Road Cycling - Downhill Speed Wabbles

I am a relatively new road biker, i have done alot of mtn biking. On my rides when going downhill every time I go over 35mph the front end starts to vibrate. This gets really bad and I am only just staying on the bike. It has never happened Mtn biking even when I am going this fast. Like when I was a kid on a skateboard going to fast downhill. Any ideas on why this is happening and how to stop it. Oh one guy at a bike store had never heard of this happening before. Is it just me?

henders-

It is not just you... it is a phenomenon called "shimmy" and it can be EXTREMELY dangerous. It typically happens in larger frames. A few things to check- make sure your hubs are tight, and also make sure you dont have reflectors on your wheels, as this makes it worse.

Also, make sure that when you are descending, you stay loose. When you are loose, your body acts as a damper, which will minimize the vibrations. If you tense up, you dont have that damping effect, which will worsen the vibrations. When the shimmy starts, try to slowly apply your brakes to slow down, and use your knees to pinch the top-tube of the frame. I have found that this helps immensely.

I have a frame that will develop shimmy at about 43-44mph, if I am tense.. if I am nice and relaxed, there is no shimmy whatsoever. When I first got the bike, it had reflectors on the wheels, and I would develop shimmy at any time above 36-37mph..

G'day,

I pretty much agree with Blue Devil.....& if the character at your LBS has never heard of this b4....change bike shops!,

cheers,

Hitchy

is your frame straight? your hubs good? lift up your bike and see if the wheels have lateral play

Thanks for the replys, the hubs seem to be good, no lateral play. Its a 61cm frame and I am 6'2. Yes the first time I riding I hit 46mph started to apply breaks and it just went wild. I will try relaxing. Easier said than done, now though. This does not seem to happen to everyone, and I see guys on big frames all the time, are they all just really relaxed. I mean how do you stay relaxed in a pelaton down hill at 50+mph?

Thanks for the replys, the hubs seem to be good, no lateral play. Its a 61cm frame and I am 6'2. Yes the first time I riding I hit 46mph started to apply breaks and it just went wild. I will try relaxing. Easier said than done, now though. This does not seem to happen to everyone, and I see guys on big frames all the time, are they all just really relaxed. I mean how do you stay relaxed in a pelaton down hill at 50+mph?

There have been several threads on this subject, this being the most recent:
http://www.bikeforums.net/showthread.php?t=54327&highlight=descending+wobble

I haven't experienced the problem myself but I tend to stay in the drops and grip the top tube with my legs - which might be attributed to a high "pucker" factor. ;) Seriously, I feel that I have much more control in the drops.

Braking lightly might also be a key.

what kind of fork is on the bike? if it is not a carbon fork, you might want to try getting one and trying it. :)

It sounds like the "shimmy" actually started when you applied the brakes? If that is the case, definatly check your wheels and/or brakes. When I have had it, it was at max speeds, usually in the wind for some reason. Applying the brakes and gripping the toptube helped it go away.

fogrider makes a good point, the type of materials in the fork and elsewhere are worth looking at.

Some materials absorb stresses much better than others, perhaps you have a harmonics problem at certains speeds. Like that famous bridge in Tacoma Washington.

Henders, I am about to commit sucide here on this forum, but what the hay, I've done that before!

If your hubs, headset, rear wheel alignment in the stay and frame is straight then there can only be two other problems. One is a wheel is out of balance, similar to a car vibrating at certain speeds (usually a 5 to 10mph window) causing a harmonic imbalance. But bike wheels are so light that this rarely happens. BUT to check deflate your tires and move the tires 180 degrees (you may have to remove the tube to do this). Then retest and see if it reoccurs. Also make sure your computer magnet is located on the opposite side of the wheel from the valve stem. If none of this does the trick then there's another issue.

The reason your mtb did not shimmy for is because the mtb's use a beefer frame. In the days gone past, except for todays custom made bikes, they use to use lugs to assemble the frame. The extra weight of the lugs was worth it because it added support to the frame; I have never heard of a lugged steel frame vibrating-no matter what size the frame is; this is a newer phenomena that has occured over the last 15 years. All that is supporting todays frames is a small bead of welding, and this offers little support for the frame. So what is happening at certain speeds this vibration or shimmy sets up due to the lack of frame support and the way to control it is to clamp you knees against the top tube, in effect providing the support the frame lacks.

