No stopping power on my disc brakes

[gregoryoftours]

The more I read about disc brakes the more I wonder what is wrong with rim brakes! They always seem to work fine and are easy to fix.

Rotors are easier and cheaper to replace than rims, discs generally have better wet weather performance than rim brakes, hydros especially can feel amazing and require a lot less effort to apply strong braking while still retaining great modulation. Good for people with weak hands. Also good if you like to keep your hipster tan wall tyres nice and clean! Disc brakes are also more finicky, heavier, expensive, can be prone to seizing up or sticking, getting contaminated, wheels are weaker. Take your pick!

[RickH]

The more I read about disc brakes the more I wonder what is wrong with rim brakes! They always seem to work fine and are easy to fix.

You could say the same about gears & wheels/tyres - all the problems people on here seem to have! People asking about solutions for problems gives no indication of the number of people not having any problems.

Anecdote alert
I have on 2 occasions found that I have worn the brakes down to the metal baking.

First time was with V brakes where I found the brakes turned almost completely ineffective. Fortunately I was on a canal towpath ride & knew the location of a bike shop not far a way to get some new brake blocks.

Second time was a disc on the tandem. This was on some steep hills. The brake still worked surprisingly well (even if it didn't sound great), although I tried not to uses it on that ride unless the other brake wasn't enough.

I know which failure mode I prefer!

[mnichols]

The more I read about disc brakes the more I wonder what is wrong with rim brakes! They always seem to work fine and are easy to fix.

I did the Fred Whitton route today on a Sonder Camino and the disc brakes were amazing on Hardknot and Wrynose. The amount of control and ability to stop dead on the bends made the descents enjoyable.

[Psamathe]

The more I read about disc brakes the more I wonder what is wrong with rim brakes! They always seem to work fine and are easy to fix.

.... discs generally have better wet weather performance than rim brakes, hydros especially can feel amazing and require a lot less effort to apply strong braking while still retaining great modulation....

I did a 40+ mile downhill earlier this year with hydro disks and i didn't think they were particularly different from my cabled disks. But it was a single ride and long downhill is somewhat different from my normal rides in Flatlandia - so I accept not a totally fair comparison.

Ian

[Brucey]

IMHO the most important thing about any brake is that you understand its limitations and compromises (including the knock-ons on the rest of the bike) well enough to make sure that

a) you are making an informed choice about using it at all and/or
b) you don't come unstuck whilst using it.

IME most 'problems' which people have with brakes stem from a lack of understanding about what they are dealing with.

One comment I feel I must make is that it is incredibly dangerous to allow disc pads to wear down to the backing; to do this you are wearing at least 1mm more than recommended off each pad. If this is combined with a disc that is off-centre in the caliper body, it is, with many brakes, possible for one of the pad backings to not be retained by the caliper body lip any more. If a pad isn't retained by the caliper body any more it may get spat out (no brakes!) or it may move and jam the brake solid. If you have not noticed your pads wearing down this far then you probably won't have spotted that the disc is off centre in the caliper body either, so......

cheers

[mnichols]

The LBS says that he's managed to sort this now by cleaning, adjusting and hard braking to bed in, but has said that softer pads grip better than the sintered ones that i have.

Thoughts?

[Brucey]

could be, but unless you either

a) compare pads (a bedded-in set vs another bedded-in set, being careful to observe the exact same pad location each time) or
b) change other stuff (eg cables) whilst retaining the same (bedded-in) pads etc

you won't find out what is going on. Bad cables can feel 'alright' but rob you of 50% of your brake effort, easy.

One of the checks you can do is to install a short, lubricated, new cable to the rear brake from dummy handlebar/lever, and to compare how that feels vs the usual cable/lever, whilst leaving the caliper etc undisturbed. If your extant cables are a year or more old and weren't particularly well-installed then I'd expect you to see a large difference.

Pads need to be bedded-in as per the brake manufacturer's instructions. You can tell how well this is working by looking at the working faces of the pads; they should be scuffed all over. Ditto the discs. Pads bed in most quickly on new or nearly new discs, and most slowly on worn discs; in extremis if the disc is worn badly, you might have to wear half the (usable) friction material off the pads before they are bedded-in, in some cases.

