Brake Cable Failure.. Brake Caliper Clearance

For discussions about bikes and equipment.
johnweightsen
Posts: 16
Joined: 31 May 2020, 3:53pm

Re: Brake Cable Failure.. Brake Caliper Clearance

Postby johnweightsen » 31 Jul 2020, 1:25am

http://smutpedaller.blogspot.com/2012/0 ... grade.html

There is something I've found which illustrates my point perfectly.


Scroll down to pictures 6 and 7.

Picture 6 shows the brake with the cable installed ready to go- plenty of clearance between the brake arch and mudguard.

Picture 7 shows the quick release open - no clearance at all between the brake arch and mudguard. This lack of clearance would be emphasised even more if the cable snapped.


FYI that isn't my blog, hopefully the author doesn't mind the free advertising!

MartinC
Posts: 1872
Joined: 10 May 2007, 6:31pm
Location: Bredon

Re: Brake Cable Failure.. Brake Caliper Clearance

Postby MartinC » 31 Jul 2020, 9:53am

Yes, the penny dropped as soon as I read the first reply to last night's post! I was, incorrectly, assuming you were talking about lateral clearance.

So I guess the root of the problem is that the distance from the brake pivot bolt to the rim is insufficient for these brakes. I'm also wondering how much brake drop you actually need to get the pads aligned with the rims, i.e. could you use short drop brakes? They may (or not) have the same problem. I appreciate you're looking for nutted brakes which may be difficult for short caliper. Other medium drop DP's may or may not have the same degree of problem.

Dia Compe are a good source of old style brakes. They still manufacture single pivot brakes, nutted DP's and centre pulls with a variety of drops. Centre pulls may be the answer - they shouldn't lose clearance when open. The Dia Compe 450 or 610 model centre pulls may fit what you want, Billys Bike Shop is a good source.

cycle tramp
Posts: 764
Joined: 5 Aug 2009, 7:22pm

Re: Brake Cable Failure.. Brake Caliper Clearance

Postby cycle tramp » 31 Jul 2020, 9:23pm

Hello There..
It's the first time I've seen this type of dual pivot brake before and it took me some time to figure out how it works.... when the brake isn't under tension, the arms extend away from the rims like any normal capillary brake, but as one of the arm extends away from the side of the rim, in doing so, the bit of the brake arm on the other side of the pivot swings downwards into the space occupied by the tyre.
Can I ask a couple of questions?
Is this your 'forever bike' or in a couple of years will you be buying another bike? Did you build the bike yourself? Was this bike you purchased and then changed the tyres? How good is the performance of the rear brake? Do you travel with alot of tools?

Brucey
Posts: 39801
Joined: 4 Jan 2012, 6:25pm

Re: Brake Cable Failure.. Brake Caliper Clearance

Postby Brucey » 31 Jul 2020, 10:10pm

cycle tramp wrote:...It's the first time I've seen this type of dual pivot brake before ….


really? Shimano have manufactured little else for at least twenty years (only recently moving away from this design) and clones are almost ubiquitous on road bikes to this day.

nearly all of them (shimano or clone) have similar issues, most often manifesting itself as a clash between one or other arm and the mudguard as the brake is used.

