travisnd

1. 2001 Z06 PTA/TTA car

2. 2004 Z06 shocks, Pfadt polys, stock springs, race allignment and T1 bars (full soft in the rear)

3. 255/35/18 A6s

When I switched to the 255s from 275s the car developed a very slight push whereas before it was perfectly neutral. My feeling is that the reduced compliance from short sidewall of the 255s is making the front swaybar effectively "stiffer" creating the push.

I have no issues putting the power down and can trail-brake the car with ease. I'd like to try and tune some of the push out if possible. I figured I could do one of two things given the rule-set:

1. Soften the front bar

2. Stiffen up the rear bar

I can't figure out a good way to soften the front T1 bar. Can't find anyone who makes softer mount bushings for it. I guess I could shim it out with some washers, but lets focus on option 2.

If I put the rear bar to the middle setting i.e. one setting stiffer than it is now what would happen? My suspicion is that it would just make the rear end more loose and unpredictable vs. giving the front end more grip which is what I ultimately need.

Thoughts?

drivinhard

Brian, I think from talking at VIR you were saying you don't run ballast right?

Anything you can do in the off season to toss weight off the nose and move rearward will help. Get it as balanced as you can get it (it's free), then go from there. With a heavy nose, at the end of the day, it's gonna be hard to get the car really neutral.

We had to go nuts to get the car to 50/50 AND keep it @ 3118 (it's a piece of cake if you run it at 3250 or 3350 like a lot of guys, just add weight in the rear). It's a LOT harder when you are trying to do it AND run the car as light as we run them (basically ~2920 sans driver). It makes a huge difference though.

RAFTRACER

The answer to the questions could differ depending if it is "tight" on the way in, middle , or off of the corner...

travisnd

I don't run ballast... I've got the car about as light as I can get it. Remember I'm 250 and I have a full fire suppression system and the cool-shirt system. With everything and a 1/2 tank of gas I was 3132 at VIR's scales so I'm right where I need to be.

I've ditched the headlights and I don't have anything under the hood I don't need. The only thing I could do is remove the front bumper foam but then that could allow a minor impact to destroy/tear the front facia. I could go lexan and remove a few pounds to put lower, but it'd probably be 15-20lbs max. I've already cut out the OEM b-pillar rollbar and trimmed the roof panel for the cage.

My car is 2870 w/o me in it!

travisnd

Car feels great on turn-in and mid-turn for the most part. It exhibits push mostly under power exiting a turn. I.e. T2 and Southbend at VIR, Turn 2, Carousel, and 9 and 10 at Summit Point, Turn 4 into the esses, 10B and 12 at Road Atlanta to name a few.

Based on that I'm not sure how lightening the front could fix corner exit push. If anything the car could use a stiffer rear spring to make it not squat under power. I've seen pics of the car coming out of turns and it looks like it's launching at the drag strip!

RacePro Engineering

Travis,

Based on your careful analysis, your ultimate cure is probably going to be stiffer springs and adjustable dampers, where you can better control F-to-R weight transfer under acceleration. BUT, before spending that $$$, we would try dialing in more rake at the front, and perhaps stiffening the rear anti-roll bar.

Out of curiousity, why don't you simply go back to what worked well - 275 tires? (I am certain you have good reason(s)).

Ed

travisnd

PTA/TTA rules... smaller tires = more modification points which allow me to run the spoiler and BBK up front. The car feels better on the 275s, but I set the TTA track record at VIR in March with the 255s with a 2:04.6 (which Mr. Drivinhard beat in October with a 2:03.9) and then set the TTA record by 2.5 seconds with a 1:17.8 at Summit Point main a couple of weeks ago.

The spoiler makes the car more stable under hard braking and more planted in high speed turns. Initially I thought the spoiler was creating the push (nothing up front to balance it), but at high speeds the car is ok... it's at medium speeds that it understeers out of turns.

Speaking of modification points... adjustable dampers and different springs are a no-go... no more points to spend and keep it TTA/PTA legal.

