Which size SKS Bluemels will fit my frame - 35, 42, 45?

Question

I'm thinking of getting full mudguards for my road bike, the Triban RC 520 , (details here) , and the manufacturer's page tells me that it comes with 700 x 28 tires and the frame is compatible with tires upto 36mm.

Searching for SKS Bluemels, I find the following sizes useful : 35 mm, 42 mm and 45mm. The 35 is good for 20-28mm tires and the 42 is good for 25-35 mm tires , it appears. The 45 seems a bit overkill for 28-38 mm.

I think I should go for the 42 mm ones for upgradability to bigger tires later, but since the frame is compatible with upto 36 mm tires only, would these "42 mm" fenders fit ?

(perhaps this is a silly question , but I'm buying online so want to be sure that they will )

Or is this measurement of the mudguard width not the same as tire width and so I can go ahead with the 42 mm ones ? For that matter, will even the 45 mm ones fit ?

NOTE : I have seen 24683 , but the question here is slightly different - what I ask is if the mudguards bigger than 35 will even fit.

EDIT 1 : Will these from Decathlon fit too ?

EDIT 2 : My bike has disk brakes, so there are no rim brake calipers in the way.

Answer 1

Choosing a maximum size for a given frame is not an exact science. It's done by either mounting or approximating the widest size tires you want to be able to run and then using a caliper or ruler to see what you've got to work with in terms of vertical and side to side clearance.

Your bike looks like it shouldn't have trouble with the 45mm. That's probably the number to choose, unless by some chance there's clearance for 50s.

Two things people often under-consider in the abstract:

• 7mm total overhang in width gets thrown around as a bare minimum, and you can see SKS follows this line, but it's really pushing it in practice. In addition to being barely adequate for spray, it makes preventing rub finicky in day to day use. How easy they are to live with is actually one of the bigger factors in choosing a width. A 28 in a 45 does not look funny or perform badly. It's actually great.
• Wider means more wrap means the tire can be more sunk in means wider may function where narrower won't even if it's not needed for coverage, if vertical clearance is limited.

Answer 2

Generally you want your guard to be wider than your tyre, to catch water that gets slung at an angle. The problem is finding if the guards will fit between your seat stays, under your rim brakes (if you have them) and still stand off your tyre's tread, while leaving enough room for stones/grit/mud to pass through.

A wider guard will keep you drier, but will stick out further and be a very little less aerodynamic.

Wider guards would also be stiffer than narrow ones, and less inclined to flex.

I would suggest fitting the widest guard that fits through the seat stays without distortion. Proper roadwater deflection requires full-length guards, and they need two support stays aft of the rear axle. The front guard only generally needs one support stay. Both guards should come down behind the tyre and finish no-higher than the axle. The rear guard should come forward to the level of the BB, and the front guard should go to at least the horizontal, or slightly further forward.

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Note the manufacturer's spec of "up to 36mm tyres" will use all the space under a brake leaving no room for the mudguard/fender.

You'll have to measure, read specs, and hope. If they turn up and don't fit, then you'll likely have to return, or if you are brave fit them by filing. Mudguards are not precision parts - some fettling is frequently required.