Speed Shakes on the Bike

I think this may come down to bike fit and form as that is the only thing that would cross over to 2 very different bikes. I have never had a 'proper' bike fitting. The bikes are in mechanically sound condition as mentioned. While I do feel like I am pushed back and relaxed I may not be similar to you 'feel' like your swim form is great, but video review shows something completely different. Also when the speed shake starts I am pretty sure that I pop upright like a panicked prairie dog as I try to reign in the speed and prepare to get launched in full panic mode.

Honestly I will just have to be more mindful of being relaxed and squeezing the top tube on all my downhill descents to to make sure I have that form instead of it trying to be reactionary.

Thanks!

As a matter of fact I do, 50K. I tend to put myself at risk on a pretty much daily basis, so its a smart idea to be covered. Then of course I take little risks like careening down twisty mountain roads on a bicycle. Ah the little things in life that bring us joy.

Cheers

I've only had speed wobble on two occasions.

The initial time was on my first "real" road bike back in the early 80's, a Trek 560 with the stock wheels, etc. Happened just as I approached 40 mph, and scared the crud out of me. Learned the trick of clamping the top tube between my knees (to the detriment of my frame pump the first time I tried it). Later, when I swapped to a better set of wheels, the problem went away.

The other time was in a time trial, aboard a custom TiCycles Softride and Specialized Tri-Spoke wheels. I'd spec'ed a Trek carbon fork (hm... Trek again) because it was lighter than the other offerings. Anyway, a hilly, long TT, and any time I got over 30mph, the front end went wild. Kinda destroyed any chance I had of posting a good time. Replaced the Trek fork with a Kestrel, and never had the issue again.

The issue is that there's some harmonic at work, and you need to do something to change the frequency. Clamping the top tube does this by essentially putting a mass at the half-way point. Same with shifting weight forward or backward. Swapping to a stiffer fork can help (as above), or even a change in wheels/skewers/etc. With the Peugeot, it may just be an issue of the flexibility of the frame itself, and not much you can do to change that.

Oh, and $50K for life insurance... You need more. Just sayin'.

This probably won't answer your question in a way that is helpful but it is the technical answer to the problem.

http://rspa.royalsocietypublishing.org/content/royprsa/463/2084/1955.full.pdf

Here is a recent article about Jim Papadopoulos in Nature and his quest to solve this problem.  He has worked on this problem since the late 80's.  He is an acquaintance of mine.  This article is readable but ultimately not helpful to a rider.

http://www.nature.com/news/the-bicycle-problem-that-nearly-broke-mathematics-1.20281

My Litespeed Tuscany had a speed wobble.  It seems to be cured by replacing the original wheels with Zipp 101 wheels.  The spokes on the original wheel were thread-like 1.5 mm.  The Zipps have bladed Sapim spokes that are .9x2.2 mm.  These dimensions give a much stiffer spring constant for the spoke and a laterally stiffer wheel.  (The radial direction is stiffer too but that doesn't matter.)

TW

When I belonged to a bike race team, one of my teammates was dealing with this with her new bike. The shop was trying to problem solve. What I got from this was some bikes it happens and some it doesn't. Sometimes they can figure out why but other times it is just the way the bike is and better to use another bike for descending fast. She would not race it until they solved the issue and if they couldn't fix it she didn't want the bike.

I find it odd that both your bikes have this happen.....old one makes more sense to me and not so much the newer one.

I'd bring the new bike back to the shop you bought it and ask them about it. Bikes aren't supposed to do this.