Need advice on Suspension Mods

[BronxBomber]

Hello to all. The other day I was hustling down a quiet road when someone pulls out of a driveway in front of me. I slammed on the brakes and I came to a stop avoiding a would be accident. Shame on me for being a little over enthusiatic on a public road. Shame on the idiot that didn't look before pulling out. Anyway, the purpose of my thread is that, while the FiST stopped with no issues, the back end felt like it lifted up and the car felt very unstable. Since HPDE is in my very near future as well as a Cyborg turbo install, I felt it was now time to invest in some chassis mods in the hope of alleviating this issue and making the car more planted when under duress. All I currently have in terms of suspension/chassis upgrades are Mountune springs on stock dampers. I will be switching to dedicated coilovers soon, but my question is more in regards to the lift and squirmyness I felt under hard braking. What would you all suggest to help mitigate this? Your responses are appreciated. In particular, if you guys that track your cars could chime in, that would be very helpful. Thanks again.

 

[jeffreylyon]

Firstly, rear anti-sway bars and torsion bars will not make any difference. Increasing rear roll stiffness with not make the rear feel "more planted" during straight line braking. I can't imagine that a rear brace will make any measurable difference, either.

You're experiencing weight transfer - momentum caused the front of the car to load and the rear to unload under hard braking. During HPDEs you'll learn how to use weight transfer to balance the car between under- and oversteer, which, IMHO, is just lovely in the FiST. You hit the brakes hard which loaded the front and unloaded the rear; because the rear tires had less grip and the front was really paying attention to your steering input, the car felt twitchy. Stiffer front springs would reduce dive, stiffer rear dampeners *might* increase rear grip a tad. HPDEs and autocross will teach you how to use a lightened rear to rotate the car and how to keep it front looping around and, conversely, how to "catch" the rear with throttle (weight transfer to the rear).

Some track rats (I say that with the utmost of respect) on this forum aren't crazy about the brake balance on the FiST. I'm not sure if mucking around would help things - we usually move bias towards the rear under lower traction situations (rain) and move it forward when we've got good traction, hence more weight transfer and less rear grip under braking. It doesn't sound like you had too much rear bias, it sounds like the same experience you'll feel with every high-speed corner entry you'll experience during a HPDE hot lap.

BTW, and I'm biased a tad - you'll really learn to master quick weight transfer in autocross.

 

[Siestarider]

Jeffrylyon covers it very nicely. I would only add that there is a huge difference between panic breaking and the quick hard squeeze I use on track to drop 70 mph threshold braking. I am still on stock brakes and pads, rear end does not squirm for me, and I am right at ABS intervention. But only doing this level of braking in a straight line, easing into trail braking with turn in, then to throttle to control attitude during apex approach and through exit. My belief that larger rear rotors would be beneficial on track is more a hunch than anything more definitive. Just have to find out by trying.

Main point is on the street, squirrely or not, you were able to throw the anchor out successfully when you had to. On track you have both time and repetition in your favor.

 

[WhiteFords]

Higher rate springs and stiffer dampers will reduce brake dive.

As said above, track experience will teach you how to use the weight transfer to your advantage.

 

[BronxBomber]

Weight transfer I understand and I've dealt with it in my other cars. It's the squirrelly part that bothers me. I stopped in a straight line, but car started darting around and the road was very flat. It didn't have grooves in it or bumps. I was just curious if any body stiffening would help. As I said previously, I am going to go with BC racing coilovers since they were great on my Subaru. Well' see if that helps. I'm also going to be getting the Pierce Motorsports 6 point chassis brace just to stiffen things up a bit more. May also go with some polyurethane suspension bushings. I'll chime in to see how each addition changes the feel or doesn't change the feel, particularly when panic stopping.

 

[FORZDA 2]

The unstable feeling is the front control arm bushings flexing and rebounding. Spring/bar changes won't affect the result you're feeling. The main cause is the abrupt and full application of the brakes (panic-type). If the brakes are applied smoothly and build to max-braking (one full second is plenty of time) the bushings will absorb the load without rebounding and the car is rock stable. Also note that Whiteline makes poly bushings and an adjustable caster bushing as well.

 

[jeffreylyon]

What [MENTION=1686]FORZDA 2[/MENTION] said - a PM or Monotune 2-point front brace will also help with toe changes as the suspension loads.

I saw Jackie Stewart give a great example of what happens when as you introduce loads into a chassis: Go up to your buddy and shove him announced - he'll stumble. Now gently push on him until you're pushing just as hard as when you shoved him - he'll settle against you and not stumble. As [MENTION=1686]FORZDA 2[/MENTION] said, if you ramp into braking you'll avoid making all the little springs (sheet metal, bushings, tires, etc.) in the chassis from oscillating. If you "shove" the chassis all the springs will go "Boing-ng-ng-ng!!!" and bounce back and forth, making the chassis feel unsettled..., because it is. You can stiffen the springs with braces and urethane (and heim joints if you're a he-man) but the chassis is still a bunch of springs.

This remains the hardest lesson for me while moving from road racing to autocross - in AC we introduce loads the chassis more quickly than in RR. I'm old and my hands and feet aren't nearly as fast as they used to be so I'm shoving the chassis when I should be loading it. When I was young and fast I was pretty dangerous in an FF or FA, but I could feel the chassis load under braking and turn in as I gently introduced brake and sheering input. Now I'm just thinking through it - I'm loading the chassis faster on an AC course with big, quick brake stabs and lots of steering and it's responding faster than can I react. Boing-ng-ng-ng and I'm reacting to the initial Boing.

 

[BronxBomber]

My main question is really whether or not coilovers and or chassis bracing and/or poly bushings, will help the deflection and extremely noticeable weight transfer issues I'm feeling. I'm not talking about changing the way I drive on a course so as to mitigate the issue that way. I'm asking if there is any hardware that will help. I never had any of these issues with my Subaru which is why I'm asking. My Subaru was a sedan, so I'm thinking there was more weight in the back to keep it more planted.

 

[WhiteFords]

Alleviating the drastic weight transfer under hard braking and acceleration will be best done with proper spring rates and dampers. The mushy suspension bushings are part of the problem, as their deflection under load is causing all kinds of movement. Dedicated track cars have very little roll and pitch due to high spring rates, aggressive damping, and few, if any, rubber bushings.

The chassis of these cars are very stiff from the factory, at least compared to the old American iron I have in my garage. A bit of bracing may help a little.