Mechanism to drive a wheel from an internal motor allowing suspension?

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  • Thread starter some bloke
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  • #1
some bloke
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TL;DR Summary
a central motor, inside the wheel, fixed to the frame. the wheel goes around the outside, but can also move up and down for suspension purposes. how best to transfer the drive?
I'm brainstorming how to make an electric motorbike, making as much use of the available space as possible to allow for as many batteries to be fitted as possible.

Batteries aren't relevant right now; my focus is on the final drive at the back of the bike.

To maximise space, I want to fit the motor inside the rear wheel - this seemed the sensible place for it, it's already round and it's where the drive needs to go. it also means one less chain / transmission to worry about, and can help make the overall look of the bike a lot cleaner.

My initial thought was simply to have the motor in the wheel, on a swingarm, but I have since learned the values of minimising unsprung weight, and the issues with having such a gyroscope on a bike - if the motor spins as fast as the wheel, it will make handling suck.

So my next step is to house the motor on the back of the frame, where the wheel will be mounted. It would then have sliders, with the suspension, mounted around it, and the wheel mounted around that.

The result would be that (taking the frame as stationary), the motor will be stationary, and the wheel will slide up and down on the suspension, with its centre of rotation passing through the motors axis of rotation at the midpoint of the suspension travel. The dimensions of the motor and wheel will be the limiting factors in the distance of the travel of the suspension, but that is another hurdle for later on.

How best can I transfer the drive from the motor to the wheel? essentially, I need a motor with the drive coming out on one side of it, which can move along a slot across its own diameter, without changing pace as it does so - suspension movement should not impact drive speed.

My initial thought was a differential, with one side connected to a rack & pinion (forming a non-turning side) and the other side connected to a vertical worm gear. The worm gear provided the drive, and if the suspension moved, both sides of the differential would roll up their respective tracks - one up the rack, the other up the worm gear - and the difference between the 2 would be nil, meaning it would not change speeds as it moved. I think that this system may be unnecessarily complicated, though.

If I can I will scan in a drawing of this to try and make it clearer. I'm sure there must be a simpler method to achieve what I'm looking for, with less moving parts to potentially fail!

Cheers,
 
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Answers and Replies

  • #2
berkeman
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Summary: a central motor, inside the wheel, fixed to the frame. the wheel goes around the outside, but can also move up and down for suspension purposes. how best to transfer the drive?

My initial thought was simply to have the motor in the wheel, on a swingarm, but I have since learned the values of minimising unsprung weight, and the issues with having such a gyroscope on a bike - if the motor spins as fast as the wheel, it will make handling suck.
I'm glad that you figured out how important unsprung weight is. :smile:

The two common drives for motorized cycles are chain drive and shaft drive. It's hard to think of a better alternative to those. Is coming up with an alternative drive mechanism (3rd best choice) a requirement for this project?
 
  • #3
some bloke
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I'm glad that you figured out how important unsprung weight is. :smile:

The two common drives for motorized cycles are chain drive and shaft drive. It's hard to think of a better alternative to those. Is coming up with an alternative drive mechanism (3rd best choice) a requirement for this project?

I don't want to have the motor in the conventional position - essentially I want to mount it inside the wheel, or possibly as a longer, narrower motor down the frame, leading to a 90° bend (essentially turning the shaft-drive into a motor). If I can move the motor to the "unused" space in the hub of the rear wheel, it will allow more space for batteries in the bulk of the motorbike, allowing a greater range.
 
  • #4
Mech_Engineer
Science Advisor
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I think your two main options are:
  1. Utilize an electric hub motor, which can be mounted in the wheel and still move with any suspension the wheel is mounted to. See here: https://en.wikipedia.org/wiki/Wheel_hub_motor#Mechanism
  2. Use a more "conventional" motor mounted centrally on the frame, and then drive the wheel using a chain or belt drive. See here: https://en.wikipedia.org/wiki/Electric_motorcycles_and_scooters
It sounds like based on your answers you would tend to want to lean towards the hub motor option.
 
