Originally posted by taz5176
i saw some of ur postings @ hobbylink regarding gear ratio
im rather new, i dun understand how u guys get the 1:1 overdrive? how do u get the figures? formula?
Taz, it's pretty simple once you know it. Which part is it are you not clear about?
For easy and quick reference, you can go to this website below;
Play around with figures (take the Impulse as an example) and see how the tire size and pinion / spur ratio will affect the final speed of the car...
For a 3 belted car drive transmission, there are 2 sections to worry about i.e. front and rear. You need to check if your front and rear internal drive ratios are the same. If they are somehow not the same, you'll either get overdrive (front pulling the rear) or the other case would be underdrive (rear pushing the front) assuming that your front and rear tire diameters are the same.
If for some reason your front and rear internal drive ratios are different, you need to play around with the front or rear tire diameters to alter the front and rear tire rollout so that they are the same in the end result. If the front spins more for every engine revolution (i.e. front internal drive ratio lower than rear), you need to use smaller front tires than the rear to make the final tire rollout for the front and rear the same.
Taking the stock 705 / Impulse PRO with 15T/18T pinions and 50T/46T spurs as an example;
Front Internal Drive Ratio = 24/18 x 24/16 = 2
Rear Internal Drive Ratio = 46/23 = 2
Spur / pinion ratio (1st speed) = 50/15
Spur / pinion ratio (2nd speed) = 46/18
Front / Rear Tire diameter = 62 mm (foams)
Final Front Drive Ratio (1st speed) = 2 x 50/15 = 6.67
Final Rear Drive Ratio (1st speed) = 2 x 50/15 = 6.67
Tire Rollout Front (1st Gear) = (pi) x 62 / 6.67 = 29.2 mm per engine revolution
Tire Rollout Rear (1st Gear) = (pi) x 62 / 6.67 = 29.2 mm per engine revolution
Final Front Drive Ratio (2nd speed) = 2 x 46/18 = 5.11
Final Rear Drive Ratio (2nd speed) = 2 x 46/18 = 5.11
Tire Rollout Front (2nd Gear) = (pi) x 62 / 5.11 = 38.12 mm per engine revolution
Tire Rollout Rear (2nd Gear) = (pi) x 62 / 5.11 = 38.12 mm per engine revolution
Both front and rear tire rollouts are equal and therefore, the overdrive is said to be 1:1 i.e. no overdrive. If front tire rollout is bigger than the rear, then it is overdrive. If the rear tire rollout is bigger than the front, then it is underdrive. Some people purposely set the car for overdrive (or underdrive) because it suites their driving styles.
Assuming there is no friction loss in the clutches and the engine is able to rev 36,000 rpm;
Top speed achieved (2nd gear) = 36,000 x 38.12 = 1372320 mm / minute = 82.3392 km/hr
Hope that is pretty clear