Brake Cable Failure.. Brake Caliper Clearance

[johnweightsen]

Hello,


I'm using Tektro R559 brake calipers.

When my brakes are set up with the pads the appropriate distance from the rim I can clear a 28 and possibly 30mm tyre.

But when the brakes are initially installed without cabling (so the arms are fully open) I can just barely clear a 25mm tyre.

I'm right in saying that if I installed a 28mm tyre and experienced brake cable failure I would ironically be pulled to a halt very quickly (and dangerously) right?


Is this a common issue with all/most dual pivot calipers?

Can anyone recommend a nutted dual pivot caliper that doesn't have this "flaw"?

Would using single pivot calipers eliminate the "flaw"?



Hope someone can alleviate my curiosity.
Cheers.

[Brucey]

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

cheers

[johnweightsen]

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..?

[Brucey]

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

th arm rubbing on the tyre won't be a very effective brake. You can probably try it and see if you like, by starting with the cable slackish and then flicking the QR open

cheers

[johnweightsen]

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

th arm rubbing on the tyre won't be a very effective brake. You can probably try it and see if you like, by starting with the cable slackish and then flicking the QR open

cheers

Good idea, I'll give that a test and see. I'm naturally quite cautious having worked in life insurance for a while!

Would the design of a single pivot caliper have the same characteristic (less clearance with the arms fully open)?

[Brucey]

Would the design of a single pivot caliper have the same characteristic (less clearance with the arms fully open)?

no, I don't think so

BTW on MTBs brake parts dropping onto the tyre can have far more severe consequences; the knobbly tyres more easily jam up. 28 and 25mm tyres tend to be smooth and won't cause a jam so easily. I guess if you are using tyres with heavy tread on them you should be a bit more concerned; you can see how badly the brake drags when pushing the bike to start with...?

cheers

[johnweightsen]

I'm now thinking that in real life the only time I'm likely to suffer brake cable failure is under heavyish braking and I would be prepared for slowing down anyway. So perhaps this isn't such a massive worry really, especially if as Brucey says you're not likely to come to a dead halt if you're using slicker tyres.

I imagine you'd be even less likely to lock up the wheel in that situation if you were using mudguards (I'm guessing the tyre would slide round the inside of the mudguard (under the caliper) even easier than on the underside of the caliper.

[MartinC]

I don't understand what's going on here. R559 are deep drop brakes with a extra wide quick release that opens for fat(tish) tyres. I've used them on LX17 (622x17) rims with 32mm tyres and with the QR open they drop out with room to spare. Prior to cabling they're open much wider like most DP and SP brakes, they even come with some black sticky tape to protect the right hand caliper from scratching when they're wide open. Without cabling they should open until the cable fastener hits other caliper arm.

Opening less wide with no cable attached makes no sense to me - what am I missing?

[johnweightsen]

I don't understand what's going on here. R559 are deep drop brakes with a extra wide quick release that opens for fat(tish) tyres. I've used them on LX17 (622x17) rims with 32mm tyres and with the QR open they drop out with room to spare. Prior to cabling they're open much wider like most DP and SP brakes, they even come with some black sticky tape to protect the right hand caliper from scratching when they're wide open. Without cabling they should open until the cable fastener hits other caliper arm.

Opening less wide with no cable attached makes no sense to me - what am I missing?

Sorry they're actually R359 which are mid reach in the Tektro range.

If you search for a pic of one you can see how when the calliper is fully opened one of the arms dips below the bottom of the "arch" of the calliper. This arm then gets squeezed in and the arm moves above the bottom of the arch creating more clearance.

[gregoryoftours]

I can't imagine that it would jam solid, and as you would be trying to brake at the moment a brake cable snapped then some amount of braking is probably better than none! In any case the likelihood of a brake cable suddenly slipping/snapping is virtually nil if installed properly and checked regularly.

[Brucey]

rear view of tektro R359 caliper with no cable attached



IIRC the relative positions of the arms are also dependant on the centring screw position

cheers

[MartinC]

IIRC the relative positions of the arms are also dependant on the centring screw position

Yes, this is the only thing I can think of, maybe this is in an extreme position. Only yesterday I was changing the cassette on a bike with R359 calipers. To drop the wheel out I used the QR on the Campag Ergo lever (i.e. the same as taking cable tension off the caliper) and it opened wide enough for the 28mm tyre (on a narrow Open Pro rim) to drop out with plenty of room to spare. My recollection of these brake calipers out of the box is that they are the same as all the rest - no cable tension and the caliper will open wide until the cable anchor hits the other arm.

I'm finding it hard to visualise what's happening with John's brakes. The caliper opening with cable tension is totally counter intuitive for a brake. Presumably it must open so far and then close or it wouldn't brake at all. I don't know if screwing the centring screw all the way in or out could cause this. If not the caliper must be misassembled or sticking somewhere.

John, can you post some photos - it may help.

[MartinC]

................when the calliper is fully opened one of the arms dips below the bottom of the "arch" of the calliper. This arm then gets squeezed in and the arm moves above the bottom of the arch creating more clearance.

Hmmm. Surely when the arm moves up the brake pad on it pivots inwards and the other pad pivots inwards too as both caliper arms are squeezed together. Just a thought - when the brake is off is the cable anchor below and inwards of the caliper arm end?

[johnweightsen]

Hey Martin, I can provide pictures at some point in the next day or two perhaps. But if you look at Brucey's useful picture and imagine squeezing the pads together, you can visualise how the "front arm" is lifted up and increases vertical clearance for a tyre.



FWIW I've noted the same thing on another set of short reach recessed nut Tektro dual pivots. This was at least a year ago so I can't remember the exact model. But I was swapping from some single pivot brakes and when I installed the new Tektro brakes the frame no longer cleared the 25mm tyre- until I squeezed the pads together (so that they sit in their natural couple of mm from the rim position) then there was an acceptable amount of vertical clearance.

At that point I decided I didn't want to risk it and harvested the corrosion free barrel adjuster and cable clamp for the old single pivots! Now I'm wondering if I was being a bit too cautious.



Maybe next time I'm changing my cables I can shoot a quick video spinning my 28mm tyre and cutting the old cable to show how the brake opens up fully, the "front arm" drops down and drags (or indeed halts?) the tyre.

[johnweightsen]

Martin rereading your posts I think you might be visualising the situation incorrectly and for that I can only blame my description, of course!

To be clear we are discussing vertical brake clearance, so nothing related to pad clearance when removing a wheel etc.

See my edit on Brucey's image- when the cable is not installed (or snapped!!) the red area is the lowest (reducing vertical clearance) but when the cable is installed and I'm ready to KOM Mont Ventoux the red area is actually slightly above or inline with the blue area, increasing the vertical clearance. So the worry is when you experience brake cable failure, which I know is unlikely, the vertical clearance is reduced and the red area is now dragging on your tyre (or locking up your wheel, which is my perhaps unnesscessary worry).