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Ariel Atom

Ariel Atom

(OP)
A member on a car forum sent me some dimensions for the chassis for an Ariel Atom for me to run a structural check on. Has anyone else done this? This chassis is torsionally very flexible for a performance car (under 1000 ft lb per degree). Is a torsionally soft chassis now considered better?

RE: Ariel Atom

For a vehicle as light weight as the Ariel atom, I don't know if softer might actually be better. It depends on the application. I remember even with dirtbikes there was such a thing as overly-stiff frames. Especially when riding on rocky trails. The bikes would deflect more when any force was transferred through the forks/shock that did not perfectly align with the suspension's direction of travel.

Do road racers have preferences in their chasis, or is stiffer always better?

"Formal education is a weapon, whose effect depends on who holds it in his hands and at whom it is aimed." ~ Joseph Stalin

RE: Ariel Atom

Go-karts essentially use the give in the chassis as the "suspension".

RE: Ariel Atom

And karts need to unload the inner rear tire enough for some differential action so the live/locked rear axle so the kart can actually turn.

RE: Ariel Atom

GregLocock I was asking a question, not making a claim.

You yourself agree that too stiff can be bad for some things (like karts).

Oh omniscient one, please answer my question as it relates to this thread.

"Do road racers have preferences in their chasis, or is stiffer always better?"

Cheers

"Formal education is a weapon, whose effect depends on who holds it in his hands and at whom it is aimed." ~ Joseph Stalin

RE: Ariel Atom

I suppose that a soft chassis could serve as a hedge against ride discomfort during off-track excursions. But I'd rather be sharing track time with the people who prefer to stay on the black stuff and design accordingly.

Try to look at chassis torsional stiffness as being about one-third of a springs-in-series problem.


Norm

RE: Ariel Atom

(OP)
The Ariel Atom I am working on is rear engined and the chassis looks like it will have a rear weight bias. Wouldn't you want to transfer some roll stiffness to counter oversteer? Will a weenie chassis transfer roll stiffness even with a rigid anti-roll bar? I looked at photos of these cars and there are no anti-roll bars on any of them. Is it because they are ineffective or not necessary on these cars?
Am I a rube engineer who has been sent to look for a left handed screwdriver in analyzing one of these cars?

RE: Ariel Atom

Panther i answered your question. It is conceivable that the lighter weight or lower cgz of a more compliant car may result in faster times, but that is not really a direct effect of the chassis stiffness. You can always replicate a given tune of a soft chassis with a stiffer chassis by softening the springs and bushes, they are just springs in series.

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: Ariel Atom

(OP)
I learned these cars are pretty quick. I have to assume they are well balanced (a la Boles) and don't need a roll transfer mechanism. Isn't this unusual?

RE: Ariel Atom

It is often said that a correctly designed car would not need anti roll bars, especially at the rear. Whenever a well respected suspension designer says that then you might as well start drawing one up, in my experience. They offer a quick way of rebalancing the car.

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: Ariel Atom

(OP)
This has turned into a search for justification for such a torsionally flexible chassis. I was looking for the “secret” to a flexible chassis but I cannot find one.

The only justification might be the Balanced Car, that is, one that supposedly requires no chassis roll stiffness. But a balanced car is perfect only in a balanced world. Operating a car around a curve is not a balanced operation. To begin with, the car is not balanced: Only two out of four wheels steer, only two out of four wheels apply power, but all four wheels brake, and the wheels that steer are at opposite ends from those that apply power. When entering the curve, two wheels are steering and four are braking. When exiting the curve, two wheels are steering and now two wheels are applying power and no brakes are being applied. These are not balanced conditions and it may be favorable to transfer rolling resistance one end to the other to counter this unbalance. Or at the very least, have a chassis rigid enough to permit this roll transfer. In summary, I cannot justify a torsionally flexible chassis. I’m open to arguments on this.

What else I’ve learned:
Off road guys tell me that a rigid chassis requires less power to drive over rough terrain. I saw some dyno charts of this.

And I got a good quote from a race car driver who was also a river rafting guide:
“Roll stiffness in a race care is like the fat guy in the boat. Sometimes you want him in the front; sometimes you want him in the rear. It depends on the boat, on the rapids, and on how fast you want to git down the river.”

