Saturday, June 04, 2011

Bike wheels and wheelbuilding

So I've had some adventures with wheels and wheelbuilding over the last year. Several people have asked about the wheels I've been using lately. Since I'm physically incapable of doing anything productive after today's long ride, I thought I'd share some thoughts, and hopefully allow others to learn some things I've learned the hard way. Pretty much everything here is my opinion and ramblings from personal experience and research, and others will have different opinions for sure.

It all started when I wanted to build a Powertap wheel on the cheap. I'd already built up a couple of wheelsets, so I got the hub, Cervo electronics pack, cheap alloy rims, and spokes and misc. supplies from various sources online. I built up the totally conventional 32 spoke wheel using the great Sheldon Brown's directions, and everything worked nicely. The problem came when racing. There's useful data from racing with a power meter, but when you're barely hanging on with regular wheels, racing with a three pound flywheel means not hanging on, which is not helpful.

The next step was to see about lightening things up. A lot. The best way to do that would be with some mid-depth carbon tubular rims. Of course the whole idea of trying to do things on the cheap is quickly going out the window, but I really enjoy learning how to build things myself, so I procured myself some generic rims from Asia, which turned out to be excellent quality. You learn much more about a carbon component by looking on the inside, not the outside. In any event, this wheel built up to be the same weight as my previous alloy "race" wheel, with the added bonus of the weight being at the hub instead of the rim.

Tubulars were a learning experience by themselves. In short, the best solution was simply using tubular tape. This made a messy, difficult job easy. Basically, peel off the backing from one side of the tape and stick it to the rim. Mount the tire, get it nice and centered, which is easy because the backing is still on the outside-facing side of the tape. Once everything is lined up, peel off the backing (which is plastic and just pulsl out from between the tire and rim), inflate the tire to max pressure to settle it in, ride around a little bit to further ensure good contact, and you're done!

Important tip: choose the right tire for your rim. Vittoria tires (and Zipp Tangente tires, made by Vittoria) have a slick latex coating on the base tape that doesn't stick to the tape. Continental tires do not. Also, make sure the tire and rim profiles match. Some rims have an indentation in the center, designed to accommodate the stitching below the base tape of most tubulars. If the rim does not have this profile, then the new Continental GP 4000 tubular, which is vulcanized instead of stitched, has a smooth, flat base tape. Be sure your rim and tire profiles match, or your adhesion will be seriously compromised.

Another tip: the deep sections of carbon rims are great for aerodynamics, but it's important again to match the tire to the rim. Traditional rims are made for 21-22mm tubular tires or 21mm clinchers. Recently, though, 23mm tires have become the norm for good reason, but it throws off the aerodynamics of the wheel. This is the whole reason behind advanced rim shapes from HED and Zipp. They are designed around today's 23mm tires. If you're using conventional V-shaped rims, though, simply using 21mm tires will give you some of the benefits of the advanced, wider rims.

Having the experience behind me, good and bad, of the Powertap wheel, I built up some 50mm carbon clincher wheels. These were laced with 20 radial spokes in the front, and 24 two-cross drive side/radial non-drive side in the back. I used Sapim Race spokes, and Sapim Polyax nipples. These spokes are very strong, and look great because they're stainless steel and always shiny! The Polyax nipples work very well, since they can come out of the rim at the various angles of the spoke, and help prevent spoke breakage from being kinked at the nipple. From another experience building a 650c Powertap wheel, I've learned that round spokes are the way to go, and the aero benefit of bladed spokes over aggressively butted round spokes is minimal.

The deeper rim depth, and the extra material required for the clincher hook and stronger sidewalls adds weight over the tubulars, but without the Powertap hub, the package was still a bit lighter. The amazing thing, though, is the benefit of the very deep rims. This is what prompted me finally to post this, because I really noticed it today, and at a couple races in the last few weeks that I've used these wheels for. At speeds above 20mph, the aero benefit of the 50mm rims is very noticeable! It really is quite amazing. I noticed it today on our long ride that averaged over 21mph: cruising along at speed was noticeably easier! At Opus and the Target Crit, not only was it much easier to hang on at those speeds, but it also left me fresh enough to make some moves and go faster that I ever could! On these rims I'm using Zipp Tangente 21mm tires by the way. Going downhill today, every time I had to coast and ride the brakes to keep from running into everyone else, who were pedaling!

It seems like the aero benefit of 38mm wheels goes up to about 18-20mph, and above that the 50s come into their own. I'm not fast enough to realize the benefit of the crazy-deep 80mm rims, but I'm sure they would come into their own at pro-level speeds.

The other nice thing about these wheels will be using them for cyclocross season. Being clinchers, changing to cyclocross tires will be easy. From experience, open tubular tires (basically made just like tubulars, handmade, not vulcanized, but not sewn together at the end) with latex tubes yields a ride very much like tubulars. The deep profile will prevent mud from sticking, I'm light enough that I can run the tire pressure pretty low, and clinchers aren't going to roll off the rim very easily, and not having to deal with gluing tires is priceless! The tape method above is great for high-pressure road tires, where the tire pressure alone is enough to keep the tire on the rim, but would not be good for low pressure cyclocross tubulars, which are also subject to much more extreme lateral force.

A final note on economics: if you're building wheels because you're trying to save money, that's a totally lost cause. It's cheaper to buy a pre-made wheelset, and if you're new to wheelbuilding, probably safer too!

So, a few personal experiences and ramblings on carbon vs. aluminum, tubular vs. clincher, rim depth, spokes and nipples, tires, and tape vs. glue.