flak monkey
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| posted on 4/6/10 at 07:57 PM |
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Rear brakes for 13" rims
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....
Sera
http://www.motosera.com
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eddie99
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| posted on 4/6/10 at 08:02 PM |
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You continue to impress me David, nice job!
http://www.elitemotorsporteng.co.uk/
Twitter: @Elitemotoreng
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NOTE:This user is registered as a LocostBuilders trader and may offer commercial services to other users
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daniel mason
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| posted on 4/6/10 at 08:56 PM |
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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!
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flak monkey
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| posted on 4/6/10 at 09:06 PM |
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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.
Sera
http://www.motosera.com
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matt_gsxr
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| posted on 4/6/10 at 10:04 PM |
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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
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James
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| posted on 4/6/10 at 10:35 PM |
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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
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"The fight is won or lost far away from witnesses, behind the lines, in the gym and out there on the road, long before I dance under those lights."
- Muhammad Ali
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flak monkey
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| posted on 5/6/10 at 07:58 AM |
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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.
Sera
http://www.motosera.com
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flak monkey
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| posted on 5/6/10 at 08:20 AM |
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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
Sera
http://www.motosera.com
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matt_gsxr
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| posted on 5/6/10 at 09:02 PM |
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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!
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phelpsa
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| posted on 6/6/10 at 10:10 PM |
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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]
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flak monkey
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| posted on 8/6/10 at 06:16 PM |
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And proof they do fit....just!
 
Brake clearance
Sera
http://www.motosera.com
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