Drum brakes adjustment

[PotatoHead202...]

So took my CZ for it's first ever MOT today after the rebuild. Clutch is working properly now. It failed on both front and rear brakes being inefficient.

I have no problem rebuilding combined ABS units but I'm not so sure on drums. I fitted new shoes to both drums and a new cable on the front.

I'll admit I was concerned about adjusting them too much and causing binding so they are relatively loose. The front lever almost comes back to the bar.

What am I looking for here? Should this just be the adjuster isn't tight enough on them? The TLS front had it's levers adjusted off of the wheel to check that it was operating both arms evenly.

The tester refused to work on the bike saying it's too specialist so he's out

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Honda VFR1200F, Honda CBF500 ABS, CZ125 now CZ175

[Easy-X]

Drum brakes are "specialist" now?! How far we've come

If you can lift the bike up so a wheel (or both) are off the ground give the wheels a spin to check for brake drag and have a play around adjusting, get feel for where you go too far and then back off a bit.
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Yamaha XSR700, Honda Rebel, Yamaha DT175, Suzuki SV650 (loan) Fazer 600, Keeway Superlight 125, 50cc turd scooter

[PotatoHead202...]

[Pete.]

Turnt he trunnion adjuster nut over the cam ends half-turns at a time until you're either happy with the lever travel or the wheel starts to bind when spun. If it starts to bind, back the adjuster off until it does not, and live with whatever lever travel there is.

If it still fails from lack of effort then you'll have to strip it down and find out why. Could be the drum is worn so the shoes are only contacting in the centre.
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a.k.a 'Geri'

132.9mph off and walked away. Gear is good, gear is good, gear is very very good

[MCN]

Drum brakes do this annoying little thing where wear in the drum allows the shoes to move to far . Even new shoes.
The cam that pushes the ends of the backing out, to push the shoes into the drum can run out of cam.
It's not easy to see coz the drum covers everything.
New drums are the solution. Not uncommon to replace drums on drum brake machine.
They're bastirts, that's why disc's a boss now.

Adjustment rule of thumb.

Wind the adjustment up until there is no freeway in the lever.
Back off the adjustment 1-1/2 turns.
It's that same adjustment for lots of drums.
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Disclaimer: The comments above may be predicted text and not necessarily the opinion of MCN.

[PotatoHead202...]

[slowasyoulike]

I would get some calipers and check whether the drum is actually still round. I had bother adjusting the rear drum on the CZ125 I had back in the day, my friendly old-skool tester had a look and said the drum had gone oval. Luckily it wasn't too bad, but it meant a lot of travel in the pedal and I was still on the lookout for a new wheel when some bastard stole and torched it.

Fast forward a few years, and a mate of mine bought a skip-load of CZ250 twin stuff from a woman whose husband had carked it before carrying out his plan of making one good one out of approximately three of them. There was tons of the stuff; three viable frames, maybe six engines and - amongst other things - about 20 wheels. The bike we assembled that actually ran and got through a test was wearing the only two wheels amongst the lot that didn't have oval drums or some other hideous deficiency. Hence my suspicions!

[stinkwheel]

I'm surprised it failed the brake test, drum brakes usually fly through even if they are totally shit. What sort of brake tester was he using? They usually ace it on a rolling road tester, the half width SLS on my trials bike passes no problem and you can push the bike forwards with it held fully on.

They aren't very good when properly adjusted anyway.

As you say, the trick with the front one is to get it so both shoes are hitting the drum at the same time. I generally back off the secondary shoe using the link rod then over-tighten the primary one until it is just rubbing and back it off a hair so it isn't. Then I do the same with the secondary shoe. Then adjust from there using the main adjuster so it has a comfortable length of throw on the lever.

I generally find drum brakes work a bit better when there's a reasonable amount of take-up on the lever. You don't want to be able to pull it all the way back to the bar but you would probably want it to come closer to the bar than most people would be happy with on a disc brake.

You may also need to re-position the brake levers at the drum end on the splines. You want the two levers on the front to be as close to paralell as possible when properly adjusted and you want the main levers to be as near 90 degrees to the cable/rod as you can get them.

New shoes are another thing. They aren't necessarily concentric with the drum when you get them, you may need to bed them in. To do this you adjust the brakes so the shoes are striking evenly, then get a stick of chalk and draw lines across your brake shoes about 1cm apart. Re-fit them and turn the wheel a couple of times while holding the brake on. Take the shoes off and sand down and high-spots whenre the chalk has been rubbed off. Rinse and repeat until it's rubbing the chalk off all the way over. Some people also like to sand a chamfer on the leading edge of each shoe to help the cam-on. NB: If they are new old stock Czech shoes, there could be asbestos in the friction material so wear a mask!

The friction material itself is also a potential factor. Are these "new" CZ shoes? In which case 1) They were made of cheap shite material when new. and 2) They have been sat about for a long time and are probably bordering on antiques now. A variety of companies will take your brake shoes and re-line them with a modern friction compound. If there is a lot of wear on the drums, some of the more specialist places will put thicker material on for you and even turn them so they are concentric with the drum on a lathe to save fucking about with chalk and sandpaper. They need the hub and wheel to do this and it's not cheap.

