DarrenW
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| posted on 24/2/05 at 12:28 PM |
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Sierra Master Cylinders - bigger capacity???
My donor Sierra was bog stated, non ABS, 2.0 DOHC GLS.
Master cylinder looks like bog standard cast item, 3 outlets and bottle on the top.
I want a new cylinder, was just wondering if there is a replacement with larger cylinder available and if they are recommended. (Hopefully that is
direct replacement - ie bolt holes and pipe connections as i used the old one to get the pipe runs laid out).
(reason im asking was cos when i rebuilt a MK1 golf gti i was advised to get a larger cylinder mk3 type that bolted straight on but gave better brake
performance, i think pinston was only 1mm larger from memory).
Cheers in advance,
Darren.
[Edited on 24/2/05 by DarrenW]
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alister667
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| posted on 24/2/05 at 01:42 PM |
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I seem to remember there were 3 different types of master cylinder fitted into the bog standard sierra (in around 1986 anyhow).
I found this out when I treid to get a M/C refurb kit. I don't remember one being mentioned as better than the others.
If you do go for a different M/C remember to check the bore/pedal leverage so it isn't as hard as a rock.
All the best
Ali
http://members.lycos.co.uk/alister667/
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clbarclay
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| posted on 24/2/05 at 01:52 PM |
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A bigger diameter master cylinder will reduce the force at the brake pads if the slave cylinder remains the same diameter.
If you don't belive me I can show you calculations proving it.
The best bet for increased braking is to use larger MC and SC keeping the same ratio, but bigger SC will mean you can use larger pabs so you have a
larger area of friction material on the disc.
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DarrenW
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| posted on 24/2/05 at 02:28 PM |
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quote:
Originally posted by clbarclay
A bigger diameter master cylinder will reduce the force at the brake pads if the slave cylinder remains the same diameter.
Thats weird, doing just that improved the braking quite a bit on my Golf. Come to think of it i did however fit rear discs (total & absolute waste
of money time and effort as i had to bias them down a lot proving that the drums were more than capable) and slightly larger front calipers. I guess
they 'consumed' more oil and therefore the larger cylinder was more essential than nice to do.
It sounds a bit acedemic anyway if its not possible for me to get a cylinder with a larger diameter - not worth worrying about it seems. The only
reason i was asking was so that i only spent the money once. If the brakes arent good enough after running in i can always dabble with grooved discs
etc etc.
From a purely acedemic viewpoint i would be interested in seeing the calcs / proof of theory.
Thanks for the help,
Darren.
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flak monkey
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| posted on 24/2/05 at 02:35 PM |
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I dont know whats actually better, larger or smaller diameter but here we go:
Formula is:
Pressure = force/area.
Reduce the area you increase the pressure, keeping the force the same.
Ie you can use a lower force and get the same pressure...therefore same braking power.
Thats the way i understand it, or i could of course be wrong...
If you use a smaller MC you will get more pedal travel obviously...and larger visa versa.
David
[Edited on 24/2/05 by flak monkey]
Sera
http://www.motosera.com
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MikeP
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| posted on 24/2/05 at 04:29 PM |
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Hi Darren, all of the formulas are laid out in http://www.7builder.com/Downloads/MikesBrakes.xls
Puhn's "Brake Handbook" is pretty good for the theory.
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clbarclay
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| posted on 24/2/05 at 07:11 PM |
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As flak monkey said pressure (P) = force (F) / area (A)
Pressure in a fluid is evenly distrobuted.
therefore the pressure acting in the SC is equal to pressure developed in the MC.
F (1)(from brake pedle on MC) / A (1)(of directly proportional to diameter of MC) = P (in the brake fluid)
P (in the brake fluid) x A (2)(of directly proportional to diameter of SC) = F (2)(force produced by SC acting on brake pad)
This condeced
F1/A1=P
F2/A2=P
therefore F1/A1=F2/A2
and F2=A2x(F1/A1)
If MC orinional has an A1 of 1, a F1 of 1 and A2 of 1, using above equation F2 equals 1
If MC bigger has an A1 of 2, a F1 of 1 and A2 of 1, using above equation F2 equals 0.5
If MC smaller has an A1 of 0.5, a F1 of 1 and A2 of 1, using above equation F2 equals 2
In short
bigger MC, force on brake pad = 0.5
origional MC, force on brake pad = 1
smaller MC, force on brake pad = 2
Before you all rush and get MC's that are the diameter of biros remember that to much force and you will lok the brake disks (not good) and to
get the suficent movement of the SC piston, the MC must displace enough brake fluid befor you run out of travel.
An ulternative to changing MC is to mount its pivot on the brake pedle closer the brake pedles main pivot for more brake pad force or further from the
pivot to reduce brake pad force.
And you thought that knocking toghether a new pedal box was a complete dodal.
If in dought then copy the dimensions from your donor car for your pedal box.
