front drum brake

I have experienced a problem on both Norton and Triumph front double leading shoe brakes.

Some grab "too well" , giving exceptional braking but sometimes fail to release for an instant when I let go of the lever. Is it just weak return springs or is something else involved? Right now it is a two finger brake but it remained on when I let go for maybe 3 seconds.. Could it be that the shoes are drawn in to the brake drum on their own? And how is it solved. I have beveled the lead edge of the shoes. Is there a potential for a front wheel lock-up? At a high speed or just low speed? Will it be cured by new springs?

Have you removed cams, cleaned shafts and bores, and re-lubed the shafts, with a spot of lube on the cam faces? Also same treatment for the pivots?

Slick

seattle##gs said:

Some grab "too well"... Could it be that the shoes are drawn in to the brake drum on their own? Is there a potential for a front wheel lock-up?

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Yes, on both accounts. Drum brakes, especially dual leading shoes, benefit from what's known as "servo action", where the shoe's friction helps to draw the shoes into the drum to assist with braking. Normally, the return springs overcome that servo action.
Were it a hydraulic brake, I would suspect a fluid leak. Believe it or not, grease/oil makes the brake much more likely to lockup than when dry.
My first car had drums at all four corners; it started losing rear-end lube through a failing seal. The first indication was when exiting the freeway, that one rear wheel completely locked up and dragged a flat spot in the tire. This was with so light a pedal pressure that the other three hardly contributed to the braking effort. The brake wouldn't release until coming to a complete stop and lurching backwards. I drove the rest of the way home on side roads, with the one tire screaming at every stop. Quite unnerving! Upon pulling the offending drum, the amount oil lube wasn't even enough to reach outside, avoiding earlier detection.

seattle##gs said:

...and how is it solved. I have beveled the lead edge of the shoes. Will it be cured by new springs?

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Reflecting back to the prior, make sure there's been no contamination to the shoes and drum surface. Drum brakes are much more sensitive to contamination than disc brakes for the above reason. If you suspect anything on the shoes, you're better off replacing with new. How do the wheel bearing seals look? The lightest hint of grease being flung onto the drum could contribute.

Nathan

I hav cleaned the drum with acetone. Wheel bearings are sealed. The drum was turned and the linings were arced though not on a lathe. Today I tried to readjust the two shoes and was able to make the problem a little worse.
Clue #2...I can push the bike foreward and clamp down on the front brake . WHILE MAINTAINING FORWARD PRESSURE and the lever RELEASED, the brake does not want to dissengage. If I stop pushing the bike and especially back it up a couple of inches it releases completely. I have ordered a new pair of springs though mine look perfectly good. I would like to try and save the existing linings as it is about $75 and a few days at the brake shop. Last time I looked they seemed pretty clean.

How does this servo action work? Are the springs the only thing that stops it? Though I haven't checked in a while all pivot points appear to be greased.


Is there someplace that can turn the brake drum? Can't find one in Seattle. Vintage brake is booked up for 4 months.

texasSlick said:

Have you removed cams, cleaned shafts and bores, and re-lubed the shafts, with a spot of lube on the cam faces? Also same treatment for the pivots? Slick

Click to expand...

+1

If the brake still grabs when all has been cleaned of rust and greased and adjusted, then look at the shoes.
It would be prudent to hand file a shallow 30 degree lead in on the leading edge of each of the end of the shoes of about .5 inch or so. This will prevent the shoes grabbing. I always do it on my bikes. HTH.

seattle##gs said:

How does this servo action work? Are the springs the only thing that stops it? Though I haven't checked in a while all pivot points appear to be greased.

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Once the shoe is in contact with the drum, the drum rotating motion continues to pull the shoe into contact due to the geometry of the curved shoe and pivot point. A self energizing effect.


I found a wiki quote/link to help you understand:
"Self-applying characteristic[edit]

Drum brakes have a natural "self-applying" characteristic, better known as "self-energizing."[4] The rotation of the drum can drag either one or both of the shoes into the friction surface, causing the brakes to bite harder, which increases the force holding them together. This increases the stopping power without any additional effort being expended by the driver, but it does make it harder for the driver to modulate the brake's sensitivity. It also makes the brake more sensitive to brake fade, as a decrease in brake friction also reduces the amount of brake assist.