FWIW the friction coefficient of pads varies, but no-one has published any figures. It will vary with disc material and (almost certainly) disc condition too. Brake pad formulation is a black art and some of the ingredients in some pads actually work as lubricants at high temperatures. This sounds a bit crazy until you realise that the primary friction couple might otherwise have a rising friction coefficient with temperature, and might tend to make the brake very grabby when the pads get hot unless something is added to the mix to mitigate this effect. However traces of one pad material may linger on the disc and interfere with the operation of other pads; this may be temporary or it may persist for some time.
Between this effect, the risk of other types of contamination, and the effects of disc wear on the length of time it takes new (hard) pads to bed in, quite a lot of LBSs change brake discs routinely, (well before they are 'worn out' per se) if they want the customer's brakes to work properly.

cheers

[Mike_Ayling]

The more I read about disc brakes the more I wonder what is wrong with rim brakes! They always seem to work fine and are easy to fix.

My thoughts as well!

Mike

[mnichols]

could be, but unless you either

a) compare pads (a bedded-in set vs another bedded-in set, being careful to observe the exact same pad location each time) or
b) change other stuff (eg cables) whilst retaining the same (bedded-in) pads etc

you won't find out what is going on. Bad cables can feel 'alright' but rob you of 50% of your brake effort, easy.

He used his bike as a control, changing pads, rotors and cables between his, mine and new. Now has new pads and cables and works. Personally i think they just needed to bed in, but tempted to swap the whole lot out for BB7 next time as i don't seem to have this hassle on my other bikes.

Not sure why they started playing up but needing sorting as I'm doing a lap of Ireland in 4 weeks. Likewise i'll leave well alone know as they are working and only got a few weeks of training left

[Brucey]

one of the main reasons for choosing sintered pads is that they last longer. This never comes without compromise, so things like noise, modulation, disc wear, friction coefficient are liable to suffer. Maybe the ones you have bought have traded friction coefficient for life a bit too much?

Another thing is that the MA of spyre varies through the stroke; the MA is highest when the arm is at 90 degrees to the cable.



On both BB5



and BB7




the arm is radiused so that its effective length doesn't vary through the middle of the brake stroke, and this (assuming the ball ramp profile is uniform) gives more consistent MA. You can see that the cable stop on the BB5/BB7 calipers is offset, so that the radiused brake arm protrusion will usually swing under the barrel adjuster support (in MTB calipers especially) instead of hitting it.

The spyre caliper body design doesn't allow for a similar approach; the barrel adjuster sits atop a vertical rather than offset support post, so even though they have redesigned the caliper arm (and BTW the SLC pinch bolt is worse in some respects) it still can't have a radius on the end of it; the arm would hit the cable stop support. What they have done is angled the arm differently in the later version of the spyre ('road' rather then spyke MTB) , so that it is more likely to be in the optimum position when the brake is applied, despite the shorter brake cable pull. The BB7 'road' has been like this for ever; the BB5 'road' isn't like this, presumably because it doesn't have a moving pad adjuster; it needs to have as long a swing on the arm as possible because this is the only way moving pad wear can be allowed for; the arm radius is so long on BB5 that the brake MA is unaffected over about 45 degrees of arm swing.

So there is a useful benefit (in terms of caliper MA) to be had in spyres if you make sure the brake bites when the arm is at 90 degrees to the cable.

I've yet to see a set of spyres that was used in the weather where the thrust bearings were not corroded and rough. You can check this by working the caliper arm by hand when the pads or disc are out of the caliper; bearing roughness will be evident. This roughness may not be disastrous for the brake, but it certainly won't improve it....

cheers

[Brucey]

the current/recent BB series range;



BB5 is practically unchanged for at least 15 years.
BB7 has been revised in the late noughties; parts are different after the change.

BB7S has an anodised body (instead of painted) and stainless steel bolts.
BB7SL is as BB7S but has Ti bolts instead of stainless steel.

Eventually painted caliper bodies start to corrode on the outside in all-weather use but I have never seen this actually cause a major problem; IME internal problems are likely well before this occurs.

