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A Flexible Chassis is Slow 3

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BUGGAR

Structural
Mar 14, 2014
1,732
This is relative to my investigation of the Ariel Atom chassis:


A Flexible Chassis is Slow.

In my quest to understand the importance of chassis stiffness of the Ariel Atom, I was talking with some off-road guys. We have a pretty big off-road community here in the desert and they’re a pretty open group. I was talking to them about chassis rigidity and they were talking about how much it slowed them down. What were they talking about? So here is a transcript of what I remember them saying, as best as I can remember and with some editing.

When you hit a bump, it’s like the bump is hitting your wheel with one of those little arrows that engineers use to show a force. It always points towards the center of the wheel because of the same laws that also keep people parallel to each other no matter where they are standing on the globe (sic).

Those force arrows pointing at the wheel can be broken down into two directions, one vertical and one horizontal. This is because engineers think in X and Y directions. In addition, those are the only directions that things can move when hitting a bump (called degrees of freedom by engineers – kind of like all the places where you can’t go if you’re under 21).

As noted, there will be an up arrow and a back arrow. The back arrow is related to and proportional to the up arrow. This proportionality changes as the wheel rolls over the bump but the back arrow is always going to be some proportion of the up arrow and it’s always going to be there until that bump is finished with the wheel. You can forget about the bump pushing back after cresting the bump because your car is flying off that bump without looking back for any help.

This back arrow forces back on the car’s suspension, and the car’s reaction is to slow down slightly. To reduce this slow-down arrow, we must reduce the up arrow, or more accurately, the vehicle’s reaction to the up arrow. This reaction to the up arrow comes from inelastic and elastic force/energy absorption: inelastic from the shock absorbers, elastic from the tires, springs and chassis flexibility.

For a given energy input from a given bump, the reaction force from the up arrow and its evil back arrow is less for an inelastic reaction than for an elastic reaction. Thus, the more energy absorbed inelastically by shock absorbers, the less the evil back arrow restraining force. If the chassis is flexible and does not permit the shock absorbers to absorb as much energy as they can, the resisting forces will be increased. That’s one reason to run multiple or progressive rate springs; they leave more work to be done by the damper than by the springs, and the slow-down arrows are smaller.


I think he’s right. I think. At least it was a great conversation!

Added by me: There are, however, minimum amounts of elastic energy that are required to restore the chassis and wheels to their neutral position. Also, experience shows that less than critical damping is “best”.


 
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It's always sounded more like "ignore effects of less than 10% as a first cut approximation" to me.


Norm
 
If I was building a racecar in my garage, the goal would be to make the chassis as stiff as possible within reasonable weight constraints. If it turns out to be less than 10x the roll rate I have in mind, I should worry about the tune-ability of the suspension. If not, I should not.

je suis charlie
 
I suspect winning cars are uilt as light as possible with a low cg as a secondary consideration, and stiffness third.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376
 
GregLockok said:
I suspect winning cars are uilt as light as possible with a low cg as a secondary consideration, and stiffness third.

I've worked on cars that were clean sheet space frames, and cars that are based on a OE BIW, and in both cases, you'd be right.

In the case of a BIW turned racer, in my experience, much of the added structure which increased chassis stiffness is put in place because of rules and regulations regarding crashworthiness. The additional chassis stiffness is a welcome, but secondary, byproduct.

Of course, at a high level, those rule-mandated structures are optimized to provide as much stiffness per weight as possible, but *most* people building cars don't have the right tools to do that type of engineering.
 
Regardless, I wouldn't compromise a 15x torsional stiffness chassis down to 7x to save 10 kg on a 500 kg car.

Stiffness mat only have third priority but the reality is everything is a trade-off - you can't compromise any metric beyond a certain point, for the sake of improving a "primary" or "secondary" one.

je suis charlie
 
True, but the point we are struggling with is that it is hard to directly quantify the performance benefit from a stiff body at a vehicle level. A lower cgz, or lower mass, directly translates into higher lateral g at the limit.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376
 
gg said:
Regardless, I wouldn't compromise a 15x torsional stiffness chassis down to 7x to save 10 kg on a 500 kg car.

I might be more concerned about where (along the X-axis) that compromise down to 7x happened.


Norm
 
Another star to Greg because he summarized my post about the torsionaly flexible Ariel Atom - why is it so fast? Is there a track that needs torsional rigidity and on which this car would not be competitive? (There is some large money on this particular answer.)
I'm looking at F1 cars and their only need for any suspension at all seems to be to provide a mechanism to transfer wheel loads diagonally in the turns which can only be done with torsional stiffness (am I wrong on this?).
 
Somewhat unrelated but I have to pass it on. Some of the guys do measure their frames for torsional rigidity. Did I say they were competitive? No stone left unturned! To eliminate transients that would possibly measure anything other then pure torsion, they suspend their cars between chains hung from a chassis lift fully extended above and chains fastened to inserts in the floor slab below. They don't even have to disconnect the suspension. Yes, the suspended cars look like a mechanics idea of S&M. They use come-alongs with cable tension meters for loading and a Craftsman protractor to measure angles. It seems to work. It cost me a couple more Dos Equis to find out, but around 3,000 to 5,000 ft lb per degree seems common.
 
> I might be more concerned about where (along the X-axis) that compromise down to 7x happened.

Why would it matter, and why would there be a particular where (as opposed to the integrated effect of all of the torsion between the front and rear wheels)?
 
""I might be more concerned about where (along the X-axis) that compromise down to 7x happened."

"Why would it matter, and why would there be a particular where (as opposed to the integrated effect of all of the torsion between the front and rear wheels)?""


I would think it was a lower order effect. There would certainly be some effect on dynamic roll response. Imagine a vehicle where the chassis was torsionally stiff between one axle and the engine (massive) and much softer between the engine and the other axle.

je suis charlie
 
OK, I suppose so, though it's hard to imagine that not being the case, given that the engine is usually very close to one of the axles and far from the other, with the lion's share of the torsional compliance occurring in the structure between.
 
As long as torsional compliance is low, the effect is of no consequence.

je suis charlie
 
How weak chassis driver can feel on transition inputs?
for example like a insufficient lateral compliance?
is difference perception front or rear chassis twist?
gives difference over all lateral g levels?

I hope makes sense

(I have only steady state cornering experience ...... understeer change )

Thanks Radek
 
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