# Bicycle Mechanics - High(er) Speed Stability Problem

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View Full Version : High(er) Speed Stability Problem

alberg37
09-27-04, 08:25 AM
In rolling hills, I used to take it easy on the downhills with the idea that I was resting for the uphill portion. Then I started riding a fixie some and learned to appreciate momentum. Now on my geared road bike I tend to keep the cadence up downhill, bring up the speed, and have an easier time on the unsuing uphill. Bottom line is that I am reaching higher speeds than I used to.

Here is the problem: at speeds around 32-33mph, the rear of the bike starts oscillating. I can brake to bring it under control, but its not comforting. Moving my weight a bit forward on the saddle also seems to help.

Judging by distance from the brakepads, the wheels appear to be within a mm of true. Is that too far out? What is a reasonable expection for a safe maximum speed on a descent?

The bike in question is a 1982 531 steel Holdsworth Special, kind of an in between road racing -training bike, 73/ 73 angles, 40" wheelbase. Never abused, no crashes.

Thanks

Tom F.

John Ridley
09-27-04, 09:20 AM
You could be starting to experience balance problems. Put the bike on a rack, put it in a high gear, and spin it up to a high speed, and see if the whole bike starts to move in time with the rotation of the wheel. If one side of the wheel is heavy, the bike will tend to move forward when that bit moves backwards, and vice versa, as the wheel tries to keep its center of mass in the same position.

You may be able to trim this by messing with spoke reflectors, trimming them in or out, or using lighter/heavier reflectors to counter the valve stem weight.

dragracer
09-27-04, 09:39 AM
I have the same problem and posted about it a while back (RIGHT HERE) (http://bikeforums.net/showthread.php?t=59576). I thought it maybe had something to do with the carbon spokes on my Spinergy wheels but I don't really know. It just seems very unstable at speeds above 30mph. I've just learned to live with it and use very gentle pressure on the bars when I have to make steering corrections at speed. It's fine as long as I don't make any quick moves one way or the other. Beats the heck out of me what makes it "wiggle" but you are right....it's NOT a good feeling.

Balance
09-27-04, 12:14 PM
Check that your wheel doesn't have a hop/flat spot in it. Also check that your tire isn't a bit deformed. This really shouldn't be happening, and you should get it solved. That's not safe. You should be able to bring your road bike over 50 miles an hour and still be solid.

sorebutt
09-27-04, 01:06 PM
If none of the above suggestions work....

I assume that you rear hangers are the horizontal type (1982 vintage). If that the case, make sure the rear wheel is perfectly centered and aligned with the frame.
My bike has horizontal hangers and at one point my wheel wasn't evenly in. This causes the bike to pull to the side and the front wheel is trying to "chase" it and keep it straight.. This feels like a shimmy or a wiggle.

To test if this is your problem, ride with no hands down a hill (no pedaling) and will see that your front wheel shakes from side to side..

If your wheel looks like it is centered and it still not tracking right, you may have a bent frame...

jeff williams
09-27-04, 01:12 PM
I would guess your wheel is not true. Imagine a wire under tension that is knocked, the vibration will travel the lenght of the wire and return to point of impact. If then a continued 'knock' at the same interval will add more energy to the wave or oscilation.
The entire bike would be the wire. The problem- your rim, or an imballance in rotational weight. IMO.

P.S..don't bug me about spelling, I've had 4.5 hours sleep and back working.

CPcyclist
09-27-04, 09:22 PM
Have you ever been able to ride thur the wiggle?? Had this happen on one bike around 50mph but smoothed out at 51mph only had to worry about the way back down. like jeff indicated think wire and harmonic resonance it could be that at 32-33mph the wiggle/wave from your wheel is resonating and amplifing thur your frame.

with my cuurent bike with the wheels I built up myself I have gone 50-57mph several times with smooth sailing all the way.