Problem with bike stability at speed

manners2210

Hi,

I have recently started to have had problems with my bike feeling quite unstable/twitchy when I am going at speeds greater than about 25mph. Some of the roads I like to cycle on have quite steep and long decents and before this has never really given me too much to worry about, however on one occasion I developed a "speed wobble" going about 35-36mph which scared the life out of me!

Wanting to ensure that there was nothing mechanically wrong with my bike (2009 Specialized Allez Elite) I booked it in with a LBS who serviced it and gave it a clean bill of health. On my next ride out I experienced the same "shimmy" and now when I get up to the 25mph mark the bike seems very unstable - almost like the wheels are about to slide from under me. I have Gatorskin tyres (700x23) on which have only seen about 150miles, inflated to 100psi (I weigh about 11.5 stone)

Reading various forums and internet sites I can say my technique is not helping me in that I immediatly tense up when the speed starts to rise (regardless if it is downhill or not), I am not on the drops (on the hoods) and I have also read that the saddle should be higher than the handlebars - on my bike it is - but only buy a few cm (my frame is a 56cm and I'm 6ft)

My questions are;
Could the tyres be inflated too much? The recommended psi is 110. Could it be that they are too hard and cause the bike to skid and bounce on the ground? Would a different tyre or psi help at all? As an experiement I lowered the psi to around 85 and this did seem to help a little.

How much higher should the saddle be over the handbars? I have a few spaces on the headset that could be removed to lower the handlebar height if that would help.

To control the speed when decending, which is the best break to use? the front or rear?

There are so many hills where I live it's ruining my riding at the moment because I'm too nervous to go down hills at any speed

Sorry for all the questions, and thanks in advance for any replys

Cheers,
Brian

chiark

this doesn't answer any of your questions directly, but have you still got wheel reflectors on your wheels?

These threw my bike around fairly significantly when doing above 20mph or so...

Spacers: shouldn't cause problems.
Height difference between saddle and stem: 5-10cm drop to step is typical
Brakes: I always used to use my rear brake mostly on my mtb, but since moving to road have been using the front more and more... Just keep your weight back

Can you feel a wobble on the bike at all when you spin the rear wheel up to a good speed without you on it? If so, something is out of balance... if not, it sounds like velominati rule 64?

mattward1979

Have you tried descending in the drops?

You have more control over the bike at speed, and you can put far more power into the brakes.

When descending I usually use the rear brake to regulate speed around bends/other cyclists, but the front to actually slow down significantly... Although that said its a process that just "happens" so I use both at the same time just in differing amounts automatically depending on what I want the bike to do...

If you are feeling a violent shaking of the frame, try pressing your knee onto the top tube and this should dampen the wobble.

EDIT:

As for saddle to handlebar drop, mine is about 2 inches after playing around with seat heights and flipping the stem, however this is different for everyone really... Some people have it more level, and some have a crazy 6 inch drop that looks like it would give you serious spinal complications over time!!

Whatever feels comfortable and controlled!! So have a play around!

markos1963

How long is the stem on your bike? A too short a stem could lead to instability along with the seat to bar drop.
More importantly your statement regarding 'tensing' up is telling. If you grip the bars too tightly you will overcorrect any wobbles and amplify them. It's easy to tell you to relax and let the bike run on it's own but obviously it's never as easy as that.
Try finding a nice straight safe hill with good visibilty and ride down it a few times trying to keep a soft grip and build up your confidense. Push your weight backwards a bit and lower your body whilst on the drops, a nice bend in the elbows will help to damp out the shocks. Put your feet in the quarter to three position and put yourknees against the top tube.
I love descending(cos' I'm crap at climbing) and look forward to long hills, possibily I ignore the danger and the wobbles and I concentrate on getting down as quick as I can.

topdude

Something has changed the dynamic of the bike, was it ok before fitting the Gatorskin tyres ?
I found that fitting lighter tyres and wheels made my bike feel unstable, it's ok now with Aksiums and 25mm Gatorskins.
Something to do with the rotating weight / gyroscope effect possibly :?

