Bicycle Mechanics - Front end wobble

?Open question to all.?
I have a front end wobble when I sit up in the seat.:confused:
I am riding a late 80's Miyata touring bike, all CroMo & lugged. It is an old bike that was never ridden until two years ago when it was given to me. The bike was vitually brand new, with only dust on it. No scratches or paint chips so I'm sure it never had any frame damage, ie bent frame.
It rides very sweet when I'm on the bar, but sometimes when I sit up the front will wobble. It seems to occuring more consistently recently. My first suspicion was weight imbalance, too much weight in the back and not enough in the front. I use a rear pannier and it seemed worse on the ride home when I have more clothes in the pannier, therefore heavier. I noticed when I sit up and it starts I put a hand on the top tube and push down it would stop. So I moved the panniers as far forward as possible and remove a spacer in the rear dropout to extend the wheelbase and it still occurs. Could it be related to the headset?
Any ideas?
Thanks
Steve

yes.

The first thing to check is that the front wheel is true. I think this may be dynamic interaction between the front end assembly and the spring frequency of the frame. The vibration can usually be damped by pressing one leg against the frame.

You can change the natural frequency of the front end assembly by moving its centre of gravity forwards or backwards. Try putting a vice- grips on your handle bars (gently), first pointing forwards and then pointing backwards and see if it makes any difference.

D*Alex, I'm sure you have lots of ideas. But can they get past Joe?

AndrewP, Thanks. I think the front wheel is true from my observation, but it is due for a professional truing. I will try your idea with the visegrip to change the front end frequency. It is not a consistant problem so I hope I don't have to ride around with it on for too long. It that does seem to rid the wobble, what would be a long term fix?
Steve

re: D*Alex > yes

Its time to rebuild your headset.

So, do you guys think that his headset is loose? Or is something else wrong with it? Do you have any suggestions for testing the headset prior to taking it apart and rebuiilding it?

Maybe, maybe, and take it to a mechanic.

http://draco.acs.uci.edu/rbfaq/FAQ/8h.5.html

The headset is a good idea so is checking your hub to make sure it's adjusted properly. But the first thing I would do (because it's the cheapest) is to make sure the wheel is balanced by moving the tire a 1/4 of a turn on the rim and retest to see if the wobble get's worse or better (mark your rim and tire first so you can get back to where it started), if the wobble changes for any reason when the tire is moved than you know you have an out of balance situation and you may need a new tire.

That "old" Miyata is a good bike. I believe it is at least a triple butted frame, the Japanese where making some very good steel frames in those days.

Check BOTH hubs for proper adjustment. A bad/loose rear hub caused bike shimmy for my daughter when she was descending at speed. I ended up replacing the hub as it wasn't repairable. (Actually, I let my daughter rebuild the wheel while I adviced and demonstrated and checked.)

Thanks all for the advice. I still haven't figured it out. Being an intermittent problem I haven't been able to reproduce it in my last couple of rides.

Sakarias - I'm pretty sure the hubs are fine. The rear was just rebuilt and wheel trued. The front may need truing. It is a possible culprit.

Froze - I will try rotating the tire on the rim soon, but they are new Hutchinson Excel tires & new tubes and the problem existed intermittently with the old tires. And yes, this is an older Miyata with "Miyata exclusive Splined Triple Butted Tubing" with a cro-mo fork and touring geometry. A real nice commuting bike.

Bikerider - That link is very interesting. I want to believe that this is the situation and that this is normal for a well balanced springy steel frame. Only I haven't been able to repeat the problem consistantly this week. I got the wobble to start for a short period and I checked the speed, 13.2mph. Then I could not get it to repeat at that speed. Later it started at 14.4mph but didn't continue. That doesn't follow the speed specific theory very well. But I do think road irregularities do tend to stop the harmonics.

1oldRoadie - It may be the headset. It is currently properly tightened but it was loose for a while and I didn't realize it. It appears fine, no looseness or feeling of worn bearings.

AndrewP - I haven't tried the "vicegrip" trick yet. I keep forgetting to bring it on my commute. Not a normal comuting item.

Thanks again for your input. If I ever get a conclusive answer I will post it.
Steve

Originally posted by sakarias
Check BOTH hubs for proper adjustment.

Yes. Similarly, loose front or rear spokes can also cause wobble. (Loose left-side rear spokes caused my whippy 1959 Capo's most recent bout of oscillation.) I can also trigger oscillation by overloading the rack over my rear wheel.

Your wobble could be caused BY LOTS OF THINGS. You could completely overhaul all the parts and straighten the frame and still have the wobble, although it wouldn't hurt to try. You may just be "set-up" in a poor "position" on the bike.
You could quit worrying about it and just stick your knee against the top tube when it happens to stop it.

Ride Straight
Pat

I'm with Pat. Check everything... can't hurt. But my understanding is that many front wheel oscillations at speed x are just a fact of life. Don't try riding no-hands at that speed, or use your knee, and worry about something else.