Potentially stupid question about wheel balancing.

Originally Posted by joshwalker094

wheel balancing on a motorbike, i have much to learn. how do you this and why?

Just let your tyre place do it. It's done so your wheel is balanced as it rotates, so there are no 'heavy spots' so to speak. Imbalanced wheels can lead to instability and a few other problems.

[Sid Squid]

Originally Posted by joshwalker094

wheel balancing on a motorbike, i have much to learn. how do you this and why?

Why: An unbalanced rotating mass will vibrate, the amount of vibration depends on speed and the mass of the imbalance, a 50cc bike with wheels unbalanced by 10 grammes probably won't be noticeable, an SV650 doing eighty with a wheel that's 40 grammes off will shake like you wouldn't believe. The front wheel is much lighter and has a lighter load upon it so any given weight of imbalance will be more problematic at the front.

How: The wheel, disc and tyre combination obviously rotate quite quickly and being three or four separate components it can be difficult to guarantee their exact balance when the individual parts are made, so typically the wheel with discs will be balanced when fitting a tyre by sticking or clipping weights to the lightest part so that the complete wheel will be within - typically - 5 grammes of imbalance. There are two ways of balancing; dynamic and static.
Dynamic balancing: This is done by clamping the wheel to a machine that spins it quickly and measures the imbalance and suggests a weight required, the weight is fixed to the wheel and spun again to remeasure until the imbalance is corrected.
Static balancing: This is done simply by supporting the wheel on low friction bearings, spinning it by hand and letting it stop naturally. Gravity will pull the heaviest part of the wheel to the bottom, so a balance weight will be stuck at the top, the wheel spun and left to stop again, if when doing this the wheel always stops in the same place then another weight will be added to the top and the process repeated until the wheel stops in differing places indicating there is no part significantly heavier then any other. A skilled operator can achieve very fine balance like this.

Bike wheels are relatively narrow and usually the weight's position side to side on the rim is not critical, as it can't be far enough from the centre axis to cause a deflection. But in the case of wide car wheels the imbalance can be only at one edge which will cause the vibration to try and twist the wheel, (if you were looking at the wheel edge on such an imbalance would make the wheel rock side to side as well as up and down), in that case dynamic balancing machines can suggest how much weight and whereabouts on the rim it should be placed.

You are not balancing the wheel, the wheel rim, the disc, or anything else made of metal. All these things are manufactured to pretty exacting tolerances, use the same thickness of material all the way round, and are near as dammit balanced before you start, unless you have a really poor one.

In 99.9% of cases, you are only putting weights on the rim to counteract a imbalance in the tyre. Rubber in a mould cannot be made to be totally uniform all the way round.

[joshwalker094]

Cheers Sid for clearing things up for me,
Ralph- that i understood straight away.
thanks guys and your mighty knowledge

[Sid Squid]

What you're balancing is the the rotating mass, all of it - which is the wheel, disc and tyre combination.

Originally Posted by Sid Squid

What you're balancing is the the rotating mass, all of it - which is the wheel, disc and tyre combination.

Indeed, because together they create a single rotating mass, but 99.9% of the time it's the tyre which is causing the imbalance within that assembly. Other parts of the assembly may change the points you put the weights on slightly, but not by enough to feel the difference when riding.

I'm sure you know that already Sid, but for the benefit of those that may need some reasoning...

Think of the size of 10 gram lead weight. 15mm*10mm*2mm approx? = 6.7mm^3

Lead is denser than the material the wheel is made of. To have a 10 gram imbalance on a wheel (which isn't that significant), you'd need to have a chunk of metal greater than 6.7mm^3 missing out of the rim, of a difference in thickness equivalent to that volume on one side of the rim.

The valve makes a bit of a difference, but it's not heavy and wouldn't be a detectable difference when riding.

Rim locks on off road bikes make a big difference, but the OP relates to an SV with a cast aluminium wheel.

So in answer to the OP's question, most wheels, and definitely discs are in balance before you start. Balancing without the discs isn't going to make any difference at all.

Anyone who disagrees, don't bother arguing with words on a forum, just post me a video of a wheel with discs and no tyre or valve, which has a heavy enough imbalance in it to rotate itself on a static wheel balancer, and settle itself with the heavy point at the bottom on it's own. You'd better have a lot of wheels, or a real Friday afternoon wheel where the aluminium didn't flow through the cast properly, to find one that lets you record that video.

[NTECUK]

You do it to stop vibration and increase component life.
You cansearch you tube to show you the old fashion static way.
Or pay someone with a flash electronic one to do it.

I balance using the spindle suspended on a pair of axle stands, it's accurate enough.

[NTECUK]

Its a tried and tested method Ralph.
Been around almost as long as me .

[yorkie_chris]

Originally Posted by -Ralph-

Anyone who disagrees, don't bother arguing with words on a forum, just post me a video of a wheel with discs and no tyre or valve, which has a heavy enough imbalance in it to rotate itself on a static wheel balancer, and settle itself with the heavy point at the bottom on it's own. You'd better have a lot of wheels, or a real Friday afternoon wheel where the aluminium didn't flow through the cast properly, to find one that lets you record that video.

I'm not interested in arguing or videoing it, but I've had one like that.

No idea why, I would expect if casting flow was a problem then there would be cold shuts etc. My guess is bits breaking off the cores used to form the hollow sections leaving a lump inside.