Track width of FSAE cars

[ThreeColours]

Originally posted by MCoach:
I agree with Sormaz that for a tilt table, RC heights won't matter. The suspension isn't moving, no jacking forces, therefore no extra loading. The way it's lifted is the ground plane moves rather than loading from one side.

I don't agree. Force of gravity is not normal to the tilt table. In my opinion, if you want to ignore RC height you must have ---> RC height= CG height or Roll rate equal to zero, that is to say, you have infinitely stiff springs. Regards

[Tim.Wright]

Originally posted by Sormaz:
'Jacking' forces have no effect on tire loading. Think of the car as a closed system, and these are internal reactions. The only way suspension travel could have an effect would be physical movement of the CG through travel (or a track width change).

Jacking forces will result in 'extra' loading of the spring, but this is being reacted by the a-arms, toe link....NOT the tire

I don't agree there. Thats wrong even before the pedantic level. You have got the theory almost completely backwards. If you have lateral loads on the tyre, (and a non zero rch) you will have jacking forces. These act between the contact patch and the chassis and the bypass the spring.

Anyway, don't take my word for it. As Erik said, do an FBD in front view and you will see your self. Apply a lateral force at the CP, and you will see you need a vertical force to satisfy force equalibrium.

[Sormaz]

Originally posted by Crispy:
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Sormaz:
Pushing forward with the pedantics...

'Jacking' forces have no effect on tire loading....

If we are being pedantic, wouldn't we say that jacking forces will effect tire loading, because of the resulting movement of the CG?
</div></BLOCKQUOTE>

Hence the 'dot dot dot' part:

"'Jacking' forces have no effect on tire loading. Think of the car as a closed system, and these are internal reactions. The only way suspension travel could have an effect would be physical movement of the CG through travel (or a track width change)."

And further,
I was mainly targeting the comment of "no jacking forces, therefore no extra loading" (implying 'extra' tire load)
I did not say no jacking forces on the tilt table. In fact, I did not mention the tilt table in that reply at all because that was not the point I was making.

Regardless, I agree with your comment and regret the invocation of that pesky word.

Z,
Two cars go into a corner with the same lateral g...do the tire reaction forces care whether load is internally reacted through spring or a-arm?
I agree that the tires may care how high the cg is, and the cg height may depend on certain geometry and resulting body roll angle....but that would just be being pedantic


Tim,
Thanks you for clearing that up. I agree with you. Now please show me where my backwards idea of the 'theory' is in contradiction with yours?

[Tim.Wright]

Originally posted by Sormaz:Tim,
Thanks you for clearing that up. I agree with you. Now please show me where my backwards idea of the 'theory' is in contradiction with yours?

Well you said 3 things, 2 of which I thought were backwards:

Jacking forces will result in 'extra' loading of the spring...

Don't agree with that. Jacking forces bypass the spring and act directly between the contact patch and the ground.

...this is being reacted by the a-arms, toe link

Agree

....NOT the tire

Disagree there too. Jacking forces are seen at the contact patch for sure. In fact "geometric load transfer" as Claude names it, is just the jacking forces which occur from the lateral forces at the contact patch.

[Goost]

Two cars go into a corner with the same lateral g...do the tire reaction forces care whether load is internally reacted through spring or a-arm?

Not sure about this Sormaz. Perhaps you would be right that they don't care in the mid-section of a long turn, but you "into the corner", so:
Even if they weigh the same and have the same CG height but different RC heights, they have very different contact patch loads during transient maneuvers: The car with the high RC immediately transfers weight (jacking) to the outside tire as soon as the car accelerates laterally. The car with the low RC only transfers a little instantly (jacking), but the rest transfers relatively softly as the car rolls into the turn.
For those few moments the low RC car is faster.
Those few moments happen nearly 40 times on an autocross track.

[Goost]

And there I've done nothing to clarify the fundamental disagreement here which is whether RC height affects weight transfer. A FBD would be the only help there. Just remember transient maneuver is the key: this is not a static problem it is a dynamic problem - it has two degrees of freedom and you have neglected one of them.

[Buckingham]

For those few moments the low RC car is faster.

In FSAE its possible there are more 'few moments' that the high RC car is faster.

If the high RC car transfers weight faster, it might achieve steady state sooner (has a lower time constant). If the course requires quick changes in direction (step inputs) the car with the lower time constant should be easier to control by the driver (faster).

I'd take the car with the high RC in the slalom (square wave) any day.

[Francis Gagné]

Originally posted by Buckingham:
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">For those few moments the low RC car is faster.

In FSAE its possible there are more 'few moments' that the high RC car is faster.

If the high RC car transfers weight faster, it might achieve steady state sooner (has a lower time constant). If the course requires quick changes in direction (step inputs) the car with the lower time constant should be easier to control by the driver (faster).

I'd take the car with the high RC in the slalom (square wave) any day. </div></BLOCKQUOTE>

With a high RC, you have a very fast time constant, granted. But you have no control over it. A low RC car can be adjusted to have a fast or slow time constant. Plus, the low RC car with fast (stiff spring/bar) can be adjusted to have good roll damping. The high RC car will always have poor roll damping.

The point Goost is making is that steady-state cornering is slower, I agree that this should be the case considering tire load sensitivity (TLS). It might not be significant with some of the SAE tires though.

Buckingham, I get the impression you are saying a soft car will change direction slower than a stiffer car (in roll). Of my understanding, it is not the case, it is only the suspended mass weight transfer that is transiently slowed. The lateral force time constant is relatively unchanged. Considering TLS, the car during transition will make more lateral force than in steady state.

Also, damping in bound, rebound at front and rear can be adjusted so to slow the transition on the front and make it faster at the front, contributing to more yaw acceleration in turn-in. (and can be adjusted for more stability in turn-exit).

This is something you cannot do with a high RC. As for the time constant, our 50mm high RC had a 10hz natural frequency if I remember well. The car was fairly stiff in roll to prevent our huge sidepods from scraping all over the place... 10hz is much faster than any human can actively control.

[MCoach]

Back to basics...

Ha, I am wrong in my original answer, as ThreeColours mentioned, RC height must be at the CG, and/or have infinitely stiff springs! Of course springs are going deflect. Doh! As an FBD shows, the tilt table represents what cornering forces are on the track because the Z-axis force of gravity is now the lateral component force when tilted with respect to the vehicle. With the RC height at CG height, all roll forces will be jacking forces.

This is the exact reason "infinitely stiff" springs are supposed to be swapped in when checking the CG height of a vehicle. The CG WILL move unless these steps are taken. In the case of the tilt table, the roll rate will affect how much everything moves, as in when lifting the front of the car, the pitch rate will affect how much everything moves there.

Freudian slip of the tongue and mind.

Z, I hope one FBD, a palm to the forehead, and 10 push-ups helps calm you down. :P

However it does make me curious as to how much that CG actually moves on our chassis when sprung...need do some more testing. I'm thinking it isn't a spectacular distance.

However, the conclusion that I'm coming to as that, even with springs deflecting, assuming that CG height change is negligible (don't have data to say whether it is or isn't right now), is that tilt table is still based on just CG height and the component Y-axis force...

[Z]

Recipe for a fit and healthy body = 100 x squats, push-ups, pull-ups, and sit-ups, per day.

Recipe for a fit and healthy engineering mind = Much easier!!! I reckon 1 x FBD per day should do it.

If someone could convince the local newspapers to put a new FBD into the puzzle pages each day (along with the xwords, sudoku, etc.), then I reckon we'd all be flying around in zero emission skycars in no time at all!

Z