Shimano BR-AT50 modifications/upgrades

[Brucey]

PACE RC-100



Aluminium frame fitted with PACE fork with dedicated Magura braze-ons. In the late 1980's this was a pretty hot ticket for a race-spec MTB (a couple of chums of mine had these bikes, and when they needed work guess who did it...? ) and they are now a collector's item. PACE come from t'other side of the lumpy bits from where you are, but it doesn't make them all bad you know...

Their many forks are listed here;

http://www.goatsurfer.com/pace_forks.html

and you can see that where possible they put the brake bosses on the back of the forks because the load path is better.

Their stem/steerer design (circa 1989?) used an integrated TIG welded steel steerer/stem assembly. This bolted into the fork crown and worked like an inverted A-Head system (but pretty much before anyone else was doing it this way, obviously). PACE later went to make all kinds of bits including their own BB assemblies but the early bikes had a modified 1" steel Campagnolo Nuovo Record headset where the threaded race had been machined to make it a sliding fit on the steerer, as their design mandated.

Whilst I didn't always think their designs were perfect or that their parts were always super VFM, I do think it is a shame that they appear not to be making their own bouncy forks any more. You can still buy spares though. You can see their present range of frames etc here http://www.pacecycles.com/ and it seems all wrong to me that their bikes usually have someone else's forks on them!

cheers

[reohn2]

Thanks for that Brucey,they totally escaped my consciousness ,though with designs like the one in the photo I don't know how and so close to home too

[Brucey]

update: My initial impressions about my modification were not far off the mark; much of the time the brakes were a little more powerful.

But....

-the rest of the time they were (once the pads were a little worn) wont to manifest some of the less pleasant quirks of canti brakes that are not quite right. There was a little squealing (not too bad) and occasionally quite a lot of juddering, especially in the wet.

I think the juddering was caused by the very thing I was aiming for (the thrust line going through the bushing rather than to one side of it) but when it was wet, the brake probably flipped between states and this caused a judder. However I couldn't know that this was the case for sure until I'd fitted a different setup using the same brake pads on the same rim.

I did the swap about two months ago (to a more conventional setup with the pad set ~symmetric to the bushing, and a thrust line ahead of the bushing as a consequence) and since then there has been no juddering, no squealing, no bad behaviour, regardless of the conditions.

So overall I'd have to mark my experiment/modification down as a failure, or at best 'of limited benefit'.

In point of fact, at one stage the juddering was so bad I began to fear for the frame and fork; the fork was twanging back and forth an enormous amount and no frameset will take that indefinitely.

Now, it may just be coincidence, (or not) but the day before yesterday, the frame broke...

cheers

[531colin]

I know we have had this conversation before......
I have been in the habit of taking front XT parallel push vee brakes apart and re-assembling them so that the pads are in front of the pivot...



....as manufactured, the front pads are behind the pivot, and the brakes scream and howl with the worst of them, but the rears have the pads in front of the pivot, and are quiet almost all the time. (I have one set on a ceramic rim which is sometimes noisy)
My front brakes with the pads in front of the pivot are blissfully quiet. It may seem extreme to modify your brakes just because of a bit of noise, but I really couldn't live with the constant racket....it was so bad i was prepared to try anything or just chuck the brakes away and go back to cantis.

[Brucey]

I wonder if that is a step too far! I also think that the PP V brakes have some special problems; shimano appear to have given up with them now. I note that your bosses look set very high, meaning that the brake block is set low in the brake. This is good for power, but with a PP type V brake, it also puts a higher load through the PP linkage.

[One common mode of squealing with these brakes is, I think, caused by the wheel sweeping through the brake at a non-tangent angle (thus pushing the brake block either up or down when the brake is on) whereas the position of the brake block in the slot tends to preload the linkage in the other direction. In the case of your bike I suspect that the block height is preloading the linkage downwards, but the off-tangent effect would have been pushing the brake block upwards as the brake was set originally. It is possible that moving the brake block forwards may have effected 'a cure' by allowing the brake blocks to work closer to tangent on the wheel rim.

I also wonder if the same kind of thing can happen with conventional canti brakes, i.e. that the cable/reaction loads push the arm one way on the pivot, but the (non-tangent) forces on the brake block push it in another direction, resulting in movement and squeal....? ]

With stud-mount (i.e. V-style) brake blocks, there is a fair amount of variation in stud position depending on the model. The 'original XTR' style has a heavily offset stud, but the current generation of shimano brakes come with a near- symmetric V-brake pad holder; it is still offset, but barely.

However, one variation I've never seen is a holder that has the stud mounted to the rear (i.e. open end) of the brake block. Were there such a thing available, I think you could perhaps have avoided refangling your brakes? Maybe a more recent (i.e. near symmetric) holder would have worked better?

I also wonder if moulded brake blocks with an offset stud (Aztec, Fibrax, Kool stop spring to mind) that can be reversed might have done the trick?

BTW with my brake mod I am pretty sure that any judder would have been amplified on the bike it was fitted to; the bike has (had...) no uphanger and a rather springy 1" steerer. I have occasionally had smaller judder episodes in the past and the same wheels etc (that caused judder in this bike, eg because of rim width variations) didn't cause judder in other bikes.

So although my mod didn't work in this bike, I may yet try it again in another!

cheers

[CREPELLO]

I wonder if that is a step too far!

Nah, not when braking becomes like pulling teeth. Anything for a bit of peace.

I too have updated the AT50's on my Dalesman (or did I mention that above?). I quite like them and they have been very quiet most of the time.

[531colin]

I wonder if that is a step too far!

Nah, not when braking becomes like pulling teeth. Anything for a bit of peace...............

^^^^^wot he said!

I was used to cantis like this....



Where the front canti (with the pad in front of the arm) was quiet, and the back one (with the pad behind the arm) squealed like fun.

Then I bought the XT Vees ....the rear ( with the pad in front) was blissfully quiet, and the front (pad behind arm) squealed. I thought there might be a pattern here, so I modified the front Vee brake, and all was well. I'm running them on 3 bikes at present, and they have been on another 2 or 3 bikes/forks. I have just one rear XT vee on a ceramic rim that can squeak a bit.
I have a couple of bikes still running cantis, on the rear I now use Tektro CR 720 (where the pad is central to the arm) because they don't squeal.

Perhaps there is a pattern, perhaps there isn't.