Disc brakes exhibit no self-applying effect because the hydraulic pressure acting on the pads is perpendicular to the direction of rotation of the disc.[4] Disc brake systems usually have servo assistance ("Brake Booster") to lessen the driver's pedal effort, but some disc braked cars (notably race cars) and smaller brakes for motorcycles, etc., do not need to use servos.[4]"

Halfway down the page.... https://en.wikipedia.org/wiki/Drum_brake

"appear to be greased" isn't good enough. Take the cams APART from the backer plate, clean rust/and or hardened grease from the shaft and hole in the backer plate, verify free fit/rotation, coat with moly grease using a q-tip. The grease nipples on the end of the shaft are ok for routine maintenance, but old grease will harden in the passages and make them ineffective, complete disassembly is required to clean/inspect/verify. As mentioned, likely where the problem is.

Don't think its servo effect if brakes still on when pushing the bike by hand, suspect it something else.

I have ordered a new pair of springs. I will pull everything apart and clean and lubricate. The leading edges have already been beveled. Probably thursday I will go through the whole routine

seattle##gs said:

I have ordered a new pair of springs. I will pull everything apart and clean and lubricate. The leading edges have already been beveled. Probably thursday I will go through the whole routine

Click to expand...

Does it have the stiffening plate modification?



http://www.britcycle.com/Products/06/063410.htm

Here is a simple 5 minute test:

Remove the brake lever and free the cable actuator cylinder from the lever. With the cable so free, any binding in the system will be evident by working the cams on the brake plate with a wrench (spanner). There should be nothing to restrict motion in either direction. Check also that the cable moves freely in the sheath.

Slick

Get rid of it and fit a single disc. I've had about four serious crashes when road racing which were primarily due to the front brake locking up. The first time I ever raced my two sons aged 8 and 10 were watching me at I approached the end of the front straight on Winton Raceway at about 100 MPH. I couldn't stop the bike and had the lever pulled almost to the bar. Then the linings heated up and the brake locked and chucked me over the front. I slid on my back across the corner and into the escape road. The good thing was I never had to watch my own kids racing motorcycles. They thought I was dead. I still carry a dislocated collar bone due to locking the AJS7R brake on the front of my Triton.
A disc brake is predictable and if you grab a handful, it will always come off when you let go. As far as I am concerned bloody drum brakes should be banned along with pudding basin helmets - that combination is what killed a lot of guys in the old days.

In Australia disc brakes were fitted as standard equipment on a few road bikes before they were fitted to road racing bikes. A friend of mine fitted a disc brake front end off a Yamaha XS550 to his TR3. All the other guys were using the Yamaha Fontana-copy drum brakes. I stood at the end of the front straight at Calder and watched three of them crash on different laps while trying to match his braking into the corner.

There is no question that a disc is better than a drum. The Commando drum isnt stellar. The late BSA Triumph drum is quite good.
However for normal road riding, if in less than mountainous areas, the Commando drum can be good enough.
We all arent road racers and the way the Authorities have criminalized any speed over the posted by 15mph the speeds at which
we are forced to ride at are not beyond the capabilities of the drum brake.
You are a track guy and not a road guy. You see things strictly from the track view point.

Commando brakes are technically not servo action brakes. They are termed Twin Leading Shoe brakes. Servo action brakes as used on drum brake cars have a smaller area leading shoe and a larger area trailing shoe. Often they will also have different lining material on each. Where the shoes meet at the top, they each sit on a fixed pin but can pivot around the pin. Where the shoes meet at the bottom is usually the adjuster mechanism. In servo action or self energizing brakes the bottom part of the shoes have no fixed pin or point to retard rotation. When the brakes are applied the leading shoe (smaller one) contacts the drum and as soon as it does the entire brake shoe assembly tries to rotate on the upper pin. As a result the secondary shoe (larger one) is rammed via centrifugal force into the drum. All this being caused by hydraulic pressure coming from the master cylinder/wheel cylinder. This increases the braking force on the secondary shoe which does the majority of the braking.

Drum brakes are capable of providing plenty of friction stopping power. The problem is when repeated heavy applications of braking is called for. The drum gets too hot and actually flexes under the loads applied to it. Hence the brakes fade and one can not stop. CRASH IMMINENT.

I cannot tell you how many drum brake cars I have worked on where the shoes have been installed ass backwards. ie (big shoe installed to the front of the car).

All this is making me very nervous about drum brakes. No idea they could lock themselves up. I always figured that all would be well by just letting go of the lever. I
could part of the problem be that linings have improved greatly? I have seen some of the old junk linings from the 60's and they look pitiful.

Another possibility...since the backing plate axle hole is a sloppy fit on the axle, could that extra play contribute to the servo effect? The trick set up by VINTAGE BRAKE is to bush that hole down to a close fit and allow little or no shifting of the backing plate.. I have heard good things about this method. 4 month waiting list at VB.

