Originally Posted by
The Golden Boy
I'm not an engineer, but my guess is that, on a level rack, 90 or so % of the weight is being borne by the vertical stays going down to the dropouts.
That being said, I do like using the racks with two struts going to the seat stays.
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.