Ritchey Breakaway Speed Wobble

[Ken Robb]

[QUOTE=Litespeed_Mike;1973832]HELP:
More detail:
I have a [year unknown blue/white] Ritchey Breakaway with a Ritchey Comp 1-1/8" headset and a Ritchey Comp Headset.

There is your problem. You have too many headsets.

[ElvisMerckx]

A very reputable frame builder once told me wobble/shimmy (at speed) is usually due to a slight misalignment of the front and rear wheels. Makes sense to me from a general understanding of physics too.

[Ken Robb]

I've had bikes that wobbled. Some just a little and could be tamed by clamping my knee(s) on the top tube and one that wobbled no above 28mph no matter what I did. I sold it to the first guy who looked at it since it only had 500 miles on it and looked new. I told him why I was selling it and insisted that he test ride it before I took his $$. I live close to the top of La Jolla Shores Drive and it's steep enough for me to get to 48mph on bikes that are stable. He bombed down that hill with no wobbles. He looked to be 20-30 pounds lighter than me.

None of my Rivendells or Serottas(CSi and Legend ti) ever wobbled nor does my Hampsten. My Kirk Terraplane and Waterford were fine as well. My Litespeed Natchez wobbled badly when I hit some rippled pavement at 35mph so I sold that one too.

My guess is my two worst wobblers were because the first one's fork was flexy and the second one had a frame not quite stiff enough for my weight. I was about 215lbs at the time.

[Peter P.]

Originally Posted by Ken Robb

My guess is my two worst wobblers were because...the second one had a frame not quite stiff enough for my weight. I was about 215lbs at the time.

Lennard Zinn proposes this as a possible cause for shimmy on larger frames. That's why he builds his taller frames with "dropped" top tubes and uses larger diameter top tubes. The lowered top tube simulates a smaller frame triangle; the larger top tube diameter resists front end twisting forces better. He seems to think shimmy is partially a resonant frequency twisting of the front end of the bike.

[Ken Robb]

Originally Posted by Peter P.

Lennard Zinn proposes this as a possible cause for shimmy on larger frames. That's why he builds his taller frames with "dropped" top tubes and uses larger diameter top tubes. The lowered top tube simulates a smaller frame triangle; the larger top tube diameter resists front end twisting forces better. He seems to think shimmy is partially a resonant frequency twisting of the front end of the bike.

This makes sense to me as my frames are 59-62cm c-c.

[rustychisel]

Originally Posted by Peter P.

Lennard Zinn proposes this as a possible cause for shimmy on larger frames. That's why he builds his taller frames with "dropped" top tubes and uses larger diameter top tubes. The lowered top tube simulates a smaller frame triangle; the larger top tube diameter resists front end twisting forces better. He seems to think shimmy is partially a resonant frequency twisting of the front end of the bike.

Lennard is undoubtedly correct in this, the unresolved issue being what sets it off.

[Mark McM]

Originally Posted by ElvisMerckx

A very reputable frame builder once told me wobble/shimmy (at speed) is usually due to a slight misalignment of the front and rear wheels. Makes sense to me from a general understanding of physics too.

Please explain the physics of this. When I turn, my wheels are out of alignment. Why doesn't my bike shimmy every time I turn?

[RonW87]

Originally Posted by Peter P.

Put me in the camp of, "the bike's too small" and could be a contributor to your wobble.

I disagree.

1. I'm not sure that a bike being to small would contribute to wobble/shimmy. In fact, the opposite may be true.

2. On these bikes the top tube slopes but with the skinny steel tubes (and corresponding 27.2 post), the bike shows a lot of post even with a proper fit. Also, doesn't look like the stem is that long. So I don't think the bike is too small.

R.

[redir]

The only bike in my stable that doesn't wobble is my Moots Compact. The rest of them are large framed bikes. Bicycletricycle summed it up perfectly, it could be any of the things you mentioned and more and there very well may be absolutely nothing you can do about it. That's why I have learned to always touch knee to TT on fast descents... problem solved.

[unterhausen]

Originally Posted by Mark McM

Please explain the physics of this. When I turn, my wheels are out of alignment. Why doesn't my bike shimmy every time I turn?

