Rear suspension squat.

[fastg]

I just came back from VIR, had a great event with the HOD group, www.hookedondriving.com. I scanned the posted pictures from eTech. Normally after your first track day you never give the pictures a glance, but there I was scanning and he had taken some good pics. One or two caught my eye that it looks like my rear suspension is a little low on outside wheel when the tire is loaded in a turn. I don't think these pictures are taken were the load is at it's max. The car is running well and I was driving faster than ever before, I did feel a couple of tire rubs just for a split second if I hit a bump when heavily loaded. The rear shocks must be the only part I have never touched on the car, Gaz struts but I have no idea of the spring rate. What do you think? 

[sf4018]

That looks like the left hander after roller coaster when the corner begins to load the car on the right side with some compression, so you’d expect to see something going on like that there. Did you find any marks where the rub happened?

[fastg]

I think the biggest load was out of Oak Tree, and the last corner onto the front straight the bottom of Hog Pen. Wow it was HOT, I normally take my H20 as coffee, tea or beer. I have never drunk so much plain water in my life. But the car is running well, it really rips, I could keep up with a different class of car.  The 6 speed box and the extra power gave me a lot better drive out of Oak Tree, I was having to back off to gave Corvettes a point by, that has never happed before. 

 

To answer your question the rear fender are black to tire marks are not obvious, but maybe it's the very outside edge of the tire between the used tread and the side wall, just on that corner. I will know more when I get the car on on jack stands and the wheels off. 

 

Graham 

[sf4018]

This is the closest pic I could find to yours, it's the hairpin at Barber. The gap between tire and fender on my RR is bigger than yours for sure, but I jacked up the rear ride height some after grounding out the lowered floors on some curbing, so I wouldn't say it's necessarily "correct" it's just good for me, just a frame of reference.

[Croc]

 

Bit surprised to hear Oak Tree is giving you tire rub issues as it is fairly slow.  However, the rollercoaster section down onto pit straight makes sense - some brutal compressions there.  

 

I don't think the photo gives you any evidence of squat.  Looks like the car is nicely set up and loaded up properly through the corner.  The evidence (if any) will be tire rub on the inside of the rear wings.  

 

I went digging through my collection of photos from past sevens events for a photo of your car to compare to others re tire to rear wing positioning.  This is Lightning Circuit coming off the 5% banking and you should be doing between 80-100mph depending on the seven.

 

 

For comparison, here is Tom Carlin in his Birkin XS.  The IRS Birkin is more likely to tire rub on track than other cars because of the nature of the design - not a criticism just an observation.

 

 

 

 

 

Then there is Show Pony  in his (then) 420R

 

 

Paul in his Birkin

 

 

 

And then there is Fat Boy in his car with the long travel rear end of a 2000 VW Passat.

 

 

And last point of comparison - my 420R at Spa-Francorchamps going through Eau-Rouge.  This is as much load/speed I have ever put on a Caterham rear end in a corner at any track.  The next closest would be the 100mph sweeper at Croft circuit....and that broke the A frame.  

 

 

Photo angles can be misleading so caution is needed to interpret.  Your car definitely is softer in the rear than front so it gives a visual "twist effect".   Front and rear are not under load in the same plane which is what you see with a Caterham or Birkin.  Thats an observation and not necessarily a problem as it may handle best that way.  But it is different to how your average Caterham and Birkin is set up.   Your rear wing is designed with a closer profile to the rear tire than the Birkin or Caterham which are wider with a fatter lip.  But even then, it does look closer in relative positioning under load than the other sevens.  

 

I think what you may be finding is that as you have upgraded the power, gained experience, know the circuits better, etc., you are now a lot faster driver and could be reaching the top end of the spring rate relative to the rear bump stop and so could justify a harder spring rate.  You can calculate what you should have and then compare that to what you have on the car now.  At the same time I would then test the valving in the shocks (or just upgrade).  Many Locosts I have seen usually have off the shelf shocks which are valved for production road cars - much heavier car weight - higher pressures for rebound control.  I know you know this already - springs always come first before shocks in sorting handling.  

[fastg]

Thanks for the pictures. Your right about Oak Tree but I adopt a different strategy that is working out well. Corner speed is an advantage our light cars have, but also initial acceleration. So at Oak Tree I slow it down, turn hard and get it pointed down the straight. While other car are bouncing over the curbing I have my foot flat on the floor. The new 6 speed trans is helping as 3rd gear is now perfect.

 

I have started to develop winter plans, and a suspension upgrade is on the table. I have the Gaz coil over setup for a Locost, which while not amazing it's not bad, it has served me well. There is not much knowledge about Locosts in the US so I am looking at Procomp Motorsports, there heavily into the Locost racing scene. http://procomp.co.uk/shockdyno.html 

 

I will get the car up in the air this weekend and see if I can find the spring rates, the ride is soft, not harsh at all, so I am expecting the rate to be on the low end of the scale. 

[KnifeySpoony]

Even with 215lb rear springs on my S3 (often considered "too-stiff' for a 7), I see a lot of rear suspension compression in track photos. After recently switching from ZZS to ZZR, I saw a evidence of a little bit of rubbing on the rear wing (very small groove on tire). 

[MV8]

For those cutting tires but happy otherwise, the travel indicating rubber ring on the shaft (if equipped) can show if/how much foam taper bumper can be used to prevent damage without a sudden spring rate increase from bottoming on a standard solid rubber bump stop. The gradual rate increase can start an inch of shaft travel before the cutting begins, so as not to upset the car. That may be all the change you want to make. Many oems (typically struts) come with foam bump stops. They are usually included with the gaiter/dust boot kit.