Shimmy

velocipede2288 · Post by **velocipede2288** » 26 Dec 2007, 3:34pm

I have recently had a wobble, especially when decenting a steep hill. I have checked the wheels for true and the axles are straight, but still have the wobble. Can anyone suggest what the problem can be?
It is an old 1984 Raleigh.

reohn2 · Post by **reohn2** » 26 Dec 2007, 5:36pm

You don't say if it has just started doing this or if its always done it, if its on one particular hill.
That said,have you checked the tyres,if they're fitted straight and running true and the pressures?
Is the frame straight ,tracking?
Does it happen when the bike is loaded?
Check headset bearings,adjustment,lubrication,etc.
Have you altered anything on the bike recently?

velocipede2288 · Post by **velocipede2288** » 27 Dec 2007, 2:26pm

reohn2 wrote:You don't say if it has just started doing this or if its always done it, if its on one particular hill.
That said,have you checked the tyres,if they're fitted straight and running true and the pressures?
Is the frame straight ,tracking?
Does it happen when the bike is loaded?
Check headset bearings,adjustment,lubrication,etc.
Have you altered anything on the bike recently?

I have had the wheels off, and the tyres off, the wheels are true horizontally and vertically. also I have checked the axles,they are not bent.
I took a spill a couple of months ago, but it wasn't a huge one, The forks seem to be in line, Also the same day as the spill, the delaileur came off and up into my back wheel, luckily no spokes were broken.but I was 30 miles from home and had no gears, so had to get a lift.
Holding the brakes full on and rocking the bike shows the head set is ok????
The bike is unloaded, just a small saddle bag.

DavidT · Post by **DavidT** » 27 Dec 2007, 4:48pm

Hi
I would suggest you experienced the phenomenon of "Shimmy" - a sort of yaw effect along the length of the bike axis and frightening as hell when at higher speeds - 25mph +.
It first happened to me about 5 years ago decending a very steep hill at 38 mph, I thought I was going to die!
Anyhow on a cheerful note - search this and other forums for shimmy and you'll be able to read up on the subject. Chris Juden is really the expert on this matter and pointed me in the right direction after my experience.
When it happens, you need to damp the effect, by placing one or both thighs against the top tube. It works, and a) can thereby save a serious injury, and b) enables you to maintain confidence over the fear of it happening again.
There are numerous theories about why it happens and how to avoid it. As said, a search under shimmy will reveal all. Remember the life saver though!
All the best

DavidT · Post by **DavidT** » 27 Dec 2007, 4:59pm

Could also try this link to CTC website Q&A section. Chris Juden has a lengthy and excellent piece on shimmy if you scroll down.

http://www.ctc.org.uk/DesktopDefault.aspx?TabID=3519

If this does'nt work, just search shimmy on CTC website.

Sorry for no direct link - I'm a bit of a dinosaur with such things, so no direct links, photos, quotes et al on my links! I did try to copy and paste the above link.....should'nt it go blue now?.....One day.....

DavidT · Post by **DavidT** » 27 Dec 2007, 5:00pm

Yippee! It did go blue, and it works...Do I get a certificate now? Happy reading, which I hope is useful to you. Don't forget to scroll down.

velocipede2288 · Post by **velocipede2288** » 27 Dec 2007, 6:32pm

Thanks again for all the help. I'll shimmy now instead of wobble

I had the wheels off again today and took a good look at them before I re ad this help line.

Post by **Graham** » 27 Dec 2007, 7:09pm

velocipede2288 wrote:Holding the brakes full on and rocking the bike shows the head set is ok????

Not necessarily.
I'm pretty sure that my major shimmy incident, a few years back, was due to a worn headset.
There was no obvious play in the bearings, but the high mileage had worn tiny dents in the headset bearing races where the (caged) ball bearings sat for most/all of the time.
Thus the steering would always tend back towards this central position. However, once the oscillation got going . . . . . :shock:

reohn2 · Post by **reohn2** » 27 Dec 2007, 11:00pm

Graham wrote:
velocipede2288 wrote:Holding the brakes full on and rocking the bike shows the head set is ok????

Not necessarily.
I'm pretty sure that my major shimmy incident, a few years back, was due to a worn headset.
There was no obvious play in the bearings, but the high mileage had worn tiny dents in the headset bearing races where the (caged) ball bearings sat for most/all of the time.
Thus the steering would always tend back towards this central position. However, once the oscillation got going . . . . .

This is classic headset bearing wear.
If the wheels are OK then it must be time to look elsewhere,if the headset is doing what Graham suggests renew it.
I would first check the frame to see if there has been any major damage after the incident you describe, perhaps starting with the rear dropouts to see if they've been disturbed or cracked(where the they're brazed to the frame) , then check other joints on the frame for similar faults.

Let us know how you get on.

Bob S · Post by **Bob S** » 28 Dec 2007, 7:54am

Are your spokes all tight and none distorted, as you had a tumble?

velocipede2288 · Post by **velocipede2288** » 28 Dec 2007, 11:03am

Bob S wrote:Are your spokes all tight and none distorted, as you had a tumble?

Yes I have checked and trued the wheels, can't find any cracks. I'll have to do a head strip down and look at the bearings, and maybe the back wheel bearings too.

PW · Post by PW » 28 Dec 2007, 6:00pm

I had a bike which only ever did this twice, the first time was when I fitted low rider front panniers, it was uncontrollable at 17 - 20mph. Took them off & went back to the old type front rack, never another shiver, despite some very high speed descending. Years later it happened when my son was borrowing it for a spin. There turned out to be a fatigue crack across the downtube/headtube lug. The moral being if it's been well behaved for years then gone bad all of a sudden give the frame a thorough check over.

velocipede2288 · Post by **velocipede2288** » 30 Dec 2007, 12:31pm

Well this week end I stipped the headset, took off the forks and checked for allignment. Re- greased the top and bottom bearings. took off the bakc wheel, and freewheel, checked axle, and re-greased bearings. While I was about it, I fitted new cables and housing, and handlebar tape. re-greaed the hubs.
As someon had suggested that the magnet on the wheel for the computer had casued wobble on their bike, I removed it also.
Re-aligned the back drops as suggested by Sheldon Brown, see below. Seems drastic, but it works.
http://sheldonbrown.com/frame-spacing.html
Today I had a good ride. Still no idea what caused the wobble ..??

reohn2 · Post by **reohn2** » 30 Dec 2007, 6:23pm

velocipede2288 wrote:Well this week end I stipped the headset, took off the forks and checked for allignment. Re- greased the top and bottom bearings. took off the bakc wheel, and freewheel, checked axle, and re-greased bearings. While I was about it, I fitted new cables and housing, and handlebar tape. re-greaed the hubs.
As someon had suggested that the magnet on the wheel for the computer had casued wobble on their bike, I removed it also.
Re-aligned the back drops as suggested by Sheldon Brown, see below. Seems drastic, but it works.
http://sheldonbrown.com/frame-spacing.html
Today I had a good ride. Still no idea what caused the wobble ..??

Fingers crossed you seemed to have fettled it :0)

velocipede2288 · Post by **velocipede2288** » 31 Dec 2007, 11:04am

Put the computer back on the bike. Didn't make any difference, bike still fine.

Woopy!

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