CSX3170 gets killed in autox

[Cobra #3170]

We took the car to a Crows Landing airport course last weekend and got beat badly. I have not tuned for concrete in quite a while and it showed in spades!
We tried the new A052 Yokahama's and they did not compare well to Bridgestones, they are expensive and slow. Looking at the data we are getting killed on the brakes pulling only .9 G stopping, Corvette competition is getting 1.2 to 1.3 G braking. We got 1.16 G acceleration at 65 mph so it is putting power down pretty well except for wheel spin. It would make Mark Donahue happy because it will spin the tires the length of any straight. The traction control pulls 20 D of timing out but still not enough to control wheel spin. We lowered the car a bunch which helped everything but braking now at 2.6" front of frame to ground and 3.4" rear. I need to add roll stiffness with front bar and increase rebound but we only had 4 runs each day so not enough time. I increased spring rates by 5% but think that hurt us too. We were off 2.4 seconds on Saturday and 1.4 seconds Sunday so some progress. We were actually very close to a McLaren 720S (.1 seconds) but 4 Corvettes got by, some brand new others C5 and C6 with big horsepower. We had too much camber on both ends which will help when we reduce it but I am not sure that is the whole answer. Oh well, we will work on on a concrete tune the rest of the summer. I am also looking at a Bosch Racing ABS system for the car. Here is a video for you to watch, look at the windshield above the red light sticky taped to dash. When you see red lights in my poor mans head up display traction control is on. Scott actually smoked the tires at 8200 in third gear bad enough that we saw it from pits. Thinking about reducing available throttle opening. The red light comes on when wheel acceleration is more than 225 rpm per .1 second. We finally figured out slippery asphalt now we need to get concrete figured out.


https://www.youtube.com/watch?v=UgG0C_6XZS4

[rodneym]

We have faith you'll figure it out.

[t walgamuth]

Well, good luck with it! I have run a lot of autocross on sticky concrete at Grissom and other places but cannot make any suggestions as it appears you know a lot more about what you are doing than I do. Are you allowed spoilers and such in the organization you are running in? Looks like that course had some high speed places where the aero would help a lot.

I'd like to add some spoilers to my cobra but am having trouble thinking how to add it without damaging my very nice paint.

Iv'e been running in X prepared scca so splitters and either spoilers or wings are allowed at rear.

[Cobra #3170]

Originally Posted by t walgamuth

Well, good luck with it! I have run a lot of autocross on sticky concrete at Grissom and other places but cannot make any suggestions as it appears you know a lot more about what you are doing than I do. Are you allowed spoilers and such in the organization you are running in? Looks like that course had some high speed places where the aero would help a lot.

I'd like to add some spoilers to my cobra but am having trouble thinking how to add it without damaging my very nice paint.

Iv'e been running in X prepared scca so splitters and either spoilers or wings are allowed at rear.

It is allowed and would help but I want to keep the body original, the wheels are the only thing that looks different than original. Since I wrote this I found we ended up with 1 degree too much camber on both ends of the car. Fixing that should help braking and improve times. The big problem is the car is too light for 200 tw tires.

[Jdata]

With your knowledge and experience.. you guys will get there! I am grateful and appreciative. Thanks

[t walgamuth]

Originally Posted by Cobra #3170

It is allowed and would help but I want to keep the body original, the wheels are the only thing that looks different than original. Since I wrote this I found we ended up with 1 degree too much camber on both ends of the car. Fixing that should help braking and improve times. The big problem is the car is too light for 200 tw tires.

Going for 2 degrees negative?

I am at one and am thinking of going to two and putting in some toe out.

I was running fairly hard on Sunday and my rear tires were in the low nineties and fronts were around 104 degrees. I have the live rear end so they are always equal across the tread in temp.

I imagine I'll get into the 120 range in front and 110 on the rear at grissom on a nice hot day.

[Cobra #3170]

Originally Posted by t walgamuth

Going for 2 degrees negative?

I am at one and am thinking of going to two and putting in some toe out.

I was running fairly hard on Sunday and my rear tires were in the low nineties and fronts were around 104 degrees. I have the live rear end so they are always equal across the tread in temp.

I imagine I'll get into the 120 range in front and 110 on the rear at grissom on a nice hot day.

Bridgestone's like 2.8 D front and 1.8 D rear, when the car is right front and rear pressures gain an equal amount. We are certainly not there yet. We are now logging Steering angle, traction control, throttle angle longitudinal acceleration, lateral acceleration, pitch rate, roll rate, yaw rate and actual roll and pitch in degrees. We have distance traveled, turn radius, engine rpm, and combined accelerations. It is a little overwhelming right now but will be an excellent set up tool when I learn more. For example we had yaw rates of 54 degrees per second which indicates over steer when compared with steering angle. Seems Scott was driving around it because the more we corrected it the faster he went. We are going to try setting the traction control at 300 rpm per .1 second at the drive shaft and see how that works.
Also going to try a taller front tire to see if braking improves.

[t walgamuth]

Taller front to get a little more rubber patch? Also will give a little less leverage to the calipers.

Traction control? I have the seat of my pants for that!