cheers
-------------------------------------------------------------------------------
~~~~~~~~~~~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~

MartinC
Posts: 1872
Joined: 10 May 2007, 6:31pm
Location: Bredon

Re: Brake Cable Failure.. Brake Caliper Clearance

Postby MartinC » 1 Aug 2020, 4:00pm

Yes, it's a pretty normal DP caliper that's constructed to open wider than most which will exacerbate the lowering of the long arm. But it's design is generic.

cycle tramp
Posts: 764
Joined: 5 Aug 2009, 7:22pm

Re: Brake Cable Failure.. Brake Caliper Clearance

Postby cycle tramp » 1 Aug 2020, 4:22pm

Brucey wrote:
cycle tramp wrote:...It's the first time I've seen this type of dual pivot brake before ….


really? Shimano have manufactured little else for at least twenty years (only recently moving away from this design) and clones are almost ubiquitous on road bikes to this day.

nearly all of them (shimano or clone) have similar issues, most often manifesting itself as a clash between one or other arm and the mudguard as the brake is used.

cheers


Twenty years, eh?... take this as a sign of just how much 'racing bikes' and their associated technology interest me.....

I guess if every type of this sort of brake operates in the same way, either the drop outs to hold the brake blocks need to be much longer so the brake arms can be mounted further away from the tyre or the manufacturer intended the bicycle user to use tyres of 25mm width or less.

If the bike is to be replaced in the next 5 or 7 years then perhaps the cheapest option would be to replace the rear cable every year and ride with a multi-tool. If the cable is replaced every year the chances of the cable breaking are much reduced. If the cable does break then my concern would not be the sudden stop but that the brake continues to jam itself against the tyre, making the bike impossible to either ride or push home.. if you have a multi tool, then at least you can unbolt the rear brake from the frame.

cycle tramp
Posts: 764
Joined: 5 Aug 2009, 7:22pm

Re: Brake Cable Failure.. Brake Caliper Clearance

Postby cycle tramp » 1 Aug 2020, 4:35pm

...however, it could be that we're shying at flies, here. Personally, I'd try fitting a borrowed or scrapped 28mm tyre on the back wheel and disconnecting the rear brake (when the bike is stationary) and seeing what does happen.

Pebble
Posts: 95
Joined: 7 Jun 2020, 11:59pm

Re: Brake Cable Failure.. Brake Caliper Clearance

Postby Pebble » 1 Aug 2020, 6:35pm

johnweightsen wrote:
Brucey wrote:I think if the cable broke the arm might rub on the tyre but not slow you down that quickly.

cheers


Thanks for the response.


I'm not sure myself. With a 28mm tyre (and no cable installed) you can't actually get the wheel all the way into the front dropouts.

But of course with the cable installed it's no problem.

So if the cable broke I imagine the top of the caliper would be enough to lock up the wheel. Or am I wrong..?

It would be quite a nice fail-safe device for the rear wheel if the cable did snap.

I once snapped a cable under hard braking and it was a pretty scary experience that I have never forgot - the rear wheel locking up would have been better than my run-away predicament.

MartinC
Posts: 1872
Joined: 10 May 2007, 6:31pm
Location: Bredon

Re: Brake Cable Failure.. Brake Caliper Clearance

Postby MartinC » 2 Aug 2020, 3:07pm

cycle tramp wrote:...however, it could be that we're shying at flies, here. Personally, I'd try fitting a borrowed or scrapped 28mm tyre on the back wheel and disconnecting the rear brake (when the bike is stationary) and seeing what does happen.


Yes. Before trying that I think I'd just try to get a feel for the front braking effect by disconnecting the front cable and pushing the bike forwards (like you do when checking the brakes or headset) and seeing how well the wheel was braked. I guess it will vary with the tyre pressure, casing suppleness and and the tyre tread pattern. I think at either end of the spectrum (locked or mild rub) I might know what the answer is but in between there's a big grey area. Losing modulation as you were braking for a corner would also be a risk. On balance getting loss of one brake is probably a more predictable problem than a random amount of uncontrollable residual braking especially at the front.

Going back to the OP. I could easily be wrong but I'm guessing John is using a R539 medium drop caliper 'cos it's one of the few available with a nut fitting even though short drop brakes would fit. I have a bike with R358 callipers (same design as R359 but without the safety catch on the QR). When I fitted them it was running on 25mm tyres and mudguards, now it has 28mm tyres, Open Pro rims and no guards. The tyres do come up at 28mm depth inflated. The pads are halfway up the slots. With the caliper QR open and the Ergo lever QR open the caliper opens as wide as it can but the arm is still 4-5mm above the tyre. It would need the pads to be at the top of the slots to foul, so I'm guessing that's the OP's set up.

I'd be tempted to find a different caliper that fitted the frame better and use a https://spacycles.co.uk/m7b0s100p2341/S ... erter-Bolt to convert it to nut fitting. A a short drop caliper (some go to 52mm or so) or a different medium drop without the wide opening feature may fit. I'd guess a single pivot Dia Compe BRS200 or a centrepull GC450 would fit (you may want an old style lever to get the best cable pull) if you can't find a DP one that works.