How to find the right gear ratio ?

Hi all, recently I've swapped the Castle Neu 1520 for the big 1717 (thanks to Castle sale) and I wonder which is the best gear ratio to use with this kind of motor.

My savage (here the gallery) is a bit heavy and has the tranny with the Flux standard gears (not the High Speed Idler gear) and bulletproof diff gears (9 / 29 ratio).

At moment I'm using a 47 tooth spur with a 25 tooth pinion, for a final gear ratio of 13.42, but I can't figure out (before testing) if I can make the ratio lower assuming the amount of torque the 1717 can give out.

Keep in maind that the FLM Forward Tranny Chassis doesn't allow for many combination of pinion and spur due to the space I need to have within the chassis to mount 6s Lipo

What do You think ? Am I lower/high on gear ratio ?

By the way, here is the custom motor mount I've built to fit the 1717





As You can see, I have made the mount higher to have the motor shaft on the same horizontal plan of the spur axle. This allowed me to maximize the distance between the two shaft and, hence, having the motor mounted as much outer as it possible with respect to the chassis, so to have enough space for the batteries

Thanks, Andrea.

[Bmr4life]

Nice mount.

What gearing and battery did you use with the stock motor?

[JERRY2KONE]

I love the mount. Nice work. Impressive for a homemade unit. I think you are just going to have to play around with it until you find the right setup gear wise to fit your needs. Trial and error. How heavy is your truck fully loaded?? I was kind of wondering about those 1717 motors. They look much bigger than the 1520, but how much more power can they deliver compared to the Neu 1521 series??

Originally Posted by Bmr4life

Nice mount.

What gearing and battery did you use with the stock motor?

My last test was with a Medusa 36-80-2000 (2.000 rpm/volts) on 5s.

With this setup, I've used 23 or 24 tooth pinion on 52 tooth spur. No heat problems at all.

The mount is made of Anticorodal (Aluminium 6062), not so strong as Ergal, but much easier to work on.

Bye, andrea.

[nitrostarter]

Andrea- Have you mounted this in yet? My concern is the thickness of the mount takes away some surface area on the motor shaft. I hope this doesn't affect the gear mesh and being able to tighten down on the pinion set screw.

If this does become a problem, I'm sure you could machine a groove in the front to fit an Allen wrench in there like the RC-M or FLM mount does.

Originally Posted by nitrostarter

Andrea- Have you mounted this in yet? My concern is the thickness of the mount takes away some surface area on the motor shaft. I hope this doesn't affect the gear mesh and being able to tighten down on the pinion set screw.

If this does become a problem, I'm sure you could machine a groove in the front to fit an Allen wrench in there like the RC-M or FLM mount does.

Yes, I've already mounted the motor (I have a photo but not on my laptop - later this evening (italian time) I'll post it) and I have enough space on shaft/mount to create correct mesh.

Bye, Andrea.

[JThiessen]

You didn't say that you had tried this, so here is the link to BrianG's calculator http://www.scriptasylum.com/rc_speed/index.html

Originally Posted by JThiessen

You didn't say that you had tried this, so here is the link to BrianG's calculator http://www.scriptasylum.com/rc_speed/index.html

Yes, I've used the calculator, but the problem is that a calculator doesn't said which is the best possible (theorerically) value given a specific motor.

I mean, I can enter different values and get a total gear ratio and, consequently, an evaluation about top speed. But how can I find the best ratio to be used ? Is, for example, a ratio of 13 : 1 suited for a motor like the 1717 ? Is the motor so powerfull that i can, lets say, lower the ratio down to 10 : 1 ?

I don't have any specific requirements about top speed. My only concern si to be able to use the motor at its best.

Bye, Andrea

Originally Posted by andreazinno

Yes, I've already mounted the motor (I have a photo but not on my laptop - later this evening (italian time) I'll post it) and I have enough space on shaft/mount to create correct mesh.

Bye, Andrea.

Here a couple of photos



(In the second photo the motor seems to be not perfectly aligned, but it is just an effect of the camera lens distortion)

Bye.

[BrianG]

Originally Posted by andreazinno

Yes, I've used the calculator, but the problem is that a calculator doesn't said which is the best possible (theorerically) value given a specific motor.

I mean, I can enter different values and get a total gear ratio and, consequently, an evaluation about top speed. But how can I find the best ratio to be used ? Is, for example, a ratio of 13 : 1 suited for a motor like the 1717 ? Is the motor so powerfull that i can, lets say, lower the ratio down to 10 : 1 ?

I don't have any specific requirements about top speed. My only concern si to be able to use the motor at its best.

Bye, Andrea

Some people shoot for a rollout ratio of 1:1 as a starting point. That usually gives you moderate speeds (a bit on the low side usually).

You can try this: http://scriptasylum.com/rc_speed/_rollout_ratio.html

Or, just plug some numbers into the top speed calc and click the little gears icon in the results area. That will bring up a table and show the pinion needed for a 1:1 rollout in bold.

[JThiessen]

I normally end up at a rollout of about 1.8 - 2.2 with my trucks.

Originally Posted by JThiessen

I normally end up at a rollout of about 1.8 - 2.2 with my trucks.

Mmh, seems a quite high value.

At moment I have a rollout of 1.35 and, considering the motor specs, I reckon I can raise up the value up to 1.5 - 1.7

Bye, Andrea.

[JThiessen]

I had thought so too, but it seems to work so far. I started using it after I put the stock Flux set up in the calculator, and it was in the upper 1.XX range (dont remember if that was before or after the BP diffs).