ChrisGamlin
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| posted on 3/5/07 at 08:31 PM |
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Calculating master cylinder size
Hi all
Can anyone explain / point me in the direction of a website on how to calculate master cylinder sizes for a bias bar twin MC setup?
I currently have Wilwod 4 pots on the front and Escort drums on the rear but Ive never been particularly happy with the retardation despite having
spend good money on things like Pagid pads, so Im now in troubleshooting mode and want to check to see if one/both of my MCs are the wrong size.
cheers
Chris
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flak monkey
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| posted on 3/5/07 at 08:33 PM |
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JAG helped me out immensly with some braking calculations a while back for my dissertation.
If you want to know how I worked it out I can run through the theory with you via msn if you like.
David
Sera
http://www.motosera.com
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ChrisGamlin
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| posted on 3/5/07 at 08:41 PM |
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Cheers David, I don't have all the info to hand at the moment though, was planning on getting the Wilwood piston diameter etc over the weekend
(assuming you need this?)
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flak monkey
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| posted on 3/5/07 at 08:43 PM |
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You can change the cylinder sizes to give you the required braking effort you need. Smaller cylinders will give you a higher braking force, for the
same pedal effort.
You could do some experimentation, depends what you have fitted now, and how much money you have.
You can get all the data you need from the wilwood site.
Sera
http://www.motosera.com
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ChrisGamlin
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| posted on 3/5/07 at 08:45 PM |
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Yeh I thought the Wilwood site had all the info when I looked ages ago, but I can't find it on there now.
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flak monkey
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| posted on 3/5/07 at 08:46 PM |
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If you look in the products it has the relevant info listed.
E.g powerlite caliper:
http://www.wilwood.com/Products/001-Calipers/015-PL/index.asp
David 
Sera
http://www.motosera.com
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ChrisGamlin
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| posted on 3/5/07 at 08:50 PM |
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BTW I have .625 on the front and .75 on the rears which is the recommendation I had at the time, the fronts are quite snatchy though with not a lot of
feel and hard to modulate, so very easy to lock up, and because of this Im thinking maybe a slightly larger piston?
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ChrisGamlin
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| posted on 3/5/07 at 08:53 PM |
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Ah OK, I was looking in the info section which is where I thought they had generic calculations before. Unfortunately they dont do my calipers any
more (Dynalite 2's) so I'll have to have a look for some with the same piston size.
cheers
Chris
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flak monkey
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| posted on 3/5/07 at 09:00 PM |
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If you know the piston size then you dont need to look up the caliper.
The next size up master cylinder is 0.7" from wilwood i believe.
I assume you have played with the balance bar a bit and backed the fronts off a touch? Might be all it needs?
Sera
http://www.motosera.com
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ChrisGamlin
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| posted on 3/5/07 at 09:07 PM |
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Yeh ive played around with it before, but it doesnt seem to improve, compared to friends who have (on paper) similar or inferior brake setups on their
BECs, I reckon I have to brake 20-30m earlier in my car than I can in some of theirs.
I also wondered if it could be partially to do with geometry? I dont run anything special though and its AFAIK corrected book front end, so it
shouldnt be too bad.
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flak monkey
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| posted on 3/5/07 at 09:09 PM |
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Geometry can have an effect on braking distances, especially the amount of camber you are running up front.
If you get all the numbers I am happy to run through some theoretical calcs for you.
Sera
http://www.motosera.com
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ChrisGamlin
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| posted on 3/5/07 at 09:16 PM |
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Ok cheers, will see what I can get at the weekend.
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britishtrident
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| posted on 4/5/07 at 06:53 AM |
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To me it sounds like the rear brakes aren't doing any work.
Your car will have a slight weight bias to the rear so the rears should do about 30% of th braking.
Who ever told you to put a 0.75" cylinder on the rear circuit on an 8" drum braked axle knows sweet fa about brakes.
You need a smaller bore master cylinder in the rear circuit.
Also check you balance bar has enough clearance to allow the bar to swivel -- but not too much that it won't work one cylinder in the event of
a circuit failure.
Also check the handbrake cable isn't over tensioned preventing the rear wheel cylinders from working effectively.
If it were my car I would start with 0.7" cylinders on both circuits and balance bar set dead central.
