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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.

  • Not very good fender as it is too narrow – paparazzo Dec 15 '17 at 19:35
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    With personal experience, I found even a reasonably long back rack wasn't long enough to keep my back from being splattered by muddy water during the rain. I added a homemade plastic extension to my rack to catch the farthest drops, which did the job but was pretty ugly. – kgutwin Dec 15 '17 at 22:43
  • @kgutwin as ugly as the one I posted in my answer? – Chris H Dec 18 '17 at 9:56
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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

bicycle with rack as a mudguard I hope you get the idea.

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    Great answer. Another thing to consider is that water can actually get flung forward and make the back of your legs and feet wet so it’s important to have some coverage behind your seat post. – RoboKaren Dec 15 '17 at 20:39
  • @RoboKaren has a point here. One solution would be to attach another mudguard on the seat tube, which will only partly be a solution, or ride wearing shorts to minimise the amount of clothing affected by water. The most elegant yet effective solution would be to use considerably wide mudguard reaching from chain stays up to seat stays. – Mike Dec 15 '17 at 20:51
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    Not just legs and feet (@RoboKaren) but the chainrings and front derailleur, contributing to wear if the water is gritty (i.e. always) – Chris H Dec 15 '17 at 21:02
  • In practice you need longer fender, because the droplets don't fly straight but roughly in a parabolical arc (with some interference from air) – ojs Dec 16 '17 at 12:36
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    @ChrisH yes, and then there's a vortex behind the rider. It gets complicated. – ojs Dec 18 '17 at 11:22
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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):

3 rear racks

(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:

mudguard tail

and some coverage at the front for the drivetrain and my legs:

front of rear mudguard

  • Concur - I've lined the underside of a parcel rack, to make it an effective half-mudguard. I extended the rack backward by lengthening the mounts and the rails though, and this did nothing for the frontmost quarter of the wheel. – Criggie Dec 15 '17 at 21:14
  • That sounds about right if the rack itself doesn't do everything asked of it. – Kevin Rowe Dec 15 '17 at 21:49

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