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Old 07-09-2004, 02:25 PM   #1
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Default Big Pinion / Small Spur - OR- Small Pinion / Big Spur -- Difference?

I don't have my chart handy right now, so my numbers may be off, but I think I can get across what I am asking...

What is the difference?

Running an 86t Spur with ??t pinion for a final drive of 7.0

Or

Running a 94t spur and ??t pinion for a final drive of 7.0

Does the size of the pinion/spur matter if the final drive comes out to be the same?

In one way I can see that it matters because I would think it is harder on the motor to turn a larger spur, but at the same time I would think the motor only sees the "load" of the final drive.
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Old 07-09-2004, 02:32 PM   #2
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We have been discussing that in the oval forums forever, basically a gear ratio is a gear ratio the difference lies in a slight amount of efficiency gain with larger gears due to less aggressive gear interference angle but this adds weight also to a small degree. In pan cars bigger gears helps to reduce unsprung weight by moving the motor forward toward the pivot point and away from the axle.
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Old 07-09-2004, 02:33 PM   #3
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Also smaller gears = less rotating mass.
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Old 07-09-2004, 02:37 PM   #4
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much like the idea with the pan cars, for touing cars as well, we use these combinations to produce a low motor location instead of a more forward location. some xray guys are often using 132/3 spurs to get the motor low while still running a fairly high fdr when they use the 18/32 pulley combo.

i think the effeciency vs. rotating mass to be an insignificant study. using the gears to locate the motor correctly should produce effects much higher in magnitude.
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Old 07-09-2004, 05:47 PM   #5
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I believe use the largest gears you can fit that are within your gearing range.

I run a 75 on my TC3. I did this because my car was light on the motor side (even after the battery location modification) so I wanted to utilize the weight I HAD to have in the car before adding weight.

the weight difference in using larger gears is SO minimal, its not even worth thinking about. Too much solder on your packs can make up for that.

I also agree about the mechanical effeciency: more teeth is more effecient because of a less steep involute interaction.

ALSO, using a larger spur gear gives a slightly wider range of FDR's to play with., because the difference between one tooth on a pinion has less effect than with a smaller spur.
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Old 07-09-2004, 10:59 PM   #6
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I agree with all the above said. It has also been said by many that smaller spur will make the drivetrain spool up faster, which would be good for technical tracks with lots of 180's.
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Old 07-12-2004, 06:13 AM   #7
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the larger the spur will supply more torque
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Old 07-12-2004, 05:54 PM   #8
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Yeah, what he said.
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