Shimmy, Shimmy, ···???

[gmcampy]

I fitted out my Old Marathon this weekend in preparation for a bikepacking trip in Dec. I mounted my sleeping bag on the handle bars, and sleeping pad under the top tube, tent is on teh back on the pannier rack. At any speed over 10 mph the front end shakes VIOLENTLY if both hands are not firmly on the bars. I swapped around a few things and same result. Bag weighs about 4#. Never have the issue without the extra weight on the front.

Any ideas???????

Thanks
Glenn

[AngryScientist]

headset nice and tight?

bearings in the hubs pre-loaded well?

[gmcampy]

headset nice and tight?

bearings in the hubs pre-loaded well?
Yup, the shaking is more pronounced when I remove my left hand...... I "guessing" that air movement across the sleeping bag is affecting steering. The shaking is not present without the bag on the bars even at 40 mph......

[Louis]

I "guessing" that air movement across the sleeping bag is affecting steering.

It might be the aerodynamics, but I think it's more likely due to the mass & inertia of the bag changing the dynamics of the system.

[David Kirk]

What is the diameter of the top tube? Is it old school steel with a 1" top tube?

dave

[jamesutiopia]

Is the shimmy really speed related-- does it shimmy when coasting and also while pedaling?

When the shimmy sets in, does pressing your leg/knee into the top tube attenuate things?

Are the wheels true, especially the rear? You can try running the headset a bit tight or loose (try both) to see if that changes the dynamics, as suggested already.

Are your tires lumpy, or knobbed? Guessing the latter.

[gmcampy]

Is the shimmy really speed related-- does it shimmy when coasting and also while pedaling?

When the shimmy sets in, does pressing your leg/knee into the top tube attenuate things? NO only gripping the bars

Are the wheels true, especially the rear? You can try running the headset a bit tight or loose (try both) to see if that changes the dynamics, as suggested already. VERY true, no issues without the loaded handlebars

Are your tires lumpy, or knobbed? Guessing the latter. Smoothe Conti Gatorskins

I thinking outloud, the sleeping bag was partially resting on the steer tube when suspended from the bars, is it possible that this was impacting the steering? Options are to reconfigure the load and put extra into my panniers for the upcoming trip, or deal with the vibration......

[Dude]

2 things

1. Get some front racks and put the weight lower down, not on hbars.
2. Echoing what David Kirk said, can the frame of the bike even handle the extra weight of everything?

[alessandro]

coco bop.

[rustychisel]

cumma cumma wang dang!


Back in the last century, we used to pack heavy low, including tent in pannier, and light high, meaning sleeping back on rack top. Lower your COG. Also, too much weight in a bar bag always makes a bike a pig.

[Peter P.]

This makes me wonder if the OP would have the same problem using a handlebar bag.

[gmcampy]

Thanks for the replies, Old skool 1" TT cro-mo can handle the weight, I will try some alternate packing methods this weekend or more likely deal with it: rolleyes:

[Wilkinson4]

Pics? It sounds like the weight up front is too high. Any idea on the trail of the bikes front end?

On my Ramboullet I had a small handlebar bag and tucked the pad behind it using bungee cords. I used a saddle bag to for the tent and everything else, sleeping back lashed to the top.

http://farm5.staticflickr.com/4051/4456530303_d7b124d9ab_o.jpg

mIKE

[gmcampy]

Bungee corded bag to bars....bag weight about 5#

http://forums.thepaceline.net/showthread.php?t=139571

[parco]

Is everything secure? Or is there movement?

[David Kirk]

The 1" top tube can more than handle the weight safely........it was done this way for decades.

The shimmy issue is another matter. The smaller old school tubes make for a front triangle that has much less torsional stiffness and this is what can lead to shimmy. Shimmy is really the act of having the head and seat tubes flex out of the same plane,wind up, and then flex back in the other direction. This makes a feedback loop and it can build to the point where it flops you to the floor.

