Technical FAQ: The future of tubular tires and the physics of high-speed shimmy

Have a question for Lennard? Please email him at veloqna@comcast.net to be included in Technical FAQ.

Dear Lennard,
Scott’s questions about the future of tubulars in your August 10 column made me laugh in recognition. I’m another geezer who wakes up with night sweats worrying about what will happen when my dwindling supply of precious tubulars runs out.

Coincidentally, just last week, when I was taping on a new pair of tires (and offering up thanks to the wizards at Effetto Mariposa for making tubular life so easy), I found yet another geezer trauma to fret about: Am I riding on my final pair of aluminum tubular rims?

I haven’t made a deep search yet, but from what I’ve seen it looks like the aluminum tubular rim is a threatened species. If so, the next wheels I lace up will probably require carbon rims.

Which brings me to my question for you to answer: In your experience, have caliper brake pads for carbon rims improved to the point where they are safe to ride in the mountains? I weigh 165 pounds. Or, is it only safe to ride carbon wheels with disc brakes? If so, I’m going to need a new frame (yet another trigger of the 3 a.m. “heebie-jeebies”).
—Ted

Dear Ted,
It is indeed tough to find aluminum tubular rims these days. At your weight, I think you’ll be fine with carbon rims and brake pads made for them. Rim heating on hot descents can be an issue, even with light riders if going fast enough (note Joseba Beloki), but it’s much more of an issue for riders over 200 pounds. And, of course, even if the tire doesn’t roll off the rim due to heating, it can slip around the rim, causing the valve stem to be cocked at an angle (you can turn the front wheel around so it slowly slides back the other way, but you can’t on the rear). But a light guy like you riding geezer-conservatively is probably not at risk of insufficient braking or extreme rim heating.

Brake squealing can be a challenge with carbon rims, and wet braking can be even more problematic than with aluminum rims. It may take a number of tries with different brake pads to find quiet ones that also offer sufficient friction. I always managed to find some that worked, when I raced for years with rim brakes and carbon rims, mostly in cyclocross.
― Lennard

Dear Lennard,
I just read Scott’s question on the future of tubulars in a recent column. I’ve been riding on them longer than Scott, and I am probably older than him. I started racing bikes at the age of 19 on this quirky cinder track race at my university. The roads around there were rough, and after destroying two sets of clinchers in my first year, I followed the advice of several older more established riders and switched to tubulars. Gone were the constant flats, and the ride was supreme. I was hooked. I agree with you that the future looks dire. I just built up a new Dogma with disc brakes, and there are only handful of tubular disc wheels available and a thousand choices for tubeless. Fearing their demise, I purchased 10 Veloflex Pro Course 28 mm tires and a couple of tins of Mastic 1. Hopefully, that will get me through the next 10 years. Then I’m going to take up golf.
— Stan

Dear Lennard,
Some thoughts on your tubular column on VeloNews:

“Paris-Roubaix has always been contested on tubulars,” you wrote. But, it was won on Michelin clinchers at least once…(Guesdon — 1997?)

“To me, this means that a rider running tubeless clinchers at low pressure who hits a sharp-edged cobblestone at high speed is certain to crack his rim sidewall,” you wrote. Are you certain? Perhaps, but a guy in Paris-Roubaix will get a new wheel (or bike) soon enough so who cares? Nobody will ever know.

“I heard attributed to UAE mechanic Beppe Archetti went something like this: ‘If they want to run tubeless (clinchers) at Paris-Roubaix, I’ll quit. It doesn’t make any sense,’” you wrote. Beppe can hang it up if/when UAE has someone capable and interested in winning Paris-Roubaix, and Campagnolo wants to promote their wheels for tubeless clinchers.

“At least 95 percent of the riding I do is on tubulars for all of these reasons, no matter whether I’m riding dirt or pavement,” you wrote. And you, Lennard, are a tech nerd/pro mechanic/bike maker rather than a Joe or Jill Crankarm who gets their bike serviced at their local bike shop where nobody has glued on a tire for years.

How many of these modern cyclists can even remove and replace a wheel with a thru-axle, let alone deal with a flat tubular on the road?
— Larry

Dear Lennard,
The math professor wrote in response to the column about high-speed shimmy:

“A linear analysis leading to resonance is appropriate for any system where there is an oscillator that is being forced at a special frequency — the resonance frequency — and when this happens, the amplitude can simply build to infinity. This is not what happens in bicycle instability for two reasons: first, there is no periodic forcing that causes the high-speed wobble (in fact, it can happen on a smooth road).”

I challenge the professor’s assumption. Isn’t the trail force a periodic force? As the handlebars/fork swings left and right in a shimmy, the centering steering force (the trail force) increases as the steering angle gets further from center and decreases as the steering angle gets closer to center, a periodic function. So the further the fork is pointed to the right, the more the trail force is pulling it back to the left towards the centerpoint.

My background is in mechanical design engineering.
— Daniel

Dear Daniel,
Here is the response from the math professor you referenced:

“Thank you for your comment. I was avoiding the technical phrases to avoid becoming too pedantic. But you are picking on a feature of what I said that requires that I expand my phrasing to be clear, that “there is no nonautonomous period forcing”. By forcing we mean an external force, and that is what the phrasing nonautonomous is for. Auto being self. Even a simple unforced spring (meaning no nonautonomous force) experiences a force which is the restoring force when a Hooke spring is displaced from its equilibrium position. You are similarly describing the autonomous restoring force of a fork wheel setup. What is the interesting feature that I described earlier, is that while a classical analysis of a linear spring system cannot produce a limit cycle oscillation, the true system is nonlinear, meaning the restoring force, in just the right way that allows the limit cycle, the wobble. And this wobble does not require an external (nonautonomous) periodic forcing.

— Erik M. Bollt, W. Jon Harrington professor of mathematics and professor of electrical and computer engineering, Clarkson University

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Lennard Zinn, our longtime technical writer, joined VeloNews in 1987. He is also a custom frame builder (www.zinncycles.com) and purveyor of non-custom huge bikes (bikeclydesdale.com), a former U.S. national team rider, co-author of “The Haywire Heart,” and author of many bicycle books including “Zinn and the Art of Road Bike Maintenance,” “DVD, as well as “Zinn and the Art of Triathlon Bikes” and “Zinn’s Cycling Primer: Maintenance Tips and Skill Building for Cyclists.” He holds a bachelor’s in physics from Colorado College.

Follow @lennardzinn on Twitter.