Tire scrub, and the fact that a turn doesn't last long enough. However, if the frame is likely to wobble, a turn can set it off. I have a late '70s Italian bike that shimmies at speed, and a high-speed turn would set it off every time. Scary. I should check the alignment on that frame, not built up ATM.

I have fixed a bad case of speed wobble by aligning a frame, so I know it is definitely a cause. I scoff at the people who say it's frame flexibility, because the people that have simulated speed wobble said they had to reduce stiffness to the point it would be scary to ride.

I suppose that the fact that the OP's seat is so far back wrt the rear wheel could be a contributing factor. Most bikes will shimmy if there is enough weight to the rear.

[RonW87]

Originally Posted by unterhausen

I suppose that the fact that the OP's seat is so far back wrt the rear wheel could be a contributing factor. Most bikes will shimmy if there is enough weight to the rear.

OP's bike has a no-setback seatpost and saddle is not really pushed back at all. I don't see any rearward weight bias.

[unterhausen]

Originally Posted by RonW87

OP's bike has a no-setback seatpost and saddle is not really pushed back at all. I don't see any rearward weight bias.

I was thinking the same. However, we don't know his weight balance. The fact that the frame is so small and relatively short wheelbase means that it's really easy to get the weight back past the point where the bike's propensity to shimmy is affected. It still probably is an alignment problem, but the weight balance probably isn't helping.

I would like to understand why weight balance matters. But my own bike collection has taught me that it does. My All-City will shimmy if I sit up, but when I'm in a normal riding position it's ok. In fact, it has never shimmied when I was in a normal riding position. Maybe I have never hit its critical speed.

[Mark McM]

Originally Posted by unterhausen

Tire scrub, and the fact that a turn doesn't last long enough. However, if the frame is likely to wobble, a turn can set it off. I have a late '70s Italian bike that shimmies at speed, and a high-speed turn would set it off every time. Scary. I should check the alignment on that frame, not built up ATM.

Please explain how tires can scrub. A tire can scrub if it is not aligned with the direction of travel - but unlike a multi-wheel vehicle, it is impossible to ride a two wheeled vehicle in a straight line without both wheels being parallel to the direction of travel.

Originally Posted by unterhausen

I have fixed a bad case of speed wobble by aligning a frame, so I know it is definitely a cause. I scoff at the people who say it's frame flexibility, because the people that have simulated speed wobble said they had to reduce stiffness to the point it would be scary to ride.

No, you don't know it was the definite cause, if you couldn't repeat the phenomenon. What did you do to control the variables? Where you able to get the bike to wobble again by return the frame to its original misalignment? An anecdote is only evidence of the existence of an anecdote.

[Flying Pigeon]

Hey OP I hope you are enjoying the bike nevertheless. I just rode my Breakaway this morning and love it every time.

For me I've never had a shimmy, and it is a very stable descender. Compared to other road bikes of mine it has high trail (comparably slack head tube and a short offset fork). It descends fine at 100kph+ and carves very well IMO. Although it isn't very stiff especially in the stays.

I don't know what to add here. Yesterday in the Tour of California on TV I watched a Jelly Belly rider get a nasty shimmy while tucking on a descent, so it happens to everybody. My worst was a 753R Peugeot.

I would be curious how a custom fit bike would look for you.

my bike:

[Peter P.]

Some people have wondered what sets up a shimmy condition.

I have a Rock Lobster ATB with a rigid fork.

The bike rides perfectly fine and I have no doubt it is straight.

It never shimmied until...

...I was riding D2R2 a few years ago, the 115k route. There's a wicked fast, not quite straight, asphalt descent down Ed Clark Road. We were warned we could hit 50mph.

As the descent started, the grade took such a sudden increase that my front end unweighted ever so slightly and momentarily. The bike immediately began the Watusi, something it had never done before. My two (@ 30lb. heavier) buddies, riding similarly equipped rigid mountain bikes had no problem.

I attributed it to the sudden rearward weight bias on the bike and whatever that did to the unweighted front wheel. I immediately clamped the top tube with my knees and the bike recovered, whereupon I let go of the brakes and got back up to some ridiculous *****-eating grin speed.

I also have a bike with a rear rack. I used to commute with a small mechanic's tool bag secured to the top of the rack which weighed probably 7lbs. . With such a high center of gravity load on the bike, riding no-hands was almost impossible because the bike would shimmy. Riding with lower, rear panniers installed however, was not a problem.