Shimano BR-AT50 modifications/upgrades

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Brucey
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Shimano BR-AT50 modifications/upgrades

Post by Brucey »

I've long used various cantilever brakes and despite the fact that they are a little bit fiddly to set up I've always had a soft spot for several models of shimano cantilevers that were produced in the 1980 and 1990s. Of these the BR-AT50 was certainly the best value and didn't seem to work appreciably worse than the more expensive versions.

Like a few other post-mount cantis these brakes can be configured to work OK on wide spaced bosses or narrow spaced bosses. On wide spaced bosses (~83mm spacing) they can be configured as mid-profile or wide-profile brakes, but on narrow spaced bosses (60-65mm) they usually can only be configured as wide profile brakes.

This is what they look like normally (NB non-standard pivot bolt here; otherwise factory fit parts);
Image

and this shows what the parts look like when the brake is disassembled

Image

I've used these brakes for long enough to know that they will eventually wear the bosses and the pads badly if the line of force from the pad is well away from the pivot boss. (Assuming a frictional coefficient of about 1.0 then imagine a line pushing out from the centre of the brake block at 45 degrees when viewed from above.) On the rear brake this is no problem; a symmetric or slightly asymmetric brake block will work OK. At the front it isn't so good; the post mount is well forwards and even the most asymmetric pad will be too far forwards and therefore twisting on the boss instead of more pushing on it (if you see what I mean). [NB. This kind of reasoning is why PACE and Thorn (amongst others) have often mounted V brakes on the back of the fork blades.] So maybe revising the pad mounting on the front brake is a good idea, I thought.

Here is the result of this approach, together with a few other modifications;

old dog: new tricks?
old dog: new tricks?


I've carried out the following modifications;

1) the slimmest cartridge pad holders that I could find are fitted; XTR pattern ones made by Clarks (model ref CP522). These are slim enough to fit between the fork blades and allow enough room for the rim (but with narrow forks, not always the tyre when the wheel is removed unless it is first deflated). They will accept any standard fit 70mm V-type brake insert.

2) The post mount hardware is reversed on the brake arm in an attempt to give a better path of the force from the brake block. The curved aluminium piece has to be inverted (so that the 'UP' arrow points downwards) when this is done, and similarly the (toe adjusting) wedge washer has the tab pointed downwards. (In one instance the opening in the arm needed a tickle with a swiss file to allow the parts to be fitted OK.)

3) Custom stainless steel bolt (Mafac pattern) and washer on the main pivot.

4) Domed stainless nut on the straddle pinch bolt.

5) Domed stainless nut on the post mount bolt.

6) Waxoyl treatment on all exposed steel parts.

7) Waxoyl treatment on the return springs (which tend to corrode in our lovely winter weather).

8 ) The curved steel washer (which originally had very thin plating and corrodes easily) has been covered in self-adhesive aluminium tape.

9) The brake arm has been polished using solvol autosol. (Note that many other shimano cantis have anodised arms that look nicer when they are new but can't be polished up the same way when the finish eventually deteriorates.)

as pictured the brake is mounted to a pair of (single hole) MAFAC braze-ons which are slightly less than 60mm spaced apart. You can see that with thin pads of this sort the arms can be elevated slightly even when the bosses are very close to one another. This isn't quite a full mid-profile set-up but as shown (with the straddle high enough to clear a lamp bracket) it is about +25% to +30% higher MA than a full wide-profile setup would be and with a lower straddle that just clears the mudguard I think you could go to perhaps + 50% or even +100% higher MA if you wanted to.

The toe adjustment can be done using finger and thumb on the wedge washer; there is just enough room for my fingers between the back of the brake arm and the fork blade. Similarly the end of the post-mount bolt just clears the fork blade; if the frame builder has reprofiled the boss too much there could be a clash here; in this event the post-mount bolt can be shortened slightly.

When the post-mount bolt is tightened, the post must be restrained in the Mafac way (i.e. by holding the post directly); the usual shimano style method (using an allen key in the end of the post mount bolt) is no longer possible because there is no access.

In use the brake is (using a high MA lever) about as powerful as I would like to have on a springy fork and is plenty enough for unladen work. Good modulation, too. I'd want a lower straddle for more power if the same brake were used on a tandem or a well-laden solo.

There is a school of thought that says that well offset pads like this can cause the brake to squeal. I thought I was in for that because the brake did squeal at first. But after a few applications of the brake it soon settled down and has been quiet since then.

