Life is expectation tempered by reality. Marriage forces a trade of the GTi for CrossPolo. You bought Capco instead of Capitec shares with your bonus five years ago. And then there’s the worst paradox of choice facing us all: aluminium frame with carbon wheels. Or carbon frame with aluminium hoops?
In an ideal world of the six-hour work day, year-round autumn weather and decipherable Discovery Vitality rewards, we’d all be riding mountain bikes where both frame and wheels are a matching unity of string and glue tension material. It would be a universal income outcome of carbon.
Unfortunately, we live in a country of 12-hour traffic commuting weeks, increasingly unpredictable weather and sickening data prices. Our only escape from all these travails are our mountain bikes, which have to conform to budget, implying a sacrifice in available carbon: you can have it as either frame or wheels.
The consensus reasoning implies that wheels have a disproportional influence on performance, so you’d be better off with carbon rims, rolling an aluminium chassis. Right? To me, it’s a very real question, because I’ve just built an aluminium mountain bike with carbon wheels, to replace a carbon frame rolling aluminium hoops. Initial trail testing has yielded unusual results.
On my carbon dual-suspension frame, the bike felt notably more forgiving over technical terrain or during high-speed riding over low-to-medium frequency trail chatter. The aluminium bike, with its carbon wheels, has required me to reduce fork and shock pressures - below the recommended minimums - in an attempt to quell the harshness. In my mind, a carbon wheelset, of similar dimensions, with matching tyres, would be superior in every way to aluminium rims, but there are very definite opportunity costs to be calculated – and I’m not merely referring to price.
Pretty is Performance. Not comfort.
Mountain bikers are peculiar about aesthetics. They might dress badly in their lives beyond cycling and have awful installations of fake rock cladding on their property, but hours will be spent agonising about matching grip and seat colours, sock choice and decal kit upgrades.
With its intricate shapes and range of finishes, carbon is a magical material offering the promise of unrivalled performance and elegant aesthetics. It’s not often that engineering components happened to both look good and work well – as is the case with carbon.
Requiring fewer spokes – due to the inherent rim strength – and with a clever layup, carbon wheels can be a percentage of grams lighter, equalling a tremendous performance gain, due to the energy sapping of effect of rotation mass.
The gains in acceleration and unflinching tracking and steering accuracy with carbon wheels, must come at a cost. And this cost is not only a loss of compliance and the debit in comfort, but perhaps something we rarely stop to consider…
Stiffness and stopping
‘What does my carbon rim stiffness have to do with braking?’ It’s a fair question, but the causality isn’t that complicated. Unlike ordering pizza toppings or being like-minded on the merits of a specific energy gel, we can all agree that carbon wheels are notably stronger and stiffer than aluminium rims. The simplest test, if you have a set of old aluminium rims, is to remove the tyres and try to do a calisthenic dip between them, using the rims as a set of parallel bars. You’ll feel the buckling deformation.
It’s this malleability of the aluminium rim which gifts it superior ride comfort, but also absorbs power transfer, robbing riders of a true conversion from their energy-to-torque output. Aluminium’s flexibility might also have an odd benefit regarding braking.
The idea is postulated by British veteran downhill racer, Enduro champion and master wheelbuilder, Robert Cooksley. His theory is that an aluminium rim deforms slightly under braking load (‘squashing’ – if you will), elongating the tyre by an admittedly tiny margin, to provide a greater area of contact with the trail through which to apply braking force.
‘But I run low tyre pressures already, my tread is elongated at the contact patch too, so what’s the point?’ Well, if you are riding carbon, perhaps not. I’ve found that slightly higher tyre pressures are the norm on my carbon rims, for fear of suffering a catastrophic rock strike. It’s a counterintuitive action – true – but I’d wager many carbon wheel riders do the same, aware that their rims are stronger than aluminium, but also impossible to dent remedy if something goes very awry.
If we consider Cooksley’s theory, the compound effect of a stiffer carbon wheel, shielded by higher tyre pressure, could possibly cede braking superiority to the softer aluminium rim rolling a tyre with less air volume in it. Even with both wheelsets running similar tyre inflation pressures, the vertical compliance of an aluminium rim should – theoretically – provide superior braking, as it deforms slightly in support of the elongated tyre contact patch its carrying.
Carbon wheels are magnificent in their combination of strength and lightness. A set of carbon mountain bike wheels are immensely accurate in their behaviour on the trail and the power transfer is terrific, but they can be harsh and potentially too accurate for the inexperienced or less committed.
If you’re a dedicated amateur racer or someone sufficiently confident to manage potential high-speed deflections in technical terrain, the potential performance value of a set of carbon wheels are indisputable. For weekend warriors, traditional ‘soft’ metal rims, with axles turning in a carbon frame – might be the better combination of chemistry and metallurgy.
To mitigate against catastrophe, I run the tyres on my carbon rims at slightly greater pressure, with less air in the suspension bits, to compensate. The consequence of it all is incredibly precise steering (leaning) response and matchless acceleration, with a noticeably reduced margin of error.
Carbon wheels have been an even greater influence of my riding, requiring more tinkering with the pressure gauge and pump, than a composite frame with alloy rims ever was. With carbon rims, the tyre pressures are up and suspension pressures down. The most peculiar consequence of rolling carbon hoops are that the raft of contemporary fork upgrades from MRP and Luftkappe, are now starting to make a lot more sense to me.