By coincidence, I happen to be in the process of replacing a conventional rim damaged after hitting a pothole the force of the impact was sufficient not only to depress the rim where it hit the hole but to push it outwards at a point either side – this process necessary to accommodate the displaced rim material. With the Ksyrium wheel in question, it is probable that the sharp edges of the female thread – Mavic calls the technology ‘Fore’ – offered an initiator for the cracks, but all rims in wheels with tensioned spokes are susceptible. Spoke pull tries to ‘bow’ inwards the hub-facing wall of the rim, putting the hub-facing exterior surface in tension. Of course, spoke tension is what created the stress in the rim that started the cracks in the first place and, like all such fatigue cracks, repeated cyclic variation in the stress is what made them grow. Now in two halves and weakened by the crack, the female thread gives way a little and the nipple pulls through a little further, reducing spoke tension and the control exerted by the spoke in the rim.
Any additional tension experienced by the spokes – as, for example, when accelerating out of the saddle and generating lateral force on the rim – tries to pull the nipple further through the thread. The angled faces of the thread – 60deg in a metric thread – create a ‘wedging’ effect under spoke tension that pushes apart the sides of the female thread. These cracks, of course, were the cause of the sensation of imprecision from the rear wheel.
Extending either side of all but one of the drive-side rim drillings was a crack of greater or lesser extent. And the rim has reached the end of its days. Si’s wheel dates from 2007 and has seen plenty of race and training camp action. Spoke tension is high, especially on the rear wheel’s non-drive side, despite Mavic’s use here of ‘Isopulse’ radial lacing in order to even-up tension with the non-drive spokes. Otherwise, the wheels work in the conventional way, with the spokes under tension keeping the rim where it is wanted. The aluminium spokes – an industry first – sit in special nipples that thread directly into the rim, thereby avoiding the need to create a thread on the spoke itself. Introduced in 1999 and regularly upgraded by Mavic over the years since, these hoops are legendary for their durability in normal use. On inspection, the rear Ksyrium wheel proved to be less easily sorted. Unless both front and rear rims happened to be offset to the same side by the same amount.Ī few minutes spent adding tension to the drive side spokes sorted the dishing issue, while a few more corrected the radial ‘hop’ that had somehow found its way into the rim. Of course, had he centred the blocks, he’d have avoided the rub but the tyres would not have been in line. Otherwise, the rim would be pulled across to sit equally between the flanges…īy applying the same additional amount of turn to all the rear wheel’s spoke nipples, Simon had effectively increased the relative pull of the non-drive side spokes, pulling the rim over so that it was closer to that side’s brake block. This means, if there is the same number on each side, that they need less tension than those on the drive side to create the same axial pull. In a ‘dished’ wheel, where the rim does not lie exactly centred between the two hub flanges, the spokes on the non-dished side – non-drive in the case of a rear wheel for derailleur gears – have a more effective axial, or sideways, pull on the rim thanks to the angle at which they act on it.
While it cured, or at least alleviated, the apparent lack of stiffness, this intervention also had the effect of pulling the back wheel’s rim over.Ī quick check with the dishing tool showed it to be offset a good 2mm to the non-drive side, which is what you’d expect in the circumstances. A sprinter by temperament and physical gifts, he was unhappy with the stiffness under hard acceleration of his Trigons and, in an effort to improve matters, tightened the spokes. The brake rub ‘issue’ was easily resolved. Simon has been having wheel trouble, manifesting as brake block rub on his Trigon carbon-fibre rim wheels and a woozy, soft, flexible sensation from the rear when riding his Mavic Ksyrium SLs – so I took a look.