Wheel imbalance

Discussion in 'Cycling Equipment' started by Magnesious, Mar 18, 2010.

  1. Magnesious

    Magnesious New Member

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    I noticed recently that my rear wheel is not balanced. By that I mean that one part of the wheel always 'drops' - put another way, if you imagine a given spoke, that spoke will typically end up in the same position if the wheel is at rest (off the ground).

    If I lift the rear end of the bike up and make the wheel go really fast, the bike tends to 'shake' in my hand with the same frequency as the wheel's rotation. It's not that pronounced, but it's noticeable.

    Before you assume it's the tube pin that's the cause of all this, it's not! The crazy part is that the wheel is actually heaviest on the *opposite* side. I experimented with counter-weighting the pin side of the wheel, and this indeed reduced the 'shaking'.

    I went into my bike store down the street and the mechanic treated me like I was wasting his time. He didn't think it was an issue to be worried about, and it probably isn't. He also mentioned it might have something to do with counter-somethinged bearings. I asked him if he had something convenient to counter weight it with, but he wasn't helpful. I'm pretty sure a wheel that is not balanced wears out bearings prematurely. Am I nuts?

    My question is really whether anyone knows if wheel makers purposely do this to counteract the tube pin. For reference, it's an Alexrims DA28.
     
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  2. waldowales

    waldowales New Member

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    That wheel is not going to be turning anywhere near fast enough for the unbalance to matter.
     
  3. dabac

    dabac Well-Known Member

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    The phrase tube pin is a bit odd, I assume you're talking about the valve.

    Rims start out as lengths of aluminium profiles, which are then rolled into a hoop and joined together. The joint is directly opposite the valve hole. Some rims are welded together while others have cavities in the profile where pins are inserted to keep the joint together. Pinned rims tends to be heaviest by the joint.

    Youre probably right in that if your wheel spent its life spinning in thin air, then the imbalance might influence bearing life. But for the bike IRL you have the whole rider weight pressing down, making the issue of imbalance pretty much a moot point in comparison.
     
  4. jhuskey

    jhuskey Moderator

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    Perplexing issue but I recommend going to a good shop and getting that rim on a truing stand and spinning it to see what happens and then draw conclusions.
    No point is guessing what is wrong just start eliminating guesses.
     
  5. maydog

    maydog Well-Known Member

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    I had an imbalance on my rear wheel that was noticeable when riding at high speeds (25+ mph). The wheel had a reflector mounted opposite the valve stem. Without the reflector, this side of the wheel assembly was already heavier (the missing material in the stem hole makes the other end lighter).

    By moving the reflector to the valve stem side, and adjusting its radial position, I was able to perfectly balance it.

    So if your wheel lacks a reflector, you could add one to both balance the wheel and to add nighttime visibility. Otherwise you can play with the position of the existing reflector.
     
  6. dhk2

    dhk2 Active Member

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    No, you're not crazy, and imo wheel balance certainly does matter. When I descend at 40+ mph, last thing I want is for my wheels to be hopping off the pavement.

    Most of the cheaper rims like the Alex are jointed with steel pins at the split (opposite the valve stem hole) so that they are heavier there. My bike has Velomax Circuit wheels, but the rims look like low-end Alex, and they were definately out of balance when new. Cranking the back wheel in top gear had the bike bouncing on the stand. I used wraps of lead solder on the spokes either side of the valve stem: just trial and error had everything smooth in a few minutes. Then I repeated the basic weight at the front, making fine adjustments by noting whether the wheel would come to a stop once or oscillate.

    Considering how important balanced wheels (and round tires) are to cars which have vibration damping between the tires and the driver, I think wheel balance and roundness are at least as important on bikes. Note: most of my riding buddies ignore the issue too :)
     
  7. tafi

    tafi Member

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    A few grammes of imballance would have to be rotating pretty fast to make the bike and rider hop off the ground. The weight of a rider and bike amounts to around 780N
    If the imballance is about 5g then the resulting centripetal force is 1.87N at 40kph (25mph). Less than 1% of the way to making it jump.
    In order to do so at 40kph then you need more than 2kg of imballance or you would have to be travelling at more than 800kph for 5g to make the bike lift.
    I grant you it is annoying to see the bike bounce in the stand (especially when you're trying to tune it) but I've never noticed an effect on the road.

