CSX3170 autox prep

[Cobra #3170]

I thought it was time for an update on Scott and my effort to get into the Goodguys shootout this year in Scottsdale. We have made a number of changes to improve the car. I replaced the Penske 3-ways with JRI 2-way shocks which was a major improvement. Evaluated 3.54 and 3.73 rear gears in place of the 3.31 and Torsen T2R, Salisbury and Tracloc differentials. We also tried some different caster camber combinations and toe settings. The car gets so low in the rpm band at Goodguys we loose intake velocity followed by an explosion of torque so driving is pretty difficult. Scott did qualify for Scottsdale by winning the Pleasanton Goodguys autox so that is out of the way early this year and did it with the 3.54 ratio. We have changed to smaller master cylinders because the brakes were too difficult to modulate. ended up with .75 front and .625 rear.
We tried two different brake pads and decided on Porterfield R4-1. We have tried Rival S, Falken 615k and Bridgestone RE71 tires. Still have not identified the best brand however, more testing is needed. I have raised the front roll center to reduce RC migration and reduced camber which helped handling and also improved braking.
The car is now at 2.8 D front camber with 5D of caster. Rear is 2.2 D. of camber, front and rear toe varies depending on the course from 0 to 1/8" total in.
The car is really beginning to work well now, I like the 3.73 with Tracloc the best because it does not under steer as you bring up the power. Brake changes are positive too, much better modulation and higher G stopping.
I still have some knock back so may float the rotors to address that.
We ran it this weekend at Crows Landing in California and it was pulling 1.1 G
acceleration at 70 mph and a solid .9 acceleration at 91 mph and 5500 in 3rd. I am still losing belts at high revs so will be installing an idler pulley on the belt this week.
All in all the car is really improved over last year but so is everyone elses car.
I like the JRI shocks, when you change a setting things happen unlike the Penskes and they have a huge range of adjustment. I will write an update on tire testing and alignment settings when we complete the sorting.
The good news is that the car is competitive on a tiny asphalt lot or a big wide open airport. The AAS series allows modern Corvettes in our class (Cam-S) even though SCCA does not and the Cobra has beat new 2015 Z06's so it is no slouch in an autox even on 200 TW street tires and no electronic nanies.

[Ron61]

It sounds as if you are making major gains on getting the car to handle like you want. Congratulations and I will be looking forward to more reports.

Ron

[Tommy]

That sounds like an amazing amount of testing to a novice like myself. Do you have access to a track just for that purpose, or are you doing it all at weekend events? ... I'm also curious as to how many changes you make at one time. If I made more than one, I'd be totally baffled by which one had which effect. But then, I don't have a world class driver providing feedback. ... I really enjoy reading your updates. Keep em coming.

[Cobra #3170]

duplicate post

[Cobra #3170]

Thanks Tommy and Ron, the America Autox Series is on an abandoned air port and they set up a test course on one of the runways. The course is marked with paint so it is always the same and it has its own dedicated timer. For $20 you get 2 hours of testing time which is an amazing deal. We usually sign up for 4 hours of testing at a time and just park the trailer at the test course and run the autox (which is different every event) when our class comes up. I have set up sheets where I record everything done to the car every time we run right down to installed spring lengths so if we get lost we can always get back to something that works.
Scott is such a good precise driver that we can immediately see if a change is better or worse and sometimes it is an improvement in one area only and not overall time. We save all the electronic data from every autox so we can go back and review it.
Yes, we make multiple changes at a time but have done so many different tests we have a pretty good idea of what will help a given problem. The car is getting so good now that even my worn out old a$$ is beating many of our competitors and Scott is always at least 2 seconds faster than me. We usually both make base line runs then sit down and review the data and mutually decide what to do. Goals are at least 1.3 G cornering,
1.2 G acceleration and 1.5 G braking and of course over all time. Two weeks ago we met everything but braking. Our maximum decell was only .7. After the master cylinder and pad change we are 1.0 but still a long way from 1.5. That is our weak point right now so we are looking at knock back springs in the front calipers and possibly replacing the 2 psi check valves with 5 psi. I would not try this on a road race car but for 1 lap stuff think it will work. Another critical point is ride height we have found the front of the lower frame tube needs to be 3.00" and the rear about 3.250. If the car under steers we go up to 3.375 if it over steers we drop it to 3.125. This really allows some quick fine tuning because you can do it on the grid and just count shock collar rotation.
Once the brakes are fixed we will start shock tuning to see if there is anything there and what happens when you make a big change.

