Dual Pivot brakes

[NUKe]

Trying to fix noise, I adjusted the brake pads I noticed that on the front( teKtro ) dual pivot brake, on one side the Pad sits distinctly lower than the other. One left( I think) is a low in the slot as will go, the other is just below halfway down. The wheel is true and not off centred I built in Wheel trueing stand and remember checking for this by tuning the wheel around. The pads sit in the same postion with relation to the rim, So why the difference in hieght and can adjust this ?

[Brucey]

you can usually adjust this by twirling the centring screw on the caliper and swinging the caliper around the mounting bolt a bit. This effectively adjusts the height of one arm vs the other.

hth

cheers

[JohnW]

I'm going to subscribe to this thread and watch it run and run, because I think that the geometry of double-pivot brakes does cause differential amounts of movement between one side and the other, and I also think that you get more pressure on one side of the rim than the other.

I have Shimano on two of my bikes and Tektro brakes on another. In terms of effective braking I have no problems with either - they were a revelation after changing from previous Weinmann 600 and Dura Ace single pivots. But I think that the differential is a product of the design.

However, I'm looking to learn
.
I tried Acor brakes on one of my bikes - never again.

[NUKe]

Thanks Brucey, I'll take it off and have a look over the weeekend

[NUKe]

you can usually adjust this by twirling the centring screw on the caliper and swinging the caliper around the mounting bolt a bit. This effectively adjusts the height of one arm vs the other.

hth

cheers

Yep that worked thanks Brucey

[sreten]

I'm going to subscribe to this thread and watch it run and run, because I think that the geometry of
double-pivot brakes does cause differential amounts of movement between one side and the other,
and I also think that you get more pressure on one side of the rim than the other.

However, I'm looking to learn

Hi,

(It took me a while to work it out, returning to bikes I'd never heard of dual
pivot brakes, except of course the symmetrical dual pivot centrepulls of old.)

If the two arms were independent, you would be right, dual pivot brakes would be awful
for the reasons stated, and simply wouldn't exist, because they wouldn't work in any sense.

What you are missing is that they are not independent, they are linked under the adjust screw.



Force on the front brake arm moving the right pad towards the wheel also moves the right
of the rear arm up and the left brake pad towards the wheel, with no force on the cable
housing of the rear arm. Because the front brake arm helps the rear brake arm, it must
have more of a lever than the rear arm, hence the differential pivot points.

The linkage also forces the movement of each pad to be the same geometrically,
and hence the forces applied to each brake pad to be the same, i.e. it works.
In practice, as implied by the pivots, at the cable end the front arm moves
more up than the rear arm moves down, as it has to, defined by the geometry.

The image shows long reach calipers. I assume for the reach the overall leverage
of the dual pivot is more than a single pivot. i.e. the arms are effectively shorter.

Hence why they were developed, to get more leverage than the physical limitations
of single pivot brakes on road bikes, i.e. a dual pivot brake that fits a bike has the
leverage of a shorter reach single pivot that would not fit the bike in question.

rgds, sreten.

[Brucey]

I think that small differences in force and pad travel from one side to the other probably don't matter that much for the simple reason that most wheels are not very stiff laterally, and in addition the caliper will find a way to rotate around its mounting bolt if it really needs to. This means that the force on each side of the rim cannot be wildly different; if it were then the wheel would flex or the caliper would move.

If you analyse the geometry then provided the ratio of lengths on each arm (from centring screw to pivot and then from pivot to pad) is the same on each side then the pad travel will be the same each side also. However this can only ever be true for one pad height; moving each pad a fixed amount in each slot is a different percentage (a very different percentage) of the distance to each pivot and therefore the pad travel on each side can vary.

