Machining rear disc ? Can I

M

musicswapshoppe

Rocker
With the help of "The Don" (Thanks mate) I have my gaitors finally on !

Also removed the back wheel as the brake disc has a lot of marks where the previous owner must have let the pads wear too thin.

So I am either replacing the disc, and the sprockets or (if possible) just taking disc to a machinist. Anyone here done this? The disc has "min th 5mm" written on it. So am assuming that is the minimal thickness the disc can be machined to. Am I assuming right?
 
B06Tang

B06Tang

Cafe Racer
That sounds right although I can't say 100%; my manuals are still sitting in some boxes that hasn't been explored into yet as we are moving in. If I was you though, I think I would just say screw it and get a new disc. They can't be that expensive and the piece of mind would be worth it. Don't wan't to be screwing with your with your breaks like that unless you have to. Am I going to have send a guardian bell out your way to keep you safe? :D
 
O

otherwise

750cc
The recommended min is usually calculated with a bit of a safety allowance. I'd say if the disc is machined, and to achieve total removal of marks going under by a few thou is required it's no big deal. On the other hand if you machine down to the recommended min and a few marks are still seen, that's also no big deal, most of my bikes end up with marked rear discs and I always stop OK. I machined my race bike discs down to about 60% of factory spec then remove the same amount from the two mating caliper halves, all in the name of weight reduction, worked on the track, thin discs especially on the back isn't a big deal. If your caliper pistons are pushed too far out packers can be used between the piston and the disc pad. Lots of things can be done, the book is conservative to protect the factory from the law. On the other hand if you aren't sure about these things go with the recommendations.
 
78gold

78gold

Two Stroke
Gaitors

Dave,
Glad to hear you have gaitors on. Mine are still sitting here in a box.
I havnt done any on Bonnie but had M/Cycle discs machined in past and never had an issue.
I have a copy of the manual if interested I can look see if any details on how it can be done and how much can be taken off.
Regards
Nigel
 
O

otherwise

750cc
tbirdsp said:
Rear brake? Unless you are just doing it for looks I wouldn't even worry about it.
Click to expand...

I tend to agree, that's what I was getting at when I said "most of my bikes end up with marked rear discs and I always stop OK". I trail brake into most corners and while I've never allowed metal to metal the discs take a hammering. It's a matter of experienced judgment, what looks a bit marked up compared with what is chewed and mangled. A gauge as to machine, change or leave, is are you going through pads too fast.
 
M

musicswapshoppe

Rocker
As it is already off. I will this time get it machined (If price not too high) As the marks are fairly deep and at least the pads will wear more even. But take yourpoint as I really dont use a lot of back brake for most riding. Thanks for input all
 
ssjones

ssjones

750cc
Here in the states, it's about $30 to be a disc "turned" as we call it. Most good shops will only turn a disc if it stays within the spec on the disc (5 mm), obviously they don't want to be lible. I gave up turning discs for the cars and just replace the disc (here, the price is similar for an autoparts disc ($30 or so). They just warp faster. My machine shop guy says most cheap discs (car) are warped right out of the box (made in China, etc.).

I just mic'd both of mine, and each is still within the factory spec w/36k on the odometer. A front disc for my America is $220 from NewBonneville, yikes!
 
KingBear

KingBear

Hooligan
How deeply scored is the disc? Just because there are a few shallow grooves doesn't mean they need to be machined. I only turn my rotors every other time I change the pads (at the most), and then only if the grooves are deep and rough. The pads wear into the grooves quickly for positive stopping power.

Shops will want to machine your rotors every time you change the pads because they can charge you for it now, plus shorten the life of the rotor so you'll have to buy a new one sooner. They know damn well your rotors might not need turning.

Anyway, just use your own good judgment.
 
M

musicswapshoppe

Rocker
The scores are fairly deep but to be honest I could get away with just fitting new pads and would not effect their life span much. I guess as the scoring was already there I would like to get rid of them. So I will (as i said) take them to a machine shop for a quote.
On another note. I am changing both sprockets as well. Normally people change chain at same time. Whats the reason for this as I was just gonna change sprockets?
 
M

musicswapshoppe

Rocker
Well took disc to an engineer and thickness is already 4.6. So is below minimum before I start lol
Gonna just keep it and fit new pads, chain and sprockets.
 
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