The long&short of canti blocks

[mattc]

Is it a "normal" thing for bikes to use the longer brake pads,such that they always spring into the forks (or stays) when the straddle cable is disconnected?

Is paint damage an issue? Am I worrying about nothing??  It LOOKS hideous, as if the wrong blocks are fitted!


[Just bought a 2nd used bike with this issue - I can report that the forks never fell off the previous one. But it still looks wrong ... ]

[mcshroom]

Yes it is, although I've recently fitted some mini v's with holders that take road pads, and I prefer that set up myself.

These are the ones that my (RX-5) brakes came with. I'm tempted to get some more as I prefer the look to normal length v brakes.

[mattc]

Thanks 'shroom!

Is there any difference in performance with long-vs-short? My physics brain says it should be all about the compound*, but sometimes theory doesn't match reality.


(*and the setup/geometry of the brakes! )

[mcshroom]

I can't see why the pad length should make a massive difference and I've not noticed any difference to braking performance compared to the previous brakes with longer pads. I fitted the pads and brakes together however, so it hasn't been an exact comparison.

You might find that they still foul the fork/stays though. Mine do: -

[Brucey]

long(er) brake blocks came in with the original 'deer head' XT I think. Prior to that the longest brake blocks were the Mafac five stud and the Scott-Matthauser brake blocks.  Then there were one-piece brake blocks such as Aztec, Kool Stop, and Fibrax versions. The 70mm cartridge  ones we are most familiar with came (I think) with a 8s XTR groupset which came with the first version of 'parallel push' V brakes.

I think the logic was that a longer brake block that was thinner would last as long as shorter fatter one, but require less adjustment in the meantime; just as well with the slightly reduced running clearance that came with V brakes.

There is an argument that the longer (and in many cases offset) brake block is more stable against the rim, and thus twists less, allowing more even wear than you might get otherwise, and perhaps reducing squeal.  There may be something in this, but that is not to say that brakes with shorter brake blocks don't work well, and that brakes with longer brake blocks never squeal.... 

One thing is clear; that is that the shorter ~55mm shimano  'road' pattern brake blocks offer the widest choice in insert compounds. IIRC Swissstop ones are no longer made in the 70mm fitment.  Annoyingly there are few options for post-mount 55mm insert holders, which is what you might prefer for a lot of older canti-brakes.

BTW the 70mm brake blocks don't usually badly obstruct the passage of the wheel on an MTB with decent mud clearance, but they do on a lot of road bikes. Then again most caliper brakes don't open up far enough to allow a fat tyre through either.

cheers

[Torslanda]

Based on purely anecdotal evidence I think that there is something in the theory that long pads are not as effective as short ones in an identical setup.

The theory goes a bit like this: the longer pad has greater surface area whilst the mechanical advantage remains the same, the result being that the longer pad exerts less force on the rim.

The anecdotal evidence is actually from the car world: MkII VW Polos were renowned for having 'crap' brakes. VW were blamed for not fitting the car with a vacuum servo which meant higher pedal pressures were required to stop the car when in fact garages were fitting the wrong brake pads. MkII Golfs used a similar pad but with a lot more material on them - but they had servo brakes. Unfortunately they were the same thickness and, even worse, they fitted. So if the garage ordered Golf pads instead of Polo (you might be surprised how many mechanics get stuff like that wrong!) no one would know unless the customer complained about needing thighs like Chris Hoy to stop the car. And even then the poor sod who supplied the pads would get the blame for 'crap parts' . . .

[Brucey]

in any given car with any given pedal load the force is always the same, but the pressure (force per unit area) varies with the pad area.

With changes in pressure go changes in work rate per unit area and therefore temperature of the brake pad. Car disc brakes often have brake pad compounds that have relatively poor cold friction and increasingly good friction as the temperature increases (up to a certain point). Compounds vary with vehicle weight; with  heavy vehicle the brakes never stay cold for long, so more servo assistance, pads that work well at higher temperatures etc are very much the order of the day.

Thus if you can fit brake pads that are meant for a big heavy car into a smaller lighter one that doesn't generate (or need) as much clamping force in the brakes, you may get a delay in the working of the brake or indeed it may not work at all well until the pedal pressure is unreasonably high.  You can test for this by simply applying constant brake pressure at the pedal and seeing if the rate at which you are slowing down varies noticeably or not. Normally the brake tends to 'come on by itself' to some extent as the pads heat up.

 Most of the tricks that improve 'cold bite' also increase disc wear, or make the brake pads highest working temperature a bit less... i.e. there is no free lunch.... .

Bicycle brakes don't work in the same way; very few brake blocks work a lot better when they are hot than when they are cold. Thus pressure per se is not so important in the usual operation of the brake. However it does cause variations in how the brake block clears water in the wet and it does change the wear rate of the brake block as well as how hard/long you can brake before the brake block overheats.

cheers

[Somnolent]

I've used the dual compound version of these for a number of years on a couple of canti-braked machines:


Very long lasting, pretty effective and no risk of paint damage as the whole thing is rubber.

If you need a bit more braking power at the back then these combined with salmon or dual compound blocks are very good but you do need to read the instructions - they are not fitted the same way as regular cantis.   SJS have been selling them off ridiculously cheap for years.

[Brucey]

worth noting that in an earlier incarnation the post-mount brake blocks (similar to the above) differed from the bolted version of the same brake block only by virtue that there was a threaded sleeve loctited to a threaded stud beneath.  With a bit of effort this threaded sleeve could be unscrewed and then used with a (much wider) variety of bolted brake blocks.

I am not sure if there are any such brake blocks (with removable posts) on sale at present, though. If you see any, the clue is that there is a hexagon at the base of the post.

Using this scheme I was for some years able to use one of my canti-equipped MTBs with a wide variety of brake blocks, most of which wouldn't have been easily available to me in the post-mount format.

cheers

[rogerzilla]

Long pads are there not for better braking but to make wheel removal easier via the "quick release" (cable unhooking) function.  It's not the length that facilitates this directly, but the thin profile.  However, if you had the thin profile with normal-sized pads, they would have an unacceptably short life, so the extra surface area slows down the rate of attrition.

A side benefit of the thinner pads is that they usually squash a bit less under hard braking.

[Moleman76]

Minor thread hijack:
It's seemed to me that the longer the pad, the less well its manufactured curve seems to match my rims.  (They do seem to be made with a curve to them, I don't think my eyes have gotten that bad yet.)
So, are they made to suit a 622 / 700c / 29er rim diameter, and thus are less curved than my 650b, 584 rims? Or am I imagining things?

It seems like the ends of the pads are closer to the sidewalls than I would like - along with the end that dangles between the fork legs wanting to hang up and not allow the brakes to fully open (original topic).

[Brucey]

most long cartridge brake blocks were originally designed for 559 rims.  Recent ones and some others may be designed for 622 rims.  584 rims are within 5% of either so brake blocks ought to fit OK on them either way round.

cheers

[rogerzilla]

Kool-Stop make two different profiles of long plain stud pads, it seems.  The squared-off ones on the Bob Griffin look more sharply curved than the more boat-ended ones on the Harry Quinn.  The former are sold as mountain pads, though.