So even in a well designed caliper the travel on each pad will only ever be equal at one pad height in the slots; I'd expect this to be when the pads are set in the middle of the slots but who knows....

cheers

[531colin]

I never really got to grips with dual pivots.....just looking at them makes my head hurt.
In the picture somebody helpfully put up, the arm closest to the camera clearly has a high mechanical advantage (MA) because its so long from the cable attachment to the pivot. That's fine, and I'm with Brucey, the brake and/or the wheel will move so the force is shared equally between the two sides.
But I'm not convinced about the other arm. They could have put the pivot somewhere a bit more adventurous than in the middle, couldn't they?....or just made it longer from pivot to cable attachment to increase the MA...? and that queer little grub screw between the 2 arms, does it really really centre the thing? And as the pads wear, one moves up the rim and the other moves down, I mean, that's just weird.
I think they should have done it like an old-fashioned centrepull......make a little carrier to take 2 pivots, so you make 2 pivots like the one for the arm nearest the camera.....sweep the cable attachments up like a vee brake with a noodle and all, and away you go.......

[Brucey]

one way of thinking about DP caliper MA is as follows;

First consider each arm in isolation, and that the centring screw does not exist. Imagine each arm pulls against a fixed stop instead of the other arm. In the pictured caliper, one arm (the upper one) has an individual MA of ~1:1 and the other has an individual MA of about 2:1. [NB for comparison a short-ish reach conventional side pull has an MA of around 1:1 or ~1.2:1]

Next consider that the arms are constrained to move in unison, because the centring screw always transfers load and motion. [If you look at the lower arm there is a hard steel plate set into it so that the centring screw doesn't dig in.] If the centring screw were not there, the caliper were rigidly supported, and the pads bore against something rigidly supported (a dummy rim), the result would be that the upper arm would generate about half the force of the lower arm and the dummy rim would see a net thrust sideways.

In reality because the lower arm shares its force into the upper arm, the motion of both is constrained, and the whole assembly isn't especially rigid, the net result is that the overall caliper MA is an average of the individual arm MAs, i.e. around 1.4:1 or 1.5:1.

hth

cheers

[NetworkMan]

.... and to emphasise what Brucey is saying I bet that if you were to take the wheel out and put a few fingers between the blocks you could pull on the lever with the other hand (ouch!) and waggle the fingers side to side, showing that there is no sideways thrust on the wheel rim even though the MA is different for the two sides.

[SA_SA_SA]

.... dual pivots.....
I think they should have done it like an old-fashioned centrepull......make a little carrier to take 2 pivots, so you make 2 pivots like the one for the arm nearest the camera.....

I agree:--- like the new 6800 or a very old altenburger synchron
http://forum.ctc.org.uk/viewtopic.php?f=5&t=87035&p=787032&hilit=shimano+6800#p787032

..,sweep the cable attachments up like a vee brake with a noodle and all, and away you go.......

Like a TRP/Tektro/Campag TT style brake?
http://www.tektro.com/_english/01_products/01_prodetail.php?pid=80&sortname=Brake&sort=1&fid=2

[foxyrider]

I think the quality of the brake makes a big difference too. Currently i've got three different 'qualities' on my bikes -

Shimano mid range long drop - these tend to always sit a little skewy and require a lot of lever movement to stop, doesn't matter what i do they always require a lot of pull to get stopping - my least favourite brake.

Campag Centaur circa 2000 - less lever movement than the Shimano but no better at staying centred

Campag Super Record 2014 - 3mm at the lever and they are fully engaged with braking power i've never had on any other brake set up. One bike is running the single pivot rear which is slightly less powerful but does seem to stay centred, the tourer (i know, a bit flash) has double dp. i did run the road bike with double SR DP's for a while - really was overkill. I previously had Veloce DP's on the 'race' bike and you can feel the difference with the SR's.

I would put Campag DP's on the Shimano equipped bike but the drop isn't quite enough on an audax frame with generous mg clearance.

Some of the Tektro brakes look the part but are terrible - our mechanic complains of them flexing and twisting and the bike is in the workstand!

[531colin]

http://forum.ctc.org.uk/viewtopic.php?f=5&t=87035&hilit=shimano+6800

Yup, missed the whole thing......the mention of "Ultegra" ensures I don't look. Tektro stuff looks interesting.....thanks for pointing it out.

