Why can't I use a carbon fiber bike with a trailer

[peterbro@hotmai]

My only triple crank bike is a Scott CR1 carbon fiber frame bike.  My guess is that the dropouts (which are mysteriously attached to the frame somehow) won't be able to handle the load and stress of a trailer.  Is that right?  There are no mounts for racks at all...pure racing bike.  What to do???

[peterbro@hotmai]

Post script:  I guess if it were a pure racing bike it wouldn't have a triple crank....but hey, I got it for a good price!

[cyclesafe]

Conventional wisdom (usually the best) says that the dropouts on a carbon bike won't stand the stress of pulling a BOB.  When I was on the Southern Tier last Spring, there were dire predictions that my Trek 5200 frame would snap in some spectacular fashion - probably on a steep descent causing my trip and possibly my life to be cut short.

What actually happened was that I effortlessly beat everyone to camp, always by a margin of at least several hours.  My triple crank gearing was sufficient for even the steepest ascents (I never once had to walk my bike up a hill), my wheels (with 24 bladed spokes on the rear) gave me no trouble whatsoever, and my 25mm Armadillos suffered only three flats in 3000 plus miles.

However, I did have poor braking on steep descents, moderate instability at high speed, and prompt crashes whenever I came anywhere near gravel.  All of these problems were addressed by slowing down and for the latter problem, avoiding any sharp turning.

Having said this, I have just bought a bike designed for touring because I do plan to ride on gravel and on otherwise unpaved trails/roads.  But if I thought that I was only going to tour on pavement, I wouldn't hesitate to use my road bike and BOB.      

[OmahaNeb]

My oldest son has a Trek with carbon rear stays and front fork.  We went on a week long tour using BOBs, with similar results.  I initially was more concerned about the wheels than the frame.  His wheels have less than 20 spokes per wheel.  No problems with the wheels or the frame on the tour.  Now he has a problem with staying on the road when riding, but I contribute this to his mother's genes, she has the same problem with riding and talking at the same time.

[Peaks]

The BOB hangs on the rear skewer.  So, the weight is transferred directly into the wheels.  Can't imagine why a BOB trailer would effect the frame at all.  

That being said, bikes are basically designed to carry just so much body weight.  Putting weight in different places, or putting excess weight does stress the bike in different ways than it was designed for.  

Myself, while I don't have a fancy road bike, did notice that my rear rim is cracked after doing a cross country trip with a BOB. I suspect it's just metal fatigue after carrying the load for that long.

[dknapp]

The rims will also crack when they are just worn out by the rubbing of the brakes.  I think it is unlikely they cracked just because of the added weight.  My wife cracked her Gary Fisher/Bontrager rims after about 7K miles of commuting.  They were just worn through on the sides.

[peterbro@hotmai]

Thanks for the comments.

[DaveB]

The rims will also crack when they are just worn out by the rubbing of the brakes.  I think it is unlikely they cracked just because of the added weight.

Rims generally crack in one of two places:  

First is just what you reported, the brake track cracks from abrasive wear.  I've had several rims fail that way in from 11,000 to 30,000 miles depending oh how they were used, how much bad weather they were exposed to and how durable the rim was to begin with.  Very light rims are, obviously, less durable and less wear resistant.

The second mode of cracking is around the spoke holes which is a fatigue failure or caused by excessive spoke tension or poor rim design/material.

The tongue weight of a trailer is unlikely to accelerate either mode of failure.    

[peterbro@hotmai]

Please keep your comments regarding my question pertaining to the carbon frame, not the rims, etc..  If you wish to discuss rims and other "metal fatigue" issues please open a separate topic.  We are getting off the subject here, thank you.