Having used these brakes a fair amount I am of a view that what I've done to them here has made them better rather than worse. Food for thought, anyway..?

[edit; update... I have since abandoned this setup, as described in a later posting 10/6/2016]


cheers
Last edited by Brucey on 10 Jun 2016, 4:58pm, edited 1 time in total.
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gaz
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Location: Kent

Re: Shimano BR-AT50 modifications/upgrades

Post by gaz »

Interesting mod, I might try it out on the commuter*.
Brucey wrote:1) the slimmest cartridge pad holders that I could find are fitted; XTR pattern ones made by Clarks (model ref CP522). These are slim enough to fit between the fork blades and allow enough room for the rim ....

Even those holders won't fit between the fork blades and the Sputnik on my Jackson.

*Edit - I am notoriously slow at trying things out. The underlying issue is that I'm happy with my braking and don't need it to be better, just ready to work next time I want to ride the bike. As such it's going to be a while before I begin any experiments.
Last edited by gaz on 1 Mar 2015, 5:10pm, edited 1 time in total.
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Brucey
Posts: 44520
Joined: 4 Jan 2012, 6:25pm

Re: Shimano BR-AT50 modifications/upgrades

Post by Brucey »

blimey! I wonder if slimmer rims might suit that bike better? If they really don't fit you must have less than 10mm between the rim and the fork blades?

BTW this thread might be of much greater value to others if we can list other similar shimano (and other) brake models that can be converted in this way and if it always provides appreciable benefit of any kind, or of there are exceptions. Theory and my experience to date are one thing (well, two things at a push...) but the experience of others may add fat to this fire.

cheers
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CREPELLO
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Re: Shimano BR-AT50 modifications/upgrades

Post by CREPELLO »

Good looking mod there. I've upgraded my AT-50's too, but not the rejig like above. As much hardware has been replaced with SS and I've also used the Clarks shoe holders. This has been for my old (Dales)man with close canti studs and a not very wide fork. It works for me because I'm using a narrow Mavic MA3 rim. I'll have to see whether I could cary out the full mod.
Brucey
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Joined: 4 Jan 2012, 6:25pm

Re: Shimano BR-AT50 modifications/upgrades

Post by Brucey »

oddly enough I just checked the setup of my most powerful canti-brakes, which are Avid shortys, (set onto wide bosses, obviously). The standard Avid cartridge pad is an offset design and the mount is also offset rearwards (on the front brake) on the arm itself, so the net result is that the front pads are positioned similarly to my modified BR-AT50 brakes.

The rear pads on the Avid brakes are a different matter; I do not think they are similarly well positioned. Just as well the rear brake doesn't need to be so powerful I guess.

cheers
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rjb
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Re: Shimano BR-AT50 modifications/upgrades

Post by rjb »

Nice one.
I've always found these brakes to be more than adequate in use. I had them on a previously owned Tandem and used a very high straddle clamp and had no concerns with rim clearance. This is how I have them set on my hack bike. Ps I have a set of these weinmanns which are a similar vintage and thought about reducing the brake shoe clamp to accept v brake shoes now that the weinmanns shoes are rarer than hens teeth :lol:
image.jpg

image.jpg
At the last count:- Peugeot 531 pro, Dawes Discovery Tandem, Dawes Kingpin X3, Raleigh 20 stowaway, 1965 Moulton deluxe, Falcon K2 MTB dropped bar tourer, Rudge Bi frame folder, Longstaff trike conversion on a Giant XTC 840 :D
rjb
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Re: Shimano BR-AT50 modifications/upgrades

Post by rjb »

Brucey wrote: [NB. This kind of reasoning is why PACE and Thorn (amongst others) have often mounted V brakes on the back of the fork blades.]
cheers


Forgive me as i am probably a cynic but i thought Thorn put the v bosses on the rear of the front forks for tandems to use the Suntour self energising brakes. V brakes have largely made these redundant which explains why they are now in their bargain basement. :lol:
At the last count:- Peugeot 531 pro, Dawes Discovery Tandem, Dawes Kingpin X3, Raleigh 20 stowaway, 1965 Moulton deluxe, Falcon K2 MTB dropped bar tourer, Rudge Bi frame folder, Longstaff trike conversion on a Giant XTC 840 :D
Brucey
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Re: Shimano BR-AT50 modifications/upgrades