But good ol' Shimano, eh? Its a new marketing ploy, so we'll introduce it at the "aspirational" end of the market (read "spendy end").....while we are at it, we'll reduce the tyre clearance to the extent that you don't stand a hope in hell of using it on the bikes where the brakes really are crap, that's the bikes with dual pivots where you stand half a chance of squeezing in a 28mm tyre and a mudguard.
In one of the links somebody said they would like to try them on their Dahon folder....hollow laughter.
None of this helps in the task of trying to get Mr and Mrs Average to accept the bicycle as a form of transport, or a way to have a nice relaxing day out.

I ride with lots of people who drag their brakes on anything approaching a descent because they are frightened they won't be able to stop. On the averagely-maintained bike, I reckon cable friction significantly reduces braking efficiency. You can design this out to a significant extent by making the cable long-travel, low-tension instead of short-travel, high tension.
But that will never happen all the while the marketing men are squeezing in an extra gear or three.
To me a swooping descent is an absolute joy....how horrible for it to be reduced to a miserable, frightening experience, to be glad when its over, instead of wishing it would last and last.

[Mick F]

Having ridden with folk down here and being aware of them, I've noticed something very interesting about descending.
I let my bike go. Gravity is my friend. Why brake unless you HAVE to slow down?

Some time back, I was up on Dartmoor and was met by four Royal Navy cyclists out for a training ride prior to an End2End. They stopped at the top of Peek Hill to admire the view and allow the straggler to catch them up. I rode alongside the straggler!

Any road up, we carried on together after that - all five of us - and whizzed through Princetown and headed for Tavistock. Long long long downhill to Merrivale from Rundlestone (1,500ft), and as you descend into Merrivale, I was at the rear and freewheeling. I overtook all four and plunged into the vale and was half way up the other side before the fastest caught me up.

Next descent is Pork Hill. (I've managed 52mph down there) Again, I was to the rear of the other four and let my bike go. I passed them all easily and flying along.

Basically, people are frightened. They dab their brakes. They are worried about speed and they don't think they can cope and are worried that their bikes won't cope either.

You have to have faith in your abilities and faith in the design of your bike and faith in the braking system ....... and relax!

[sreten]

I never really got to grips with dual pivots.....just looking at them makes my head hurt.
In the picture somebody helpfully put up, the arm closest to the camera clearly has a high mechanical advantage (MA) because its so long from the cable attachment to the pivot. That's fine, and I'm with Brucey, the brake and/or the wheel will move so the force is shared equally between the two sides.
But I'm not convinced about the other arm. They could have put the pivot somewhere a bit more adventurous than in the middle, couldn't they?....or just made it longer from pivot to cable attachment to increase the MA...? and that queer little grub screw between the 2 arms, does it really really centre the thing? And as the pads wear, one moves up the rim and the other moves down, I mean, that's just weird.
I think they should have done it like an old-fashioned centrepull......make a little carrier to take 2 pivots, so you make 2 pivots like the one for the arm nearest the camera.....sweep the cable attachments up like a vee brake with a noodle and all, and away you go.......

Hi,

Hi,

Your completely missing the point. Unlike single centre pivot, dual pivot cannot cope with warped wheels
and swivel to equalise forces, your completely wrong that the brake or wheel moves to equalise forces.
And your misquoting Brucey, he is not saying what you want to think you are agreeing with.

The rest of your post is just nonsense based on refusing to comphrehend how it really works.

The grub screw adjusts the static gap, and the brake is overall twisted to suite.

The fact that the rear arm pivot is in the middle, you claim to be unadventurous,
just shows how cluelessly you understand the brake. In reality, as already
described the pull of rear arm must be the same as single pivot, the pull
of the front arm is more, but you end end with more leverage still
within the sort of scope of the pull for standard single pivot lever.

rgds, sreten.