Gear ratio

[Timr100 1]

Hi all is there is there a formula to work out gear ratios to get the approx top speed of my tt02, 

[DK308 219]

As long as you have all the numbers you need, just use a gear ratio speed calculator. The math is simple, just boring.

The trick is grtting the actual speed correct. I like to divide the theoretical speed with the motor efficiency and I tend to calculate motor rpm from 75% pack capacity.

So get these numbers and use the following calculator.

Motor kv

Motor efficiency

Batt voltage

Tyre dia

Motor gear reduction

Final drive reduction

http://www.apexgarage.com/tech/gear_ratios.shtml

[nbTMM 433]

unloaded wheel speed (kmh) = V * K * Tpinion / Tspur / Rdiff * Wdia * pi * 60 / 1000

Where:
V = battery voltage
K = motor kv
Tpinion = number of teeth on the motor pinion gear
Tspur = number of teeth on the spur gear
Rdiff = the differential gear ratio
Wdia = the wheel/tyre diameter in meters
pi = 3.1415

As an example we can calculate it for my TT02 which has a 5130kv brushless motor, 2S Lipo pack, 61tooth pinion gear, 69tooth spur gear, and touring car wheels/tyres which measure about 65mm diameter. Therefore:
V = 8      <---- 4 volts per cell is a good guesstimate for a freshly charged lipo, accounting for some voltage sag under load
K = 5130
Tpinion = 61
Tspu = 69
Rdiff = 2.6    <----- this is the standard diff ratio for a TT02. Unless you use over/under-driven 'drift' spools with different numbers of teeth of the diff crown/pinion gears, it will be 2.6
Wdia = 0.065

unloaded wheel speed (kmh) = 8 * 5130 * 61 / 69 / 2.6 * 0.065 * 3.1415 * 60 / 1000 = 171kmh

So if I hold the car in the air and floor the throttle the wheels will spin at 171kmh. Will the car actually go that fast? No. In reality the fastest it's gone on GPS is 100kmh. This is because DC electric motor power peaks at half the unloaded rpm.so the motor power is starting to reduce when the car exceeds about half of the unloaded speed, ~85kmh. At some point, the amount of power the motor is producing will become exactly equal to the forces of aerodynamic drag and friction from the drivetrain and the tyres rolling on the road. At that point, the car will not go any faster and you want this to occur where motor power is maximum to achieve the fastest possible speed for your setup. You can calculate drag and motor power and so on, but honestly the simplest way to work out if you have the right gears is just to do a speedrun and then compare the actual top speed to the calculated unloaded speed.

Since my car already goes faster than 85kmh it means that I'm undergeared - the motor power has already reduced somewhat when it hits the maximum speed of 100kmh. I could put a pinion with more teeth and/or a spur with less teeth to achieve an unloaded speed closer to 200kmh, so motor maximum power would occur around 100kmh where my car currently tops out. Unfortunately I physically can't fit a bigger pinion or smaller spur so the only solution for me to go faster is to get a motor with higher Kv, or wheels/tyres with a larger diameter. A more aerodynamic body shell might get me a few kmh too.

Similarly, if my car topped out at 60kmh with the current setup it would mean that i'm overgeared - the motor wouldn't be reaching maximum power before the effects of aerodynamic and mechanical drag became too much. In that case, a smaller pinion and/or larger spur would be required to go faster. 

If it topped out at exactly 85km/h the gears are probably just right and the only way to improve it is a more powerful motor, more aerodynamic shell or higher voltage battery.

Be warned that running your motor full throttle for long periods near half it's unloaded speed or less will cause it to get very hot! Most RC cars need to be undergeared to keep the motor rpms up and therefore motor temps under control. The gears that achieve your maximum top speed will generally not allow you to run the car for very long before overheating the motor/esc. The TT02 is very undergeared out of the box (22t pinion 70t spur) so you will find that the actual top speed with those gears is very close to the unloaded speed. Effectively the top speed out of the box is more limited by the gear ratio and kv of the motor because it's going so slow that aerodynamics and motor power are barely a part of it.