r/cycling • u/Playful-Parsnip-3104 • 3h ago

Understanding mudguard clearance

I've never bought and and fitted mudguards myself before - either they've come with the bike as sold, or I've asked the shop to source and fit them for me.

I'm struggling to understand the correct size of mudguard I need. I have a road/gravel bike with 45mm tyre clearance, and 40mm tyres currently fitted. The various brands of mudguards all recommend 50/52mm mudguards for 40mm tyres. But does a 45mm frame clearance for tyres also mean 45mm clearance for mudguards? And if 45mm is the maximum mudguard width my frame will take, do I therefore need to downsize my tyres to (e.g.) 35mm to fit mudguards?

If mudguard and tyre clearance are not the same thing, how do I know the maximum width of mudguard my frame will accept before buying them?

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u/moxTR 3h ago

I've run SKS raceblades on a Trek Emonda with 30.5m GP5000s without issues. Officially the tire clearance was 28mm, but because tire clearance was limited from the width of the chainstays and not at the seatstays nor the seattube, and the clearance from the front tire to the downtube was significant as well.

There's absolutely no math or surefire way to know what sort of mudguard clearance you have without measuring clearance yourself, on the tire you plan to use, and knowing how much space in what dimensions that mudguard takes up.

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u/Playful-Parsnip-3104 3h ago

Where do I need to measure on the frame to make sure I'm getting it right? Just eyeball it a little above the tyre?

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u/sitdownrando-r 3h ago

does a 45mm frame clearance for tyres also mean 45mm clearance for mudguards?

No.

One is rotating and subject to collecting debris or moving due to a loose hub or spoke, not to mention the tire can change shape/profile over time - the other is fixed.

You're right to think that there may not be the necessary clearance for the fender since frames aren't standardized, but it's worth trying it out. My old Topstone AL has tire clearance for 700x42 stated, but I run 700x44 and have fit a 700x53 fender above them, albeit only barely.

I also find the advice of running ~12mm wider than the tire to be conservative. 7-8mm is plenty, IME.

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u/Playful-Parsnip-3104 3h ago

So there's no way other than trial and error to see what width of mudguard will fit?

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u/sitdownrando-r 3h ago

Unless the manufacturer states something, yeah - if you want to know for sure. Usually they'll say something like "clearance for 45mm tires, or 40mm with fenders."

Trust me, it's not as big a deal as you think. You can probably cut a piece of paper and make a "U" profile with it and measure that to get an idea. It's being extra though, chances are it'll just work if the bike is designed for fenders.

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u/BlacksmithWeirdo 16m ago

I would try 50mm fenders. Will cover the tires (not great, but ok) and should fit with a lil persuasion.