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• Quick question for the suspension geeks.
• IanMunro
Member

Does anyone know if there has ever been any examples of high unsprung mass issues being overcome by way of counterbalancing?

I could be wrong, but I think the emphasis in ‘unsprung mass’ is on the mass part, as opposed to thinking of the mass as weight. So If you had a counter mass you now have two masses to accelerate as opposed to one. In counter directions so one has gravity on it’s side, but only up to accelerations of 9.8ms^2. You also then have to accelerate them back in the opposite direction to get to your start position.

jackthedog
Member

Makes sense so far.

So, ridiculous hypothetical scenario; suspension e-bike where all other factors remain same, heavy motor directly in rear hub vs heavy motor on opposite end of see-saw swing arm (belt driving standard hub) that conveniently perfectly counterbalances the weight of the rear wheel. Which is better?

IanMunro
Member

Assuming that that the swing arm extension was massless then, I’d say the other end.
However I think it would make much more sense to mount it at the pivot point and just take it out of the equation.

Wibble89
Member

that conveniently perfectly counterbalances the weight of the rear wheel. Which is better?

I think its not just the counter balance that need to be looked at. the shorter the counter balance lever the closer the mass will become to unsprung, the longer the lever the worse the unsprung effect will be. Reasoning although moments wxl=w1xl1 cancel out, but the accelerations may have something to do with v^2

jackthedog
Member

Decided to put this in Chat as it’s not really a bike specific question.

Unsprung mass. General rule, high unsprung mass is undesirable as we know.

Does anyone know if there has ever been any examples of high unsprung mass issues being overcome by way of counterbalancing?

Do any scooters work like this perhaps (swing arm mounted engines is what triggered my curiosity)

Heavy motor in hub. Don’t think of unsprung masses as weights which need to be lifted when you hit a bump but as masses with inertia which need to be accelerated when you hit a bump. Two perfectly balanced masses on either end of a seesaw have twice the rotational inertia so will track high frequency bumps half as well as with just one mass on one end of the seesaw.

jackthedog
Member

Right, mass isn’t weight. That clears it up a bit. It’s about accelerating that mass, rather than lifting a weight.

So the hypothetical hub motor ebike, but reconfigured so the batteries see saw with rear wheel would be worse than having those batteries mounted to sprung main frame.

Makes sense. Thanks chaps. 🙂

IanMunro
Member

Yup, but I like your lateral thinking on the subject.

maxtorque
Member

Probably better to think in terms of Inertia tbh. I.e. the way that the mass acts to oppose an applied force. In this case, the direction of the applied force matters.

For example, two large elephants carefully stacked at the very centre of a seesaw could easily be rotated (by lifting one end of the seesaw) by a person, but move those elephants out to sit on the ends of the seesaw, and suddenly takes a lot more force to rotate them (or more accurately to change there rotational velocity around the pivot point)

jackthedog
Member

This helps. So you might be able to wiggle that seesaw relatively easily with the elephants near the pivot, but with the elephants sat on the ends of the seesaw you’d have trouble getting them moving, and then stopping them once under way. Which explains why large unsprung mass makes suspension ‘unresponsive’.

You can tell i’m no physicist right? 🙂

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