Is it safe to buy carbon rims?
Let’s get one thing straight from the start; carbon fibre is a stronger material than aluminium. So you don’t need to worry about it being structurally strong enough for use in a rim. However, unlike aluminium, it is not so good at dissipating heat and temperatures can get much higher if you use a carbon fibre braking surface.
This does not affect all carbon fibre rims. If you are running disc brakes there will be no heat build up and no concern. This applies to both tubular and clincher rims. If you are running tubular rims – even if the temperatures do get too high – where the tyre is bonded on and there is no sidewall, you don’t need to worry about sidewall failure – the tyre can naturally run at a higher pressure without concern, so if pressures rise from heat, it may feel harsh but it does not present a safety concern. All that will happen is a change in the surface of the sidewall. The rim may be damaged but it will not ‘fail’ (ie: your tyre will not blow off). So if you are using either disc brakes or tubular rims you do not have to worry about heat.
Most people on road bikes do not use disc brakes and they also do not use tubulars. So heat failure should be a consideration for most riders… There are a number of ways you can guard against this failure.
Various high-end carbon fibre rim manufacturers have used various techniques to overcome the issue of heat failure. Heat failure occurs when too much heat goes into the rim from braking. This heat causes the air inside the tyre to expand, increasing the pressure. As the resins in the carbon fibre start to denature the carbon fibres come apart with the forces applied by the tyre. ENVE carefully manage the temperatures inside their rims (ie: they have designed the rims in such a way that not very much heat builds up in the first place). Zipp use high temperature resins that can tolerate high temperatures without boiling. Various other manufactures weave other materials into the braking surface to help with the dissipation of heat. Another solution, pioneered by Xentis, is to remove a lot of the resin from the braking surface, which helps to prevent heat build up and also has the benefit of improving braking performance (carbon fibre offers an inferior braking performance to aluminium). Xentis, Zipp and ENVE rims are all available to purchase through DCR – contact for details.
I sell a range of all-carbon clincher rims under my own brand (DCR). These use a combination of high temperature resins, generally titanium reinforcement in the bead and a basalt layer to overcome the risks of heat failure. The high temperature resin can tolerate high temperatures without failing. The titanium reinforcement helps to dissipate heat and provides additional structural support that isn’t reliant upon the resins remaining intact. The basalt layer helps to improve braking performance and dissipate heat. The rims themselves have no internal bladder and no foam core. This hollow cavity inside the rim also helps to manage heat as most rim failures occur when this cavity acts as an insulator.
To add a further layer of caution, I do not recommend these rims for riders over 100kg nor for prolonged descending at speed. If you do undertake a big descent, it is wise to reduce your tyre pressures first, often it is pressure, rather than pure heat that causes any issues. Thankfully issues are now rare. I recommend checking these rims regularly for signs of bubbling. However as long as you heed my guidelines there should be no reason for you to have any issue with braking.
The only brake pad I recommend using with DCR rims is Gigapower. That will produce a lower overall temperature than most pads as well as offering very good braking performance. All of our rims have a UD (unidirectional) matte finish. They have all been tested to withstand tensions of 300kgf – far in excess of what any spoke or hub can tolerate.
Some may question the quality of DCR carbon rims based on their competitive price. It is usually safe to assume the case with bicycle parts is that you get what you pay for. Quite simply in this case, these rims come to me direct from the factory. There is no separate budget required for marketing, branding, there are no decals, there is no cut for an importer/distributor. The profiles are based upon the sorts of profiles that were used by high-end manufacturers a few years ago before the patents expired so although they are not wind-tunnel tested, they are sound. They are made using a standard Toray T700 carbon fibre – common for use in rim applications but not considered an exotic carbon fibre. The layup is stable and makes no creaking or cracking sounds when stressed. The material is used in generous quantities to make the spoke beds nice and thick allowing them to tolerate massive tensions if required. T1000 (an example of a more exotic carbon fibre) would allow you to build a lighter rim but it is more brittle and tends to make for a harsher ride. The material itself is also more expensive. The weight gained by any extra material with the T700 can be made up for by the lower spoke count it allows and the aerodynamic benefits of the profiles.
The available carbon fibre rim options are available here
Still not convinced?
Carbon rimmed wheelsets are available with rims from:
WR Compositi
Xentis
ENVE
Dymag
Zipp (only with Tune hubs – still built by DCR)