Putting the weight where you did no doubt added weight in the right spot so as to act like a weight at the end of a lever that helps set up the shimmy and to make it build.

I think you'll find if you lower the weight as much as possible it will help...........so will making the load solid and secure to the bike. A weight that is high and allowed to move will help set up the frame twist and shimmy.

FWIW - I spent countless hours researching and studying the causes and cures for shimmy and some of the things I have learned go against the popular notion of what causes shimmy. Usually folks will suggest that a loose headset or front hub bearings will cause the shimmy..........or that the frame is misaligned and this is causing the issue. I can tell you that in the countless bikes I tested back in the day that suffered from shimmy none of them acted up due to a headset, front hub, or frame misalignment. Not once.

I worked with the problem a lot back in the early 1990's with Serotta and we had real issues with some of the bikes having shimmy. I measured and tested almost all of them and got to ride them down hills to make it happen. Being a test pilot for this was sketchy at best.


In brief - here is what I found -

* shimmy occurs when the frame's front triangle twists and winds up and then returns and overshoots center and sets up an feedback loop and oscillation. The handlebars are at the top of the swinging lever and therefore the weight of the bars/stem/levers greatly affects the system. We had a real issue when STi levers were first introduced due to their added weight. The numbers of bikes that had the issue went through the roof when STi levers became popular.

* headset bearings, either too loose or too tight never seemed to be the issue. One could damp the system with a VERY tight headset but you couldn't ride the damn bike with it this way. Loose hub bearings (with in reason - a small amount of free play) did not seem to have any effect.

* frame alignment was not the issue. In fact the better the alignment the more likely the bike was to shimmy and if the bike was put out of alignment on purpose (by cocking the wheels in the frame and/or fork) the shimmy often stopped. This is due to the fact that cocking the wheels made the bike pull to one side and it would be less prone to flexing/twisting past center and back again. If it just pulls to one side it tends to stay there - if it is straight it can more easily twist past center and back again setting up shimmy. This is super easy to test - cock the rear wheel way off in the rear drops so that the tire is very off center and in most cases the shimmy will stop. Don't get me wrong - I'm not suggesting that this is a cure - just an interesting experiment.

* shimmy is the oscillation of the frame head tube and seat tube twisting relative to one another and the longer the top tube is the more prone it is to shimmy. The longer levers make the weight swinging at the end of them more effective at setting up the pattern. So big bikes do it more often than smaller bikes. The vast majority of bikes I saw shimmy back then were size 58 or above. Making the front end stiffer with larger tubes almost always changed/fixed the issue.

* having wheels that are way out of balance can set up a shimmy. High profile alloy rims have heavy seams and they can set up a shimmy at the right road speed. Balancing the wheels. or putting in a different set, can often make it go away immediately. Balancing the wheels is easy BTW - a little weight and double sticky tape and you can do it yourself.

* lastly - ALL bikes will shimmy given the right set of circumstances. Every single one of them - no exceptions. Now here is the catch...........they will usually only do it at a speed that makes it irrelevant. By this I mean that they would be prone to doing it at speeds that are so slow that we have no issue with damping it or it will happen at a speed we can't get to - say something like 70 mph. The issue comes when the speed that the magic combination comes together to set up the shimmy is at our typical cruising speed. If the bike shimmies at 70 mph who cares? If on the other hand it does it at 23 mph then we have issues.

I fully realize that no one asked for this info in this thread but I always take advantage to share what I know about the subject in an effort to help dispel the idea that if your frame is slightly out of line or your headset is loose that it will case a bike to shimmy. My experience tells me otherwise.

I apologize for the thread drift and thank you for reading.

dave

[alessandro]

In brief - here is what I found -
...
I apologize for the thread drift and thank you for reading.

Not at all--you looped around and brought it back to center. Very informative!

[aingeru]

nice post, man !

[gmcampy]

Great post! Thanks for the insight. Best to get the load off the bars and into my Panniers for this trip I suppose. No load = no shake :banana:

Glenn

[SpokeValley]

Great post, Dave. Thanks!