Vlocity

I'm running 275 Kumho Victoracers on the front and 315's on the rear for my track days. I don't have any of the instability issues that you mention. I really don't think that it is necessary to pin the rear axel if the rear bushings are in acceptable condition.

For correcting with tire pressure changes only the conventional thinking to get a feel of more Understeer and less Oversteer would be to lower the front tire pressure and/or raise the rear tire pressure. Have you checked your HOT tire pressure after a session to see where the tires are? I like to get to a close hot tire pressure like 39/38 or 40/39 front/rear and then see what that equates to with a cold starting temp on my Kumhos....street tires obviously will be different. But you might be surprised to see a larger stager on the pressures with them hot then you had cold.

If your shocks are adjustable ...again to get to a feel of less understeer and more oversteer you would want to make the front shocks harder and the rear shocks softer.

If your front sway bar is adjustable you would want to shorten it/stiffen it.

If your shocks and sway bars are not adjustable you could play with ride height....you don't say if you are running a stock height or not. My car really likes being set on the factory minimum height front and rear and on occassion I have went up 2 turns from that setting on the rear to add a little bit more rear grip from mid corner off.

One last thought with regard to your alignment and that is that the Bridgestones might also like a little bit more negative Camber in the rear. I also have just about all of the Caster that I can get dialed in. There is the old adage that Camber makes the car happy and Caster makes the driver.

What ever you decide to do... just make one change at a time until you find the right combination. I keep a notebook so that I can remember what I have done from one day to the next.

Keep us informed.

Ken

mark kibort

http://www.youtube.com/watch?v=zxImi48nv-A (thats 125mph before the braking point for turn 2)
http://www.youtube.com/watch?v=1yusCDwWJz0 (thats 135mph before the braking point of turn 1)

Ispeed

I have been thinking of replacing all the rubber in the rear suspension and new upper A arms in the front, then everything will be new.

Vlocity

I put in a set of Carl's poly upper control arm bushings and it made a tremendous difference.

+1 on the front bushings.

Ken

Okv

I am also struggling with the same issue.
I once concluded it is not the weissach, but I may be wrong.
And, it may also be due to something up front.
I have a pair of new Carl-type upper arms around, perhaps I should put them on now.

(Sorry for posting on this old thread, but, seeing as it is not yet resolved..)

Lizard928

Okv,

I am interested in seeing what the actual issue could have been myself as well. However I have a strong feeling that it is simply a properly performed alignment is needed.

Mrmerlin

I have been working on a rear handling issue with my 84 Euro the rear end seemed loose.
So far I have installed a used set of the rear upper control arms from a newer car and also changed out the the control arm inner bushing thats connected to the rear crossmember.
I am also fitting new tires and wheels so i will report back with these changes

Vlocity

Don't ignore the front of the car when you are looking for a loose condition.

Worn out sway bar bushings or weak front shocks are worth a look. Stiffness in the front will reduce the inclination of the rear to step out. Below is a standard cheat sheet.

Guide To High Performance Handling

Adjustments Decrease Understeer Decrease Oversteer
Front Tire Pressure Higher Lower
Rear Tire Pressure Lower Higher
Front Tire Section Larger Smaller
Rear Tire Section Smaller Larger
Front Wheel Camber More Negative More Positive
Rear Wheel Camber More Positive More Negative
Front Wheel Toe Toward Toe-Out Toward Toe-In
Rear Wheel Toe Toward Toe-In Toward Toe-Out
Front Wheel Caster More Positive More Negative
Front Springs Soften Stiffen
Rear Springs Stiffen Soften
Front Anti-sway Bar Soften (Thinner) Stiffen (Thicken)
Rear Anti-sway Bar Stiffen (Thicker) Soften (Thinner)
Weight Distribution More Rearward More Forward

Regards,

Ken

mark kibort

You dont need the rear weisach to be pinned or modified in anyway. Ive built up 4 race cars that all handle spectacular and have NO handling issues, ranging in hp from 200rw to 380rwhp and running rear tires from 275s to 335s DOTs. Ive also assisted racers with loose lower control arms and my own upper control arm issues, that were either fixed or replaced with stock components. Loose handling is usually due to tires, driver induced, too stiff of a rear spring setting or too soft of a front suspension set up. ride hight is also a part of the equation as well as weight balance. if you are too high in the rear, the car will probably be very loose as well. Dont forget alignement. too much rear toe out will cause this issue. Ironically enough, PINNING THE REAR WEISACHE will make the car loose too!

