# Gearing for efficiency

I want to make one of my boards as efficient as possible for my longer commutes to get as much ragen as possible. I have some long stretches without any interruption and I want the cursing speed to be around 30 kmh / 19mph. Are there some king of uniform formel to determine gearing? Which parameters are important. My guess is that it’s all a mixture of motor kv, wheel size, gearing and battery parameters.

For this specific board I run. 200kv, 150mm wheels, 10s which leaves me to the gearing to mess around with. The gearing is right now 14/42.

I want to understand this so I can mess around with efficiency on different setups in the future. Anyone got some knowledge on this?

3 Likes

Check dis

1 Like

Imo get big gearing ratio and short width gears for less resistance & the best grease

1 Like

Thanks for the link. He does some cool stuff!
The thread is mostly about his geardrive. I want to understand the relationships between the different parameters to determine what gearing would be best in different scenarios.

2 Likes

Was thinking the exact same thing☝️

1 Like

What exactly makes gear drive more efficient? How does a belt create so much more drag?

No slippage mainly

Get a belt and flex it with your hand, you have to apply a bit a force and work for it to flex, now imagine that you are continuous bending the belt, that and the friction of each teach are the main culprits, a gear doesn’t have any flexible element that requires power to bend, you still have the other loses, such as teeth friction, bearings, air resistance, viscous loss due to grease

6 Likes

So the belt tension is mainly at fault?

Tensile member bending (not tension) and contact friction. Which one contributes more, I cant say for certain, but in my mind as an engineer it would be contact friction (especially with larger belts/pulleys) but realistically the only way to know that is to run some tests. My hunch is based on the fact that slightly used belts can be flexed with virtually no force after they are worn in, but friction typically stays the same.

edit - if by “belt tension” you mean how tightly you tension the belt when you mount it, then yes that contributes a lot to how much resistance there is in your belt drive… You can test it just by tightening it way up and feeling how much harder it is to rotate. Another reason I think its mostly friction is because the bending stress the belt feels is constant regardless of how tight the belt is, but it still gets harder to rotate as you tighten it hinting that friction is the primary issue.

3 Likes

Belt tension does do a lot to consumption, riding with a single drive that requires a lot of belt tension so it doesn’t jump teeth I managed to get 10 Wh/km, the same board, but in dual motor with the belts as loose as possible, with higher top speed, does 9.1 Wh/km

5 Likes

Great real world test! Crazy to see almost 10% improvement just by loosening the belts. Let alone dual motor vs single. I’d imagine the actual improvement is ~15% considering loses from the dual drive.

3 Likes

For me, this is only about 30mph, but I’m kind of an old wuss, I guess. After that, it’s pretty much all cursing, which I assume keeps me from streeting face and increases efficiency.

3 Likes

And that is in sub optimal conditions, the high belt tension was just cruising around my house, the low is with plenty of hill and lots of hard accelerations

I keep my belt pretty loose, so loose that it will skip on hard braking. Where is the friction? The rubber in the pulley teeth?

You shouldn’t have it that loose. Either way, yea, the rubber rubbing as it “enters” or “dives” into the tooth profile, and back out which is dynamic friction. The tighter the belt tension, the greater the normal force between the surface of the belt and the pulley. Friction force is Ff=(mu)*Fn. Mu being the kinetic coeff of friction, Fn being normal force.

To answer your question; max efficiency is typically found at ~75% motor load, with pretty big losses in eff below 50% load. What RPM this equates to is dependent on a lot of things, assuming you had knowledge of what your peak load rating is of the motor (aka manufacturer spec max current/power) you would be able to calculate what power is ideal for acceleration to reach near that 75% load.

Obviously, beyond that, you want the greatest gear reduction you can get that gets you near the 75% load rating under acceleration. This is because a higher reduction requires less torque and less power from the motor at cruising speed. Just don’t go too high on the erpm, and you should be good.

edit - here is a document with some info: https://www.energy.gov/sites/prod/files/2014/04/f15/10097517.pdf

1 Like

Something I haven’t seen others mention is tyre choice and size can make a big difference.

I recently switched from a 6.5" tyre to 8" and have seen around 15-20% gain in range. I run both at the same psi and both are street tires. I can’t be sure if the gain is from having larger wheels that go over bumps easier or if it’s something to do with changing the motor rpm at cruising speed but the difference is significant.

5 Likes

@SeanHacker

4 Likes