I have a 9 speed Sensah drivetrain coming, and considering replacing the chainring. I can't find anyone talking about an oval chainring, so I'm guessing it's not a good idea. I just wanted to confirm. It's going to be for a Bafang Ultra. The Sensah forces me to stick with 40t to be ideal.
Is an oval chainring a no-no on an ebike?
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AHicks
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I don't think I'd try that on a mid drive (especially an Ultra!!). More inclined if we were talking a rear hub.
Pretty sure the oblong shape is there to enhance when "people power" is in use.....
Difference being how the motor delivers it's power to the rear wheels. Mid drives supply power through the chain ring, where there would be a definite downside due to the surging action caused by the shape of the chain ring. Especially when considering an Ultra (with it's torque monster personality), pretty sure it would make you nuts, as well as being hard on an already over stressed drive train.
Pretty sure the oblong shape is there to enhance when "people power" is in use.....
Difference being how the motor delivers it's power to the rear wheels. Mid drives supply power through the chain ring, where there would be a definite downside due to the surging action caused by the shape of the chain ring. Especially when considering an Ultra (with it's torque monster personality), pretty sure it would make you nuts, as well as being hard on an already over stressed drive train.
Stefan Mikes
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The oval chainring is counter-productive on a mid-drive motor e-bike, as the torque sensing requires uniform torque loading. The oval ring provides variable torque to the motor.
harryS
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Elliptical cranks were designed to be used with pedals to maximize the leverage of your legs. I have one on my 1992 Trek that I converted with a hubmotor. I first thought the crank was warped when I glanced down while pedaling, but it was designed that way.
Will be counterproductice for a motor. Edit: middrive motor,
Will be counterproductice for a motor. Edit: middrive motor,
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AHicks
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New members are allowed silly questions. Actually ALL of us can come up with them on occasion. Far more important is to feel welcome to participate -
tomjasz
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Man if you knew the questions I asked in 2014 you'd feel like a genius! It's how we learn.
antboy
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Is that right though, on a torque sensing mid-drive?
In the "silly questions" department, if an elliptical chain minimizes the dead zone by smoothing out the torque delivery to the rear wheel, wouldn't it also do the same for motor output (less spikey, especially on a climb)?
AHicks
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Motor has the same output for 360 degrees. There's no big cycle of power delivery like when YOU are delivering the power.
Stefan Mikes
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Just think about it. The variable chainring diameter creates a torque wave at the crank axis (torque depends on the ring diameter, which is variable here). This really confuses the mid-drive motor's torque sensor and controller. No ill effects for the hub-drive motor though, as the hub motor is independent of the drivetrain.
AHicks
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Sure, no issue there...
antboy
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I'll admit, I had to scratch my head at some of the answers, as they didn't quite make sense to me. 
After looking at a couple of emtb forum asking the same question, the big issue would be that the chainring moves independent of the ring (the chainring doesn't move when pedaling backwards), so it's basically impossible to keep the arms aligned with the proper point of the oval.
After looking at a couple of emtb forum asking the same question, the big issue would be that the chainring moves independent of the ring (the chainring doesn't move when pedaling backwards), so it's basically impossible to keep the arms aligned with the proper point of the oval.
AHicks
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Hadn't considered that, but that's another reason it would be a bad plan!I'll admit, I had to scratch my head at some of the answers, as they didn't quite make sense to me.
After looking at a couple of emtb forum asking the same question, the big issue would be that the chainring moves independent of the ring (the chainring doesn't move when pedaling backwards), so it's basically impossible to keep the arms aligned with the proper point of the oval.
kmccune
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Hard to reinvent the wheel.It's rather interesting to see how old tech can hang on( I give you hydraulic rim brakes) for instance.
Alaskan
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With a mid drive bike the chainring is not directly connected to the cranks but rather freewheels in the backward direction. As such, the position of the bulge in an oval chain ring shifts in relation to the cranks. For an oval chain ring to be effective it has to remain in a fixed rotational position relative to the crank arms, such that the widest diameter matches the most powerful downstroke position of the crank arms.
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Stefan Mikes
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So obvious answer, Richard. I have even not thought about it! Thank you!
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