Wobble worry!

Gunth

Just looking for some advice from people about the following problem which I recently experienced. I was cycling downhill on a very wet road when suddenly my bike started to vibrate pretty hard. The best way to describe it is if I was cycling at speed over a series of rumble strips. I have cycled this particular stretch of road on a number of occasions previously and have not encountered this problem. At the time I firmly gripped the handlebars firmly and braked gently hoping not to crash. The vibation which originated from the front of the bike ceased and I continued cycling but as nervous as hell. I checked a bike maintenance book and Google and the closest match I found is an effect know as a 'shimmy'. Has anyone experienced this and if so what is the best remedy that they found? Also will this be a recurring problem as I might not be so lucky next time? This episode has really dented my trust of the bike as it is only a few months old.

Lumen

Loose headset?

emty

Your headset could be loose.Check the forks to see if there is any give in them.Also,if it was just a speed wobble your bike frame is kind of acting like a tuneing fork.Clamps your knees around the top tube(crossbar),that should do the trick.Not a nice thing to happen,it can give you a good fright.

Gunth

I checked for ripple effects in the paintwork that would indicate damage but none to report. Also I checked for looseness but everything seemed fine. Thanks for the tip re the knees against the bar.

Ryaner

Headset usually the problem although I had it from over-tightening the headset on mine the last time I cleaned it out.

Raam

Possibly a bearing gone in your headset.
It can also occur on larger frames due to steerer length. Fork rake can also be a factor.
Leonard Zinn has some opinions on it:http://velonews.competitor.com/tag/shimmy

When the bike starts to shimmy, pinch the top tube with your knees.
Your body will stop it. There is a better article somewhere which I cannot find right now.

Edit: ah beaten to it with the knee tip.

lescol

If it ever happens again then clamp your knees to the bar and loosen the death grip on the handlebars, the bike will then "calm down".

http://www.sheldonbrown.com/brandt/shimmy.html

Raam

Here is the article I was talking about: http://www.prodigalchild.net/Bicycle6.htm
Scroll to near the end.

poochiem

Have had this a few times seemed to be at high speed (downhill) and on certain surfaces - thanks for the tips

ROK ON

I used to get this on one of my bikes (heavy vibration when descending). Then one day the back chainstay broke. It could be a problem with your frame.

WakeyTyke

Had the most frightening experience in September, descending from the Sally Gap on the Dublin-Paris cycle. My Giant TCR began to vibrate alarmingly, so much so my vision became blurred and as I crossed a small bridge the bike went into a massive wobble, how I managed to stay upright I will never know (those following behind thought I was a goner). I would imagine I was up over 70kph but I couldn't read the speed because of the vibration:eek:

The fact that I came so close to crashing at that speed has shaken my confidence on descending big time. I wish I had known about gripping the top frame with my knees!

Gunth

Thanks guys for the replies so far. Reading through the posts it seems that the 'shimmy' effect could be the result of a number of issues. I will get a professional mechanic to check the headset and hopefully that will be the end of it. The only problem is that I will be waiting for this to reappear. I won't be wearing white bike shorts in case it does happen again!

Raam

@Gunth: I'm in the habit of pretty much always pressing a knee against the top tube while free-wheeling when I am descending. I find it makes the bike a little more stable. I don't need to do it as much with my carbon bike, but with my alu one I find it a great help. Now that's not saying that you won't get the problem with carbon, just that I experience it a bit more with my alu bike. That could also be due to the fact that I have ridden the alu one more than the carbon one.

Murph100

Hey Gunth,



I had the same problem a few months ago, went through all the suggestions helpfully given by the lads here, and yes, knees against the top tube, weight off the saddle a bit and loosen the grip on the bars does the trick .... BUT... you'll be bloody wrecked if its a long descent and its a right feckin' pain having to do it on every fast descent.

Once you rule out loose headset, worn bearings, bad wheel alignment, dodgy forks etc. you end up with a perfect bike that still gives you a speed wobble, why ? Well the main theory is that riders who have this problem on a particular bike tend to exacerbate the bike's inherent vibration frequency ( all bikes have one ), whereas most other riders actually suppress it, this is due to factors like body composition, proportions etc.

Bike components also have an effect on this frequency, which is why in some cases changing to a different wheelset can stop the problem. In my case I changed the wheelset and definitely it did help BUT I still could get a lil' bit of shimmy trying to develop on some descents, nowhere near as bad as before though. I then found a Look fork in Cyclesupertstore for a giveaway price, thought 'what the hell', fitted it, and voila NO MORE SPEED WOBBLE !! Same headset, same bearings just different fork, bike handles like a dream now.

HTH and sorry for the long post.

Gunth

Thanks for the advice Murph100. Will be trying the knee against the top tube and a slight adjustment on body position for future downhill parts and hopefully this will rectify the problem.

LastGasp

Gunth wrote: »

I will get a professional mechanic to check the headset and hopefully that will be the end of it.

Hi Gunth, Checking the headset is straightforward enough. Stand beside the bike, pull the front brake on and rock the bike back and forward. You shouldn't feel any movement in the front forks. If it's loose you'll feel it moving slightly.
If it is loose adjustment is easy too. Slacken off the 2 clamp bolts that go horizontally through the back of the headset. Then tighten the bolt that goes down through the steerer tube a little at a time until you can no longer feel movement as above. Don't overtighten it or steering will be stiff. Then tighten the clamp bolts again and that's it.

Gunth

LastGasp wrote: »

Hi Gunth, Checking the headset is straightforward enough. Stand beside the bike, pull the front brake on and rock the bike back and forward. You shouldn't feel any movement in the front forks. If it's loose you'll feel it moving slightly.

I tried this and no movement was noticed in the forks. At least this rules out the headset being an issue.

Raam

Gunth wrote: »

I tried this and no movement was noticed in the forks. At least this rules out the headset being an issue.

Careful there, I've no play in my headset but the damn thing still shimmies. An experienced cycling friend told me to get the bearings checked. I'm doing so next weekend.

In my case it can occur even on the flat, especially when riding with no hands, so my problem may not be the same as yours.

Sean02

Forget the knees lark might be ok for Kelly at 60 m.p.h. Once you have that terrible experience and theres no apparent reason like a loose bearing it registers with the computer and will happen again and again at even slow speeds. Fair play that you had the presence of mind to check the speed, all I thought of doing was praying!!!. Sorry for not being more positive. I still have night time mares Tullyesker Hill, Belfast and Cuba. Dump the bike.