HPI 39T High Speed Idler Gear... Increase of up to 20mph!?

[Bmr4life]

Originally Posted by Finnster

An idler gear is a gear that does not offer gear reduction. Its like a spacer between two transfer gears. 20T or 40T, no change in gear ratio. Same concept as a chain between sprokets on a bike. Longer or shorter chain, no diff.

However, this idler allows you to use a different set of transfer gears, which have a lower gear ratio. That is what drops the final drive ratio and allows for more top end. Same effect as if you put a larger pinion on the motor.


This was released bc the gearing was already a bit funny in the flux. Std pinion was 20T. Not a huge room up for bigger pinions, and spur can't get much smaller than the 44T on there now. Just adds flexibility, plus its a stronger material than the other idler, so a more bulletproof. Nothing magical otherwise.

Just wanted to add.

I have the high speed "idler" and it does make a difference. It replaces a 44t gear with a 39t gear. The 44t gear used to ride on the 18T part of the 18/23 drive gear. The 39t gear rides on the 23t part of the 18/23 drive gear.

This effectively changes the transmission ratio from 44/18 x 29/32 = 2.22 to 39/23 x 29/32 = 1.54

That explains how my flux can hit 50mph on 4s with the stock spur and pinion on truggy racing tires.

Differential Ratio: 3.3076923076923075
Transmission Ratio: 1.5366
Other Ratio: 1
Spur Tooth Count: 49
Pinion Tooth Count: 20
Total Voltage: 14.8
Motor KV: 2200
Tire Diameter (inches): 5.65
Tire Ballooning (inches): .5
Motor Current Draw: 0
Motor Coil Resistance: 0.006
Spur/Pinion Ratio: 2.45 : 1
Total Ratio: 12.45237 : 1
Tire Circumference (inches): 20.89 inches (530.65 mm)
Rollout: 1.68:1
Total Motor Speed: 32560 RPM
Vehicle Speed: 51.73 mph (83.1 km/h)
Effective KV Value: 2200
KT constant: 0.61 oz-in/A
Motor Torque: Amperage not specified...
Final Torque: Amperage not specified...
Final Power: Amperage not specified...