Jukka

Now that I have rebuilt brakes and suspension, I still have one unwanted feature in my car. The nose dive under hard braking is not what i want from my car.
Main features of my combo:
12" ls1 brakes with wilwood adjustable combo valve, eibach pro-kit, Kyb gr2 struts and kyb agx rear shocks(now adjusted to level 4, scale is from 1 to 8 where as 1 is soft and 8 is firm). Would lca relocation brakets or panhard relocation braket help to adjust rear suspension's geometry to more braking friendly? How about tubular torque arm?
I have 12" ls1 brakes in all corners, and wilwood adjustable combo valve. Would adjusting brake bias towards rear do any good? I know at the moment my brake bias is a bit too much towards front. i also know that adjusting rear shocks has an effect on this. Could this problem be fixed with koni yellows?

3.8TransAM

Decoupling the torque arm from the trans has a huge effect.

Also u want the fronts to lock up slighlty before the rears, so keep that in mind as you adjust your bias.

later
Jeremy

Edited due to my own stupidity

Shagwell

Brake bias plays a BIG part in nose dive. The more rear brake you have, the less nose dive you have, but as Jeremey said, you don't want the rears to lock up to easily. Usually about 10ft before complete stop(as stated, just before fronts).
- Also, how are your front struts? If they're worn/too soft, they'll allow excessive nose dive.

laiky

your nose dive is due to the soft gr2's and soft prokit springs, brake bias adjusting isn't going to help the situation. You need good struts, at least then stiffer springs.

Secondly, for a street driven car you DO NOT wan't the rears to lock up before the fronts EVER. Once the rears lock you can't steer and your car will become unstable likely to spin. The fronts do 75-80% of the braking if you lock the rears first you lose directional stability before you have reached your maximum braking potential. If you were to lock the front you would not be able to steer but understeer is stable and predicatable. You simply slightly reduce your brake effort and regain steering. If you let off to unlock the rears so you don't spin you are even further away from maximum braking increasing stopping distance and fighting for control.

ahhhFlubbernuts

You are going to HAVE to drop your rear roll center at least 1" when you start getting brakes that work properly on a 3rd gen. Factory setups will not have enough force coming down from higher speed to jack the rear of the car enough, but once you better the brake performance then there is nothing more you can do even wth separate rebound and compression adjustments to control the aft to fore weight transfer and rear jacking under increased braking force. The only solution at this point is adjusting roll centers and recalculating springrates and swaybars acordingly.

83 Crossfire TA

iTrader: (2)

Huh, in what way? Common sense would say that you’d get more dive _but_ also more usable braking from the rear tires.

Wow, that sounds scary. Ever pull a parking brake lever while moving? How hard did the car try to swap ends.

Secondly, when you’re “adjusting bias” with a proportioning valve you’re really just limiting the highest pressure available to the rear brakes, you’re not actually having any effect on brake bias until you reach the point where the prop valve is limiting the pressure available to the rear brakes. You can keep pushing that deal but as you do you end up with a combination that is more and more frighting with less then perfect road surface conditions (dust on the road, gravel on the road, wet road…)

Lowering the rear roll center should reduce the amount of dive but also require a stiffer rear suspension (or less stiff in the front, but that will make dive worse again), so it is a step in the right direction.

Personally, I HATE the whole big rollbar thing that most f-body guys go with in trying to get some handling out of these cars. Yes, you do have to limit the front suspension travel, but you can do that with stiffer springs or bigger roll bars, and stiffer springs have the added advantage of controlling brake dive better. Unless you’re restricted by class rules (can’t run stiffer springs) I don’t even understand why people would run the 36mm front bars at all, by the time you balance everything else out and get it working right both ends of the car are so stiff that you skitter over every crack in the pavement.

Alternatively, if you really believe your bigass sway bars and relatively soft springs are helping you in some way, there is always the potential of dialing in some antidive into the front suspension. I’m not sure why I’ve never seen anyone do it, it would be simple enough, just relocate the front LCA pivot down slightly or the rear one up (weld the original shut and drill a new one where you want it). I’d bet that a 1/4” on either one would make a dramatic difference.

Jukka

I understand why rear brakes shouldn't lock before fronts but right now my rear brakes are a bit too weak i.e. I could have more pressure on the rears.

Could further lowering of rear suspension help this problem? I could either run with out rear spring isolators or cut rear springs to lower rear. I have this friend who has had no problems when he has ran with out spring isolators. I understand that this would mean that there would be more road noises inside the car but that is not an issue. You propably agree that lightening the car would help performance in general, but would it help nose dive if I lost some 50lb from the front? I know I could lose even more with out making this a completely uncomfortable ride, but losing weight in general is a different topic.

another thing that I forgot to mention. This is not a race car but my own personal toy for my own personal "quality time"

Dewey316

That would be ideal, so long as class rules would allow for it (for someone racing). If you are allowed to change suspension mounting points.

We could run the math on it, but I do think that 1/4" would make a huge diffrence, our cars don't have very much anti-dive dialed in to them in stock form.

As to the original poster. I think better struts would really help you. Dump those KYB's and toss a set of Koni's on there, I bet it would be a night and day diffrence. Also, as Mark mentioned, going with less front sway-bar, and more front spring should help things also. The sway bar isn't doing anything under braking, if you go with less sway bar, and more spring, it should dive a little less, but you can probably keep the same roll resistance.

ahhhFlubbernuts

How'd this get to a swaybar discussion?

