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vanhelmont

Good to hear you are feeling better.

Jim said:

Wisey

Yes, and the other trick is to transfer your weight off the saddle and onto the pedals. Again, this would shift more weight forward.....

sandranian · Site Admin

Tall people problems....

verktyg

the70sbike

Jim, sorry to hear about the accident and hope your recovery goes well.

I just read this topic. I ride 60-62cm frames and have never had a shimmy problem, but I will be a bit more careful about screaming down hills from now on! I have hit 40 mph on Hill Street in San Diego, think I will cool it from now on.

overfedlonghairedgnome

Hello all and thanks again for all the ideas.

I think I understand how the aft weight distribution may adversely affect the shimmy system, like a bobweight around the steering axis. It makes sense since I got more shimmy after sitting up, even though I was taking weight off the saddle. I will have a nice discussion with my flutter and dynamics engineers at work.

Have a great new year! - Jim

verktyg

Jim,

Happy New Year and glad to hear that you are doing better. Best wishes for a quick recovery. Wink

logarto

I ended up with a mid-grade sport touring Trek in a 61 ish size made from Reynolds 501. I got the frame for free because under its original owner it definitely had the shakes. I had witnessed it myself from a few bike lengths back-this after the original owner had complained to us about it. At the time Trek said that their stuff was "laser aligned;" using crude methods like the "string test" I was able to determine that is was close. Chances are if someone was suing someone and the frame got put on an alignment table it would have been perfect or close to it. It looked just as good as anything I've ever seen with the H tools for dropout alignment.

So I eventually built it up as a fender bike with huge tires, stuck a long mountain bike seatpost in that came up against the butt in the seat tube (and thus was very hard to remove years later when the time came,) and tried to avoid any serious descending in the 12-15 years that it was part of the stable. It never once wobbled for me and I probably never went faster than 35 on it either.

When I went to sell it I finally discovered the cause.

The bottom headset area had been milled just a little bit too wide in the head tube, every alloy hs cup I stuck in there could be easily popped right back out just with thumb pressure. The original owner had been using a Tange Levin or something similar and I believe that the bottom HS cup was pretty much oscillating inside of the head tube on him at a certain speed. Of course we were all using pretty narrow tires back then too.

I was looking to stick a cheap ball bearing headset in there and keep the plastic stronglite delrin that I had been running. I ended up pretty much gluing the stronglite back in there and then taking it out on some really screaming descents to check it, even though the plastic cup fit better than any of the alloy parts did to begin with.

FWIW right after bike computers came out in a very ill-advised experiment I got a Colnago with a 75 head angle up above 62 MILES per hour with just a 53-13 to work with- so it had every reason to be shaking like a leaf. No helmet, on cheap training tubulars too and just above a piece of road where someone dumps a motorcycle and usually dies from it about every 2-3 years because it's a very deceptive progressive radius curve as you enter it in that (downhill naturally) direction.

So I get to spend the rest of my life remembering how profoundly stupid that stunt was- not that the helmet part would have made much of a difference. Nowadays with the bikes I own I start coasting and using my ample torso as an air brake at about 40.

My previously mentioned Trek 720 (the maroon 531ST one with the horse collar seat lugs) had a shaking problem when fully loaded that Bicycling magazine kind of hints at during their review (given the amount of print space that Trek was buying every month back then,) and one other blogger who ended up with a Surley mentions this same phenomonen too, so I'm pretty sure it wasn't just my bike. I sold it last spring and if I were to end up with something like this again it would be the earlier pewter one or else a Specialized Expedition or Miyata 1000.

Oddly enough Bicycling would have had to have loaded up the bike during the test process to even say what they did say about it-unloaded this bike rode like a stretch limo.

Selling it was a conscious decision that I was more likely to tour on 26 inch wheels if it ever happens again at all. The current owner is aware of the potential but he rode it from Santa Cruz to BC last spring with regular front racks and not low riders and didn't have it happen once.

Christophe

Hi ! I'm new to this forum. First of all, I hope Jim as recovered well from his accident last year.

He asked for experiences about shimmy with 70's Gitanes, and here's mine.
I started looking for french vintage bikes as commuting bikes a few years ago. As a Frenchman, I have no problem finding hundreds of them in classified or on ebay.
Two years ago, I bought a 54 cm 70's blue Gitane, with 3 reynolds 531 tubes. No model indication, a sticker on the left base would just say " Racing team ". It has mavic tubular rims with these lovely Normandy hubs, a sugino cranckset and simplex levers, 5 speed.

I assume this is a 74 TDF n° 1835. I picked it for about 40 $, a fair price consedering some paint chips .
I immediately noticed it would be fairly harsh riding, and would turn easily ( too easily ?) which I thought was due to a short fork trail.
In a word, it 's a squirrely bike.
The first time i rode down a light slope it started shimmying seriously, at about 25 km/h. It would also shimmy, at about 20 kmh, when I ride without hands, as I like to do on my way back home after work.
I couldn't even think a racer could have used this bike like that, and thought it had a problem, such as a front chock that would have bent the fork a little, thus reducing the trail.

As I really like the way these little beasts look ( know what I mean ? Italian half-chromed forks and so on ... ) I started to look for another one I'd be more lucky with.
For 30 $ I found what I think is a 76 Interclub in pretty good working and cosmetic condition. The paint quality is far superior to the TDF as well.
Same size and geometry ( didn't check the fork rake and trail though ), same wheels, but nervar crank and huret derailer, and of course no Reynolds and a bit heavier ( didn't wheight it either) ...
Comparing with the TdF, I noticed a smoother ride (with same wheels, same saddle ), and it seemed at first to track better, being more stable.

I went for a test ride on my favorite slope and .... a little shimmy around 35 km/h. Way less than the TdF, as it disappears when holding the handlebar, but it is present, or at least, the will to wobble is there ...

So ... I thought I had found a trustable workhorse keeper, but I'm disappointed. I will keep it because for it's a perfect commutting bike and it looks so classy ! But I will not invest any money on any other Gitane.

My main road bike is a 2001 titanium - CF Colnago with Ambrosio Excellence rims I got second hand four years ago. My LBS told me it would be comfy ( I have back problems ) and I must say it dampens the road vibrations like I'm riding on velvet.
But it also tracks well, and feels absolutely safe , even in mountain roads. My fastest speed has been 76 km/h with it, and I never felt the faintest will to wobble, shimmy, quiver, shiver or anything like that.

What makes a bike shimmy like hell and another one just a little bit, considering the geometries are the same ? I thought the weight of the interclub could have modified the resonance frequency in a good way. I'll try to measure out precisely the trail of the forks and compare, but it doesn't seem easy.

Anyway, no more Gitanes for me. My project is now to find one sample of each main french company, namely Peugeot, Mercier, Motobecane, as well as some local makers such as Courtois, Lejeune, Méral ... Just one of each !
There's a pink Mercier staring at me right know, i'll see if it shimmies ....