High Speed Wobble - Causes & Cures
As many of you may know, a couple of weeks ago I toppled off my bike at high speed and ended up in the hospital with a fractured hip socket (acetabular).
I was entered in an event called Death Ride near Lake Tahoe California and had even got credit for finishing. The 15,000 feet of climbing and 5 mountain passes took me just over 8 hours. On the way down the last peak, heading toward town for post race activities, my front wheel started to wobble uncontrollably for about 10 seconds, causing me to go flying over the handle bars.
I'm home happy and healing very well and trying to figure out what caused the high speed wobble and what I would do differently next time in the 10 seconds I had to come up with a solution.
It seems the more I read about it, the more complicated it appears.
There are a couple of articles here and here
I was thinking it may be a mechanical issue, but many of the articles appear to rule out that cause. My bike is about 5 years old, mostly original parts, with the exception of handle bars, pedals, saddle and rear wheel. It's a Cervelo soloist (S1) aluminum frame, known for it's great handling. I was thinking of taking it to the LBS and having the front wheel replaced and having the forks and other front end components looked at and possibly replaced. Now I'm not so sure.
As to bike handling, I ride quite a few hills, live next to a 3,000 foot mountain and am comfortable climbing and descending. I typically hold down hill speeds below 35 mph as I'm kind of a chicken past this speed and I believe age and kids have dampened my need for adrenaline rushes. I always descend with hands in the drops and fingers close to or on the brakes. I have gone in excess of 45 mph at times, but very rarely and only on very clear roads with great visibility and no traffic.
On this particular day, I had already descended over 14,000 feet very comfortably. When the wobble started I was at about 35 mph going straight down a very long hill. I applied brakes quite hard. It didn't help so I let go of the breaks and put my knees to the top tube to try and dampen the vibrations. No luck either. I kept my knees tight and then started feathering the brakes without success. By the time my 10 seconds were up, I was probably going over 40 mph.
So my long winded approach to the question; has anyone ever had this happen to them and did you manage to correct the wobble before crashing? If so, what did you do? Any input as to causes? Is this a rider error or can the bike/components be at the heart of the issue.
Any and all thoughts gladly accepted!
Tom
Comments
Tom - glad to hear you're doing well. I have actually thought about this a little recently. We get alot of front end shimmying around here descending down mountain roads. It is most prevalent when the sides of the road are tree lined, there is a cross wind and you have any sort of deep rim. Whenever we hit an open spot in the trees the wind blows the front end all over the place. I also have thought it may be caused by wheel balancing (such as a car wheel with a weight on the rim). Is it possible to be going so fast that the wheel gets out of balance and starts to shimmy? I mostly do what you did - sit up and feather the brakes until it comes under control. Other times I just hang on tight and get low but it's mostly my comfort level on that day. Just a couple thoughts; interested to hear what others have to say also.
I used to experience this a fair amount, back when I wanted to see how fast I could go. We've got several really long descents, like from 5500-6500 feet down to near sea level. I heard a lot of explanations for the wobbles, and I read those links, but I don't think anybody really knows what's causing it. I bet it is some sort of harmonic phenomenon, in the sense of several different recurrent vibrations adding to each other. Seeing your post made me realise I don't get this anymore, and I think there are several reasons, mostly things which made me go slower down long hills. If I want to go 45+ mph, I'll do it on a short 300' hill, one where I know the run out and traffic patterns. Anything longer, I stay under 35 mph d/t: (1) being in several races - e.g., IM Calif 2001 - where guys died going downhill; (2) riding down mountains with my wife on a tandem - if I want to stay married, we go slower, and got a disc brake for her; (3) Mountain biking in Xterras, which showed me just how bad people can mash themselves up by not paying attention on downhills; (4) just getting older, and worried about the strength of my bones, and how long it might take to heal; (5) doing more riding on my TT bike - even mountain descents - which makes it real obvious when instability is starting. And maybe, I've got a better sense of fore/aft body stability, coupled with the benefits of keeping one's center low when going fast downhill. SItting up on the hoods will slow me down, but also make me more unstable.
Advice for the future? None that will help you avoid the problem or go faster. Anything over 20 mph becomes increasingly dangerous down an endless hill, and really, we're not getting paid for this, so why risk anything?
Oh Tom, I know that terrible feeling. I used to be a fearless descender until I developed a shimmy going down Marsh Hill at Timberman; scared the living sh*t out of me, for years. As you know all too well, at those speeds and grades when you fall off you don't just tumble to a stop; quick unconsciousness is the only mercy.
I, like you, tried to read up on the subject, only to be stymied by conflicting "expert" opinion. Here's what's worked for me: clamping the top tube with my knees, sliding back to unweight the front end, doing the seemingly impossible: relaxing i.e. not trying to control the thing through muscular effort.
I finally conquered the downhill dragon at the race we met at: IM CdA 2009. I was able to roll a few of those downhills at better than 46mph.