Lighting up the tires at 8200 rpm? I have my rev limiter at 6000 which is too low but not knowing exactly what is in the motor I don't want to blow it up.

[Cobra #3170]

Originally Posted by t walgamuth

Taller front to get a little more rubber patch? Also will give a little less leverage to the calipers.

Traction control? I have the seat of my pants for that!

Lighting up the tires at 8200 rpm? I have my rev limiter at 6000 which is too low but not knowing exactly what is in the motor I don't want to blow it up.

I am hoping the longer footprint, taller sidewall and 1 degree less negative camber will improve decel to a competitive level. The problem with these cars is they are short and narrow and transfer lots of weight very quickly. So when it is decelerating the front tires are doing a big % of the work The biggest front tire I can run with a stock front fender is a 295/30/18 but Bridgestone does not make anything that will fit other than a 285/30/18 or 275/35/18 so we are stuck. Just need to make them perform the best in this platform. Lowering the rear, reducing camber and going to a tall tire as well as more weight in the rear all help. Last weekend peak acceleration was 1.16 G, peak cornering 1.37 G and peak stopping .9 G so it illustrates the problem.

Traction control: to quote Scott "the car spins the rears when I look at the throttle" We just pull spark timing but it does soften the motor a little I also have a very slow throttle cam and 6.5" of throttle travel. He rarely goes over 1/2 throttle so we have too much power for the tires we have to run but that is what makes it fun. Believe me, no one could control this thing with their foot alone. Over 690 normally aspirated instant on RWHP in a 2400 pound car on 200 tw tires.

[t walgamuth]

I have a buddy who may be able to help me set up something to measure the G forces etc. It will be interesting to see what I am doing.

[Jdata]

What logging software are you using telemetry again?

[Cobra #3170]

Originally Posted by Jdata

What logging software are you using telemetry again?

I use an interface I designed so we don't load the TPS or send a 12 volt pulse in to a 5 volt input. That unit connects to a Race Capture Pro MK III, the race capture sends all the data including solid state giro to a Samsung tablet running Solo Storm with blue tooth. I do not like Race Capture software, it is clumsy, full of bugs and difficult to use. Solo Storm is a breeze to use and very intuitive. It can run alone with a GPS receiver but this year I want lots more data so we are putting Race Capture in. We just pull the tablet after a run and take it out and look at what ever data we want, there are 23 channels of data recorded on every run. We are still learning how to use steering input vs yaw rate but the potential for big improvements is certainly there.

[t walgamuth]

Sounds impressive!

[Jdata]

What size brakes are you using? And also what type meaning material

[Cobra #3170]

Originally Posted by Jdata

What size brakes are you using? And also what type meaning material

I have 13" front rotors with Baer 6 pot calipers and 12" rear rotors with Wilwood 4 pot calipers. I wish I had gone right to Brembo calipers but hind site now. I have tried lots of different compounds from Wilwood, Carbo tech and others, currently use a Porterfield compound. The master cylinders are AP with a 7/8" front and 3/4" rear and adjustable bias bar. Brake modulation is good just too easy to lock. I just took 1 degree of camber out of both the front and rear this week. Hopefully decel will be better now. Rotors are very high grade Stop Tech front and Wilwood's best in rear. Front pads are Corvette size so tons of material choices rears Wilwood so not as many choices. Bigger rotors just add rotating inertia so that is not the answer either maybe ceramic.

[Tom Kirkham]

I am surprised you are not logging the shocks.

[Cobra #3170]

Originally Posted by Tom Kirkham

I am surprised you are not logging the shocks.

We have linear pots for that just have not installed, they are delicate enough that I don't like leaving them on shocks. We have dyno curves for all possible settings on our shocks and have a pretty good handle on what we need with our spring rates and unsprung corner weights. We use the shocks more as a dynamic tuning tool than something to monitor. We do not go out side our specific range of adjustment though because the car can get very bad very fast if we do.

[t walgamuth]

Thanks for sharing your particulars Tom!

[Jdata]

I would think the short wheelbase doesn't help with the rapid weight transfer during braking. Softening the front wouldn't help?

[Cobra #3170]

Originally Posted by Jdata

I would think the short wheelbase doesn't help with the rapid weight transfer during braking. Softening the front wouldn't help?

That is true but then you hurt transitional response. It is a game of trade offs, we are changing springs in 25 pound increments now and have found that the car is close to ideal for its weight distribution. We have been changing spring rates for 5 years now and it seems we always end up with what tested well two years ago. The short wheel base and narrow track help because the car
is physically smaller than its competition but it also hurts because we overload the tires easily due to that rapid weight transfer. The 200 TW tires compound the problem because the car is light enough that we do not get as much mechanical grip from surface deformation of the tread rubber. Getting the right balance of grip and transitional response time is what we try for when setting up at an event. I am hoping all the data we take now will lead us to the right answers quickly. Steering input in degrees/sec vs yaw angle in degrees/sec should tell us if we are right or wrong once we have a data base of good setups to compare to. Right now we do know that if we are not over 1.35g in lateral grip we need to start working. We got over 1.45 on Kuhmo's and they were very fast but are illegal now. New Bridgestone's are about 1.37 if the car is right. I am hoping we will see the same type of relationships in other parameters.