Too Much Grip for the CG?

[mfain]

Here's something for the racers in the crowd to comment on. Since my project track car is taking forever to finish, I bought a NASA Spec Ford Focus to build current track time in the interim. It is a full cage, SVT suspension, professionally built car - 2300lb s/146 HP on Hoosier R7s. I have worked the suspension until it is very neutral and, despite its low HP, is a very fast "momentum" car. Yesterday, at Wild Horse Pass in Phoenix, I took a fairly tight sweeper at 75-80 mph (on the edge). With the car being very neutral, I expected it to break loose at both ends if I exceeded the limit - it didn't. Instead it rolled 30 degrees (inside tires 3 feet off the track) AND the outside rear tire came off the ground 6 inches. The thing tried to "pole-vault" over the outside front tire which still had grip. Luckily I was exiting the sweeper and straightening the wheel. I stayed on the throttle (FWD) and the car came down hard with only a minor 2-wheel "off". Any thoughts other than the obvious CG height comments?

Pappy

[Ron Sutton]

Here's something for the racers in the crowd to comment on. Since my project track car is taking forever to finish, I bought a NASA Spec Ford Focus to build current track time in the interim. It is a full cage, SVT suspension, professionally built car - 2300lb s/146 HP on Hoosier R7s. I have worked the suspension until it is very neutral and, despite its low HP, is a very fast "momentum" car. Yesterday, at Wild Horse Pass in Phoenix, I took a fairly tight sweeper at 75-80 mph (on the edge). With the car being very neutral, I expected it to break loose at both ends if I exceeded the limit - it didn't. Instead it rolled 30 degrees (inside tires 3 feet off the track) AND the outside rear tire came off the ground 6 inches. The thing tried to "pole-vault" over the outside front tire which still had grip. Luckily I was exiting the sweeper and straightening the wheel. I stayed on the throttle (FWD) and the car came down hard with only a minor 2-wheel "off". Any thoughts other than the obvious CG height comments?

Pappy

Hey Pappy !

I suspect it is a combination of the CG being high and the front roll center being low ... maybe below ground ... and migrating far to the outside of the front tire ... which makes the moment arm (distance from CG to RC) be the lever that it is ... except the lever is causing the inside tires to lift.

In professional race cars, we work hard to make the front roll center migrate the opposite direction ... and migrate towards ... or past ... the inside front tire. We do this to work the inside tires mo'better ... and not lift them off the ground.

So ... even when you're not lifting the inside tires off the ground ... that car is not loading the inside tires much, if any.

Have fun & be safe !

:cheers:

[rustomatic]

I think a picture of the car's stance (ride height) would help here, especially relative to Ron's idea. I had an '03 SVT Ficas, I mean Focus, and while it handled great, what was interesting was that it was the only front-drive car I'd driven that would willingly hang the rear out in a turn (in a very manageable way). It's interesting how good tires might change this dynamic . . .

Secondarily, I remember the control-blade rear as being very difficult to lower for some reason--it might have been that bushings would bind as the car got closer to the ground, which would serve to prop the car up . . .

[mfain]

I think a picture of the car's stance (ride height) would help here, especially relative to Ron's idea. I had an '03 SVT Ficas, I mean Focus, and while it handled great, what was interesting was that it was the only front-drive car I'd driven that would willingly hang the rear out in a turn (in a very manageable way). It's interesting how good tires might change this dynamic . . .

Secondarily, I remember the control-blade rear as being very difficult to lower for some reason--it might have been that bushings would bind as the car got closer to the ground, which would serve to prop the car up . . .

Here are a couple of pictures. The rear suspension has the SVT set-up with Massive Speed Systems adjustable camber and toe links, Eibach (soft) springs, and Koni adjustable shocks. I'm sure Ron is right about the roll center migration. Also, I was low on fuel and the tank is located low on the passenger side, so more fuel weight would have been good ballast.

57616

57617

[mfain]

Hey Pappy !

I suspect it is a combination of the CG being high and the front roll center being low ... maybe below ground ... and migrating far to the outside of the front tire ... which makes the moment arm (distance from CG to RC) be the lever that it is ... except the lever is causing the inside tires to lift.

In professional race cars, we work hard to make the front roll center migrate the opposite direction ... and migrate towards ... or past ... the inside front tire. We do this to work the inside tires mo'better ... and not lift them off the ground.

So ... even when you're not lifting the inside tires off the ground ... that car is not loading the inside tires much, if any.

Have fun & be safe !

:cheers:




Thanks Ron! I'll have to think about roll center migration with a strut suspension.

Pappy

[Ron Sutton]

Thanks Ron! I'll have to think about roll center migration with a strut suspension.

Pappy

Frankly ... I wasn't thinking about it being a strut car. We typically get less roll center migration with strut front ends ... but they still migrate. How much depends on the dynamic LCA angles.

But what you describe the car doing ... sure sounds like a roll center migrating to the outside ... which leads to lifting of the inside tires. Combine that with a high CG & sticky tires ... and that is "probably" the cause.

:cheers:

[mfain]

Frankly ... I wasn't thinking about it being a strut car. We typically get less roll center migration with strut front ends ... but they still migrate. How much depends on the dynamic LCA angles.

But what you describe the car doing ... sure sounds like a roll center migrating to the outside ... which leads to lifting of the inside tires. Combine that with a high CG & sticky tires ... and that is "probably" the cause.

:cheers:





Ron, The car rolls quite a bit due to the soft (spec series) springs. Would stiffening the springs proportionally - front and rear- and keeping the same roll percentages reduce LCA angle changes (dynamic) and help prevent some of the roll center migration? I could turn it into a Go-cart if necessary. LOL. But the transition from on-the-wheels to on-the-lid might be a little more abrupt.

Pappy

[Ron Sutton]

Hey Pappy,

Ron, The car rolls quite a bit due to the soft (spec series) springs.
I see that from the photo. I don't think the car is high enough. Could we raise it 12"-16".

...

...

JK. That puppy has a HIGH CG. Any chance the rules will let you get it lower?

Would stiffening the springs proportionally - front and rear- and keeping the same roll percentages reduce LCA angle changes (dynamic) and help prevent some of the roll center migration?
Yes. It would reduce the roll angle & the migration. Can you do this with a bigger sway bar in the front & stiffer springs in the rear?


I could turn it into a Go-cart if necessary. LOL. But the transition from on-the-wheels to on-the-lid might be a little more abrupt.
If you get it to run significantly flatter, I think your "tippy" problem will go away.


Pappy

:cheers: