Dale
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| posted on 17/2/05 at 05:36 PM |
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Dual master cylenders nessesary?
I am just wondering if its worth the bother for a 99% of the time road car to go with dual MC or just use the single donor brake cyclender and a
porportioning valve. My factory brakes were made to stope a 3300 -3800lb car so they should be up to the task.
BTW whats a good pedal ratio to use if I am removing the power brake booster.
Thanks
Dale
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clbarclay
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| posted on 17/2/05 at 07:08 PM |
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In short, no.
the main advantage of a dual master cylinder system is that with a balance bar, the for/arft braking can be adjusted. the sort of thing that is only
needed when quick changes of brake balance is needed
An alternative to the above for brake balance is a fixed or variable restrictor in the rear brake line. most comercial cars have these (fixed) already
fitted.
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niceperson709
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| posted on 17/2/05 at 10:00 PM |
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Hi Dale
I think you will find that your rta will mandate a duel circut brake system so that failure of one curcuit will not mean you have no brakes at all
.
Best wishes
Iain
Best wishes IAIN
life is not the rehearsal , it's the show so don't sit there thinking about it DO IT NOW
http://iainseven.wordpress.com/
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Dale
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| posted on 18/2/05 at 12:04 AM |
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It does have to be duel circuit here in Canada but they consider a single mc capable of running duel circuits. It does offer a single point of
failour but I have never seen a mc die all at once-- its usually quite gradual as they start to bypass.
Dale
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clbarclay
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| posted on 18/2/05 at 12:15 AM |
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Failure to the pipes is more likely and more sudden than failure at either master or slave cylinders.
Single master running dual circuit have two pistons inside, and the pipe work also has a valve arangment so that if one line losses pressure, the line
with pressure in it works the valve, closing the line with a leak so that the fluid in the reservoir doesn't leak through the master cylider and
out of the damaged line.
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Avoneer
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| posted on 18/2/05 at 12:44 AM |
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Stick with single - makes the job easy.
If you're using drums on the back, you probably won't need a rear proportioning valve either.
Pat...
No trees were killed in the sending of this message.
However a large number of electrons were terribly inconvenienced.
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MikeP
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| posted on 18/2/05 at 12:50 AM |
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Hi Dale - my opinion is that most donor tandem master cylinders aren't suitable for locosts - the ratio is wrong. I know your car is a
different design, you'd need to find out your front/rear weight split and CoG to be sure.
Also IMO, it's even more important for the street than the track. On the track, poorly balanced brakes cost you a few seconds. On the street
there are many more things to run into, and a cars length could mean the difference between life and death.
Dave Horsley and I worked out the formulas and built http://www.7builder.com/Downloads/MikesBrakes.xls that shows why the donor parts may
not work. We used the "Brake Handbook" by Fred Puhn as our main reference. The spreadsheet can help select master cylinder sizes, figure
out pedal pressures, etc, but only if you've got disc brakes front and rear. Unfortunately we weren't able to find the formulas for drum
brakes.
HTH!
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Dale
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| posted on 18/2/05 at 01:01 AM |
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I am using disks front and drums rear,
My weight measurements so far lead me to believe that with only a driver and some fuel I should end up being probably be close to 45/55 split for
front to rear -- could actually be a bit heavier to the rear but not by much. I have some spare MC's but they were from an even heavier
donor.
I wonder if it makes a difference if I go with diaganal braking such as the older VW rabits used for each circuit.
Dale
[Edited on 18/2/05 by Dale]
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britishtrident
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| posted on 18/2/05 at 08:52 AM |
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Some misinformation floating about here so lets start from the beginning.
Most builds use pedal ratios between 4.5:1 and 5.5:1
Dual mastercylinders and balance bar really are best left for track use it is very easy to get the set up of this system wrong and end up with either
too much back brakes or a system in which if one circuit fails the pedal goes to the floor without operating the remaining good circuit.
Dual circuit master cylinders used in 99% production cars are fairly simple no magic valves inside is just two pistons of the same dia in a common
bore pushed apart by a return spring.