RE: Ariel Atom

Another shortcoming of a torsionally floppy chassis is the inability to tune roll transients by adjusting damping rates eg to improve turn-in, you can shift damping stiffness to the rear.

je suis charlie

RE: Ariel Atom

Any "secret" to a torsionally flexible chassis probably lies in its "lightness".


Norm

RE: Ariel Atom

I'd agree with Greg, that is really really soft. I have driven an Atom extensively, and found it very very easy to drive fast, and very very easy to tune.

I'd be shocked if that quoted figure is accurate, although I have no reason to doubt the skills of the OP in calculating it.

RE: Ariel Atom

(OP)
jgKRI, I am looking for drivers' experiences with these cars. I did the numbers and I'm looking for verification because I can't believe the flexibility I came up with either. A second analysis from another engineer is welcomed (I was able to enter the coordinates and materials into Risa and play with it within a day.) I watched videos of these cars on the track but how can you "see" flexibility without telltales? I have correspondence to a couple of kit car people but their knowledge of engineering seems to be limited to cad drafting(?!)
How effective was tuning your car with anti-roll bars? I don't mean to ask race secrets but did adding/subtracting the bars front/rear have a lot of affect. Thanks.

RE: Ariel Atom

The only time I've seen a race car changed to make if less ridged, was during the coil bind era in NASCAR, circa 2007 or so. We stiffened the chassis like we would for a sports car, but with no suspension compliance, it had to be added back to the chassis. It did not follow conventional thought...neither does coil binding though :)

-Dave

NX 9, Teamcenter 10

RE: Ariel Atom

Quote (BUGGAR)

jgKRI, I am looking for drivers' experiences with these cars. I did the numbers and I'm looking for verification because I can't believe the flexibility I came up with either. A second analysis from another engineer is welcomed (I was able to enter the coordinates and materials into Risa and play with it within a day.) I watched videos of these cars on the track but how can you "see" flexibility without telltales? I have correspondence to a couple of kit car people but their knowledge of engineering seems to be limited to cad drafting(?!)
How effective was tuning your car with anti-roll bars? I don't mean to ask race secrets but did adding/subtracting the bars front/rear have a lot of affect. Thanks.

I am happy to share my experience.

For background, I am a reasonably experienced driver- in a past life I was a national-level Solo II competitor. I'm not a superstar but I know which pedal makes the car go so to speak.

I do not own an Atom, but I have a friend who does. I have spent approximately 10 track days (and counting) co-driving his Atom with him, at Gingerman here in Michigan or at Mid-Ohio. I have never (and never will) 'compete' in an Atom so I have no secrets to hide.

The car I have seat time in does NOT have antiroll bars front or rear. I know that there are atoms out there which have them- this particular car does not. It does, however, have the upgraded Ohlins damper package which was available at the time, and it has also been changed to a single coil vs. the stock helper-spring setup.

The base characteristic of the car out of the box was toward mild understeer at turn-in- at low speeds and with even halfway decent tires, it would molest the bump stops mid corner.

Removal of the helper springs aided this somewhat, by getting away from the rising-rate spring behavior that induced a lot of roll. The single coil is also one step stiffer on the chart of options available for the Atom- I would have to dig with my buddy to remember what the actual rates are front and rear.

I've found the car to be relatively easy to tune- after fiddling with damper settings over several sessions, the car is at a point now where mid-corner balance is easy to tune by adjusting bound damping in the rear.

RE: Ariel Atom

(OP)
Excellent write up. Thank you.

If your spring rates at the wheels are anywhere near 50 pounds per inch, then the chassis I'm working on meets the 10 times "rule" (chassis should be stiffer than 10 time the roll stiffness of the suspension), and all is good.

RE: Ariel Atom

Adjusting dampers to tune the F-R distribution of transient roll resistance relies on MOI(x) reacting the transient roll moment at each end.

With a torsionally stiff chassis, both F and R roll moments are acting on the full MOI and both ends will be quite sensitive to damper settings.

