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I have been riding an aluminium frame hybrid bike for 20+ years and it is coming to the end of its life. Having tested a couple of carbon hybrid bikes and been very impressed with the responsiveness of the ride I'm considering taking the plunge - something like a Canyon Roadlite or Trek FX tick most of the boxes. It will be used primarily on my short (5 mile) but hilly commute, as well as longer weekend rides.

I've read several posts that discuss the fatigue and impact resistance of carbon frames vs aluminium vs steel, and I have a couple of specific concerns that I haven't found addressed in detail elsewhere:

  1. I am concerned about possible damage caused by securing a carbon frame with a D-lock (== U-lock). I store a pair of D-locks at my workplace where we have semi-vertical bike racks. I will not be mounting the locks on the frame. However, it was suggested both by a mechanic in my LBS and Canyon's online chat assistant not to let the lock rest on the frame. This is suprising - each lock is insulated with thick plastic and weighs perhaps 1.5 kilos. I thread one lock through the front wheel and let it rest on the down tube, and use the other one to secure the back wheel to the stand. I wouldn't expect a carbon frame to have any trouble bearing a small load such as this, as long as there is no forceful impact when locking/unlocking (or of course from accidental damage from others removing their bikes). From what I've read it takes a major impact such as a crash or forceful hits with a hammer to cause impact damage.

  2. I've read a few posts suggesting carbon bikes are less suitable for commuting for several reasons (harder to fit racks and mount locks; more alluring to thieves), bit also due to the weather conditions they will face. As I'm based in the UK there will be no shortage of rain and roughly half my commute is on roads. Road salt is known to corrode aluminium over a long time, but are there similar concerns with carbon, or is it just people being more protective of carbon bikes because they're more expensive?

Thank you.

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    Road salt doesn't corrode aluminium like it does steel - instead there's a white bloom of aluminium oxide which doesn't progress further into the metal like ferrous oxide. – Criggie Apr 7 at 0:20
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    The locks themselves aren't a problem, it's what happens when someone knocks your bike over or pulls the lock by mistake. A carbon tube is immensely strong parallel to its axis but much weaker normal, so the force of the lock pulling against it can lead to cracking. – awjlogan Apr 7 at 10:28
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    That bloom can build up internal stresses and crack the aluminum similar to the way water freezing can crack rocks. There is also the problem of contact with other metals which can cause the aluminum to act as a sacrificial anode. en.wikipedia.org/wiki/Galvanic_anode – Fred the Magic Wonder Dog Apr 7 at 15:04
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    My experience with carbon construction and salt water is limited to windsurfing gear, but there carbon held up to daily salt water immersion much better than aluminum. Both needed regular fresh water rinsing for long life, but the carbon gear could stand a weekends neglect much better than aluminum. – Fred the Magic Wonder Dog Apr 7 at 15:09
  • Perhaps a combination D-lock with threaded cable lock would mitigate the weight issue whilst maintaining a good level of security as per Ichabod's reply. – pdm2011 Apr 7 at 22:18
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TL;DNR - All else equal, an aluminum framed bike at the same price as a carbon bike will be a better bike.

My general feeling for Carbon frames are they are overrated, or more accurately, quality modern aluminum frames is underrated. Aluminum frames cost alot less to build than the cheapest carbon, let alone a quality carbon frame. With Hydro forming, the designer has a lot (not all) of the design flexibly of carbon, and a lot more than the days they build frames from tubes. Savings building the frame can be spent on components, making a better bike at the same price point. Obviously at the "Money is not issue" end of the market, carbon makes a better frame for riding, but care must be taken because at this end fo teh market carbon layup is done in a way to maximize performance weight, with less concern to robustness.

As far as robustness and longevity, a quality, but not race focused, carbon frame will be more than robust enough.

Using the N rule, you really should have two bikes, which will probably be more cost effective longer term. One for commuting which is cheapish (its only 5 miles, a few hundred grams weight saving won't make any difference to you commute times or effort even if it is a bit hilly) and robust. Then have a weened bike for longer rides that you keep the miles down. After a year, the weekend bike is still in great condition rather than having been used in all weather and banged around in the work bike racks by other riders.

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  • I only have storage space for one bike at the moment, but when that changes the idea of a leisure bike definitely appeals. I'm also considering an aluminium bike with the same componentry as the carbon one. The difference in both price and weight is modest (£400; 1 kilo) but what I really enjoyed about the carbon bikes I tested was the sharpness of the ride and how efficiently it converts drivetrain energy into thrust. I haven't tested a recent aluminium build, but it sounds as if the bike world has moved on a lot in the last 23 years and I may find similar improvements in aluminium. – pdm2011 Apr 7 at 22:13
  • A kilo is a lot more than I expected. A light Aluminum frame is just over 1kg. Most (all?) of the difference you feel has nothing to do with the frame material. – mattnz Apr 8 at 0:11
  • Please link or explain the "N rule." ;-) – Michael - Where's Clay Shirky Apr 13 at 17:34
  • Refer Rule #12 - velominati.com :) – mattnz Apr 13 at 19:34
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I can't speak to the durability or reliability of carbon fiber frames for commuting, but it seems to me that a bike frame should easily withstand the weight of a lock on the down tube as you describe. A water bottle in the cage being jerked around by the bumps on the road would probably generate more stress. And if the frame is that delicate, do you really want to trust your neck and spine to it? ;-)

You may want to reconsider your locking arrangement, however. A D-lock through the rear wheel to a solid object and a D-lock holding the front wheel and frame together means that a thief could easily have a bicycle frame with front wheel while leaving the rear wheel hanging in the rack. If you were to drop the front lock and replace it with a stout cable, you could have better theft protection. Get a straight cable, not one of the curly kind, and loop it through the front wheel and put the free end through the cable eye so that the frame and front wheel are secured by this loop. Pull the free end on through the bottom of the frame and down (or up) to the D-lock for the rear wheel, and put part of the D-lock through the cable's eye. Someone could still cut the cable and make off with everything but the rear wheel, or cut the D-lock and have the entire bicycle, but you won't have that concerning amount of weight on the frame, and the bike will be more secure against an opportunist. FWIW, I've cut through a D-lock (we call them U-locks in the US, but it sounds like the same thing) with a common hacksaw in under a minute.

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  • The main problem with a carbon bike could be it's absence after your working day- Since a carbon frame would probably be more desirable to a thief he might go greater lengths (and risks) to break the lock. – Carel Apr 7 at 17:34
  • Thanks for the locking tips, using a cable lock would certainly reduce weight on the frame. I've always considered cable locks a lot more vulnerable to attack than D-locks but a lot must depend on the build quality of the individual lock. – pdm2011 Apr 7 at 21:49

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