MountainMonster

Could something possibly have happened that would cause you to have been uncomfortable or worried before the speed wobble came? Sounds like me like your not that relaxed, and rather than it being a problem with the bike, it lies with you.

Maybe try loosening up, and being more relaxed while on the bike!

MountainMonster

Could something possibly have happened that would cause you to have been uncomfortable or worried before the speed wobble came? Sounds like me like your not that relaxed, and rather than it being a problem with the bike, it lies with you.

Maybe try loosening up, and being more relaxed while on the bike!

unixnerd

When descending put your knee against the top tube

+1. That should reduce it as your body is acting as a damper.

A higher tyre pressure can help too. Only had it once, scared the cr*p out of me. It happens more easily on very smooth bits of road as the resonance builds up more easily, rougher roads break the vibration up.

JRooke

Hello, sorry to be the barer of bad news, this is not a curable thing with stems or headsets, I used the exact same frame for a winter bike and sadly it was absolutely insane down hills, the bike would start some crazy wobble and I would have to brake heavily to try control it causing my companions to splay themselves across the road to avoid me. It is on the other hand controllable to a certain extent, the wobble only occurs when the brakes are applied, so it requires a huge amount of bravery to prevent it from happening, I'm not saying never brake down a hill, but caution would be advised.

The problem can also be helped with stiffer wheels, as the wobble transfers in to the wheels. I used a pair of aksiums (and still do) through winter, renowned for reliability but not stiffness, I gave one ride a go on some of my better wheels Fulcrum racing 1's just as a test and the wobble was distinctly better.

There is a press release from Specialized somewhere on the tinternet denying these frames have any kind of speed wobble, which for me, just confirms they do =/

centimani

Funny, ive been experiencing this myself (shimmy)
Some of it related to the bike and some of it is in the mind i think.
First time, on a 35mph downhill ive ridden many times without problem, was the first time i'd gone down there on my winter bike, an old 531. There is a tiny bit of play in the headset and the Tiagra wheels are known to be flexy, so i put it down to that.
Next time i was on my Bianchi which had never had a problem before, i got a terrible shimmy on again. I put that down to me being a bit tense after the last time.
But, then it dawned on me, i'd fitted a new computer and not put the magnet opposite the valve to balance it all out. Possible that was the reason. Ive since rectifed that.and been dowhill almost as fast, not that same one though, no problem.

manners2210

Thanks for all the replies.

I have gone down fairly big hills previously without any real problem, however this particular hill has a slight camber and because it's a fairly busy road at time I am always worried about coming off because of the cars behind me. Hills I have previously gone down have been country lanes, I'm certainly more aware of the cars which is probably causing me to tense up and grip the bars too tightly perhaps.

I'll put the recommended psi in the tyres and see if that helps, also when decending should I have most of my weight forwards or backwards? I've read that having the pedals horizontal and pushing down on them slightly out of the saddle is a good position?

hugh1982

Stanley222 wrote:

When descending put your knee against the top tube

+1 to this. This should help quite a bit. Also, relax your death grip on the bars and keep some flex in your elbows.

High speed shimmy used to happen to me a lot but following the tips above have all but eradicated the problem for me.

Bar Shaker

The knee may help but something seriously isn't right if this is happening in the first place.

Peddle Up!

Like many other things in life, you're never quite the same after you've experienced it.

If you have a hybrid or tourer, try a few descents on that. The greater mass makes them more stable and builds confidence. Worked for me, although I'll never be the flying downhill lunatic.

ThatBikeGuy

I used to have something similar except not just downhill...Turned out to be a loose headset. :oops: Fixed now and the bike glides down hills effortlessly. 8)

thescouselander

Have you checked for the obvious things like play in the axle/hubs. I had the cone nuts come loose on a bike once with similar results.

manners2210

thescouselander wrote:

Have you checked for the obvious things like play in the axle/hubs. I had the cone nuts come loose on a bike once with similar results.

I havent checked this yet no, I just thought that it would be checked as part of the bike service? I will check this tonight and the other suggestions made to see if there are any loose bits hanging off the bike

Peddle Up!

Another thing that helped me. Keep your elbows tucked in. It helps the aero profie, of course, but it also seems to stabilise the bike too.