quote:

Originally posted by JPosey:
I wish there was a rule of thumb on R/T vs rollout.

Typically, 1" of tire diameter is worth .007 in reaction time.

Thanks for the info botmbulb! With this knowledge, tire swapping is going to be a wasted endeavor in my application. I did a Google search this morning found on Class Racer one racer said 2 inch shorter was worth .015 in reaction time. I guess in need to practice a couple extra bumps from shallow and try that.

So when the car is actually leaving the line would'nt the rollout be different then when its just being pushed out by hand? Because of the effect the weight transfer has on it ?

quote:

Originally posted by Mark Cmar:
So when the car is actually leaving the line would'nt the rollout be different then when its just being pushed out by hand? Because of the effect the weight transfer has on it ?

Surely, but lower tire pressure will give you a more shallow stage position to begin with. A flatter tire will be wider 2" off the ground than a fully inflated tire.

i get that but wouldn't as the tire comes up under launch actually be smaller diameter than if it were if it had more air in it ?

Probably, but the difference is tiny compared to the difference at 2" with the weight on the tire.

Inflate one front tire on your car to 30 psi and one to 20. I'm guessing you can visually see a difference in the tires, specifically the shape at the bottom.

Now take them off the car and lean them up against the shop wall. Can you visually see the difference now? Guessing not.

not seeing a difference against the shop wall is my point

meaning more than half of the 2 inch difference is gone

quote:

Originally posted by Mark Cmar:
meaning more than half of the 2 inch difference is gone

The difference was there when you staged the car, is my point.

im missing something here

what im saying is , if you get 2 inches more static rollout while staged and 1 inch of it is gone when the back of the tire exists the stage beam . the amount of slowed reaction , assuming around 12 inches of total rollout that you'd get from the first inch is pretty small . im guessing maybe 7 thousands

quote:

Originally posted by Mark Cmar:
what im saying is , if you get 2 inches more static rollout while staged and 1 inch of it is gone when the back of the tire exists the stage beam . the amount of slowed reaction , assuming around 12 inches of total rollout that you'd get from the first inch is pretty small . im guessing maybe 7 thousands

Sounds like you get it just fine. When I say 2" I am referring to the starting line beam height; I don't think you're typically getting an extra 2" in tire width from pressure adjustments.

I prefer to adjust R/T with RPM rather than pressure because there isn't a lot of room to work there, IMO.

Ike and i went through all this stuff in the 70s and 80s Hence the first accurate and reliable Delay Box was born

It was always a private joke between us the He needed more rollout and i needed less. And the top bulb with his quartz timed Box took care of us both LOL

I miss my friend

quote:

Originally posted by Mark Cmar:
i get that but wouldn't as the tire comes up under launch actually be smaller diameter than if it were if it had more air in it ?

Maybe because at a lower PSI, the centerline of the spindle is lower (closer to the ground), so to lift high enough to clear the beams requires more vertical travel of the spindle, (which would also slow the R/T at lower PSI).

This is a front heavy 8 sec car with street radial tires. The frt. radial tire will squat so much if you lower air pressure it will widen and lengthen the footprint and increase the roll out. Inflating the tire to max pressure will help with RT but hard to say how much with a 16" radial.
I would try and borrow a set of wheels and tires from someone and see what difference it makes. Surely someone has a set of mounted front runners you could make a couple of passes with