How thick are brake discs

Ingah · World Chat Champion Joined: 10 Apr 2009 Karma :

I just wondered if anyone happens to know or have a good estimate of how thick brake discs are when they're new and unused (if it helps, it's for the single front disc of my CB500).

I know that the minimum thickness (for my CB500) is 4mm, and i'm trying to work out if it's worth buying a used 5mm disc to replace my slightly-warped-probably-at-the-wear-limit one, or whether that would be wasting cash that could be going towards a new/newer one.
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-- Ingah

rodeobob · Two Stroke Sniffer Joined: 23 Nov 2009 Karma :

Not sure about in the UK but here in Aust the brake disc is meant to have the minimum thickness stamped on it. If it does not it is not for road use.

You can look up anything you need to know here.

https://www.metalgear.com.au/

Cheers, Bob.
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Robby · Dirty Old Man Joined: 16 May 2002 Karma :

I would worry less about the thickness and more about the nature of the wear and level of warping.

You should be able to tell how worn it is by the depth of the wear lip - the pads never fully cover the face of the disc, so you get a raised lip around the inside and/or outside. Compare this with the thickness in the middle for an idea of wear. Scoring is bad too, once a disc is score or ridged it'll never wear flat again.

I would go with the used disc. New ones are very expensive, used ones are cheap. Some people have had success machining car disc to go on bikes. Given the price of car discs and the price of decent machining this seems to make sense.

A new disc is £90+. A car disc in a suitable diameter will be £20 or less. I just paid 80 quid for an engineer to do a hell of a lot of work on a cylinder, so machining a disc down should be cheaper.

May work out even cheaper to buy a wheel with a disc and used tyre on ebay. Then you have a wheel and used tyre to sell and recoup some cost.

rodeobob · Two Stroke Sniffer Joined: 23 Nov 2009 Karma :

Sorry but i beg to differ.

It is all about the thickness.
If the disc wears too thin and the pads wear right down, the pads can fly out and trash your whole world. Seen the aftermath of this on a KX250. Ruined the caliper the disc and the rider.

Second, never ever machine up any car disc for a motor bike. They are made out of different grade steel. Car discs can be big and heavy and are made out of far crapper grade of steel. Machining them down below their stated minimum thickness weakens them considerably. Very very bad idea. Im sure the machinist would tell you so.
Hence your 70 quid difference.
Car discs are cast, very few motorbike discs are and if they are its highly likely that its a totally different grade of material.

If you are fitting a new disc fit new pads as well.

Sure they are expensive, both good pads and discs. But would you like nice brakes that stop you reliably, or would you rather crash into stuff and fall off all the time???

Cheers, Bob.
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chris-red · Have you considered a TDM? Joined: 21 Sep 2005 Karma :

rodeobob · Two Stroke Sniffer Joined: 23 Nov 2009 Karma :

If i remember right im sure some of the old bikes dont have pins.

Not sure what was up with the KX, only one pad came out.

On an old bike with a 6mm+ disc if its allowed to get that worn.

Dont know why you guys are picking points.
The point im trying to make is, cut corners and you die.

Need a better reason to do it right???

Bob.
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Various IT/DT175 engines for 'projects'

Robby · Dirty Old Man Joined: 16 May 2002 Karma :

chris-red · Have you considered a TDM? Joined: 21 Sep 2005 Karma :

Willson · Traffic Copper Joined: 26 Jun 2010 Karma :

I agree with Bob, don't skimp on your brakes.

Buy a decent disc it will pay you dividends, instead of buying second hand discs etc, and finding you've got to replace them once a month, or worse.
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rodeobob · Two Stroke Sniffer Joined: 23 Nov 2009 Karma :

Paxovasa · World Chat Champion Joined: 25 Apr 2010 Karma :

I know that brake pads have to have a minimum of 3mm on them for a MOT.

Does anyone know what the limit is for discs?
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rodeobob · Two Stroke Sniffer Joined: 23 Nov 2009 Karma :

Paxovasa · World Chat Champion Joined: 25 Apr 2010 Karma :

Teflon-Mike · tl;dr Joined: 01 Jun 2010 Karma :

rodeobob · Two Stroke Sniffer Joined: 23 Nov 2009 Karma :

Paxovasa · World Chat Champion Joined: 25 Apr 2010 Karma :

OK Bob,

My Suzuki GSF600 K3 (2003), what is the wear limit?
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rodeobob · Two Stroke Sniffer Joined: 23 Nov 2009 Karma :

A 600 bandit.

