Mark - I've got a couple questions for you - I'm coming to you from a similar world - MacStrut Sentra (which I've sold but my discussion envelops all cars really)

Everything I've ever read about raising the roll centers shows that there will be larger jacking forces at the tires and it will decrease the grip of the inside wheel along with increasing load on the outside tire. I'd like to hear your opinion on the tradeoffs/compromises of all this with modified control arms.

I understand the huge help in bumpsteer and camber curve. But I find it hard to believe that moving the kinematic roll center and creating more geometric pro-roll is such a good thing. Have you thought about just going with super super short struts, lowering the car a whole lot to get the CG down and correcting the bump steer?

Problem I have with all this roll center correction stuff is that I've never seen any real data that says its better. No datalogger numbers and no suspension analysis screenshots. Seat of the pants feel doesn't fly with power and it shouldn't fly with suspension tuning either.


And for others - don't point me in the way of the recent SCC articles about suspension tuning with roll centers/roll axis stuff. I know Mike K - I'm not sure if he really doesn't know what he is talking about or he was just trying to dumb it down so much so that the average person could understand the basics - but most of what is in that article about roll centers is just plain wrong.

 

All I did with the roll centers is get them into the ballpark of where they were at normal ride height for the chassis. The problem is that when you lower the car 2 inches like I did with a strut suspenstion, the roll center takes a serious dive. This causes several problems, which I am sure you are aware of. It is a balancing act though, because like you said, you don't want the roll center to get too close to the cg. I like to think of it this way. As you bring the roll center height closer to the cg height, you need less spring and bar to control body roll, at the expense of the the forces being applied at the tire tread will be more in lateral than vertical. I like the front roll center to be just a tick above ground level, then use spring and bar to control body roll from that point. It is a trade off. The real problem with the roll center getting too low, is that with even a small amount of body roll, the roll center migrates very quickly both down and over. This creates kind of a feeling of the nose flopping over too easy and requiring even stiffer springs and bars to control. All a give and take balancing act. If it were easy, there would be a manual with instructions. Hope that give a little insight though.

 

If you have a strong math background and want to cut to the chase so to speak, this book is a must have:

Race Car Vehicle Dynamics (Milliken & Milliken)

It's a race car engineering textbook published by the SAE.

 

Got it Steve


You should try this one:

http://www.amazon.com/Introduction-R...8&s=automotive


Shows you how to apply everything Milliken talks about.


Mark - http://www.neohio-scca.org/comp_clin...Center2007.pdf

Happy reading!

 

Sitting within arms length of me as I type this.

You know how it goes though. As Mark pointed it, it's all trade offs. After all, we're just asking more performance than we should out of econobox sedans, right?

 

Interesting read. The wrench in this is that a strut suspension has a roll center that moves much more that a double a-arm setup. So as to not make this a overly complex discussion, basically on the front strut type suspensions, you want to keep the lower a-arm level, or slightly downward sloped towards the wheel. This is to keep the roll center that you are stuck with on a strut setup from moving greatly with body roll.

 

I need mod money, lol.
Mark, nice runs Sunday.

 

Thanks, I plan on running again there this sunday. Made some changes at the rear of the car, pulled about 40lbs out(race seats), and will be running race gas for more boost. Hopefully the weather holds out. I can't wait to find out how much the changes are worth. Next step will be pull the a/c, and put on the lightweight wilwood brake setup, and lightweight hood. (another 85lbs off the car bringing it to 2850lbs. Will have to wait a few more weeks to test it in that configuration. Then I will be trying out the Evogreen turbo, and see what that does over the stock one I have now. I am not going to spend the money to get the car under 2830 until I am sure that I won't be running a 2.3 stroker motor which would be a minimum weight of 2817lbs to be legal.
Mark

 

Damn, no way can I compete with all that weight reduction. I'm at ~3350 with driver. SM is no place for a normal street car, but I'll do my best!

 

Wow, I never thought an EVO would be able to get close to the minimum weight for SM!!

Would you be willing to share where you sourced a hood lighter than the stock aluminum one? All I've seen are heavier except the ones Robispec had on his site. But I thought those weren't available anymore. Unless it's an aluminum skin with all the bracing removed??

What wheels are you running? CCW's?

 

EVOlutionary, man, you can't call it the "Red Rocket." That's already the nickname for something you don't want to be associated with.

PS. My dog has a "red rocket."

 

hey mark i might show up to just hang out. i wont be able to drive out that morning.

i'll be at a scca event tomorrow instead. see u then.

 

Jarrod- I am getting the hood from Robispec. 10lbs according to him.

Manny- Cool, I will see you then. I would go to etown tomorrow, but this thing called work is more important.

 

And you'll be 3400+ with that roll bar your looking into

 

Yeah, guess I'll just need more power...