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Technical FAQ: Electronic for ’cross, TPI explained, and more

  • By Lennard Zinn
  • Published Oct. 8, 2013
  • Updated Oct. 30, 2014 at 5:31 PM EST
The diameter of this handlebar is larger on top than at the ends and in the hooks, and the transition in diameter is at the upper part of the main curve of the drops. Photo: Lennard Zinn | VeloNews.com

Cyclocross season is hitting its stride and this week we take a look at a number of reader questions specific to the off-road discipline. One reader also writes in to advise those racing and dealing with rashes from their chamois to check up on their creams and consider requesting Therapeutic Use Exemptions.

Electronic shifting for cyclocross? Really?

Dear Lennard,
I see riders racing cyclocross on Shimano Di2 and some ’cross bikes come with it now. Why would anyone use Di2 for that? Wouldn’t it get wet and stop working? And isn’t it awfully expensive to risk breaking in a crash?
—Alonso

Dear Alonso,
Actually, I think it is more of an advantage in ‘cross than on the road. I am the only guy I’ve ever seen using Campy EPS electronic shifting in ’cross, and on some courses, I feel that it’s a really big advantage. I haven’t used Di2 in cyclocross, but our own Chris Case wins elite local races on it.

A couple of weekends ago, we had a race here at the nearby Flatirons Mall on a grassy hillside above the Denver-Boulder Turnpike. One corner was an uphill buttonhook around a tree after dropping off of a downhill sidewalk and descending along the sidehill. I noticed most riders in my category repeatedly pumping their lever to shift from their smallest cog to their largest in anticipation of the buttonhook that required coming to a near stop and then turning sharply left uphill. They couldn’t pedal hard down toward the corner due to the decreasing gear as well as all of the hard effort from their right arm. I, on the other hand, could pedal most of the way to the corner in a high gear, and just as I started applying my brakes, I could just hold down the right downshift lever and keep turning my feet. It doesn’t require nearly the force, concentration, or time to make the shift all of the way from one end of the cogset to the other, and I closed some gaps on that downhill that way.

The same thing could be said about many points on that course that abruptly went from steep downhills to steep uphills with a barrier or a sharp corner, or both, between them.

Also, I have yet to have my derailleurs to get too mucked up to work. In fact, they shift so powerfully that they overcome a lot of mud that might thwart regular derailleurs. And, of course, there is no cable to get sticky.

With one exception, riding in rain or wet mud has also not been a problem. In a huge mountain rainstorm, my electronic system once failed and I was told by a Campagnolo representative that it would be a warranty item because the housing on the Power Unit that contains the battery must have a crack in it that allowed water in. Since it hasn’t happened before or since (that was over an hour in hard rain, and I haven’t found myself in a similar situation with that bike), I have not done anything about pursuing that. Simply watching pro teams on electronic shifting racing for hours in the rain leads me to believe that rain is not a problem with electronic shifting. And a battery inside the seat tube, which Campagnolo now offers, would probably eliminate even that concern.
―Lennard

Explaining tire TPI, in depth

Dear Lennard,
Can you please explain TPI on bike tires? How is it measured? And how does it affect riding road and cyclocross?
—Jim

Dear Jim,
TPI of tire casing is Threads Per Inch. However, it’s not such a simple answer, because it is measured in a particular way, and there is misinformation out there.

A casing is generally made of threads laid down adjacent each other and stuck together with latex or compounded rubber. It is cut on a 45-degree bias, flipped over, and laid over another strip cut on the same bias, so the threads in the two layers are perpendicular to each other. Mark off an inch longitudinally on this casing and measure the threads within it. The TPI is the number of threads in an inch of casing on the top plus that on the underside. In other words, one layer of casing is actually two layers of parallel threads that are laid at angles on top of each other, and the threads in both layers are counted.

When comparing tires, though, make sure you’re comparing apples to apples when it comes to TPI. The important point is that folding a casing over does not increase TPI; it’s still the same casing, just folded over. A good example is this Continental Grand Prix4000 S II tire. Notice in the diagram how the casing wraps around each bead and then the edges overlap under the tread, making three layers of casing under the tread. You can see in the chart below the illustrations that Continental calls out “3/330” for the TPI, which means that three layers are counted to come up with the 330 TPI. It should be clear from what I have said that this is a 110 TPI casing, and that overlapping it does not change the actual casing TPI.

