Rack and Ruin

The three-year-old frame of my electrically-assisted mountain bike cracked where the top tube meets the seat tube. The dealer and manufacturer replaced the frame. However, the dealer noticed I use a seatpost rack and suggested it could have been responsible. I use the bike for commuting, dropping off the odd kerb. I carry 6-10kg on the rack. Should I switch to a permanently mounted rack with stays attached to the frame?

Phil Wilson

In short, yes. Seatpost-mounted racks of the type you describe are very popular for plenty of reasons, including ease of fitting, especially where a frame has no provision for mounting a conventional pannier rack. Compared with said conventional rack, they – and the practice of hanging a large bag from the saddle – are an inferior structural solution that puts a large bending load on the seatpost and, in turn, on the frame.

Given the reliability of most seatposts today, this should not be a problem with a light load of, say, 2.5kg and gentle riding style. But dropping off a kerb with 10kg on the rack is asking a lot of the frame.

A conventional frame-mounted rack attached at the rear dropouts will reliably take a lot more weight (check rack specification for precise figure) without over-stressing the frame. Inspect fixings often if you do lots of kerb drop-offs.

Richard Hallett

​Cycle’s Technical Editor

This Q&A was published in 'Cycle' the magazine for members of Cycling UK. To contact the experts, email your technical, health, legal or policy questions to editor@cyclinguk.org or write to Cycle Q&A, PO Box 313, Scarborough, YO12 6WZ