difference between aluminium and carbon frame

thebourke

whats the difference between an aluminium frame bike and a a carbon frame bike apart from the price ?
i have a hybird sirrus bike which i commute to work with...
i notice some bikes now have only carbon forks and the rest of the frame is aluminium...which is better?

07Lapierre

Weight and comfort.

reilig

I don't think there is much of a weight saving on carbon over alu, but the biggest difference that I know of is comfort. I recentlychanged from a full alu bike to an alu with carbon forks. You can visibly see the forks flex when cycling on a tar and chip road. The comfort difference is immence. With a full carbon bike, i imagine that the whole bike will flex for comfort as you cycle along.

Beasty

There's an article in today's Times "Building a better bike" with comments from Terry Dolan on the move from metal to carbon frames

The main reason for using carbon fibre is its incredible strength compared to its weight

The reason some bikes just have carbon forks is that this is the area where you get most benefit from absorbing the shocks from riding on rough roads

Jawgap

Rigidity too. CF is stiffer therefore you lose less power from the frame flexing - it may not improve your top end speed by much but it improves acceleration.

I'm not sure if it's to do with frame geometry or CF itself, but I've found that the handling on my CF bike to be noticeably sharper than other bikes and cornering speeds are (feel?) faster.

Hungrycol

Jawgap wrote: »

Rigidity too. CF is stiffer therefore you lose less power from the frame flexing - it may not improve your top end speed by much but it improves acceleration.

Really?

Lumen

Hungrycol wrote: »

Really?

I think it's more that for a given level of comfort and weight, CF will be stiffer. Or for a given level of stiffness, CF will be more comfortable and lighter.

So regardless of whether you're prioritising stiffness or comfort or lightness, there is some lower bound for the other things which makes an AL fork worse overall.

fat bloke

Rigidity can be engineered into a frame build during design and manufacture. So it's not black and white -aluminium is one thing, carbon is another. There are flexy carbon frames and there are super rigid ones. Just as there are super light ones and relatively heavy ones. I think a good frame is a good frame, regardless of material.

Hungrycol

Lumen wrote: »

I think it's more that for a given level of comfort and weight, CF will be stiffer. Or for a given level of stiffness, CF will be more comfortable and lighter.

So regardless of whether you're prioritising stiffness or comfort or lightness, there is some lower bound for the other things which makes an AL fork worse overall.

I would have thought gramme for gramme alu would be stiffer albeit less comfortable and marginally heavier. But I agree with fat bloke that the frame building and design is likely to be of greater consequence.

Beasty

Hungrycol wrote: »

I would have thought gramme for gramme alu would be .... marginally heavier.

Hungrycol

Beasty wrote: »

My bad. Two thought processes one sentence!

Jawgap

According to Wikipedia (so it must be true) the specific modulus of Alu is 69 GPa, CF is 181 and steel is 200. On a per volume basis, I think CF wins.

Although this is a crude measure and I agree that design and construction can contribute signifcantly to the stiffness or otherwise of the bike.

More info here (including some pretty graphs)

http://www-materials.eng.cam.ac.uk/mpsite/interactive_charts/spec-spec/IEChart.html

All I can say is the Boardman Team Carbon is stiffer than the Alu bikes I have, if that helps the OP.

opti0nal

You can make shapes with carbon that you can't with Alu, so it's possible you'd get a more comfortable fit for your physique and ride style.

Also, the 'resonance' of vibration seems dampened when using carbon.

But, in crash, you'd probably write-off a carbon quicker than an alu.

Lumen

opti0nal wrote: »

You can make shapes with carbon that you can't with Alu, so it's possible you'd get a more comfortable fit for your physique

petethedrummer

Jawgap

An Alu toilet seat is used in the B1 bomber - costs about $600 though