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Rear fender mounts

Old 01-31-20, 11:21 AM
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BigPoser
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Rear fender mounts

I going to be working on a build soon that will need some rear fender mounts. It will have disc brakes so I know I'll need to have a mount at the SS bridge as well as the CS bridge. My question is regarding a specific distance or spacing along both the SS and CS. Fenders are new for me, so I'm totally clueless, but don't want to just throw some mounts in there.

And would I just a H2O boss for the mounts?

Any help on this is appreciated.

Brandon
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Old 01-31-20, 05:35 PM
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If the brake is tucked inside the rear triangle, you shouldn't have any problems due to discs. If it's SS mounted, it might give you problems with the stays.

First thing to consider is if you have horizontal dropouts, track ends, sliders, rockers, etc. Unlikely with discs but you never know. If so, the CS bridge needs to be placed to clear the tire, even if it gets less than an ideal fenderline, especially with horizontal dropouts, even more so if you want to be angle to remove an inflated tire. It's always possible to space out the CS bridge if chainstay length isn't constant so wheel is more important than fenderline here. If you have horizontal dropouts and want to use something like a VO spring thing, then the CS bridge hole has to be a through hole. Take a look at a vintage Raleigh Sports and you can see the funky fenderlines due to horizontal dropouts to tension the IGH.

You can use bottle bosses, but L-brackets on fenders tend to be sized for M6 not M5 because that's what rim brakes use.

I'm not sure what's trendy for fender clearance nowadays, I've seen examples built with as little as 10mm, the rando crowd likes ~20mm, and fenders shoehorned into road bikes where they barely fit. You should get the wheel you plan on using, put on the tire, let it stretch at high pressure for a few days, then measure the circumference and calculate the radius. Or you could just guesstimate. Half of bead seat diameter plus tire width gets thrown off if tires are wide or knobbly or inaccurately sized and rim width can also affect it a little.

You take your radius, add desired clearance, maybe add a bit for a rubber/leather washer and make sure your bridges clear them (radius to the wheel side of the bridge, not center). If your tire diameter is say 678, then radius is 339, adding 20mm of fender clearance makes that 359mm.

CS bridge should be drilled radially to the wheel. SS bridge tangentially if using a L bracket, radially if using direct attachment. Direct attachment looks much cleaner if not running rim brakes but means the fender can't be reused on a different bike. If you don't drill through the bridges and use bosses and direct attachment, it looks extremely clean with the fender on, and barely noticeable with the fenders off.
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Old 01-31-20, 06:15 PM
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you can use a bottle boss for the mounts, doesn't take much of a washer to cover up the gap between 5 and 6mm bolts. I use metal fenders, so I use fender washers on the mounting.

I feel like you should aim for 20mm, especially if it's a bigger tire. 10mm gets filled up fast, and there is nothing more annoying than crap being dragged around inside your fender. 15mm might be workable, but you have to be really careful. What you want to do is aim to put your bridges at the same distance from the axle center.

I assume you are using vertical drops or TA. If you are using horizontal, all bets are off.
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Old 01-31-20, 08:07 PM
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I like the idea of a fender washer!!
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Old 02-01-20, 06:13 AM
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This is an open ended bottle boss in the CS bridge, which also doubled as a vent hole for brazing.

I used a bottle boss in the seat tube instead of a chainstay bridge. There was not enough clearance to run the fender all the way to the chainstays and still have the front derailleur function.

The finished frame with color matched fender and leather fender washers..
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Old 02-01-20, 09:55 AM
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Clearance over tires- Old school skinny was 10mm min. Smart amount and not be too much is 15mm. This is based on the largest/widest tires the bike will ever see.
Rear attachment- Radially drilled/direct mount at the brake bridge is cool but far less adjustable and a bigger pain to set up. L bracket allows some position options.
Eyelets- 5mm mounting bolts are fine where're talking fenders not racks. (And any well laid out bike that also uses a rack will have separate eyes for that and are far enough from the fender eyes to have all play nice). I've used bottle mounts, actual eyes, SS rack mounts (hour glass shaped), unthreaded tabs and self made eyes with a stand off.
Fender line- I don't get too worked up about how the tire and fender follow each other. I do try to have the fender about the same height off the tire but if it's off a tad so what? Also this can be adjusted to a degree with struts that are length adjustable.

Both images show a separate eye mounted to a short stand off for our touring bikes. Likely these https://www.cycle-frames.com/NOVA-EY...tegory_id=1574 Andy



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Old 02-03-20, 03:09 PM
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Thanks for the help on this. We'll be using Paragon Rocker drop outs. My customer is a wheel builder and wants something that he can ride, but also show off his builds. The Rocker will give him the ability to put just about any disc wheel he wants to show off including single speed.

DSAUL - I really like the mount on the back side of the ST. We may go that route. That looks clean.
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Old 02-04-20, 12:06 AM
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The traditional way to deal with fenders on bikes with adjustable CS length is just to suffer poor fender line and hope it doesn't look too offensive. Personally not a fan on adjustable CS length on fendered bikes. Eccentric BBs are a SS/fixed workaround for fenders. It would also look much cleaner than a chunky rocker dropout when showing off wheels. Of course eccentric BBs have their own drawbacks. Otherwise, obviously clearances need to account for variations in effective CS length, and fenders need adjustability to maintain ideal fender line. That means you would need excess fender stay to account for adjustment, and a way to adjust the CS bridge attachment point.

One idea I've played around with in my head is brazing a CS bridge with a socket in the middle, allowing it to trap a nyloc nut for a spring thing. Even though you don't need a spring thing for wheel removal, it would make adjusting the fender at the CS bridge as simple as turning a screw without the need to messing with spacers.

Despite the drawbacks of an eccentric BB, I'd take a moment to consider it if wheels are the highlight. Clean and simple fender attachment with perfect fender line to frame the wheels every time with clean minimal dropouts allowing you to see the hubs. No kludging or distractions anywhere near the wheels. It's certainly more photogenic for wheels.
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