Alloy road bike vs carbon road bike: which should you buy?

If you’re looking to buy a new road bike, there are two major choices for the material that the frame is made of: aluminium alloy or carbon fibre. You’ll find a smattering of bikes made of titanium too. Until the 1990s, all bikes had steel frames and steel is still an option you can find for some new road bikes.

So how do you choose between an alloy and a carbon bike and what should you be looking out for? You'll find examples of both in our guide to the best road bikes.

First up, aluminium alloy frames are cheaper to make than carbon ones, so that bikes at the cheaper end of the market will have an alloy frame. Top spec carbon is expensive, so the priciest road bikes are made of carbon fibre.

Somewhere in the middle, there’s an overlap where carbon starts to predominate. That’s roughly between £1000 and £2000, although there are outliers.

But don’t discount an alloy frame bike if you’re looking in that price range; there are some excellent alloy framed bikes out there.

How is an alloy frame made?

The tubes that make up an alloy frame need to be thicker at their ends, where they’re welded together and there’s more stress, than in the middle. A frame made of tubing that’s of different thicknesses along its length will be lighter than one where the tube thickness is the same throughout.

Different shapes and thicknesses in different parts of the tubes allow bike makers to tune the ride characteristics of the bike too.

So you’ll see terms like “triple butted” and “hydroformed” bandied around, describing the processes by which the tubes are shaped to purpose, shaving weight where it’s not needed without impacting the frame’s strength.

Alloy frames carry a reputation for riding harshly. But there’s no reason that should be the case and sophisticated alloy frames like that of the Boardman SLR 8.8 will ride as well as a carbon one.

How is a carbon frame made?

A carbon bike frame is made up of layers of carbon fibre embedded in an epoxy resin matrix. The layers are built up, usually by hand, around a template to produce the frame’s desired ride characteristics.

Once all the layers are in place, the frame is heated in a heavy metal mould to fuse them together, developing the frame’s strength and ride characteristics.

There are different grades of carbon fibre. You’ll often see the term “high modulus” describing a frame. High modulus fibres are stronger, so you need fewer of them and the frame can be built lighter, whereas you need more lower modulus fibres to achieve the same strength, resulting in a heavier frame. But higher modulus carbon fibre is more expensive than lower modulus carbon, so frames branded high modulus will be more expensive than “mid-mod” frames.

In practice, even a frame made predominantly from high modulus carbon fibre will include some lower modulus fibres in certain parts of the frame, to deliver the ride characteristics that the bike needs and so that it’s not too harsh to ride.

Often you’ll find that major brands offer the same bike with an alloy and two different grades of carbon frame as the price rises. So Cannondale, for instance, sells the Synapse endurance bike with an alloy frame, the Synapse Carbon and the Synapse Hi Mod with prices from £1000 all the way up to £7000. 

Carbon or alloy - which should you choose?

If you’re looking for the best road bike costing over £2000, you’ll probably be looking at a carbon frame, while if your budget is under £1000, most road bikes will have a frame made of aluminium alloy. 

It’s in the space between that you are likely to have a choice and where an alloy frame might be a better option than a cheaper carbon one.

First up, because the alloy frame is cheaper to manufacture, you might find a better spec on an alloy bike than a carbon one at the same price. That’s most obvious in a higher spec groupset - Shimano 105 instead of Tiagra for example.

Disc brakes give better stopping power than rim brakes, but they’re more expensive, so you might get discs on an alloy frame for a similar price to rim brakes on a carbon frame. Likewise, wheels, tyres and other parts might be better on an alloy framed bike than on a carbon one. All of that might give you a better ride experience from an alloy bike than a carbon one at a similar price. 

Bike brands like Cannondale, Trek and Canyon have a lot of experience designing alloy frames that are lightweight and ride well. Their alloy frames have a quality look too, with smooth welds and sophisticated tube shaping, which may include aero features.

The frame might handle and ride better than a lower priced carbon frame. Built to a budget, a carbon frame may not have had the engineering input that a quality alloy frame has had.  In some cases an alloy frame will be lighter than a carbon frame for a bike at a similar price.

Bangs and bashes

Particularly if used for commuting or around town, bikes can pick up a lot of dings. Both carbon and alloy frames can shake off minor scrapes, but a more major bash or crash needs to be looked at more carefully.

It’s fairly obvious if an alloy frame has been damaged to the point where it’s unsafe to ride as, like any metal, there will be a visible dent. Smaller dents can spread over time though, as the metal fatigues under stress, so check regularly that a dent hasn’t led to a crack which spreads across the tube. 

With carbon frames, damage may result in delamination inside the frame tubes, which may be invisible from the outside and can only be picked up by X-raying the frame, which is a specialist service and expensive. You can get the frame repaired or strengthened where damage has occurred, but again this is expensive and may not be economical for a cheaper frame.

Regardless of frame material and however slight the damage might look, you should take your bike to a shop for examination, to make sure it’s still safe to ride.

Open mould frames

Whereas name brand bikes will usually be engineered to the bike brand’s specification, smaller brands might have bought a so-called open mould frame. That’s where a factory in the Far East has designed and manufactured a frame which the brand buys and labels up with its own logo.

So multiple bike brands might offer the same frame but named differently. That’s not a bad thing in itself, but some open mould frames are better than others, so you might not get the same quality that you would from a larger marque’s frame.

Choose wisely

So the choice of an alloy or a carbon frame for a bike at a similar price isn’t clear cut. It’s worth examining your options, what spec is offered and – of course – riding the bike you’re considering before parting with your cash.