which wheel is unbalanced

[Roger Mills]

That's a reasonable way of getting a static balance, but it could be out of balance dynamically. That may not matter for a scooter.

A balancing machine at a tyre depot will provide both static and dynamic balance. It matters which rim is used for the weights.

Imagine that you have a perfectly balanced wheel. Now put a 20g weight on one rim and another 180 degrees round on the other rim. The wheel will still be ok as far as static balance is concerned but it will be out of balance dynamically because the weights will try to make the wheel 'flap' as it rotates at high speed.

[Jonathan]

heel is unbalanced. Reasoning that the vibes would be greatest directly ov er the wheel in question, I downloaded a recording accelerometer app onto t wo smartphones and put them (i) in the boot over the wheels and went for a spin down the local dual carriageway then (ii) put them in the footwells an d did the same.

em is that the levels are extremely similar above all four wheels and none of them stand out as being worse than the others.

low the suspension springs. Or could it be that all 4 wheels are a bit unb alanced.

Our 1953 Rover used to do that and I discovered the cure was to get the tyr e pressures exactly right, then the problem went away.

Jonathan

[The Natural Philosopher]

must have been terrible tyres whose whose tread length increases by 10% on being inflated.

[Dave Plowman (News)]

Some designs of suspension are prone to giving wheel balance symptoms when it's actually wear in the suspension that is causing it. An obvious one is the fluid filled bushes that BMW use in the front suspension - quite difficult to check for wear as there is some 'give' when perfect.

[Tim Lamb]

In message , "Dave Plowman (News)" writes

Too right! Sliding pillar front suspension on a series 2 Morgan. Cross ply tyres on 16" steel rims. Used to shake like a wet dog at 55mph.

New shocks, suspension bushes and eventually widened 15" wheels with proper balancing cured the problem.

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[Fredxxx]

Anything that described a circle orbit such as a tyre, static and dynamic balance occur at the same time.

Anything that doesn't describe simple harmonic motion may have a harmonic content. The only place I can think of offhand is the motion of a piston in an engine, I am sure there are others, but certainly not perfectly round uniform tyres.

[NY]

Wrong! If the wheel/tyre was an infinitely thin disc, then static and dynamical balance would be the same.

But if the wheel has thickness (as they do!) then two weights placed suitable places on opposite sides of the wheel could set static balance but the wheel could still wobble about its access because the balancing weights may not achieve dynamic balance, causing a turning moment. An earlier poster has described this.

[Fredxxx]

I think we are talking of two entirely different things. You can statically assess the balance of a tyre by considering it has two infinitely thin disks, separated by a known distance.

Dynamic balancing implies some speed dependant imbalance. For a circular object that is assumed not to deform at speed balance can be always be assessed statically.

[Roger Mills]

No it doesn't! With your 2 disk model, the wheel will be in static balance as long as the overall centre of mass is on the axis of rotation. But if one disk is unbalanced statically and the other is also unbalanced by an equal amount but shifted by 180 degrees, it will still be in overall static balance but it *won't* be in dynamic balance because there will be a rocking couple between the two disks. If it's a wheel which can steer, it will flap about its steering axis - with some interesting knock-on gyroscopic effects.

[Fredxxx]

That rocking couple can be determined statically. It would manifest as a moment on the axle which rotates with the wheel. Measurements through one turn of the wheel is all that is required. It is not dependent on speed.

[Johnny B Good]

I'm afraid Roger's right on this one. What's worse is that you're wrong about being able to detect this type of imbalance using your method.

[Tim+]

So why does my tyre shop always spin my wheels to balance them?

Consider a very wide wheel, or better still, a train axle with welded on wheels. A weight fixed to the top of one wheel and the bottom of the other wheel could well be in static balance but be terribly unbalanced when spinning.

Tim

[harry]

Another example is the "boxer" IC engine.

[Tim Lamb]

As one who struggles with the Telegraph balancing birds problems, I hesitate to step in, but.

Surely you could statically balance a rotating object by fitting double the necessary mass at half the distance from the rim?

Some dim recollection of centripetal forces tells me the relationship is not linear but 2nd power for different distances from the centre of rotation? c=mv2/r?

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[RobertL]

One possibility: For each of the the driven wheelsjack the car so only that wheel can spin; chock the other wheel. Then it should be clear which one vibrates. (Don't lock the diff)

Another possibility: perhaps it's not balancing at all but a 'flat' on all the tyres resulting from the car having been left parked in one spot for along time.

Robert

[Dan S. MacAbre]

Probably a silly idea, but I feel compelled to mention it. I've no idea how good they are, but you can buy a thing that passes itself off as a wheel balancer for something like £50. They're called bubble balancers, and use a spirit level. They only do static balance, of course, and things might be quite different when moving. But it'd be interesting to hear from someone has actually has used one. There are a few videos on YouTube by people using home-made balancers.

[Roger Mills]

You'd need some incredibly sensitive measuring equipment. If it's that simple why do you think that tyre depots go to the expense of balancing machines which rotate the wheel at high speed?

[Roger Mills]

Perfectly true. But it has little to do with dynamic balance - where the balance weight needs to be not only at the right angular position but also in the right plane. [Car wheels sometimes need equal and opposite weights on the two rims to achieve dynamic balance even though they are in static balance without that.]

[The Natural Philosopher]

Exactly.

[Vir Campestris]

I was surprised at the term, and looked it up.

Dynamic balance is a commonly used term for things like this.

Andy