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Painting By Numbers

by Alan Milligan — last modified Oct 01, 2018 02:53 AM

Even when judiciously following a plan, things can go catastrophically wrong ...

It turns out that in the world of track cycling one is not spoiled for choice when it comes to getting aerodynamic and deep dish carbon wheels. Front axle diameter differs from the road, and the rear axle length differs.  In a pure track situation, brakes are not run so a carbon brake track is fine, but I am quite skeptical about this for durable road use: the resins used in carbon are really only good for 160-180C - which can be easily reached on a downhill; and the thermal dispersion/heat sink properties of resin are non-existent compared to any metal; this leads to deformed rims and quickly wearing down the brake track.

The only thing to do is therefore - to build your own from scratch.  Get some rims with aluminium brake surfaces (front at least), get some track hubs, measure them all up, get suitable spokes, and put them all together.  In the unlikely event this proves to be non-trivial, invest in a wheel truing machine and a spoke tensioning tool. 

Naturally, we are not at all bereft of information technology in all of this.  A collegue, Karl Stoerz, has a PHP-based website to make spoke measurements, and when it comes to tensioning them all up, there is the excellent Wheelwright desktop app which calculates overall wheel tension as you record the tension on individual spokes (see diagram below).

Mistake (i) - opting for blade spokes with an old-skool 32 hole hub; these may be great for modern radial hubs, but the cross-overs lack tension as there's less material required to bend over.  This also affects the spoke tool calculations, and I ordered spokes that proved to be 4-6mm too long.  So elected to go for some DT Suisse Competition round spokes (double butted 2.0/1.8/2.0mm).

Mistake (ii) - not settling for spoke tensions something of the order of my existing Mavic Open Pro rims (and same Miche Primato hubs).  Rather than use the usual Newton unit for this tension/force, the bicycle industry uses a measure, Kilogram Force (Kgf), to describe this.  A rim tension may be anywhere from as low as 80Kgf perhaps all the way up to 230Kgf - and a property of the materials/construction of the rim - and the limit probably known by the manufacturer.  A fairly standard target tension to aim for is 110Kgf.  These tension forces actually vary based upon the cross-sectional diameter of the spokes used - the wider the spoke, the more material contained within and the less deflection it'll go through to reach the required to reach the Kgf.

The Teny Bike tension meter I bought on ebay appears to be of fine build quality and consistent.  It works by measuring the amount of deviation when a known spring force is applied perpendicular to the spoke.   However it utilises it's own scale whereas the industry has defacto adopted the TM-1 scale as popularised by Park Tools (and their own spoke tension meters of course).  Teny have (possibly helpfully) provided their own charts to map their tension/spoke deviations to Kgf.

 Teny tension chart

Upon first go, I built and trued the wheel by feel, and discovered I had a Teny tension of ~4.7 which according to their translation chart above, would map to about 70Kgf - which appeared way under-tensioned.  I was kinda in two minds as to how to equate the multiple-diameter DT Suisse spoke to the chart - with it rather falling somewhere between the 1.8 and 2.0 columns.  With an eye to the 2.0 column I was aiming for ~5.8 Teny tension (which I though would give me ~ 110Kgf) which with brass spoke nipples was proving very difficult to tighten to.  

Alas I sheared a nipple at a 6.0 Teny tension during final wheel truing.  I elected to cut the spoke and relace a fresh one; but immediately upon cutting it, the rim spectacularly failed where that spoke had been.  Not only that though, on closer inspection, many of the other spokes had ripple/blister/deformations on the carbon fibre as well.  It's a little unclear from the radar diagram below, but the Teny tension numbers range from 5.8-6.0.

Broken rim Wheel building tools Wheelwright tensions

So now I'm in post-mortem mode trying to discover which/what numbers I had way too much faith in.  The spokes on my Mavic rims are 2.3mm with a Teny tension of ~5.0 - which alas doesn't even register on the Teny tool chart - hardly confidence inspiring of Teny numbers.  It would seem to me that a DT Suisse brass nipple is on the absolute limit of usable tension at a Teny 6.0 which probably suggests a Teny 5.0-5.25 as good working range.  I think that since 3/4 of the spoke length is 1.8mm diameter, I should have translated directly off that chart - but then even at Teny 5.3 this would only have given me a 100Kgf tension.  But then maybe 110Kgf is too much tension in the first place?

Since this was all rather destructive, I cross-sectioned the rim with my hacksaw and the following photo shows it's construction.  There are two layers at the top with 4mm of carbon and resin and a single sidewall layer of 1.3mm; just some very tiny voids between layers at the top.  The resin does look well bonded to the aluminium rim track.

Rim cross-section

Now that all of the information has been collated - off the various parties for comment and a new rim to try again ... 

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Alan Milligan

Alan Milligan

Alan Milligan

Location: Sydney, Australia
Alan Milligan
Alan is the principal technical architect of Last Bastion Network solutions in Australia. Alan's background is in application development with a number of global titans of retail and investment banking. Alan also has a history of CIO roles for a number of start ups where he delivers business value with open source solutions. Talk to Alan about how you can deliver critical infrastructure while mitigating risk and managing your existing vendor relationships.