A rim oscillating due to wind striking the wheel at an odd angle is hoey. I got 36 spoke rims that grab a lot of air, and I've ridden on windy mountain roads and through strong pass gusts and exceeded 60mph on the bike and never experienced anything like that in 30+ years of riding, nor heard of anyone that happened to back in the lug days.

Anyway that's one jerks opinion!

What rims are you riding? I have had a problem like that with deep aero rims (cxp33) just a thought....

Phil

Thanks for evryones replys. I looked at the other sites recommended. It sould not be the fork, its carbon so I'm assuming it could not be out of line. I will rotate the tire 180 to see about this. I did notice that the front brake was slightly out of line. The left side would be applying more pressure than the right. Could this be a factor? Well I fixed it anyway so we will see.

Ditto Blue Devil's reply. I also find that being cold/shivering on a descent will cause a shimmy because it's so hard to relax.

I almost always keep a knee on the TT when I descend.

I had the same problem with downhill high speed shimmy this spring. I had the front wheel rebuilt/trued and it helped a little but I still had the shimmy. My LBS guy took the fork in to the factory (only 30 minutes away) and had them put it on a jig to check it... shure enough, it wasn't straight (one dropout was a little forward of the other. They trued the fork to specs and I've had no problems since. Had a 36 mph downhill today, smooth as glass.

My riding partner was recently experiencing a consistent down hill wobble. We ended up tightening his headset a bit more and the shimmy is no more. (so far)

I have never heard of a lugged steel frame vibrating-no matter what size the frame is; this is a newer phenomena that has occured over the last 15 years.

Not meaning to hijack a thread but my Serotta Colorado II with lugged steel vibrated horribly at around
40 MPH. Never did isolate the problem. OTOH, I have never had an aluminum framed bike, new or old, that oscillated...

I think it would be an interesting survey, on another thread, to collect the data on what kind and the manufacturer of bikes that have experienced the dreaded shimmy.

If your brake pads were not centered--if one hit the rim before the other--that would push the wheel slightly over to one side when you apply the brakes, and I imagine it could cause a shimmy that starts when you brake. Stand next to your bike and squeeze the brake lever. If you see the wheel move to one side, grab the calipers and twist slightly to center the pads, or use the centering screw on the calipers. I bet, though, relaxing, avoiding a death grip on the bars, and keeping your elbows bent and your shoulders loose so your upper body acts like a shock absorber will help calm the bike a lot. If descending fast makes you tense up, then slow down. It's good to work within your comfort level, and if you want to extend your comfort level, do it a little at a time.

Henders, I am about to commit sucide here on this forum, but what the hay, I've done that before!

If your hubs, headset, rear wheel alignment in the stay and frame is straight then there can only be two other problems. One is a wheel is out of balance, similar to a car vibrating at certain speeds (usually a 5 to 10mph window) causing a harmonic imbalance. But bike wheels are so light that this rarely happens. BUT to check deflate your tires and move the tires 180 degrees (you may have to remove the tube to do this). Then retest and see if it reoccurs. Also make sure your computer magnet is located on the opposite side of the wheel from the valve stem. If none of this does the trick then there's another issue.

The reason your mtb did not shimmy for is because the mtb's use a beefer frame. In the days gone past, except for todays custom made bikes, they use to use lugs to assemble the frame. The extra weight of the lugs was worth it because it added support to the frame; I have never heard of a lugged steel frame vibrating-no matter what size the frame is; this is a newer phenomena that has occured over the last 15 years. All that is supporting todays frames is a small bead of welding, and this offers little support for the frame. So what is happening at certain speeds this vibration or shimmy sets up due to the lack of frame support and the way to control it is to clamp you knees against the top tube, in effect providing the support the frame lacks.

A rim oscillating due to wind striking the wheel at an odd angle is hoey. I got 36 spoke rims that grab a lot of air, and I've ridden on windy mountain roads and through strong pass gusts and exceeded 60mph on the bike and never experienced anything like that in 30+ years of riding, nor heard of anyone that happened to back in the lug days.

