High speed wobble

I know what you mean! Our group had completed a 54 mile loop and on the last leg, a down hill, I was doing somewhere around 40 MPH and gaining when all of a sudden my bike developed a shimmy. I thought for sure I going to go over the side of the hill. I was able to apply the brakes and stop with out blowing out a tire or going into a skid. I was rather nervous for the rest of the ride, but it didn't happen again. Never figured out what the $%&@* happened or what caused it. At the time my weight was 180. So who knows.

Quoted post said:

Originally posted by kranz
Hello All-

This is my first post on this forum. I'm hoping y'all can help me make some sense of a rather un-nerving experience I had this week.

I pretty much exclusively ride on a former railway right of way with very gradual grades. So I don't have much experience with hills. But earlier this week I rode my #1 bike on the road to my mechanic's to pick up my car that had been in for service.

I coasted down a hill which wasn't particularly steep but was rather long and picked up what for me was quite a bit of speed. I suspect I was easily doing 30+ mph, perhaps more. Everything was fine, but suddenly without warning, the bike started to wobble uncontrollably. I managed to brake to a stop with the bike continuing to wobble till almost stopped. It was almost like the bike hit some sort of resonant frequency and then started to oscillate. Needless to say, it scared the bejeezus out of me. I inspected the bike and could not find anything obvious. No play in the axels or headset. Axels firmly attached to the frame. Frame appears straight and intact. I climbed back on and finished my trip without further incident. I even went down another hill, although not as steep and not as fast.

My bike is a late 70s Raleigh Supercourse that I have just refurbished. All axels, headset and bb have been rebuilt. New 700c x 28 Continental Ultra 3000s w/wire bead on Weinmann concave rims and maillard low flange hubs. Rims reasonalbly true. I normally inflate these to 100+ pounds. I am a heavy rider @220 pounds.
I didn't check tire inflation before I left as I had inflated the tires a couple days before and they seemed quite firm to the touch.

Does anyone have any ideas about what might have caused this. I am a bit nervous about repeating this incident and want to be sure I am riding safely. Any ideas, advice and/or conspiracy theories appreciated.

thanks,

jim

What was the road surface like? Is it a road that would have a bit of heavy vehicle use. (Like large trucks for example.) Other than that, I can't think of anything.

Quoted post said:

Originally posted by Ratty
What was the road surface like? Is it a road that would have a bit of heavy vehicle use. (Like large trucks for example.) Other than that, I can't think of anything.

Road surface was well maintained asphalt, quite smooth. Road is heavily travelled mostly with commuter car traffic. Incident occurred just before mid day on a warm humid day.

Quoted post said:

Originally posted by concord
I know what you mean! Our group had completed a 54 mile loop and on the last leg, a down hill, I was doing somewhere around 40 MPH and gaining when all of a sudden my bike developed a shimmy. I thought for sure I going to go over the side of the hill. I was able to apply the brakes and stop with out blowing out a tire or going into a skid. I was rather nervous for the rest of the ride, but it didn't happen again. Never figured out what the $%&@* happened or what caused it. At the time my weight was 180. So who knows.

Concord- Did you continue to ride that bike under similar conditions after that day?

I had a friend who experienced the same exact thing you described. He attributed the problem to a bad carbon fiber fork and the problem went away after replacing it. No one has told me a decent explanation of how that happens but I theorize it has to do with slight misalignment of the wheel/fork interface causing resonance at high speeds.

Here is a clear explanation of the problem and some tips to minimize it:

http://www.roadbikerider.com/yourturn2.htm#Speed%20Wobble

I hope it will be helpful.
Good luck!!!

Quoted post said:

Originally posted by cuervo
Here is a clear explanation of the problem and some tips to minimize it:

http://www.roadbikerider.com/yourturn2.htm#Speed%20Wobble

I hope it will be helpful.
Good luck!!!

Thanks Cuervo, that link is really an eye opener. My bike and I will be better prepared next time I go charging down a hill.

jim

Quoted post said:

Originally posted by kranz
Concord- Did you continue to ride that bike under similar conditions after that day?

Yes indeed I did and have. As a matter of fact I have done this ride (same route) more than two dozen times and it hasn't happened again, but it freaks me out every time I start that down hill... so far so good.

This has happened to me on two quite different bikes, a Motobecane Grand Jubile (touring) and a Centurian Ironman (racing style geometry). I always keep my head sets tight and my hubs and wheels in good shape so I was mystified as to why this occurred. In a way I am relieved to see that it happens to others as well, and that there seems to be no sure answer to its cause. I prevent it from starting by NEVER making any erratic motions on a fast decent, not even to shift. I keep the wheel steady and my body well balanced. I do my best to soak up bumps like a shock absorber. For me this prevents the onset.

My guess is that my wheels, while true and adjusted, can still be out of balance, just like car tires. If the out of balance front and rear happen to coincide things get hairy.

I've been riding for over 35 plus year and rode on steep mountains where the downhill speeds reached 60mph on all type of road surfaces and never experienced any high speed wobbles. Why this happens to some and not to others is strange. How tall are you? I have known riders that rode on 60cm plus frame sizes having those issues-but only rarely and you said it only happened once, so I wondering if your tall and frame is the issue. But your doing good by reading that RBR website and try to prevent it or stop it the next time it occurs.

kranz said:

My bike is a late 70s Raleigh Supercourse that I have just refurbished. All axels, headset and bb have been rebuilt. New 700c x 28 Continental Ultra 3000s w/wire bead on Weinmann concave rims and maillard low flange hubs. Rims reasonalbly true. I normally inflate these to 100+ pounds. I am a heavy rider @220 pounds.

FWIW. I think that your (front wheel's) spoke tension may be the problem ...

How are your wheels laced -- how many spokes per wheel AND with what type of spokes are they laced with?

alfeng said:

FWIW. I think that your (front wheel's) spoke tension may be the problem ...

How are your wheels laced -- how many spokes per wheel AND with what type of spokes are they laced with?

There have been some reports, howbeit limited, that bladed spokes caused similar problems. And also, again limited reports, that radial laced wheels had similar isssues.

cuervo said:

Here is a clear explanation of the problem and some tips to minimize it:

RBR reader feedback, p. 2

I hope it will be helpful.
Good luck!!!

Thanks for sharing the link - very helpful 🙂

beekeeperfred said:

This has happened to me on two quite different bikes, a Motobecane Grand Jubile (touring) and a Centurian Ironman (racing style geometry). I always keep my head sets tight and my hubs and wheels in good shape so I was mystified as to why this occurred. In a way I am relieved to see that it happens to others as well, and that there seems to be no sure answer to its cause. I prevent it from starting by NEVER making any erratic motions on a fast decent, not even to shift. I keep the wheel steady and my body well balanced. I do my best to soak up bumps like a shock absorber. For me this prevents the onset.

My guess is that my wheels, while true and adjusted, can still be out of balance, just like car tires. If the out of balance front and rear happen to coincide things get hairy.

I had the same problem with a late 80's vintage Centurion Ironman.
My weight was similar to yours at the time. For me, the problem occurred if I sat up and rode no hands, or otherwise unweighted the front wheel. Same downhill coast.

I suspect the OP changed the tuning of the steering, by reducing the mass of the rims and tires, or changing the stiffness, or spoke tension.

This situation is DANGEROUS. I know people who have crashed from this.