BMXroadie

My Canondale R3000 is unstable at high speeds. When I brake at speeds around 40mph or higher, the bike wobles back and forth violently. It feels like some misterious force is trying yank the handle bars out of my hands. When it happens, I have to relax and brake carefully. I replaced my old wheels (the rear rim was shot)this year with Mavic Ksyrum Elites. I thought the new wheels solved the problem, but the other day during a high speed decent, it happened again. I checked the bike over carefully - no cracks in the aluminum welds, no deep scratches in the carbon fork, but my new rear wheel is a bit out of true (guess it needs tuning now that its broken in).

If this problem continues, it's only a manner of time before disaster strikes. The rear wheel is going to the shop tomorrow. should I consider replacing the fork? Any other ideas?

AEO

speed wobble.
https://en.wikipedia.org/wiki/Speed_wobble

009jim

Check for wheel alignment by running string lines down the sides of both wheels while bike is sitting in the normal riding position. You might need an assistant. Do you have disc or rim brakes? Check your forks are not bent and no looseness in the head tube bearings.

Shimagnolo

https://davesbikeblog.squarespace.com...e-visited.html
https://davesbikeblog.squarespace.com...-steering.html
https://velonews.competitor.com/2010/...-poorly_121162

Dan The Man

The simple fix is to press your knees inward so that your legs touch the frame of the bike.

hueyhoolihan

i have had that experience too.

from my reading it appears that it is not all that uncommon and occurs in motorcycles too. it is often described with references to the contributing causes. some of which are rider/luggage weight, frame flex, wheel weight, alignment, etc, etc,..

anyway, you might just try running a heavier rim or tire combo and see if it fixes it. i have had two bikes with the problem, one would occur when putting a different wheel on the front and another one was fixed when i removed some weight (permanently) from the handlebars.

rjr2008

It could very well be the wheel out of true. Maybe be nothing more. It doesn't take much.

pat5319

,
Your fork, frame or steer tube is probably bent or twisted. I'm not sure it can be fixed as Alu (Aluminum) loses strength dramatically when bent. Alu deforms when welded and I'm sure a completely "Square/Straight" Alu frame is virtually non-existent, I've worked on thousands of bikes and have never seen a "Square/Straight" Alu frame nor met a mech who has.
The GOOD news- I had a bike that did the same thing and no one had the frame tools to fix it where I lived, nor could I afford to pay for the fix it if they had. I found that if I put my knee againts the top tube it "settled " down. I rode it like that for years, often exceeding 50 mph with no worries!

rumrunn6

in addition to grabbing the top tube w/ your knees, you can also get out of the saddle (stay low w/o standing) this will lower your center of gravity with your weight on the pedals and front bars.

I used to have a bike that did this when my rear rack was loaded heavy. it was a weird rack that attached on the seatpost and braced w/ low rear eyelets. I tried countering that with a homemade front rack but that was annoyingly bothersome having stuff racked up front.


I stayed away from heavy loading that rack and going 40+ with that bike

good luck and thanks for sharing.

wrk101

+1 Had this with a 1984 Yamaha Venture Royale I had years ago. Right about 45MPH, it got what Venture riders called "The Death Wobble". You learned to accelerate quickly to 50MPH, or slow down to 40MPH. All kinds of home remedies for it. The lesson back then was the wobble could be caused by a variety of things, everything from a poorly adjusted headset, wear on swingarm bearings, weight distribution, etc. I can tell you it was quite scary.

fietsbob

If its a frame misalignment you cannot cure it, because the Aluminum frame
is heat treated in a whole .. to neutralize stresses from the welding sequence..

speedevil 

My experience with motorcycles is that the "wobble" is most often due to an out of balance or or out of true rear wheel.

There's a good chance that truing the rear wheel may minimize it. There are some motorcycles that are more prone to it due to head angle and trail. I'm sure the same is true about bicycles as well - the same forces, head angle and trail all factor into it.

BMXroadie

Thanks for the links. They were very informative. I think the problem is I'm putting my weight to far back. I'm a little scared of height, so on a steep hill my tendency is to put my but behind the seat. Although I'm not a big person (5-10 and 155 lbs.) it would seem this is my biggest problem, since it throws off the balance. On my last ride, I made a point to keep my weight centered. It seemed to work, although I didn't reach a high speed.

vendorz

So... you're suggesting shorter trail might be inclined to become unstable on high speed descent, correct?

Andrew R Stewart 

Ah, speed man's shimmy... I agree with most that others have said. My understanding is a harmonic vibration in the system (the bike/rider). As mentioned, changing the stiffness, center of gravity, the masses, and your shorts will all effect it. The comment about braking at speed inducing it might make one think that there's too much trail... but if it were that simple no bike would exhibit this. After a few millions of bikes before it would have been figured out. But that's not the case. Part of the problem is the repeatability, another rider on the same bike at the same speed, doing the same inducing action might not have the same shimmy. The way that i have dealt with it is to increase the torsional stiffness of my bikes as I build new ones. Funny though that you've got one of the stiffer frames already... like I said if this was understood... Andy.

davidad

https://draco.nac.uci.edu/rbfaq/FAQ/8h.5.html

ultraman6970

Why in the world people has this? Have used several bikes, raced for 12 years but never ever had this problem. The only thing i can think of it is a problem of balancing weight and position over the bike. But the bike geometry is another thing that can contribute to the problem.

AEO

it's pretty hard to get the harmonics just right in everything so that they amplify each other.
that's why they only happen at certain speeds and most people never experience it.

hyhuu

I experienced it once but hasn't been able to reduplicate it yet. It happens when I was coasting down hill at around mid 40 mph on a brand new smoothy road.

lostarchitect 

I had this once. Turned out the headset was loose.

speedevil 

Yes, all other things being equal. As was noted here a misadjusted headset can be part of the problem.

Think of a chopper-style motorcycle - it's very stable to the point that it's difficult to turn (significant amount of trail). Now think of a racing motorcycle, where the trail is minimal. Very maneuverable, but not nearly as stable. Turning is as quick as thinking about turning.

Descents by themselves are not the cause of instability, it may just be that a descent is where you reach the speed that it comes into play. I know that I can't ride fast enough on the flats to see any stability issues.

IthaDan 

Besides loose bearings, check the toe on your brake pads.

humboldt'sroads

Very good. More info than I ever wanted to know about shimmy (which I've never encountered), but very enlightening.

onespeedbiker

Most the posters here are properly addressing a harmonic wobble, but I am wondering if it is something else; the key being it only occurs during high speed braking. If so my first question be does the wobble stop when you let off the brakes or does it continue? and is it less pronounced when "you relax and brake carefully?" If so there may be a problem with your brake pads were on hide is braking harder than the other. This can be the result of worn pads. A dual pivot brake combines a single pivot arm on one side and a canti arm on the other; this usually results in uneven pad wear and if let go for to long can result in one side having very severely wear while the other looks to still have more life in it..