Better hub brakes (floating trailing shoe?)

[SA_SA_SA]

The recent "How long before disc brakes are standard on road bikes?" thread and its link to http://www.63xc.com/dws/hubbrake.htm (highpath super powerful hub brake) made wonder if a simpler way to boost hub brake power by adding float would be to:

allow the trailing shoe to "float" a bit by making its hemisphere more flexable than the leading shoe. This would reduce brake efficiency when cycling backwards but hopefully increase it forwards because the less worn trailing shoe could flex allowing the lead shoe to bite easier.

Would this work or be worth it? An advantage would be that it could retrofit-able.

And why did Sturmey not make V brake lever compatible hubs like Sachs did(briefly) I wonder.

[Brucey]

not sure it would work; the problem is allegedly that;

1) the shoes are unevenly loaded (mainly because one is leading and the other trailing)
2) the shoes wear unevenly
3) once worn the shoes are unfavourably loaded for powerful braking subsequently

In order for this to be mitigated, either or both of the brake shoe fulcrum points must be able to move independently of the hub axle so that they can exert equal pressure even when the shoes are unevenly worn.

The argument is that the self-servoing action on the leading shoe will create an unbalanced force between the two shoes.

However this is not the only unbalanced force; with any reaction arm there also is a net thrust force on the brake plate as well as a torque, and with a shorter reaction arm this thrust force gets bigger. In addition the axle will flex a little under the (weight transfer...) braking load and this may throw the brake plate off-centre too.

I dismantled one of MY SA hub brakes today and I discovered that (unsurprisingly) when the brake was applied the brake plate wanted (in the absence of being bolted to the axle) to be centred elsewhere, as might be expected with slightly unevenly worn shoes. However, to my considerable surprise the brake plate was off-centre in such a way as to suggest that the trailing shoe was worn, not the leading shoe.

The conclusion I have come to for now is that it isn't as simple as I thought, even though I didn't think it was very simple to start with....

cheers

[WrightsW5]

They are v lever compatible, S.A. originally made them for caliper levers but it turned out they worked even better with v levers.

[531colin]

I think its really simple, you just need the shoes to self-center to the drum.
The only cable operated drum brake I have played with in the last 30 years was a moped front hub (Atom brand!) that I built into a tandem wheel....the brake plate was held on the axle by "cone locknuts" for want of a better description....I opened up the hole a bit, and put a fibre washer in there so the brake plate could move, and away it went.
You need the brake plate free enough to move under braking forces as the leading shoe wears, but held enough not to drop down when you go over a bump.

[robc02]

And why did Sturmey not make V brake lever compatible hubs like Sachs did(briefly) I wonder.

The ones on my drop bar "tourer" are operated by the Tektro drop bar V-brake levers (RL520, I think). I tried them with Campag Athena (old type) aero levers but ran out of travel, especially on the rear, and the brakes were quite spongy. They are fine with the Tektro levers.

I dismantled one of MY SA hub brakes today and I discovered that (unsurprisingly) when the brake was applied the brake plate wanted (in the absence of being bolted to the axle) to be centred elsewhere, as might be expected with slightly unevenly worn shoes. However, to my considerable surprise the brake plate was off-centre in such a way as to suggest that the trailing shoe was worn, not the leading shoe.

The conclusion I have come to for now is that it isn't as simple as I thought, even though I didn't think it was very simple to start with....

I wondered at one point whether simply enlarging the backplate mounting hole and arranging suitable spacers and, perhaps, a wavy washer might do the trick by allowing the backplate to float; but a couple of things I have read make me, also, think it might not be that simple!

Here is another interesting link on drum brakes.

EDIT - I've just seen your last post, Colin. Was your Atom hub markedly better following your mod? The design of the cones on the SA hubs might require a different approach but it wouldn't be too difficult.
I spent part of this afternoon working on two different drum brakes and, while I was at it, I tried operating them with the backplate pulled partly out so that it cleared the extension of the hub cone on which it sits. Each combination I tried (a couple of different shells and backplates) appeared to centre as normal - certainly not in a radically different position. One backplate had new shoes - so that is what you would expect, but the other has quite well worn shoes.

[Brucey]

I wondered at one point whether simply enlarging the backplate mounting hole and arranging suitable spacers and, perhaps, a wavy washer might do the trick by allowing the backplate to float; but a couple of things I have read make me, also, think it might not be that simple!

I have a similar arrangement on trial at present. I have a controlled clamping load on the brake plate of a couple of hundred lbs, but the bulk of the axle bearing clamping load (about a tonne or more) passes through the spacers etc.