    Car wheels/tyres suffer from much greater imbalances because they are much heavier to start with and the high speeds of highway driving leads to a vibration. More important, however, is the fact that cars have a pair of front wheels and imbalances can lead to the steering being pulled back and forth, so the steering wheel will wobble from side to side at speed. With only one front wheel, a bike is immune to this effect.
     
  8. dhk2

    dhk2 Active Member

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    tafi, understand there are plenty of doubters. Appreciate your fine analysis, and agree hopping off the pavement might be a slight overstatement. The weight I added to my Circuit wheels may be a lot more than 5 grams though I don't have a scale to actually check it. However, I still think a bike with balanced wheels is smoother at high speeds, just based on my on perceptions. Hard to imagine that the vertical bounce I observed wouldn't be felt through the butt or arms, but again, I've got no "double-blind" testing to confirm the improvement.

    It seems to me that unbalance in the front wheel could induce dreaded speed wobbles. And wouldn't a balanced wheel roll more efficiently? The vertical energy that is induced has to be dampened by our bodies, meaning it's wasted.

    So, I still believe balanced wheels are important to the smooth ride of a bike, just like a car. But I don't expect to make any converts....particularly not here :)
     
  9. maydog

    maydog Well-Known Member

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    I agree that wheel balance is very unlikely to be an issue of safety or component longevity. The analysis above is a good first order approximation of forces involved.

    However, there are higher order effects involved. The imbalance is not a simple force upward, but a rotating vector with a regular period. The oscillations could magnify the force to a point where they are at noticeable to the rider.

    In my case, the issue was easy to correct by moving accessories mounted on the rim. This resulted an a perceivable reduction in bounce and it took me less than 10 minutes.

    Now when I am servicing my wheels, I take an extra minute to check the balance. Most times, no adjustments need to be made. There may not be a huge benefit in balancing most wheels, but it is easy to check and takes little time to correct.
     
  10. tafi

    tafi Member

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    Yes it is a rotating vector (and I never said it wasn't :D). It is found from the definition of a centripetal force after all. The question I was addressing was that surrounding the ability of this force to counteract gravity (and cause the bike and rider to hop), in which case only the maximum force on the vertical axis makes any sense at all.

    To outline my logic for looking at this statically:

    You could look at the rider/bike as a mass and the tyres (or some combination of them with part of the frame) as springs and try to model the effect here of the rotating forces in each wheel, but I think you'd find that the natural frequency (thanks to the high stiffness of such a system) would be much higher than the highest possible driving frequency. We then are safely operating in the region below the resonance where the ratio of input force to the force on the mass (transmissibility) is approximately 1. So the static force case I used above should apply reasonably well.

    I have to admit, dhk, (after a rethink) that speed wobbles are a different kettle of fish all together since the vibration itself is partly rotational (in a different axis to the wheel) and some component of the imbalance force may lie in the plane of vibration.:eek:
    To prove any contribution, the frequencies of wobble and rotation (of the wheel) would have to be the same. Then the phases of these motions would need to be checked (expect approx 90 degree phase difference at resonance as proof).

    Now I've probably posed more questions than answers :rolleyes:

    The upshot is, no-one likes unbalanced wheels, and the correction of the imbalance is relatively simple (as long as the weights don't come off and take someones eye out :rolleyes:) but the degree to which they need correction is debatable.
     
  11. alienator

    alienator Well-Known Member

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    Speed wobbles are completely different as a speed wobble is the result of an excitation of a harmonic specific to a given bike/rider system and geometry. Given the number of things that excite initiate speed wobble, you probably cannot rule out wheel oscillation as an initiator, much like you can't rule out a loose headset or loose quick releases.
     
  12. Resistorlead

    Resistorlead New Member

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    Tires are compliant (thank goodness) so you're going to feel plenty of vibration long before the bike starts hopping up off the ground. Apply the forces calculated against the spring rate of the tire and see what deflection you get. Now, using the special test equipment, measure the low frequency sensitivity of the butt...

    There may also be an audio component involved.

    Seems too much to bother with, but IMO out-of-balance is detectable and it certainly doesn't hurt anything or cost much to correct it.
     
  13. tafi

    tafi Member

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    The tyre actually represents a pretty stiff spring (as mentioned above) and as I already state the forces at work here are very small, so any deflection of the tyre purely due to imbalance will pale into insignificance compared to even the usual surface roughness of the road.

    You can find an order of magnitude estimate of the tyre's spring constant by knowing your weight and the amount of deflection when you get on the bike.

