Instant centers

[Francis Marquis]

Hi,

Me and a friend are starting to design a FSAE for our school. We are new to car engineering, but we are both at our 4th year in engineering. Like many of you, we are wondering about instant centers. I have read the "race car vehicle dynamics" chapter on this subject.

But there's something I can't understand. Milliken is talking about "jacking effects". But since the lateral forces on both tire are always in the same direction, the moments generated around the instant center won't be the same on both sides of the cars, thus generating different effects ( jacking the car on one side, lowering it on the other side ). Am I right?

Plus, I am wondering: If we consider that the lateral forces on tires generate moments around the IC, why don't we consider that the vertical forces are doing the same?

[Francis Marquis]

Hi,

Me and a friend are starting to design a FSAE for our school. We are new to car engineering, but we are both at our 4th year in engineering. Like many of you, we are wondering about instant centers. I have read the "race car vehicle dynamics" chapter on this subject.

But there's something I can't understand. Milliken is talking about "jacking effects". But since the lateral forces on both tire are always in the same direction, the moments generated around the instant center won't be the same on both sides of the cars, thus generating different effects ( jacking the car on one side, lowering it on the other side ). Am I right?

Plus, I am wondering: If we consider that the lateral forces on tires generate moments around the IC, why don't we consider that the vertical forces are doing the same?

[Drew Price]

Francis,

Start with this article by Bill Mitchell, it should answer some of your questions:

Mitchell Roll Center Article (http://www.neohio-scca.org/comp_clinic/hand_out_reprints/Mitchell%20Roll%20Center2007.pdf)

Best,
Drew

[Tim.Wright]

I have that article but I havn't got around to reading it. From a brief look, it looks like a pretty good treatment of the subject.

To answer your question from my own point of view - The outside wheels will typically be taking most of the hoziontal forces (at limit cornering) and therefore will contribute most to the jacking effect of the chassis. But you are right in that the inside wheel will have a lowering effect due to its horizontal contact patch forces.

I know the answer to your last point in my head but can't quite articulate it in a way that makes sense. Not while I'm at work anyway.

-Tim

[L B0MB]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Timo:
the jacking effect of the chassis
</div></BLOCKQUOTE>

The 09 chassis is going to inspire some jacking effects...

[Tim.Wright]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content"> I am wondering: If we consider that the lateral forces on tires generate moments around the IC, why don't we consider that the vertical forces are doing the same? </div></BLOCKQUOTE>

Ok, I've had a bit of time to think about it.

Conventional roll theory (geometric roll centres et al) typically ONLY takes into account vertical forces at the contact patch, ie when calculating roll angles at some lateral G force. Therefore jacking forces are looked at in isolation and superimposed.


<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content"> The 09 chassis is going to inspire some jacking effects... </div></BLOCKQUOTE>
When I'm not hanging around you guys ALL my non-working hours, I actually reach a stage where I can write jacking without laughing like a school girl.

If the 09 car loses the phallic like qualities of last years car then you might just be right.

Tim Wright
Curtin Motorsport
Compulsory Life Member