Rear Spring Rates

Like a lot of you I currently have 350lb springs in the back of the car (550's in the front). Have any of you experienced any shock bottoming with this package in the car. We were doing some measuring in the shed last night and comparing to photos taken of the car racing. It appears that the car is getting close to, if not actually bottoming the left rear shock out. I have experienced some mid corner snap oversteer in a fast 4th gear sweeper, this lead us to start looking at this issue.

We are looking at using either a 400lb linear rate spring or a 350lb rising rate (progressive) spring. Anyone tried either of these combo's? How did it pan out?

Kel.

Kel, does it happen at the beginning of a session or more as the session goes on? I haven't experienced that but I have experienced the handling going away as the session went on. So much so that I thought it was my tires getting too hot and going away. That stopped once I put Penskes on the car. A local circle track shock guru put one of the Konis on his dyno and their performance falls off dramatically as they heat up.

Kel, does it happen at the beginning of a session or more as the session goes on? I haven't experienced that but I have experienced the handling going away as the session went on. So much so that I thought it was my tires getting too hot and going away. That stopped once I put Penskes on the car. A local circle track shock guru put one of the Konis on his dyno and their performance falls off dramatically as they heat up.

It happened about 3 hours into the last 6 hour. I have to confess Jim that I am 30kg heavier than the other 2 drivers (at svelte 220lbs) so it may just be additional heat in the left rear shock and tyre - where most of the work happens on this track. The other drivers found the car fine and did not experience the same issue. Both rear tyres are rather glazed so have been running hot. Pressure was reduced in the left rear after my stint.

Another factor is that I am not running a rear bar Jim, so this will be causing more weight transfer onto the left side particularly in the 2 x fast right handers on the track.

With the stock driving school suspension I was scraping frame coming through Hogspen @ VIR. With the new setup and a rear bar it's no longer an issue. But I'm only running 25-30 min stints.

FWIW I'm 235lbs.

With the stock driving school suspension I was scraping frame coming through Hogspen @ VIR. With the new setup and a rear bar it's no longer an issue. But I'm only running 25-30 min stints.

FWIW I'm 235lbs.

this was happening to me in the same place - I moved the 550s to the rear, disconnected the sway bar and went with 750's up front - car feels great now.

oh, and I am mere 168lbs.. since we are sharing weights..

Greetings .... I also went to 750 f. / 550 r. Very Happy w/ the set-up .... :naughty:

Everyone - How did you determine that 750/550 was the pkg? did you try others?

Paul - Do you run a rear sway bar or did you take it off like Devin?

I have not tried many (testing the springs is rough). I have run a 900 fr and 400 r but that created a bunch of push in the nose.

Devin, got the idea from me! ;-) i also run no sway

But the car feels great with that setup, but I have only run slicks on the car, not sure the car would handle as well with DOT's on it.

Devin, got the idea from me! ;-) i also run no sway

But the car feels great with that setup, but I have only run slicks on the car, not sure the car would handle as well with DOT's on it.

Yep - Eric suggested it when we were at VIR - at the time I had 900/750 springs and was getting all sorts of push.....

Listened to Eric and disconnected the sway and with the "softer" spring rates -- mucho better!

so devin, panoz paul, and eric - you guys went firmer (OK)...anyone go softer with more bar just to compare? Did all 3 of you try the 550f/350r with a bar? I so want to go and try a doz. diff. combinations and see whats up.

Thanks for all the feedback. I am testing the car this coming weekend. Have not made any changes as yet to the rear springs. We will do some more measuring to confirm the shock bottoming theory this week.

Thoughts at this stage are to either fit progressive rate springs or a light bar on the rear of the car.

Kel.

yeah, i had the car with 550/350 with a rear bar.. it felt to me as if it had a bit of oversteer.

Went to the 750/550 with rear bar and it seemed to push a little, disconnected the rear bar and it was awesome, predictable, easy to rotate with moderate throttle, but not overly twitchy.

yeah, i had the car with 550/350 with a rear bar.. it felt to me as if it had a bit of oversteer.

Went to the 750/550 with rear bar and it seemed to push a little, disconnected the rear bar and it was awesome, predictable, easy to rotate with moderate throttle, but not overly twitchy.

You may be interested on how I measure shock travel. Put orings on the shock rods , they move up as the rod enters the body. Very simple and easy. I run the 550/350 spring set up with the adjustable sway bar and am very happy. The car is very netural and will very to a little loose with track temps. I have no bump stops on the shocks.

For what it's worth... Gary Jones highly recommends no rear sway bar. Pomroy and Eastman have tried it both ways and are convinced no rear bar is the way to go. Light setting on the front bar. Both guys were very fast with the rear sway but they say the car hooks up much better without it. It's something you have to try to know. I've just recently swapped in Penskes along with 600 fronts, 350 rears and am happy with the car. It's a different car from the Konis and stock springs.

Jim, which Penskes shocks did you chose? Thanks Bill

Bill, the ones I and another guy are using are here:

http://www.penskeshocks.com/Sports_Car-Formula_Car-7500_Series.php

They are the single adjustable, coil over shocks. They are red and look just like the Penskes on the GTS car and I'm sure are the same thing. Penske appears to only have a few different options with this stuff. We got them used from a local guy who does shock work for circle track guys. Apparenty it's very common for those guys to use these shocks. He rebuilt them, has a shock dyno, etc. I don't know what he did for valving but we like it. They are expensive though, even used.

Jim, thanks..Bill

Greetings ... the car came w/ 350 / 200's ; first step was 550 up front & moved the 350s back & installed a sway bar ... Tommey Archer ( Mr. Viper ) took me for a " real " ride !!! He thought the car was to soft ; on his advise I went to the 750 / 550 & kept the bar ... Gary Curtis @ lead instructor BIR ; retired GTS Racer ; states he can put the car any where he wants ... I still believe I don't drive the car at full capacity ... Low 1:53's on the 2.6 / 14 turn coarse at BIR ; Gary can turn 1:46's ... Hope this Helps ... Paul See Track @ www.birperformance.com (http://www.birperformance.com)