Carbon wheels. Aluminium frame. Or the opposite?

Light. Strong. But there are features of riding carbon wheels that are never really discussed. Let’s have a chat.

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?

 

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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.

 

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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


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An attribute of the stiffness inherent to carbon wheels, possibly affects an element of mountain biking performance that is misunderstood and underappreciated: braking.

 

‘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.

 

Under Pressure


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.

 

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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.




10 Comments

BikeHub Plus, Jul 04 2017 08:49

friends don't let friends buy a cross polo

NicoBoshoff, Jul 04 2017 10:19

One thing I've been considering of late is the raft of tyre inserts available, specifically Huck Norris and CSixx FOAMO.  They're light enough to merit using them with your carbon hoops, and perhaps a little less armour on your sidewalls.

 

So e.g. instead of going for the DD casing on those Maxxis, I can drop 150g to the normal Exo casing with the insert (at about 100g an insert), allowing me to run a similar weighing set up, run lower pressures and still enjoy the benefits of carbon circles.

 

It does however add R600 to your wheel setup, but then again, destroying a rim makes that seem more than worth it.

nonky, Jul 04 2017 11:21

Great, balanced review - not the usual: "IT'S CARBON, BUY IT".

 

thank you for this, which is probably the most helpful wheel comparison i've read online.

Headshot, Jul 04 2017 11:36

I dont think the massive money justifies the benefit, double edged as it is.. If I rode carbon rims they'd be written off - as it is, all the  rims have is a few small dings despite a couple of instant flat tyre punctures and lots of rocks.

 

Get that pressure wrong or suffer a slow flat you don't notice until its too late and bang, its new carbon rim time. The deformation that ali rims "suffer" from is actually their greatest benefit.

NicoBoshoff, Jul 04 2017 11:43

I dont think the massive money justifies the benefit, double edged as it is.. If I rode carbon rims they'd be written off - as it is, all the  rims have is a few small dings despite a couple of instant flat tyre punctures and lots of rocks.

 

Get that pressure wrong or suffer a slow flat you don't notice until its too late and bang, its new carbon rim time. The deformation that ali rims "suffer" from is actually their greatest benefit.

Ja but with the tire inserts of late I think one can build a wheel that really has all the benefits.

 

Granted, to get the trail buzz out of carbon one would have to go for the heavier inserts that aren't practical unless you're a proper racer, but I reckon you'd get bloody close.

 

Having said that, I ride carbon rims because they came with the bike.  I'd have been equally happy rocking some Flow MK3's on a good set of hubs.  Certainly not replacing them with more carbon if they poop (hopefully never).

Bakkies, Jul 04 2017 02:41

ok. we need a phase 2 of this test. performace vs performance of the 2 setups. spending the same amount on 2 different bikes and then compare the wattage required to climb a steep hill. example. a carbon giant anthem vs a anthem advance. the carbon whith alu rims and the alu frame with carbon rims..... I hope you get the just of my idea. my money will be on the alu frame with carbon rims

Beefy, Jul 04 2017 02:56

ok. we need a phase 2 of this test. performace vs performance of the 2 setups. spending the same amount on 2 different bikes and then compare the wattage required to climb a steep hill. example. a carbon giant anthem vs a anthem advance. the carbon whith alu rims and the alu frame with carbon rims..... I hope you get the just of my idea. my money will be on the alu frame with carbon rims


Lance may need to apprehend the perpetrators who nicked his bike first.

Beefy, Jul 04 2017 02:57

What wheel rims must I use on my Steel frame bike?

Headshot, Jul 04 2017 03:44

‘It is more tolerant & forgiving, offering increased confidence and more cornering grip. If ever there was a bike to make you smile, this is it!’ –

 

 https://dirtmountain...OV6FyYFghM1L.99

 

This is what the steel frame dual suspension builders say about their product. The bike tested by Dirt is equipped with Stans Flow rims. On their version, carbon wheels and frames would probably be "less tolerant and forgiving, offering less confidence and less cornering grip...." And fewer smiles.

 

It really does depend on who you talk to in the bike industry doesn't it...

MoreTrails, Jul 07 2017 12:52

What wheel rims must I use on my Steel frame bike?

Bamboo....

 

Or thermoplastic if you really want to bring the bike bang up to 20th century