Try setting the balance by using the balance bar if you have to set the bar more than a few turns off centre and still find the fronts lock
excessively early put the 0.625" cylinder in the rear circuit but with drum brakes there is a limit to how much more braking effort you can get
by cranking up the pressure.
[Edited on 4/5/07 by britishtrident]
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RazMan
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| posted on 4/5/07 at 07:23 AM |
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A lot of your 'feel' problem could be to do with your pedal ratio & disc size so don't forget to put those into the equation. I
have the same front calipers as you and ended up with a 0.625" m/c on the front with a pedal ratio of 4.5:1 which gives me a very firm pedal but
requires a strong right leg!
To reduce the braking effort required I will now have to consider fitting larger discs..... or book more nights down the gym
[Edited on 4-5-07 by RazMan]
Cheers,
Raz
When thinking outside the box doesn't work any more, it's time to build a new box
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ChrisGamlin
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| posted on 4/5/07 at 09:12 AM |
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I cant remember where I got the information on the bore sizes from now, might have been one of the kit manufacturers (won't mention names as I
don't know which one!).
Its tricky to say if the rears are working or not, certainly if I lock a wheel the front would be the first to go, and thinking about it Ive never
managed to lock a rear even when playing with the bias bar although its tricky to test because intentionally snatching a brake on the road to test the
balance is a bit precaious on the road, and Ive not done a lot of playing on track due to just wanting to make the most of the track time.
What would the result be if I swapped the MC's around? I assume more rear braking along with a less pedal travel on the front circuit? Would a
.625 be big enough for the slave cylinders on the rear drums though?
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jimgiblett
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| posted on 4/5/07 at 09:19 AM |
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Hi Chris,
I think you have the same STM pedal box as me. I would try swapping the m/c around so that you have 0.75 front and 0.625 rear (as it is a no cost
change). 0.75 on the front of mine works well on mine but I have disks on the back so not so easy to comment. STM used to use the 0.75 on the
Dynalites IIRC.
- Jim
quote:
Originally posted by ChrisGamlin
BTW I have .625 on the front and .75 on the rears which is the recommendation I had at the time, the fronts are quite snatchy though with not a lot of
feel and hard to modulate, so very easy to lock up, and because of this Im thinking maybe a slightly larger piston?
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ChrisGamlin
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| posted on 4/5/07 at 09:21 AM |
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Oh and as for pedal box, I have an ST floor mounted pedal box, not sure what the pedal ratio is but will look at the weekend.
Ive also got a rear disc conversion in the garage using Peugeot 206 rear calipers which I should get round to fitting at some point, but to fit it I
need to mod the trailing arm pickup points on the axle a bit, so its not a straightforward swap unfortunately.
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ChrisGamlin
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| posted on 4/5/07 at 09:22 AM |
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Cheers Jim, yeh I have the floor mounted STM pedalbox, so might give that a go as a quick fix over the weekend.
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ChrisGamlin
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| posted on 8/5/07 at 10:32 PM |
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I checked my Wilwoods this evening, they've got 1" 3/8 pistons compared Outlaw M16 replacement 1.5". This is a bit odd as they were
sold at the time as an M16 replacement, yet the caliper piston area is significantly smaller than a standard 2 pot M16.
The M16 has a piston area of 4500mm2, as do 4 pots with 1.5" pistons like your Bremsports, but with 1 3/8" pistons the piston area is
3850mm2, so more than 15% less piston area, therefore being less powerful and needing a smaller MC I guess? I see that Hi Spec use the same size
pistons though for their M16 replacement, so maybe there's a good reason to reduce piston area despite the loss in power?
Ive also measured my pedal and it's approximately 5:1 ratio with the pedal 190mm long from pivot to centre of foot pad, and ~38mm from pivot to
the centre of the bias bar.
So, any revisions on the above sizes?
I found a link to this brake calculator spreadsheet
here, its been mentioned before by RazMan,
has anyone else used it? Having a quick look at it Im not sure what units (I assume all imperial) and also not entirely sure what it tells you at the
end anyway!
Chris
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RazMan
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| posted on 8/5/07 at 11:14 PM |
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35mm pistons are the same as mine (Dynalites) and it's quite a coincidence that we are experiencing the same problem - not enough rear braking.