In the randoneurring world many bikes have been made better by switching to needle bearing headsets, despite the naysayers.
I hope your healing is swift and complete, both physically and emotionally.
Tom, I have been were you were descending a big grade flying like the wind and bang out of nowhere you get a vibration from the front that takes over the whole bike. This happened to me twice on an assault on Mt Mitchell century ride in NC.
Once on my road bike, going downhill about 40mph, the front wheel was jumping left-to-right about six inches. The very next day on my tri-bike down a different hill, same thing with the front wheel. And, most recently, at Toc 2010. The rider behind me was convinced I was going down.
The harder you brake, the worse it seems to get. The knees on the top tube helped a little, but not a whole lot when 200lbs (me) + gravity were working against it. Relaxing is easier said than done for sure.
I understand the theory of harmonic vibrations, but to have them on two different bikes, under similar situations, kinda leads me to believe it's also user (me) error.
Knowing that it scared the crap outta me, has severely limited my descending speed - and confidence - especially on crappy road surfaces and blind corners. Scary stuff.
I freaked out, especially since the forecast was for winds on race day. The wobble hadn't happened without the wheel cover during camp. I read the Sheldon Brown article linked to above. I used my regular front wheel and took off the wheel cover. I also had the race mechanic check out my headset at bike check-in. (Not a minor matter for those who know Felt Bayonet forks--most people can't adjust them.) Nevertheless, during the race, I still felt the wobble, and I clamped the top tube between my knees and kept the weight on the pedals instead of the saddle. That kept everything under control. I haven't been back on the tri bike since the race--I've switched to my road bike to get ready for Savageman.
I've had my bike blown around by cross winds before, and this is much, much worse.
@Matt, interested to know how you would go about adding weight to balance out the valve stem issue. Would stem length make a difference? I've been buying longer stems to accommodate my various wheels. Maybe I should stick with shorter ones where I can.
@Colleen. Interesting thoughts on the mix of wheels. My PT on the back is built into a DT Swiss 1.2. Not as deep as a 404 I don't think, but definitely deeper than my front "work" wheel.
I'll have a bunch of the bike parts looked at from cones, skewer, springs and other items. I'll have the trueness and balance looked at. It may be a simple fix.
Tom, I would definitely look into the mechanical solutions first: have the frame, wheels, headset, etc looked at.
On wheel balancing – focus primarily on the trueness of the wheel – sided to side and roundness. The valve stem weight issue I pointed to was mainly to say that the rotation system we all ride is natively unbalanced. If the other elements in your system are solid , I would not worry about the valve stem. On metal rims the joint where the rim connects to its self is often a heavier point than the valve stem. If you want to balance these weight issues out, I have seen a couple of approaches used. One is to use small wraps of lead soldier added to the spokes. Another is sand added to epoxy and this pasted on the rim to add weight to the light side. This is often done on carbon rims.
Matt
As Rich mentioned, first have the bike looked at to make sure there are no structural issues. If everything checks out, there are a number of things that you can do to lessen the impact. You already mentioned clamping your knees to the top tube. A couple of other things to try if you begin to notice the front end of the bike vibrating.
- Start peddling again. By maintaining a constant cadance on the bike you can counteract some of the issue with the front end. You just need to slow peddle soemto pick up the gyroscopic effect.
- Come out of the drops and move your hands to the brake hoods. Being in the drops forces your arms to be more rigid which transmits the vibration into your upper body and just reinforces the vibration and results in instability. This is one of the issues with a Tri bike. The brake hoods are a little farther forward and force your arms into this stiff position. The important thing is to relax your arms and upper body so that they absorb the vibration and dampen the effect.
- Adjust your position on the saddle. Sometimes you may have to move forward on the saddle, or back on the saddle. See what will help, For me I tend to move back off the saddle. I know others that feel more comfortable moving forward.
If you feel like you are getting out of control and freaked out, the first thing to do is sit up. This is the bike equivalent of pulling the parashoot. It will help to either slow you down, or slow down how quickly you are accellerating. You can also rotate your knees out to increase the surface area. Think of this as going anti-aero.
Start applying the back brake. Don't slam it, just slowly apply it. If you are using cork brake pads with a carbon rim this probably won't help much. Then you can start to feather the front brakes to further retard your speed. Again, do not slam on the fron brakes or use them to heavily. There is very little weight on the front wheel and they will lock up easily causing you to flip over or lose control of the front end.
A couple of important things to keep in mind. DON'T PANIC. The bike wants to stay up as much as you do. A properly adjusted bike will naturally remain upright. Keep your upper body loose, it is your shock absorber. If you tense up your arms, the front end of the bike will become much harder to control.
Keeping this in mind has made a world of difference for me when cornering and descending. If you lock up your ams it extends the vibration. Lightening up in that situation is easier said than done but that and pinching down low onto the top tube has saved me.
P
@John and others, thanks for the ride position advice. Definitely something I'd like to practice. Maybe someone can invent a crash simulator, I know I'd pay for that now!