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What is warping my brake disks? (GPZ500S)

Guest Jel Harry

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I picked up a 1995 GPZ500S last year as a winter hack so my shiny GSXR could stay, well, shiny. After fixing a number of faults and riding it throughout the winter, I took it for it's first MoT and it failed due to a warped front disk. The Kawasaki part was £80+Vat so I opted for a pattern part (prob Chinese) and returned to the testing station. It failed again. Same reason. Grrrr. Then I rebuilt the brakes and lubed up the runners so the calipers slid better and also filed away any oxidisation from the disk seating in the wheel. Returning to the MoT station, it passed - but only just. The brake has got steadily worse since then and is now so bad it's unsafe to ride.

A couple of extra things to point out are that the bike had failed it's previous MoT for a cracked front disk (replaced before I bought the bike) and the bike went into tank-slapper wobbles when decelerating under engine braking between 60mph and 50mpg. I hadn't spotted this on my test ride (D'oh!) but the tyre was knackered anyway so I replaced it and had the wheel balanced and the wobble completely disappeared.

What I don't know is WTF is causing the disk to warp so badly (if that's what it is). I just don't know and I don't want to chuck £80 at it unless I'm sure that's the right thing to do. I tried replacing the bearings but I couldn't get the old ones out so gave up. Any ideas???


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I'd have my money on sticking calipers causing the problem, but cheap discs tend to be more prone to warping anyway. Try a complete strip, clean, and rebuild of the front brakes replacing parts as needed.

In the past I've used performance Kevlar pads (on my car) which were awesome but they caused warpage something to do with Kevlar not being suitable on standard pattern discs, probably not relevant to your problem though...,

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I would only (personally) try to figure out the problem once you had an original disk on. Chinese stuff is cheap, but it is cheap for a reason. My first inclination is that the replacement disk has warped because it is second rate metal. As such it doesn't dissipate heat as well as an OE one and warps.

Stopping is important, buying cheap can be false economy.

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