Anatomy of BB5 and BB7 brake calipers

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Brucey
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Joined: 4 Jan 2012, 6:25pm

Anatomy of BB5 and BB7 brake calipers

Postby Brucey » 12 Nov 2017, 10:12pm

Avid BB series brake calipers have been around for about two decades now and have been through several iterations, including the 'BB' model and the BB3. Current models include the BB5 and various versions of the BB7 model, which has a moving pad adjuster as well as a fixed pad adjuster.

They are called 'BB' because they have ball-bearings in them; three 7/32" balls (~5.5mm) ride on ramps and translate a cable pull into the rather shorter movement of the brake pads.

Although not perfect, they are basically simple and reliable mechanisms which do benefit from maintenance. In UK conditions a particular issue is that the fixed pad adjuster is liable to seize up. I plan to add further details to this thread but the following is a start;

This is a parts listing for the BB5

Image

BB7 (there are variations with date of manufacture BTW)

Image

Service manual

http://cdn.sram.com/sites/default/files/techdocs/gen.0000000003513_rev_b_2012_bb7_bb5_service_manual_english.pdf

Installation manual (2006 vintage)

http://cdn.sram.com/sites/default/files/techdocs/2006_BB5and7_English.pdf

There is a page on the (usually excellent) Park Tools website which covers installation and adjustment of BB5/BB7 brakes, posted in 2015. I would link to it, but it appears to be wrong; in what I presume is a misinterpretation of Avid's recommendation that the disc be offset in the caliper body gap (for BB7 only), they presently (as of Nov 2017) recommend that the pad to rotor clearance is set unevenly in both BB5 and BB7 brakes.

cheers
Last edited by Brucey on 18 Nov 2017, 9:28am, edited 2 times in total.
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Brucey
Posts: 35596
Joined: 4 Jan 2012, 6:25pm

Re: Anatomy of BB5 and BB7 brake calipers

Postby Brucey » 13 Nov 2017, 9:52am

further to the above, this is a well-used BB5 that has been stripped down

Image256.jpg
BB5 in bits


The only parts that have not been removed are

a) the return spring tension adjuster; this takes the form of a grub screw in the brake arm. I have never had cause to want to adjust this tension.

b) the fixed part of the ball ramp; this is a press-fit into the caliper body.

If you use BB5 or BB7 on a commuting machine, I think it is a very good idea if you keep a third brake caliper and that (as the pads wear out) you rotate this between the two calipers that are on the bike, instead of simply replacing the pads. This means that you will never be stuck without a pair of working calipers, and it allows whichever caliper is off the bike to be properly overhauled 'offline' as it were. The spare caliper can be overhauled and fitted with new pads, ready to be installed. Installing a whole caliper takes about the same length of time as new pads only.

The pictured caliper had a seized fixed pad adjuster (FPA), which is commonplace. There are two modes of seizure; salty water gets into the screw thread and corrodes everything together, or the threads in the caliper body get filled with crud and the FPA simply won't screw into them. When finger pressure fails to move the FPA, a torx key is used and the fitting soon rounds out. Removing the FPA knob (something that is made easier by removing one finger, so that a jeweller's screwdriver can be inserted into the gap) allows a 8mm socket to be used on the back of the FPA instead. If this doesn't shift it, heat/penetrant/tapping with a hollow drift usually shifts the FPA. The threads in the caliper body can be cleaned using a pointed tool. You will know (roughly) when you have enough clean threads by simply testing as follows; with new pads installed in a loose caliper (in BB7 with the moving pad adjuster backed out) , it should be possible to move the FPA far enough that the clearance between the pads is almost zero. If you cannot wind the FPA in far enough to do this then it may be that the threads are still clogged with crud. A light coating of copper ease on the screw threads when the FPA is reassembled ought to prevent a recurrence.

However having sorted out the fixed pad adjuster, it isn't a bad idea to overhaul the rest of the caliper. I'd suggest it is a good idea to do this about every other pad change.

In BB7 you pop off the moving pad adjuster (MPA) knob and then start to disassemble the brake arm. The brake mechanism comes out of the caliper through the FPA adjuster bore or the caliper body can be split.

BB5 has fewer parts but has one or two tricks up its sleeve; the plastic cover just pops off the arm (pry with a jeweller's screwdriver) but the bolt that secures the caliper arm has a bi-hex head on it and needs a matching bi-hex tool to release it. If you are lucky an 8mm bi-hex ring spanner will do the job; if it is oriented correctly the spanner shank sits in a small recess in the arm (that is there to 'time' the cover) and allows the spanner over the bolt head even though the bolt head sits in a recess. Otherwise you will have to get a bi-hex 8mm socket with a thin wall to work this bolt. BTW when this bolt is refitted, it isn't a bad idea to use threadlock on it.

A smear of (non melting, non-separating) grease on all the ramp/ball parts isn't a bad idea, and the 7/32" balls can be replaced easily enough.

