RC Underground in Braking

[James Montupet]

Hey Folks:


Our old suspension geometry has our roll center going about an inch underground under hard braking. I can think of two interesting things happening due to this RC.

1) Upon corner entry (say braking towards the apex), we get negative jacking due to IC heights which (in this context) will provide good turn in, especially as it gives some normal force to the inside wheel/tire.

2) The bad news: With our RC underground, the instantaneous geometric weight transfer is negative if the driver brakes towards the apex. Farther in the corner, as the RC rises and elastic weight transfer occurs, weight transfer will switch to its normal direction. I don't think the tires or the driver would much like this "weight transfer reversal", so to speak.

So, my questions are whether or not I've described the effects correctly (unlikely) and whether or not the good jacking effects due to IC location outweigh the nasty geometric weight transfer.

Thanks for your time.

James Montupet
Frame and Suspension Co-leader
Duke University

P.S. Sorry to make my first post a question; I'll try to put up some mildly educated opinions in the future.

[James Montupet]

Hey Folks:


Our old suspension geometry has our roll center going about an inch underground under hard braking. I can think of two interesting things happening due to this RC.

1) Upon corner entry (say braking towards the apex), we get negative jacking due to IC heights which (in this context) will provide good turn in, especially as it gives some normal force to the inside wheel/tire.

2) The bad news: With our RC underground, the instantaneous geometric weight transfer is negative if the driver brakes towards the apex. Farther in the corner, as the RC rises and elastic weight transfer occurs, weight transfer will switch to its normal direction. I don't think the tires or the driver would much like this "weight transfer reversal", so to speak.

So, my questions are whether or not I've described the effects correctly (unlikely) and whether or not the good jacking effects due to IC location outweigh the nasty geometric weight transfer.

Thanks for your time.

James Montupet
Frame and Suspension Co-leader
Duke University

P.S. Sorry to make my first post a question; I'll try to put up some mildly educated opinions in the future.

[mtg]

The below ground front roll center in corner entry will make the front end of the car less responsive during turn in.

Above ground roll centers will have approx. instantaneous geometric load transfer to the outside tire when you turn the wheel, increasing the "bite".

Another point to keep in mind is the roll inertia. The car rolls around the roll axis (obviously), and as it travels further from the Cg, you will have a higher roll inertia, which also slows down the transient of the car turning into the corner (and rolling out of the corner).

You're trading ressssponse for sta-bil-ity http://fsae.com/groupee_common/emoticons/icon_smile.gif

[James Montupet]

Matt-

I agree with you about a greater roll moment leading to greater stability and less response.

What I still worry about is some instability due to weird weight transfer. I've heard that geometric weight transfer is negative with below ground RC; your answer seems to imply (correct me if I'm wrong) that it's zero if the RC is below the ground. I could see some odd things happening to the tires with quick loading and unloading (or vice versa) due to negative and then positive weight transfer.


So, what I'm thinking is that I don't really know if below ground RC is more stable, due to these two opposite effects. I think putting the RC up might do some good, as it might lose some unpredictability due to negative geometric weight transfer and increase responsiveness due to changes in roll inertia.

James

[mtg]

The geometric load transfer is only zero when the RC is on the ground (or if there's no lateral acceleration). If the RC is Z below the ground, the geometric load transfer is the same as if the RC was Z above the ground, just opposite sign (same magnitude, different direction).

What will probably give you the best answer is calculating the magnitude of the loads you're asking about. I would guess that you're not going to put a negative load on the outside tire with a below ground RC, rather the vertical load will just decrease. The whole time, the load will be flying all over the place anyway (ever see any pushrod strain gage data?). If the magnitude of the load change from the RC position is neglible compared to the load change from everything bouncing all over the place..... you might be better off deciding what color to paint the car. Or it could be a big deal.

So what I said above is true, but to figure out how important it is, you need to know that magnitude of those loads.