BBSHD efficiency + RPM

moped-dan

10 mW
Joined
Apr 12, 2022
Messages
20
I have some questions about the BBSHD, particularly when I should be shifting for best efficiency and least stress on the motor.

For example I have it geared so it will hit right under 20mph in the lowest gear, and as I wind it up the watts drawn slowly drop as I approach max speed. Typically I will shift up just as the watts starts to drop.

But say I keep the throttle maxxed out even after the watts drops down (meaning max rpm I believe?) am I stressing the motor too much? I have read these motors like to spin fast, but how fast exactly? And should I always upshift once the watts begin to fall on full throttle?

Will I hurt it or negatively effect reliability by continuing to ride at full throttle even if the watts drawn have fallen way down (ie I have it programmed to a max of 1000w but at 20ish mph or whatever in the lowest gear, that will fall down to 2-300ish watts).

Cheers. I love this motor and would like to keep it running for as long as I can.
 
if youre just trying to make the motor last I dont think you have to worry about high rpms or high torque or anything else and just keep the motor from getting too hot. motors are pretty durable, as long as the bearings are eventually replaced, and they arent damaging the bearing seats, and there's nothing vibrating or causing friction to short the phases or the windings heat destroying the magnets or eventually breaking down the magnet wire insulation after many many hours is all that's left.

less watts for any given speed is more efficient so shift on.
 
ebuilder said:
My favorite motor in the industry is the BBSHD.
To better answer your questions, how about some background on your bike, how much you weigh etc?

What gear ratios do you run? Cassette? Front chainring size?

Do you run a gear shift sensor?

Do you run a brake/motor cut off sensor?

What type of bike? What tire size?

When you are out riding, how much do you pedal versus how much do you use the throttle?

Do you ride mostly on flat land or up hills? How big are the hills?

What is your average speed when out riding? What is the top speed of your bike and what size battery do you have?

I can fill in the blanks a bit if you provide a bit more information.

The bike is a pretty run of the mill hardtail mountain bike. Luna 42t sprocket on front and 11-32t cassette out back. 29er with mtb tires.

I typically pedal quite a bit (i only have an 8ah battery) but my question is about times I use only the throttle since my commute involves a few busy intersections where i use only throttle and I cant keep up with the cadence of the bbshd.

Am I damaging anything holding the lower gear at full throttle, ie when the bbshd is at what I assume maximum rpm?
 
I thought was asking about the motor not the gears. More torque is going to be tougher on the gears so spinning the gears as opposed to doing higher torque is going to be easier on them and also. higher efficiency from the motor.
 
The motor operates more efficiently at higher rpm and lower torque. I keep mine in the lowest gear that will give the speed I want. Keeping the motor in too high of a gear increases the torque/load on the gears and causes the motor to heat up more. It's actually best when the chain ring is spinning faster than you can pedal, but there is no human input at that speed.
 
Yeah I should have been more clear, curious about the motor itself not my drivetrain.

I keep hearing higher rpm is better and electric motors like to spin, does that literally mean holding it at full throttle is ok for some amount of time? In a low enough gear where it’s limited by the rpm of the motor.
 
U could run the grin motor simulator. At some high speed the motor will be more inefficient due to the drivetrain losses and the switching losses in the motor.
https://ebikes.ca/tools/simulator.html

I don’t think the simulator accounts for drivetrain losses though.

Put ur bike upside down and spin it as fast as possible and look at ur wattage used. That’ll tell the losses to both the drivetrain and switching at that speed. Gear it down (up?) and compare. The more gearing and speed of the motor the better… until it isn’t.
 
moped-dan said:
I keep hearing higher rpm is better and electric motors like to spin, does that literally mean holding it at full throttle is ok for some amount of time? In a low enough gear where it’s limited by the rpm of the motor.

Right. The idea is to keep the motor near the peak of the efficiency curve, which is when it's lightly loaded. You can run it at full throttle until the battery runs out. At higher loads when the rpm slows down is when you can get destructive heating. The plastic gear is typically the first thing to fail.
 
Thanks all for the info. I will definitely be playing with that grin simulator. I always wind it out in lowest gears if im using only throttle.

fechter said:
You can run it at full throttle until the battery runs out. At higher loads when the rpm slows down is when you can get destructive heating. The plastic gear is typically the first thing to fail.

Ok that is good to know. I also see that now in the simulator where it gives you the time until it overheats, depending on the stress on the motor. I don't think once Ive gotten the motor more than warm to the touch.

ebuilder said:
PS. your small amp-hr battery means you likely only have 2 or 3 battery cells in parallel. This limits amperage at full power and is also harder on the cells within your battery.

Good point - my battery is 14s2p of the Samsung 40t. I also limited the bbshd to 20amps to help compensate for this even though I had no real issues at the stock 30a setting.
 
The OP brings up something I've thought about many times as my BBHSD hits 35+ mph. What issues does it face at high speeds? At the extreme end of things - Can you take the chain off a BBHSD and let it spin at 100% throttle (52v) with no issues or does it self-destruct? If running 40+ mph is centrifugal force of the chain around the freewheel/derailleur an issue? It's rare I hit these speeds but it does make my wonder.....?
 
With the chain off, you can run it full throttle with no issues. If the bike isn't moving, the motor will eventually overheat due to lack of air flow, but imagine running full throttle going downhill you could do that as long as you want.
 
Diggs said:
If running 40+ mph is centrifugal force of the chain around the freewheel/derailleur an issue? It's rare I hit these speeds but it does make my wonder.....?

I’m curious how the drivetrain efficiency is effected. Easily answered spinning it upside down and comparing current drawn at high speed with chain on vs off. If it’s really inefficient at high speed maybe all the lost energy is going towards destroying the rear derailleur
 
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