>>1736287firstly by tire diameter and tire width,
then you have to deal with the Frame Geometry and mounting points of your bike
and avoid compromising the performance of any brakes that may be in the way
most racks have a way to adjust the length and angle of the upper support arms
these supports mount either to a hole in the seat stay bridge (same hole as calliper rim brakes)
or onto threaded braze ons located on the seat stays above the Chain stay bridge
if there are no such mounts its possible (but not advisable) to secure the upper supports to the seat post clamp, or to the seat post its self
or P clips can be used if the frames tube shape allows, and the load light enough
the lower support legs of slightly more expensive racks can be adjusted in length, while not strictly necessary this is a nice feature to have
the fork ends / drop outs of a non racing bike may have threaded Eyelets built in, one for mounting a rack another for mounting fender supports
if no lower mount is present, some racks can be mounted to the axle of the wheel, but this is an old or unusual feature
not all racks are suited to carrying pannier bags
to stop the pannier bags from wobbling about and getting caught in the wheels, an extended frame is needed
the rack in
>>1736309 has L shaped extensions built in
each bike is so different in shape and size its not really possible to recommenced a rack without knowing the bikes dimensions
fully adjustable racks avoid some of this issue, but tend to be more expensive and fragile
as a side note steel racks are far more durable, many times I have seen the welds on alloy racks fail
where as a steel rack can flex and bend allot more before failing entirely
