Bike wobbles at high speed, bad wheels?

[acrustycat]

I'm new to cycling, so I'm looking for some help. I just put a new set of wheels on my bike-- mavic open pro 32 spoke rear with Powertap SL, and Mavic aksium front wheel. I was out riding today and as I came down a hill the bike started wobbling pretty badly. I actually felt like I had lost control of it. I slowly applied both brakes and after I slowed down a bit it stopped. I have never felt anything like this before, the only change made to the bike was the wheels. I am 210 Lbs and am riding a cervelo R3. Both wheels were just trued by bike shop. I was riding on the stock shimano wheels before this with no problems. The only thing I can figure is that I am too heavy for these wheels. Does that make sense?

 

[boolakeel]

Sounds like you experienced "shimmy". If it occurs again, relax, don't grip the bars tightly, bring in your knees to the crossbar, it will likely stop.

Have a read of this http://www.sheldonbrown.com/brandt/shimmy.html

 

[daveryanwyoming]

What you experienced is called 'speed wobbles' and it can happen for numerous reasons with wheel issues being one of them.

Basically it's an oscillation of the front end of your bike that hits resonance at certain wheel speeds. Often rough roads contribute to the onset of speed wobbles as they provide the initial vibrations that then build into full blown trouble. It's a case of an under damped system and it's often hard to pin down the exact cause but contributors include:

- Tight or loose headset
- Tight or loose wheel bearings
- Out of true wheels
- Too much tire pressure for the road conditions
- Too much or too little weight on the front end due to positioning issues
- Very rough roads
- Wind conditions often coupled to deep aero wheels

The only thing you changed was the wheels but that doesn't mean the wheels are at fault, it could be for instance a loose headset or weight distribution issue but your previous wheels or tires dampened the system enough that you didn't have the problems. OTOH, the simple fix is to avoid using those wheels or perhaps to run a bit less pressure up front or perhaps larger tires to better dampen road vibrations. Don't be surprised if it happens at some point with your original wheelset, perhaps at a slightly higher speed or perhaps on a bit rougher road as the fact that it happened at all with recently trued and presumably well adjusted wheels implies the system as a whole is marginally dampened for fast riding conditions.

If it happens, start by trying to shift your weight distribution, ride the drops if you were up on the hoods, rise up to the hoods if you were down in the drops. If you can't get it to go away then try clamping the top tube with both knees to dampen vibrations and try to slow down by sitting high and catching air in your chest before very, very carefully applying the brakes to get down below the speed range that causes problems.

Speed wobbles suck but many very good bikes exhibit the problem in certain situations and there is no definite cure. Your best bet is to make sure all mechanical issues are resolved as in bearing preload adjustments and to think about running a bit larger tires at slightly lower pressures if you ride on rough roads a lot and then perhaps have a bike fitter double check your position with attention to stability at speed.

Good luck,
-Dave

 

[acrustycat]

Thanks, I appreciate the reply's. After speaking to some people and reading the posts above, I think it was probably a combination of things that made it happen. It was a pretty rough road and my tire pressure was 120psi. I was also peddling hard to accelerate as fast as I could for the uphill that was coming after the down-- which means I was probably a little "jerky" and pulling on the drops to peddle harder. Maybe I was able to get away with riding like that in the past because my old wheels were heavier. Some people indicated that heavier wheels and tires will make the bike less susceptible to shimmy. On all the descents after that one I didn't feel it, but I was deathly careful and for the most part just coasted and avoided any movement.

 

[tonyzackery]

Speed wobble with a Cervelo?! Damn, IME speed wobble has only occurred with a cheaper carbon bike I had. From my research, structural harmonics is the culprit - the front end (fork) is not handling the loads placed on it at a particular speed. I'd straightaway replace the fork with something more robust (i.e. heavier)...

 

[Eichers]

Hi acrustycat, does your bike have time trial bars? These have been known to cause aero issues ...

 

[Peter@vecchios]

Originally Posted by acrustycat .

I'm new to cycling, so I'm looking for some help. I just put a new set of wheels on my bike-- mavic open pro 32 spoke rear with Powertap SL, and Mavic aksium front wheel. I was out riding today and as I came down a hill the bike started wobbling pretty badly. I actually felt like I had lost control of it. I slowly applied both brakes and after I slowed down a bit it stopped. I have never felt anything like this before, the only change made to the bike was the wheels. I am 210 Lbs and am riding a cervelo R3. Both wheels were just trued by bike shop. I was riding on the stock shimano wheels before this with no problems. The only thing I can figure is that I am too heavy for these wheels. Does that make sense?

I've seen this on more than a few bicycles with Mavic bladed spoke front wheels. Try a standard, round spoke, type front wheel. I DO think a stouter rear rim would be a better idea for you tho, like DT 585..but that has nothing to do with shimmey.

 

[acrustycat]

Thanks for the input, I just did another hilly ride today. I purposely pushed the envelope on the descents and couldnt get it to happen again.... who knows???

 

[Phil85207]

Try reading this.
http://sheldonbrown.com/gloss_sa-o.html#shimmy