32 replies to this topic

[orcadian]

Can I run this by you all?

I've been repairing/restoring cars and motorcycles for over 40 years and have frequently come up with the age old problem of removing a drum over a wear lip or drawing pads from calipers with a similar problem caused by the lip on the edge of the disc. In some extreme cases on some vehicles, some garages have had to resort to smashing a drum off when the self adjuster has seized. I know this is not such a great problem on Minis - unless the adjuster is seized solid.
Now, I'm only a time served Aeronautical Engineer and not a mechanic, but why is a brake drum wider than the swept area of the shoe? Wouldn't it make much more sense if the last few millimeters of the drum at the outer edge were bevelled at say 30 degrees? OK, the lining would eventually wear with a small lip, but at least the drum would come off.

Same situation with the disc - if it were smaller than the swept area of the pad by 1mm then the pad would end up with the lip (and that crusty/rusty edge would be constantly kept clean by the pad lip) OK I will concede that when the pads were really worn on a non-vented (thin) disc the pad lips could touch, but a fragile piece of pad material would not prevent the pads gripping. Plus point - it would also reduce unsprung weight, minus point - there would be less material, so the brake temp could increase marginally.

Anyone got any thoughts on this?

[Cooperman]

Sounds a great idea to me, as a fellow former aeronautical engineer and Mini enthusiast.

[lrostoke]

I suppose when manufactorers design these things they expect regular maintenance to take place, that way the situations you describe shouldn't be an issue, because drums would be changed and so would discs.

[bmcecosse]

In fact if you do the excellent Fiesta Mk I caliper conversion - you get just that - the pads are very slightly overlapping the disc if you turn it down to 7.9" diameter. On the rear drums - you can always fit the wider shoes that were specified for the 'twin leading shoe' front brakes. But the best plan is to keep the adjusters nicely greased up so they don't seize! Simples.

[orcadian]

Thanks for the quick replies folks,

Even with regular maintenance this wear lip causes problems in the trade at relatively low mileages and WAY before the wear limit of the drum or disc has been reached. On self adjusting types where the shoe is continually being moved to almost touch the drum, you only need a few thou of an inch lip to prevent you removing the drum (ask anyone in the trade who has worked on a twin wheel Transit!!) As I said, this is not much of a problem on Minis - my question was more general than that to see if anyone could come up with a very sound engineering reason for me not modifying mine! I have a Harrison 140 Lathe in my workshop - well capable of chamfering my drums and reducing the diameter of my discs - a nice little job during the winter perhaps? Thanks bmcecosse for sharing the info about the Fiesta conversion - if its good enough for them..........

[Shifty]

I can't think of a reason why this wouldn't work. This leads me suspect that theres some very good and obscure reason that this wouldn't work!!

Its such a simple idea that would surely have occured to to an automotive engineer at some point over the last 30 odd years. Even just from a cost saving point of view, losing a few mm of the diameter of a brake disc would mean the componant would be cheaper to make and hence more profit.

I know we have a few automotive type engineers on board here so, 998dave,mk3coopers and sukidawg if you're reading this, why hasn't this been done??

[Big_Adam]

Now, I'm only a time served Aeronautical Engineer and not a mechanic, but why is a brake drum wider than the swept area of the shoe? Wouldn't it make much more sense if the last few millimeters of the drum at the outer edge were bevelled at say 30 degrees? OK, the lining would eventually wear with a small lip, but at least the drum would come off.

You do know you're on about BMC right? BMC where cheap and badly managed. They got something that worked they kept it, for a LONG time. We've also got manufacturing processes and tooling along with mass production issues.

Sure, you could make the drum closer to the correct width, but there more room for error on the day Terry is off sick and the new boy has to adjust the machine.

Least that's my theory.

[MRA]

That almost sums it up, there is also more cooling with more metal, pads should never overlap the disc, because if they do contact in the middle then you won't get a lot of braking, in years gone by when machining was a lot less accurate than it is today the "stack up of tolerances" had to be catered for so you often found that the pads sat well inside the outer diameter of the disc, this way they would never be over the edge. Drums also need to keep as much water off the friction area as possible, water between the shoe and drum, gets pretty hot, so hot in fact that it can create gases from super heated steam that will give you the same effects as "aquaplaning" therefore your stopping distance may be the car in front.

Next time you drive anywhere near a ford.... no not a ford car, a ford as in river or stream crossing, quite a few will have the sign "now try your brakes" this is to clear the water.