That info above is confusing. maybe its the way it printed out.


mk

BC

The 928 was always the most stable car mid corner for me. It communicated exactly what it was doing, no matter if I was wrong or the car was wrong - I knew.

Squirly handling at the rear could be a softer sidewall. I immediately notice this in cars when driving them - especially the 928 when you are intune with it. Thats why I don't like kumhos.

Vlocity

Here we go... fixed it I think. Everytime I made a column it took the spaces out.

Ken

mark kibort

I remember Don Hanson had that posted in his trailer. Its all MOSTLY wrong and backward. I forgot where that was posted/published, but it is mostly incorrect. For example, a pushy front end will get worse, not better, with higher front pressures, unless you are too low to begin with . But, starting at a reasonable air pressure, lowering the pressure increases grip to a point.
same thing on the rear. you pump up those rear tires at 40psi and watch how skatey your rear end becomes. the chart is way way off.

Some thing are correct , as in the areas of the swaybar stiffness and springs, but the toe, tire pressures and camber are mostly wrong. too much front camber, for example, will make for a pushy car, as well as not enough. Rear toe increases, reduces oversteer and too much toe out will cause more, for another example.

Another fact is just because you induce looseness, doesnt mean you fix understeer. You end up, often times, with just car with a push and is loose for a nice slide around turns.

mk

Guide To High Performance Handling (bold and Italics denote mistakes in chart)



Adjustments ............. Decrease Understeer ........Decrease Oversteer
Front Tire Pressure.............. Higher ............ Lower
Rear Tire Pressure .............. Lower ............ Higher
Front Tire Section .............. Larger ............. Smaller
Rear Tire Section ............. Smaller ............ Larger
Front Wheel Camber............. More Negative........... More Positive
Rear Wheel Camber.............. More Positive .......... More Negative
Front Wheel Toe .............. Toward Toe-Out ........ Toward Toe-In
Rear Wheel Toe ............... Toward Toe-In ......... Toward Toe-Out
Front Wheel Caster............ More Positive .......... More Negative
Front Springs ............... Soften .......... Stiffen
Rear Springs ................ Stiffen ......... Soften
Front Anti-sway Bar........... Soften (Thinner)......... Stiffen (Thicken)
Rear Anti-sway Bar ........... Stiffen (Thicker) ........ Soften (Thinner)
Weight Distribution .......... More Rearward ......... More Forward


((

Vlocity

Mark,

The chart is published by BF Goodrich under their proper care and feeding of the R1 and I copied it from the Tire rack. http://www.tirerack.com/images/tires...ce_R1_care.pdf

I have used it to good effect and believe it to be accurate, but I hate to see this thread go another 12 pages. LOL.

As for the tire pressures, I think the distinction that they are trying to make is increasing and decreasing the contact patch. Similar to your experience, I was always under the impression that raising tire pressure is similar to using a stiffer spring. Numbers thrown around by some shops say 1 PSI is equal to about 10 # plus of spring rate. So that is why you tune with springs and shocks and then do very fine tuning with tire pressure as a last step.
Ken

RKD in OKC

I like these...

mark kibort

Yep, that chart has been around a long time, but it doesnt make it any less wrong! .

The 1psi equals 10-50lbs of springs depending on who you talk to or the type of tire you have. Right there, that coincides with the spring effects which contradict the pressure advice of the chart. Nascar uses tire pressuer for fine tuning spring rate, but their side walls are huge compared to other road racing tires. The tire doesnt grow to any measureable size differnece with pressure, and the contact patch will actually be reduced with greater pressure. The pros know how to tune with pressure, and they dont follow the advice here in any way, shape, or form.

Personally, i have quite a bit of experience this area, and the few times Ive had too much pressure front or rear, was the few times my car has been loose or pushy because of it. I also remember a crew of NSX's that were at a DE event. all 6 of them were spinning out at different places on the track. (buttonwillow). I asked one of them, (the leader of the pack) what pressures he was running. He said he set them at 44 cold. I asked him to measure them now, and they were near 50psi. I had ALL of them jack down the pressure to 32psi and none of the group spun again the rest of the day. serously, it was funny. at one point there was 4 of these cars all spun out at the exact same time!

go ahead, use more toe out on the rear and see if it makes your oversteer go away!! LOL (and Ive never used LOL before on any thread!)

mk

PS pics below at a 2001 Speedvision race at sears showing the front tire action out of turn2. Andersons rear camber got messed up by contact with the Vet, yet he still finished the race and gave him a battle!