The 7" front brake on my M21 plunger has never worked well, in fact I get more braking from the rear wheel. The front drum was machined and the brake was set up by a local wheel builder that a lot of the BSA club members here use. Both shoes have good contact with the drum and both shoes engage the drum at the same time.

His current advice is to move the brake operating arm down to give more leverage on the cam.



This has improved things slightly but images of this brake on the web show this arm closer to the 8 o'clock position on the majority of bikes with this brake.

He also says that the cup and cone head stem bearings can contribute to vibration in the front wheel when braking. I've been paying more attention to what's happening to the front wheel when braking hard and the front wheel does oscillate so an order of the bearing conversion is under way.

Maybe my expectation is too high? I don't expect this brake to set the world on fire but I don't think that it should be so wooden.

Those drums should give sufficient brake power.
I can bring my M20 to a total halt with 2 fingers on the front and this with hauling a side car with no brakes.
On the other hand (there's always the other hand you know) – while speeding it takes me more then a couple of seconds, yes?
So, although we are not dealing with modern ceramic disk brakes and the technology is ancient, I still think one should feel that kind of “bite” when pulling a carefully adjusted brake’s lever.
What about the friction material?

You will never be able to do a "stoppie".

Mine are not brilliant at all even with new pads. I was thinking of trying one of those rod conversions.

I probably need a skim too.

Regards

Pat

email (option): sacombsashtrees@hotmail.com

"Those drums should give sufficient brake power.
I can bring my M20 to a total halt with 2 fingers"

Michael,
This is what I'm hoping for.

The wheel builder sent the shoes out for re-lining and is happy with the job they have done. I take it that they have been re-lined with the same material that they would normally use for road bikes. he does brakes for race bikes as well.

Brake reliners often use a material that's a little too hard...A change to a softer lining material will improve things but will have the downside of more rapid wear....Perhaps that's a small price to pay if it avoids an accident though!...

You could also make a slightly longer brake operating arm to improve the mechanical advantage. Think about the cable run if you do that though...the misalignment can't be too extreme...

Fork bush condition is also a factor to consider with regard wheel vibration etc. under braking loads....Ian

email (option): ian@wright52.plus.com

looking at the picture I think it would work better if you connected the cable to that chrome lever thingy!

Hi Matty,

Have you examined you brake cable?
The quality of cables can have catastrofal influence on your (braking) performance.
An easy test could be to check the movement (tension) of the cable when braking. You don't have to be riding during this test.
If the cable moves it will certainly move during a ride and that is a lost of energy that could well be used for braking.
I've also seen conical 'iron ends' (don't know the exact english word) that will squeeze the inner cable when braking. That's also something you could check.
On my Norton fixing this resulted in a huge improvement of the braking performance.

Good luck,

Sven

email (option): snvosselman@gmail.com

A very good point Sven that I completely forgot to mention!....

Poor quality cables,particularly the outer cable, do indeed have a great effect on braking performance..A fairly heavy inner cable should also be used...Ian

email (option): ian@wright52.plus.com

Actually that cable looks like a throttle cable to me. Clutch and brake cable is around double the thickness.

Hi first of all you should never have your drum machined before or after a wheel build I have repaired and had 100s of wheels rebuild by so called wheel builder's it is a highly skilled job i cannot do it many say they can but many do not do it right test your spoke tension by tapping with a spanner should be a dull sound I bet you can play a tune on them slacken higest pitch spoke only a touch then go around wheel again testing then repeat if wheel was rebuilt then machined after its scrap if you want to live do not use it I restored a 1963 norton 650ss my wheel builder retired he must have been in 70s or early 80s Mr Eric booth of Coventry so customer got them built by a well known company around 6 months later had his mates around my shop accusing me of nearly killing him waited till he came out of hospital went round his house he stated going down a long a straight road a car pulled out on him he yanked brakes on and the front slider snapped in half and he went over the top bars done his back in he told me the same thing you're saying when he applied his brake the front end started to vibrate this was stressing the slider which under heavy load gave up don't ride it if this happens get it sorted nothing to do with bearings. Check also you have the right lever arm as you can get the square hole in different positions it may be off another bsa you have to get the leverage point right + pattern levers are no good M.A.L.MOTORCYCLES

email (option): malmotorcycles@gmail.com

Mal I'm very sure you mean well and I have tried to read your post, but some punctuation would make it a lot easier to read. Regards Ron

email (option): ronpier@talk21.com

Hi Matt,
The laws of physics apply here. Maximum mechanical advantage is achieved with the cable at right angles to the brake lever and drop off rapidly once the right angle is passed.Hence the "eight o'clock" position mentioned. That is as close to a right angle as is practical. It will give the best result combined with staying in adjustment and not passing a right angle for the longest period.
Disregarding all the other fork, steering head and cable concerns and talking solely of the brakes, from your description I would say the linings are too hard. It's a common problem. You need soft linings. Very soft linings by modern standards. Modern brake shops rarely deal with mechanical braking systems. It is all hydraulic systems using far higher braking pressures. What they consider 'soft' is still much too hard for your application. It's not their fault. They just aren't familiar with the needs of ancient thumpers.
One of the best brake shops in Melbourne that understand this exceedingly well are BGT out in Dandenong. Derry has been lining shoes for vintage and classic bikes and cars with mechanical braking systems for a long time. You can discuss the issue with him and he will provide an appropriate lining. It is best you take the complete front wheel.

Finally, there are other simple and discreet tweaks you can do that will improve your braking even further but pointless to do until you get the basics right.

Cheers, Bob

email (option): bobmcgrath21@hotmail.com

Thanks everyone for the input.

Ian, going for softer shoe lining is definitely on the cards, fork bushes, stanchions and seal are new with no or minimal play.

Sven, good idea on the cable, it's operation is smooth and free but I can't remember where I got it from and I don't know the quality, I'll look at replacing it.
John, the inner cable is 2.0 mm dia.

Mal, not sure what you mean by "should never have your drum machined before or after a wheel build". The drum was machined before the wheel was built. I'm pretty sure that the operating lever is original but I'll look in to that.

Bob, I think that the wheel builder is more concerned with the operating arm being parallel with the cam than the angle of the cable to the arm as long as there isn't too much cable deflection as it leaves the cable adjuster.
Thanks for the BGT contact, hadn't heard of them, will talk to them when I exhaust my options with this fellow.

Well mine stop the bike dead in it's tracks, when stationary at traffic lights & intersections.

As for machining.
lacing a wheel badly can and usually does distort the drum.
Thus the drums should only be machined after the wheels have been fully rebuilt for a month or longer.
Big arguements about weather you should machine them bearings in or out, I can see the logic in both.

Mechanical brakes have to use the rotation of the wheel to force the shoe into the drum.
To do this the radius of the shoes should be larger than the radius of the drum so when the shoe contacts the drum, it flexes into the drum. This is called self servo actions and very few understand it and fewer still know how to set it up to happen.

As mentioned previously , most of the information around is for hydraulic or even worse power assisted hydraulic braking systems.
For some unbeknown reason people have a bad habit of cutting from a car site & posting on a motorcycle site.
Go to almost any motorcycle web site and search for braking and you will see dozens recommending lining the drum with emery paper and rotating it against the shoes to "radius them to the drum" which is correct, only if you have power assisted brakes.

Also all the data about brake efficiency post dates our bike by decades.
While it is true that 75% of the braking is done by the front wheel, this ignores engine braking because it comes from late model bikes that have almost no engine breaking.

Back off on any old British bike and you nearly stop dead.
Back off on a modern rice burner and you just keep sailing along at the same speed.

