24tracktape

Okay, this Guerciotti used to be fine going down any hill at any speed (and I'm in Southern Vermont where there are some pretty long descents).

I switched to Schwalbe Ultremo R 23mm from the Vittoria Rubino Pro tires I had been using recently, and it now it seems like the bike just wanders (a very low frequency shimmy, sort of like a random crosswind type feeling). It's very disconcerting, and I feel that I am not able to trust the bicycle anymore on the steeps. (I'm not having this problem on my other bicycles, by the way)

Wheels are Campy Ventos, and I'd say there is less than 1mm of vertical runout on the back wheel and less on the front wheel. Laterally they are almost perfect. I've not yet trued them to be perfect, but can a small amount of runout cause high speed wobbles?

On the other hand, an a workstand, i can definitely notice some "pulsing" that can be felt through the frame when the back wheel is rotated at top speed (in top gear). Something out of balance? I'm using Michelin Latex tubes...if that makes a difference.

I've already checked the headset (Chris King) and the alignment of the wheels in the dropouts. Ditto the tension on the hubs and quick releases, and there is no interference with the brakes/frame in any way.

I'm really thinking tires or vertical runout, but what other options are there that I might have missed?

Is it possible that the contact patch of the Ultremo R 23mm is just much smaller than the Rubinos and I'm just not being accurate with my balance?

Thanks for your input!

-Rich

coldfeet

There is a recall notice on some of these, may be your problem.
https://www.bikeradar.com/news/articl...-r-tyres-21793

24tracktape

Thanks, I was aware of that recall, but mine were produced after the bad batches according to Schwalbe (they assured me mine were fine, and the tires do not exhibit the bulging defect).

I appreciate your response, however.

Best-
Rich

Hydrated

Did you check the condition and adjustment of your hubs? I had the same "wandering" problem on one of my bikes and it turned out to be poorly adjusted hubs.

edit: Oops... sorry... I just read your post again. Saw that you'd already checked the hubs. Sorry.

24tracktape

The hub seemed fine when I checked them, but when I pull the wheels to true them, I'm going to make sure.

Another thing I realized is that I took off my cyclocomputer and associated magnet. I wonder if the magnet could have been balancing the front wheel?

FBinNY 

Total runout doesn't always tell the whole story.

Think of a wheel with runout as a large cam. The shape of the cam makes a difference. A cam with a long slow rise will gently lift and lower the axle once per revolution, while a cam with a short rapid rise will cause a pulsing feeling. Given that you say you can feel the vibration, I suspect your runout is more of the latter and could be the cause of your shimmy.

Tire pressure could also be a contributing factor, especially with out of true wheels. Ultra high pressure exaggerates the effects of the cam by reducing the vertical compression of the tire. If for example you have a rapid rise and fall cam, the wheel will partly lose contact after the high spot passes. A softer tire would absorb some of the height change and mask the cam effect.

Note too that while wheels are aligned based on the rim, you roll on the tire, so even a perfectly aligned wheel with a poorly made, or poorly seated tire can have runout at the point of contact and cause handling problems.

Lastly, check that the headset is tight but free, and likewise the hub cones. Free play on either the hubs or the fork, or an overly tight headset can also cause problems like yours.

DannoXYZ 

Check your headset bearings too.