Centralising front brake - spindle nut clearance.

CAB · Post by **CAB** » Fri Jun 08, 2012 9:58 pm

Hello.
I'm trying to improve the performance of the front brake on my G12 CSR. I have fitted new shoes from the club and centralised the front brake shoes by loosening the outer spindle lock nut, applying the brake and tightening the locking nut as per the manual. Presumably there should be enough clearance between the brake plate hole and the shank of the outer spindle lock nut to allow the plate to move radially, thus allowing the shoes to contact the drum equally and centralise the plate. Once centralised the plate is held in this position because it clamped between the inner and outer nut. I have had to replace the brake plate as well as the lower fork leg after the brake anchor strap failed so it could be a problem with the combination of parts.
Not being happy with the performance of the brake, I have stripped it again and one shoe seems to have bedded in more than the other as if the plate has not centralised. There doesn't seem to be much clearance between the spindle hole and the shank of the outer spindle lock nut,thus preventing the plate from moving radially.
Can anyone advise on the amount of radial float there should be on the brake plate?
Thanks
CAB

1608 · Post by **1608** » Sat Jun 09, 2012 12:10 am

The fact that one shoe is showing more wear than the other is because one shoe is leading and the other trailing. The leading shoe is working much harder. I believe the centralising of the shoes ensures that the whole surface of each shoe bears against the hub. Sorry but I can't remember how much radial movement there should be, but its not much.It might take a while for both shoes to show even wear. How about trying the old and tasted mod of easing the trailing shoe a little.Edited by - 1608 on 08 Jun 2012 11:11:59 PM

SPRIDDLER · Post by **SPRIDDLER** » Sat Jun 09, 2012 1:00 am

In direct reply to your question you can, if really necessary, relieve or open up the backplate hole to increase the amount of radial adjustment.
However...
The question of front brake improvement keeps cropping up and coincidentally in my enforced convalescence and to save repeatedly doing the same Forum Search to reply to questions I decided some time ago to do something (hopefully) useful and put together info from my and others' posts on this subject (and other topics) with the intention of submitting them when finished as Technical Articles.
Here's a small extract from my unfinished draft:
With the brake shoes assembly out of the wheel chalk an arrow on each brake shoe in the direction of wheel rotation. (See pics at end) Be careful when marking the arrows as you'll be looking at the brake assembly 'back to front' i.e as a mirror image of when it's installed.
The arrow pointing away from the brake cam/expander is on the leading shoe and the arrow pointing towards the cam is on the trailing shoe.
The leading edge of the leading shoe and the trailing edge of the trailing shoe are both at the cam end of the shoes.
Because the fixed brake shoe pivot doesn't move when the brake is operated the cam ends of the shoes inevitably touch the drum first until the linings are ‘worn in’. Hence disappointment when a relined brake performs no better or maybe worse than the old ones.

Ideally, when the brake is operated we want both shoes to contact the drum at the same time and with equal force but since it isn’t easy to know that this is the case it is essential that when the brake is operated it is the leading shoe and not any part of the trailing shoe which makes first contact with the drum. This will ensure that the brake benefits from the ‘servo’ effect on the leading shoe as a result of the drum dragging the leading shoe outwards towards it. The trailing shoe is in effect being 'pushed away' from the drum. Now, if, upon braking, the trailing edge of the trailing shoe touches the drum before the leading shoe you lose the 'servo' effect of the drum tending to 'drag' the leading edge of the leading shoe outwards towards the drum (look at the chalk arrows). Keep up at the back, there’ll be questions later.
To give an extreme and hypothetical example you could remove either shoe completely, refit the assembly to the wheel, and the brake would still work, although it would not be possible to fit the pull-off springs.
Furthermore, you also want as much area (length) as possible of both linings to contact the drum. Once one shoe (either shoe) has contacted the drum the cam will not be able to turn any more therefore you want to ensure that it is the leading (servo effect) shoe which makes contact first. Brake drum/lining skimming which matches the drum and linings contours is a further option to improve braking, especially if the wheel has been rebuilt which can have distorted the drum.
I arrived at a total of one and a half inches removed from the trailing edge of the trailing shoe by taking off about a quarter of an inch at a time (see pic at end), refitting the wheel and trying it out on the road. I put the shoe in a vice, hacksawed across the 7/8" width of the lining down to the shoe casting then used a chisel to carefully 'lever' or slice off each ¼” of the lining material starting from the cut and going towards the end of the linings as I didn't want to risk disturbing the adhesion of the lining that was to remain. The amount of your lining which will need to be removed will not necessarily be the same. I would only modify the bonded, not riveted type of linings.
Adjusting shims can be fitted under the stems of the removable ‘mushrooms’ at the cam end of the shoes on some models to compensate for lining wear by bringing the linings closer to the drum when ‘at rest’, thus eliminating lost lever action before the linings make contact. Ordinary steel washers, trimmed to fit, of various thickness can be used here. Each shoe mushroom may need a different thickness of washer. Obviously, washers cannot be fitted on models which have the ‘castellated’ type of wear adjusters.
You can also rotate the wheel on the bench repeatedly applying the brake, disassemble and look for high spots or non-contact areas on the linings. A bit of chalk dusted onto the linings can help disclose these non-contact areas and give you a clue as to what’s going on when the brake is applied. You will probably be surprised at how little of the linings shows drum contact. It's worth repeating that centralising the shoes inside the drum after replacing the assembly into the wheel is essential. Do this on the bench with wheel on its side by loosening the fixed pivot, locking the brake on hard (a tube over the brake arm helps to apply sufficient force), then tightening the fixed pivot and brake backplate locking nut.
I recognise the inevitable risk of poking my head above this technically contentious parapet and it is not intended to be comprehensive, definitive nor authoritative (it may not even be relevant to your CSR brake design) but if it helps anyone to understand or to fettle their front brake so much the better.
There are other factors to consider; Modern asbestos-free linings are harder/have a lower coefficient of friction than the originals, handlebar lever pivot pin to nipple centre distance, outer cable construction....
O.K. you can go now.