I have today ordered 4 pot Superlites (with handbrake) & discs which SHOULD sort me out
What pads are you running? On a Seven you will need the softest compound you can find and all I could get were EBC Green Stuff which are too hard and
need quite a lot of heat before getting grippy. Wilwood do a 'Smart Pad' which I think is on the same level as Greens. Same story on the
rears and maybe playing with different pads might help you here too.
Personally I think your m/c sizes are spot on and you will only make the rears less effective by swapping them - but the choice is yours.
[Edited on 8-5-07 by RazMan]
Cheers,
Raz
When thinking outside the box doesn't work any more, it's time to build a new box
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britishtrident
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| posted on 9/5/07 at 07:21 AM |
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Yes the rear drum brakes will work with a 0.625 this would give almost a 50% increase in pressure to the rear brakes compared to the current set up.
However I think it might result in the car having too much rear brakes and having to set the balance bar way off centre.
0.7" master cylinders on both circuits should be an ideal starting point. If that dosen't work you have the option of 0.7" on the
front and 0.625" on the rear.
The difference in caliper piston area is a bit of a red herring Catherham for years used Girling P14 calipers on the front which are a lot smaller
than Cortina M16 calipers.
Other rear brake options are
Fit larger wheel cylinders
Fit 9" drums from the Capri range
Convert to rar discs.
[I] “ What use our work, Bennet, if we cannot care for those we love? .”
― From BBC TV/Amazon's Ripper Street.
[/I]
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britishtrident
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| posted on 9/5/07 at 07:40 AM |
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One thing I should point out is that with drum brakes you can't assume a 25% increase in hydraulic pressure wil give a 25% increase in braking
effort,
Because of factors such as self servoing action and elastic distortion of the the brakes shoes and drums at higher hydraulic pressures the friction
characteristics of drum brakes are very non linear. As result when push to the limit an increase of say 10% in hydraulic pressure might result only in
a 1% increase in braking effort.
When racing Imps we originally used Hillman Avenger P14 calipers on the front and Imp wheel cylinders on the rear. Imp rear brakes are practically
identical to Escort Mk1 8" brakes.
Like all true rear engined crs Imps and Davrians normally need a 50%-50% front rear braking distribution we could get this using a 0.7"
mastercylinder in the front and a 0.625" cylinder in the rear circuit.
When we fitted larger calipers in the front we couldn't get enough adjustment on the balance bar so tried putting larger wheel cylinder in the
rear brakes, but couldn't get anymore out of the rear brakes becasuse the drums and shoes were distorting under pressure. When the rear drums
were ditched in favour of discs sanity was resumed.
[I] “ What use our work, Bennet, if we cannot care for those we love? .”
― From BBC TV/Amazon's Ripper Street.
[/I]
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ChrisGamlin
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| posted on 9/5/07 at 11:52 AM |
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Cheers guys, I do have a set of refurbed 9" drums in the garage that I got with an axle I bought, so is that worth persuing?Will the 0.75 rear
be OK with the (presumably bigger) wheel cylinders on those?
As mentioned above I also have the disc conversion but that needs a bit more fabrication to fit, and as I have a trackday a week Sunday I'd
like to get something in place by then.
As to pads, I have Pagid RS15's on the front which should be about as good (or expensive!) as you can get, I got these in an attempt to fix the
general poor braking I had with other pads (Greenstuff and Wilwood PolyMatrix), but it didn't help much.
If I were to go with a 0.7" piston, where's the best place to get them from? Ive seen a few no-name reproduction ones for ~£20 but
I'd prefer a genuine Girling if possible. Are they a standard fitment on anything so I could get one from a local factors? Im using the remote
reservoir ones similar to this
Chris
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RazMan
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| posted on 9/5/07 at 12:55 PM |
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The Girling & Wilwood m/c are the same fitment wise - just watch the length if space is tight (Wilwoods are slightly longer)
Your pedal ratio is about the same as mine so I suggest a few visits to the gym might be in order to strengthen up your right leg 
p.s. When you mentioned that you were being outbraked by other drivers, were the brakes fading or just not enough to lock up the wheels?
Cheers,
Raz
When thinking outside the box doesn't work any more, it's time to build a new box
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britishtrident
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| posted on 9/5/07 at 01:48 PM |
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Ditch the 0.75" much too big drum brakes were always used with smaller mastercylinders than discs.
ISTR the 0.7" dia Girling master cylinder was last used on the Series 3 Land-Rover clutch
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