I am wanting a mudguard that is quite long for the rear. However I don’t want the mud to flick onto my back.
I have already tried the ‘Ass Saver Fendor Bendor’ but it slips on my frame. Would you be able to recommend me one that would work?
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Mudguards / fenders work by intersecting an imaginary line between your wheel and you/the bike frame.
There are two main reasons for having them and they're similar but different.
In addition mudguards might offer an increase in aerodynamics, and somewhere to hang more lights/reflectors.
So the best mudguards will look something like this:
The only thing missing is a flap on the rear guard - this is more for the benefit of someone riding behind you though.
A second downside, this style only works on rigid bikes. If you have suspension then your guards can't be too close.
Notice the rear guard is fastened to the rear triangle, so as the suspension flexes, the guard stays in the same place. The front guard is much shorter because it can't use stays for support, and is a very long way from the tyre. The tyre/guard clearance has to be bigger if the bike is likely to be ridden in mud too, else there can be a buildup of mud enough to stop the wheel turning.
The MTB guard has to be much wider too, because the tyre is wider. Knobblies catch and throw a lot more water/mud/stones than smoother road tyres.
As such, this offers much less protection from water and mud.
Materials will probably be either metal or plastic. There are wooden ones, but they're not common. Metal ones can suffer from stress fractures due to vibration, so more stays work better. Plastic ones are a bit more forgiving, but they crack slightly easier than metal and can embrittle with UV/age.
A final option is a home made hack. All you need to do is intersect the flying droplets. So search and you'll find fenders made from strips of plastic, old bottles, even tyres with the bead cut off. If you have a rear rack, simply line it with flat plastic and you've got protection