Motor noise - why are some motors silent?

benjamin84

100 W
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May 20, 2010
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This may be a silly question, but why do some e-bike motors make a lot of noise, while others are perfectly silent? Are there fundamental differences between the way these motors work, or are some motors simply tuned/built/designed more quietly with casings and bearings that cause less noise?

There is a large difference between hub motors and mid drive motors for example, where something like a QS205 hub motor like I have on my Qulbix (admittedly mid mounted) is completely silent, compared to the motor in most mid-drive kits, or even the Sur-Ron, where you hear a very audible whine every time any power is applied.

I've tried to google this and apart from finding out that some controllers (sine wave vs square wave) are quieter than others then I can't really find a clear answer to this. Please enlighten an EV-noob :)
 
Its all in the rpm of the motors, a hub motor sees maxx arround 1100rpm thats why he is silent.
The middrives are much higher in rpm ,and then be geared down. The surron is arround 4000 to 6000rpm, when rpms raise the wining begins. I have a 138 qs middrive on my senda and he's compleetly silent under 1000 rpm, but the higher it goes the higher the wining. That why i choose to make a surron middrive with a qs 205 just for silence :lol:
 
benjamin84 said:
This may be a silly question, but why do some e-bike motors make a lot of noise, while others are perfectly silent? Are there fundamental differences between the way these motors work, or are some motors simply tuned/built/designed more quietly with casings and bearings that cause less noise?

There is a large difference between hub motors and mid drive motors for example, where something like a QS205 hub motor like I have on my Qulbix (admittedly mid mounted) is completely silent, compared to the motor in most mid-drive kits, or even the Sur-Ron, where you hear a very audible whine every time any power is applied.

I've tried to google this and apart from finding out that some controllers (sine wave vs square wave) are quieter than others then I can't really find a clear answer to this. Please enlighten an EV-noob :)


There are a couple things going on there.

Firstly, like you said, commutation type makes a difference. With the same motor, sine wave commutation is going to be much quieter than square wave.

Then, with the differences between mid-motors (some) and large hubs is RPM and gearing. Mid-drives generally have some type of reduction gearing to change the rpms down to bring up torque. The motors run faster as well as transmitting the drive through gears that make noise as rpms rise.
 
Also the composition and cut of the gears, if there are any, makes a huge difference in noise. Nylon gears are quieter than metal ones, helical cuts are quieter than straight cuts.
 
Thanks for the replies chaps :) I believe the gearing is probably the main contributor to the noise, and now that you mention it the noise from a Sur-Ron is pretty close to what you get from a straight cut racing gearbox under load!

Not the best video as he barely gives it any gas, but that whine is just like the whine from bikes like the Sur-Ron, Alta Redshift, or the KTM Freeride-E. And probably due to the gear reduction being noisy.

[youtube]a-k3YAjIQzI[/youtube]
 
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