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
Generally (case for MNR and others) the .75 is the rear, .625 is the front.
Rgds
Rich
The numbers relate to bore size (inches) and the smaller bore usually goes on the front (less movement, more pressure)
What brakes are you running
If disks on rear put the small bore (0.625" on front -- if drums on back its more tricky.
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)
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]
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...
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]
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!
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]
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.
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.
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!)
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]
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.
quote:
Originally posted by britishtrident
Sometimes you have to work with what you have got
quote:
Originally posted by MikeP
less weight under breaking.
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.
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.
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
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]
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]
quote:
Originally posted by 02GF74
how can you be against me??!?!?!?
quote:
Originally posted by Lippoman
However it does not work like an automatic bias bar.
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.
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.