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Fork-mount racks and carbon forks

ratherbeintobagoratherbeintobago Posts: 636
edited March 2012 in Workshop
Anyone able to help on this? I am looking to buy a roof-rack bike carrier, and like the look of the Thule 861, which secures the bike using the fork dropouts with the front wheel off (I am a bit nervous about the downtube clamp kind with a carbon frame).

This website suggests that fork-mount racks shouldn't be used with carbon forks, but there's no mention of this on the Thule product page. I can't really see why there would be a problem, but is anyone able to advise/clarify?

Ta

Andy
On twitter

Posts

  • deswellerdesweller Posts: 5,175
    I'm not a big fan of roof racks in general. You get big aero loads on the frame that it's not really designed to support, plus you're blowing wind and rain through the headset and BB at high speeds for unusually protracted periods of time, which seems like a really good way to clean the grease out of them to me. And there are numerous stories about people totalling their bikes by driving under low bridges, having forgotten about their pride and joy riding up top. I'd go for a boot-mounting rack myself.
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    On Strava.{/url}
  • arlowoodarlowood Posts: 2,554
    Hi there

    Check out the comments on a thread I started some time back on the merits etc of various roof mounted racks. Back then the Thule product which fixed at the front drop-outs was the 561 - probably new re-branded as the 861.

    http://www.bikeradar.com/forum/viewtopi ... highlight=

    You will see that at that time Thule were happy to recommend the 561 as long as the front forks had steel or alu drop-outs even though the forks were carbon.

    You may have to confirm whether your particular bike has that feature before you could use the 561 type rack. If the forks are all carbon (ie including the drop-outs) then Thule don't recommend the 561 - and probably the 861 also. Might be something to do with the lateral forces on the fixing points when your bike is buffeted by side winds etc when on the rack.
  • ratherbeintobagoratherbeintobago Posts: 636
    Thanks for that - I shall go & have a look at my fork (sadly, I think it's got carbon dropouts)

    Andy
    On twitter
  • Monty DogMonty Dog Posts: 20,614
    Try a Google search, but there's no evidence that I've seen where a frame has failed due to use of a fork-mounted roof rack. The loading on a bike ridden at speed with rider is greater than any unladen frame on a roof rack - unless you're driving at a ridiculous speed excess of something like 300kph. What kills bikes on roofracks is driving into low height clearances!
    Make mine an Italian, with Campagnolo on the side..
  • deswellerdesweller Posts: 5,175
    Monty Dog wrote:
    Try a Google search, but there's no evidence that I've seen where a frame has failed due to use of a fork-mounted roof rack. The loading on a bike ridden at speed with rider is greater than any unladen frame on a roof rack - unless you're driving at a ridiculous speed excess of something like 300kph. What kills bikes on roofracks is driving into low height clearances!

    <idle thought>

    Roughly what air speed would be required to flip a bicycle?

    </idle thought>
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    On Strava.{/url}
  • Monty DogMonty Dog Posts: 20,614
    Some simple physics - Force = Mass x Acceleration - work out the rest using the weight of bike and rider vs an unladen one.
    Make mine an Italian, with Campagnolo on the side..
  • deswellerdesweller Posts: 5,175
    I don't know what the centre of pressure is or the cross section or the centre of gravity.

    EDIT: I don't think I could estimate the aero load from flow rate and cross section anyway.
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    On Strava.{/url}
  • satanassatanas Posts: 1,303
    Be careful driving into garages. if you get a rear-mounted rack, be careful reversing. I know people who've totalled frames with both these sorts of racks. Inside the car is safer!
  • bus_terbus_ter Posts: 337
    Monty Dog wrote:
    Try a Google search, but there's no evidence that I've seen where a frame has failed due to use of a fork-mounted roof rack. The loading on a bike ridden at speed with rider is greater than any unladen frame on a roof rack

    I've been thinking about this. Where the fork fixes to the carrier, the bike itself is like a long lever and any sideways movement of the bike will put a *lot* of sideways force right at this fixing point. Force in a direction that it is not designed to support.

    When riding the bike there is virtually zero sideways force here. The bike pivots at the point where the tyre touches the ground, and the fork at the point where it connects to the wheel is free to move side to side as the bike leans.

    To illustrate the point, If you were to sit on a bike connected to a fork carrier and then lean over to one side you can easily imagine the forks snapping right off at the bottom..


    I'm still undecided whether to buy a fork mounted carrier, or a clamp mounted carrier. I want the safest option for my carbon bike.
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