Investigating Chain Skipping and Shimmy

Sometimes, you can keep riding with a broken chain and not even know it’s broken!

Dear Readers,

I wanted to share a couple of observations from my ride last Thursday and Friday that could be of use to you as cyclists. On Thursday, a buddy and I rode west over Old Fall River Road(dirt) in Rocky Mountain National Park and on over the Continental Divide and down to Grand Lake.

On Friday, we rode back through the national park, this time over Trail Ridge Road, the highest continuous paved road (12,183 feet above sea level) in North America.

Chain failure

The first observation is about chain breakage, in this case a SRAM 12-speed Flattop AXS chain. My chain came with the first-generation SRAM Force XPLR group that’s on my bike. It is like this Rival AXS chain in the sense that it doesn’t have the cutouts in the link platesthat the Red, Force and new Rival chains have. I have many thousands of miles on it, and, thanks to regular chain waxing, it still registers no measurable wear on a three-prong Pedro’s Chain Checker Plus gauge.

Near the end of the Zinn Fondo mountain gravel ride on June 21, my chain was intermittently skipping on the larger cogs. I assumed it was due to a slight position maladjustment of my rear derailleur, since a day or two prior, I had demonstrated on my own bike to a customer how easy it is to make position adjustments of SRAM eTap wireless electronic rear derailleurs. While riding the following day, I tweaked my derailleur adjustment slightly to the outside, and it seemed to fix the problem. A few days later, however, it started skipping intermittently again, but this time on the smaller cogs.

I continued to fine-tune the derailleur adjustment, and the skipping didn’t go away. That’s when I started checking the chain wear with the three-prong Pedro’s Chain Checker; again, it showed no appreciable wear (the only chain gauges that work with AXS Flattop chains are ones with three prongs; standard two-prong gauges are inaccurate with these chains because of their oversized rollers).

Since my chain was not at all elongated, I figured that cog wear could not be an issue, but just in case it might be playing a role, I switched wheels to one with a new 12-speed AXS cassette of the same 10-44 size. That did nothing to resolve the issue, however; it still skipped intermittently on the smaller cogs.

A few days later, my chain came off my front chainring twice on my 3-mile ride into the shop. Other than when the chain had become packed with ice in a cyclocross race, I had never had a chain come off an X-Sync 1X chainring with its tall, fat-thin-fat teeth, so this came as a shock to me. The following day, I went on a ride in the mountains with Davis Phinney, and twice on the way to his house and once while on our ride, my chain again came off the single chainring.

Later in the week, I was going to do the two-day ride I described going back and forth over the Continental Divide through Rocky Mountain National Park, and I didn’t want any chain problems. Again, I checked the chain elongation at dozens of spots in the chain with the Chain Checker. Again, no elongation. So I put my reading glasses on and inspected every single link. And lo and behold, I found something I had never seen before that explained why my chain was derailing—one inner link plate was broken cleanly in half!

The opposite inner link plate was intact, so the chain kept working mostly decently save for the occasional skip and thrown chain. But I think I would have broken the other link plate on one of the climbs over Rocky Mountain National Park.

The link plates on 12-speed chains are super thin. Chain breakage normally happens at a chain pin, where an outer link plate is pried off the end of the pin during a shift. A chain plate being torn in half just riding around (JRA) is not something I had seen before and didn’t think to look for. And mine didn’t even have the big cutouts in both the inner and outer link plates that Red, Force, and Rival chains now have. From now on, I’ll carefully inspect the entire chain whenever I have mystery chain skipping, and I recommend you do the same.

Shimmy

The other subject that came up on my ride up high in the mountains is high-speed shimmy. I had not even a hint of it, but I have seen it happen over the years on the east-side descent of Trail Ridge Rd. The winds up there above treeline tend to be high, often pulled down by low pressure over the plains to the east. There is nothing up there to slow the wind, and there is even one place where the road is on a ridge dropping off on both sides. Winds can be swirly there, pushing you one way and then the other while you roll at high speed.

I have done this ride a lot on deep-section wheels and have often felt a bit like a potato chip in the wind up there. We were fortunate to have light winds that day. I was talking to Rolf Dietrich about how rock solid the 16-spoke Rolf Prima Ares 4 wheels I was riding felt, and his explanation seems worth passing on.

If you have read my VeloNews columns over the years, you know that I have many times answered questions about high-speed shimmy. As a framebuilder who got into the field largely because I had a bike that shimmied uncontrollably at high speed, I addressed it primarily by stiffening the frame to raise the resonant frequency of the frame so its resonance would not be set up by normal environmental factors when descending, like wind and bumps in the road. Shimmy especially plagues tall riders on tall bikes (which I am), and the heavier the rider, the greater the chance of developing it. Here’s one example of a column I wrote on the subject with links to prior columns on the subject. In it, Rolf Dietrich discusses paired spoking and its effect on shimmy. Since I just happened to be riding Rolf paired-spoke wheels on my ride up there last week, I discussed it with him, and this is what he said. He doesn’t have wind as a variable in his list; I would certainly add that one in as well as fork stiffness and alignment.

