karlspackler1

Hi there,
At about 15 mph the front end on my bridgestone 500(63cm) wiggles if I take pressure of the handlebars. Headset is good, any ideas what it could be? Here is a picture of the bike, for no reason in particular:

FBinNY 

Front end shimmy is the bane of lightweight bikes. Almost all bikes suffer it to some degree, though with most it usually happens at much higher speeds.

Usually it's a resonant phenomenon, whereby a small displacement travels the length of the bike, and returns in time to reinforce the next pulse until it builds up enough tom make control difficult.

The first thing to do is look for underlying cause of wiggle, like a misaligned wheel (or tire), loose hub bearings, a loose (or sometimes too tight) headset, bent fork, or poor front to rear frame alignment. Also if you have a tire with a center ridge and notice that the wear zone is near the edge of the ridge, change to a tire without the ridge.

The next step is to look for ways to dampen any vibration, like increasing front end weight (move saddle & bars forward) or using a wider front tire at lower pressure. Sometimes even using a stiffer grease in the headset helps.

Lastly if you've solved all obvious and still have a high speed (30+mph) wobble, change the harmonics of the frame by changing the pedal position, transferring weight to the pedals, or by setting one or both knees against the top tube.

Given that your problem is happening at very low speed, I focus in the first step, and solve the obvious causes of tracking change.

gyozadude

What does it feel like if you take the front wheel out and spin it while holding the ends in your hands? Is the spinning balanced? If not, then it's a start towards figuring out what's going on, what's in the tires, tubes, or rims that could cause the imbalance.

karlspackler1

the problem seems to be in the wheel. i spin it and if i tip it a little while it is spinning it really starts to wobble unevenly. What can cause that? I'm guessing a number of things. There is no rattle. It feels almost like the wheel itself is unbalanced. Its true though.

really appreciate the help

gyozadude

That wobble is natural. The wheel has some mass and once it gets going, it's like a gyroscope. If you try to shift the plane of rotation, the wheel will try to counter that (Newton's Law of Motion - Inertia) and shift back. The question we want to answer is if the wheel has an imbalance as it revolves around the hub that can trigger and exacerbate the side-to-side wobble when you hit a certain speed (mentioned above by FBinNY - resonance frequency). Just keep the wheel in the same plane, and spin it and does the wheel bounce up and down and wobble if you don't try to move it? If there is a natural imbalance, especially out of the mid-plane, (sort of like a car tire being imbalanced), then this is likely to lead to wobble of the handlebars and this feeds back on the wheel trying to right itself and stay rotating in the same plane, and that can lead to wobble.

karlspackler1

It seems fine when I spin it and hold it steady. im gonna put some heavier tires on and see how it rides.

karlspackler1

put some new tires on and dropped the stem down a bit - it seemed high. it is much better now.

Ira B

Wobble at that speed tells me that something just ain't right with your bike.

Trouble shoot:

With the bike on the ground, lock the front brake, squat down beside the bike and wiggle it front to rear.
Do you "feel" any clunk-clunk or click-click through the handle bars?
If yes: stare down the fork/crown joint on each side as you repeat this. If the fork is failing or loose at this point do not ride the bike.
If the fork blade/crown joints are good, repeat but now stare down the drop outs.
If one is loose do not ride the bike.

If all these points pass inspection and you do feel a clunk-clunk, your headset may look adjusted but be a tad lose of have bad bearings. Before tearing the headset apart:
Try to move the rim from side to side to check for loose wheel bearings. (both wheels)
Spin it to check for trueness of both tire and rim. If needed true the wheel. If the tire looks like it swallowed a rat, replace the tire.

When you allow the wheel to stop spinning. Does it slowly come to a stop and may go back and forth a tiny bit or does it have an obviously heavy spot that quickly moves towards the ground? If so you may have a big glob of Slime or some other sealant in the tube. Replace the tube.

Are the spokes sloppy loose? (both wheels)
True wheel and tension spokes

If you still have not found the issue, go over the frame with a fine toothed comb looking for cracks or damage.
At this point, overhaul the headset.

Still no luck?
Call an exorcist.

HillRider

That is a BIG frame and a "standard diameter" steel frame at that. Historically shimmy has been much more of a problem with big frames made of small tubing diameter tubes and this one qualifies in spades. Lennard Zinn specializes in making big frames (he's something like 6'7" or so) and has written several articles on the phenomenon.

karlspackler1

I have been through all the troubleshooting stages and found nothing. It seems ok on some short trips around the neighborhood - at least better.

I do think it is in part just the nature of the size of the bike and the tubing diameter. This is a taller bike than I usually ride. I usually go with a 60 cm or 61. But on the older steel bikes the top tubes are shorter. Bridgestone didnt start making the longer top tubes till later I think. For that reason I thought the bigger bike would be good.

Now I just need to take it out and really test it with the lower stem and seat. Find some hills.

Thanks for the Zinn tip, I found a good velonews article and it made a lot of sense.

karlspackler1

Final diagnosis - bent fork.

I feel stupid but it still just wasnt right, brought it in to LBS, first guy thought it was a bad headset. I have had bad headsets before, and though it was indexed ever so slightly, I loosened it up a bit and it was then no longer indexed.

Still had issues, brought it to another LBS, who noticed the bent forks, which for some reason I was unable to see until he showed me. Also, there is a bulge in the frame on the downtube near the head tube. So this baby is bent. Time to see what the guy who I bought it from - who I have bought several solid bikes from, and I consider somewhat of a friend- thinks. Am I a sucker and should lick my wounds? or should I expect some retribution?