I should test the car on 255s w/o the spoiler and with the spoiler with the rear bar on the middle setting, but I don't want to go changing the car before the race in a few weeks at Road Atlanta... don't have any way to test different setups in a controlled environment.

EDIT - As for rake I have the front as low as it'll go on the stock springs w/o cutting the bushings on the spring adjusters. I may try raising the rear of the car up a little more now that you mention it. With all the weight taken out of it I couldn't get it low enough with the stock spring adjusters (looked like a moonshiner) so I have longer bolts in the rear spring. Maybe I'll put the stock spring bolts back in and raise the butt end up another 1/2" and see what that does.

sperkins

There's a big difference in a 3130lb car with no ballast and a 3130lb car with 250lbs of ballast. Like Mark said, get as much weight off the nose as possible and move it where you want it.

sperkins

Has your car been corner balanced? If you swap the rear spring bolts, you've just screwed up your corner weights.

travisnd

There's nothing much left for me to take off the front of the car. Remember I'm probably 50 lbs heavier than you and 70ish than Mark If you guys had your car at my car's weight you would have to add back 50/70 lbs of ballast to make weight. Unfortunately I am the ballast. Only things I can think of to still come off the front end are:

1. Relocate my Odyssey battery (already much smaller than stock) to the rear, but then I'd have to run longer battery cables and re-work my cut-off switch etc. etc.

2. Remove my stock fans and install one small fan (On the winter project list)

3. Remove the stock bumper foam, but I don't want to do that since it supports the front facia.

4. Remove the front signal housings. Already tried this and found that by the time I had created an adequate hole filler out of some tinted covers the weight savings was negligible.

travisnd

Nope... before the cage I had it weighed and the cross-weights came in nearly perfect w/o touching anything.

There's a good bit of fine tuning my car could benefit from, but I'm not paying the local shop $90/hr. to do it and I don't have my own scales yet.

Jason

And that is why I have no desire to add aero and adjustable shocks.

I'd try jacking around with air pressures first. Cheapest and easiest. See if I could drive around the problem, then go to the rear bar. Just be ready for on entry.

drivinhard

small rad fan (or no rad fan)? Z16 hood? small clutch? (this is huge) dash/HVAC stuff (I know you have some stuff still in there, can't remember what). small alt. wipers. lexan. wiring.

Any nut/bolt/washer from the dash forward that doesn't make the car stop/turn/go...toss. I'm ~205 w/ gear (only 50 less than you) and pretty much run 250 lbs of ballast.

not trying to derail your thread but altering the basis dynamics of the car makes a huge difference. you are putting more weight over the drive wheels, and allowing the front tires to work better. every caged C5Z I've done still comes back from the cage shop 53/47 or 52/48 @ 3118

PushinTheLimit

My car is about 53/47 balanced this year, but that's with almost full dash. I have no idea what if anything much has been pulled from it. Since its an old T1 car, I doubt much has been removed. Sometime I need to pull some of that out.

travisnd

1. Small fan - I mentioned I'm doing that this winter.

2. Z16 hood - $$$$

3. Small clutch - $$$$ and my stocker is still fine

4. I have zero HVAC stuff and no dash other than a piece I cut off to cover my gauge cluster vs. making metal cluster supports.

5. Don't want to go lexan up front since it scratches so badly and I'd have to fabricate/add a brace. Need wipers in the event of PTA in the rain.

6. I'm not getting into cutting up wiring when I don't know what I'm doing on that front very well. The only extraneous wires in the engine bay are things like the wire that went to the washer reservoir pump.

7. I'm not sure what my weights come out to be, but I'll bet mine's not as nose heavy as others. I removed a bunch of weight from the roof structures and my fire system and cool-shirt system are mounted in the rear tub vs. passenger side floor-board like most do.

8. I don't have scales to futz with all this stuff in my garage. I really want to buy some though

travisnd

Done that... the 255s need enough air in them to keep from destroying the outside edge too quickly.

Been driving around the push all season... but yeah the rear end being too happy is what I want to avoid. I really like how the rear of the car feels and how early I can put my foot on the floor... don't want to change that.