Pressure limmiting, reducing and regulating valves come several different types. The simplest and most suitable for home builders are simple shut off
valves. When line pressure reaches a preset value the port feeding the rear wheels is shut off and the rear brakes see no futher increase in
pressure a simple example of this type of valve was used on early Minis (see Dave Andrews Robin Hood site). Some european cars and vans use a
variation of this in which the vavle connected to the rear suspension allowing increased rear brake pressure when the vehicle is laden. This type
of load sensing arangement isn't suitable for a Locost but the valve can be very easily adapted to act as an adjustable shut off valve . The
Fiat Uno valve has been used by many locost builders to do this.
The next category of valve are presure distribution proporting valves-- these are more sophisticated i that they used a stepped piston. Under light
braking pressure they operate exactly like the the simple shut off valve but once the preset line presure is reached instead of capping the presure to
the rear brakes at the preset, the stepped piston comes into play creating a lower rate of pressure rise to the rear brakes. This type of valve is
used on a number of modern cars. One of Fred Phuns books has a picture of one of these valves (I think from a mid 70s Corvette) adapted to be
adjustable.
Many FWD cars with diagonal split systems use a dual circuit version -- two proporting valves sharing the same valve body.
Another type of valve is the "G" sensing valve used by BL on the old 1800 and Montego and by Ford on the Sierra setting one of these up
for a Locost is really too much work.
Most front engine rwd road cars have 75% front 25% rear braking effort split with a book the Locost this would be nearer 66%-33% so the back
brakes are doing a greater percentage of the stopping.
Mixed disc/drum systems throw a spanner in the works because drum brakes are self servoing they respond differently to hydrauilic pressure than
disks. Also the mechanical nature of drum brakes makes them less efficient at high hydraulic pressures due mechanical distortion of the shoe and drum.
Putting this all together this means is at low pedal pressures drum brakes tend to bite harder than discs.
The best solution is to go for a simple front rear split system with a simple adjustable cut off type valve in the rear circuit. Test the car in dry
and wet condition and if the rear brakes are still too powerful fit smaller dia rear wheel cylinders.
[Edited on 18/2/05 by britishtrident]
[Edited on 18/2/05 by britishtrident]
[Edited on 18/2/05 by britishtrident]
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MikeP
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| posted on 18/2/05 at 02:59 PM |
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Hi BT - I agree with everything you say in your post, except for this bit:
"Most front engine rwd road cars have 75% front 25% rear braking effort split with a book the Locost this would be nearer 66%-33% so the back
brakes are doing a greater percentage of the stopping. "
One of us has it back to front - pardon the pun  .
The problem, as I see it, is you won't get *enough* braking force on the rears if you use a donor system on a locost. As you point out, a
locost has more weight on the rear tires and less weight transfer to the front under braking than the donor. So on a locost, the rear tires can do
more work than on the donor.
If you use the donor system unchanged, that system limits the force to 25% on the rear, when the locost could use closer 33%. So you won't stop
as quickly as you otherwise could.
Like I said, as I see it - am I missing something fundamental here? I do realize that too much rear braking is dangerous on the streets - but some is
necessary, or we wouldn't use rear brakes at all .
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britishtrident
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| posted on 18/2/05 at 03:27 PM |
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Leaving aside heat related effects on braking for now
Using drums on the rear has some advantages for street use at light pedal pressures drums are more "efficient" than disc at converting
pedal pressure in to stopping power. But under hard braking a drum brake tends to distort very slightly and the shoes also distort and as a result
drum brakes don't stop as well as discs when put to serious use.
On a light car using brakes from a much heavier production car the rear drum brakes will do more of the initial braking than the discs at the front
--- with a Seven style car weight distribution this is good when combined with a presure limmiting valve to prevent rear locking.
The big advantages of disc brakes are mechanical stiffness which makes them work better at high pedal pressures than drums and much better heat
disapation allowing them to recover from fade before the next corner :-)
Some years back I tried getting disk front drum rear to work on a rear engined car using a balance bar setup -- it was never satisfactory even when
different sized master cylinders and rear wheel cylinders.
I considered fiting an adjustable cut off valve in addition to the balance bar but in the end switched to discs.
When the car was switched to discs at the rear it became much easier to get the brakes balanced for any race conditions --- 1/2 turn difference
towards rear in the wet.
For road use I think a simple adjustable cut off valve is the way to go once set it is fairly full proof.
[Edited on 18/2/05 by britishtrident]
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