I've had this on my mind for a while:

Since a small wheel curves downward from the fork crown more sharply than a large wheel it seems to me that the brake pads are not only getting pulled horizontally, but also down. So I'm wondering if brakes for small wheels would be better mounted at a downward angle in order to align the brake bolt with the tangent of the smaller wheel?

Of course this assumes that having the brake forces aligned with the brake bolt like on a larger wheel is a good thing, but perhaps forces perpendicular to the brake bolt (and parallel to the brake arms) actually enhance brake operation? After all I would think there is more flexure in the brake arms than the brake bolt, but the generally poor performance of rim brakes on small wheels makes me skeptical.

What do you guys think?

Your are trying to fix something that is not broken. Roger

Insufficient brake performance- check
Wheel curvature requiring excessive brake drop- check
Bent brake mount bolts- check

Oh it's broken.

1: improperly setup brakes. I can lock my front wheel with my dahon, and this is with rim brakes. The rear is sluggish due to the amount of cable housing length, can't be helped.
2: huh?
3: never seen this. Pics or it didn't happen.

I think the curvature of the smaller wheel does not have anything significant to contribute to downward bending forces on the brake bosses. It is the brake bosses you mean, isn't it?

I am going to assume your talking about brake bosses, too. The forces whether horizonta or vertical would both cause bending on the bosses, small wheels or large. I think in any case the vertical force on brakes is small enough to be ignored.

In a perfect world the center line of the brake bolt should be perpendicular to a line that goes through the center of the wheels axle. Most bikes are close but not perfect. Roger

1: V-brakes or caliper brakes? V-brakes are powerful enough to be sufficient even if they are crippled. Plus the geometry is different and I think the mounting bosses are stiffer than a regular caliper.
2: For a 27" wheel, 8" ahead of the fork crown the rim is basically still at the same height as it is directly under the fork crown, but for a 16" wheel, 8" ahead of the fork crown the rim is located all the way down by the axle. So for a brake which protrudes x mm ahead of the fork crown the brake arms need more "reach" to reach the rims on a smaller wheel. The smaller the wheel diameter the more pronounced the problem.
3:
19735469232_ORIG.jpg

No I'm talking about caliper brakes mounted with a bolt through the fork crown. See pic above.

...and the smaller the wheel the less close it is.

So first I notice the brakes are crap no matter how much adjustment. Second I get some new brakes, but they don't reach even though the "reach" measured from the bolt to the brake pads is the same as the old ones, reason: the arms of the new brakes are farther ahead of the fork crown where the rim is lower due to curvature. Last I notice the old brake bolt is bent down. So I'm wondering if the cause of all these problems is that the brake bolt is not "perpendicular to a line that goes through the center of the wheels axle" (ie not tangent to the rim).

Make it tangent = solve the problems? I dunno and I don't like to make destructive modifications unless I know they will work.

most peculiar.
and yes, I assumed you were talking about V-brakes.
properly setup calipers have the same stopping power as V-brakes.

Now, if this was a common problem, you'd think all the people with modified R20s, bromptoms, moultons and bike fridays would have come forward already.

Maybe they're not as fast as me or maybe their wheels aren't as small as mine.

Also, if your brake ability exceeds the limits of tire traction how can you tell if the performance is reduced unless it drops below the threshold of tire traction? So if you have a very high quality brake as one surely would on a bike friday or moulton and/or if you only use the best brake pads (koolstop) then perhaps you might not notice? But I've seen lots of complaints about the brake performance of Bromptons and other folders and if it is indeed related to wheel curvature then I wouldn't expect folders with larger wheels like R20s to be as affected. The braking action of V-brakes and calipers are also different in their vertical profiles, so that may be another variable obscuring user experiences.

Interesting observation. I have not seen this nor seen a discussion on it ... ever. If you don't mind, what bike is this? What brakes did you use?

Do you think that your use of the bike is rougher than the typical user?

While I am no engineer or physicist, my experience with Brompton-like folders (a Brompton and a Merc) is that these bolts do not bend like that under normal use. Even if you were heavy and doing "stop-ees" all of the time I would not expect the bolt to bend in that manner. Perhaps the top of the brake hit something while it was being stowed or something fell on top of it.

Bike is Pacific Carry-me. Bent bolt is the stock brake. Replacement brake is KCNC, which has the same vertical "reach", but won't reach the rim due to being offset further ahead of the fork crown (I guess to help the brake pads clear very narrow forks?).