Note that Avid quote 'caliper only' weights; this doesn't include pads or the mounting bolts/ISO adaptor, plus their scales appear to be, uh, 'optimistic'. The dirty little secret is that, when measured in the same way, BB5 seems to be even lighter than even BB7SL. Probably if you just want the Ti bolts, you can buy those and upgrade a more standard caliper and it will be cheaper than the SL model.

Both BB5 and BB7 calipers share the same fixed pad adjuster (FPA) that can corrode or get fouled with brake dust. You should dismantle the caliper and clean/lube it from time to time.

Both calipers use CPS (caliper position system) washers on the post mount bolts; this allows the caliper to fit onto frames with perfect alignment even if the mounts are a bit wonky. However the CPS washers are aluminium and have been known to crack. Stainless steel versions are available (at a price) as aftermarket fitments, but washers from V-brake blocks work quite well, and some of these are made in steel not aluminium, so you don't have to spend a fortune to repair or improve the strength of the assembly.

The big differences between BB5 and BB7 are that
- BB7 has two adjustable pads which most folk agree is easier/better than the BB5 barrel adjuster/FPA arrangement
- some users complain that BB5 pads, once worn, often sit cocked at a funny angle (*) and are more likely to rub.
- the BB5 pad is smaller than BB7 which means a shorter wear life, and easier pad overheating with BB5.

On the last point I have found that with sintered pads, things are quite tolerable with BB5.

(*) this isn't always seen and it is not a simple issue, but I think it can be addressed when it does occur.

FWIW in common with most mechanical disc calipers, the 'road' versions of these calipers are (according to SRAM/Avid) suitable for use with their road levers. This means that the caliper MA is slightly lower than one that is optimised for NSSLR levers. Thus there is less leeway for variations in cable condition or pad friction coefficient if you are running recent (most new models after 2008) shimano STIs. If you do find power is a bit lacking, even with new cables and carefully selected pads, and easy solution is to fit a bigger disc; going from 140mm to 160mm or from 160mm to 180mm ought to compensate almost exactly for the reduced MA of NSSLR brake levers.

Bottom line is that when buying new brakes, unless you are a on a tight budget and/or are very worried by weight issues it is probably worth spending the extra on BB7 over BB5. However if you already have BB5, shelling out for BB7 mightn't be 'worth it' per se; there may be other ways of getting a bigger bang for your buck.

cheers

[mnichols]

Thanks Brucey for taking the time to write that comprehensive response.

I'm going to read it a few times to make sure i understand all the points, but the one that stands out is the cable angle, because i did change this. When the pads wear i tend to compensate for this by undoing the allen bolt that clamps the cable and shift it up the cable a bit. It's a quick easy adjustment on tour and i find it easier than turning the tension nut (might be the wrong name for it). Maybe i caused the problem by doing this?

[Brucey]

if you have the original style of spyre and the bite point is set so that the arm hardly moves before the brake comes on, you are probably going to lose about 10-15% of the brake MA vs having the arm at 90 degrees to the cable pull at mid-bite. Not a lot, but something rather than nothing.

Given that these brakes work well with SRAM levers, I do wonder if they are bit low on caliper MA for use with shimano levers; you could be losing another 10-15% there too. Likewise if the pads are a bit low on friction coefficient for some reason, and/or are not bedded in, plus the caliper mechanism being a bit rough.... it all adds up...... well actually, it multiplies together, so if you have five things each of which deprives you of 10% braking force your brake efficiency becomes

0.9 ^5 = 0.59

in other words you will get a little over half the brake force this way. If each thing deprives you of 15% then

0.85 ^ 5 = 0.44

cheers

[mnichols]

Thanks that's useful to know

[rmurphy195]

Having the arm at 90deg as Brucey suggested is, as I recollect, TRP's recommendation for setting up these brakes, and for the reason mentioned - this is in much the same way as the bridge cable angle of Cantilever brakes.

These videos may be useful to you

https://www.youtube.com/watch?v=wPTK0yEUI5s

https://www.youtube.com/watch?v=1T2RLgQoyvs