Un-sprung > Sprung Weight?

[InfiniteWheelie]

I've been thinking about alternative e-wheel designs and this question crossed my mind.

What would happen if only the rider, side panels, and pedals were sprung weight? Meaning the motor, rim, tire, battery, controller etc. were all connected rigidly to each other like a non-suspension wheel, and moved together when hitting bumps.

Say you had a large wheel (120lbs), with a light rider (100lbs). With more un-sprung than sprung weight, would the suspension still work, or does the sprung weight have to be greater for the suspension to function?

I can't wrap my head around it. Would the only downside be less responsive suspension due to the high un-sprung weight?

 

Edited February 5, 2023 by InfiniteWheelie

[techyiam]

The ideal suspension would be to have almost zero unsprung weight, and infinite sprung weight if that is possible. 

Why?

Because what you want is for the sprung weight to travel along unperturbed while the unsprung weight to react instantly in order to follow all the undulations in the terrain, all the while supporting the sprung weight.

The more massive the unsprung weight is, the more inertia it has, the more sluggish it would be to faithfully and instantaneously follow the terrain.  

The more massive the sprung weight is, the more inertia it has, and the more difficult it is to perturb it. 

[InfiniteWheelie]

@techyiam I know that adding un-sprung weight makes suspension more sluggish, and isn't desirable. What I'm really asking about is...

You never see a case where the un-sprung weight is actually higher than the sprung weight. I want to know if a suspension can even function in this situation.

Maybe I'm wrong, but intuitively it seems like the un-sprung section would need something heavier than itself to push against. Normally the rider stays level while the tire moves up and down when hitting bumps, but can it still do that when the weight pushing down is less than the weight being flung up when hitting a bump?

Edited February 5, 2023 by InfiniteWheelie

[techyiam]

2 hours ago, InfiniteWheelie said:

You never see a case where the un-sprung weight is actually higher than the sprung weight. I want to know if a suspension can even function in this situation.

In real life applications, you want the "highest sprung weight to unsprung weight ratio".

You are right.

You do need the inertia of the sprung weight for the unsprung weight to push against, and you don't want the momentum of the unsprung weight to be high enough to perturb the sprung weight.

Basically, you want the unsprung weight to do all the shaking, and not be able to disturb the sprung weight. And the sprung weight thinks it is on a perfectly smooth road. To achieve this, the sprung weight has to be much more massive than the nonsprung weight.

If an electric wheel crashes into a dump truck, would the driver even feel it?

The conservation of linear momentum explains this.

Edited February 5, 2023 by techyiam

[Paradox]

2 hours ago, InfiniteWheelie said:

What would happen if only the rider, side panels, and pedals were sprung weight? Meaning the motor, rim, tire, battery, controller etc. were all connected rigidly to each other like a non-suspension wheel, and moved together when hitting bumps.

I believe the V11 suspension is like this.

[mrelwood]

Bus and tractor driver’s seats are suspended, in which case the sprung weight is much lighter than the non-sprung weight. And they work very well. It’s only about matching the spring rate to the sprung weight.

 And yes, the V11 suspension is exactly what you described in the opening post, where only the pedals and the necessary joining structure are suspended with the rider. Motor, battery and mainboard housing are not suspended.

 What makes the situation interesting with EUCs is that the rider’s knees function as suspension as well. If a V11 rider rides with soft knees (like everyone should), the weight that the wheel’s suspension pushes up against can be only a tiny fraction of the total sprung mass. So the mass against the springs varies more than it does on other suspension wheels. No matter how much you give at the knees on a V13, the springs carry almost 40 kg of mass on it’s own. On the V11 it’s maybe 1 kg.

 This makes the V13 suspension work at a much more optimal spring ratio at all times, and the minimum amount of 40kg of sprung mass makes the wheel ride much smoother.

[InfiniteWheelie]

On 2/5/2023 at 6:21 AM, Paradox said:

I believe the V11 suspension is like this.

On 2/5/2023 at 9:57 AM, mrelwood said:

Bus and tractor driver’s seats are suspended, in which case the sprung weight is much lighter than the non-sprung weight. And they work very well. It’s only about matching the spring rate to the sprung weight.

Very interesting, I didn't realize the V11 uses this design, and the bus seats are a good example too. It seems it's possible even on heavier wheels.

Do you think this design could perform well on a 50kg+ wheel? Assuming you used high quality suspension parts unlike the V11.

Edited February 6, 2023 by InfiniteWheelie

[mrelwood]

14 hours ago, InfiniteWheelie said:

Do you think this design could perform well on a 50kg+ wheel?

Sure, but never as good as a system where the batteries and the wheel body are suspended as well.