rusty
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| posted on 13/2/06 at 09:58 PM |
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Brake Master Cylinders
Evening all
We now have our pedal box and the two master cylinders but I am unsure which one is which. One is marked .75 and the other 625.
Can some one please tell me what these numbers mean and which is for the front and which is for the rear.
Thanks
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RichieC
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| posted on 13/2/06 at 10:06 PM |
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Generally (case for MNR and others) the .75 is the rear, .625 is the front.
Rgds
Rich
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RazMan
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| posted on 13/2/06 at 10:08 PM |
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The numbers relate to bore size (inches) and the smaller bore usually goes on the front (less movement, more pressure) 
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 13/2/06 at 10:56 PM |
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What brakes are you running
If disks on rear put the small bore (0.625" on front -- if drums on back its more tricky.
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02GF74
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| posted on 14/2/06 at 11:41 AM |
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quote:
Originally posted by RazMan
The numbers relate to bore size (inches) and the smaller bore usually goes on the front (less movement, more pressure)
I don't think the last bit is 100% right.
or rearranged gives:
Force = Pressure x Area
Pressure is not going to change in the fluid (incompressible) hence a smaller area means less force; or more piston travel ignoring any effects to
piston resistance.
i.e. the smaller bore = more movement, less force)
I hope I've got it right
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Syd Bridge
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| posted on 14/2/06 at 12:06 PM |
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Smaller bore ALWAYS to the front.
Major percentage of the braking is done by the front wheels, thus more pressure needed.
In the p=f/a, if f stays the same, and a is decreased, then P increases. ie. Big area=low pressure, small area=high pressure.
It's the reverse at the wheels. Larger piston bore gives more clamping/braking force for a given pedal pressure.
Volumes moved do not come into the calculations whatsoever.
Syd.
[Edited on 14/2/06 by Syd Bridge]
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Lippoman
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| posted on 14/2/06 at 02:12 PM |
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The force at the MC is the pedal force used to create pressure, thus less area equals higher pressure.
F/A = P
At the wheel cylinders/calipers greater area equals more clamping force as the pressure is constant (already created at the MC).
F=P*A
The MC is the pump driving the system.
The only instances when volumes come into the equation is when pedal travel is excessive, usually drum rear with incorrect adjustment or applications
where the MC is mounted below the horizontal plane of the calipers and fluid drain back occurs due to gravity and vibration. These however need to be
corrected by other means.
The only instances I can think of, where the smaller MC could be for the rear, are if the front calipers have huge piston areas as compared to the
rear (say six pot fronts and drum rear?!?) or where the pad compounds used are very different.
Please correct me, if I've been disinformed...
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gttman
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| posted on 14/2/06 at 03:49 PM |
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Surelly It will all depend on the fluid movement required to move the caliper pistons. the whole point of the fluid is that it doesn't compress,
so more fluid movement = more caliper piston movement.
Pedal length and piston size will vary the amount of fluid moved for a given distance the foot travels.
So surelly bigger pistons will mean more fluid movement and thus more clamping force?......
[Edited on 14/2/06 by gttman]
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7 in a fancy suit
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| posted on 14/2/06 at 04:05 PM |
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You're getting amount of movement and force mixed up. I agree with 02GF74, lippoman and sydbridge. The smaller the MC piston, the greater
pressure you will be able to apply.....until you reach the point where the volume of fluid shifted isn't enough to take up the free play between
the pads and the brake surface. At that point the pedal hits the floor before the pressure is transferred....and you're in trouble!
A man gets lonely in the desert.......and bored
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britishtrident
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| posted on 14/2/06 at 05:24 PM |
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quote:
Originally posted by Syd Bridge
Smaller bore ALWAYS to the front.
Major percentage of the braking is done by the front wheels, thus more pressure needed.
In the p=f/a, if f stays the same, and a is decreased, then P increases. ie. Big area=low pressure, small area=high pressure.
It's the reverse at the wheels. Larger piston bore gives more clamping/braking force for a given pedal pressure.
Volumes moved do not come into the calculations whatsoever.
Syd.
[Edited on 14/2/06 by Syd Bridge]
Normally on a Locost yes but not always on every car -- depends entirely on weight distribution what is fitted at each end. Try balancing the
brake efort on an Imp racer with discs on the front and drums on the rear.
