UKC

Hi,
has anyone got any experience of this occasional hazard of road bikes ? My partner got a new CF road bike last year and in the course of 1 year has had 5 serious shimmy that easily could have sent her down the road at 30 mph. I have checked wheel bearings and steering headset - all of which are tolerable, and the stem length is ideal in terms of view of the front wheel axle... there may be scope to increase this by 1cm . This has happened on a range of road surfaces from rough to smooth tarmac, normally downhill, but there has always been some wind to initiate the problem.

Could it have anything to do with a woman on a mens geometry frame ?

cheers Bill

In reply to Aberdeen Rambler: I could type some incoherent nonsense instead you might want to take a look at this; http://www.cyclingtipsblog.com/2011/03/speed-wobble-when-the-bike-shakes-it...

In reply to Aberdeen Rambler: I occasionally have it happen. It's basically dependent upon the stiffness and mass of the rider and bike, and hitting a particular resonance. I'm not sure there is a lot you can do geometrically to sort it - you'll probably just move the resonance, but unless it ends up at a frequency you'll never hit you're just going to be moving it.

A lot of people suggest bracing your knees against the top-tube when it starts, which changes the effective stiffness. I have done, but mostly just relax my grip and allow the bike to speed up (I've only ever had it on descents) or gently squeeze the brakes to take your pace away from the critical frequencies.

In reply to Aberdeen Rambler: Lots of discussion here:-

http://www.ukclimbing.com/forums/t.php?t=506886

http://www.ukclimbing.com/forums/t.php?t=500345

http://www.ukclimbing.com/forums/t.php?t=555223&v=1#x7404215

Personally I've had it descending with side gusts and I've worked out it was always me gripping the bars too tight when a gust hits initiating the wobble. When I relax and don't grip too tightly I never have a problem.

In reply to Aberdeen Rambler:

As above, it's a speed wobble. It'll stop if the rider slows down or speeds up.

Apologies if the above sounds pretty unsympathetic, I've experienced them and they're not pleasant but beyond the above, there's not much to say about them.

In reply to Aberdeen Rambler:

30mph sounds awfully slow to get speed wobble. Have you checked that wheels are true? Also, see how balanced they are... does she have a magnet on either of them for a speedo? Is it on the same side ofthe wheel as the valve?

What about the headset? Any play in there?

In reply to balmybaldwin:

I got wheel wobble at about 54kmh on a new set of wheels. Spooked me out for a while. I'd been doing about 64kmh just before and think it was a combination of a side wind coming through a gap in the hedge and hitting a bump in the road at roughly the same time.

As others have said, it's the resonant frequency of bike and rider that's been triggered so you need to change that with a combination of one or more of the following things picked up from the interweb: relax; change speed significantly; get off the saddle and stand on the pedals; press one or both knees against the top tube.

For me, it's not happened again so I've not had to put the above in to practice - phew!

ALC

In reply to a lakeland climber:

Thanks All for replies. This seems to confirm the general picture, they are known to happen and most people with a few years road cycling have had one or two, but in this case Kirsty has had 5 in one year. This implies she really is the wrong weight for that frame.. The challenge now is to find a frame that suits her weight.. or get her to eat more cakes!

I had this on a recent vintage build. It would happen consistently at around 25mph for me and almost caused me to lose control on several occasions the first month or so of riding the bike.

As you said, a bit of digging found that it's the bars, forks and wheel resonating about the headset so it's a function of the rider and geometry/weight of the bike. I nearly started changing components such as the stem to see if it made a difference.

In the end, changing my descent technique did the trick. I found if I put less weight on the saddle and leaned into the bars more to weight them it solved the problem. I also descend with my feet at 6 and 12 o'clock rather than 3 and 9 which seems to help.

That's just my experience, but I hope you have some luck and save yourselves the expense of a new bike. It really does put you off a new bike when it starts happening.

In reply to Aberdeen Rambler:

the year 2011, just back from Yosemite and never ridden a road bike in my life, The misses wants me to do JOGLE with her. Go straight out to evans cycles and spend a grand on a road bike. take it out a few times after work fine. I

It was a Thursday, and were cycling to my mums for tea. 23 miles each way. The road was a bit rough, it was a bit damp, and it was a very fast road. I looked at the speedo at 43 mph. Next moment the handlebars are flicking back and forth. I thought I was going to be in hospital. Scary.

Basically, its about all the stars coming into alignment.....the weight of the rider, the geometry of the bike, and the speed your going.

The bike starts to flex, meaning the front wheel is actually moving up and down from the road. This allows the handle bars to flick back and forth.

Pressing you leg against the top tube effectively stiffens the frame and stops it dead.

Im still riding the same bike. I honestly think even tire pressures make a difference to the overall stiffness and there natural resonance of the bike.

In reply to Aberdeen Rambler: I had this a few times but not once since getting the PSI in my tyres to 100. Maybe they deform too easily at lower pressures? No idea, but it worked for me.

In reply to steveej:

Thanks for the feedback, obviously this speed weave is a combination of factors including bike and rider. I get the impression that in this case the bike and rider are close to a resonance point that is easily triggered by crosswinds etc.. Details could be changed like front tyre type and pressure but the combo will always be close to resonance.

The idea of buying a new frame and fork is challenging when I cant really work out what to look for in the new frame.