    5mm is pretty high amount of deflection for a tyre so lets take that as an upper limit estimate. Last time I checked my weight at sea level was about 800N (or about 500N on the rear wheel). That's a spring constant of 100N/mm. As determined above a typical imbalance is a few grams and an upper limit of this is about 5N or so at speed. A 5N force induces 0.05mm deflection in our tyre.

    Typical surface roughness of roadways is at least in the several mm range. 100 times greater than the deflection above. I would doubt your ar$e's ability to detect that.
     
  14. tafi

    tafi Member

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    Further to that I've just stumbled across a reference:

    Wang, G. M. and Hull, M. L. (1983), "Analysis of Road Induced Loads in Bicycle Frames," Journal of Mechanisms, Transmissions and Automation in Design, 105(1):138-145

    These guys measured the tyre spring constant at around k=150 N/mm at 688 kPa (100psi). So the estimate above is pretty good on the orders of magnitude.
     
  15. BHOFM

    BHOFM Active Member

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    Well, I have been called a "Smart Ar$e" so maybe that is why I think I can tell
    the difference??:D:D:D

    I balanced mine to near perfection this week end and today it seemed to be
    a smoother ride. The rear was 3/4oz out. When the bike is upside down and
    you spin the wheel fast it is smooth as glass. Before it would almost jump off
    the table.
    I ride trails for tlhe most part and they are very smooth surfaces. Just a tiny bit
    of tire noise when you corner.

    I agree, most of the streets are far rougher than the trails.

    [​IMG]
     
  16. jackson123

    jackson123 New Member

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    i don't like rough road
     
  17. sufiankane

    sufiankane New Member

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    Something most people don't take into consideration, the extra weight you have added to your bike will also require more power to push. I think I read somewhere that the extra energy required to push the extra weight is actually more than having the wheel unbalanced. Don't take my work for it, but something to think about.
     
  18. Resistorlead

    Resistorlead New Member

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    Very true- when I want to eliminate useless extra weight from my bike, I just get off the d**n thing.
     
  19. dhk2

    dhk2 Active Member

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    tafi's analysis is difficult to refute. Even if my wheel needs 21g to balance, as BHOFM found, that still would produce a tire deflection much smaller than the road imperfections, so it should be undetectable to the hands on the bars.

    My only nagging doubt however is that the same analysis could be used to suggest that balancing a (much heavier) car tire with 1/4 oz weights is also unnecessary. Perhaps the key difference here is the suspension, which provides for wheel travel and relatively much lighter damping forces vs the bike, and also raises concerns about spring resonant frequency.

    I recall the same issue with motorcycle wheel balancing decades ago when I worked as a Honda mechanic. Honda motorbikes didn't use wheel weights then, and our shop never messed with wheel balancing when we changed tires. But IIRC, some italian bikes (eg, Moto Guzzi, Ducati) did have balance weights.

    From a quick google search, it appears the issue is still not settled. At any rate, here's a video clip: [ame=http://www.youtube.com/watch?v=QbVuwjX0q5g]YouTube - ‪Tire Stripper Motorcycle Wheel Balance‬‎[/ame]
    With our bikes of course, the wheels can be left on the bike, and the added bonus is that the rear wheel can be quickly spun up to verify results and make final corrections.
     
  20. BHOFM

    BHOFM Active Member

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    This may be a bit wordy, so get your coffee and sit back:

    I ride for enjoyment and to improve my health.
    I ride a 35lb mountain bike with Kevlar radial road tread tires
    I carry the following equipment:

    Ceil phone
    380 Beretta
    Pepper spray
    Head light
    Tail light
    horn
    Speedo"
    27oz water
    Small towel
    Locking cable
    apx. 8oz raisins, nuts or dry cereal
    Small seat bag
    When weather dictates, light wind breaker and
    dry socks

    The bike has full suspension, maybe that makes the imbalance more
    noticeable?

    The 1 1/4oz needed to balance the tires seems to improve the ride, even
    at lower speeds and also seems to smoothed out a brake pulsing.

    I feel better about my bike! I enjoy my ride more! And on the same ride with
    about the same conditions my average speed is up almost .5mph

    I have considered adding some weight to the bike to help get a little more
    work out factor. I have lost about 15lbs in three months and the ride is
    almost too easy now.

    At first a five or six mile ride was taking almost four hours, now I can do thirty
    in about two hours. My normal is about twenty five.

    The bottom line, I feel better about my ride, which I feel makes me ride a bit
    harder and longer.

    Is this not the bottom line for all bike riders??
     
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