Drilled disk cracks - when to change
Discussion
Hi all
My MY17 C63 AMG went in for service and the pads need changing as they've got too hot and the material looks a bit crumbly on the edges, plus wear. No issues with feel, vibration or performance.
But my discs have tiny hairline cracks coming from the drilled holes on the fronts. Mercedes reckon they need to be changed, but they aren't at the minimum thickness and the cracks are only surface cracks, ranging from 2-5mm long. They don't stretch to the outside and none of the drillled holes join.
What's people opinions on changing them? I see a lot of performance cars with light surface cracks on drilled discs, especially all ages of modern Porsches.
Although I've not done many miles, they are consumables and it's obvious they aren't designed to withstand the heat from 15 minutes of hard use.
Waiting for pictures from dealer or good light to take some myself.
Thanks in advance
My MY17 C63 AMG went in for service and the pads need changing as they've got too hot and the material looks a bit crumbly on the edges, plus wear. No issues with feel, vibration or performance.
But my discs have tiny hairline cracks coming from the drilled holes on the fronts. Mercedes reckon they need to be changed, but they aren't at the minimum thickness and the cracks are only surface cracks, ranging from 2-5mm long. They don't stretch to the outside and none of the drillled holes join.
What's people opinions on changing them? I see a lot of performance cars with light surface cracks on drilled discs, especially all ages of modern Porsches.
Although I've not done many miles, they are consumables and it's obvious they aren't designed to withstand the heat from 15 minutes of hard use.
Waiting for pictures from dealer or good light to take some myself.
Thanks in advance
Don't use drilled discs, unless they are OEM and expensive - even then, you're best IMO avoiding them. They're a popular fashion accessory but they're much more susceptible to thermal stress cracking than plain/grooved discs. The vast majority of discs are cast iron, which isn't strong enough to tolerate much stress and will crack anyway if they get very hot, but will crack much quicker if drilled. You can get much stronger discs made from steel but if you're using these you will also be paying a lot more.
If you have a problem with pads overheating then grooved discs are all you should need - and it only needs half a dozen grooves per side to do the job. If you don't have an overheating problem then plain discs will be cheaper, give better braking, less wear and less noise.
Having said that, fine cracks as you're describing are quite common on any type of disc that has been overheated and I wouldn't be concerned about them.
If you have a problem with pads overheating then grooved discs are all you should need - and it only needs half a dozen grooves per side to do the job. If you don't have an overheating problem then plain discs will be cheaper, give better braking, less wear and less noise.
Having said that, fine cracks as you're describing are quite common on any type of disc that has been overheated and I wouldn't be concerned about them.
100% agree, on my (previous) e36 track car I tried drilled from different manufacturers & ALL had cracking issues, ditch them & get grooved, problem solved.
Even without my previous experience I would change them immediately, just what if the cracks get worse between you checking & the disc fails????
Not worth thinking about.
Even without my previous experience I would change them immediately, just what if the cracks get worse between you checking & the disc fails????
Not worth thinking about.
As above, we need pics.
But cracks unless they do run width of the disc and out to the edge...not usually anything of much concern.
But definitely check pad material for integrity. All too often pads have a habit of falling apart or coming undone from the backing.
I'd be more concerned about that than the actual disc. And of course it depends how hard you intend to use them in the near future.
But cracks unless they do run width of the disc and out to the edge...not usually anything of much concern.
But definitely check pad material for integrity. All too often pads have a habit of falling apart or coming undone from the backing.
I'd be more concerned about that than the actual disc. And of course it depends how hard you intend to use them in the near future.
Mercedes have a spec for the crack lengths, they're being lazy not looking it up, I'm sure it was something like 13mm, I know long, but really it was generous, and came about from enquiries on crack visibility back on sl55 and cl55, people trying to claim defective discs when they were serviceable.
CrutyRammers said:
Having "acceptable" cracks in a spinning component? Ugh, no thanks.
If the discs have got hot, they'll probably have fine surface cracks. If you took the view that no cracks were acceptable, you'd be replacing the discs after every trip. That would be a waste of time and money. In practical terms fine surface cracking is not unusual and these cracks don't matter until they get big enough to weaken the disc significantly.CrutyRammers said:
I think I'd be pretty unhappy seeing a 1/2 inch crack in a brake disk though.
Me too!You have to be reasonable about it but I agree it's better safe than sorry - I've seen cars written off after a catastrophic brake failure when the driver got complacent about the state of their discs.
It's really a matter of practicality. The alternative materials e.g. steel or ceramic are massively more expensive and can introduce issues of their own. Cast iron is cheap and easy to make and has properties well suited to brake discs. They do suffer this problem when overheated, but then so do brake pads, brake fluid etc. These are all consumables and need replacing sooner the harder the brakes are used, you just need a bit of common sense about how aggressively you are going to maintain the brakes.
Thanks for all the replies. Taken me a while to get time and light on my side for decent pictures, albeit it iPhone. I'm thus far not too concerned about the fronts as the cracks are so light, but as intended, open to more experienced opinions.
I am yet to find any cracks on the rear disks which Mercedes reported. So I kind of feel they don't really know their product and just trying to make more work (they where quiet and did quote RRP).
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I am yet to find any cracks on the rear disks which Mercedes reported. So I kind of feel they don't really know their product and just trying to make more work (they where quiet and did quote RRP).
[url]|https://thumbsnap.com/XMJfarbl[/url][url]|https://thumbsnap.com/lTdS5qS7[/url][url]|https://thumbsnap.com/WkKn94a2[/url]Nothing particularly bothered me apart from that second to last one OP where the cracks are joining up between the two drilling holes with another crack propagating out the other side towards the next hole. I’d be wary of that one, although most likely still ok
They’ve not worn spectacularly well though, I’d bed in gently if you’re not going to change them
They’ve not worn spectacularly well though, I’d bed in gently if you’re not going to change them
FunMeterAMG,
Should have gone to SpecSavers?
There are many cracks in your discs. The third one down shows them going through at least two holes.
Plus holes filled with disc dust/mud, so inoperative.
If they operate anyway. They are supposed to "vent gases heated by the pads" but as the pads cover both ends of the drilling, how?
And bedding-in to conform the pads to disc scoring is a minor effect. Far more important is to condition the pads by heating them, to the highest temperature they will encounter in use, but progressively braking from higher speeds until you reach the highest you will use them at. takes minutes only, if you can find the right toad to do it on.
John
Should have gone to SpecSavers?
There are many cracks in your discs. The third one down shows them going through at least two holes.
Plus holes filled with disc dust/mud, so inoperative.
If they operate anyway. They are supposed to "vent gases heated by the pads" but as the pads cover both ends of the drilling, how?
And bedding-in to conform the pads to disc scoring is a minor effect. Far more important is to condition the pads by heating them, to the highest temperature they will encounter in use, but progressively braking from higher speeds until you reach the highest you will use them at. takes minutes only, if you can find the right toad to do it on.
John
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