The caliper pictured had a very worn body bore; there were clear signs that the moving part of the cam had been scuffing the caliper body bore. Surprisingly there was little evidence that the pad backings (although a loose fit) had been chomping away at the caliper body bore in the same way. Note that in both BB5 and BB7 there are three positions that the cam can sit in (three ball positions), but in BB5 only one position is correct and allows the brake arm to be fitted at the correct angle. IIRC in BB7 the arm has a hexagonal fitting and can always be fitted correctly.

When these calipers get worn, the moving pad often doesn't draw back square to the disc any more, which makes the brake prone to rubbing. There are many reasons why this might happen;

1) the pad springs don't push back on the pad centres (in either BB5 or BB7) which can cause the pads to cock slightly
2) the pads may bind in the caliper body bore and not move smoothly
3) the back plate that sits behind the moving pad is able to articulate more than usual (e.g. through wear) and allows the moving pad to sit at a jaunty angle.

I have not yet come up with a good scheme for stopping this from happening in brakes that are badly worn. The BB5 caliper in the photo is prone to this behaviour that unless I can do something clever, it won't see use again.

BTW in either 'road' or MTB versions of the caliper, a full stroke of a (matching) brake lever causes the moving pad to move about 1mm. The FPA thread has a 1mm pitch. This allows the running clearance of the pads to be set accurately (and evenly) if required.

It isn't a bad idea to

a) put a marker pen mark onto the FPA wheel, so that you can keep better track of adjustments and
b) to mark the caliper body of BB5 'road' calipers (as per the intact caliper in the photo) so that you know when the brake arm is about to run out of travel.

On the latter point, the brake arm typically stops dead in BB5 road well before it does in BB5 MTN or BB7 variants. This may lead to an sudden loss of all braking so it is a very good idea to know that you are well away from this point. In fairness if the brake is set up properly to start with and the FPA is used correctly, the moving pad would have to be worn by about 1.75mm (i.e. more than is recommended before the pads are changed) before this will happen. However if the FPA is not used correctly, and the cable tension is the only adjustment that is used, it is possible for the arm to run out of travel well before the pads are worn out.

cheers
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amediasatex
Posts: 714
Joined: 2 Nov 2015, 12:51pm
Location: Sunny Devon! just East of the Moor

Re: Anatomy of BB5 and BB7 brake calipers

Postby amediasatex » 13 Nov 2017, 11:33am

If you use BB5 or BB7 on a commuting machine, I think it is a very good idea if you keep a third brake caliper and that (as the pads wear out) you rotate this between the two calipers that are on the bike, instead of simply replacing the pads. This means that you will never be stuck without a pair of working calipers, and it allows whichever caliper is off the bike to be properly overhauled 'offline' as it were. The spare caliper can be overhauled and fitted with new pads, ready to be installed. Installing a whole caliper takes about the same length of time as new pads only.


Glad I'm not the only one who does this! :-D

I have to confess to an even better/worse regime...

Some time ago I had the opportunity to buy a bulk lot of BB7s when a model year swapover occurred and a distributor was selling them off. I bought 20 brakes and for several years I was just fitting a new brake when the pads wore out, and then selling the used brake on eBay with a note about needing new pads, offten the used brake sold for more than it cost me new so had 'free braking' for several years !

When these calipers get worn, the moving pad often doesn't draw back square to the disc any more, which makes the brake prone to rubbing


I have battled with this on a small number of brakes too, I can't work out why some seem more prone to it than others. It seems to be partially alleviated with better lubrication early on before they get too worn, but it's pain if you end up with 'one of those ones' where the moving pad simply will not return in a consistent or proper manner.

It's worth noting too that the BB5 and BB7 use different pads, annoyingly!

Brucey
Posts: 35596
Joined: 4 Jan 2012, 6:25pm

Re: Anatomy of BB5 and BB7 brake calipers

Postby Brucey » 13 Nov 2017, 7:42pm

'disposable calipers'? That is a new one!

Regarding the bore wear issue; it seems that the return spring exterts a force on the cam plate (piston) that is part torsion and part thrust, i.e. the return spring acts partially like a compression spring. Because there is tension in the cable when the brake is at rest (whenever the arm does not return to the end stop when the brake is off) it may be possible for the load on the three balls to be unevenly spread when the brake is off. If there is also insufficient axial load on the cam plate, perhaps it is possible for the least loaded of the three balls to settle and/or to ease itself into a new position, so that when the brake is used, the initial thrust on the cam plate isn't perfectly axial. If the balls are not well lubricated, perhaps the cam plate remains off centre, and this is when the bore wear occurs. BTW the cam plate isn't in any way a tight fit through the fixed part of the cam; there is only contact (and load) at five places on the cam plate/arm assy; where the cable mounts, where the spring mounts, and where the three balls touch.

Regarding the pads that won't pull back straight; it seems to me that the pad pusher plate will always freely articulate slightly within the amount allowed by the fact that the snap ring is (an increasingly large amount) slack in the groove in the pusher plate. In addition this amount is altered not only by wear but also by the build up of wear debris where the pusher plate fits into the cam plate. In at least one brake the pusher plate wouldn't allow an even amount of articulation in all directions until it was removed, cleaned and regreased. I plan to reset this brake so that the free movement of the pusher plate is limited in the direction in which the pads typical go askew (i.e. the pad springs push off centre etc) and maybe the pads will pull back square.

