jnbrown

I am looking for a rear rack to use on my Calfee tandem.
The two mounting options I prefer are brake mounting bolt or seatpost clamp.
I will be using Arkel T-28 panniers which on the smaller scale of size.
I would like the option of using rack top bag which I also have but honestly will most likely be using panniers.
The stoker's seatpost is a Specialized carbon and I am concerned about using a seatpost clamp like the Axiom product here:

https://www.axiomgear.com/products/ge...k-seat-collar/

This would allow a more secure attachment with a two arm rack compared to a brake bolt mounting. However we won't by carrying much weight because our touring will be done credit card style. Mostly we will be carrying extra cycling and street clothes and other necessities, so the brake bolt mounting might be a sufficient, viable and better option.

Some racks I am considering are the Tubus Fly and the Axiom Streamliner. I have read the Fly is too narrow to accept a rack top bag. The Streamliner has 4cm of setback which provides for better heel clearance which I am not sure I need since the Stoker has small feet and 165mm cranks. The rear spacing is 145mm so whatever rack I get I will need to spread it at the dropout mounting points.

I would appreciate any suggestions about which rack and mounting would be best, or if there are other options I should consider.

Thanks.

WillJL

I take it you're considering those mounting options for your tandem because it lacks rack braze-ons on the seat stays...Is that the case?

If so, have you considered p-clamps like these? Of course, if the frame is carbon like the seatpost, this would be out of the question...

Why not just change out the stokers seatpost to an aluminum one to allow for your seatpost-clamp rack that you like?

Chris_W

It's not clear to me why the material of the seatpost would affect whether you could use that seatpost clamp. I've used one of those clamps before with a carbon frame & carbon post, please tell me why I shouldn't.

WillJL

Its my understanding that clamps are inappropriate on carbon components and frames because the tubing is not engineered to withstand stresses applied in that way. Apparently carbon can crack and catastrophically fail. On the other hand, I can imagine that it might be okay in some situations, but I wouldn't want to risk it.

Maybe since a seatpost is designed to withstand the force of the seatpost clamp, it would be safe...

The information I've read about not using clamps on carbon components probably had to do with clamps that don't disperse the load over as wide of an area. I'll look around and see if I can find where I read that and post a link.

FOLLOW UP: After reading this, its lead me to believe that as long as the clamp going on the carbon doesn't have any sharp burs, and particularly if the clamp is rubberized, that there should be no problem.

SECOND FOLLOW UP: and after reading this, I will never consider buying, using, or recommending a carbon component.

jnbrown

The frame has small diameter carbon stays so I can't use seat stay clamps.

I built the bike with the carbon post for possible added comfort and don't want to be changing seatposts all the time. Since most of the load will be on the dropouts I would think it is ok to use clamp on the carbon post as long as I tighten it to the same torque as the normal seat post clamp. Like I said I wouldn't mind using the brake bolt mount as long as it will strong and stable enough.

jnbrown

If it was a seatpost beam type rack I wouldn't use it on a carbon seatpost because of the large cantilever force and the torque needed to keep the rack from swaying. Having the dropout mounting points is a totally different situation I think.

Chris_W

(Sorry that it took a few days to respond to this)

I was referring to the seatpost clamp link to in the original post, link here. It replaces a standard seatpost clamp, which goes on the one place on the bike where a carbon frame and carbon post are SUPPOSED to be clamped. With a rear rack, most of the weight is on the lower mounting points, the upper points mainly keep the rack in position. Your response seems to be saying not to add clamps to carbon anything, which I generally agree with, but what about a clamp that already has to be there, is designed to have a clamp there, and to which you are adding a small amount of additional forces?

sstorkel

If I post a web page that says gravity will cease to exist tomorrow, will you add seatbelts to all your furniture and carpet your house with velcro?

Just because some idiot posts it on the Internet, doesn't mean it's true...

professorbob

I still go back to the last day of the tour de france a couple of years ago when the world saw Jens Voight (I think) riding around the Champs Elysees with a carbon fiber knife edge pointing at his privates.

mijome07

Good one.