TECH TALK
(reprinted from Dirt Rag Magazine)

Wheelbuilder Dave Thomas on wheel building

I am constantly being asked for my opinion about which hubs, spokes, rims, and lacing patterns are best for a given individual. This comes as no surprise as I make my living constructing bicycle wheels. Although I can only make specific recommendations for wheels that I personally build, there are some aspects that will be consistent enough to be considered good general information.

The first thing to remember is that front and rear wheels are really two different animals. Front hubs have a symmetrical "center to flange" dimension that is plenty large, (7Omm between front flanges means 35mm center to flange dimension for each side). Rear hubs are generally shoved off center to make room for the gear cluster. This makes for an imperfect structure which unfortunately is subject to even greater stresses than a front wheel due to drive torque and brake slides during cornering. Logic dictates that strength factors must be added to the rear wheel in order to maintain a strength-balanced wheelset. These factors include heavier rims or spokes, and greater numbers of spokes. Bontrager rims are offered in two different weights, one for rear wheels, one for fronts. When equal numbers of spokes are used, this makes sense as does using heavier spokes for the rear. I feel that the best solution is to add spokes to the rear wheel as compared to the front, while maintaining identical rim sections. This will provide consistent dent resistance while increasing resistance to the typical rear wheel failures, mainly spoke breakage,(?) difficulties, and "potato chip" syndrome. (You can call it wheel taco, pretzeled, or any other food that isn't round). Lighter riders may find it appropriate to use 28 spokes on the front while heavier riders would be well advised to have 36 for the rear. Before you say "Whoa, thirty-six spoke wheels are dead," consider this point: 36 15-gauge spokes actually weigh l0gms less than 32 14gauge spokes. More, lighter spokes will also be stronger because there will be less unsupported area at the rim and the lighter wire (spoke), beingmore elastic, will reduce the possibility of cracking the rim at the holes.

Another point for rear wheels is the disparity in tension of drive vs. non-drive spokes. In order to center the rim over the usually offset rear hubs, drive spokes must typically have 60% greater tension than non-drive. For good results, spoke gauges must be mixed, otherwise drive spokes will be pushed to the elastic limit while non-drive spokes could end up too loose. Both conditions contribute to premature spoke breakage. (Heavier spokes go on the drive side).

Another trick that reduces the effect of hub offset up to 50% is the mixing of "heads in, race lace" and "heads out one cross" on drive and non-drive sides respectively. Unfortunately, these techniques are too difficult to explain verbally so I think it's time for me to say adios until next time. Note: if you don't know what the hell he's talking about, find Dave's ad in this issue and give him a call. -Ed.

RESPONSE LETTER TO ARTICLE FROM KEITH BONTRAGER

Good article (See Tech talk by Dave Thomas DR#26). I like it when someone who has seen the problems and knows how to fix 'em talks about it.

To add fuel to the retrogrouch debate, this year's 8 speed hubs require more dish, and we all know what that means. Santa Cruz has a bunch of good Mexican restaurants, and taco availability has never been a problem. I don't think we need more.

We actually build wheels and destructively test to ensure taco resistance. I've personally crunched 50+ rear wheels to date!

Your ideas on spoke count seem good. I would personally emphasize the benefits of mixed gauge spokes.

Our experience is that the rear wheels require heavier sections in order to balance dent resistance. Since ten weight distribution of a typical bike is 60% rear, 40% front (actually this is as much weight as is likely to be put on a front wheel; it typically is 65,35 or more) a bump that hits both wheels will result in a larger force applied to the rear wheel. So, the brake horns and side walls are thicker on the BCX rims.

Keep up the good work,

Keith Bontrager