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Front end shake, what causes it?(9 posts)

Front end shake, what causes it?Breakfast
Dec 27, 2002 9:31 PM
Today I put on a new chain on a bicycle in a workstand (clamped seat post) and when it was finally on and I spun the rear wheel in the 53t/12t at a high rate of speed I noticed how the rear wheel created quite a lot of imbalance and shook the whole bike in the workstand. This got me wondering whether this spinning imbalance of the rear wheel is the cause of front end wobble, or shake, at moderate and high speeds when coasting with your hands off the bars.

Couldn't this imbalanced rear wheel oscillate through the frame and manifast at the steerer head?

It seems the blame is always directed at the headset, the front wheel, or the frame alignment. What about this theory of mine, is it valid?
Is the wheel "true"merckxman
Dec 28, 2002 6:49 AM
....both laterally AND vertically???
Not perfectly, within normal range though.Breakfast
Dec 28, 2002 8:00 AM
Even if it were perfectly true with no discernable runout laterally or vertically the tire and tube will make it have enough imbalance to induce a wobble at a high spin while mounted in a workstand. Try it yourself and see.
why don't you blame it on stand? nmcyclopathic
Dec 28, 2002 9:43 PM
Something you would do? ???(nm)Breakfast
Dec 29, 2002 7:20 AM
No single causeKerry Irons
Dec 29, 2002 6:25 AM
But certainly there are many possible triggers. Think of the bike/rider as a complete mechanical system that has a number of resonant frequencies. Even if this system is prone to resonate, it needs a trigger to get it going. So your rear wheel vibration could be the trigger. Generally, bikes with stiffer head/top tube structures will not oscillate, but whippy forks can be part of the problem too. Most of the things you talk about are things to work on to reduce the trigger vibrations - they don't reduce the bike's natural tendencies. Also, the back wheel does not necessarily vibrate. I'm guessing you have an out of true wheel, a lumpy tire, a rim that has a very heavy weld, etc. I can spin up the back wheels on my bike and my wifes bike, and the stand is rock steady. If I do the same thing on my commuter bike, it bucks like a bronco.
why was the wheel so unbalanced?Alexx
Dec 29, 2002 7:50 AM
Did the tube have 6 patches in one area? Has a tire liner slipped and bunched up in one place? Or worse, has the owner put a sh!tload of sealant in, and let it solidify in one place?
It isn't ! All wheels exhibit some high speed imbalance...Breakfast
Dec 29, 2002 4:49 PM
...if you don't believe it then put your bike in a workstand or hold it up and with the chain on the 53t/11t (or 12t)and spin the cranks up to get the wheel spinning super fast. It will wobble!

I have yet to see a wheel that is super true and still doesn't wobble with a tube and tire on it.

Anyway, the point here is whether or not such oscillations originating from a slight rear wheel imbalance contributes to a front end wobble when coasting with your hands off the bars at a fairly fast speed, especially on downhills.
I'll bet it isn't the wheel....Alexx
Dec 30, 2002 5:06 AM
...but rather a sympathetic vibration caused by lateral inputs from your hands on the pedals. Rotating machinery can cause all kinds of vibrations like this....
BTW, the answer to your original question is yes, it can affect the steering, but not as much as a front wheel problem will.