they are likely to be purchasable thru various outlets in quantities needed. In this case, it's tires likely to fit in the wheelhouses without wheel travel restrictions. But, run what you brung if that's what your budget permits.

RE: Slip and Axle Stiffness

(OP)
Im a bit limited for tires for the sti as far as R-comp, not sure a non track tire would last out there. I’ll probably end up trying all the track rated stuff before too long.. I went through a practically new set of stock front pads in 2 20 min sessions, completely gone.. needles to say im hard on the car. Upgrading to large cooling ducts and Ferodo race pads for next session. Tires with temps in the 180 deg range max(i have a IR temp monitor mounted).. what would a street rated tire do with 180deg temps?

RE: Slip and Axle Stiffness

Is that an outer, inner, or center temperature? A single channel is NFG (IMHO) if the working section of the tread is not identified. Here is the kind of display it takes. I've had to alter the load scaling factors because manufacturers DO NOT approve of lab test data distribution. Hopefully you get the idea. Road tire temp data is available, just nobody usually cares about it. If the tread comes off, it's too hot.

"Being hard on a car" is generally NOT considered to be an attribute. "Slow & Smooth is Fast" usually prevails. Figure-8 Racing is the exception. More fun, too. Owners generally prefer to have their entries finish, then top 10, then top 3 without the thrashing that tire, suspension, and engine abuser's can be noted for. Watch some Rally car videos to see what I mean.

RE: Slip and Axle Stiffness

cibachrome,

I am glad you chimed in. I wondered if any OEM tested off the shelf tires, especially hard-core grip tires. I would like to hear from others.

But my point stands. OE tires are different than aftermarket tires and people need to be aware of that.

RE: Slip and Axle Stiffness

Absolutely, but my local Audi dealer will put them on an R8 without hesitation. Just a warning about wet weather. The new hires here put them on an S-10 "race truck' to autocross. Wound up with hoos your daddy's, Wide Rims matter. Hey ? WRX = Wide Rims Xtra ?? Subaru= Substantially Underpowered, But Accelerates Uphill.

This is just 1 degree data but you get the idea. "Yes or No" as Claude would say ....

RE: Slip and Axle Stiffness

(OP)
Google turns up a quick description of shredded street tires at track temps.. looks expensive and dangerous. Not sure if this is a good case of the exception vs the rule, but the saying “bring a knife to a gun fight” may apply..

https://www.s2ki.com/forums/wheels-tires-56/bridge...

RE: Slip and Axle Stiffness

The guns I take to gun fights are hollow points or wad cutters. There won't be any ties. Because some manufacturers forbid testing of their tires, you have to sneak them onto a Flat-Trak on the weekends.

BTW: Make sure any race tires you 'acquire' have the exact same build date. They are like Grandma's chicken soup. You know it's 'chicken soup' made that day, just not sure how much chicken you'll be getting next week.

RE: Slip and Axle Stiffness

Greg Locock said: " ..... One that people don't think about is trailer tires. ...."

OK, I gotta ask: Why?

GEspo: That link took me to a discussion page. The tires in those photos look to me like cold tires driven hard. I've seen this before on some street performance tires driven on a track.

RE: Slip and Axle Stiffness

People pull trailers with their vehicles. Trailer dynamics gcan greatly affect the tow vehicle's dynamics. Most trailer tires are made at sausage factories. Then there is the braking topic. Trucks now have built-in trailer brake controllers. You need to know that the tires are capable of providing FX forces. Tires made in some countries usually disintegrate when they see stopping loads as well as cornering loads.

Then the hitch design: Bumper, load equalizing, goose-neck, and fifth wheel. Now add to this list a drunk driver whose empty wine bottle gets trapped under the brake pedal going down a hill. The resulting death(s) injuries and set up all manufacturers for liability requirements (Deepest Pockets). The drunk has no money but the OEMs do.

RE: Slip and Axle Stiffness

cibachrome,

So you are saying that computer modeling of vehicles with trailers is something that is done - and for that, you need trailer tire data.

OK, makes sense.

RE: Slip and Axle Stiffness

Sure, ADAMS or some rule of thumb type simple calculations on the stability margin. Apparently Carsim has a trailer module, so we could run playstation simulations of trailer-tow. Double lane change would be fun.

here's some ADAMS trailer stuff, https://www.researchgate.net/publication/264812314...