I am yet to see any real brake testing that takes engine braking into account.
In most cases you are required to attain a speed then declutch and apply the brakes
Thus ( and it is just a guess ) I would guess up to 50% of our breaking is done by the engine and then you could split the wheels 60:40 in favour of the front.
In the thesis that Koerner used to eventually write his book a lot of time is devoted to the conflict between the governments and the motorcycle makers about breaking ( or rather or lack off brakes).

email (option): bsansw1@tpg.com.au

['The laws of physics apply here. Maximum mechanical advantage is achieved with the cable at right angles to the brake lever and drop off rapidly once the right angle is passed.Hence the "eight o'clock" position mentioned..']

I imagined the degree of mechanical advantage was produced by the force applied to the handlebar lever and the 'lever' effect given by the length of the operating arm to produce the force applied....

Of course the cam position inside the hub and the effects described by Trevor also come into play...

Frankly I can't quite get it clear in my mind exactly what is going on in terms of braking forces...I'll have to think about it a bit more!...

Machining hubs AFTER a wheel has been built is a well established method so I can't really say I agree with Mal on that...The key thing is that the work is done correctly and accurately...I've done a few myself over the years...Obviously the thickness of the drum must be considered...The WD M20 for example, has a pretty thin drum and the amount that can be removed is very limited...

Also, fork bushes and steering head bearings can't just be ignored as Mal appears to suggest....Their condition does have an effect on braking so it would be foolish to just assume the wheel is at fault...

Another thing that can be done to improve the mechanical effect is to use handlebar levers with a greater distance between the pivot and the centre of the nipple...There are various levers out there..Ian

email (option): ian@wright52.plus.com

A QD brake drum does not matter because it should be unaffected by the tension on the spokes as it is bolted into the hub independent of the spokes.

As for cables & levers, it is a simple matter of vectors, just the same as for a con rod on a crank.
While the force applied by the tension on the cable does not vary, at the lever it splits into x & y vectors.
Now you can take them using either the lever axis or the cable axis as the base refference it does not matter.
The outcome is the same only when the cable is at right angles will 100% of the tension in the cable be transferred to the rotation of the lever ( one of the vectors will be 0 ).

The apparent mechanical advantage is the distance the brake lever travels divided by the distance the brake shoe moves x 2 ( 2 shoes )..

This will be a reasonable amount higher than the actual braking force but it makes the maths a lot easier.
The pressure on the friction material is 40 kg ( your grip strength ) X the mechanical advantage divided by the surface area of the shoes.
From that you can work out the heat generated by the shoes on the drum using the dynamic R values of the material ( usually provided as a graph or spreadsheet ).
Divide this figure into the kinetic energy of the motorcycle and you get the amount of time it will take for the brakes to convert the movement of the motorcycle into heat and for the bike to stop.

You can then work out the time taken for the engine to consume the same amount of energy ( spring compression + gas compression )
Note this assumes your finger is on the kill switch.

This second figure will give you rotations of the engine to stop.

With a little giggery pokery you can get a direct comparison between the two, plot both on a graph Speed vs Time and note where the two deceleration curves cross.

Or do the same thing on a piece of road.
Stoping distance font brake only
Stoping distance back brake only
Stoping distance both brakes only
Stoping distance backing off only
Stoping distance engine killed only

Then the same with various combinations.

makes fo a fun thing to do on club days , particularly if you get the members to guess what the figures will be for their bike

email (option): bsansw1@tpg.com.au

When I said "the wheel builder is more concerned with the operating arm being parallel with the cam" I should have said right angle to the cam.

A lever type dial indicator on the drum shows a runout of .002" or .003"

I would have thought that runout was pretty good, but would be interested in other peoples thoughts.

In my mind there are so many things that come into play even if the drum is perfect; uneven spoke tension, slight play on bearings, wear on cams. The process of riveting liners must alter the profile too.

So I cannot really see how the end result can be CNC accurate.

I guess that's why soft liners come into play.

email (option): sacombsashtrees@hotmail.com

Thanks for the detailed reply Trevor...I shall take my time later to absorb the information....Ian

email (option): ian@wright52.plus.com

Hi Matt the brake cable is the right thickness at 2 mil. Looked thinner in the photo. Regards John

I remember reading that machining pressed steel drums is to be avoided. When the drum is distorted it seems to me more logic to adapt the tension of the spokes to get it round again than to machine it round again.

