Originally Posted by Northeast Racer
rollout= (tire diameter x 3.14) / (spur/pinion)
not sure what 3.14 equals in MM though, sorry
25.4mm/inch, so:
3.14 X 25.4 = 79.76mm
BUT (and this is a BIG but) I don't believe you have to convert pi to metric
, just the tire diameter. Pi is pi. Or pi are round, or whatever it is mathematicians say. Pi is a fixed ratio (thank goodness or highschool Geometry would have been even MORE fun). If you convert both pi and the tire diameter you'll end up with a number that likely won't show up on any roll-out charts. Or if you convert pi and not the tire diameter you'll be similarly in error. Unless your tires are 3.14" in diameter...but I digress.
The tire diameter is all I need as I've got roll-out charts that accept both inch and mm diameter inputs. BUT...if you're really jonesing for some math you can do it all yourself.
Examples?
1.7" (tire diameter) X 3.14 (pi) = 5.34" (tire circumference) X 25.4 (convert to mm) = 135.59mm (tire circumference).
Now, let's suppose your buddy told you the guys at your track are using about a 44mm roll-out (1.73") for their layout. Circumference (135.59) divided by desired rollout (44) gives you your target gear ratio (3.93:1). Further, let's pretend you've got a 100-tooth spur gear on your car... take 100 (spur gear teeth) divide by 3.93 (target gear ratio) and you get 25.45 (desired pinion size). Now, I've HAD pinions that have a half tooth, but they're really noisy and not terribly efficient, so in this instance you'll have to round up or down. OR if you're REALLY anal retentive you can try a different spur gear and see if it lines up better with your target gear ratio. A 96 tooth spur would give a target pinion size of 24.42. Hmmm...not much better. A 92 tooth would give 23.4. Well, if you're THAT much of a perfectionist I guess you'll have to true your tires down a bit. We could do the math to tell you how much to take off of them, but it's making my head hurt.
Take the lazy way out...measure tire od and plug it into the chart and compare with what your buddy tells you is a good roll-out number. Assuming HE knows what he's doing...
Scottrik