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Teach me about final drive ratio

Teach me about final drive ratio

Old 02-26-2011, 10:52 PM
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Default Teach me about final drive ratio

I have heard a lot of guys saying if I learn about final drive ratio I won't be guessing my pinions all the time just wondering if someone will take the time to explain in detail how I can use it to my advantage

Thanks Matt
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Old 02-26-2011, 11:11 PM
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Higher FDR = more acceleration, less top end
Lower FDR = less acceleration, more top end

To calculate

Spur/pinion x Transmission ratio.

Ex:

75 spur / 25 pinion = 3 then mulitple by 2.6 (ex; associated trans) = FDR of 7.8

It gets more detailed with onroad folks, they factor in tire rollout too as it makes a bigger difference for them.
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Old 02-26-2011, 11:15 PM
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Originally Posted by GizmoJunkie View Post
Higher FDR = more acceleration, less top end
Lower FDR = less acceleration, more top end

To calculate

Spur/pinion x Transmission ratio.

Ex:

75 spur / 25 pinion = 3 then mulitple by 2.6 (ex; associated trans) = FDR of 7.8

It gets more detailed with onroad folks, they factor in tire rollout too as it makes a bigger difference for them.
And there you have it! Well said. The only specific info you need to know is what is the internal ratio of your transmission. What car is this for?
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Old 02-26-2011, 11:17 PM
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Well when I get the final ratio has does that tranfer to the track I get the gearing i.e. smaller pinion more torque larger more speed but I don't understand how my b4.1 setup sheet said a 17.5 turn brushless is supposed to 28 pinion and 75 spur right now I'm running a 28 pinion and an 84 spur with not heat issues what so ever

I just don't get how the final drive ratio tranfers to the track I have always went buy the teemp after changing the pinion
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Old 02-26-2011, 11:18 PM
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B4 is 2.6 internal
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Old 02-27-2011, 12:02 AM
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Ok

With my example above. I equated out to 7.8. This means the motor turns 7.8 times to turn the rear tires once.

With the 75/28 the motor turns 6.96 times to turn the tires once.
With an 84/28 the motor turns 7.8 Times to turn the tires once.

Speaking of which you would get the same ratio running a 75/25 You would actually have less rotating mass (not that it equals much) with the 75/25.

The more revolutions a motor makes to put power to the ground, the easier it has to work. It balances itself out with getting more of its torque to the wheels/ground. It is kinda like adding leverage to the ground.

On the other side of that, less leverage, but more speed. The closer you get your tires to rotate to the motor rpm (making the ratio smaller) you lose some of the torque, almost as if you were adding standing weight, as the smaller ratio has a harder time moving the car initially, but when the motor spools out, you end up with a higher top speed.
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Old 02-27-2011, 06:53 AM
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Thanks for the input guys it has helped me understand a lot better
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Old 02-27-2011, 07:04 AM
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Just from my own knowledge having run a b4 with 17.5 you should be geared closer to 72/32ish
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