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Discussion Starter #1
Hi all,

I recently put my bike into a dealer to have the front brake callipers refurbished. One of the pistons had been sticking. When I got the bike back they pointed out that one of the discs was cracked and would have to be replaced. The bike has done just 17k miles and this is the second time that disc (Left side) has been replaced. Anyone got any clues as to the possible cause of this? The crack runs from one of the outer holes to the rim.

By the way, for UK riders, MTC bikes of Reading did the calliper refurb for just £60 and were very helpful.

Michael L.
 

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This is not a common problem I have seen discussed on here before. The disc on mine have 91,000+ miles on them now and have shown no sign of cracking.

I don't have a clue to what caused it other than maybe a dragging pad causing it to over heat or the bolts holding the disc on not being torqued correctly or evenly.
 

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Have them make 110% certain the mounting points on the wheel are clean, and have no burrs or deformities that would place stress on the disk rotor when it is torqued down...
 

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Michael

I am really sorry to learn of this, and sorry not to have noticed it while you were down here.

I do not recall any reports of craked discs on here. There are two things to spring to mind.

1. Was it an OEM (genuine part that was used).
2. Maybe the sticky piston has been sticking for a while and has been inducing a side loading/overheating/disc oscillating situation which has lead to a fatigue crack.

£60 is a fair price for the work by the way - pity they did not call you for permission to swap out the disc while they had the calipers off.
 

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Discussion Starter #5
Hi Norman,

Yes it was OEM fitted by a Suzuki dealer at 6k miles. Actually its the left disc i.e not the one relating to the sticking piston. MTC couldn't replace it at the same time because at the moment there are no replacement discs available in UK. They will have to get one from Europe somewhere.

The odd thing is that the crack is near the outer edge of the disc, not near any of the fixing bolts which is where I would expect any stress to start.

All the best

Michael
 

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Very strange - could be a manufacturing issue.

However, this link suggests that cracks are not unusual in service - linky and this from Wiki :

Cracking is limited mostly to drilled discs, which may develop small cracks around edges of holes drilled near the edge of the disc due to the disc's uneven rate of expansion in severe duty environments. Manufacturers that use drilled discs as OEM typically do so for two reasons: appearance, if they determine that the average owner of the vehicle model will prefer the look while not overly stressing the hardware; or as a function of reducing the unsprung weight of the brake assembly, with the engineering assumption that enough brake disc mass remains to absorb racing temperatures and stresses. A brake disc is a heat sink, but the loss of heat sink mass may be balanced by increased surface area to radiate away heat. Small hairline cracks may appear in any cross drilled metal disc as a normal wear mechanism, but in the severe case the disc will fail catastrophically. No repair is possible for the cracks, and if cracking becomes severe, the disc must be replaced.


We do know you had a defect and maybe you have been unlucky.

It may be the caliper is a bad one in the sense that even though rebuilt it has a tendancy to become lazy off stroke.
 
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