Pitting discs

[JohnC]

I noticed my rear disc had some pitting after the 6000 mile service, now the front discs are going the same way at 8000 miles. Is this a manufacturing flaw or environmental damage? I have a Kwak with spondon discs and they are like new with over 20K on them. Is it worth mentioning to my dealer?

 

[Doughboy]

Looks like a porous casting to me - yes a manufacturing flaw. Personally I WOULD go back to the dealers.

 

[thornley]

Bike stood in one position for some time? The pitting matches your pad profile by the look of it.

Classic grain boundary pitting, not a porous casting. These discs are made from profile cut stainless plate, Its caused by the material not being in the correct solution annealed condition, this causes migration of the nickel and chromium from the grain boundaries during the rolling process, leaving the boundaries of the grains lacking any of these protective alloys. Solution annealing should be the final part of the process. material is heated to1050 to 1150 Centigrade dependent on grade and quenched, putting the alloying materials back into solution, it is then pickled to remove any surface scale.

It basically rusts at the grain boundaries if left for any time opening up tiny pits as it goes. Stainless steel will rust, most noticeable on welded joints that have not been pickled and passivated to remove the surface iron after the heating process.

Not environmental, although you could dry your discs and turn the wheels regulary to remove moisture. Ask the dealer if he believes that 8,000 miles is the life of a disc. I personally think that properly solution annealed they should last a lot longer. Poor manufacturing process and quality control on raw material.

 

[JohnC]

Thanks for the replies both, Thornley, the bike is used everyday, weekends included so no, it never sits for long. The pitting is all over the rear disc, not just in that position and the left front disc has hardly any pitting. I'll speak to the dealer this week. If this is the life of GS discs then I'm afraid it will need to go as I'll be doing 20K + pa and I'll be buggered if I buy two sets of front and rear discs every year. My Audi A4 has 100K on it with the original (non-pitted) discs, and as stated my 20K spondon discs look like new. Ultimately how does this affect the performance/life of the disc?

 

[thornley]

Your Audi will have cast ventilated discs. they are not stainless steel.
Pitting will cause faster pad wear. Ultra thin bike discs also suffer from heat build up, cooling and heating rapidly which can increase surface pitting.

Discs on your bike are a balance between cosmetics and function.

 

[JohnC]

Thanks again, forgot the car would be cast iron. There is no noticable wear in the discs, i.e. no lip or ridges. Would this add to the 'faulty manufacturing' theory? Do you work in metallurgy btw?

 

[thornley]

Thanks again, forgot the car would be cast iron. There is no noticable wear in the discs, i.e. no lip or ridges. Would this add to the 'faulty manufacturing' theory? Do you work in metallurgy btw?

Ex Production Manager Haynes International, Specialist alloys for Chemical and aerospace applications. Apprentice mechanical Engineer through ONC, HNC, few detours (9 years HMP) before BSc (hons) and C.Eng. Before moving on to several other Material companies, Managing Director and cop out to running the French house.

Fault I believe is nothing to do with wear and everything to do with incorrect heat treatment of the stainless used in the manufacture of the discs. Pitting can also be caused by overheating the disc during use causing the same grain boundary migration of the nickel and chromium at the surface leaving the iron behind around the grains to rust.This happens at a macro level and can only be seen initially under a microscope, but increases rapidly once it starts.

Quite simply stainless steel is not the best material for discs but it is a balance between good looks and practicality. Remember cast iron discs on bikes?, some still fit them to the airheads as replacements, superb grip but look crap once out in the rain. Mileage not such a factor here as much as age, pitting is a slow process but increases rapidly with the application of heat and exposure to salts, damp storage etc.

I would ask you dealer to state if he thinks that the mileage you've had is reasonable.

 

[JohnC]

Thanks again Thornley, I'll let you know the response.

 

[Packer]

Similar problem with my 1150gs, front discs replaced twice under warranty.

Out of warranty now and they have done it again but I'll just monitor it and if it looks like it's getting too bad I'll replace them with non BM parts as all the BM items seem to be very poor quality in this respect.

 

[JohnC]

Packer, it must be something in the air up here m8, i.e. the huge amount of piss that falls on us

Seriously, what was your mileage, wear rate, braking style, condition compared to mine etc, and who was the dealer (pm if you want) and what was their overall response with regard to the pitting. I'd be asking for a good will if this is the third time they have pitted. I'm with Thornley in that it's a manufacturing defect. I had a close look at the left front disc and it seems fine atm, so it might not be a generic problem. I'm sure BM will sort it out anyhow

 

[JohnC]

Well I took the bike in to the dealer today. No decision as yet, it's with the MD atm but I'll let you know soon how I faired.

 

[JohnC]

Discs to be replaced. No quibble. They have seen only a few BM discs pitting.

 

[thornley]

Excellent, well done. They really couldn't argue that it was reasonable for this to happen.

 

[JohnC]

Again, thanks for the info thornley. My dealer, Motorrad Central West is replacing all three discs on the bike with all the fittings, 'and' all the pads. It's refreshing to know that MC are so pro-active when faults are found to be genuine, to the extent of replacing all the discs and pads when perhaps others would go down the 'fair wear and tear' route. I offered to supply my
own pads and was told, not required ‘that’s what your warranty is for’. Well done all at MC