Car roof carriers and carbon bikes.

baldy124

Folks,
A question for those of you that own full carbon road bikes, specifically those with carbon dropouts on the front fork.
How to you transport it on the top of your car?

Half the things you read say not to clamp the carbon frame around the down tube for fear of crushing it (or you've got a fancy frame like a Cervelo 'S' series and it's simply not a round downtube).
The other half of things you read say don't clamp it by the front fork (if you've got carbon dropouts) for fear of damaging the fork. Thules own roofrack instructions say this, and owning one of these, a Thule 561, I can see how it could damage the dropouts. But the instructions also say "The recommended roof option for transportation of bikes with sensitive frames (e.g. Carbon)"

So what do you do, which one do you believe? I'd hate either the front dropouts or downtube to fail whilst riding a new expensive bike!
Or does everyone just put it on the back of their car (or inside)?

With a new family and a couple of bikes for the family, putting a towbar mounted rack isn't really an option at the moment (especially as I already own both type of roof mounts, a Thule 561 & a 591).

Cheers

proto

I've often got two bikes on the roof of my car, a LOOK 585 and a BMC SCL01 Pro Machine, and both bikes have carbon fork dropouts.

The carrier I use is also a Thule 561 which clamps the front dropouts. I've had no issue at all in the three years I've been using the carrier, which includes a long and fast trip to Italy and back. Tested at speeds up to 110 mph or so.

I do the clamps up tight, and regularly check for any damage the the dropout faces. None detectable. The only other issue I can imagine that Thule might be concerned about would be the joint between dropout and fork blade, but again nothing to see there, and frankly I fail to see how the loads on the dropout/fork/ from wind blast at 100mph+ and the side loads from cornering can be anything like as severe as loads from a fat bastard like me sitting on the bike and descending Chinnor Hill at 50mph.

I'd be interested in listening to other's experiences.

bompington

All this stuff about fragile carbon is just nonsense, of course you can fling one on your roof any way you like, as long as you remember to cover it so it doesn't get wet.

nickwill

I have slightly negative thoughts towards fork mounting bike carriers. To me they put a lot of stress on the dropouts and round the headset area. I have a thule carrier which mounts on the down tube. This has the advantage that there is a bit of movement of the front wheel and therefore the stress is spread, rather in the way that a tree moves with the wind and therefore is less likely to be blown down.
I'm probably not expressing this idea very well, but it makes sense to me!

ozzzyosborn206

i went away in the summer with my mate and he had thule fork mounted roof racks and my tarmac and his tcr have both been fine, one thing i would say though is have you filed the little nobs on your downtubes off so it is quicker to take a front wheel off? i have and think that it probably helps give a safer clamp as it is all flat. As said above i doubt if produces much if any more force than hitting a corner flat out or riding over cobblestones........

Pross

I would be more concerned on the stresses applied to the dropouts and forks that to the downtube as there is more of a cantilever affect due to forces potentially being applied further from the point of fixing. The downtube fastening is more of a risk for overtightening and crushing the tube but there shouldn't be any reason to tighten to that degree as the bike can't really go anywhere even if quite lightly tightened.

baldy124

It actually less the stresses on the drop outs that worry me as they're designed for some decent loads (tho in compression from rider weight, not tension as they are on the car roof when a gust hits it). But what worries me is the sharp ring on the thule mountings put a 'dent' crack in the carbon dropouts. They're sharp enough to mark alloy drop outs and I'm worried what they'd do to a carbon drop out, there's also the worry if they don't line up neatly with the lawyer lugs. Pretty much you put a dent, indent in carbon I'd be worrying quite a lot about it.
Am I just unlucky with the 2 thule 561 racks I have, with them having sharp mounting rings that go against the fork (guess it helps the grip but doesn't seem ideal)?

proto

Certain that the Thule clamp doesn't clout the 'lawyer's lips' on my forks.

The knurled/serrated ring of any decent QR skewer will leave an impression in alumiunum dropous, no different from the Thule clamp. The clamping force should be similar too, no need to overdo it on the Thule clamp.

Maybe an engineer can do a calculation, but with such a small frontal area, I can't see how the load on the frame from wind blast can be high enough to cause problems, and if it is, then it would be as bad with frame tube clamping as well.