I do have the stiffening outrigger plate. Everything is new or VG condition. This is a sunday bike

aceaceca said:

Commando brakes are technically not servo action brakes. They are termed Twin Leading Shoe brakes. Servo action brakes as used on drum brake cars have a smaller area leading shoe and a larger area trailing shoe. Often they will also have different lining material on each. Where the shoes meet at the top, they each sit on a fixed pin but can pivot around the pin. Where the shoes meet at the bottom is usually the adjuster mechanism. In servo action or self energizing brakes the bottom part of the shoes have no fixed pin or point to retard rotation. When the brakes are applied the leading shoe (smaller one) contacts the drum and as soon as it does the entire brake shoe assembly tries to rotate on the upper pin. As a result the secondary shoe (larger one) is rammed via centrifugal force into the drum. All this being caused by hydraulic pressure coming from the master cylinder/wheel cylinder. This increases the braking force on the secondary shoe which does the majority of the braking.

Drum brakes are capable of providing plenty of friction stopping power. The problem is when repeated heavy applications of braking is called for. The drum gets too hot and actually flexes under the loads applied to it. Hence the brakes fade and one can not stop. CRASH IMMINENT.

I cannot tell you how many drum brake cars I have worked on where the shoes have been installed ass backwards. ie (big shoe installed to the front of the car).

Click to expand...

"Commando brakes are technically not servo action brakes. They are termed Twin Leading Shoe brakes"

Correct. But the principle is the same, and the OP wanted to know how it worked.



"When the brakes are applied the leading shoe (smaller one) contacts the drum and as soon as it does the entire brake shoe assembly tries to rotate on the upper pin. As a result the secondary shoe (larger one) is rammed via centrifugal force into the drum."
Centrifugal force does not act upon the shoes, they are not rotating.


"Drum brakes are capable of providing plenty of friction stopping power. The problem is when repeated heavy applications of braking is called for. The drum gets too hot and actually flexes under the loads applied to it. Hence the brakes fade and one can not stop. CRASH IMMINENT."

Brakes fade because the friction coefficient of the lining material relative to the drum (or rotor) goes WAY DOWN at those temperatures. It is not caused by the drum flexing (although it can, an insignificant amount). Fade feels similar on disc brakes and it's not the rotor collapsing. :mrgreen:

peter12 said:

Don't think its servo effect if brakes still on when pushing the bike by hand, suspect it something else.

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The servo effect happens as soon as there is motion. Normally, it's not noticed, at the walking speeds described by the OP, but the (likely) jamming due to cams/pivot not moving freely causes it to STICK in the "brake applied" position.

Also, be sure the cable hasn't been overlooked..... http://www.motosport.com/product?psrefe ... tchType%7D

Seattle##gs- I had the same problem with freshly rebuilt drum breaks on my 71 as well. Everything was new or refurbished as described on your bike. When applying the breaks they would feel like they would "cam" tighter, making the front forks dive and stopping abruptly. I never found out exactly what was causing the problem sorry to say. I did notice it happened at lower speeds. After checking to make sure everything looked right I took the bike out on an open road and aggressively applied the break from higher speeds several times, like a racer coming into a turn, really heating them up. The problem went away. I chalked it up to the fresh break pad material was grabby and needed to be broken in a bit. I have not had the problem since, but like I said I can't be sure what the problem was in the first place.

Even on a single leading shoe brake there is a self-servo effect. It happens because of the distance between the centre of the can contact area, and the point of contact of the lining with the drum. It is a leverage effect assisted by the friction of the lining. If the cam spindles are in the wrong position, you can brake into a corner and when you try to get back onto the gas the bike will take you straight out into the boonies because of the steering geometry. When it happened to me I thought I had a mental problem. I persevered and crashed all over the landscape 4 times in one day. The whole situation is compounded when the lining is hard and then heats up and becomes sticky. With my 7R AJS brake I used a soft lining on the front shoe and a hard one on the rear shoe. For the first three laps of a race I'd have fair brakes while the front one was working, and fair brakes for the last lap while the rear one was working.
Who needs that shit ? - just buy yourself a standard commando disc, caliper and master cylinder and fit asbestos linings and use it safely without anxiety. If you sell the drum, you will probably cover the cost of the disc brake.
On my Seeley, I've got two Suzuki high speed steel discs, Lockheed AP calipers with asbestos linings, a Lockheed master cylinder designed for a single disc. It is one-finger operation and it is deadly. It needs to be like that because of the other idiots who race against me. I have to be careful I don't pop-in in front of anyone then out-brake them. It is totally predictable and extremely safe.
With motorcycling you always need to have your life in your own hands - drum brakes don't allow that. You can be as careful as all get-out and still have the brake crash you.