I bought a pannier rack but it seems way too narrow to fit on my MTB bike (in terms of the axle width), I can sort of push apart the two legs (of the rack) but even then the spacing is only 10CM (when streched) between both legs which seems a 2-3cm too narrow for what is required.
Potentially this has been bent during transit - but I'm not entirely sure and thought to confirm in case I bought a wrong item.
Is there a standard set of sizes? What are they?
That rack has been crushed, presumably during shipping. For a rear wheel it should be somewhere between 120mm and 135mm between the axle mounts, with perhaps 5-10mm additional tolerance for going outside the dropouts.
A front rack would be a little bit less - most front dropouts have an OLD of 100mm.
That yours measures at about 70mm means something's off. Check the angles between each side and the deck - it should be symmetrical. Or stand the rack so both axle mounts are on the floor which should leave the deck level side-to-side.
Whether you can cold-set the rack depends on if it is steel (maybe) or aluminium (probably not) If you just bought it, talk to the shop and ask about what happens with shipping damage.
Since you paid retail-new price, you expect a good useable part to arrive. Is a different matter if it came from the $1 bucket at the local swap meet.
Tubus has some nice technical drawings for all of their racks: https://www.tubus.com/en/products
They also state the allowed width. For the classic model rear racks it’s from 150mm to 180mm. For the newer models it’s 140mm to 170mm. It doesn’t look like they are offering “wide” or “narrow” racks.
Pletscher states all kind of dimensions but not the allowed width.
I don’t think there is a “standard” size. But rear dropout spacing pretty much only goes from 126mm to 142mm (157mm for super boost). The mounting holes for racks will have a similar deviation (just add ~20mm or so for the seatstay width) so it’s relatively easy to support all of them.
