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Old 01-24-2015, 01:33 PM   #1
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Default FDR Equivalence Question

Amongst our club members, we have a debate going that I would like to get others opinion on.

Given a specific final drive ratio, say 3.7, are there any differences in the performance of a car using different combinations of spur and pinion to achieve the same fdr?

For instance, an FDR of 3.7 can be created using a 96t spur and a 49t pinion, or you could, for instance, use a 92t spur and a 47t pinion. Would there be any differences in performance? Is there a difference in load placed on the motor, such that one gear combination might run cooler?

Our members have a number of different opinions on this, so any thoughts are appreciated.
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Old 01-24-2015, 01:51 PM   #2
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No - to all intents and purposes there is no difference whatsoever.

There are some theoretical efficiency improvements by having the spur and pinion as close as possible to the same size. There are more teeth in contact which spreads the load and reduces tooth bending. in practice the face width of the spurs and pinions we use are so grossly over-engineered it makes no difference.

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Old 01-24-2015, 02:13 PM   #3
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There is he argument for "Lower rotating mass" with smaller sized combos, but any advantage gained is so minimal hat a well placed fart in the opposing direction would negate such effects.
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Old 01-24-2015, 03:02 PM   #4
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Rotating mass has also a gyroscopic effect giving a bad influence on the handeling of the car and Larger or smaller gears will move the weight of the motor. I think it will have a more effect on the handeling and not with the accelerating or topspeed
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Old 01-24-2015, 03:36 PM   #5
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In terms of acceleration and top speed, zero measurable difference. Larger could see some improvements in efficiency while smaller may have some improvement in mass but it's a very small difference.

Motor position could make a big difference in handling though especially in pan cars.
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Old 01-24-2015, 05:07 PM   #6
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Yes there is a difference in the way my car feels.
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Old 01-25-2015, 08:47 AM   #7
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Originally Posted by skypilot View Post
Yes there is a difference in the way my car feels.
That will be the change in motor position, not the gears sizes.
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Old 01-25-2015, 08:23 PM   #8
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Center of gravity will change if you change only the spur or the pinion. If you want to keep the car handling the same, you can calculate the total number of teeth on both spur and pinion and change both at the same time to keep the same total number of teeth. As long as the total number of teeth on both spur and pinion stay the same, the motor location stays the same.

It is also helpful to know the minimum and maximum total tooth count for the two limits of the motor mount slots. This helps one know what gear combinations will and won't be able to mesh when ordering them on-line.

For example on my TRF 419, combinations of 64 pitch gears that add up to less than 134 teeth won't mesh. Combinations that add up to 143 put the motor in the middle of the adjustment range.
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