Thorn's rear-mounted fork brake bosses: Why?

[Danneaux]

Hi All!

This topic came up in a For Sale post and looks ready to grow legs, so I copied my post from there to here to avoid off-topic thread fragmentation and invite others to contribute with thieir thoughts pro and con on Thorn's unique front brake placment on some models.
=====
Repost:
I recall a past post here where Thorn designer Andy Blance spoke to the matter. If I recall correctly, the goal of this placement was to reduce brake judder and squeal by pulling the mounts, brakes, and pads inward under brsking rather than outward as in conventional designs.

I recall seeing some early Thorn-sourced photos that showed cantis mounted on the rearward-facing fork bosses. They were actuated by a Sun Tour bell crank -- a little teeter-totter device. The cable was routed through one side of the bellcrank and secured to one canti brake arm, the bellcrank served as a bedstop for the housing. The other side pulled canti through a short straddle cable. Equal and opposite forces were split evenly between the arms with disconnection as easy as a v-brake. Sometimes, a similar setup is used to actuate rear cantis from below on an open frame.

There are times my good memory is a curse, so here's a truly esoteric bit of trivia that may have inspired Andy's rear-side fork brake boss placement: I can't speak for Andy, but as a longtime roadie and with Robin Thorn starting with bike repairs...

In the early 1970s, road rims were available with flat-sided/"straight"/parallel or angled sidewall designs, where the rim edge was wider than the base. Most sidepull calipers of the day had pretty flexible arms that were pulled forward by the brake pads.

On the rear, when used with angled rims, this had the effect of pulling the pads from the wider to narrower part of the rim, reducing braking. Canny bespoke framebuilders and mechanics would reverse the rear sidepull caliper, mounting it on the forward side of the seatstay brake bridge. Under braking, the pads would be drawn forward and upward into the wider part of the angle-sided rim, creating a mild self-energizing effect and reducing judder and squeal.

I have a couple bikes dating from 1970s originally equipped with angled rims and sidepulls and, yes, there was a noticeably positive difference to reversing the rear brake. I'm guessing such influenced Andy, at least for a time, but of course confirmation would have to come from him.

Whew! Enough historical trivia and esoterica from me for the moment; I'm tapped out!

All the best, Dan.

[mickeg]

My issues with the rear facing cantilever (which I abreviate as canti)  posts on the Nomad Mk II fork:

I already mentioned in a recent thread, the design means that conventional canti brakes can't be used, there is no place to put a cable hanger.

Someone else in that recent thread mentioned that the brakes catch a lot of dirt and mud, but for me that is not a big deal when I have fenders on the bike.  But I can see where it is an issue.

Sometimes you may want to mount something else on those posts, such as a small rack, example in first photo is my Sherpa with a small rack on those posts.

Probably my biggest gripe is that the brake rotates on a post that is far from parallel with the orientation of the rim where the brake pad contacts it.  See second photo.  When you actuate the brake, the rear part of the brake pad instead of moving directly towards the rim and slightly down, instead mostly moves down and somewhat inwards towards the rim.  This makes it hard to adjust the brake very well while making sure that the brake pad will not contact the tire sidewall when using wider tires like those in the photo.

Since the brake arms are not directly perpendicular with the fender, the fender under the cable is further from the canti posts, these are the longest brakes that I could find yet my brake cable still rubs on the fender.  If the posts were parallel with the fender, I would have had several more mm of space.

When I disconnect the cable and try to remove my wheel, the aft ends of the brake pads are much closer together than the foreword part of the pads, a bit harder to get the tire in and out of the fork than if the canti posts were in front so that the pads would be very close to parallel.

I have the S&S version of the bike.  I have to remove the fork to pack the bike.
 When I pack the bike in a tiny little case, it would be easier if the posts were on the same side of the fork as the curve of the fork blades.  Makes it harder to pack a fork that has a thicker shape.

If Thorn was concerned that the brake posts would flex the wrong way when you brake, by that I mean the outer part of the posts could shift further apart from the brake pressure on the rim, there are other ways to deal with that.  I do not recall what those plates were called that could be added to the canti posts, were they called brake boosters?  Those plates could reinforce the canti posts so that the posts would flex less.  Those could have been provided when selling the bike or the frames.

That is all that comes to mind at the moment.

[Danneaux]

... the design means that conventional canti brakes can't be used, there is no place to put a cable hanger.

That's right, George. In the older Thorn-sourced photo I saw, possibly of a Nomad Mk2 prototype, the rear of the fork crown carried a boss like the front (where a headlight bracket is generally mounted) or perhaps was through-drilled. In any event, the conventional cantis were then actuated by a SunTour Power Hanger (bellcrank) like the one shown in the attached photo. SJS Cycles still sell one made by Tektro, shown here...
https://www.sjscycles.co.uk/brakes/tektro-power-hanger-brake-enhancer-for-cantilever-brakes/?geoc=US

Best, Dan.

[Andyb1]

With the brake behind the fork I can imagine that that any water / dirt on the rim is more likely to end up hitting the rider.

I was recently given an old SWB Velocita recumbent that I am working on to make rideable again and that uses a bellcrank on the canti front brake so that the cable can enter it from the side - it can not come in from the top due to the frame above.

[mickeg]

That graphic with the Suntour gizmo makes it more clear how it works. 

On my rando bike I use a canti cable hanger that is installed on the fork crown instead of at the top of the head tube, I did it that way because my second stem for the handlebar bracket would interfere with cable operation if installed at top of headtube.

Yes, I could have had the cable mounted slightly to the side of the second stem, but I am an engineer, engineers like symmetry.

But the cable hanger I used would be too tall to fit on the back side below a downtube, thus only works in front.

[PH]

I think Dan has covered the theory and thinking behind the idea.  In reality, I don't think it made much difference to most people, and made for an awkward cable run, plus having the uninitiated thinking you'd put the forks on backwards!  That Thorn didn't persevere with the idea for long, indicated that whatever the theory, it wasn't a popular feature. 

[Danneaux]

Nicely summarized, Paul!

Best, Dan.