― Lennard

Dear Lennard,

Conventionally laced LOW spoke count wheels can induce SHIMMY and I know of a death caused by SHIMMY during a State of Wisconsin State Road Championship when the rider was thrown straight into a tree, hitting it head first straight on at high speed. The wheel was ROVAL branded, old and well used and known by the riders' friends as having trouble staying true and chosen by the rider because of its lightness to increase the rider's chance of victory.

A number of variables can and do cause shimmy in conventionally laced wheels depending on their varying amplitude.

These variables are:

1) spoke count

2) spoke tension

3) spoke ductility

4) rim stiffness

5) bike frame resonant frequency

6) rider weight

7) vehicle speed

8) road surface condition

There may be more variables, but No. 1 above is most important. Low spoke count conventional wheels are true only in the trueing stand during building. As they are mounted in a bike and as the rider climbs aboard the bottom spoke at the Road Contact Point ( RCP ) becomes de-tensioned. This pulls the wheel out of true, deflecting the rim in the opposite direction of the bottom pointing spoke at the RCP. If the spoke is a RIGHT spoke, the spokes LEADING & TRAILING this spoke are each LEFT spokes and pull the rim to the LEFT. And conversely, as the next spoke rotates over the RCP it is of opposite orientation, pulling the rim to the RIGHT!

In this condition the LATERAL left & right force vectors of the spokes acting on the rim no longer balance, keeping the wheel true as in the trueing stand. In LOW spoke count conventional wheels there is considerable unsupported radial distance between LEFT & RIGHT spokes, aggravating the differing LATERAL deflections between LEFT and RIGHT spokes as the wheel rotates under the rider under way.

The LOWER the conventional spoke count, the greater the varying LATERAL deflection. Concurrently the remaining variable, 2 to 8 above, will affect the onset of SHIMY in varying degrees.

With ROLF Paired spoke wheels the varying LATERAL spoke force vectors of 1 ) above cancel each other; they disappear. They cannot induce SHIMMY. Thus, the remaining variables, 2 ) to 8 ) above, are much less likely to come into play.

I hope you read this analysis slowly and carefully. You might then understand that the theoretical LOWEST spoke count for ROLF Paired spoke wheels is 6 TOTAL spokes for radial lacing. I comfortably ride such wheels.

Important is to know that the UCI has approved a MINIMUM spoke count of 12 for TENSIONED WIRED WHEELS for MASS start events as for the TdF, for all tensioned wired wheels and has specifically tested and approved a 12 spoke ROLF Paired spoke wheel for MASS start events. Also to be understood is that Campagnolo marketed a 12-spoke CONVENTIONAL wheel and discontinued it shortly after its introduction.

Rolf

Dear Lennard,

I've often wondered why I've never experienced even a hint of the dreaded shimmy during my 55 years of riding, and I do wonder if it's because I do every downhill with my weight fully on the pedals, hands floating on the bars - butt floating over the seat - a habit I developed riding dirt bikes (you can't ride a 200 lb. motorcycle any other way). A bicycle being an inherently stable device, it feels like I'm just letting the bike do what it does best, whereas with weight on the seat and bars, it feels like it would be easy to destabilize the bike. I believe that jibes with something you mentioned years ago - that rider input is a big factor when it comes to shimmy. The one cure for shimmy that seems to work is clamping the seat with the knees, but I've wondered how much of that is the clamping, and how much of that is that it forces the rider off the seat and bars, and onto the pedals. It's pure speculation on my part, but I do wonder.

Steve

Dear Steve,

Could be. It’s a hard theory to test, because it requires setting off a shimmy at high speed that may build to the point that it is uncontrollable.

― Lennard

Finally, for those of you who asked about the gate my wife and I were working on during our staycation, it’s done! Check it out! It’s such an improvement over the gate we had before; it’s hinge post had been broken for decades, and it was a heavy lift to open and close it. This one swings like a dream!

― Lennard

Subscribers can send brief technical questions to Lennard at: veloqna@comcast.net.

As a frame builder, Lennard Zinn has been designing and building custom bicycles for over 42 years; he founded Zinn Cycles in 1982 and co-founded Clydesdale Bicycles in 2017. His Tech Q&A column on Substack follows his 35-year stint as a technical writer for VeloNews (from 1987 through 2022). He is a former U.S. National Cycling Team member and author of many bicycle books including Zinn and the Art of Mountain Bike Maintenance, Zinn and the Art of Road Bike Maintenance, and The Haywire Heart. He holds a bachelor’s degree in physics from Colorado College.

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