Roll Center & Tire Load

[Brian_Murphy]

Hey guys. I just discovered this forum and am going through quite a overload of information between a book that i am reading and the posts on this site. Unfortunately i decided to go to a tech school (which i will not name for embarrassing reasons) and not to college and am now forced to teach myself the best i can with books (and most of all) field experience. I feel like i have a decent understanding on plotting Roll Center and all that which i know has many more factors than you can plot but i feel i would have a little better understanding on the whole situation if i had one basic question answered (and this is going to obviously be geared toward stock car racing). Lets say I'm trying to increase LF tire load (basing this on a conventional set-up, not coil bound) would i want to move the RC to the right or left. I have been given 2 different explanations on this question...

1. You would move the RC to the left so that the moment arm/load is pushing down more on both tires as the chassis rolls.

or

2. You would move the RC to the right. The demonstration given to me was this...the guy took a lets say 3FT long by 1FT wide (a quick chassis) piece of aluminum. He placed a 1" round piece of steel (the "roll center" which i know is not connected in this way) under and to the right of the centerline of the aluminum. He said placing the roll center to the right gave the left had more leverage to load the tire and vice versa. Kinda like a teeter todder.

I realize this may be a simple and stupid question and im sorry for that. Thank you very much for the help!

[Brian_Murphy]

Hey guys. I just discovered this forum and am going through quite a overload of information between a book that i am reading and the posts on this site. Unfortunately i decided to go to a tech school (which i will not name for embarrassing reasons) and not to college and am now forced to teach myself the best i can with books (and most of all) field experience. I feel like i have a decent understanding on plotting Roll Center and all that which i know has many more factors than you can plot but i feel i would have a little better understanding on the whole situation if i had one basic question answered (and this is going to obviously be geared toward stock car racing). Lets say I'm trying to increase LF tire load (basing this on a conventional set-up, not coil bound) would i want to move the RC to the right or left. I have been given 2 different explanations on this question...

1. You would move the RC to the left so that the moment arm/load is pushing down more on both tires as the chassis rolls.

or

2. You would move the RC to the right. The demonstration given to me was this...the guy took a lets say 3FT long by 1FT wide (a quick chassis) piece of aluminum. He placed a 1" round piece of steel (the "roll center" which i know is not connected in this way) under and to the right of the centerline of the aluminum. He said placing the roll center to the right gave the left had more leverage to load the tire and vice versa. Kinda like a teeter todder.

I realize this may be a simple and stupid question and im sorry for that. Thank you very much for the help!

[PatClarke]

Brian,

You need to read this paper from Bill Mitchell on Force based roll centres. Lots of things will become clearer.

http://www.neohio-scca.org/com...e%20Dynamics2007.pdf (http://www.neohio-scca.org/comp_clinic/hand_out_reprints/Vehicle%20Dynamics2007.pdf)

Cheers

Pat

[exFSAE]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Brian_Murphy:
Unfortunately i decided to go to a tech school (which i will not name for embarrassing reasons) and not to college <span class="ev_code_RED">and am now forced to teach myself the best i can with books (and most of all) field experience.</span> </div></BLOCKQUOTE>

Believe me this is no different when it comes to many things in engineering - particularly in motorsport. Was for me anyway. So you're not at a disadvantage. Besides, being a drive self learner is good thing.

Now then, onto "roll centers." All they are is a way of describing lateral jacking forces. Let's set aero aside for now - jacking forces will not put more total load on the tires. They can only change a little of how they're proportioned - much like springs and bars.

Anyway. Sounds like you have an oval track application. Let's say you want more LF load. Why try to do it with front geometry? You can't get more steady state front axle load through jacking forces (again, aero aside here folks) so that's out of the question. You aren't going to make any primary change in reducing lateral load transfer so that's out of the question (assuming we're not jacking the CG down any appreciable amount). What's left? Changing your dynamic cross weight.

But if that's the only option to get more LF load (at the expense of say RF).. there are easier ways to do it than front geometry. Pull static cross weight on the scales.. change some springs.. smaller front bar.. hook a rear bar.. track bar up.. etc etc.

Really the main advantage (IMO) to jacking forces on the front end of a stock car is to control body attitude for aero. That's it. If you want to change your crossweight trace there are better/easier ways of doing it than a geometry swap.

[Brian_Murphy]

Ok. I read that article before but after you sent it back to me i realized i did not read/think hard enough on it. Now that it i have i feel like i am almost there but a little confused. His diagrams are from the rear where the LF is on the L and the RF is on the R correct? If thats the case then what Im seeing is lets say you move LS points only to move the FAP lower, you will increase jacking but decrease load?

I was gona say, other than coil binding or bump stopping, jacking seems to play no big roll in a conventional set up?

The only reason why im asking about geometry is because everyone around here (probably due to Cup influence, they are affected by aero 1000 times more than we are) is concentrating so much on it.

[exFSAE]

Bear in mind that the effects of truck arm location (split, height) and track bar height are all from jacking forces - so I wouldn't go so far to say jacking forces have no big role in "conventional" (assume you're meaning no stops or coilbind? Has a different connotation in Cup) setups.

There's just no magic to it ("the trick is... there is no trick!". It's tool in the box for adjusting what your cross weight trace is (aero aside). I think the best way to conceptualize it is to just imagine all the forces going through the virtual swing arm. What many people neglect in this is that the jacking effects are very much dependent on your tire forces... not just a function of the control arm orientation. Geometry gives you jacking ratios.. adding tire forces to the mix gives you the actual jacking force.

If aero really isn't an important item for you, front geometry is probably more a tuning point for getting your dynamic camber where you want it through travel, rather than trying to dick too much with lateral jacking and pull down.