As a rule of thumb, max tire clearance on a fender-ed bike is 10mm narrower than the frame is designed for without fenders.
For example, the Outback is designed around 700c x 48mm or 650b x 2.0” (~51mm) so with fenders, that comes out to 700c x 38mm or 650b x 41mm, although with 650b you get some leeway due to the smaller diameter of the wheel. You might find that you can get away with a 650b x 45mm tire and fenders. Your mileage may vary depending on what tire and rim combo you use and factors like measured width and tire height/profile.
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In general, you want your fenders to be at least 10mm wider than the widest tire you're going to ride.
https://ridepdw.com/blogs/everydaycycling/fitting-your-fenders-a-brief-guide
Velo Orange Fender Basics
https://velo-orange.com/pages/choosing-fenders
Riding with fenders can be transformational. You'll find your bike far more useful. You won't get a wet streak up your back every time you ride through a puddle. Your feet will no longer be soaking wet just because the road is damp. And you'll stay dryer in anything but heavy rain. Your clothes will be cleaner. It's not surprising that most non-racing bikes in Europe and Asia have fenders.
All our fender models are metal - mostly aluminum with a couple models in stainless steel. They come in sizes for 26", 650b, and 700c/27" wheels.
Most experienced cyclists prefer metal fenders because they provide better weather protection than plastic and are very durable. They also look much nicer than plastic. In fact, a nice pair of fenders can make even an ordinary bike look exceptionally elegant.
One of the reasons that plastic fenders don't work well is that they are too short. The front fender, must come down close to the ground to keep spray off. All the fenders we sell are made to our specifications to ensure that they offer adequate coverage.
Measuring for Fenders
The basic rule in fender fitting is that the fender should be at least 8mm wider than the tire; 10-12 mm is better. So if you're running 32mm or 1 1/4" (for 27") tires, get 45mm fenders. If you're running 42mm tires, get 50 or 52mm Fenders. The reason for this clearance is that otherwise little pebbles, sticks, and other debris that are thrown up by your tire will get lodged between the fender and tire. Of course many tires are smaller than their advertised size, so it's wise to actually measure the inflated tire.
A fender's width is measured on the outside, not inside. So to determine if a fender will fit you'll check clearance at the brakes, in the fork crown, and in between the seat stays.
To check the brakes apply them, so the pads touch the rims. Now measure the width between the arms. Also check that you have at least 9-10mm vertically from the tire to the brake. Short reach caliper brakes typically can accommodate 35-37mm fenders. Grand Cru Long Reach Brakes can fit 43-45mm fenders. XL reach brakes like Tektro R559 can fit 50-52mm fenders.
Next check the width and height of the fork crown. Finally check that the seat stays are far enough apart and that the brake bridge is at least 10mm above the tire. You may have noticed that I've not mentioned the chain stays. Those are easy because you can always trim the bottom of the fender to clear.
Of course metal fenders are flexible and can be bent, so if the fit is close you can always squeeze the fender in a few millimeters at the offending spot.
It's important to also get fenders of the proper radius. 27" and 700c wheels are close enough to use the same fenders. 650b and 26" fenders are close and some folks do use one size on the other, but you may need to adjust your fender line as the size is close but not identical.
Fender line refers to the fender following the radius of the tire. In other words, the gap between the fender and the tire should be constant along the whole length of the fender. It's one of those things that may not matter much in practice, but it effects the bikes appearance a lot.
As for fender length, that's easy: get the longest fenders possible. Long fenders keep you a lot drier. And if you live in the Northwest, get a mudflap for the front fender too. If you ride brevets or in pacelines, your fellow riders will appreciate a rear flap as well.
https://www.renehersecycles.com/fenders-for-different-tire-sizes/
Which fenders for which tire size?
Generally, fenders should be about 40% wider than your tires. This allows them to wrap around your tires without encroaching on the required clearances. This works well for tires up to 42 mm wide, which are best used with 58-60 mm-wide fenders.
However, you cannot scale up fenders indefinitely: Fenders wider than 60 mm do not work with “road” drivetrains, as the chain hits the fender in the smallest gears. For tires wider than 42 mm, stick with a 60 mm-wide fender. Choose a model that does not wrap around the tire very far, and mount it higher above the tire to provide the required clearance. You get a bit of “air” showing between tire and fender, but this “motocross” look is inevitable if you want to run ultra-wide tires with a road drivetrain. (Mountain bike cranks sit further outward and have room for wider fenders.)
Here is a list of Rene Herse tires and recommended fenders:
700C x 26 mm tires: H30 700C, 41 mm wide
700C x 28 mm tires: H30 700C, 41 mm wide
700C x 32 mm tires: H40 700C, 45 mm wide
700C x 35 mm tires: Rene Herse 700C, 50-51 mm wide H50 smooth or H47/H51 fluted
700C x 38 mm tires: Rene Herse 700C, 50-51 mm wide H50 smooth or H47/H51 fluted
700C x 44 mm tires: H80 700C, 62 mm wide
650B x 38 mm tires: H79 650B fluted, 59 mm wide
650B x 42 mm tires: H80 650B smooth, 62 mm wide or H79 650B fluted, 59 mm wide
650B x 48 mm tires: H98 650B, 75 mm wide (for MTB one One-By drivetrains); H80 650B XL, 62 mm wide (for Road drivetrains)
650B x 55 mm tires: H98 650B, 75 mm wide (for MTB one One-By drivetrains)
26″ x 1.8″ tires: H80 26″, 62 mm wide
26″ x 2.3″ tires: H98 26″, 75 mm wide (for MTB one One-By drivetrains); H80 650B smooth, 62 mm wide (for Road drivetrains)
For tires not listed above, we currently don’t offer fenders.
Rene Herse fenders include illustrated step-by-step installation instructions, including how to indent the fenders for fork crown and chainstays (don’t cut aluminum fenders!) and how to mount them free of stresses, so they will give decades of silent, trouble-free performance.
Will Rene Herse fenders fit on my bike? Click here to find out.
https://velo-orange.com/collections/650b-fenders
https://velo-orange.com/collections/650b-fenders/products/vo-snakeskin-fender-50mm-fenders-650b They're 50mm wide, designed for 650b wheels, and work with tires up to 42mm wide.
https://velo-orange.com/collections/650b-fenders/products/vo-polished-52mm-zeppelin-fenders-650b?variant=53108232647 These fenders feature an art-deco, zeppelin profile and are available in high polished silver and smooth Noir. They're 52mm wide, designed for 650b wheels, and work with tires up to 44mm wide.
https://velo-orange.com/collections/650b-fenders/products/650b-smooth-fenders-60mm-silver-and-noir These fenders feature a fantastically simple Smooth design and are available in high polished silver and smooth Noir. They're 58mm wide, designed for 650b wheels, and work with tires up to 50mm wide.
https://velo-orange.com/collections/650b-fenders/products/vo-wavy-58mm-fenders-650b These fenders feature a gorgeous Wavy pattern inspired by a classic French design and are available in high polished silver and smooth Noir. They're 58mm wide, designed for 650b wheels, and work with tires up to 50mm wide.
https://velo-orange.com/pages/re-radiusing-fenders
https://www.rodbikes.com/blog/tech-talk-fenders/