Which is why my main question is really if the rear bar stiffness would do anything to increse front end grip. My guess is that it won't... it would only change the relative grip between the front/rear. If that's the case, then a stiffer rear bar would make for a car that wants to oversteer coming into the turn then push it's way out.

Timz06

switch to bud lite and add ballast in the rear

RacePro Engineering

Not exactly. GENERALLY SPEAKING, when the front of the racer is the main complaint ON CORNER EXIT, as you have identified, one needs to focus on the front suspension. From the photo you posted above, we can see that the front of the car has become unloaded as you accelerate out of the turn.

You have already said that the most direct and effective solutions - [1] wider and lower front tires/wheels; or [2] stiffer springs, and being able to regulate bump and rebound resistance; or [3] lower front ride height; are not options.

So now we are a little deeper into compromise territory. A higher-rate anti-roll bar does not REDUCE grip. It simply redirects where the available grip is applied, and how quickly. So, what the stiffer rear anti-roll bar WILL do is allow you to rotate the car easier under power. If you can predict where in the turn the understeer starts, you will need to transition with the throttle to mild oversteer just before that point, and maintain that degree of yaw through track-out. You will NOT be able to reduce throttle without severe consequences! It will be disconcerting until you grow accustomed to it.

Ed

96CollectorSport

Brian,

I would leave the front bumper foam in there just because it's pretty light and if you ever end up using it you'll be glad it's there. I think Jason Berkley said he was glad he didn't take his out after the big T1 crash at the 2010 Run-offs.
PTA will constatly be a compromise - I think that adjustable shocks would really help your push on corner exit.
The way I look at it is, if you stiffen the rear bar you are taking grip away from the rear - essentially loosening the rear of the car up to balance a front end that isn't gripping. (Even though that's opposite of what Ed is saying - that's still how I look at it). I would try to fix the end that isn't working vs messing with the end that is working well.
There is a never ending compromise in PTA - you have to look at your set-up as a whole and figure out what points/parts are worth the costs. You chose a spoiler/brakes vs shocks.

I'd have to disagree with Mark and Scott though, if you put more ballast to the rear, it will make the car push more on corner exit (basically he already has enough grip in the rear on corner exit, what he needs is a little more front end grip). So if he would/could add rear spring rate (or band-aid PTA/TTA style stiffen the rear shocks) it would help keep more weight on the nose and keep the front end planted on exit. Although I do agree ballasting is a great tool for balancing the car, in Brians case he may just need to add right side weight.

My recommendation would be to make sure there is zero bind in the front end - especially the front T1 bar. Don't underestimate how much that will help. Also get the car corner weighted when you can - I hear Scott doesn't always lock his garage.

travisnd

Thanks for the good post Ed.... as for that last part. It's like I always tell my hi-po rwd students, "When in doubt more gas... the answer is always gas"

I plan on it... not only is it safer, but not having it there could allow a minor impact to destroy the bumper cover vs. just being a minor impact.

I agree, but I'm locked into '04 Z06 shocks currently.

This is how I look at it as well... the rear ain't broken so why try to fix it. I like the spoiler because of the drastic braking improvements I get... I have some really need traqmate graphs showing spoiler vs. non at VIR and it's a big difference for me. I went with front brakes to survive W2W weekends w/o going through 50 sets of pads etc.

That was my thought as well... more rear weight will make the push worse. I'm sure I could use right side weight though. I have NASCAR bars on the driver's side vs. a pyramid X on the passenger side. Also the seat, me and all the safety equipment is on the left side. I installed my fire bottle and cool-shirt cooler in the rear tub to get the weight on the rear and to keep the passenger floorboard open so I have the option of putting my passenger seat in if I'm instucting.

How can I do this? Do you mean pre-loaded when stationary or are you refering to the bar being able to rotate in the bushings freely? With the stock T1 bar bushings and the OEM front bar brackets it's really tight when torqued to spec. Can you shim the front bar out with washers like you can on the rear? I've got the rear shimmed with one washer per side as per the feedback from this forum.

I live too far from Scott/Mark etc. to raid their garages