With a very floppy chassis, F and R will each react against a different value for MOI since the front and rear of the chassis are decoupled to some extent. (imagine a chassis with a longitudinal hinge joint in the middle of the chassis) So a car like the Atom (mid/rear, transverse engine) will have a much larger MOI connected to the rear axle than the front. The consequence is much higher sensitivity to rear damper settings than front.

je suis charlie

RE: Ariel Atom

Quote (buggar)

If your spring rates at the wheels are anywhere near 50 pounds per inch, then the chassis I'm working on meets the 10 times "rule" (chassis should be stiffer than 10 time the roll stiffness of the suspension), and all is good.

Suspension roll stiffness (a moment stiffness, just like chassis torsional stiffness) needs more information than just the spring rate (a translational stiffness, reflected to the tires). Big hint: things like consistent units and track width probably have something to do with it.


Norm

RE: Ariel Atom

Quote (NormPeterson)

things like consistent units and track width probably have something to do with it.

The other thing about an Atom is that it's a pushrod car- so if the design relies on the main springs only to provide roll resistance, without the aid of a sway bar that would act in a linear fashion relative to differential suspension displacement, than the roll couple is also affected to some degree by where the car is in its suspension travel- because the spring behavior, when applied through a bell crank and pushrod, is less linear than, for example, a MacPherson strut car without sway bars might be.

This is evident in tuning an Atom- you can directly and noticeably impact the weight transfer rates (in roll) with small ride height changes. This is also useful as a tuning tool.

Also, BUGGAR: it looks like the car currently has ~40 N/mm springs front and 45 N/mm rear.

I fooled around just now with MS paint and a photo of the rear of an Atom, estimating where the rear roll center is (this thread is making me curious) and it appears to be approximately half way between the floor pan and the ground. The real question is where the CG is located. I suspect it's relatively low, but it's hard to estimate.

The low rear roll center doesn't surprise me too much- the car is so over-powered relative to its weight, I suspect it would be very twitchy with a high rear roll center.

RE: Ariel Atom

(OP)
Wheelbase: 103", track: 60" (specific vehicle I am working on).
Main tubes: 2" x 0.065 wall, d/t = 30.
Tube material: 4130(or 4140?) steel. One manufacturer says DOM, another ERW.
My scope right now is elastic rigidity analysis only; analysis for overstress won't occur if this chassis can't prove itself more rigid. The so called 10 times rule is just a reality check. I can present all my Risa files but independent verification would be better.

I'm not getting paid for this; I thought these cars "looked cool" and wanted to try engineering one. Probably a very bad life choice.

I still think these cars are structural weenies but if this is the way to go fast, I would like to know this.

RE: Ariel Atom

(OP)
That was another question: Why use helper springs (I presume for a progressive rate) when you can set rate progression with bellcrank angles? I noted you found it desireable to eliminate the rising rate springs.

RE: Ariel Atom

Quote (BUGGAR)

That was another question: Why use helper springs (I presume for a progressive rate) when you can set rate progression with bellcrank angles? I noted you found it desireable to eliminate the rising rate springs.

I don't know for sure but I believe the helper spring setup is only present on the more streetable versions of the car, to provide at least some compliance on the road. I'm pretty sure that once you 'upgrade' to a more track-oriented option set, the helper springs go away. I also suspect that on-road manners are the reason why initially the car was underdamped. As I said earlier, right off of the showroom floor, it was a mildly scary car mid-corner on a bumpy course because it was already so close to being on the bump stops steady state. A simple increase in front bound damping (within the range of the shocks- nothing has been re-valved on this particular car) was enough to kill this behavior.

Quote (BUGGAR)

I still think these cars are structural weenies but if this is the way to go fast, I would like to know this.

It seems that 'structural weenie' is an apt description.

I think this is A way to go fast, maybe not THE way. This car is so stinking light, and is vastly over-tired and over-powered, that I think it gets away with a lot with regard to the chassis being less than optimal. If the goal was to turn an Atom into a fast autocross car that had to compete at a similar power-to-weight ratio as some other cars, I suspect it would be very hard to make it bleeding-edge fast.

RE: Ariel Atom

buggar - I understand the sanity check nature of "10 times". But you need to start with the right number before you multiply it by 10 or else it's a case of GIGO.

http://www.ismasupers.com/downloads/tech-talk/Tech...

