Descending 'shimmy'

[gbb]

Here's a funny thing. Part of my ride into Rutland (or thereabouts) take me on a long descent into Harringworth, hitting 37 mph, long sweeping bends right then left, road surface a little poor (B road i should think)
Up to a few weeks ago, i could descend it ok on my Bianchi. Two weeks ago i took the Raleigh Clubman and got a distinct shimmy on. Dont think i was gripping too tight or anything, assumed it's because its a much older bike, i reset the rear triangle to accept 8 speed wheels etc, and maybe its not entirely true but only shows at very high speed.
Last week i took the Bianchi...and got the same shimmy. A few weeks ago i had the front forks out and regreased the headset. Not quite adjusted well enough maybe ? and causing the shimmy ?
I even thought maybe (and this is very very tenuous) that ive recently fitted a computer...is the wheel magnet throwing things out ?

Any thoughts ?

 

[fossyant]

It can happen - sometimes can be caused by wind conditions. I had a 531 that would shimmy at over 40 with mudguards on when it was windy. My best bike has never done it, but that's a handbuilt steel bike. My Ribble has only done it once, at 50 mph coming down Woodhead with strong cross winds.

 

[Zoiders]

Does the clubman err on the side of audax/touring geometry?

 

[Fab Foodie]

Chapter and verse:

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

 

[zacklaws]

There's a steep hill near me, every time I go down it, I get a shimmy, I don't know why as it does not happen on any other hill, I find it very strange. The first time I noticed it was one day when I was doing a few circuits and I had to desend it a few times, it would happen soon as I hit 15mph, then it would stop as I sped up, and then again when I hit 30mph which made me clench the saddle with the cheeks of my bum till I slowed down a bit and got out of it, to once again have it kick in at 15mph again.

Usually just pressing a knee against the top tube eradicates a shimmy

 

[Zoiders]

I think some hills cause it because the actual road surface is a bit wrinkly from the road surface sliding slightly with gravity as the tarmac is layed.

 

[Tynan]

shimmy on the first bike due to the bike geometry lead to the rider being twitchy on the good bike seocnd time around?

 

[gb155]

My CAAD 9 does it every time I pass 40 mph

 

[Fattman]

Just back from hills of Southern Spain, previously-experienced shimmy gone on this trip after moving magnet on rear wheel to be on spoke opposite the valve.

 

[gbb]

Does the clubman err on the side of audax/touring geometry?

I don't know what the Clubman was , racer, tourer...but it is a 'long' looking bike. The rake on the front forks probably amplify that 'long' look.

Chapter and verse:

http://www.sheldonbr...ndt/shimmy.html

Very interesting. It seems the wheels have most impact on shimmy'ing, but it can also relate to frame length.

There's a steep hill near me, every time I go down it, I get a shimmy, I don't know why as it does not happen on any other hill, I find it very strange. The first time I noticed it was one day when I was doing a few circuits and I had to desend it a few times, it would happen soon as I hit 15mph, then it would stop as I sped up, and then again when I hit 30mph which made me clench the saddle with the cheeks of my bum till I slowed down a bit and got out of it, to once again have it kick in at 15mph again.

Usually just pressing a knee against the top tube eradicates a shimmy

Yeah, i forgot that. Must put brain in gear next time it happens.

shimmy on the first bike due to the bike geometry lead to the rider being twitchy on the good bike seocnd time around?

Hmmm...it did occur to me

Just back from hills of Southern Spain, previously-experienced shimmy gone on this trip after moving magnet on rear wheel to be on spoke opposite the valve.

I thought it was tenuous, but then reading Sheldon browns bit above, and wheels having so much impact on shimmy'ing, maybe my bit about the magnets not so far fetched....your experience maybe confirms it.

 

[zacklaws]

I thought it was tenuous, but then reading Sheldon browns bit above, and wheels having so much impact on shimmy'ing, maybe my bit about the magnets not so far fetched....your experience maybe confirms it.

I always thought it was common practise to fit magnets opposite the valves to balance it out, I know others have mentioned the practice of doing so on the forums and I've always done it.

I used to have an old Raleigh Milk race and it was a death trap desending hills, soon as I got a bit of speed up it would start to shake itself to bits in a very terrifying manner. But I have a feeling that the problem was caused by the big buckle in the back wheel.

 

[gbb]

I always thought it was common practise to fit magnets opposite the valves to balance it out, I know others have mentioned the practice of doing so on the forums and I've always done it.

I used to have an old Raleigh Milk race and it was a death trap desending hills, soon as I got a bit of speed up it would start to shake itself to bits in a very terrifying manner. But I have a feeling that the problem was caused by the big buckle in the back wheel.

Just goes to show, however old you are, however much you think you know (and i don't know that much)...i never heard about placing the magnet opposite the valve. Well there you go

 

[buddha]

Just goes to show, however old you are, however much you think you know (and i don't know that much)...i never heard about placing the magnet opposite the valve. Well there you go

For my wheels, I thought that was true. However, when I invert the bike and spin the wheels, the wheels always stop with the valve at the top - not at the bottom as you would generally expect. So I now put the magnet near the valve.
I have 6 different pairs of alloy wheels, and it is the same for all of them.
This, of course could be a load of bo!!%$&s, so check for yourself.

BTW: the knee on top tube thing worked for me.

 

[asterix]

My old bike used to shimmy from new. The only bit I hadn't replaced were the hubs (Campag). Eventually the LBS said they could no new cones for them and so I replaced the wheels (I'd had new rims a couple of times). No more shimmy. Ever. At all. It must have been the hubs, no?

None of my current 4 bikes shimmy. 2 have Shimano hubs, 1 Campag and one a Schmidt dynahub. Never had any problems with computer magnets, after all if they could cause it, why not valves?