Changing Discs & Pads, why both?

I'm looking to change my discs. Whats the reason for having to change the pads at the same time? Obviously you can change the pads without changing the discs so what difference does it make with the discs?

My discs are fubarred but my pads are nearly mint on the rear. I'm looking to get a 2nd hand set or possibly a new set but i have been told if i go with new i need to change the pads. I take it the same can't be said with the 2nd hand option?

I can't get my head round all this lol.

Not sure mate.

No doubt I will be corrected, but it may possibly do with the fact that if you change pads on the same disc the pads can easily form to the disc maintaining a good contact ratio and thus performance. The other way around and any imperfections, such as grooves from the previous discs etched onto the old pads, will only act to ruin the surface of the new disc and reduce the contact area of the pad on the new discs? In this case reducing their efficiency.

All speculation and you have no doubt thought the same, but thats my opinion. Other than that much of it is probably just precautionary.

That was along the same train of thought as myself but it kind of contradicts itself. New pads will form to the disc yet old pads will form the disc?

Only OTHER thing i can think of is that after the pads have worn a little they may have glazed a bit firmer than when they are new hence the reason they MAY tend to wear the disc a bit more.

I'm clutching at straws though. I just don't want to do something because everyone says to do it with no logical reason why lol.

I see your point about the contradiction. I did a quick search on the inter-web and nothing came up of use. I would probably stick with the idea about the glazing and reduced performance and accept for peace of mind changing pads with discs is just the done thing.

Get your old rears off and checked (dunno where!) to see if they have started to assume shape (any indentations etc.) of old disc. If safe enough, I would have thought they'd be ok to keep using?

My rear discs were mauled in the end and the pads I had took a beating because of them and had dips. The reason I didn't use these - still relatively new - pads was that they were indeed missing contact area, so wouldn't work right and could have harmed new discs.

Depends how the face of the pad is for ya I guess really Scott!

Agreed, if you swap disc or pad individually you will have a mismatch until you wear them in which can take a very long time. If you're worried about it just rub your old pads flat with emery cloth on a flat surface. Doesn't take much and will also remove any glazing that may have occured.

Agreed, if you swap disc or pad individually you will have a mismatch until you wear them in which can take a very long time. If you're worried about it just rub your old pads flat with emery cloth on a flat surface. Doesn't take much and will also remove any glazing that may have occured.

Bingo, thats exactly what i did in work tonight lol. I'm still thinking i might just get new pads anyway, money is tight but i really can't be arsed having the calipers in bits (as they are just now) only to have to strip the wheel off and mess around with the pads at a later date.

Might as well do it once

I think I've changed front disks twice as often as the pads, so as long as they aren't a wreck, go for it.