Front end wobble

[island andy]

A few months ago I scared myself rotten going down hill at about 32 mph on my 25 year old 531 Raleigh as my forks started to shake from side to side despite all attempts to correct. Much to my relief and surprise I came to a stop still in one piece and later found lack of headset maintenance to be the likely cause. I've recently built up a Genesis Equilibrium which is noted as being steady as a rock down hill but today on the same hill same thing happened albeit slightly less dramatically. Both times into a head wind, today on the drops but not gripping hard, I enjoy the hills and after years of mountain biking find the road quite relaxing. Same wheels ( Mavic Open Pro on 105 ) same 28c Gatorskin. I think I'm the problem some how, but how ? Later on my ride today I topped 30mph arrow straight perfectly secure. Any help gratefully received.

[531colin]

recent thread http://forum.ctc.org.uk/viewtopic.php?f=5&t=67770&hilit=shimmy

edit....actually I had forgotten all about this......I never got a shimmy out of my bike...I will try again (if I remember!)

[island andy]

Thanks for the link Colin, my situation fit Tehs description of tall rider going down hill gathering speed on a bad surface perfectly. Amazed I am, just never knew bikes could be so dangerous when ridden correctly, rightly described as a violent death wobble in one reply. All my bearings are good so now going to ride down same hill with different wheels/ different tyres. Doubt I'll ever manage the same speeds down it now though, the consequences if the wobble happens could be extremely grim.

[teh]

It would be interesting to know just how tall/heavy you are. FWIW I've got my shimmy problems under control by getting as low on my bikes as possible. I fitted 200mm cranks (lowering the saddle somewhat) and my (14cm)stems are as low as they can go. Regaining a degree of confidence has also helped.

Is your handlebar/stem configuration "normal"?

[Brucey]

recent thread http://forum.ctc.org.uk/viewtopic.php?f=5&t=67770&hilit=shimmy

edit....actually I had forgotten all about this......I never got a shimmy out of my bike...I will try again (if I remember!)

if you are using a roller bearing headset, you may find it difficult to provoke a shimmy since there is enough friction in many such headsets to damp the oscillation except perhaps at exactly the right speed.

Swapping to such a headset has worked for some people as a 'cure' for a shimmy.

cheers

[island andy]

I'm 6'1" about 11 1/2 stone, I did think that a strong head wind was helping to unweight the front end. My Genesis has got a 10mm spacer under the stem but I'm getting rid of that v soon any way, still fine tuning it.It's not what I'd call a shimmy, a motor cyclist would call it a tank slapper and knowing how to deal with these I tried easing my weight forward first time it happened but it made no difference. I must of travelled 200 yards almost completely out of control expecting to be off all the time. Bit less dramatic today but still a lot more than a shimmy. I'm going to try spinning the front wheel and checking for balance. Also going to fit another pair of wheels with 23c Gatorskins and see what happens. Bit tricky to fly down a hill that appears to have my name on it now though. Some kind of CaneCreek headset is fitted, not sure if roller or not.

[island andy]

Tried to write tank slap--r we are a bit sensitive here chaps.

[531colin]

Yes..."flapper bracket" is OK though...I guess the program doesn't know what it means!

[Brucey]

Tried to write tank slap--r we are a bit sensitive here chaps.

I think the deletion is automatic. IIRC you can write that phrase as a single word?

cheers

[island andy]

Sorry Teh didn't see the question. I've got a 100mm almost flat stem and v v shallow drop bars. Trying to work out how long cranks would help ? I normally go down hill with cranks horizontal but even if vertical surely one leg lower is evened out by one leg being higher. Don't think logic is neccesarilly needed here though. Still can't rule out me being the cause unconsciously some how. Same problem with two different bikes seems odd. I'm going to ask other cyclists about this hill, surface is quite rough.

[531colin]

longer crank = lower saddle....

[meic]

I'm 6'1" about 11 1/2 stone, I did think that a strong head wind was helping to unweight the front end. My Genesis has got a 10mm spacer under the stem but I'm getting rid of that v soon any way, still fine tuning it.It's not what I'd call a shimmy, a motor cyclist would call it a tank <i>[derogatory word removed]</i> and knowing how to deal with these I tried easing my weight forward first time it happened but it made no difference. I must of travelled 200 yards almost completely out of control expecting to be off all the time. Bit less dramatic today but still a lot more than a shimmy. I'm going to try spinning the front wheel and checking for balance. Also going to fit another pair of wheels with 23c Gatorskins and see what happens. Bit tricky to fly down a hill that appears to have my name on it now though. Some kind of CaneCreek headset is fitted, not sure if roller or not.

Did you try touching the cross bar with your leg, though?

That is the first thing that a cyclist (as compared to a motorcyclist) should try.

[reohn2]

Yes..."flapper bracket" is OK though...I guess the program doesn't know what it means!

Nor does anyone else with no motorcycling experience

[reohn2]

It's begining to sound like the problem is a combination of road surface/headwind/speed on that particular stretch of road and not bike related exclusively.
Tyre pressures ?

[Brucey]

Yes..."flapper bracket" is OK though...I guess the program doesn't know what it means!

Nor does anyone else with no motorcycling experience

I've ridden motorcycles most of my life, but that phrase clearly passed me by; I had to look it up. Was/is it in common use in some parts of the country and not others, I wonder?

cheers