Look it up yourself.

The link is there.

Bob.
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Paxovasa · World Chat Champion Joined: 25 Apr 2010 Karma :

m0l0t0v · World Chat Champion Joined: 20 Sep 2006 Karma :

rodeobob · Two Stroke Sniffer Joined: 23 Nov 2009 Karma :

Wanna try again there m0l0t0v, sorry i read it a few times but i cant make sense of it.

I gather you are saying that an experienced bike rider wouldnt be having front brake failure because he would be sorting things out before the got that far.

Paxovasa, Wear limit would be the minimum thickness wouldnt it???

Bob.
____________________
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Various IT/DT175 engines for 'projects'

m0l0t0v · World Chat Champion Joined: 20 Sep 2006 Karma :

How often do the front brakes fail? Hardly ever. A huge number of members on this forum and I can only think of one instance where the front brakes failed. It's very rare.

You're saying an experienced rider wouldn't agree. So by that logic an experienced rider has several front brake failures. Not because he would be sorting them out, rather did he create the problems that created the failures?
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Pete. · Super Spammer Joined: 22 Aug 2006 Karma :

Most discs have the minimum thickness stamped in them. If they are on or below that then there's an increased risk of warping or even complete failure. The mass isn't there to soak up the energy from braking causing the disc to over-heat.

Put simply if your used disc is appreciably above it's wear-limit (minimum thickness) and not warped there's no reason why you can't use it.

I never heard of anyone machining a cast-iron car disc to fit a bike but that doesn't automatically make it a dangerous exercise. That would have to be decided on a case-by-case merit.
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rodeobob · Two Stroke Sniffer Joined: 23 Nov 2009 Karma :

m0l0t0v, whats your point???

Are you just trying to make argument for the sake of making argument. You are still not making much sense im afraid. Maybe im too dumb. Quote something or whatever, im not following you.

Theres only one way to fix a bike, thats the right way.

As far as the comment about the use the rear brake comment that Robby made. Front brake failures are extreemly rare, that is true. And in that instance you have no other option other than to use the rear brake. 99.9% of the time that will result in an accident as with the panic the rear weel will lock, most of the time you'd run out of braking room as you were braking for a reason and the rear brake is at best only about 20% efficent and with the back wheel locked the rear would slide out and you would have a big crash, or maybe a small low side.

Perhaps i was taken out of context, not trying to victimise Robby for his comment, but i was more getting at the fact that with his brake solution he offered up, he would more likely than not be experiencing such and accident for the sake of 70 quid. I got the impression that he was expressing that this would be a fine out come, save 70 quid and have a front brake failure.

I dissagree.

Bob.
____________________
DT200R '91
DT175 '91
Various IT/DT175 engines for 'projects'

Pete. · Super Spammer Joined: 22 Aug 2006 Karma :

Ingah · World Chat Champion Joined: 10 Apr 2009 Karma :

Seem to have sparked a little debate here...

So on reading through, to summarise what i've taken from it all...:
* As i don't know what i'm doing with brake discs, stick to the correct bike discs. NP, i'm not one of those people that feels safe trying to get someone to bodge me a replacement anyway.
* Scoring / warping / thickness / size of the lip, are all important factors when buying a used disc.
* That 5mm thickness disc is probably not going to have much life left in it. So i'm probably best to look for another one instead.
* It'd probably be wise to fit the new pads i require at the same time as the new (used) disc. When i can get ahold of one.
* If my front disc fails i'm probably dead, because my rear drum brake is absolutely rubbish.

The problem is that new OEM disc is £110. TBH, i can't nearly afford it yet - because i've spent too much money on OEM components when i could've been saving it elsewhere. So i'll just end up leaving it for now and be on the lookout for a good used disc. I now understand that a new disc would be a safer bet, but sometimes you just can't afford a new one there and then. And if i leave it until i can, i'm more likely to come a-cropper.

I didn't understand that site either:
Part No 20.028
Description MGear Brake Disc - Solid
Fitment Front right

OD Ø mm ID Ø mm BHC Ø mm No of BH BH Ø TH min. TH Offset
296 144 166 6 10.5 5 4 0

What does all these unexplained abbrevations mean? (i guess TH min. is obvious as it matches the 4mm i'm aware of.. but the rest is gobbledegook to me).
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-- Ingah