Originally Posted by
bikemig
P clamps work fine if you don't have the braze ons.
Originally Posted by
Dfrost
I AM an engineer (mechanical degree, retired after 45 years in the aerospace biz working in mechanical design and test), as well as an avid cyclist and occasional bike tourist.
The seat stay attachment points are under very little load, primarily from braking forces and rack loads that are not centered over the dropout attachment. The dropout attachments are taking all the dead weight plus dynamic loads from rough roads.
I see and experience no problem with a single point attachment on the brake bolt. If the bike has dual attachments already I’d use them, but wouldn’t go to any great expense to add them.
Far more critical to rack strength and stability is triangulation in the vertical struts (typically visible from behind). It doesn’t take much triangulation to add significant lateral stiffness which is important to the handling of a loaded bike. The double attachment at seat stays only helps very slightly to reduce sway in a rear rack.
And it’s worth noting that [MENTION=381793]gugie[/MENTION] is also a working mechanical engineer, with waaay more bike experience than me.
Well, I'm getting paid, that's for sure, and I do have "engineer" in my title. I don't know about waaaaaaay more experience than Dave, I'd bet he's got a lot more miles on bikes than me.
Dave's analysis is quite correct. People use single stay rear racks all the time for short rides and long tours. The single stay type ones are made to attach to a brake bridge, and all the production ones I remember seeing have an adjustable attachment so you can level the rack. That's typically 3 screws that can potentially come loose.
With a real heavy load on the back, I'd want a CrMo rack and would much prefer 4 points of contact over 3 (wouldn't you choose 11 over 10?) As a sidebar, I've seen a few aluminum racks crack and break while out on the road. Decades ago, on a bike trip down the California coast with a buddy, the only reason we finished the trip is that I was carrying a small role of duct tape in my kit. As the "steel is real" crowd likes to point out, steel almost never fails without some warning.
Would I modify a bike just to add two attachment points over one? No. If I were painting a bike and wanted to use a rear rack would I (if you had easy access to a framebuilder type)? I think it's low hanging fruit.