My new wheels arrive next week so I have started to have a look at fitting rear discs under the rims.

CXRs have the shallowest tyre well of any 13" rim, so they have the biggest internal bore of any cast 13" rim. This gives a radial clearance of 153.5mm inside the wheel.

Sierra calipers on standard mounts are around 163mm or more without any modifiations. In addition the standard 253mm disc is massive for the rear brakes (fronts are only 265mm on the wilwood kit)

So I decided to fit wilwood powerlites to the rear with the built in handbrake mechanism. Caliper bore is 25mm compared to the 32mm ones fitted at the front.

Rear brake conversion kits use 240mm discs, though I couldnt really determine which ones. And these are all designed for drum brake hubs, which I want to do away with so I can retain the bolt on drive shafts for the LSD.

Comparison of the Fiesta Mk3 240mm disc and the sierra 253mm disc - quite a difference in weight as well.


Sierra and fiesta discs

And the fiesta disc has a 10mm deeper bell


Sierra and fiesta discs 2

This is handy if, like me, you need to run high offset wheels to keep them under the body. As the sierra caliper and the wilwoods will foul the rims with anything over an ET30 offset on standard sierra discs.

In order to fit the fiesta discs you have to turn down the sierra flanges to 134mm


Modified hub

The downside of such a small disc is that the hub then requires modification to get the caliper close enough to the centreline. I ground off some of the original caliper mounts to allow the caliper in closer. Hopefully there is still enough meat around the hole to have enough strength.

Then I made up some aluminium blocks to bolt the calipers to.


Caliper bracket

And finally the caliper mounted


Caliper mounted


Caliper mounted

It could do with going another 2mm closer to centreline, but theres not enough clearance.

Even so this set up should give around 3mm clearance to the inside of the wheel without any further modifications to the caliper or handbrake mechanism.


Measurement

And there you have it. A bodged together small rear disc conversion....

You continue to impress me David, nice job!

so the cxr's have a larger inner diamiter than my r500 wheel? what are my options for rear brakes? and what does the caterham use? help is much appreciated thanks!

The CXR has the legal minimum depth tyre well which is set by ETRTO so you might find wheels with the same inner diameter, but nothing bigger on a 13" rim.

Not sure what rear set up caterham use - I have a feeling they are 240 or 250mm discs and use AP Racing calipers.

David,

Nice write-up. I think you have done the right thing, as converting to drum braked hubs is a lot of bother.

So those are Fiesta disks. I have mk3 Escort fronts, which are also 240mm diameter, but actually heavier than the Sierra disks.

Are these ones the BDS4439?
if you wanted to move the disk a further 10mm inboard then you can use the BDS4045, which is an escort Mk3 front disk with similar dimensions. I have these escort disks and they are a bit heavier than the sierra rear disks (boo).

I am planning to go to the even smaller Fiesta mk1/2 small engined disks which are 221mm (plenty big enough in a light car), but I will need to turn down the hub a bit further.

Sorry for the ramble (post pub),

Matt

Fantastic work as ever Dave!

Any chance you could weigh those callipers so we could compare them with the Mk4 Golf callipers that are becoming popular please?

Thanks!

James

Matt - the disc part number is BDC3611 from quinton hazell. The disc is 10mm further inboard than the sierra one (43mm overall height compared to 33mm). If you went any further inboard you would have trouble making a caliper bracket to suit as the caliper bolts would be inline with the bracket on the hub - I see the escort discs are 58mm overall height, which I think would be too much. Though with access to a milling machine you could probably knock something up. I also assume you are using 240mm vented discs if they are heavier than the sierra ones?

James - I will weigh the calipers etc later today and see what the different weights are between the new set up and the sierra set up, that will also allow you to compare the golf ones.

Weights as requested

Sierra Caliper with bracket 2668g
Sierra Disc 3405g
Modified drive flange 1812g

Fiesta disc 3243g
Wilwood caliper 1118g
Caliper bracket 135g


So, for both sides, I make that 12.146kg for sierra calipers and discs or 8.992kg for the 240mm wilwood set up so a saving of 3.154kg for the set up for both sides.

Not a lot. But hopefully enouh to bring the weight of disc brakes to the same as drums....

David

David,

Thanks for those part numbers. I agree on your analysis

In my case I have mk3 escort with that bigger offset. These are on drum braked hubs with adapter plates and fit together fine.

These aren't vented, but are still heavier than the Sierra ones. They must just be a bit chunkier than the Fiesta ones.

Matt

p.s. you didn't include the -ve weight that you gain when you machine down the hubs!

I did the same thing but used 205 GTi calipers and the higher offset discs. I had to notch the uprights as well due to the caliper design but it all seems to work!





[Edited on 6-6-10 by phelpsa]

And proof they do fit....just!


Brake clearance