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Old 04-12-2012, 10:26 AM   #1
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Default Sizes of Spur Gears

I have an ECX Torment 1/10 SCT, which came with 48 Pitch 87T Spur Gear. I want to go to a Spur Gear with a larger tooth count...I bought a Losi 48 Pitch, 90T Spur Gear after being advised that it was "very close" to correct size...and found that "very close" resulted in the diameter being just enough larger that I am unable to replace the gear cover.

Does the tooth count of a Spur Gear determine it's diameter? Would two 48 Pitch, 90T Spur Gears from two different manufacturers have the same diameter?

If the Tooth Count determines the diameter, does that mean the gear cover will limit the tooth count? I went up 3 teeth, kept Pitch the same, and it was too big for the cover.

I am brand new to all of this R/C stuff, so I am probably overlooking something really simple that others who've been doing this for awhile consider "common knowledge".

Thank you.
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Old 04-12-2012, 01:36 PM   #2
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In order to maintain the same pitch, but have more teeth, the diameter must increase. Generally you'll want to replace the pinion instead of the spur gear if you're trying to adjust acceleration vs top speed/motor temp.
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Old 04-12-2012, 03:01 PM   #3
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Quote:
Originally Posted by justpoet View Post
In order to maintain the same pitch, but have more teeth, the diameter must increase. Generally you'll want to replace the pinion instead of the spur gear if you're trying to adjust acceleration vs top speed/motor temp.
Poet, dont mean to sound like a smarty pants, but it's not entirely true that diameter increases with number of teeth.

On 2 speed nitro, they use MODULE and this adjusts the number of teeth without changing the diameter. In EP gearing the diameters can change so they arent module.

Try a combination of pinion and spur change to compensate. If you need to go lower than permitted, might see if there's an option to change the trans pulleys
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Old 04-12-2012, 05:42 PM   #4
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Quote:
Originally Posted by justpoet View Post
In order to maintain the same pitch, but have more teeth, the diameter must increase. Generally you'll want to replace the pinion instead of the spur gear if you're trying to adjust acceleration vs top speed/motor temp.
Good answer for the question asked.

BandS, to add, assuming you have the stock 20 tooth pinion, going to 19 tooth 48 Pitch pinion will achieve almost the same gearing change as switching the spur from 87T to 90T.

87/20 = 4.35
90/20 = 4.50
87/19 ~ 4.58

Most any brand of 48 pitch pinion should work, just avoid any that say metric 48P. (metric 48P is really module0.6, and won't work with your spur gear)
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Old 04-13-2012, 11:24 AM   #5
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Thank you all, for the replies and information. I understand it a bit more now...I'd mistakenly thought that the increased number of teeth would just be squeezed into the same diameter Spur Gear, until I rcvd. the 90T Spur Gear and realized I haven't a clue, LOL.

I'd increased the tooth count on the Spur Gear because someone advised that the higher the tooth count on the Spur Gear, the easier it would be on the motor; so if I increased the Spur Gear and the Pinion Gear (to keep the ratio about the same) it would have approx. the same speed and acceleration, but be easier on the motor...but the part about the diameter of the Spur Gear increasing was omitted.

Thank you again for the informative replies.
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Old 04-13-2012, 02:18 PM   #6
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I think someone is wrong. Given the same gear ratio I don’t think small size changes are going to make much difference, unless it’s to avoid really small tooth counts, say below 12~14 or so.

I guess if one were really looking for a difference, there would be a slightly lower reaction force on the bearings of the motor and tranny topshaft, due to the longer lever arms. But I routinely use small gears to keep the motor up by the tranny, and have no bearing issues despite minimal maintenance in that area.

Last edited by Dave H; 04-13-2012 at 02:48 PM. Reason: kant rite
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