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Swingarm mounted Hub Motor

7956 Views 28 Replies 11 Participants Last post by  Functional Artist
I would like all of you engineers and designers to comment on this design of bike. Tell me what you think the ridability would be like considering the hub motor is mounted on the swingarm.

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Hi Ripperton
My tuppence worth
The motor is very close to the pivot point so the effect of it's mass on the suspension is minimised

Also because the motor is on the swing arm throttle/suspension effects are minimised - both the effects of the motor torque on the suspension and the effects of bumps on wheel torque

Overall I would think it would be pretty good
 

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You might find the swing arm is too flexible, twisting wise. The large diameter, short piece of round tubing near the forward middle of the swing arm may be an undesirable flex point. Replace it with a triangulating tube of similar cross-section of the rest of the swing arm tubes and increase the size of the shock mount cross tube to increase the overall torsional stiffness. This is of course only necessary if the swing arm is too flexible.

Asymmetrical forces from the pull of the drive chain may also twist the swing arm excessively in its current design.
 

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Discussion Starter · #10 · (Edited)
I don't know Rip. May be a weak upper shock mount pushing into the batteries? Too much anti-squat built-in with the downward angle of the swing arm?
Half right.
The bike has chronic "jacking" or antisquat under acceleration but not because of the suspension geometry.
Its because the motor is mounted to the swingarm.
The torque reaction in the hub motor axle feeds into the swingarm and effectively turns it so the suspension extends almost to top of stroke.
If the motor turns forwards the axle and swingarm turn backwards.
You also see the forks jacking but that is due to weight shift.
When the throttle is chopped the whole bike squats back down, front and rear.
I recently made a rear end for a trike with a Kelly motor mounted to the swingarm and it did the same thing.
Watch the KL9 in this video at 32s for an example of torque reaction.
https://www.youtube.com/watch?v=esHDaxz06N4
 

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So, if the SA were angled up so that the rear axle was above the level of the pivot point, could tire thrust be used to counteract the jacking effect? Although, this would be bad for ground clearance as the bike is currently configured.
 

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Discussion Starter · #12 ·
Although, this would be bad for ground clearance .
Yes.
You cant fix this. The motor has to be moved to the frame.
BUT...that all depends on who is riding the bike.
You might have someone who likes Vodka and doesnt care about the finer aspects of suspension performance and they wouldnt even notice the rear end pogoing up and down.
But then you might have a pro downhill racer and he would get off the bike in 2 seconds saying wtf is wrong with this thing...unridable.
I would ride the bike only to find out how unridable or annoying it is ie try to quantify the problem.
 

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I assume that the shaft comment was intended to be humourous, but sure... why wouldn't it work just as well (and be just as complex) as with a gas engine?

It doesn't matter much how the motor is mounted - the orientation is important to the effect of motor inertia, but makes no difference to drive torque reaction (something still turns that shaft). Where it is mounted does matter, to how the torque reaction is transmitted to the bike (how it affects the suspension).

Harley-Davidson's Project LiveWire bike mounts the motor longitudinally, but then turns the drive 90 degrees for a belt drive to the wheel... it is a Harley, after all. Seems like the worst possible technical design.... it is a Harley, after all. ;)
 

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I'm thinking with an electric motor, with the max torque of the motor available almost instantly, the counter-torque reaction through the frame or swing arm can also be almost instantaneous. This is the reaction the rider has to compensate for to keep the bike from tipping over in the case of a shaft drive or pogo sticking in the case of a swing arm mounted motor. With a ICE, the rider has more time (say a second or 2 or more) to compensate as the clutch is slipped and the RPMs and torque build up.
 

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Hi Rip
Re the torque reaction - yes you will get a torque reaction with the motor on the swingarm
BUT you will also get a torque reaction with the motor on the frame

That bike looks as if the rear swing arm is about half the length of the whole bike - tire contact point to tire contact point

So with the motor on the swing arm you will get about twice the effect of having it on the motor
As the effect is reversed you will get about the same amount of movement - but in the opposite sense!

So basically much the same movement

The advantage is that wheel movement (from bumps) will not put any torque into the drivetrain
 

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The motor torque will not transfer to the frame except from the wheel which Is always the case no matter where the motor is mounted. Having it on the swing arm eliminates need for chain tension issues. If the motor is mounted too far out on swing arm it can cause issues with too much mass on the end of the fulcrum especially when off roading which is a common problem with hub motors. But by the looks of it I see no relation, as long as you are OK with your motor swinging up and down with your tire and not your frame
 

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The motor torque will not transfer to the frame except from the wheel which Is always the case no matter where the motor is mounted. Having it on the swing arm eliminates need for chain tension issues. If the motor is mounted too far out on swing arm it can cause issues with too much mass on the end of the fulcrum especially when off roading which is a common problem with hub motors. But by the looks of it I see no relation, as long as you are OK with your motor swinging up and down with your tire and not your frame
Nope the motor torque is ALWAYS reacted out through the frame ALWAYS - Physics 101
 

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The wheel torque is reacted to by the frame through the swingarm. Physics 101.

The motor is invisible to the frame if it's centered on the swingarm pivot. All forces are purely between the motor and wheel and the reaction to the motor torque is by the swingarm lever acting on the axle pin. Not the swingarm pivot. The torque of the motor is translated to the wheel. You coupd argue that the motor is pulling on the chain which lifts the frame at the swingarm pivot, but that is defined as wheel torque.

So, he was correct.
 
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