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MikeP
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| posted on 15/2/06 at 01:34 AM |
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It might be better to get the specs on the system before recommending which master goes where. The vehicle could be a locost using the front and rear
brakes from a single donor. Then the larger master would indeed need to be for the fronts to bring the system back into balance.
I guess that puts me with BT and against 02GF74, lippoman and sydbridge  ? There's more to it than just making sure the fronts does most of
the work. Both ends of the car should be set up to contribute as much as possible to stopping the vehicle.
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Syd Bridge
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| posted on 15/2/06 at 09:22 AM |
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quote:
Originally posted by Syd Bridge
If you've gotta put the big mc on the front, you've sized the brakes wrong to start with.
If the racing Imp needed more pressure on the rear, it needed bigger rear brakes to start with.
Syd.
Mike, Which bit of that statement are you failing to understand??
If you need more line pressure to the back brakes than the front, you've got the basic setup WRONG to start with.
All you are doing by swapping the cylinders is fixing the SYMPTOM (not enough rear braking), and not the PROBLEM, incorrect sizing of
the components fitted.
Syd.
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NS Dev
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| posted on 15/2/06 at 11:03 AM |
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Surely you're not trying to imply that energy dissipated is proportional to both frictional area, co-efficient of friction AND force applied are
you Syd?
(sorry, just in a generally amused and mildly sarcastic mood, nobody take anything personally for goodness sake!)
Retro RWD is the way forward...........automotive fabrication, car restoration, sheetmetal work, engine conversion
retro car restoration and tuning
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Lippoman
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| posted on 15/2/06 at 11:24 AM |
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Brake Hydraulics 101
The hydraulics in the brakesystem act as a LEVER, multiplying the force input by the driver's foot.
A smaller MC or larger wheel cylinder both give higher clamping force at the same force input (and both require longer pedal travel to achieve same
amount of movement in the pads).
When brakes are applied a forward weight shift occurs. So even on a 50/50 weighted car there is more weight on the front wheels during braking. This
increases the grip available on the front tires (similar to aerodynamic downforce on racing cars) while reducing the grip at the rear.
The front brakes on most RWD vehicles normally handle about 60- to 70 percent of the brake load. But on FWD cars and minivans as well as RWD and 4WD
pickups and SUVs, the percentage handled by the front brakes can be as much as 90 percent of the load.
To overcome this a standard brake system on a production car usually incorporates a "proportioning valve" to reduce the pressure to the
rear brakes.
Also, if lock-up is to occur in braking you generally want the front to lock-up before the rears; braking understeer is bad, but braking oversteer is
much worse.
[Edited on 15/2/06 by Lippoman]
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britishtrident
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| posted on 15/2/06 at 12:08 PM |
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quote:
Originally posted by Syd Bridge
quote:
Originally posted by Syd Bridge
If you've gotta put the big mc on the front, you've sized the brakes wrong to start with.
If the racing Imp needed more pressure on the rear, it needed bigger rear brakes to start with.
Syd.
Mike, Which bit of that statement are you failing to understand??
If you need more line pressure to the back brakes than the front, you've got the basic setup WRONG to start with.
All you are doing by swapping the cylinders is fixing the SYMPTOM (not enough rear braking), and not the PROBLEM, incorrect sizing of
the components fitted.
Syd.
Sometimes you have to work with what you have got --- fitting bigger drums to the back of Imp is very difficult , fitting Imps with rear disc only
became practical (without a mega budget) from about 1978 when "top hat" discs started to appear on smaller production cars, before that
you had to work with what Lord Rootes gave you.
I might add we were running the smallest available Forula Ford caliper on the front and the bigest possible wheel cylinder bore on the rear and were
actually being limited by the the mechanical stiffness of the brake drum --- had we been forced to run another season with rear drums we were
planning to heat shrink a steel band round the outside of the drum to stiffen it at trick picked up from Mike Hawke (he of MGCC VSCC).
[Edited on 15/2/06 by britishtrident]
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MikeP
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| posted on 15/2/06 at 01:05 PM |
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quote:
Originally posted by britishtrident
Sometimes you have to work with what you have got
Like what BT says. A quick look at the forum title confirms we're still on the "Locost" (low-cost) forum, and not the "high
dollar clubman assembly for chequebook builders" forum .
