What makes a good wheel: Q&A with Bill Mould

Yale-educated scientist and master wheelbuilder talks about what’s important

Bill Mould enjoys teaching (and continuously learning) the physics of bike wheels

Bill Mould knows more about bicycle wheels than most people on the planet. He has produced a series of video tutorials, hand built more than 4,000 wheels, and passionately pursues the sharing of knowledge, whether that is teaching others in person or in videos, or continuing to learn himself through classes and conversations with experts around the world.


Mould is a Yale-educated scientist who served as an Air Force colonel, college professor, and civilian pilot, among other things, before ‘retiring’ into his passion as a master mechanic and expert in bicycle physics and wheel building.

We checked in with Mould to talk about what makes a good wheel.

BikeRadar: I understand that until a few years ago you were a chemistry professor teaching organic chemistry at the college level.

Bill Mould: That’s true. I really enjoyed it, but I got so busy with wheel stuff that I had to give up something. It sure helps me understand carbon composites and certain things about material science, though. And, I didn’t have to start my wheel research by learning the metric and SI systems from scratch. I wish the U.S. would get on board.

How did you get so deeply involved in building wheels?

Well, I started out many years ago as a bike mechanic. I learned how to build wheels, really enjoyed it, and my company decided to have me focus exclusively on wheels. I went to a number of schools and really dove into it, particularly the physics and engineering aspects of bicycle wheels.  Fortunately, I have the formal scientific education to get pretty deeply into it.

How many wheels have you built?

About 4,000, of every conceivable type and purpose. There are myriad ways in which a hub, rim, spokes, and nipples can be strung together to make a wheel.  The optimal combination for a given situation is a matter of informed judgment that takes quite a while to master. There is no formula that will spit out the right answer.

Things that are counterintuitive intrigue me, and spoke tension is one of those

What makes a good wheel?

A simple answer is a wheel that meets the needs and expectations of the particular rider. But, of course, it’s more complicated than that.  What you have to take into account for each wheel is the rider’s budget, body weight, type of use, a balance between strength and light weight, and more.

I picture myself juggling all these variables to come up with the right combination of depth of rim, cross-sectional shape, number of spokes, thickness of spokes, type of nipple, round or aero spokes, hub dimensions, and more.

How important is spoke tension?

Extremely important. Things that are counterintuitive intrigue me, and spoke tension is one of those. Many people believe that a common cause of spoke breakage is too much tension on the spoke. Actually, the opposite is true. I have studied this very closely, and there is no doubt.

What about rims?

Interestingly, my studies tell me that when it comes to supporting a vertical load, the potential wheel strength depends on the rim. That assumes that a competent wheel builder built the wheel, which unfortunately is not always the case.

The shape of the extrusion matters and so does the amount of material in the cross section. I’ve done a lot of compression testing in laboratories that demonstrate this relationship very clearly.

Wheels cannot be a shock absorber and still be maximally strong at the same time

I’ve built wheels for guys who weigh in excess of 500 pounds. Wheels are not collapsing, and spokes are not breaking. And, of course those folks are getting some beneficial exercise.

What are you thoughts on wheel stiffness? Is stiffer always better? Or should a wheel offer some compliance, too?

Let’s first decide what we mean by vertical compliance and just define it rather loosely as how comfortable the bike will be in absorbing some of the worst road roughness, thereby making the ride more comfortable.

My belief is that we should look to the frame, saddle, seat post, biking shorts, etc., for comfort and not the wheels. The wheels cannot be a shock absorber and still be maximally strong at the same time.  Any springiness in the wheels can only come from spoke tension that is too low to provide efficient power transfer and a strong, durable wheel.

There is room for debate on this subject, but that’s what I think. If some of your readers have a different opinion, I would love to hear from them.

Do you like carbon rims?

I do! Carbon is light and strong. If you take two wheels, an aluminum wheel and a carbon wheel with the same depth and cross section, the carbon rim will weigh considerably less, perhaps 50% less, because of the much lower density of carbon.

That’s why deep section aero rims from companies like Zipp, ENVE, and Boyd are all carbon. Their weight would be off the charts if they were made from aluminum alloy. For your British readers, maybe I should have said aluminium!

I should add one thing, though. I can tell a lot about the quality of an aluminum rim by a close inspection. Not necessarily so for a carbon rims, so it is important to know the pedigree of a carbon rim, particularly who made it and what your recourse is if there is a flaw. Buying rims off the internet from an unknown manufacturer is a crapshoot in my opinion.

You like teaching, don’t you?

Yep, sure do. I’ve made lots of instructional videos about wheels, and I teach people how to build wheels in one-on-one classes, just one student at a time.  What I would actually like to do is start sending you short videos on a range of topics about wheels, some general and some pretty nerdy. People often like to see something explained in a video instead of just reading words that describe it.

Well, that’s a good start for us.  How about sending some of your educational videos for our site?

Sure. I’ll pick some out and start sending them to you. Your readers might be relieved to learn that I avoided using calculus. I’ll try to keep them short enough that they won’t crash the internet.

Stay tuned for more with Mould. In the meantime, you can check out some of his videos at www.billmouldwheels.com.


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