I have a feeling that the two issues above may be related, in that if the cam plate is misaligned, this may force the pad to pull back whilst misaligned, and may force the pusher plate to articulate fully in one direction, which moves the snap ring such that further articulation of the pusher plate in one direction is made more likely.

cheers
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Brucey
Posts: 35596
Joined: 4 Jan 2012, 6:25pm

Re: Anatomy of BB5 and BB7 brake calipers

Postby Brucey » 17 Nov 2017, 7:42am

regarding the brake that won't pull back square; in BB5 the moving pad always tilts the same way if it can (presumably because the pad spring doesn't push quite centrally) so the pusher plate articulation needs to be set to that it limits the movement of the pad under the pad spring forces.

In bench tests a rear BB5 caliper was converted from one where the moving pad always pulled back on the tilt, to one where it pulled back square, by simply using the brake to reset the pusher plate tilt range. This was done quickly and easily by simply inserting the disc so that only the first couple of mm was between the pads, and operating the brake (not hard).

However whether this readjustment will last in service remains to be seen. Also if the front brake needs to be reset, it may not be as simple as that; it may tilt in a slightly different direction, and may need to be reset in a different way.

In BB7 if the moving pad doesn't pull back square it might be for different reasons, eg if the pad spring is deformed or something.

cheers
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mattsccm
Posts: 2702
Joined: 28 Nov 2009, 9:44pm

Re: Anatomy of BB5 and BB7 brake calipers

Postby mattsccm » 17 Nov 2017, 5:22pm

I use several sets of BB7s. I find that if I quickly clean out a carefully grease the fixed pad adjuster every time I replace the pads it will stay moveable by hand.

Brucey
Posts: 35596
Joined: 4 Jan 2012, 6:25pm

Re: Anatomy of BB5 and BB7 brake calipers

Postby Brucey » 17 Nov 2017, 5:57pm

have you encountered the 'pads won't pull back square' syndrome with your BB7s?

cheers
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GarethF
Posts: 87
Joined: 16 Sep 2008, 9:00pm

Re: Anatomy of BB5 and BB7 brake calipers

Postby GarethF » 17 Nov 2017, 11:12pm

Well I had an 'interesting' experience with my BB7s this morning. One pad dropped out of the rear caliper as I went over a particularly ropey bit of tarmac leaving the remaining pad rattling around and the notorious (and unreliable) retaining spring playing a tune on the rotor. Since it was the middle of rush hour I stopped eventually to remove these also and continued to my meeting but abandoned the fallen pad to the traffic. Fortunately the meeting was just over the road from Evans cycles where I was able to get a fresh set of pads. I think perhaps the spring (which is unavailable as a separate spare?) may be the greatest drawback with these brakes. Thankfully the front caliper is a bit more upright so it is less likely the pads could drop out, which wouldn't really bear thinking about.

Brucey
Posts: 35596
Joined: 4 Jan 2012, 6:25pm

Re: Anatomy of BB5 and BB7 brake calipers

Postby Brucey » 17 Nov 2017, 11:47pm

IIRC the recommended wear limit on BB7 pads is that you can wear about 1mm off the pads (total thickness 3.0mm) and then they ought to be replaced. To the casual observer the pads look 'less than half-worn' at this stage.

Wear them any further than that and not only do you risk that the friction material peels off wholesale from the backing (as with any disc brake) but with BB7 brakes it is very likely that the spring will start to chafe on the disc itself and will wear through.

The final outcome of all this is that the worn pads are so thin that they can fall out of the caliper; the gap between the disc and the caliper body will easily let a very worn pad fall through, even when the disc is set in the caliper body gap as per the recommended instructions. If the pads are changed at the recommended time this won't happen.

cheers
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mattsccm
Posts: 2702
Joined: 28 Nov 2009, 9:44pm

Re: Anatomy of BB5 and BB7 brake calipers

Postby mattsccm » 18 Nov 2017, 6:40am

Can't say I have noticed but never really study them. When I replace pads I usually split the caliper to clean it out. Maybe that helps. 2 allen bolts. The bikes with these brakes are used for gravel bashing. A normal ride, about 4 times a week is 1.5 miles of tarmac and 23 of gravel. Thats about 3 or 4 pairs of pads a year.

Brucey
Posts: 35596
Joined: 4 Jan 2012, 6:25pm

Re: Anatomy of BB5 and BB7 brake calipers

Postby Brucey » 18 Nov 2017, 9:33am

mattsccm wrote:I use several sets of BB7s. I find that if I quickly clean out a carefully grease the fixed pad adjuster every time I replace the pads it will stay moveable by hand.


it is a good idea for sure to clean the FPA threads out. I'm not sure it is a good idea to routinely split the caliper body though; Avid recommend that these screws are threadlocked BTW.

In addition I would note that very many greases are not suitable for use on the FPA threads; they may well separate and/or melt when the caliper body gets hot, which leads to oil contamination of the pads. A lubricant/anti-seize that is designed for high temperature use is a better idea; copper-ease is my preferred choice.

cheers
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