I confess that I don't understand why its vital that the front line pressure be greater than the rear pressure. I would think it doesn't
matter (within reason) as long as the net effect is the right stopping force is applied to each end. (And yes I get that higher pressure applies more
force, and the fronts need more force, but force also comes from the "leverage" due to different slave vs master piston areas).
Lippoman, good explaination, but some clarification is in order. Yes, more force is needed at the front even in a 50/50 weighted car. But certainly
less than a more typical 60/40 donor, plus a locost usually has a lower CoG so also transfers less weight under breaking.
The proportioning valve isn't to reduce rear brake line pressure. It'd be simplier and cheaper to increase the size of the rear slave (or
decrease the size of the master) if that were the goal. A proportioning valve introduces a "knee" in the brake force slope to more
closely follow the ideal pressure curve. This works better for the street with varying road conditions and driver applied pressures - the rears
contribute as much as possible without locking before the front. This helps ensure maximum stopping force as well as keeps them clean under light
braking, wearing evenly, etc.
The valve works like an automatic bias bar - it does more or less the same thing as tweaking the bar toward the rears under wet conditions on a race
car. Instead the valve tweaks the pressure in real time by reducing it only after the applied pressure reaches a preset point.
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MikeP
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| posted on 15/2/06 at 01:07 PM |
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quote:
Originally posted by MikeP
less weight under breaking.
lol, no pun intended . I guess I should proof read before posting intead of after .
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Lippoman
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| posted on 15/2/06 at 01:56 PM |
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Proportioning valve
MikeP,
The title of my last post was to reflect basic courses in school, and as in them the refinements were left out.
The only thing the proportioning valve can do is reduce the pressure to the rears. It does it however in the manner you describe, at a certain set
pressure it enters into action. However it does not work like an automatic bias bar.
The load sensing proportioning valves however have a function that acts similar to an automatic bias bar as it reduces pressure only as it
"feels" the rear wheels go light.
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02GF74
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| posted on 15/2/06 at 01:58 PM |
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quote:
Originally posted by Syd Bridge
quote:
Originally posted by Syd Bridge
If you've gotta put the big mc on the front, you've sized the brakes wrong to start with.
If the racing Imp needed more pressure on the rear, it needed bigger rear brakes to start with.
Syd.
Mike, Which bit of that statement are you failing to understand??
If you need more line pressure to the back brakes than the front, you've got the basic setup WRONG to start with.
All you are doing by swapping the cylinders is fixing the SYMPTOM (not enough rear braking), and not the PROBLEM, incorrect sizing of
the components fitted.
Syd.
how can you be against me??!?!?!?
my comment was in response to the quoted bit and then to go on to explain why I thought it was wrong.
The pressure is in the fluid; the force produced by any pistons pushed by the fluid depends on their area. You cannot argue against that (P=FxA).
I did not go any further than that becasue as you and a lot of others know,. there is more to braking systems than that.
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Syd Bridge
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| posted on 15/2/06 at 02:23 PM |
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OK, back to the initial question.
Assuming a Sierra donor; the larger bore mc feeds the back, the smaller bore feeds the front.
No exceptions and ifs and buts and maybe's. If it's not Sierra donor, 'suck and see', trial and error. But, it's a big
advantage to know the basics of what you are dabbling in.
Syd
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Lippoman
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| posted on 15/2/06 at 02:36 PM |
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Hydraulics
The hydraulics in the brakes use the piston sizes as hydraulic leverage. The force applied at the smaller piston (MC) is multiplied by the difference
in area to the larger one.
F1/A1 = P - at the MC
P * A2 =F2 - at the wheel
if A1 < A2, then F1 < F2
where
F1 = force input by driver
A1 = area of MC piston
A2 = area of caliper piston(s)
F2 = clamping force
P = system pressure in the braking system
The pressure in the system is created by the driver's foot pressing on the pedal. If that pedal is connected to a small piston the pressure that
can be achieved with the same amount of force is higher than if he had to press on a larger piston. The pedal travel needed to move the wheel
cylinders/calipers a certain amount is larger in the same proportion.
What creates the clamping force at the calipers is the pressure and piston area, not the amount of movement.
[Edited on 15/2/06 by Lippoman]
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britishtrident
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| posted on 15/2/06 at 02:50 PM |
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quote:
Originally posted by Lippoman
The hydraulics in the brakesystem act as a LEVER, multiplying the force input by the driver's foot.
