Ariel Atom 1

[BUGGAR]

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?

 

[GregLocock]

No. 1000 is ridiculously soft, things will fret and it will be very difficult to tune the suspension.

Cheers

Greg Locock


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[Panther140]

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

 

[GregLocock]

Trouble is, as usual, you don't know what you are talking about. I agree with bikes and karts that too stiff is bad, but I have never seen a car that benefited directly from a soft chassis.

Cheers

Greg Locock


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[BrianPetersen]

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

 

[Tmoose]

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.

 

[Panther140]

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

 

[NormPeterson]

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

 

[BUGGAR]

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?

 

[GregLocock]

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


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[BUGGAR]

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?

 

[GregLocock]

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


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[BUGGAR]

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.”

 

[gruntguru]

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

 

[NormPeterson]

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


Norm

 

[GregLocock]

1000 ft lb / deg is not legal for kit cars in the UK or Australia. In oz there is get-out clause if the car has been 'professionally designed'.

Cheers

Greg Locock


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[SwinnyGG]

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.

 

[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.

 

[Dave K]

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

 

[SwinnyGG]

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.