Roof rack and fork w/carbon drop outs

[chakatrain]

I've heard mixed comments about using the Thule roof rack (http://www.thuleracks.com/product.asp?dept_id=8&sku=518) system I have on my car to mount a bike ('07 Look 585) with a carbon fork that has carbon drop outs.

Some say using a fork-mounted roof rack on a fork w/carbon dropouts puts dangerous stresses on the dropouts which can lead to fork failure. Others say don't worry about it.

Any thoughts?

[max_powers]

the stress from clamping would probably be about the same as a wheel. The stress from aero drag would increase exponentially with speed. throw in some strong side winds and there may indeed be issues. But don't they build 200+ mph F1 cars out of CF?

[alancw3]

the primary reason why i bought a thule 916 t2 rack. no frame or fork contact. imho the best rack ever if you can use a hitch mount. the only drawback was that i had to spend an additional $275 to have a hitch put on my car on top of the $303 for the rack.

[Richard]

Personally, I would be concerned. Not about the side winds or the drag, but the bumps and jolts that send shock and motion that was not part of the design through the fork. I had a rock shox fork break at the fork tip while it was in the car mount. Better there than on the trail, but I always ascribed the reason to the sideways motions that bumps and turns put on the tips.

[David Kirk]

It would make me a bit nervous. The fork would not normally see the side/twisting loads that it will see on the roof rack. As one side of the car goes into a dip the car tilts and the bike has to follow this tilt on top of the car. A little tilt at ground or even seat level isn't much but it gets exaggerated in big way on top of the car. This puts a big side load on the dropouts that they don't see on the road.

The fork designers might have thought of this and designed with that in mind but it's hard to say for sure.

dave

[zap]

I wonder ............ if it can't handle a roof rack, would I want to ride the thing.

The frame is not going to get much in the way of road shock, it might have more side loading but a road car doesn't go around corners all that fast and aero drag should not be an issue unless you drive 120 mph or more on a regular basis.

I don't see how it's a problem but I could be wrong. Give the good folks at Look USA a call and ask them.

Then post their response here.

[chuckred]

Personally, I would be concerned. Not about the side winds or the drag, but the bumps and jolts that send shock and motion that was not part of the design through the fork. I had a rock shox fork break at the fork tip while it was in the car mount. Better there than on the trail, but I always ascribed the reason to the sideways motions that bumps and turns put on the tips.

If a mountain bike fork (or road bike fork) can't handle that, I'd be scared to ride it for sure. I have wondered about strong cross winds though...

But, from what I've heard/seen, the more likely thing is that when the bike is put on the rack, if it's tilted to one side with one tip in, then leaned up straight, it's possible to lever the fork tip into bending/breaking. (Or when taking it off, if it leans off when releasing the restraint). The proper way is to be sure that the fork is put on straight and the weight is even on both sides - and it should be fully seated on the axle... When taking it off, be sure to lift it straight off, instead of leaning the bike.

With a heavy mountain bike on a high roof, that can be tough!

[Mark35]

Is anyone planning on sitting on the bike while it is on the rack while driving at 80 mph? The load the bike takes at 40 mph with a 170 lb person in a high G-force turn, is exponentially higher than any normal use on a roof rack.

[chakatrain]

<snip>...I don't see how it's a problem but I could be wrong. Give the good folks at Look USA a call and ask them.

Then post their response here.

I did indeed email Chas Beldon of Look USA today. His response is below:

"You shouldn’t have a problem mounting the HSC5 fork to your roof rack; just make sure you don’t overclamp the quick release. We have mounted many bikes on roof racks without problem, although a roof rack does put more stress on the fork’s dropouts that a wheel does. When in doubt, I would recommend checking out manufacturer’s guidelines for the rack system you are using."

It sounds like I'm fine to run this in the fork-mount as long as I take care load the bike on the rack carefully and don't overtighten the quick release tightening mechanism. I'm not planning to drive offroad with the bike on the rack and will take care when on gravel, dirt pullouts, etc.

As an aside, would this even matter if my carbon fork has aluminium or ti droputs instead of pure carbon?

[Louis]

The load the bike takes at 40 mph with a 170 lb person in a high G-force turn, is exponentially higher than any normal use on a roof rack.

I may be wrong, and I haven't seen any numbers, but my intuition agrees with this (except for the "exponentially" part).