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Edited by - SPRIDDLER on 09 Jun 2012 12:46:49 AMEdited by - SPRIDDLER on 09 Jun 2012 08:51:54 AMEdited by - SPRIDDLER on 09 Jun 2012 08:53:30 AM

rex.webb · Post by **rex.webb** » Sat Jun 09, 2012 10:11 am

THANKS NOW ALL IS CLEAR !!!

RIDE with CARE and LIVE LONG !!!!!!!!
USE your MIRROR and LEAVE room to
S T O P !!

thirdwheel · Post by **thirdwheel** » Sun Jun 10, 2012 7:58 pm

This modification works well - as does firstly having slightly oversize linings bonded on and mounting the back plate and shoes in a lathe and turning them to the correct drum dia. Ata very slow speed and with a light cut. your pictures show the mod well - pictures are so much better than words. It will still take a little while to all settle down and bed in and you will get used to the improvement - until you have a ride on the same setup without the mod and have to re set your brain for braking. And yes it needs to be in the tech. area.

itma · Post by **itma** » Sun Jun 10, 2012 9:13 pm

He's a clever lad our Spriddler, if only we can keep him in one bit..............

CAB · Post by **CAB** » Mon Jun 11, 2012 9:27 pm

Thanks gents.
I forgot to say that I tried the above mod on the first set of replacement shoes that I had after the anchor strap let go. These were not supplied by the club (they were relined by a specialist that advertises in the classic bike comics but I can't remember their name) but there wasn't a noticeable improvement. This might have been because of the lack of clearance stopping the brake from centralising. I have tried shimming but I can't get the assembled brake plate into the drum with even the thinnest set of washers that I can find. The first replacement set then became contaminated with grease and a new club set went in (along with new hub seals). I tried shimming these but again they wouldn't fit. The brake seems better but that could be due to the replacement of the contaminated shoes. I haven't carried out the above mod on the new set yet but I will if things don't improve. I have tried a heavy duty cable and made sure that I have the correct 7/8" from the lever pivot to the cable but it feels spongy with either cable. Dad has a ratty old thin cable on his 600cc Super Clubman with insulation tape on it, the incorrect levers and his brake is better and lever solid when you apply the brake..... Seriously thinking it's all down to the insulation tape....
I have have also noticed that there is significant movement of the new lever mountings on the bars when I apply the brake. The new levers don't feel very solid so I could be losing a bit here as well.
The latest shoes haven't done much mileage at all, literally a couple of miles. I will give them a chance to bed in and will then investigate the clearance and perhaps carry out the above mod again. I have another brake plate plus the old smashed one and I will compare each of these to the plate fitted at the moment.
CAB

Tolly · Post by **Tolly** » Mon Jun 11, 2012 10:28 pm

It's best to have the new linings well bedded in before you do the mod. New linings do make the lever feel "spongy" sometimes until they are fully bedded in.Edited by - Tolly on 11 Jun 2012 9:29:02 PM

bigwol · Post by **bigwol** » Tue Jun 12, 2012 10:17 am

Is the lever a good quality item? You say there is "significant movement" at the mountings? I test rode a dog of a G11 I looked at from flea bay last year and the levers the current owner had fitted were chinese things that were awful. Both brake and clutch were suspect and these things fitted so badly they actually rotated on the bars with vibration despite being tightened so much i think the mounts were stretching! I think braking on that thing would have been better if cycle brakes were fitted to the rim!

CAB · Post by **CAB** » Wed Jun 13, 2012 11:50 pm

Hello
I'm pretty sure that the levers came from a reputable supplier and were made in England.....
CAB