And here's some real fun, not too sure about the health and safety, but damn it makes the point (although it is more complicated than it looks as they've changed the towball load as well).

https://www.youtube.com/watch?v=i2fkOVHAC8Q

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

RE: Slip and Axle Stiffness

(OP)
Does a similar principal apply here: a higher moi is more resistant to change in direction but can correct more easily than a lower moi, ie mid vs rear engine loosing it.. Wouldnt the toy car on the treadmill more easily correct with the higher moi? (it needs a tiny driver)

RE: Slip and Axle Stiffness

Quote (GregLockok)

(although it is more complicated than it looks as they've changed the towball load as well).

That's the point of that demo - to illustrate why how a trailer is loaded matters a great deal, and that a trailer can be very dangerous even if it's loaded within limits.

RE: Slip and Axle Stiffness

(OP)
(car/trailer system differs from car.. makes sense)

Why is a mid engine harder to control once traction breaks vs rear engine?

RE: Slip and Axle Stiffness

I thought mids and rears were both less stable than front engine with rear being the worst.

Keen to hear from the experts on this and setup changes that reduce the effect on mid-engine cars.

je suis charlie

RE: Slip and Axle Stiffness

All else being equal (i.e. same tires and inflation pressures front and rear) - The heavier end of the vehicle deflects the tires more (i.e. increases the slip angle) for a given "g" lateral loading. For a front-heavy vehicle, that's understeer. For a rear-heavy vehicle, that's oversteer.

The Chevrolet Corvair attempted to offset this via specifying very low front tire pressure. People didn't read the manual and inflated all of the tires to a "normal" same-ish-front-and-rear pressure. Oops.

Staggered tire sizes (i.e. differing tire specifications) front and rear - obviously wider ("stiffer") in the rear - can kindasorta compensate for this. Many sports cars are like this.

I've never heard of mid-engine being "worse" in any way than rear-engine, in terms of handling characteristics.

RE: Slip and Axle Stiffness

(OP)
Something makes sense that a mid engine is harder to get to break traction than a rear engine but harder to get corrected when it does, possibly because the higher moi rear is quick to break traction back the other direction? ie pendulum.. The math must be there to back this up because ive heard this from a few racing guys(one being one of the oldest porsche mechanics in the country)..

Does the 911 GT3 holding the record for the double lane change confirm this? (last i read it did, HIS excluded)

RE: Slip and Axle Stiffness

Having driven a Lotus Esprit at /my/ limit, once, I'd far rather be fanging around a gravel road in a Ford Falcon, noisy thing at the front, huge polar moment of inertia. Very easy to set up a stable drift. I've never tried driving a 911 fast, they are always borrowed cars. Modern 911s are hugely overtired at the rear, or if you prefer, they use pram wheels at the front, because they have a functionally silly architecture.

The fastest car through the difficult double lane change (the moose test) is some old Citreon thing, not a 911.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

RE: Slip and Axle Stiffness

(OP)
citreon is HIS.. 2008 911 GT3 RS is 2nd, supposedly

newer 911 strays from the original.. its said 997 is the last true 911.. just cant afford it, 2006/2007 gt3 engine alone is $45k, add another $20k for the gearbox, they pretty much giving the body away..

RE: Slip and Axle Stiffness

(OP)
interconnected suspension

RE: Slip and Axle Stiffness

(OP)
On the 911 architecture there have been tests comparing 911 to cayman.. on accel weight at rear wheels 911 74% vs 67% cayman.. stopping 58/42 for 911 vs 63/37 cayman..

so if stop and go goto the 911.. and correcting a skid to rear engine, 911 wins?

im a fan.. who ironically has the exact opposite car

RE: Slip and Axle Stiffness

Qashqai is second, beating all the Porsches, of which they have tested far too many.

Of course there is the old engineering observation that when you start measuring something the measurement becomes the target. Nurburgring lap times comes to mind.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

RE: Slip and Axle Stiffness

If the car is turning, it is rotating in the yaw axis and has rotational momentum. Higher MOI (ie rear engine) means that rotation is more difficult to stop and control or stop and reverse, not less difficult.

If you've ever driven an air cooled 911 at speed, you know this. More 911s have slid into armco backwards than any other platform in existence.