It's possible the brake was damaged while stowed, but if it is due to wheel curvature then I need to know so I don't compromise the vertical support of the fork when I modify it...and I will be making modifications because the KCNC brake cost me almost as much as the entire bicycle and I'm determined to make it fit (and it would fit if the wheel weren't so damn curved!). At the very least I need to drill out the back of the fork for the recessed nut, but I'm also going to either:
1. Make the holes into slots by extending them to the bottom of the fork crown. This will allow me to lower the entire brake so the pads reach.
or
2. Stick a long screwdriver in each brake hole and bend it so it angles downward to match the tangent of the wheel.

If vertical forces are a problem then modification #1 will only make it worse by removing the vertical support of the fork crown. Although Sheldon Brown has advocated mounting brakes to only the front hole of forks, which provides even less vertical support, I don't think he tried it with small wheels.

I suppose modification #2 is more easily reversed, but I'm wary since the fork is aluminum (so even if it survives being bent to an angle it might not survive being bent back straight to try #1) and it's probably never been done before in the history of bicycling (an impressive feat or a bad sign?).

Now that you mention it, it also seems strange that the bolt is even somewhat bent in the middle (not just at the front where the brake is actually mounted). Is this possible with the bolt secured at both ends or does it indicate that something acted on it from inside the fork crown?

One possibility is that along the inside of the fork the stem riser is actually tied to this bolt with a cable ("brake" type cable). The purpose is to prevent you from accidentally pulling the stem riser out while unfolding, but it's not supposed to bear any force (that's accomplished by a regular quill type expanding wedge). Surely this cable couldn't bear enough force to bend the bolt if the wedge wasn't sufficiently tightened and the rider was yanking on the bars? Surely braking forces themselves would be the far greater and more likely candidate?

I mangled my fork today so the brake would mount at an angle. If this doesn't make the brake work super then I'm just gonna go with a fixed gear.

Looks like our friend Giuseppe mounts his brakes completely perpendicular to the brake surface:

So maybe I'm wrong about orientation being an issue.

BTW, check out Giuseppe's blog, he's great!
https://mini125rd.over-blog.com/

"lthough Sheldon Brown has advocated mounting brakes to only the front hole of forks, which provides even less vertical support, I don't think he tried it with small wheels."

The 2006 Dahon Hammerhead had the front brake mounted on the rear of the front fork. It looked nuts. (20" wheels) A fix for too short a reach is a drop bracket. Sheldon made some.

I made mine out of 1/8" flat alloy bar and put short reach Ultegra brakes on the Hammerhead. They didn't stop any better than the back-to-front OEM caliper. Sold the bike. No brakes = nuts.

Drop bolt won't work for me because there isn't enough clearance under the fork crown. Don't you see for a small wheel the problem is that the required reach varies as you move ahead or behind the axle? So under the fork crown the required reach is very small, but 1" ahead of the axle (where the brake arms are) the required reach is very large.

Also, for a normal front mount, fork rake improves the situation because it moves the axle more underneath the brake arms. But for a rear mount like your hammerhead the rake has the opposite effect and makes the problem worse...so maybe that's why you had brake performance problems even with a relatively large 20" wheel?

".so maybe that's why you had brake performance problems even with a relatively large 20" wheel?"

No, it was a bad batch of levers.

I can understand that as you brake move away from the fork they need more reach. You need to accound for this in buying brakes so that the pads can hit the rims correctly. However, it won't have much effect on the bending forces, in your case I think it lowers the forces. It adds a downward component where horizontal effect on your brake rotates the brake upward. In any cases these are seen as tension and shear forces in the bolt and not bending. Post pics if you can.

Yeah after further research and thought the conclusion I came to is that it shouldn't affect braking other than that you might need brake arms that are either longer or set closer to the fork crown.

What pics do you want to see?

caliper brakes don work as nice as v brakes ... no secret here

some caliper brakes have this " aero " syndrom to move the pads forward as they use longer pads and need clearance from the forklegs ..... old style caliper brakes usually are more straight down ...

Now I feel your pain ..lol ( not really .. but I have benn there ... )

can you try to mount the brake behind the fork ? This changes the geometry of the brake arms ....and usually give you more power ... if you can find a couple of shims between the brakepad and the arm which can rest against the forklegs you also take a lot of the flex out of the system .... and get better more linear modulation ...

Remember Sommatsch with her Brompton .. I got her maybe the very last in this world Magura "D" sized caliper mount bracket for the hydraulic brakes..... never heard from her again after that ..I sure hope it wasnt the brakes which failed..lol but this might be a viable ( exotic) possibility to make the bike slow down ...

Thor