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OK, any given motor has a powerband. It is the point that the motor starts spinning, to the point the RPM stops increasing. Thanks to the design of electric motors, the powerband is massive, but you still need to gear the car correctly to make best use of the powerband. For example, a motor geared for high top speed will be really sluggish out of corners, and may never hit the sweet spot of the powerband before you need to turn.
So, how do you gear a car correctly? First, you need to know the gearing you're on.
Electric cars usually have one pinion and one spur gear. The smaller gear is the pinion, the larger one is the spur gear. To work out the gearing, you can use a great website such as http://www.gearchart.com/ but if your car isn't listed, you'll need to do a little maths. Its Spur Gear divided by Pinion Gear times Internal Ratio equals Final Drive Ratio.
Spur
(----------) x IR = FDR
Pinion
So, with some random numbers:
89t
(----------) x 2.1 = 8.49FDR
22t
To make it a little easier if you're away from a calculator or a computer, a lot of cars these days come with a gear chart in the manual.
Here's the possible pinions and spurs that fit a Tamiya TT01, and the motor mounting holes you'll need to get the right mesh, that won't strip the gears.
The numbers in the middle of the chart are the Final Drive Ratio, along the top are the spur gears, and the pinions are down the side.
OK, you now know your FDR, but what does that mean to you?
Well, the FDR number is the amount of times the motor turns for one revolution of the wheel. Using our example above, the motor turns 8.49 times for every one complete turn of the wheel.
Lower FDR's mean more top speed, higher FDR's mean more acceleration.
In general, when you fit a faster motor, you fit a higher FDR. Fit a slower motor and you need a low FDR. That doesn't take into account the space you've got to run the car in, so smaller areas mean a higher FDR as you're not going to be able to reach the top speed with a low FDR, so its always a balance of top speed and acceleration that match the motor and the area. For touring cars at least, FDR's of 7.5 or above (smaller number) are considered high.
Its a lot easier to gear a motor for a track, simply gear the car so the car stops accelerating about two thirds (2/3s) to three quarters (3/4) of the way down the longest straight on the track
So, in summary....
If you are:
fitting a faster motor
driving on a tight track or small space
using more voltage/cells
fit a smaller pinion/bigger spur/high FDR.
If you are:
fitting a slower motor
driving on a open track or big space
fit a bigger pinion/small spur/low FDR.
But remember, everyone has a different driving style, different equipment setup etc, so no-one can say ''Use 'xyz'FDR with this motor or that track'', its really a case of trial and error. Look on eBay and buy a selection of pinions and try for yourself!
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