Brake Drums - wear limits

Post by **Dominic** » Wed Mar 21, 2007 11:51 am

Hi Folks
Has anyone any idea of what the wear limits on brakedrums are? I'm running a 1098, so 8" fronts and 7" rears. If they need replacing, how necessary is it to replace the current (not far from new) shoes?

Thanks for your help

Dom

Post by **Kevin** » Wed Mar 21, 2007 4:42 pm

How are your worn ?
Normally its not just wear that finishes them off you often find the fronts especially go a bit oval with age probably caused by heat over the years or scoring from letting the shoes wear down to the rivets.
Are you saying that you have just replaced the shoes and its not making much difference ?

Post by **Dominic** » Wed Mar 21, 2007 6:12 pm

Kevin,
They work well at the moment, but there is a discernble step between worn / unworn area of the inner drum. I am however very much of the "replace before a problem occurs" school of thought, and wondered if there was a measurement generally accepted as the lower limit of wear.

Dom

Post by **bmcecosse** » Thu Mar 22, 2007 10:24 am

As long as not seriously scored - just rub away the 'step' with a file and/or emery paper. Wear is not generally an issue.

Post by **Kevin** » Thu Mar 22, 2007 6:41 pm

just rub away the 'step' with a file and/or emery paper

Sorry to disagree but that is a very bad engineering practice.
If within tolerences the drums can be skimmed on a lathe so that the concentric shape is kept in an accurate profile using a file especially in untrained hands is asking for trouble.
I would have thought that once the wear exceeds 1/16" its time to consider new drums as the overall wear is 1/8" which means the shoes are not in ideal contact over the surface area maybe Jonathon can be more specific on wear tolerances.
As new drums are around £20 each I dont imagine that having drums skimmed in a workshop with current labour charges would be any cheaper so I know what I would opt for as its a small price to pay for safety.

Post by **bmcecosse** » Thu Mar 22, 2007 7:10 pm

I was only meaning the little outer ridge of rust/dust/muck that builds up on the edge and can make getting the drums off past the shoes difficult. Definitely not 'bad' to remove rust! Don't seem to get the problem with nice big 9" drums !!

Post by **gairlochrosie** » Thu Mar 22, 2007 7:13 pm

I've never had a problem using this method.

Post by **wibble_puppy** » Thu Mar 22, 2007 7:21 pm

Kevin wrote: I would have thought that once the wear exceeds 1/16" its time to consider new drums as the overall wear is 1/8" which means the shoes are not in ideal contact over the surface area maybe Jonathon can be more specific on wear tolerances

Yeah, I'd be interested to hear about wear tolerances too. This is an interesting thread. My drums are a bit rusty on the inside, very rusty on the outside, but they have no wear - at least, there is no "step" of any depth on them, so I think they are fine (and more importantly so does Cam) so am planning to just gently remove the rust and bung 'em back on (having checked they aren't becoming oval).

I'm totally religious about brakes and always want to triple-check with really sound authorities before deciding what to do about them

Post by **bmcecosse** » Thu Mar 22, 2007 7:42 pm

When you remove that rusty lip - you will almost certainly find that in fact there is NO wear on the drums at all.

Post by **gairlochrosie** » Thu Mar 22, 2007 8:30 pm

I agree with this...in over 40 years of using this method, I've never had or caused a problem.

Post by **Dominic** » Thu Mar 22, 2007 8:43 pm

Maybe I'll speak to my local supplier and see if he can measure the internal diameter of a new drum for me, then I can calculate how much (if any!) is worn away. I think that the fronts are probably ok, the rears looked somewhat worn, but they all work at the moment! Before anyone else says it, far better to replace when still working than to wait for failure! When I have to apply the brakes prior to starting the journey is when it is the time to change them I guess....

Post by **Onne** » Fri Mar 23, 2007 12:27 am

Apparently there is no problem with wear with the original linings on the shoes, asbestos that is. The new materials (whatever they may be) are more likely to wear out drums.

I need a full set of new drums, as mine are scored

Post by **bmcecosse** » Fri Mar 23, 2007 7:34 am

Scoring adds to the surface area in contact between shoes and drums - increases efficency!

Post by **wibble_puppy** » Fri Mar 23, 2007 9:33 am

bmcecosse wrote:Scoring adds to the surface area in contact between shoes and drums - increases efficency

How odd! Do you mean scoring as in, grooves rubbed away in the inner surface of the drums? as I would have thought that in that case it would be removing some of the contact surface, and therefore making the brakes less efficient?

perhaps i've misunderstood something technical here

Post by **Packedup** » Fri Mar 23, 2007 9:51 am

Provided any scoring is perfectly concentric bmc is right - the shoes (or pads in the case of discs) are soft, and so will wear till they're a perfect fit in the grooves. Meaning increased surface area.

This doesn't take into account any wobble in the bearings or shoes (which there shouldn't really be, but in old components is possible) as the grooves/ scoring won't be perfectly aligned throughout each wheel revolution.

The grooves on "uprated" discs aren't concentric however, and do reduce contact surface as the pads have no chance of wearing/ beddign into them. However their job is to vent the exceedingly hot gasses away from the pad/ disc contact, and they should be formed in such a way as to cause a negligeable drop in real life braking performance (with an increase in performance when hot).

And I've run drums of god knows what age, with rather large lips, and never had a problem other than getting the damned things off!

Post by **bmcecosse** » Fri Mar 23, 2007 10:11 am

I was being slightly 'tongue -in cheek' - but yes the grooves will increase the area of contact as p-up explains.

Post by **Packedup** » Fri Mar 23, 2007 10:22 am

What I don't understand is how the scores in discs never seem to be perfectly round. I bought a Metro a few years ago that had obviously been driven with just the pad backing plates for many o mile as the discs were chewed beyond belief. Going by the increased area theory this shouldn't have been too bad (other than the scoring was deep enough to be nearly through in places!), but somehow the whole surface was randomly ruined rather than "enhanced".

I won't pass comment on how I really feel about people who fail to realise the hideous grinding noise coupled with a vibrating pedal and reduced stopping power might mean there's something fundamentally wrong with a vitally important safety component...

Post by **wibble_puppy** » Fri Mar 23, 2007 10:38 am

Packedup wrote:Provided any scoring is perfectly concentric bmc is right - the shoes (or pads in the case of discs) are soft, and so will wear till they're a perfect fit in the grooves. Meaning increased surface area.

I see. Cheers

The grooves on "uprated" discs aren't concentric however, and do reduce contact surface as the pads have no chance of wearing/ beddign into them. However their job is to vent the exceedingly hot gasses away from the pad/ disc contact

Yup, I'm on board with the whole "vented discs" deal

And I've run drums of god knows what age, with rather large lips

Aww don't do yourself down, dude

Post by **Kevin** » Fri Mar 23, 2007 2:28 pm

Provided any scoring is perfectly concentric bmc is right - the shoes are soft, and so will wear till they're a perfect fit in the grooves. Meaning increased surface area.

But not increased braking surface area as the shoes would not be giving any braking effort on the sides of the castellations and the life of the shoes would be reduced.

Post by **Packedup** » Fri Mar 23, 2007 3:00 pm

That depends - If the gooves were traingular then there would be an increased swept area, but of course if they were squared then you're right.

Either way, it's probably better to not have knackered/ scored/ chewed brake parts