‘AFFORDABLE’ CARBON TRAIL BIKES

With £4k to spend, is a composite frame worth the compromises or are you better off sticking with alloy?

Carbon fibre is seen as the ultimate frame material, but are more affordable composite bikes worth the extra cash and kit sacrifices, or are you better off buying alloy if you can’t afford to spend big money?

Individual carbon fibres are exceptionally strong in terms of tensile (pull) strength for their weight. Using these fibres in an overlapping, multi-direction ‘lay-up’ of resin-impregnated ‘prepreg’ sheets, which are pressed into a mould and then baked to harden them, can create a very light but strong frame. Done right, carbon chassis are also as robustly crash proof as metal ones. They’re more repairable too, and don’t corrode or oxidise. So, getting a carbon bike if you can is a no-brainer, right?

Well, that depends on the quality of the frame. You can get prepreg sheets in a massive range of fibre-toresin ratios (often quoted as a TOW or K number with some sort of vague mid/high/ultra-high ‘modulus’ tag) and cheaper, lower-grade carbon is weaker, so you need to use more of it. The lay-up also needs balancing between brittle higher and more robust lower-modulus sheets to survive mountain bike life. Not only do designers need to get that lay-up right, but factories have to use the materials they’re supposed to. Neither of which is guaranteed. How carefully the sheets are laid up in the mould and whether non-fibre fillers are used to speed up/smooth out the process can also massively affect frame integrity and ride feel.

This all makes the gains of paying extra for carbon a lot less clear cut, and if you compare some fibre and alloy bike weights, there doesn’t seem to be any advantage at all. It’s hitting the trails, not the scales, that tells you how good a bike is though, so that’s what we’ve done with four of 2018’s carbon contenders.