Glazing of the drums is another potential issue, the proper way to fix this is to put them in a lathe and do an thin skim on them with a boring bar which removes glaze and resets them to properly round and paralell. The old dodge was to glue some emery paper to an old set of shoes and adjust the brake until it is just rubbing them spin the wheel until you get an even sanding noise all the way round. People used to do this with the engine running and the bike on the mainstand which you try at your own risk! If it's like a Jawa, the front and rear wheels are interchangeable, the front has a trim plate pressed on over the cush-drive vanes.
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“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.

[MCN]

Drum brakes are s4ite.

Both of my BSA 250 singles require any braking to be Pre-Booked a couple of minutes in advance.

There is a direct relationship between engine power and braking efficiency.

Big engines push a vehicle faster so need big brakes to slow it down.

My BSA 250 has brakes perfectly balance for the power unit's output.
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Disclaimer: The comments above may be predicted text and not necessarily the opinion of MCN.

[PotatoHead202...]

Thanks again.

The shoes are brand new EBC, come to think of it they did look pretty shiny. I'll have to look at the cam adjuster as well.

If the drums are oval would it still pass if adjusted? I have two other spare wheels (one rim is shot) so could always use those drums I guess.

He had one of those roller in the floor things. Tbh I did start to worry when he couldn't work out the ignition and was trying to start it whilst in gear.....
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Honda VFR1200F, Honda CBF500 ABS, CZ125 now CZ175

[Pete.]

If the drums are oval it will pulse on the brake rollers. The tester should notice it through the lever.
____________________
a.k.a 'Geri'

132.9mph off and walked away. Gear is good, gear is good, gear is very very good

[stinkwheel]

I genuinely have to wonder if the tester did the brake calculation properly. They will be used to disc brakes stopping the roller and it being a pass. If it doesn't stop them, you have to do a calculation to see if they are good enough which includes the front to back weight bias. I've twice had them weigh the front and back wheels on a machine that was very sketchy in that regard but they both still passed (can't remember if it was with or without the rider on). He'll have a set of official MOT tester scales gathering dust in a corner for this exact eventuallity which i bet he's never used. From my previous experience, if they will even vaguely slow the bike down when pulled, they are usually a pass.

They would need to be epically terrible to have failed a rolling road test. Like to the point you would probably wouldn't have bothered taking it for an MOT and certainly wouldn't have ridden it. I've never had one fail that test and some of them were, even in my own admission, pretty borderline functional and a work in progress.

As above, ovalled drums would cause pulsing at the lever.

Mind you. With new shoes taken straight to the MOT, they might not have bedded in at all. You might find only a few mm of pad is touching the inside of the drum until they have some wear on them.
____________________
“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.

[PotatoHead202...]

[Pete.]

[PotatoHead202...]

[WD Forte]

The tester does sound a bit of an arsehole and perhaps he tested for discs not shoes
but it may well be just a case of some fettling and settling in needed to get the shoes
to conform to the drum and make full contact.
The last all drum bike I took for MOT felt crap to me even after much fettling and settling
but passed OK.
Then again, my MOT guys are decent sorts.
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bikers smell of wee

[MCN]

[MCN]

Thought I'd post 2 times.
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Disclaimer: The comments above may be predicted text and not necessarily the opinion of MCN.

[PotatoHead202...]

Got it back this morning. Funnily enough the bloody delivery driver was more helpful in what to do than the mechanic! Suggested as above, removing the glaze from the shoes etc. He thought the cable was too long but as far as I can tell it exactly matches the measurements of the manual (it's new old stock iirc).

Took some photos of it as it currently is.
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Honda VFR1200F, Honda CBF500 ABS, CZ125 now CZ175

[stinkwheel]

Looks pretty close to how you'd want it.

You are aiming to have both shoes striking the drum at exactly the same time. That's the most important thing.

Then you want the two red lines as close to parallel as possible and the two green lines as close as possible to a right angle. This is not always possible. Striking evenly is non-negotiable. Paralellness is next most important, the two levers work as a unit. Perpendicular cable to lever least important.

Oil your cable and trunnion. Have you lubed the cam pivots (light smear of grease)?
____________________
“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.

[PotatoHead202...]

Had another go yesterday evening. Cleaned the drum out as best as I could and roughed up the (very) shiny shoes. I haven't ridden it yet as the twat of a mechanic left the key in and drained the battery. I got it on it's rear wheel and span the front and occasionally grabbed the brake. It does seem better but seem to be getting an intermittent bind (not every revolution) and the brake doesn't want to release properly. I can't quite get the cable adjustment right - the cable is definitely the right one and the right length.
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Honda VFR1200F, Honda CBF500 ABS, CZ125 now CZ175

[WD Forte]

We cant assume the drum has worn evenly or is true but as pic3'after test' shows contact at the edges of the
shoe and hardly any at the centre I'd say the drum has worn more in the centre then edges.
Given time, new shoes would probably wear in but I'd be tempted to cheat a bit and glass paper
the shiny bits off a smidge to help the process as well as the afore mentioned attention to setup.

Its a CZ and the bike probably had crap to indifferent brakes from new so you're never going to do stoppies on it and
finding an MOT tester more sympathetic to the bike and its capabilities and limitations might be worth looking into.
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bikers smell of wee

[PotatoHead202...]

[WD Forte]

the shoes
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bikers smell of wee

[PotatoHead202...]