Here is another interesting link on drum brakes.

I wonder how you do Acres Geometry for pinned shoes, like SA ones?

And how do you do it at all if the abutment is flat, i.e. there is no intercept of the abutment thrust line with the thrust line from the cylinder or cam?

cheers

[531colin]

^^^^^^^Errrr....Pardon?

Yes, my modified Atom hub was a different animal after I got the brake plate to float.
With no float, it would slow the tandem, like a drag brake.
With float, it became the main brake.

[robc02]

With no float, it would slow the tandem, like a drag brake.
With float, it became the main brake.

That's more or less what I was hoping you'd say .

Let's see if Brucey's experiment yields the same result.

I wonder how much movement away from the existing centre would be required over the lifetime of the brake linings and whether that movement would cause the back plate to interfere with the drum? - The clearance between back plate and drum is pretty small - I shall have to measure a couple.

[531colin]

I suppose half the thickness of the lining? Plus whatever inaccuracies in the brake manufacture, bend in the axle, etc? Never thought about it before, but I never wore out any linings either, on a tandem used for taking the kids out on a Sunday morning.

[robc02]

We need to know the wear rates on a modified hub. As previously discussed, it could take many years to discover this from scratch! Alternatively, hearing from an owner of a Highpath hub might help.

I've just measured the backplate to drum clearance on a 70mm Elite ST. It varies slightly around the perimeter but I can normally just get a 15 thou' (0.38mm) into the gap. Furthermore, the backplate and drum have machined grooves that fit within each other to help keep dust and moisture out. It seems likely that for the floating backplate method to work long term, modifications to it and/or the drum would be necessary and that these might worsen the dust/moisture resistance.

Modifying a backplate to allow the cam assembly to slide would eliminate the problem of the backplate interfering with the drum. It might necessitate an addition to the cam mountings to prevent it tending to fall over (for want of a better term!) when operated - though the pressure from the pads might do this anyway.

[Brucey]

the brake plate has about 0.5mm clearance on a labyrinth like Rob says. If the brake plate makes contact it makes a foul sound and wears. I have seen a brake plate from a hub with damaged bearings almost worn through this way (not on my bike).

In my formative years I had cause to service many SLS and TLS drum brakes on cars etc and very rarely did the linings wear evenly. On those brakes the lining thickness was a smaller proportion of the diameter; on brakes where this is not the case (like SA brakes) the mismatch between the shoe bonding surface diameter and the inside of the drum means that the lining is bound to wear thin on the crown; the question is whether one end or the other of the shoe will also be worn.

On a standard SA brake the linings can't wear evenly, or allow one end of the shoe to wear, because the fixed pivot (fulcrum) won't let that part of the shoe near the drum.

Incidentally I have had a thought which is that there is always friction in the cam itself; on a SA front brake this friction means that the force vector from the cam thrusts the trailing shoe contact point radially outwards, but provides a near tangential thrust to the leading shoe. This might explain why the trailing shoe was more worn on the brake I looked at.

On a rear brake, the situation is reversed, so I'd expect -if this effect is significant- for the leading shoe to wear double-quick.

Depending on how things work out, I'd expect to have to reverse the shoes part-way through the life to even out the wear with a floating brake plate.

cheers

[531colin]

I have a vague idea some (motorbike?) brakes have the shoe pivot (? whats it called....the other end from the cam...) floating on the brake plate, which means the drum to brakeplate "fit" is not compromised....?

[Brucey]

a lot of car brakes have flat or angled abutments so that the shoes can slide sideways. If they get their sums right re. friction coefficients etc the wear can be realtively uniform; more often though 'something happens' and wear is not so uniform after all.

cheers

[dave wrath-sharman]

Hi, I've got a few Highpath SwingCams! There are a lot of questions and ideas on this post - I hardly know where to start.

The latest development of the swing cam has the cam mounted in the backplate which revolves around the axle - heavily damped by pressure from a wave spring. The back plate has a cut-out to clear the fulcrum pin which allowes it to swing forward 6º and backward 2º. The 80mm mk4 SwingCams on my own on/off-road bike are have done in excess of 20,000 miles and The backplate is still well clear of the fulcrum pin.

Ask any questions you like - I'll try to help.

Dave

[531colin]

Hello Dave, and welcome to the forum!

For me, a good place to start would be a drawing/photo of your brake, or a link to a drawing, etc.......I know they work, but I can't visualise it at all!
The backplate has a cut out to clear the fulcrum pin........is the fulcrum pin mounted on a separate bit?