mads
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| posted on 27/2/11 at 03:11 PM |
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question about disc sizes on front and back
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
We gain knowledge faster than we do wisdom!
Life is not a journey to the grave with the intention of arriving safely in a pretty and well-preserved body, but rather to skid in
sideways, thoroughly used, totally worn out and loudly proclaiming... "f*ck, what a trip!"
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tomgregory2000
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| posted on 27/2/11 at 03:17 PM |
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bigger on the front
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mads
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| posted on 27/2/11 at 03:27 PM |
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here are some pics in case they help...
255mm disc
 
255mm disc
270mm disc
 
270mm disc
We gain knowledge faster than we do wisdom!
Life is not a journey to the grave with the intention of arriving safely in a pretty and well-preserved body, but rather to skid in
sideways, thoroughly used, totally worn out and loudly proclaiming... "f*ck, what a trip!"
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mookaloid
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| posted on 27/2/11 at 04:37 PM |
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Think of it as leverage - the bigger disc has better leverage and goes on the front.
"That thing you're thinking - it wont be that."
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pekwah1
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| posted on 27/2/11 at 04:45 PM |
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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?
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phelpsa
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| posted on 27/2/11 at 05:18 PM |
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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.
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adithorp
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| posted on 27/2/11 at 05:27 PM |
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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.
"A witty saying proves nothing" Voltaire
http://jpsc.org.uk/forum/
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MakeEverything
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| posted on 27/2/11 at 05:43 PM |
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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.
Kindest Regards,
Richard.
...You can make it foolProof, but youll never make it Idiot Proof!...
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mads
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| posted on 28/2/11 at 07:13 AM |
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cheers folks. looks like on the front they go!
We gain knowledge faster than we do wisdom!
Life is not a journey to the grave with the intention of arriving safely in a pretty and well-preserved body, but rather to skid in
sideways, thoroughly used, totally worn out and loudly proclaiming... "f*ck, what a trip!"
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hughpinder
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| posted on 28/2/11 at 09:44 AM |
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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.)
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