Bike wobbles at speed

pearceygy

I'm riding a giant defy 1 and whenever I get to 35mph + (descending) the wheels start to wobble, I've recently upgraded my wheels but I'm still left with the same result.
I have been biking for 6 months now really enjoying it and I'll be doing my first sportive next Saturday but I would really like to break the 40mph barrier can anyone offer advice?

jakemiller

I would just say relax, make sure youre not too tense and you should be fine although theyre might be another factor to why your getting it like maybe the lateral trueness of the wheels or something of sorts

goonz

Errr is everything tight?

pearceygy

Yes I seem to be relaxed and the bike has been checked over, but on any decent descent as soon as I edge near 40 mph keeping nice and straight on a good road the wheels seem have a mind of their own, I nearly shit myself today thought I was going to come off totally lost control for a moment.

zx6man

maybe you are getting blown around, gets scarey at the top end of 30 :-)

declan1

Try touching the top tube with both knees, use the drops and as said above really try to relax.

Kieran_Burns

Golden rule:

relax.

http://youtu.be/REkXfVVFG4c

If you tense on the bars you transfer all the lateral movement through the bike and it will wobble. It's very difficult to do and you kind of need to build up to it (I've hit over 50) but if you relax, you can easilty hit the speeds without incident.

sungod

sounds like speed wobble, aka shimmy, it can be scary, which can tense you up, which can make it worse - your reflexes aren't fast enough, you try and correct it via the handlebars but the lag means you actually amplify it

the system of you+bike has a resonant frequency, if energy is pumped in at/near that frequency then oscilation can start and build up fast

damping the system (knee on top tube), changing the resonant frequency (take weight off saddle), reducing input energy (relax grip), and changing the input frequency (pedal hard to speed up, or gently apply rear* brake) are all options

as others say above, concentrate on keeping a relaxed grip on descents, keep one or both knees firnly against the top tube, once you've done a few descents without problems you'll get confidence and be less likely to have problems in future

the wind can trigger wobble, or shivering if you get chilled, but you can control it the same way

*avoid front braking once a wobble starts