Specifically I want to know if the right one would stop the rider's back from getting splattered with rain. That way it does double duty as a fender and a rack.I want a light duty back rack for day to day cargo. If I am getting bulky groceries or doing the laundry I can always use a backpack. I like the minimalist lines of some of the ones I have seen. They suit my bike.
I'd say it depends on the length of the rack you're going to attach.
It's all about the physics - the water droplets, when they detach from the tyre they move perpendicular to the perimeter of the wheel. If the rack is long enough, the droplets that might reach your back are hitting the rack (acting as a mudguard).
The ideal situation would be when the rack reaches to the furthest rear point of the wheel. Then only the droplets travelling vertically are not covered by the rack.
However when the rack gets shorter, the more droplets travel to the front, thus direction of your back.
Short story long - have a look at the picture:
- black is the bicycle (obvious)
- red is the rack
- blue is the most front-travelling droplet that might hit your back
Any rack that has a plate on top, however it's mounted, can be a partial mudguard. I've found a tail made from the end of a cheap plastic mudguard improves the coverage a lot. On one of my bikes I've taken this a step further with mudguard (in two pieces) from behind the bottom bracket, past the brakes and up to the rack in addition to the tail. It's not pretty, but this is a bike that's meant to look even older and cheaper than it is.
On other bikes I've managed to get a full mudguard underneath.
The further a mudguard is from the tyre, the wider it needs to be to do any good. This is a further limitation.
Some from my fleet (all bikes have been ridden in mud recently):
(click for bigger pictures)
- Top: a Topeak Beam Rack DX stops a little mud, but not much (as can be seen from the spatter on top of the rack). Just after taking this I fitted a Crudcatcher mudguard.
- Middle: a Topeak Super Tourist DX is much better (partly because it fits closer to the tyre) but I've added plastic.
- Bottom: A Tortec Velocity Hybrid does nothing (no plate) -- note the full mudguard underneath.
The mods to the middle one consist of a tail, which makes a big difference:
and some coverage at the front for the drivetrain and my legs: