Originally Posted by Rock'sTC3
could someone please explain gearing to me. I am running the stock spur but what I am wondering is what pinion to use under what conditions? Why changing a spur would be benefical? How to calculate the final drive ratio? What the ideal temps would be after a run? If too hot should I add a smaller pinion? larger? A smaller pinion means more torque? larger means more top end speed?
Hi Rock, I think the best way to understand gearing is to explain from the beginning:
The pinion drives the spur, which drives the top-axle. This top-axle drives the belts. The belts drive the diffs. Diffs drive the wheelaxle, which drive the wheels, which drive the cars.
This is how an internal gear ratio can be calculated. Check the number of teeth of the pulley of your top axle, and also do this by checking the number of teeth of the pulley of your differential. I don't know the values of the FK05 anymore, so I'll take my TC5 as an example. On my TC5 I've got 20 teeth on my topaxle pulley, and 40 on my diffpulley. Which means the internal drive ratio is 40teeth:20teeth. 40:20 is equal to 2:1 (Or it should be 20:40 --> 1:2 . I don't know which way around you are supposed to say it, but you get the idea
) This internal ratio means your diff will turn half a circle, while your topaxle will turn a whole one (when the 20th tooth of the topaxle is used, also the 20th of the diff is used, which means half of the circle).
Than you get to gearing with your spurs and pinions. Most of the time a good reason for my to change to another number of teeth is the simple reason that the pinion I want to use does not fit with that spur gear.
But t explain about that, let's say you start out with a 90T spur and a 30T pinion (just an example). This means your spur will drive 1/3 of a circle while your pinion turns the whole way around. 1/3 turn of the spur will also mean 1/3 turn of the top axle (which is logical, the spur is attached to the top axle) this means that 1 turn of the pinion will mean 1/3 turn of a spur/top axle. Which means 1/6 turn of the diff/wheel. This means the pinion as to make 6 rounds to make the wheel turn 1. This means the final gear ratio is 6:1 (or 1:6
When you realise the you have a lack of acceleration you have to use a lower number of the pinion.
when you realise that the top speed is reached to early on a straight, you have to use a higher number of pinion.
This is because with a lower pinion, let's do it extreme for the example and go to 20T. You will get a final drive ratio of 9. 90/20=4,5 and 4,5 x 2 = 9
This means your pinion will need 9 rounds to make the wheel turn completely, because it's easier for the motor to get to a high rev the accelleration will be better. But you'll have a lower top speed.
Let's say your motor can drive 50.000 RPM. This means that with the final drive ratio of 6 that your wheel will drive 50.000/6=8.333 rounds at top.
When using the final ratio of 9 it will me 50.000/9=5.555 rounds at top.
For top speed it is the other way around.
Most of the time when a motor gets too hot, it is overgeared (which means the pinion is to big).
Also note: when using older batteries lower the number of teeth by one. This is because old batteries have more problems supplying enough current to give the high peak to make the motor give the power to accelerate fully.
Also be aware not to overgear a little by thinking you go really hard the first 3 laps, when overgeared a little, you will have a high top speed in the beginning but your current drops quickly. The battery can only supply this "overdrive current" for the first 2 or 3 laps. So gear a little lower. You will be a bit slower the first 2 or 3laps, but you can remain driving this laptimes longer.
Also when driving in wet condition lower the number of teeth 1 or 2. Because in rain the surface has more resistance, which means the motor has to work harder to accelerate.
Final note: This comes to the reason why different motors use different ratio. Because the max number or RPM is different, not only my the number of turns , but also the brand. So try a little with different ratio, but when not sure don't go to high, when overgeared to much/to often the motor will lose performance.
Hope I made a bit understandable with this story. When you don't understand it yet, just let me know