[Brooklyn-427]

Racing Jaguar XKE`s have piston knock back issues which is caused by torsional flex of the front uprights, maybe this is what your experiencing.

[ACademic]

Originally Posted by Cobra #3170

..and the Cobra has beat new 2015 Z06's so it is no slouch in an autox even on 200 TW street tires and no electronic nanies.

That statement alone says a lot about this car's set-up and potential. The new Z06 out of the box is a very formidable track car. Congratulations!

[Cobra #3170]

Originally Posted by Brooklyn-427

Racing Jaguar XKE`s have piston knock back issues which is caused by torsional flex of the front uprights, maybe this is what your experiencing.

Yes it is the issue, I fabricated my own uprights but used original bolt in spindles and bearings which are not as stiff as they should be. There wasn't much of a problem with the 7/8 bore master cylinder but it really shows up with the short stroke AP 3/4 bore cylinder.

[Brooklyn-427]

They use a machined steel sleeve that slides on to the spindle shaft, thus increasing the shaft diameter and positively locating the inner and outer bearing so the main retaining nut can be wound up fully tight also. Its a system that works well.

[Dominik]

I highly reccommend a toothed belt arrangement. Are you sure you are not having an imbalance somewhere? Belt shedding could be a sign of that...

Are those tires you mention sliks?

0.9g at 5.500 in third is awesome!

[fordracing65]

When is this race, I want to go watch...

[Cobra #3170]

Originally Posted by Dominik

I highly reccommend a toothed belt arrangement. Are you sure you are not having an imbalance somewhere? Belt shedding could be a sign of that...

Are those tires you mention sliks?

0.9g at 5.500 in third is awesome!

The belt goes into harmonics at extreme rpm as seen on a GOPRO camera.
We disconnected the alternator and it still jumped off or turned upside down.
Converting to a toothed belt or serpentine would be a major deal and require engine removal so I am desperately trying everything else.

No, they are BF Goodrich Rival S 315x30x18 200 TW street tires

Peak horse power is at 7600 and peak torque is 5700. The car went from 67 mph to 91.7 in 2.2 seconds on treaded street tires including a shift to third.

[Cobra #3170]

Originally Posted by Brooklyn-427

They use a machined steel sleeve that slides on to the spindle shaft, thus increasing the shaft diameter and positively locating the inner and outer bearing so the main retaining nut can be wound up fully tight also. Its a system that works well.

Brooklin427,

Good idea, that is how my rear hub carriers are done so the bearing is perfectly loaded. I have the spindle nuts so tight that I repack the bearings and check them about every other time out but still have the problem. I actually think it is flex in the spindle shaft and upright causing the issue.

Ford Racing,

November 14 and 15th at West World in Scottsdale

[Dominik]

Under breaking you are not achieving the desired numbers, but are the tires already locking up? 1g decel sounds quite decent for a threaded tire, but I am not familiar with latest (threaded) tire capabilities.

I shall dig out the acceleration / coast down test I did in 1995 with a 496 cui and hardtop. Your values remind me of that time. My max rpm was 6500.

Good luck with your V belt! My fan belt also turned inside out. I changed belts and location of the alternator and it seemd to help - for whatever reason.

[Morris]

Bruce

Good luck, your post are great to read.....and I wish I could help you more....but all my experience is RR......

BTW....what size are your rotors .....and what about the inclination angle of the spindle
,...just thinking out loud....
Keep the thoughts coming.....

[Cobra #3170]

Originally Posted by Morris

Bruce

Good luck, your post are great to read.....and I wish I could help you more....but all my experience is RR......

BTW....what size are your rotors .....and what about the inclination angle of the spindle
,...just thinking out loud....
Keep the thoughts coming.....

Thanks Morris,

Rotors are 13" really tight package because I have a .5" scrub radius the upright is inside the wheel, the caliper clears the wheels spokes by less than .1" and the tie rod end is right on the brake disk for Ackerman. I wanted to use a cartridge bearing design but could not package it. with everything else.
SAI is 13.5 Degrees. These little master cylinders are not helping either (AP with 1" stroke). I did find that the pistons are not perfectly centered on the disk so I will shim to correct but doubt that will help knock back. Should have gone Brembo in the beginning and would not be fighting it now.
Tried to save some $, now will probably have to pay again to fix it.