Fender clearance
 

I asked these questions elsewhere but didn’t get much traffic and the context is in long distance cycling. So I’ll ask here.

Do you run fenders on your bike? How much clearance between fender and tire do you have?

What would you consider a minimum acceptable clearance?

Yes, I've got fenders. Fun story: 30 miles from the end of a rainy century, a guy at a rest stop offered me $100 for my fenders. I should have taken him up on it -- they only cost $35 and it didn't rain the rest of the way.

I just eyeball the clearance, maybe 1/4-1/2"?

Two things to consider. What's the largest tire you'll run on a given bike? -- make sure you've got room for that under the fender. And where are you going to ride? I've very careful about getting off clean, paved roads. You don't want to get a large piece of gravel or a stick jammed at the fender, even if it's got a quick release it's jarring. I'm not sure what I'd do on a brevet with long gravel stretches.

It's really nice to have 10mm or more. With bigger tires, more works better. If you only have 6mm or less, it's likely you will find yourself fussing with the fender pretty often.

When I build frames, it always seems to disappear somehow. Especially with forks. One thing I found was that dual pivot side-pulls actually reduce the clearance available as they are actuated. On my one bike, I just got tired of messing with it and got rid of the front fender. Prior to that, I had switched to a Mafac Racer, which helped a little.

Like Unterhausen, I aim for 10 plus mm of clearance. That said, right now I have a bike with 45mm fender over 37mm tires and another bike with 65mm fenders over 57mm tires.

I have some older Zefal fenders that have fairly tight stays on the side, that 45mm fender works well with 32mm tires, but not 37mm.

I have one bike with fenders, I'll second or third the ~10mm of clearance. The only time I run into issues with that is with wet/sticky snow gumming up between the tire/fender, but that is only a couple days of the year. Haven't had issues with rocks/debris getting caught in there.

I'm a bit curious what brought this question on. A bike has the clearance that it was made with. If you are getting a frame built, tell them 20mm of clearance, unless it's going to be used a lot on gravel, in which case tell them 30mm
I don't think I'm the only builder that has aimed for 10mm and gotten less. It's okay, but I hate the sound of gravel scraping the inside of the fender.

it’s a bike that was likely never intended to accommodate fenders. But I had the brake bridge moved and the chainstays crimped for larger tires. With a Paul Racer M, I have 13-14mm clearance with a 32mm tire, minus the fender thickness. I figure that’s probably sufficient. With a 35mm tire it’s probably getting a little close.

Fork clearance is about 12mm with 32’s minus the fender. That’s at the front of the crown, at the rear it’s significantly more. ( Can’t measure it at this moment)

okay, that sounds like plenty. Berthouds have a dimple for the crown, not sure if other metal fenders do. That increases the average clearance a little. Which is also important.

If you don't like gravel, you should try mud. A while back I hit a muddy road with fenders, probably an average of 15-20 mm clearance. It sounded like the bike was hawking and spitting all the way down that stretch: mud accumulating on the tires and scraping the fender (haw-w-w-w-k) and the flying off (spit). I was happy to get off that stretch!

I'll put in my $0.02
If you are running 32s you should hope there is room for 50mm fenders. I had 40mm Bertouds on my bike with 28s and it was doable with the caliper (BR 650s) squeezing the fender a bit. I have since removed the fenders and put on centerpull brakes and 32s. I don't think 50mm fenders will fit without some crimping and cutting. Also consider the fender inside width as the metal fenders get narrow because of the folding of the edges while plastic fenders have more room inside for a given "width".

I have Berthoud 40mm on my bike with 32mm and it works, but it would be a lot better if there was a 45. I need to order some 50mm rims.

About 1/4" where the tire enters, increasing from there. That's the goal anyway.

I run 38mm RH tires on my bike with 45mm longboards, and I've never had a problem. They measure out at 35, so I'll go along with the 10mm minimum recommendations. I tried to run 40mm duranos and there was too much fiddling.

I have Gilles Berthoud fenders on two of my bikes and a pair of vintage stainless steel Weinmanns on another.

• 42mm tyre + 50mm fender + vertical dropout = perfectly fine > Gazelle Lausanne. Designed for 37mm tyre.
• 35mm tyre + 45mm fender + Shimano dropout = deflate tyre to take the wheel out > Batavus Randonneur GL. Designed for 32mm tyre
• 37mm tyre + 50mm fender + reversed horizontal dropout = deflate tyre to take the wheel out > Koga-Miyata SilverAce Designed for 32mm tyre

My preference from this is 37mm tyre with 50mm fenders that are about 10-15mm above the tyre.

I suppose I should have been more specific. I’m really after what clearance you guys feel comfortable with at the top of the tire. In my mind, side clearance is less crucial. I’m prepared to be corrected on that however.

I think you get more play side to side, so you need more space to accommodate that.

Aesthetically, closer is better. But closer means more time fiddling with rubbing noises. Closer also is more problematic in muddy or snowy conditions. Most important in those conditions is that space increases from where crap enters to where it hopefully exits. You also want to discourage sticks from entering by having it closer at the entry point.

Also at the top of the tire you likely have fasteners that eat a few mms.

One thing to keep in mind that with plastic fenders is a stick of just the right size that jams between the tire and fender will accordion the fender up into the fork crown, the front wheel will stop rotating with respect to the bike, and you will find yourself flying over the handlebars in the amount of time it takes for the front wheel to rotate 1/4 turn. You can do the math but take it from me, it happens very very quickly.

So be sure to install the quick release features on the front fender stays. Or mount metal fenders.

The fenders are going to mount where they're going to mount. That's determined by the frame and fork. So the question really is, "How big a tire can I run under my fenders?" Simple- experiment. I've seen road bike fender clearances of as little as 1/8". I like more, as above, maybe 6mm minimum, for the reasons given above. Thing is, you don't really know how big a tire is until you inflate it on your rim. It'll still grow a bit for a few days. OTOH, maybe the OP has the tires and wonders if they can run fenders over them. Try.

https://cimg6.ibsrv.net/gimg/bikefor...0f2659f636.jpg
I run 45 SKS longboard fenders on the road bike with 28- 32 mm tires. They did not fit under the calipers, so I cut them and made aluminum brackets that mount into the Sheldon nuts for brake calipers. I used to run SKS 35 fenders, but 45mm work better. need to make better pictures of the brackets, but that is the only picture I have available now.