SteveR_100Milers

bompington wrote:

All this stuff about fragile carbon is just nonsense, of course you can fling one on your roof any way you like, as long as you remember to cover it so it doesn't get wet.

+1 for the first part, as for the last bit....in case it rusts..????

shove it on the roof, clamp it up, the stresses are b*ggr all compared to riding it unless you are strapping it on top of a Boeing 737

Slow1972

proto wrote:

Maybe an engineer can do a calculation, but with such a small frontal area, I can't see how the load on the frame from wind blast can be high enough to cause problems, and if it is, then it would be as bad with frame tube clamping as well.

The application of the load is different though. When you are sat on your bike and lean over the bending load on the drop puts will be minimal as the tyre rotates on the tarmac. In the rack the dropout is rigidly fixed so when there is a side load the point where it is fixed will be subjected to a higher bending moment.

Not sure drop-outs and the lower part of the forks are designed with that in mind.

mostly harmless

Slow1972 wrote:

....

The application of the load is different though. When you are sat on your bike and lean over the bending load on the drop puts will be minimal as the tyre rotates on the tarmac. In the rack the dropout is rigidly fixed so when there is a side load the point where it is fixed will be subjected to a higher bending moment.

Not sure drop-outs and the lower part of the forks are designed with that in mind.

Think about the load under heavy front braking which will be applied in roughly the same direction.

SteveR_100Milers

Slow1972 wrote:

proto wrote:

Maybe an engineer can do a calculation, but with such a small frontal area, I can't see how the load on the frame from wind blast can be high enough to cause problems, and if it is, then it would be as bad with frame tube clamping as well.

The application of the load is different though. When you are sat on your bike and lean over the bending load on the drop puts will be minimal as the tyre rotates on the tarmac. In the rack the dropout is rigidly fixed so when there is a side load the point where it is fixed will be subjected to a higher bending moment.

Not sure drop-outs and the lower part of the forks are designed with that in mind.

I'd be impressed if by driving there is anything approaching the same lateral forces on your frame than by cornering aggressively. I am 92 ish Kgs and haven't come close to snapping a set of forks or dropouts, and yes I realise that during cornering the centripetal force will act in compression on the forks, but not that much in slow aggressive turns. The biggest risk might be dropouts that are not sufficiently well bonded to the forks, in which case I would imagine the build quality is so bad it would be dangerous to ride such a frame at all!
Has anyone tried to break a CF tube? If you have ever cut down a seat tube you'll get an idea how strong it is - there is a lot of bllx written about CF. My road frame has survived impacts that would have bent an ally or a steel frame, I know its stronger, its just that its failure mode is somewhat more dramatic which lead people to think you can almost crush it between your fingers!!

proto

Not sure that these are in any way relevant, but impressive nonetheless:


http://www.youtube.com/watch?v=-lsDXEEU ... re=related


http://www.youtube.com/watch?v=SDVpRSNt ... re=related

SteveR_100Milers

There are some odd people around:

http://www.youtube.com/watch?v=3cZiYnIi ... ature=fvwp

Monty Dog

Since the introduction of carbon forks / frames I've never seen evidence of a frame failing this way. Given that the stress calcs on a frame are probably done with a combined bike + rider weight of 100kg plus at say 40kph, then to exert the same stresses on an unladed 7kg bike on a roof rack, you're going to be driving an F1 car then some! I think you'll find the reason why they say 'don't use carbon' is an exclusion cause to cover the rack manufacturer's backside in the event of a claim rather than something based on any evidence. FWIW the biggest threat to any bike on a roofrack is hitting height-restrictions and garage roofs! IMO a fork-mounted carrier is the safer option - putting a solid material under compression is far better than compressing a hollow tube.

teamtraveller

We carry 6 bikes on our team's rack. Half of the bikes are full carbon and we've never had a problem in 4 years. The only time we had anything break was when a Thule tray broke when we hit low branches on a tree. Switched the bike to a new tray and off we went. The bike was fine. We've never seen any evidence of damage from the clamps. The only problem is remembering your clearance, and not driving into the garage.