The 'moose test list' 911 fanboys talk about being at the top of is an ad hoc list published by a Swedish magazine. It is not a complete list of every car in existence. Porsches perform well, but so do numerous other wide/low sports cars and GTs.

RE: Slip and Axle Stiffness

Just because the weight distribution from having the engine way out over the back is favorable for acceleration and braking doesn't mean it's favorable for going around corners without ending up backwards through the hedges!

RE: Slip and Axle Stiffness

(OP)
Theres no question the rear will loose traction first on the rear engine, question is what car will be able to “swing back” the other direction to right itself.. Im wondering what motivates people to say “mid is better in turn but when you loose it you cant get the car back”.. ie what math describes this

the concept of “more predictable” is probably floating around here

thanks all for entertaining this btw

RE: Slip and Axle Stiffness

(OP)
im going to take a stab at this: the reason theres NOT alot of mid engine drift cars, when the rear brakes loose on the rear engine theres still some traction on the fronts so the car can be controlled a bit.. when traction breaks on the mid engine the more evenly distributed grip is lost both front and rear..

RE: Slip and Axle Stiffness

I suspect the statement "mid is better in turn-in" is probably relative to a front-engine car, not a rear-engine car.

The concept of the car "swinging back in the other direction to right itself" implies that the driver has lost control. Maintaining control in the first place means having a margin of stability and that means having understeer margin. Having engine weight hanging way out over the back is not conducive to having understeer margin.

If the car does go unstable, the other part of it is whether what needs to be done in order to recover is possible within humanly realistic reaction time (or stability-control-system reaction time) and whether the driver is given sufficient information to be able to judge what the right recovery action is. Full steer-with-a-pinkie-finger power steering is not conducive to doing that. 4 turns lock-to-lock steering isn't, either.

Vehicles with a really long wheelbase, wheels pushed all the way out to the corners, seem to slow down these reactions.

RE: Slip and Axle Stiffness

There's not a lot of mid-engine drift cars because drifting requires losing traction at the rear and keeping it at the front. 90/10 weight distribution with a monster engine up front and rear axle drive with a limited slip diff is the best recipe for that. Besides, there's not a whole lot of mid-engine cars out there that are suitable and cheap enough to be hacked up.

RE: Slip and Axle Stiffness

(OP)
enter “more predictable”: when the rear starts to slide on the rear engine youll notice it and still be able to control the car

I suppose id like to test all this since so many variables, tires etc.. ie when youre making huge slip angles there lots of factors to whether a mid is less controllable in slide

RE: Slip and Axle Stiffness

Just a few comments: Short wheelbase cars don't do well at speed for many reasons, Ackermann gradient for 1.

For 2, MOI is not the principle 'driver', Izz/Wt is. Barbell or Sputnik.

3), Drifting cars need mostly front grip, especially in the tire Mz world. Watch the videos. You turn the car via steering by applying steering system moments. Then you vector the power with drive wheels.

Put your car's rear wheels on grease plates and do the spinnerama play.

RE: Slip and Axle Stiffness

There seems to be some idea that the moose test is just some sort of journalistic prank. It's actually ISO 3888-2 and is used to evaluate ESC systems and limit handling in Germany, the USA, Australia, and no doubt elsewhere. It's used by the Feds to check whether an ESC system is more than a light on the dashboard, you have to be 2 mph faster through the gates with ESC on rather than off.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

RE: Slip and Axle Stiffness

I wasn't implying that the 'moose test' itself is not highly valuable or standardized.

What I was saying is that when Porsche fanboys talk about Porsches dominating the 'moose test record' they're talking about one specific list published by a magazine, compiled from tests with different drivers under different conditions over many, many years. The magazine list itself doesn't mean all that much; that isn't an impeachment of the test itself.

As far as why drift cars are almost always front engine/rear drive... I think that has way, way more to do with the fact that front engine rear drive 'sporty' platforms are abundant and cheap, while mid engine rear drive platforms are rare and expensive, than it does with the last 1% of handling optimization.

RE: Slip and Axle Stiffness

Front engine, rear drive cars have "emergency brakes" /parking brakes on the rear wheels. A necessary element of drifting or the "Reverse 180" spinnerama maneuver.

"Honey, do you love me, or is that the hand brake handle ?"

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