  • #5
essenmein
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I don't want to have the motor in the conventional position - essentially I want to mount it inside the wheel, or possibly as a longer, narrower motor down the frame, leading to a 90° bend (essentially turning the shaft-drive into a motor). If I can move the motor to the "unused" space in the hub of the rear wheel, it will allow more space for batteries in the bulk of the motorbike, allowing a greater range.

Problem is if its in or near the wheel, its going to have to move with the wheel, ie increasing un-sprung weight.

Now its not actually the "weight" that matters but the moment, the swing arm is turning around a pivot, so having mass A at the end of the swing arm is not the same as having that same mass half way between the wheel axle and the pivot. So you can reduce the moment by moving any of the weight closer to the pivot of rotation for the swing arm.
 
  • #6
essenmein
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Also from an electric machine perspective, torque production is based on its shape, low speed high torque motors are more pancake like (eg hub motors), ie large diameter rotor with less depth, vs high speed machines (eg spindle motors) are small diameter rotor and axially long. Basically a motor that looks like something that would fit where a driveshaft is located would fit into the long and skinny high speed machine category and would not produce the type of torque speed curve needed for what would basically be nearly direct drive.
 
  • #7
berkeman
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If I can move the motor to the "unused" space in the hub of the rear wheel, it will allow more space for batteries in the bulk of the motorbike, allowing a greater range.
Have you tried sketching a chain drive from a fixed internal motor to the suspended wheel's interior circumference? The challenge is to keep the chain length constant as the wheel moves up and down with respect to the fixed motor axle. I sketched a couple possibilities with idler sprockets in different places, but didn't work on it enough to convince myself one way or the other whether it is possible...
 
  • #8
some bloke
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89
Thankyou for all the replies!

The original idea was to use a hub-motor, with the motor moving with the wheel on the suspension, but I am now trying to invent a method to combine the convenience & space-saving of the hub-wheel with the added benefits of having said hub motor move with the frame, and not the wheel.

The theory is that if you have a wheel with a hollow centre to accommodate the motor, diameter D1, and the motor is smaller than this, diameter D2, then the maximum extent of travel of the suspension would be D1-D2, or the gap around the motor when it's inside the wheel.

Thankyou for the information about motors - I haven't started researching the details of which motor to use yet, but I know then that a long, thin motor will require a significant gear reduction to deliver the needed torque. it's good to know.

I hadn't considered a chain to the outer circumference of the wheel. I am now considering having a swingarm and chain mounted within the wheel, with 90° of travel (centred about horizontal) to allow the motor to remain central to the pivot. essentially as if you took the conventional swingarm and shortened it until it was housed inside the wheel, then increased the angle of movement to compensate for the reduced travel.

The only issue with this is it altering the wheelbase of the bike as it moves on it's suspension. I don't know if this will affect handling in any way - I know that the main reason the swingarm is used as it is is to maintain the length of the chain, which would still be the case.

There may be alternative methods available which I haven't though of yet - I thought of a clutch plate, but realized it would wear extremely quickly.
 
  • #9
some bloke
263
89
I've done some more research and my eyes lighted upon the Reuleaux Coupling:
https://en.wikipedia.org/wiki/File:Luch2_greifer.gif
using 90° of rotation and a mechanism to maintain the box as parallel, I have a system to create linear motion without using sliders (which I am always concerned will jam). This is a bit of a side-note to the issue but may yield some answers.

I have noted that for the duration of the movement, the centre line (from the corner of the pivot of the reuleaux triangle) always intersects the central point of the box. As such, if I can put the drive along this line, perhaps as a long cog, the drive can be transferred from the fixed pivot in the corner of the triangle to the moving section at the centre of the box.

I will mock up a design in CAD today, if I get the chance, and put a picture up to try and illustrate my ideas.
 

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