The formula is almost as easy to derive as it was to find it online. Personally, I hate having to swap back and forth between feet and inches so I keep it all in terms of inches and use 0.008727 as a numerator coefficient instead of the 1375 you'll find in the link.


Norm

(edited for boldface formatting)

RE: Ariel Atom

(OP)
"I think this is A way to go fast, maybe not THE way. This car is so stinking light, and is vastly over-tired and over-powered, that I think it gets away with a lot with regard to the chassis being less than optimal. If the goal was to turn an Atom into a fast autocross car that had to compete at a similar power-to-weight ratio as some other cars, I suspect it would be very hard to make it bleeding-edge fast."

This, too, is my conclusion after beating up on this subject. Maybe I didn't learn much but now, maybe I know in which races I should compete with these things.

RE: Ariel Atom

BUGGAR,

Wouldn't the total mass of the car affect the way the car flexes? Is any street car anywhere near the mass of an Arial Atom?

--
JHG

RE: Ariel Atom

(OP)
I'm targeting torsional rigidity, independent of mass. The mass plays in indirectly through the spring stiffness requirements and the resulting suspension roll stiffness, which is where the 10 times factor comes in.

Norm, I am using the spring rate at the wheels so I use the 60" track width between spring elements (wheels), which will be lower than the actual spring rate. Let me know if I pulled a Nancy (Sinatra - sayin' somethin' stupid).

RE: Ariel Atom

GregLocock,

How about I modify a Dodge Challenger (4000lb) so that it has the same performance as an Ariel Atom. Obviously, I need way more torque and power. The suspension is going to see way higher loads side to side and fore and aft. The need for suspension rigidity should be very much greater.

--
JHG

RE: Ariel Atom

Quote (drawoh)

Wouldn't the total mass of the car affect the way the car flexes? Is any street car anywhere near the mass of an Arial Atom?

Structural frequencies would be affected (and not expected to be well damped). I don't think the Mark I Sprites were much more massive, either.


Norm

RE: Ariel Atom

Drawoh, sorry I'm missing your point. You asked a question. I answered it, off the cuff. Here's another stab-

If you had two cars otherwise identical, and merely turned the engine power up on one, then I doubt you'd worry about stiffening the body as a first step. Far more likely you'd fit bigger tires and retune the suspension to be sportier, in which case a stiffer body would help, due to the whole springs in series idea.

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: Ariel Atom

Quote (BUGGAR)

I am using the spring rate at the wheels so I use the 60" track width between spring elements (wheels), which will be lower than the actual spring rate

50 lb/in on a 60" track gives something like 130-ish ft-lb/deg roll stiffness, against less than 7.6 times that in chassis torsional stiffness.

Looked at from a different angle, 50 lb/is wheel rate is pretty soft for the intended purpose of an Atom . . . suspension ride frequencies in the vicinity of 1.1 Hz and an implied roll rate that could be as much as 3.5°/g both seem softer than what I've seen from an Atom on track.

http://www.mustang6g.com/forums/picture.php?albumi...

On edit, most laps, that's about a 1.1g corner.


Norm

RE: Ariel Atom

Is that 3.5 corrected for gravity and tire stiffness? I repeatedly run into problems with the latter, as it is such a large proportion of the total roll compliance number. I'm not really a fan of the total roll gain number for a circuit car especially, suspension roll is far more useful as it feeds directly into your roll steer and understeer budgets.

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: Ariel Atom

That was just a very quick first cut, taken as an estimated roll moment divided by total suspension roll resistance only; I didn't even go into my datalogs to line up that particular corner on that particular lap with speed or g's. I'm guessing that tire compliance effects would add maybe half a degree per g to that, as an Atom likely has relatively stiff tires compared to its weight and track width.

So for the Atom in that picture, perhaps 4°/g or somewhere between 4° and 4.5° total is what I'd expect from 50 lb/in wheel rates and no sta-bars - or more than it looks like the Atom in my picture is showing. That 1.1g comes from actual datalogs taken in my own car, corrected downward for about 3.1°/g roll, which includes about 0.7°/g in tire effects.


Norm

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