Why do brake discs/rotors warp

ngiles

ngiles

Senior member
Location
Sydney, Australia
My Jota has had an irritating judder when coming to a halt.  The issue seemed pretty obvious because the discs on both sides of the front wheel had a slightly discoloured section which seemed to indicate the point where the brake pad is not coming into contact with the disc.  Using a dial gauge confirmed this - they have warped at the outer edge, not so the inner edge or the carrier.

So I guess the simple answer is to buy a new set, but I am curious why the things warp in the first place as I wouldn't want a repeat, all the more so as both have warped (almost at 180 degrees opposite one another).  The bike was fully restored and my very vague recollection was that the brakes did not judder just prior to stripping the whole thing down - but I only rode it a couple of times so I may be wrong.

Secondly stainless or cast iron?

Cheers, Nick
 
Apparently its the solid mounting, any expansion from heat only has warping as a place to go. That's why semi and fully floating discs were invented, so the expansion could happen and the force could be disapparated. As to cast iron versus stainless steel. And here is my favourite word again, APPARENTLY, cast iron works better when it rains. But I have had piss poor brakes in pissing down rain in both materials, so I have no ideas. I was told recently I need to deglaze my discs with a scotch bright. We will see. It's a rare event as it takes massive rain for this effect. BTW I have no warping on my solid 280mm discs fitted with 4 spots and remote reservoir that work powerfully as I try and avoid using them much, I try and maintain corner speed and a little bit of engine braking to reduce charging into and out of bends. Less chance of speeding tickets helps as well.
 
So that's close to Sydney, the brakes get hammered when well away from the public, eg Oxley ect
 
Did you check if the hub mounting surface is is true and  no buggered up raised areas around the stud holes from over torquing the carrier bolts? A small deviation at the hub is  much more at the outside of the disk.
 
Fully restored, were the wheels painted where the disc rotors mount?
 
I've heard that parking the bike while the disks are still hot can cause warping. The theory being that the bit of disk covered by the pads stays hot while the rest of the disk that's exposed to air cools down, and you can end up with a permanent warp. I dunno if the theory has any merit, but I suppose it's something to think about. If you've been hammering the brakes on a bit of twisty mountain road, ease up on the last couple of km to give the disks a chance to cool down evenly before you stop at the pub. With racing you usually get a cool-down lap after crossing the finish line.
 
Thanks for the responses.  I will pull the wheel apart and check for any uneven surfaces on the wheel or carriers - assuming there is nothing evident I'll contemplate replacing them.

I developed a habit of not pulling brakes on for long periods if I can avoid it - on push bikes long downhills create a lot of heat on the rim and has been known to pop a tyre, so it makes sense to let the rim cool briefly between braking

Ta, Nick
 
I don't brake very much.
When we first met, my future wife who had a 125 at the time and would follow me asked why I never braked. She hardly ever saw my brake light.

My Gilera Saturno had 93 000 km on it when it was incinerated. Still on the original front brake pads. I still have a spare set.

Paul
 
If the warp is only a few thou, you may be able to get the disks ground. The old cast iron disks on Laverdas have plenty of thickness. I think you can take about half a millimeter off them and still be within the thickness limit. That's what I did with mine several years ago and the braking feel smoothed right out. The bloke who did mine is retired now, but it shouldn't be too hard to find someone who can do it.
 
I spoke to Melissa from Metal Gear in brizzy, she advises they are about to do a production run of replacement jota discs, they will be available mid to late august.  They are $250 each.  They also offer a disc grinding service, $100 per disc. 
 
Thanks for all the responses - so I'm going to buy a pair and at the moment have the options of:


- brand new goodrich with carriers but these are floaters and use a different, gold carrier - look nice but move away from the originality of the bike
- brand new stainless discs from Ducatipaddy - these are 5mm thick - he advises the reduction in thickness is compensated for by the material - stainless steel
- brand new cast discs from metal gear - due in sometime soonish
- a well used cast pair which have worn to 5.5mm - not yet fitted so may have their own issues


Price wise the differential is not the biggest deal - I'm probably inclined to the stainless ones - only question is the 5mm measurement, where the wear limit for the original casts is stated at 5.6mm, although that is for cast iron.

2 questions:
1. Is there any difference between left and right discs - some places list one version to fit both sides - the stainless ones are a single version
2. Any views on the difference between stainless and cast, and in particular is the thickness and issue??
 
ngiles said:
- brand new cast discs from metal gear - due in sometime soonish

1. Is there any difference between left and right discs - some places list one version to fit both sides - the stainless ones are a single version
Click to expand...

Metalgear rotors are stainless, I have bought a few sets from them and have 4 sets on order

Metalgear do left and right hand versions, when they have them in stock so the drilling pattern is correct
 
Dellortoman said:
They drill the left disks with a left handed drill bit.
Click to expand...

Can't you just turn the disk upside down? Or are we talking one-piece disks with the carrier integral to the braking surface?

bazzee

Where I work we have pillar drills driven by 3-phase motors, they are in fact reversible. One day a student complained that the drill bit was blunt, because it wasn't cutting  into the workpiece he was drilling. Turns out he had the motor in reverse...
 
Stop asking sensible questions bazzee. There's no place for that sort of behaviour on this forum.  :D

I think the inside edge of the big hole in the disk that fits over the carrier has a chamfer on one side to accommodate the inside radius on the shoulder of the carrier. But there's no reason you couldn't chamfer both sides of the disk to make it reversible.
 
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