Howguy

Yesterday, I was riding down a long steep hill on a wet day. I have a Cannondale Synapse road bike that's about 5 years old. I was tucked down on the drops for a good aerodynamic ride. I hit, I'm guessing about 40 mph, and the front wheel began to wobble and I couldn't control it. This has never happened with this bike on dry pavement. I always test the wheels before riding to make sure they are true and they were. It was somewhat terrifying because I was going pretty damn fast on pavement and felt I was going down. I grabbed the brake handle with my left index finger and slowed the bike down a bit and the wheel stopped wobbling and I regained control. But for a half a minute there, I was truly frightened. Anyone ever have this experience and what could have caused it? The tires are typical thin road bike tires and were inflated to about 120 lbs.

prathmann

Shimmy on fast descents is a pretty common problem and I've never seen any evidence that it's associated with wet roads so I suspect that it was just a coincidence that your first encounter with it happened to be on a wet road.

There's a detailed discussion at Sheldon Brown's site:
https://www.sheldonbrown.com/brandt/shimmy.html

I've found that putting my knees solidly against the top tube is a pretty effective method of stopping the shimmy.

rumrunn6

that's the "death wobble" look it up and has nothing to do with the wet conditions. but dude - 40 on wet roads? seriously?

control the death wobble but getting out of your saddle to put your weight on your pedals lowering your center of gravity, grab the top tube with your knees to stabilize the dynamic shake, press down on the handle bars adding some weight to the front wheel, oh and slow down, I think that's the emergency list for the wobble ... the 1st time I got it I was going down hill pedaling my ass off trying to see just how fast I could go and I think I got up to about 40 and then wobble kicked it - it was not a pleasant experience and I vowed never to pedal like a maniac going downhill ever again!

spock

It might be that you had too much weight on the front wheel and your grip was too tight and the wheel might have hit something to cause the instability.

It also might be that the wheel wasn't tightened securely to the forks or the axle cones got loosened.

Howguy

Wow! Death wobble! I tell you, that's an appropriate name for it, because for a time there, I felt I was going to have a serious accident.

dougmc

It's a resonant phenomena -- so anything that changes the resonant frequency will stop it -- speed up, slow down, hold tighter, hold looser, stand up, sit down, etc.

I've run into shimmy myself, and asked about it on rec.bicycles.tech. This thread was the result. It seems that the whole thing is pretty controversial, though Jobst's article is quite good (and covered my situation exactly with my death grip.)

It's scary as hell when it happens.

fuji86

Some think this is an imaginary problem, balancing a bicycle wheel that is. I feel for the peril that you went thru. As for the wet road, 40 mph downhill might have been exacerbated by hydroplaning on high psi inflated tires. Ride safer for the conditions is the advice I can offer there.

prathmann

Wet roads certainly reduce traction and make it more important to slow down, but high-pressure bike tires are not subject to hydroplaning at normal speeds. Hydroplaning has been studied and found to require a speed (in mph) of at least 10 times the square root of the tire pressure (in psi). So for a bike tire inflated to 100 psi the speed would need to be over 100 mph.

fuji86

That and perhaps a tire that has higher air pressure (maybe even over inflation) has less tread contact with the road. Less surface area contacting the road and enough water in between and the tire lifts and has even less contact with the road. Water doesn't compress, it's channeled away if there isn't enough water channeled away and you'll hydroplane regardless of how fast you're going. There are other factors involved is all I'm saying. Under inflation can put more contact on the road but also float, as is the case with slicks and balloon tires.

dougmc

The other factors are mostly in our favor in the case of bicycles -- our tires are usually large in diameter, narrow, rounded with high pressure.

If you're looking for more details -- https://en.wikipedia.org/wiki/Hydroplaning_%28tires%29 -- the section about motorcycles pretty much applies to most bicycle tires as well, though considering how much slower we go, it's even less likely. But we certainly can simply lose traction and crash because of that -- you don't need to hydroplane for that to happen.

Ultimately, bicycles almost never hydroplane. The only situation that might come close would be a low pressure tire with a flat bottom like a car wheel (which just doesn't work well on a bicycle -- it makes turning difficult -- so it's rarely done on practical bicycles) going really fast down a long, steep hill -- and even then it seems very unlikely.

Howguy

This is absolutely fascinating to me. I had never heard of this phenomenon before it happened to me yesterday and reading about it today. Thanks so much for all the input.

In a way, it was a blessing in disguise. I could have been hurt very badly or even killed, but thanks be to God, I wasn't. So now I know about the phenomenon and have some idea what to do if it happens again. I hope also that maybe some others who have not had this experience may be made aware of it from these posts.

Thanks again guys.