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

Hi everyone....

I been mentioning the notion of a high speed idler gear that was released by HPI...
Now when it comes to gearing i am a little ignorant and need some help...

People on here have been saying that an idler gear doesnt do anything hence "idle" but in a Savage Flux single speed tranny, it def has more of a role....

Gearing for a savage flux 44ts/25tp nets you around 63mph top speed

Proof... Plus i know his gearing as we have been chatting
Savage at 62mph... 44/25
http://www.youtube.com/watch?v=JuTMW8UPqlE

First run using 44/25 PLUS hpi 39t high speed idler gear 66mph
http://www.youtube.com/watch?v=FxdmPlT3C-g

4th Run with the same gearing... He has now wrapped flishing line around his tires to stop ballooning... Speeds reached : 74.3mph.
http://www.youtube.com/watch?v=eG_KQWR4bHY

Most recent 5th run... Hasnt changed anything since the last run... And has managed to his 79.3mph!!!!
http://www.youtube.com/watch?v=Q8pydXrZqng


I think this is really impressive... Could someone explain how this idler gear works? because it is obvious its not what people thought. People say its like having a 2 speed tranny, and instead of the flux driving in the first gear it is now stuck in 2nd gear... Yes it takes a little of your bottom end away, but considering the torque some of these motors have, i doubt that would make much of a difference anyway... If anything it would make these trucks more drivable at low speeds with less wheelies.

Anyone care to chime in... Here are the facts... need an explanation.

THANKS

I dont have time right now to look at those vids, but I have some reservations about it. An idler gear is just a gear that sits in between two other gears. In a tranny, its basically "locked" or sandwiched in between the input shaft and the output shaft, thus you can't change the tooth count on it (that would require one of the other shafts to move since a change in tooth count equates to a change in diameter).

The savage tranny has 4 gears, the 2 center are both connected to the "idler" shaft. The gear is not an idler, it is a transfer gear, IMO. Basically this puts the flux in second gear, compared to stock where it is in 1st gear (ratios from the nitro 2 speed tranny).

My only issue is that something has to change on the gear this near gear meshes with, as the 39t gear is different diameter than the gear it replaces, and the shaft location does not change. The input gear needs to be changed as well. Just an observation on my part.

Thanks will look into it linc...!

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.

Thanks for that finnster... i get it completely now... I also saw a tutorial of how its fitted to the sav... Think after installing this i will be able to gear my pinion down a little

I was going to put mine in this weekend...Thinking the 18 tooth pinion should level it off nicely...

I'll just monitor temps with tires/ surface I'm running that day and report back..

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...

It also puts additional strain on the entire motor system as well

No doubt, its like running a larger pinion.