A handmade tubular or “open tubular” clincher tire is generally made by folding the edges over and sewing them together (tubular) or folding them over a Kevlar bead (open tubular clincher). But the doubling over of the casing along the edge is not added to the thread count. When it says 290TPI or 320TPI on a high-end handmade tire of this type, that is what’s in one layer of casing (which is two layers of threads).

As for how it rides, the casing is more supple the thinner the threads are (assuming they’re not stuck too tightly together). It more easily absorbs small inconsistencies in the riding surface, and this reduces the rolling resistance and increases the grip by increasing the amount of contact between the tire and the riding surface.

The threads in a 290TPI or 320TPI tubular are very thin — 1/145 or 1/160 of an inch thick, respectively — and they are only held together with latex painted onto them; the casing is not vulcanized, which sticks the threads together more strongly and stiffens it. A 60TPI tire (even if it is called a 180TPI by counting three layers folded over under the tread) has thick threads that are 1/30 of an inch thick, and they are vulcanized together; that casing deflects less easily over road imperfections. These effects are exacerbated in cyclocross, due to larger imperfections in the riding surface.
―Lennard

How do I keep my white tape clean?

Dear Lennard,
How do I keep my white bar tape clean during cyclocross season?
—Todd

Dear Todd,
There was a time when bar tape was so cheap you just changed it when it was dirty, but those days are long gone (the tape is better now, too!).

First, make sure you use tape with a slick top surface. Tape with a matte or foam top surface in any color other than black will constantly be a bear to clean.

Second, I suggest picking a different color. I have enough trouble keeping the red or orange tape on my CX bikes clean, and that is with shiny tape.

Third, clean the tape (and the saddle) before cleaning the rest of the bike, as you may forget to use a clean brush and can smear grease from the drivetrain on them, making your task all the harder. Start with dish soap and water. If that doesn’t do it, I’ve had good luck with bike cleaners from ProGold, Motorex, Finish Line, Pedro’s, and Simple Green.

Important note before taping bars for cyclocross: Before wrapping handlebar tape, make sure your levers are tightened very securely to the handlebar. This is particularly important with bars that have ergonomic shaping on top or otherwise increase in diameter above the hooks. I also recommend not pre-taping the cable housings down. If you hit a bump hard with your hands atop the levers, one or both of the levers can slide downward; this will yank on the cable but not on the housing, which will apply your brakes. I have seen riders stop themselves dead in their tracks this way. I’ve had it happen to me with a bar that decreased in diameter from a wide top section, even with the lever fully tightened. Problem was, I wanted the lever as high as it could go on the bar, and so the upper part of the band clamp was on a larger bar diameter than the lower part. So, once it moved, it was on the skinnier part of the bar, and it was now very loose. So any bump with my hand atop the lever would now apply my brake!
―Lennard

Try Seam Grip instead of Aquaseal

Dear Lennard,
I just saw your recent columns on thinning Aquaseal. I just wanted to give you a heads up to try Seam Grip. It’s made by the same company as Aquaseal (McNett) and it is the same product just a thinner consistency. It works flawlessly and we have had no issues with it; this will be our third season using it on Dugasts. It is also available from QBP in big tubes (part no. OA2516).
—Zach Edwards, North Service Manager
Boulder Cycle Sport

Get a TUE for rash creams

Dear Lennard,
Just noticed that since the rash issue began (http://velonews.competitor.com/2013/10/bikes-and-tech/technical-faq/technical-faq-even-more-on-hypoallergenic-chamois-and-diluting-aquaseal_304742), no one has mentioned that topical creams to treat a rash require a Therapeutic Use Exemption, since most have corticosteroids in them.

I have no professional advice beyond that, but since there are a lot of sanctioned racers that read your column, it might be advisable to remind them of their due diligence.
—Michel

FILED UNDER: Bikes and Tech / Technical FAQ

Lennard Zinn

Lennard Zinn

Our longtime technical writer joined VeloNews in 1987. He is also a framebuilder, a former U.S. National Team rider, and author of many bicycle books, including Zinn and the Art of Mountain Bike Maintenance and Zinn and the Art of Road Bike Maintenance, 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 degree in physics from Colorado College. Readers can send brief technical questions to Ask LZ.

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