A smaller MC or larger wheel cylinder both give higher clamping force at the same force input (and both require longer pedal travel to achieve same
amount of movement in the pads).
When brakes are applied a forward weight shift occurs. So even on a 50/50 weighted car there is more weight on the front wheels during braking. This
increases the grip available on the front tires (similar to aerodynamic downforce on racing cars) while reducing the grip at the rear.
The front brakes on most RWD vehicles normally handle about 60- to 70 percent of the brake load. But on FWD cars and minivans as well as RWD and 4WD
pickups and SUVs, the percentage handled by the front brakes can be as much as 90 percent of the load.
To overcome this a standard brake system on a production car usually incorporates a "proportioning valve" to reduce the pressure to the
rear brakes.
Also, if lock-up is to occur in braking you generally want the front to lock-up before the rears; braking understeer is bad, but braking oversteer is
much worse.
[Edited on 15/2/06 by Lippoman]
I remember years ago roller testing the brakes on MK2 Minis with single line brakes -- the spring pressure regulator valve used loose tension wih age
and after a few years it was quite common to see less than 10lbs braking effort from each wheel even although the drums shoes and wheel cylinders
were in pefect order --- usual tweak was to strip the valve an put a single washer under the spring to increase the preload, this would double the
brake contribution from the rear and still leave well short of the lock up point.
A while back I worked out the weight transfer for a Locost under 1g braking and came up somewhere between 70/30 and 67/33 front rear split.
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MikeP
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| posted on 15/2/06 at 03:45 PM |
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quote:
Originally posted by 02GF74
how can you be against me??!?!?!?
If you me me, sorry, poor attempt at a joke - I was poking fun at 7 in a fancy suit's comment. I like to think I'm much funnier and
friendlier in real life .
quote:
Originally posted by Lippoman
However it does not work like an automatic bias bar.
It's an analogy that helps me to understand how they work. As I'm sure you know, a bias bar is (properly) used to adjust to track
conditions and changing vehicle conditions (e.g. fuel load). On a slippery track the bar is moved for more rear braking since there's less
weight transfer to the front.
Same with the proportioning valve - with less foot/line pressure (e.g. in slippery conditions on the road) there's no proportioning, so more
braking force is provided by the rears - better balance, quicker stops. With more pressure in stickier(?) conditions proportioning is provided and
required as more weight is now on the front. Load sensing proportioning valves respond in a similar way but use chassis dive to detect it.
To me quite elegant in contrast to a bias bar which can only be set for one condition - usually maximum stopping force. I've heard it said that
a proportioning valve is of no use if you have a bias bar, but I would disagree if we're talking about street vehicles.
I came up with somewhat different numbers BT: for my locost I get 63/37 at 1g, while my donor (corolla GTS) needs 72/28 at 1g. I put together a
spreadsheet to work this stuff out: Braking system spreadsheet.
Page 2 (weights) shows how I came to my weight xfer numbers, page 4 (design) has a graph at the bottom that shows what happens when the bias is
changed or a proportioning valve is used. If I've got anything ack-basswards I'd appreciate the feedback.
Mike
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7 in a fancy suit
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| posted on 15/2/06 at 05:41 PM |
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Mike P said:
{If you me me, sorry, poor attempt at a joke - I was poking fun at 7 in a fancy suit's comment.}
Was I being too direct? I've been working in Iraq for 2 years 8 months non-stop now and I've been told that I am getting a bit
aggressive, snatchy and tend to point my finger a lot. Goin' home for good in 11 days time... Woo Hoo... and will try and soften into the
caring, sharing human being I used to be. Its bad when you cause offence and even when you read it twenty times over, you still can't see it.
A man gets lonely in the desert.......and bored
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britishtrident
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| posted on 16/2/06 at 10:56 AM |
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Proportioning valves, shut-off valves, load snsing ---- all slightly different in action ---- load sensing vaves I hate with a vengance have had
really bad moments while road testing customer cars in which discovered the load sensing valve was stuck full on.
For a while I have held he theory that the best setup for all conditions would be combination of a shut off type valve or a dual rate proportioning
valve and bias pedal box.
Setting it up would be quite tricky but once set up it should give an almost ideal pressure distribution for wet or dry conditions.
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