Anyway that's one jerks opinion!

Suicide? ok your dead :)

I think I remember a conversation like this before and My old tange 2 lugged frame set shimmied like the devil at 51mph and scared the bjesus out of me. Now granted this is a touring frame and I probably had no business going 51mph on it but non the less it is lugged built in 1985 and shimmies. Having said that i did remove the rack and then on fast decents always cinch the TT with my knees and have no problems. I no longer ride this bike fast and don't care to ever test the situation out again. I think that all the aformensioned situations can cause shimmy but I like the first response about the frame size. Mine is a 61 also and if you think about it the bigger it is the more flex it potentially has. All structures have there resonance frequency and so if you hit that on your bike you amplify it and bam there is shimmy. It may have nothing at all to do with a head set or untrue wheel but then again mabey it does? I can tell you this, I now have an AL/Carbon frame and it is rock solid at 50+ mph...

Have had hi-speed wobble (harmonic vibration) on an aluminum bike, a titanium and a full carbon fiber bike.
The only time it caused a crash was last year on my ti bike at 36 mph coming down a canyon (smooth pavement); sat upright to catch the wind (and slow down a bit without touching brakes) and clamped both knees to the top tube, to no avail. Finally gently applied rear brake; oscillation on the bike dramatically increased and crashed at 30+ mph, breaking shoulder in 2 places.
Reason/solution for the problem is difficult; seems to be a combination of bike/rider/road/weather circumstances. Have experienced this on very windy but also calm days. Hi-speed doesn't help, but also experienced it at about 12 mph!
Try installing a wider front tire, which would increase the trail of fork somewhat; that could alleviate part of the problem. So if you're using lets say a 23 mm tire go for a 28 mm or even wider if the fork clearance alllows.
Had harmonic vibration with a brand new carbon fork (after 4,000 miles of no problems). Now using a carbon fork with a bit more rake. Time will tell.
Have ridden extensively for over 30+ years and it's absoltely scary when it happens!

i recently went to the tour de france and had the wobbling coming down steep mountains. several times i was at the edge of wiping out. i am sometwhat scared to bike downhill now. i have a frame with a short wheel base. a mecanic at my lbs said it was from the short wheel base. i am considering getting a new frame. a new frame is cheaper than a trip to the emergency room and the doctor.

any comments

I wouldn't be caused by a short wheelbase. As one poster mentioned his was a touring bike which has a long wheelbase and his vibrated. I think this whole shimmy business for the most part is something that has only been going on for the last 15 years. So I asked my bike shop mechanic about it and he said they first go through all the possible mechanical causes: headsets, hubs, improperly adjusted wheels, bad tires, and even frame alignment (surprising, they have found bikes new out of the box misaligned with no box damage, just poor factory alignment and the majority of those misaligned frames come from Taiwan and China). When they have eliminated all the mechanical possiblities they then know it's one of two things, either the newer wheels with fewer spokes (like Shimano) is allowing the wheel to vibrate at high speeds or it's the frame. Most Aluminum bikes "should" not suffer from this due to the frame because of their larger diameter tubes should be stiff enough to prevent it; so if this occurs on a AL bike they recommend trying new wheels and this solves about 95% of the problems. But the LBS said that they seem to be seeing this more and more as the years have gone by, and could be (also) related to frame tubes getting thinner and thinner to reduce weight so they can sell all of us that light weight bike we need to have.

The reason Mckenna experience the frame vibrating was probably due to the larger 61cm frame he rode on, even though it was steel with lug construction it still did not have enough support to prevent it due to it's larger size. In the average and smaller frame sizes (58 and less) I cannot recall, nor could my LBS, having this problem with steel lugged bikes prior to 1985 that could not be resolved by adjusting a mechanical problem.

I would venture to say that if frame construction has something to do with some of the reported vibrations, then it may be possible that even todays custom made lugged steel bikes could also vibrate IF they are using the thinnest (lightest) tubing available.

It would be interesting as Mtnman said, to see a survey of which manufacture experience shimming by riders and what frame material was used; we might find a interesting corelation.