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Thanks SjB. I'm probably being a bit picky, but as a motor engineer myself, It's as easy to do it properly as not, and I expect it to be right, not nearly right. I am just very slightly worried that the balancing technology might have advanced sufficiently to change what I knew to be fact 3 yrs ago!. As for bikes, well I think as a motorcyclist myself, there is a bit more respect for tyres and the job they do in keeping you alive, than the average car fitter or driver has, so I'm not suprised at all of your experience. My bike is old enough though, to wrap solder round the spokes to balance the wheels! Mike
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the kids that fit tyres these days have all the machinery to fit without any physical help needed apart from jacking and carrying the wheel and have therefore had lots of risk assesements done on tyre changing but never actually been shown the basic basics of rudimentary tyre fitting and its finer points..............
my theory anyway.................and i was learnted properly ;-)
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I agree SjB.
There is no new technology that means that weights placed at various points around the circumference of the rim are required.
In order to balance a wheel and tyre, both statically and dynamically, a light spot on both the front and rear of the rim are all that needs to be corrected - so, there should be one clump of wieghts on the front of the rim, and one on the rear.
So please don't accept any nonsense about some weights being for static balance, and others being for dynamic!
Usually, numerous weights on one rim mean that when the fitter spun the wheel up after adding the first weight, he found some remaining imbalance elsewhere. The correct thing to do (if the angle between old and new locations is less than 90 degrees) is to remove the first mass, and then put a new one of the same mass in place between the old one and where the machine now indicates - for angles of more than 90, the mass should also be reduced. The wrong, and all too common, response is simply to bang a weight on everywhere the machine suggests!
Obviously, if the tyre has been fitted in the correct orientation, re aligning the dot, then the total mass of balancing weights required will be less.
Number_Cruncher
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Thanks for the confirmation. I will take it back in the morning and ask them to refit the tyres and re-balance. I then suspect that the mass of weights on the worst wheel will be somewhat less than the 60 grammes on it now! Think of all the fuel I'll save over the years not having to accelerate that lot dozens of times a day!! My experience is exactly as number cruncher says, if you need weights in more than one place something is wrong. On re-balancing used tyres, I often found this happening, and used to find if I took the tyre off, there were hundreds of rubber 'ball bearings' inside moving about which were obviously moving the goal posts every time you came to check the balance. Thanks again.
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I'd never get tyres and/or fitting at a main dealer anyway. (except bike tyres).
Martin
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I would agree totally with you martint123, but I bought the tyres from a quite well known company on the internet, and chose this dealer as my local fitting agent, so I did not have any choice! They are a very reputable dealer usually! I have used them before and been very happy.
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If a spinning mass has say two light spots, not necessarily opposite each other, would one weight be sufficient to balance it?
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If the two light spots are in one plane (front rim bead or rear rim bead), then, yes, because this is a static balance case.
Mathematically, in the case of static balance, you are aiming for the sums of the vectors of imbalance mass * imbalance radius to be equal to zero. Whatever the system of static imbalance, each heavy spot may be represented by a vector, pointing out from the hub in the direction of the heavy spot - the length of the vector, proportional to the imbalance mass and the imbalance radius. If you then take each vector, and place them all nose to tail, you only ever need to add one more vector to close the loop - i.e., you only need to add one balance mass to make the sum of the vectos zero.
Stated another way, if there is no brake drag, even if there are numerous light spots (or equivalently numerous heavy spots), the wheel will always come to rest with the total, or resultant heavy spot pointing down, and one balance mass at the top of the wheel will balance this out.
When you want to balance on object dynamically, you are aiming for the sum of the vectors of mass * radius * z to be zero, where z is the distance along the axis of rotation. You can, if you wished constuct a similar vector diagram to the static case, and you find that to get both static and dynamic balance, it is sufficient for there to be one mass on the outer rim, and one mass on the inner rim.
For objects with more depth in the z direction, like long turbine shafts, for example, it is necessary to balance in more than two planes because the long shaft cannot be assumed to be a rigid body, and imbalance half way along - although it could be fully corrected at the bearings could still cause local deflection and accompanying rotor dynamics problems.
Number_Cruncher
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I'm amazed at what you learn on a daily basis. Even in my job, with 12 years experience, I still learn new things about the role I'm playing.
This thread intrigues me as I've never ever thought about the whole tyre fitting process, I assumed that the tyre/rim combination was unique each time for balance and therefore whatever way it was fitted onto the rim, would require balancing.
Now I know to look for the Yellow dots lined up with the valve. I'll certainly be looking next time I take the car in for new tyres.
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VW Bora (51) 2.0 SE
VW Touran (54) 1.9 TDI
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I didn't see any dots on my new Conti Premium contacts ??
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Without wishing to sound rude, but does it *really* matter where the dot is in relation to the valve and where the wheel weights are; as long as at the end of the day you don't get any wheel vibration whilst driving?
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It's more a question of sloppiness on the fitter's behalf - it costs nothing extra to get it right.
In a simplified case - If your rim is, say, 15 grammes out of balance, and your tyre is also 15 grammes out, by putting them together properly, you might avoid having to add any mass at all. If you put them together badly, then up to 30 grammes of ugly mass needs to be applied.
You are right to say that the wheels can still be balanced with dots misaligned, and weights all over the place, but I wouldn't want that on my car.
Number_Cruncher
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then up to 30 grammes of ugly mass needs to be applied.
I must stop taking for granted that weights cannot be applied to the external edges of my alloy wheels and therefore the ugly mass is hidden out of sight.
I did wonder what the dot on the tyre was for though. Reading this thread has revealed the answer.
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It may have been important once upon a time but as DD says if they work what on earth is the problem.
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Andy, I think it's a matter of getting it right if possible. I cannot imagine why a tyre manufacturer would go to the expense of dynamically balancing a new tyre and marking it, with a yellow spot, and measuring the point of maximum deformity, the red spot, if it did not matter one jot. The fact that most people would never know does not seem a good enough reason to me. Anyway, thanks to everyone for their contibutions. I's much appreciated.
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