Post by Brucey »

there is mention of the reversed bosses here

http://www.thorncycles.co.uk/thornpdf/ArriveeSportTourTestAutumn2006web.pdf

and the claim is that it reduces squeal. However I note with interest that AFAICT this feature does not appear on the forks in the current Raven brochure....

http://www.sjscycles.com/thornpdf/thornraventourbrohires.pdf

but it does appear in the current Nomad brochure

http://www.sjscycles.com/thornpdf/ThornRavenNomadBroHiRes.pdf

I do think it provides a better distribution of forces in the fork.

cheers
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Bob999
Posts: 39
Joined: 22 Jan 2015, 7:59pm

Re: Shimano BR-AT50 modifications/upgrades

Post by Bob999 »

I hope this post is not too much of a deviation from the thread subject. After reading the thread, I decided to try the AT-50 brakes on a friend's bike I am storing ("for a couple of months" he said five years ago) on a similar vintage but larger frame. Three are working fine, but the fourth does not move at all after I tighten the mounting bolt; finger tight and it still works. I swapped one of the other brakes on to it, same problem, so I suppose it is the boss not the brake creating the problem.

The only difference I can see on the boss is that the paint is more worn on the wider-circumference bit of the boss at the frame end - there's more primer showing. I guess this might point to previous owners of the frame having a similar problem but I don't understand how the boss diameter could be bigger on that particular one unless it has a thicker coat of primer or was badly made.

My next step will be to sand off the primer altogether but I thought I would check for any advice here first - thank you.
Brucey
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Re: Shimano BR-AT50 modifications/upgrades

Post by Brucey »

on BR-AT50 brake there are only two ways that something can stop the arm from rotating;

1) that the brass sleeve binds on the boss (eg because the boss is too short, or bent) or

2) that the spring/plastic spring cover doesn't seat correctly and it then binds on the brake arm.

So if you try the brake arm without the spring/spring cover you should be able to tell what is binding and where. A misplaced blob of braze metal might well foul some brake types and not others.

cheers
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reohn2
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Re: Shimano BR-AT50 modifications/upgrades

Post by reohn2 »

rjb wrote:
Brucey wrote: [NB. This kind of reasoning is why PACE and Thorn (amongst others) have often mounted V brakes on the back of the fork blades.]
cheers


Forgive me as i am probably a cynic but i thought Thorn put the v bosses on the rear of the front forks for tandems to use the Suntour self energising brakes. V brakes have largely made these redundant which explains why they are now in their bargain basement. :lol:


Quote right,and IMO just left them there for no apparent reason :?

The Shimano Canti's in the OP and your other post are the best canti's I've ever used,we had them on '90 Dawes SG tandem,great brakes :)
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"All we are not stares back at what we are"
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Brucey
Posts: 44520
Joined: 4 Jan 2012, 6:25pm

Re: Shimano BR-AT50 modifications/upgrades

Post by Brucey »

reohn2 wrote: ...just left them there for no apparent reason :?


Although the SunTour brakes were convenient on a tandem back in the day, they say 'less squeal' (and I can believe it with some linkage style V brakes) and logically speaking the braking forces are better distributed. I don't think you need to invoke more reasons than that TBH. I don't think PACE ever used SE brakes, did they?

cheers
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Bob999
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Re: Shimano BR-AT50 modifications/upgrades

Post by Bob999 »

1) that the brass sleeve binds on the boss (eg because the boss is too short, or bent) or


Thank you - the boss is too short by a fraction of a millimetre - I can also now see that compared to the other bosses. I can't see any little blob of metal that would be the cause. Would it be OK to try to tap out the brass sleeve so I can file it down a touch?

Thanks
Brucey
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Joined: 4 Jan 2012, 6:25pm

Re: Shimano BR-AT50 modifications/upgrades

Post by Brucey »

you could try a shim washer on the boss end or better still filing down the shoulder on the boss slightly.

The brass sleeve may come out of the brake OK but even if you succeed that will be the only brake that will work on that boss.

So I'd favour filing the shoulder on the boss as the best solution.

cheers
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reohn2
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Re: Shimano BR-AT50 modifications/upgrades

Post by reohn2 »

Brucey wrote: I don't think PACE ever used SE brakes, did they?

cheers


PACE?
I'll need educating
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"All we are not stares back at what we are"
W H Auden
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