shimmy shim shim

[merckx]

I have read about the woes of front end shimmy with curiosity because it wasn't something I had experienced...…. until now. My frame is a mid-trail (72 X 55) and the fork is equipped with a small integrated handlebar bag rack. It rides and handles fabulously. Sans a bag on the rack, there is no hint of shimmy when holding the bars, or when sitting up hands-free. However, once I add a front bag, even a small Berthoud Singer bag with only a windbreaker in it, the front shimmies when I sit up and ride no-hands. Yet, there is no shimmy when I hold the bars in any position, and ride at any speed. I really enjoy sitting up to peel off a jacket, or to unwrap an energy bar so this syndrome annoys me.

There has been some discussion about using a needle bearing headset to damp the steering a bit and mitigate shimmy. Has anyone tried this? I currently have a King mounted. It seems that not many needle bearing headsets exist. Years ago I used Stronglight headsets on my racing machines, but those don't exist any longer. I see only Tange/Ird and maybe Miche needle bearing headsets when I do a search. Any others out there? Is this worth a try, or am I exercising futility?

[Mark McM]

Is this a high frequency shimmy, or more of a low frequency wobble? High frequency shimmy is typically caused by a harmonic oscillation within the bicycle itself (frame, fork etc.), whereas a low frequency wobble is more often caused by an oscillation of a mass attached to the bike. In your case, the mass is the bag, which is attached via the rack. The most common solution to bag/rack related low frequency wobble is to stiffen up the connection between the bike and the mass, which usually means stiffening up the rack (a stiffer rack, or better bracing of the rack).

[unterhausen]

maybe a Cane Creek Viscoset?

[merckx]

Originally Posted by Mark McM

Is this a high frequency shimmy, or more of a low frequency wobble? High frequency shimmy is typically caused by a harmonic oscillation within the bicycle itself (frame, fork etc.), whereas a low frequency wobble is more often caused by an oscillation of a mass attached to the bike. In your case, the mass is the bag, which is attached via the rack. The most common solution to bag/rack related low frequency wobble is to stiffen up the connection between the bike and the mass, which usually means stiffening up the rack (a stiffer rack, or better bracing of the rack).

This is a great response. I would classify the shimmy as high frequency. The rack is rock solid. It has four attachment points to the fork, and is stoutly built.

[merckx]

Originally Posted by unterhausen

maybe a Cane Creek Viscoset?

Interesting headset. Unfortunately it is not available in a threaded version which is what I need.

[adampaiva]

@merckx I had some shimmy on my Lyon with skinny lightweight tubing and 42 mm trail. It always came in at something like 17 or 18 mph. Originally I had an Ultegra headset on there but I swapped out to a vintage Stronglight needle bearing headset and it got rid of the shimmy. If I set up the headset too loose, the shimmy comes back. The steering feels pretty damped in the work stand but on the road feels just like normal.

King HS would be similarly undamped so I'd recommend giving one of the needle bearing headsets a try. Be aware that there are newer Stronglight headsets that don't use the needle bearings, so don't get one of those.

[Peter P.]

I have to play the contrarian.

If the mass on the front rack is the cause, then I feel a damped headset is not the solution. If it is, they why not just crank down a bit on the King headset and be done with it? Premature bearing wear is a small price to pay. Heck; at least do it once and see what happens; it would be a cheap experiment.

From what I understand, a lot of those randonneur bikes are specifically designed around front fork loads. If I recall, they handle better with a load on the rack vs. unloaded, and I think the result is low trail. I have no idea whether one can design a front end to handle shimmy free both loaded and unloaded. Your solution may be either a fork with a different rake or merely try squeezing the top tube with you knees while riding the bike no-hands with a front load. That may be the simplest way to accommodate your bike's quirk.

It could also be the high center of gravity that's the cause, and not the simple front load itself. Can you try a rack or load that sits closer to the front tire?

I also wonder whether swamping to a front tire with more mass might do the trick.

FYI, I have a bike with a rear rack that behaves similarly. Without a load on the rack it handles great. Place a high load (something resting on top of the rack) on it, and when I sit up it starts to shimmy.

Post back with what worked for you.

[unterhausen]

rear loading starts shimmy, that is well known. That's why a lot of bikes do it no-handed and not otherwise.

it's steering geometry related. Borrow a smart hammer and convince yourself it's not a structural resonance. Maybe this one https://www.bksv.com/en/products/tra...t-hammers/8210

Spoiler alert, you will find the first natural frequency of the frame to be above 10hz.

[merckx]

Originally Posted by unterhausen

rear loading starts shimmy, that is well known. That's why a lot of bikes do it no-handed and not otherwise.

it's steering geometry related. Borrow a smart hammer and convince yourself it's not a structural resonance. Maybe this one https://www.bksv.com/en/products/tra...t-hammers/8210

Spoiler alert, you will find the first natural frequency of the frame to be above 10hz.

I thought about the rear loading issue when I sit up, but it doesn't shimmy when I don't have a bag attached. I do sit pretty damn far behind the BB so perhaps the slight weight of a bag triggers the shimmy when I sit up. My smallest bag, the Berthoud Singer previously mentioned, and a lightweight wind breaker stuffed inside of it is enough to start the shakes. This is only an additional 400-500 grams.

[merckx]

Originally Posted by adampaiva

@merckx I had some shimmy on my Lyon with skinny lightweight tubing and 42 mm trail. It always came in at something like 17 or 18 mph. Originally I had an Ultegra headset on there but I swapped out to a vintage Stronglight needle bearing headset and it got rid of the shimmy. If I set up the headset too loose, the shimmy comes back. The steering feels pretty damped in the work stand but on the road feels just like normal.

King HS would be similarly undamped so I'd recommend giving one of the needle bearing headsets a try. Be aware that there are newer Stronglight headsets that don't use the needle bearings, so don't get one of those.

This is promising. I may have to hunt for a needle headset.

[Spoker]

Personally I hate the feeling of a tighter headset. A fix for a problem that should not be there in the first place.

[CiclistiCliff]

Threaded 1”?

[merckx]

Originally Posted by CiclistiCliff

Threaded 1”?

Correct

[oldpotatoe]

Sans a bag on the rack, there is no hint of shimmy when holding the bars, or when sitting up hands-free. However, once I add a front bag, even a small Berthoud Singer bag with only a windbreaker in it, the front shimmies when I sit up and ride no-hands.

Have you tried MORE weight in the bag? A lot of these type bikes/frames are designed around weight on the front rack..maybe MORE weight will help...

[marciero]

Originally Posted by Spoker

Personally I hate the feeling of a tighter headset. A fix for a problem that should not be there in the first place.

I hear ya. For some reason though, I like the Viscoset idea, even though it's still works by friction. I would try that on a new low trail I am building if they had 1" threadless. Otherwise I sort of just accepted no-hands-upright shimmy on my last low trail bike.

Originally Posted by oldpotatoe

Have you tried MORE weight in the bag? A lot of these type bikes/frames are designed around weight on the front rack..maybe MORE weight will help...

Low trail bikes are designed to handle well with front loads, but handling well and reducing/eliminating shimmy are two different things. For shimmy-prone designs my sense is that designers have certain parameters-bike and rider- within which shimmy tends not to occur. I read (on PL in fact) that Mike Kone will not build thin-wall skinny-tube longer than 57.5, for example. They build around those parameters but without guarantee, and then they just sort of hope for the best.

But having said that, I think you have a point. On my low trail bike I sometimes experienced no shimmy and it seemed to be with a certain amount of load. It always shimmied with no load and always with a high load. Usually with medium load but sometimes...