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clbarclay
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| posted on 24/2/05 at 07:12 PM |
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Borring young fart aint I, still it's useful for stuff to know if your going to get into an argument with a college lecturer and win.
Any way Darren you did ask.
[Edited on 24/2/05 by clbarclay]
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flak monkey
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| posted on 24/2/05 at 07:17 PM |
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quote:
Originally posted by clbarclay
As flak monkey said pressure (P) = force (F) / area (A)
Pressure in a fluid is evenly distrobuted.
therefore the pressure acting in the SC is equal to pressure developed in the MC.
F (1)(from brake pedle on MC) / A (1)(of directly proportional to diameter of MC) = P (in the brake fluid)
P (in the brake fluid) x A (2)(of directly proportional to diameter of SC) = F (2)(force produced by SC acting on brake pad)
This condeced
F1/A1=P
F2/A2=P
therefore F1/A1=F2/A2
and F2=A2x(F1/A1)
If MC orinional has an A1 of 1, a F1 of 1 and A2 of 1, using above equation F2 equals 1
If MC bigger has an A1 of 2, a F1 of 1 and A2 of 1, using above equation F2 equals 0.5
If MC smaller has an A1 of 0.5, a F1 of 1 and A2 of 1, using above equation F2 equals 2
In short
bigger MC, force on brake pad = 0.5
origional MC, force on brake pad = 1
smaller MC, force on brake pad = 2
Before you all rush and get MC's that are the diameter of biros remember that to much force and you will lok the brake disks (not good) and to
get the suficent movement of the SC piston, the MC must displace enough brake fluid befor you run out of travel.......
Thats just a fancy way of saying what i said  
Sera
http://www.motosera.com
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clbarclay
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| posted on 24/2/05 at 07:19 PM |
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Yeah but theres no desputing the facts when there laid out like that.
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britishtrident
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| posted on 24/2/05 at 08:44 PM |
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The early Golf had a very specific problem with the brake pedal crossshaft layout on RHD cars VW had to put a lot of effort in to getting a decent
pedal feel/travel on later models this involved a number of tweaks. A Sierra master cylinder is already on the big side for running without servo
-- normally cars with disk front drum rear without a servo run 0.75" dia master cylinders, the Sierra master cylinder will be I think
0.81" perhaps more..
[Edited on 24/2/05 by britishtrident]
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MikeP
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| posted on 24/2/05 at 09:49 PM |
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Or the intuitive approach if you don't like math:
Pushing on a smaller master cylinder forces less fluid into the slave than a larger one would. So the smaller master is like a longer lever arm - you
can lift a heavy weight a shorter distance. The formulas are almost the same.
Modifying the pedal pivots points is the same as changing the master cylinder size (as it's a lever too, same equations). More leverage moves
less fluid for a given pedal travel. Make sure you've got enough travel without bottoming out!
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DarrenW
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| posted on 25/2/05 at 11:09 AM |
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Blooody hell eh!!! Its all good stuff. Thanks for that.
So to be topical its like trying to walk on snow in high heels or snow shoes. Same force, different area, high pressure on high heels = sink, low
pressure on snow shoes = walk on top.
It was a while since i was at college etc so you forget the maths and theory. I since moved into Project Management where my job is to make people to
do more quicker for less £ so i rely on others for the detailed expertise these days. Its always good to get into the nitty gritty.
As Britishtrident said - the cross link was a problem on all golfs and new cylinder helped to make the brakes as good as they are on the LH cars.
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Peteff
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| posted on 25/2/05 at 12:16 PM |
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P (in the brake fluid)
Won't this affect the properties of it drastically  . Probably the colour as well. My theory is that if you try to force the same amount of
fluid into a smaller area the pressure is going to go up but it will need more effort to do it. Smaller diameter slave cylinders will increase your
brake pressure at the wheel but you'll have to press the pedal harder to achieve it. Try knocking your car into neutral on an empty road while
travelling at 20ish mph and then brake left footed. You'll soon see how keen your brakes are. Sometimes it's just you getting used to them
that makes them seem weaker.
yours, Pete
I went into the RSPCA office the other day. It was so small you could hardly swing a cat in there.
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clbarclay
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| posted on 25/2/05 at 12:22 PM |
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Don't make the mistake my lecutrer made, which is to confuse fluid in a pipe and solid (non fluid) objects.
In a fluid system the pressure is equal through out.
In soild objects like stiletos or more commonly drawing pins, the force is equal through out and the pressure changes.
The object of a drawing pin is to have low pressure aginst your finger so it is not penetrated and high pressure against the wall etc. so that the
drawing pin penetrates that instead.
[Edited on 25/2/05 by clbarclay]
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clbarclay
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| posted on 25/2/05 at 12:26 PM |
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As for the effect of pressure on brake fluid, the reson you use Dot 4 or similare quality brake fluid is that it can with stand the high operating
pressure in the brakeing system.
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