He needs to simply lower his rear roll center (roll center is not a sway bar and I know you know this Mark) but in doing so he is increasing his rear leverage and will have to proprotionately increase his lateral roll resistence to keep the same cornering balance but improve the braking characteristics. I never said anything about "bigass swaybars"

Jukka, this is what you need to buy and weld onto your car. This will lower the rear roll center up to an adjustment range of 3/4" to about 2 3/4" range.
You will need it adjusted with the panhard rod mounted probably in the 3rd or second hole from the bottom for about a 1 1/4" to 1 3/4" roll center drop in the rear.

JEG'S High Performance - 550-41055

There is an imginary line that runs from the front roll center to the rear roll center and it is too inclined. Justy lowering the cars ride height does not help this- in all actuallity it hurts it more since the front roll center lowers at a faster rate than the rear suspensions roll center does on a 3rd gen. That will incline the roll axis even further casing even more "rear jacking' and subsiquent front nose dive due to rotation of weight transfer on the front axis.

Shagwell

...I've also got to argue about the kyb's and springs causing his issues. The por-kit springs may be lighter than my sport kit, but I run kyb's on the front and have very little nose dive. The car is very stable under braking. - This was all on my stock brakes(PBR rear-disc stuff) with 3 shims in the stock prop valve, upping my rear psi. From a 125mph shutdown, planting the pedal, my rears would lock about 5 feet before complete stop. At that point, vehicle speed was already low enough that it didn't induce any steering issues, but it probably only did this because my front brakes would fade at that point(hence the new 13" C4 stuff)

83 Crossfire TA

iTrader: (2)

Honestly, it’s a simple enough deal to do that I don’t see much reason for calculating hypothetical anti dive percentages rather then just doing it and trying a few things to see what feels right.

As far as class rules, some one creative with a welder, grinder and torch could very easily make it look stock enough that even someone familiar with the cars wouldn’t be able to tell what was done unless they had 2 cars disassembled right next to each other and took a caliper to both.

Man, this is how you’ve managed to get yourself perpetually banned, you keep looking for an argument where there isn’t one.

I agree with you WRT to rear roll center and I agreed with you in my first post (I think that you and I are the only ones on this board that have played with this, although I don’t religiously keep up with this forum), and just mentioned that another possible approach is to stiffen springs and use less stiff roll bars to maintain the same roll stiffness and at the same time prevent dive by increasing front spring rate.

And lowering rear roll center will require you to increase the rear roll stiffness, which if done with stiffer springs (and matching shocks/struts) would also help.

I’m not sure that I’m following… pro kit springs sit higher but they are also stiffer then the sportlines.

laiky

Hey flubbernuts, it took me a min., Missed ya! Hope all is well.


I still can't believe how many people on this board run eibach's. God help me there junk!! If your looking to spend 250 bucks on springs just get the Guldstrand slalom kit. It blows the eibachs away and doesn't actually cause problems.

Based on all these great rear suspension posts, i bought a second 9bolt over a year ago, so i could rebuild it put the relocating brackets on, phb bracket, LS1 brakes and attempt to camber it. So far it's just accumulated more rust, one day when i have time and money i guess

83 Crossfire TA

iTrader: (2)

Check who made those springs for Guldstrand, you’ll be surprised… same with GW, Koni and a ton of others.

The problem with the eibach stuff as they sell it is the progressive rate rears (on the 3rd gen stuff, a lot of their stuff has progressive rates front and back)

laiky

The guldstrand springs are moog or trw, but there cut to fit and a good rate. i knoe you can get them from moog if you know exectly which one they use and then you have to trim them. I put the slalom package in my brothers 84 back in 90. Never had a problem with them. Should have just gotten them for myself too.

Shagwell

...I was saying I didn't know if the pro kit or the sport kit was stiffer, but that I didn't have any issues with the sport kit and KYB's. My car also has a lot of weight in the trunk(stereo, 12's weigh a little over 40lbs each+boxes+amps/etc) so that could help rid nose dive. - and if you wanna get real peticular, my aluminum heads would put less weight on the nose, thus lowering the amount pressure against the springs.

3.8TransAM

lol

I meant to say "U want the fronts to lock up slighly before the rears do" not the other way around like I stated.

I'm allowed bad days too or fingers that type on their own.

Seriously though, I know that by doing the "math" it appears that decoupling the torque arm from the trans shouldnt do it, it does.

This could also be due to the fact that old and used factory torque arms are mushy and flexible compared to ones with half the miles on them.

It made a noticeable difference on my Formula.

later
Jeremy

83 Crossfire TA

iTrader: (2)

Heh, fat fingers…

I’m not sure why decoupling was ever really tried really, torque arms in the range of the stock length ones are so close to right that if you take the flex out of the system they should work just about perfectly both ways, oh well…sure, making a telescoping (effectively infinite length) link for the top makes it effectively infinitely long to optimize action during deceleration, but I’d be surprised if there is a significant % difference if you really calculated it.

3.8TransAM

lol

Hey we all have fat fingers or the occasional brain farts :-)

I think it is just a matter of taking the years and abuse out of the stock flimsy arm.

Formula at 100k I could literally bend it back and forth easily with my hand

GTA under 50k and it hardly moves with the same force.

Plus you also have the arm solidly mounted vs. the action it looses thru the trans mount itself.

It did make quite a difference on the Formula even including the fact it now has the larger Ls1 front brakes on it.

later
Jeremy

83 Crossfire TA

iTrader: (2)

Well, I'm about to be swapping a new tranny and rear into my formula... I intend to keep as close to stock geometry as I can (well, making room for a 4L80e tranny and 9" might not let me do it 100%), but make the TA and the ta mount much stiffer, so I suspect that I'll know for sure by sometime later this summer...

i may just take the stock TA, brace it and swiss cheese what is left of the original just to make my life easier...