Technical FAQ: Cantilever profiles, broken crank arms, and more

Cantilever brake profiles

Dear Lennard,
I’m 14 and have been doing junior races for five seasons now, and I’ve recently built up a single-speed cyclocross (SSCX) bike. I’ve been looking for a new set of cantilever brakes for it. While looking at new cantis, I’ve seen three different profiles — wide, medium, and narrow. What are the pros and cons of each one?
— Alex

Dear Alex,
Wide cantilevers provide the most mud clearance. Their downsides are that they have the least power and the least heel clearance when used on the rear. Narrow cantilevers give good heel clearance (you’ll be unlikely to kick one into your spokes when mounting or dismounting), and they maximize the power of the brake. The downside is that mud clearance is not as good. Medium profile is a compromise.

Here is the detailed explanation.
― Lennard

Base tape for tubulars

Dear Lennard,
So after several years wrangling all manner of tubulars, here’s something that only occurred to me the other day: Why do tubulars need base tape? Why can’t you just glue the tire right to the rim?

I know that base tape “protects” the tubular stitching — but protects it from what? It seems to me the base tape is just there to make trouble. You have to be sure the base tape is securely glued to the tubular, and then you need to hope the glue you are using does not contain some incompatible solvent that might cause the tape to release from the tire. When you pull a glued tubular from a rim, the base tape always gets at least a little wrinkled and messed up. And if you fix a flat — and I know almost no one does these days — the base tape inevitably gets stretched, so you have to cut it and stick it back down.

Wouldn’t the world be a better place without base tape?
— Ted

Dear Ted,
For the 1979 junior world championships, national team head coach Eddie B. had the mechanics remove the base tape for the team time trial, and the Americans came away with a bronze — a big deal at the time. However, that was only one race, and you want your tires to last a lot longer than that; base tape lengthens their life.

The base tape prevents rim cement from seeping through the stitching and damaging the inner tube, and it takes some of the load off the stitches in opposing the air pressure in the tire. It also prevents the casing from contacting the edge of the rim, which could wear into it with repeated flexing while cornering and while simply rolling along. Base tape is more resistant to cutting and abrasion than tubular casing fabric, which could survive for a while before failing from the repeated indenting action of the rim on the casing.

The base tape on very old-school, 19mm tubulars (like Greg Lemond, Ron Kiefel, Jeff Bradley, and Greg Demgen rode to that 1979 TTT bronze medal) was too narrow to reach the edges of 20mm rims (we believed back then that the smaller and harder the tire, the faster it was). Due to the overly narrow base tape, the tubular casing itself would contact the edges of the rim, which within 1,000km would abrade the casing until the tubular exploded. That’s what would happen to any tire without base tape.

Alberto De Gioannini, the founder of Effetto Mariposa and a former engineer at Vittoria, told me that Armando Pagnoncelli, who was responsible for Vittoria tubular production for 30 years (first in Italy and then in Thailand), used to say that the base tape is to a tubular what a belt is to trousers, “making sure the trousers do what they’re supposed to do.” I suppose keeping one’s ass covered is akin to keeping air in one’s tires.
― Lennard

Follow up on forgoing sealant for tubulars

Dear Lennard,
At the end of your recent column about gluing cyclocross tubulars with Carogna, you mentioned that you no longer recommend sealant in tubulars. Could you elaborate a little more on your reasons?
— Jason

Dear Jason,
While sealant in a tubular protects it from thorns, if thorns are not a problem where you ride, the downsides of the sealant overcome the added security, in my opinion. Unless you have a tubeless tubular made by Tufo (Tufo and Clement), you will be injecting sealant into a latex tube. Some sealants will burn the tube, and for this reason, the only sealant that premium tubular maker Dugast recommends is Effetto Mariposa Caffé Latex.

Even if you don’t burn the tube, you will have other hassles. The sealant will inevitably clog the valve and make it hard to pump the tire or check its pressure or let air out. You can replace the valve core, but if you have deep rims with valve extenders, the tiny bore of the extenders where they thread into the valve stem will clog as well.

If you leave the valve open, the sealant will rapidly dry out in a blob at the bottom. You must keep the valve closed at all times, especially over the summer. It will still dry out, but it will take more time.

The fix for this is to get the sealant out, but unless you have a Tufo-made tubeless tubular tire, you won’t be able to remove the sealant at the end of the season. I have ruined tires trying to do so. My removal method was to repeatedly rinse out the inner tube with water and then try to let the tube dry out without sticking together inside. I described it here. I stopped doing it because many of those tires were not usable the following season. The problem is that you must keep the tire constantly inflated, despite the fact that latex tubes slowly lose air. If you leave it unattended over the summer long enough that the tire deflates completely, the inner tube will be completely stuck together at its wrinkles, and inflating it will tear the tube; your nice tubular will be an expensive piece of garbage.

I found that once I stopped using sealant as a preventative measure, I was much happier. The tires were lighter and the valves worked as they were supposed to. I saved time and my tires lasted longer.

While sealant once saved my tubulars from flatting due to goatheads that were taking down many of my competitors in one race long ago, I never had a thorn puncture in many following years of racing without sealant. I did explode a tire at a most inopportune time in a championship race because I had let the casing get too worn, so I replace tubulars before they get to that point. With thin, superlight cotton cyclocross tubulars, this means replacing the tire while the tread shows very little wear, therefore making it maximally resistant to punctures. And unlike butyl tubes, latex tubes can take a tremendous beating at low pressures without pinch flatting, further reducing the need for sealant.

I do use sealant once I develop a slow leak or get a puncture in an otherwise good tubular. I just no longer use it as a preventive measure.
― Lennard

Broken crank arms

Dear Lennard,
I read with interest your recent answer discussing fatigue of carbon fiber and metals. I recently experienced the failure of an aluminum crank arm during my morning commute (see attached picture). Fortunately, I was in an area with light traffic and was just barely out of the saddle when it happened, and I stayed upright.

Examination of the break reveals a dark-colored area which seems to indicate the presence of a crack that existed for some time. Do you agree, and if so, do you have any advice on how to inspect for and detect faults like this?
— James

Dear James,
I have seen cranks break like this many times. While you are correct that the darkened area indicates a crack existed for some time before finally completely giving way, it is not easy to detect a crack like that. If they happen at the pedal eye, the pedal axle obscures the crack. And up in the middle of the crank like yours, they are still hard to see. There is little movement at the crack until it finally gives way, and the crack in its pre-failure stages is a very fine line that, in most cases, you are unlikely to see without inspecting it under high magnification. And scratches from the shoes rubbing past the crank face tend to obscure cracks.

Aluminum, particularly high-strength aluminum with an anodized coating, tends to not give a lot of warning before failing due to fatigue. The anodization (aluminum-oxide coating) is harder and more brittle than the aluminum underlying it, so crack propagation is rapid once it forms in the anodized coating. It is akin to a piece of bread coated with a thin layer of ice. Without ice on it, the bread slice flexes easily and bends quite far before it breaks. With a layer of ice on its surface, the bread slice resists bending much more strongly, but flex it enough that a crack forms in the ice, and that crack will immediately propagate right through the bread.
― Lennard