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question about disc sizes on front and back
mads - 27/2/11 at 03:11 PM

hey up all,
loads of questions from me this weekend (sorry!)...

Got the discs from RD when I participated in the Wilwood brake kit group buy.. I cant remember which one they said goes where though and they are closed today.

The two sizes are 255mm and 270mm. The 255mm is a deeper disc so thinking these should go on the rear, particularly as they are smaller. However, the 270mm ones have less contact space for the pads, so should they be going on the back?

Cheers,

mads

tomgregory2000 - 27/2/11 at 03:17 PM

bigger on the front

mads - 27/2/11 at 03:27 PM

here are some pics in case they help...

255mm disc

255mm disc
255mm disc


270mm disc

270mm disc
270mm disc

mookaloid - 27/2/11 at 04:37 PM

Think of it as leverage - the bigger disc has better leverage and goes on the front.

pekwah1 - 27/2/11 at 04:45 PM

i know as a rule of thumb you want more braking on the front, but is this always the case?
I have found that on all of the RWD cars i've owned, they've always had bigger discs on the rear than the front, not by much but slightly bigger.

I gues this is down to the drive being through those wheels and needing the extra force?

phelpsa - 27/2/11 at 05:18 PM

quote:
Originally posted by pekwah1
i know as a rule of thumb you want more braking on the front, but is this always the case?
I have found that on all of the RWD cars i've owned, they've always had bigger discs on the rear than the front, not by much but slightly bigger.

I gues this is down to the drive being through those wheels and needing the extra force?


Often to do with the handbrake. You need a nice big disc to get enough leverage for the handbrake to hold the car on a lot of modern cars, or the centre of the disc is a drum for the handbrake (like subarus) requiring a large disc.

adithorp - 27/2/11 at 05:27 PM

quote:
Originally posted by phelpsa
quote:
Originally posted by pekwah1
i know as a rule of thumb you want more braking on the front, but is this always the case?
I have found that on all of the RWD cars i've owned, they've always had bigger discs on the rear than the front, not by much but slightly bigger.

I gues this is down to the drive being through those wheels and needing the extra force?


Often to do with the handbrake. You need a nice big disc to get enough leverage for the handbrake to hold the car on a lot of modern cars, or the centre of the disc is a drum for the handbrake (like subarus) requiring a large disc.


They've probably equiped with a load valve so do work the rear brakes hard when fully laden.

In this case the big ones go on the front. If they're Sierra convertions from drums then you'll have to have the rear hub flanges reduced in diameter for them to fit.

MakeEverything - 27/2/11 at 05:43 PM

Although the larger discs have a smaller surface area per pad, it has a greater radius. You should be able to work out the area of each discs surface contact, but the larger discs should go on the front IMO. Even if they do have a thinner contact surface, the extra diameter will help prevent fade under heavy braking.

mads - 28/2/11 at 07:13 AM

cheers folks. looks like on the front they go!

hughpinder - 28/2/11 at 09:44 AM

Be careful - the braking force is proportional to:
The area of the piston(s) in your caliper, and the disk radius of the disk to the centre of the pad.
Its inversely proportional to the diameter of the master cylinder.

The reason the bigger disk is sometimes on the back is to make the handbrake work more easily. You will find the rear caliper piston diameter is smaller so that the foot brake does not overbrake the rear.

You should be able to offer those disks up to the hubs and hold up the calipers - they'll only fit with one set of calipers, as they are quite different in diameter and offset.

Regards
Hugh

(to give a reasonable assessment we would need the above info, plus the weight balance of your car.)