Some while ago I had two new tyres on the front wheels, which clearly needed balancing as judged from the severe steering vibration. I put them on the back.
Having just fitted two new tyres on the back, I have swapped the half-worn pair back onto the front, fully expecting to have to have them balanced.
I am surprised to find that they are now in perfect balance, with not a trace of shimmy at any speed.
So is the following theory true ?:
Wheels are imbalanced because the circular periphery of the tyre is slightly eccentric with respect to the centre of the wheel. Running the wheel for a while on the rear, where this might not be noticed or matter, causes slight preferential wear in a way that re-centralises the tyre. Like spinning it on a lathe.
|
If they need balancing on the front they would have needed balancing for the back.
You have got another problem imo.
|
Why is that? The car has a traditional rear axle, and I have never noticed that wheel balance makes any difference at the rear. I thought the accepted wisdom was that one only balanced rear wheels on performance cars or if the imbalance was great enough to be noticeable.
My observation was that after a short spell of running, an imbalanced tyre appeared to have corrected itself. What problem are you suggesting ?
|
can't state any particular problem but i suggest there is something amiss.
The idea of balancing a tyre to a wheel (and ideally to the car, but very very few places are able to do that) is because of manufacturing intolerances on a wheel and a tyre. They are not made perfect and the use of weights (balancing) is to equalise these differences so the wheel doesn't spin with a centrifugal effect.
Not many tyre places even fit the ytre to the rim as accurately as it should be (ask the average tyre fitter at a fast fit to explain where the tyre is marked for fitting etc!)
if it requires to be balnced on the front, it equally needs balancing for the rear. The tyre and wheel is the same where ever it is fited.
|
What people don't realise is there's two ways that a wheel/tyre can be out of balance. An off the car machine only cures one of them. This is lateral (side to side) distortion. The wheel must be balanced in this way first.
To cure dynamic distortion (up and down) a finish balancer is required, and the wheel is balanced on the car, with weights applied to either the front or rear rim only. This also balances the wheel, disc/drum, and hub together.
In 99.9% of cases off the car balancing is quite good enough, but yours might just be the 0.01% that isn't.
|
So are you saying, Railroad, that probably the two wheels were already balanced laterally (by pure luck), but that the original imbalance I noticed was only the rarer dynamic distortion? If so, was it really the case that this disappeared by itself over time, by the tyres adopting a tiny wear variation so as to counteract the imbalance?
How else would you explain the fact that an imbalanced wheel/tyre cured itself?
|
You'd tend to notice an out of balance wheel on the front more than you would on the rear, as a vibration would be felt through the steering wheel. You'd feel a vibration though your seat if a rear wheel wasn't properly balanced, but it's effect doesn't seem quite so severe. It may simply be a case of one's more noticeable than the other.
If the wheels have been balanced off the car (which it seems thay have) it might pay you to have them balanced at a garage with a finish balancer. We had one in the garage I used to work in, but I left in 1995 and it ceased business in 2001. If you find one make sure that they DO NOT remove any weights that have already been applied to cure the lateral distortion, otherwise you'll be back to square one.
|
|
|
|
Technically, this thread covers the theory of static and dynamic balancing in a reasonablr rigorous way for a non-mathematical forum - there isn't any more to it than the description in this thread. Most off-car machines since the 70's or so can do both types of balance at once.
www.honestjohn.co.uk/forum/post/index.htm?t=42992&...f
It's not unusual for tyres to change as they wear - for tyres which are initially OK to become unbalanced, or vica versa.
I can imagine if you have put the tyres on the back axle, the back axle, being heavy, will tend to stay still, and may help even out any imbalance, by forcing the heavy spot to wear more, where the lighter independant front suspension will be more likely to be shaken around - but that's just an educated guess.
Number_Cruncher
|
|
|
