Hi
I posted this other thread "scary cobra stories" asking some more questions about all these tire spin issues I read about from time-to-time.
I have recently gotten my 521 Cobra on the road and it (so far) has behaved very nicely. Fast fast acceleration, and I can spin the tires...but I have to make it do it (i.e. rolling start, slip the clutch, hit the gas fast, dump the clutch/etc).
During the build process, I spent time doing some simple modelling of the engine, car, tires,weight/etc. I did this in a 'fancy' engineering program (I own an engineering company) to try to deterime the effects of static/dynamic friction and torque (grip or slip points). My kids and I were actually outside in the summer with some different tires to make static-friction measurements on the street...
Anyway....I was not able to create a situation using a single torque vector that would produce tire spin. Meaning, I could only simulate a tire spin if the applied torque was something very large like 4000 foot-pounds (figure a 500ft-pound engine, through the tranny, and rear ~ you get alot of torque multiplication).
So this got me to another point, which may be more to the root issue. Its actually called "jerk" and here is a quick link on it.
Jerk (physics) - Wikipedia, the free encyclopedia
Jerk...is basically the rate of change of acceleration. When you apply this to a rotating object, like a tire, it becomes the rate of change of torque essentially. If I start creating a model of an engine that has some quick changes in its torque output, then the model I made spins the tires all over the place.
So....it would be really interesting to see a chassis dyno report on some of these engines to see what their torque curves look like. I think it could be that some of the smaller engines that have been cammed-up and stroked, may have some fast transitions in their torque curves.
So lets say your engine is @ 200 foot pounds at 1500 RPM and then goes to 400 foot pounds at 2500 RPM. This is a 200 foot pound change in 1000 rpm. Now lets say it takes 1/2 second to get from 1500 to 2500 RPM. You now basically have (400-200) / (0.5 * 0.5) which is 800 footpounds of dynamic torque IN addition to the 400 foot pounds of torque you reach a 2500 rpm. Once you get to 2500 RPM (and lets assume you stay at 2500 RPM), the 800 footpounds of dynamic torque goes away because there is no more change in torque output.
So I think in many of these cases...there may be rapid pulses of dynamic torque that snap the tires loose. One the tires break loose, the friction drops significantly and then the tires are much easier to spin.
From the reports I have seen on big blocks with stroker setups (like my 521) they are supposed to have a fairly flat torque curve from idle and up. So with a flatter torque curve, you limit these fast torque "jerks" on the tires.
Of course...my simulation model could be wrong as well....I need to meet more folks around here with Cobra's to do some burnouts with for scientific analysis :P
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