[midridge2]

if i onderstand the question right, there is a very simple reason why this will not work on discs and pads.
if the outer edge of the pads overlap the discs, when the pads in contact with the discs start to wear down the outer unworn pad lining will make contact with each other and the pistons will not be able to push the linings against the disc and you will have no braking very quickly.

because of this your pads will only last a short time, you will have no indication of brake wear eg long pedal travell, and no visual indication of worn disc as you cannot see a lip on the outer edge on the disc.

[Dan]

There would also be a massive heat differential in the friction lining between the area being worn and the area not. I suspect this would lead to fractures in the material very quickly and you'd start to loose chunks of it. Friction lining obviously does not conduct heat well so the heat energy wouldn't evenly distribute through it as it can through the disc or drum. I must admit that is only speculation though.

[orcadian]

Thanks again for the thought provoking replies, as I said at the beginning I understand fully that the lip produced on the pad edge COULD eventually contact the one on the other side but I only go through pads on my everyday car after a long period of time and on the Mini would have had the wheels off many times for cleaning/suspension greasing etc in that time.
I also appreciate the bit about 'tolerance build up' but we're talking specific here - my car has it's own dimensions I can work to, as does yours. I said earlier that I might reduce the disc diameter to perhaps 1mm inside the pad edge and that little piece of pad would definitely not produce a temperature gradient sufficient to crack a pad.
Also the comment about wet drums was noted, but I'm talking about a chamfer - not leaving the edge of the lining waving in the air. Therefore the lining will wear to touch this chamfer but as I said it will then not prevent drum removal.

I also have to dismiss the comment about not knowing you have brake wear due to 'long pedal travel' You do not get a longer pedal travel on a disc set up anyway as the pads wear, the caliper pistons follow the wear. As for having no wear lip on the outside, that's the whole point of the exercise, but the lip will still be present on the inner edge of the disc and I would never rely on a visual wear lip to tell me the disc was worn - I would always use accurate measurement.

Strange that the guy with the Fiesta disc conversion has exactly what I'm suggesting at the front, do you have any problems BMCECOSSE?

[tommy13]

After spending over an hour this afternoon trying to remove the rear drums from a Saxo, I finally succeeded after hitting them very hard with the intention of smashing the drum, it jumped off ripping the linings and hold down springs off with it.
It is standard practice at our garage to remove the lip from the drum with the angle grinder (no lathe available), and after 30 years of daily practice I have got quite good at putting on a chamfer as you describe. I can only say it is an excellent idea to have a chamfer as it not only eases the removal of the drum but also allows brake dust and / or water to drop out instead of being retained by the lip. I suspect manufacturers don't do this because it is one more machining operation adding to costs, coupled with the fact that service schedules and lazy dealerships never remove the rear drums for the first 3 years after which period many owners move away from dealer servicing so they never see or are effected by the problem.
Regarding the front discs, MRA Minis is correct in that the pads can touch together in the middle if you arrange a overhang. I have only ever seen this once, and virtually all front braking effort was lost, There is no advantage in keeping the edge of the disc shiny, and the pads can easily be removed by first levering them back a little in the calipers.

[orcadian]

Thanks tommy13 for the reassurance about the rear drum (definitely on my 'to do' list) and I will take on board your experience about the touching together of the pads when worn - which I did mention in my first post (and subsequent ones) about this. I entirely agree, but really think that a small sliver of pad would crush if necessary, if I ever left them that long!!. I have sometimes removed pads (from various cars) where the pads were seized in the caliper and there is sufficient force to actually BEND the pad during braking. This can be seen if you check such a pad with a straight edge - it's hollow in the middle!
Anyway back to my solution, which might sound a bit tedious, after doing the drums, I intend to put a chamfer on the edge of the disc, the outer edge of this (where the pad sweeps) will be inside the swept area but the disc will be left full diameter, so it will actually come to a shallow point. This way the pads can never touch, but there will be no outer lip.

[MRA]

If as you say you are setting your brake system up for your requirements, then why leave a thin sliver at all ? why not siply fit the caliper/pad right on the edge ??

[mk3 Cooper S]

As already been touched upon, heat managment and structural integrety.

As many I found out the hard way as an apprentice by dressing my front discs and removing the lip. They always faded and warped very soon after.
Presumably the drums is a simular thing to retain the strength and avoid ovality?