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How round can a bike wheel be?

Ben Sharp -

I pride myself on the precision of my wheels and I’m fortunate to spend enough time on every build to ensure they are the best that they can be. One question that I frequently face is “How Round are my wheels?”, or “What tolerance can you build my wheels to?”. Because rounder is better, right?

This seems like an easy question to answer, which would prove beyond doubt to the client that my wheels are the roundest, or the best. In reality it’s a lot more complicated, and I feel it’s beneficial for clients to understand how round their wheels really are, why this is, and the effort that goes into the finished product.

P&K Lie truing stand for measuring wheel tolerances.

The simple fact of the matter is that every wheel is unique. Each one is built to a slightly different tolerance, and even two wheels built with the same specification of components will be different from one another. To this end, every wheelset I build comes supplied with a Birth Certificate detailing its specific runout, tensions and vital statistics. This is an interesting document for the client, which goes a little way to showing the hard work in every pair of wheels, but it is also crucially important when I receive wheels back for regular servicing or repair. It enables me to track the performance of components over time and understand how or why a component has failed and decide if it is safe to repair.

When I first started building wheels the quality of my work and the tolerances radially, laterally, and in spoke tension that I could achieve were limited by my experience. This is probably the case for all inexperienced builders, and those who simply don’t build on a regular basis. As the depth of my experience increased and I began to understand in greater detail how my manipulation of certain spokes affected the wheel, I was no longer limited by experience, but by the quality of the components I was using. In a high-end wheelset this can be mitigated against by using good quality components, BUT in the age of mass manufacture there are always going to be differences in wall thickness of rims and thickness of spokes, or the quality and consistency of rim joints. They might be minute fractions of a millimetre, but they will all have an impact on the tolerances of the finished wheel.

For an inexperienced builder it is very possible to build a wheel with excellent lateral and radial runout without paying close attention to spoke tension. Without taking any measurements it would be easy to judge this wheel as being better or rounder than another. The problem here lies ultimately in the longevity and safety of the wheel. Even spoke tension ensures the wheel is balanced, stable, and all spokes are sharing the stress of pedalling torque, braking force and impact. The more even a spoke tension you can achieve, the longer a wheel will last under hard riding conditions, and the better it will cope with a severe impact without totally disintegrating at the rim or hub flange. To this end, good wheel building is all about compromise. It is about looking at the set of parts you have in front of you and understanding through depth of experience the ideal combination of lateral and radial runout and spoke tension that will enable the wheel to perform at it’s maximum for as long as possible. Each wheel will have a slightly different combination of traits, but the end goal is the same.

Tensiometers used to ensure even spoke tension.

If I had to attach hard figures that I try to achieve in all my builds they would be as follows:

Lateral runout: as close to +/-0.05mm as physically possible. This goes for alloy and carbon rims, although some lighter alloy rims will “flutter” at high tensions.

Spoke tension: within +/- 8%. Although in most cases the overall tension of the wheel will exceed this standard, there will in some cases be a greater intolerance at the join of the rim where the rim tends to be heavier due to a glued and sleeved or pinned joint and needs a little extra help to be pulled into shape. For this reason, we separate out this metric on the birth certificate.

Radial Runout: +/- 0.2mm. This is for the overall or average radial runout. I split out the radial runout metric on the birth certificate into two parts, so the client can understand that there will be a greater intolerance at the join where the two sides of the rim may not meet perfectly, or there may be mould lines or “steps” on carbon rims from the moulding process. Sometimes removing these intolerances completely is either impossible or would have too detrimental effect on the spoke tension.

These are targets for every build, which will often be exceeded. Of the three tolerances, radial runout will usually be the main source of compromise in the build. This probably has the least effect on the overall performance of the wheel, due to the lesser standard of manufacture of tyres (which aren’t perfectly round), and less detrimental effect (pulsing) on braking. Regardless, the tighter you can get these tolerances in a built wheel, the “rounder” it will stay under the force of a highly inflated tyre. It is all about minimising losses in tolerance further down the chain.

In conclusion, every pair of wheels I build are “as round as they can be”. Each pair is subtly different, but within each pair is contained every ounce of knowledge and experience that I have, ensuring that those specific components are performing at their absolute best and will continue to do so for as long as possible. It would be too easy for me as a skilled builder to tell every customer that their wheels have perfect spoke tension and are radially and laterally built to +/- 0.05mm, but it would be an oversimplification. I hope that honesty with my clients will engender trust and educate them. It’s important to try and understand what you ride, and it’s important to know that each wheelset is truly unique.

Please get in touch if you’d like to learn more or have any specific questions to ask.

If you found this informative, please spread the knowledge by sharing.

Ben Sharp.