If that is not possible without keeping the rim running true, the rim should be suspected, it might be bend and the wheelbuilder could have tried to true a bend rim with the spokes and neglected the influence on the drum.

Distortion will (axially seen) be mostly on the outer side of the drum is the inner side is very rigid as closer to the flange. So with machining you have to take of a lot of 'good' material on the inner side.

Also a wrongly placed bearing can cause a big oscillation in the drum. Better check that with a dialclock before lacing.

When rebuilding a wheel with "drum-hub" I put a dialclock inside the drum to keep an eye on the ovality.

Machining a drum might be an established method (and maybe sometimes quick solution??) but it would be my very, very last option and certainly not a logic step after a wheelrebuild.

Good points and I certainly wouldn't rush into machining the drum if I didn't think it needed it...Apart from ovality another factor in that decision may be the condition of the braking surface.....

It should be noted that BSA (in the case of the M20) approve of 'cleaning up' the drums, both front and rear but say only .010" (.020" overall) should be removed, so there is little to work with....Other hubs constructed differently will stand a fair bit more that that without problems in my experience...Ian

email (option): ian@wright52.plus.com

I have to agree with Trevor. My M21 7" brake is brilliant at stopping the bike, when it's already stationary at traffic lights: otherwise it lives up to it's reputation for being a poor brake. See how many early M21s have the later 8" brake retrofitted

email (option): j@clogmaker.co.uk

So far no one has mentioned centralising the back plate to the drum?
Richard

email (option): richard177smith@btinternet.com

Thanks for the input Richard.

The shoes appear to be centralised. When operating the brake arm by hand there is a pronounced knock as both shoes contact the drum. Wheel builder says if there was a problem with shoes not contacting together there would be a softer sound and more give in the arm as the shoes contact the drum separately.

I have some head stem bearings winging there way here from Ian., will fit them when I get the chance and see where we're at.

I was reading Triumph Manual 99-0837, first printed December 1956 and they say that if the drum is scored or oval, then the drum should be skimmed (max 0.010in) off the wheel and then the wheel re-built and trued up.

Regards

Pat

ps I wasn't reading all of it, just a casual glance once Jeremy Wade's River Monsters had ended.

email (option): sacombsashtrees@hotmail.com

A "pronouced knock"? I think that this requires investigation!
Richard

I tend to make all my cables and in the brake, clutch department go up at least one size in diameter to the originals provided they can fit, my reasoning is that the thicker inner and outer will compress a tad less that the thinner ones and give me a bit more applied pressure. Having said this I have just spun my W/ng front brake in the lathe to see if it can become more than ballast. kind regards

I recall some years ago there was a post on this forum of how to convert M20 front brakes to hydraulic. (Now I can't find it and it does not seem to appear in the technical help section.) If apparent originality (instead of actual originality) is sufficient for you, maybe whoever posted that old instruction post can do it again.

email (option): jonny.rudge@verizon.net

If originality is not a concern fit an 8 inch brake from an M21,B33 or A10. Paint the brake plate black and most people won't spot the difference. I have one fitted on my girder fork M33.

The only problem is that you would lose the speedo drive. This isn't a problem for me as my speedo is driven off the gearbox. I use the same rim setting as a B33. I had a new spindle made up I will search out the dimensions. Once I sort out uploading photos I will put them on the site.

The original post referred to a tele fork model so the speedo issue doesn't apply there....

However, the fork sliders and wheel spindle would need to be changed and the 8" brake torque arm would be needed...Additionally, attachment of the mudguard stays and bridge might also be an issue and another brake cable would also be required...Ian

email (option): ian@wright52.plus.com

My mistake as most on here deal with WD bikes I assumed it was one fitted with girders. I have fitted a 8 inch brake in a B31 if you don't mind leaving the cable adjuster on the fork slider. The left hand thread is the same in the offside and the nearside just needs the spacer removed for the 8 inch spindle to be used. Also as you have said you need the torque arm and a new cable.