I recently had an interesting discussion with Hangdog98 via PM on the subject of rear suspension deigns. He is a pro in this field and has designed bike suspensions for major manufacturers. I thought I would share what he told me on this thread :
« Hi fractal. MTB suspension is often shrouded in myth and mystery and the best designs all have one thing in common. That is, to isolate pedalling forces from the suspension forces. One design did it in the 80's and has been protected by a rock solid patent which is licensed to Specialized. They call it the FSR Link and it was patented by a man in Laguna Beach called Horst Leitner who created AMP Research and it is also known as the "Horst Link". Pretty much every other design seeks to emulate the Horst link or find a way to achieve the same result and that is, to stop the suspension bobbing as you pedal and stop the rear brake from grabbing as you go over bumps and to allow the wheel to follow the ground without being affected by those other forces.
There are two main things to know about swingarms.
1. The path of the rear axel.
2. The position of the pivot.
Before I discuss those, it is important to note that motorcycles don't suffer from pedalling induced bobbing and you'll notice that their swingarm design is pretty much standard across each discipline of motorcycle design. If your desire is to have an E-Bike that behaves well under power, then the design process is much easier.
I should also dispell a myth about swingarms. Having the pivot close to, or even around the BB shell doesn't isolate the swingarm from pedal forces. The pedal forces come from the chain pulling a line across the top of the chainring to the rear sprocket. A line drawn from where the chain joins the chainring (at the top) to the pivot point center, is the lever that acts on the swingarm. This leverage changes with each gear change. If you run multiple fron chainrings then the leverage over swingarm action changes even more. The most effective pivot position to reduce pedalling forces is to have the pivot point behind the chain at the top of the front sprocket.
If you imagine the swingarm as a radius, the path of the rear axel is part of a circle. The longer the swingarm, the easier the curve (arc). The ideal situation is to have a long swingarm and as straight an axel path as you can manage. The vertical axel path should be vertical when the bike is on level ground and not pointing down a hill as some designers have tried (and failed). The purpose of this is to stop the wheelbase changes as the swingarm moves through its arc. On downhill bikes there was a school of thought that wanted to lengthen the wheelbase under compression, and/or to make the axel follow a vertical path (relative to level ground) when the bike was pointing downhill. This meant moving the pivot up quite high which necessitated the use of a jack-shaft to bring the chain up to the pivot point. They weren't very successful. Balfa is an example. They were supple in a straight line but weird and wobbly around the corners because the wheelbase kept changing which affects the steering.
The 2nd thing about the axel path and wheelbase changes is that when the wheelbase changes, the wheel must either speed up or slow down to keep up with the changes. If it speeds up because the wheelbase has shortened then the tire has greater traction demands placed on it. This causes it to break traction. If you are also applying the brake, the brake rotor changes speed with every bump and causes brake chatter (hopping and skidding). This is why I advocate a floating brake caliper which seperates the brake action from suspension action. (MadRhino has never heard of this and confused something like a Hope floating disc as being a floating brake system) I have attached a picture to illustrate the floating brake. The idea is that the brake caliper is not fixed to the swingarm, but fixed to a bracket that is free to rotate around the axel. It is then secured by a torque arm to a position on the frame to create a parrallelogram action. As the swingarm moves through an arc, the brake caliper stays constant with the rotor and matches the wheel speed, thus isolating the braking from the suspension. 99% of performance motorcycles have this.
Here is a pic of an excellent floating brake on a long travel e-bike
This is an excellent set-up for motive power adapted to an existing swingarm. If you had the freedom to fabricate the swingarm you would simplify the shock linkage system. The four bar system employed by Kona in this bike is nothing more than a designers attempt to make it look like a Horst link without infringing patents. The links and bars and shock linkages do nothing special nor isolate the pedalling action, but they're fine for motive power.
Now on the subject of frame mounted engines and hub motors. Because both legs and frame mounted engines apply torque along the chain which affects the forces on the swingarm, they are very very different from the forces exerted by a hub motor. The hub motor will drive forward and cause the frame to pitch upwards. This is a desireable trait as the end result is to improve traction by forcing the tire into the ground, and during cornering under power, will steepen the head angle and help keep the front tire planted. Tyring to isolate this action would be a mistake.
Finally, the weight of the swingarm, wheel brake and shock linkages is all unsprung weight. The difference between a cro-mo home-made swingarm with a heavy DD hub motor, 12g spokes, fat rim and heavy duty tire/tube, derailleur, linkages etc etc will probably weigh much more than a modern 125cc motorcross bike rear end anyway.
An e-bike with a powerful motor is a motorbike. Try to be inspired by how the motocycle guys do it rather than the pedal pushers. There's less BS in motorcycle suspension designs because they're designed by engineers and not graphic artists.
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«The spring calculation method is loosely based on the weight on that wheel (front or rear including rider, use bathroom scales under the wheel). Divide axel travel by shock shaft travel and multiply that by the reading on the rear scales. The shock will need to be attached to the swingarm for this measurement. This will give you the ball park figure. Test ride with an O ring or cable tie tight around the shock shaft to see how much travel you're using and go up or down from there. You want to use about 80% in normal off road riding saving the last 20% for unplanned big hits. Remember, despite what many people think, winding up the pre-load adjuster doesn't change the spring rate, only a spring change will do that.
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