Lotus Elan

.75 master cylinder bore and +2 brakes

PostPost by: Tahoe » Sat Jan 22, 2011 10:11 pm

Just posted in things in common and identified my Master Cylinder as a Datsun dual master with a .75 bore. If I go with +2 front brakes will this be a good combo? Pedal is hard now which is understandable with a large bore and original brakes. I know the pedal pressure should decrease with the +2 calipers, but will it decrease enough, or should I source a slightly smaller bore, say .7? I've read most of the old threads on this subject, but I'm not getting a definitive answer if this is agood combo, so I posted again.
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PostPost by: types26/36 » Sat Jan 22, 2011 10:41 pm

Do you have a servo? If I remember correctly a servo assisted car has a .7 master cyl and a non servo car has a 5/8 cylinder, unless you are one of those people who don't like servos why not fit one.....that should help with the pedal feel.
(This should kick off the old chesnut......."take off servo" debate.... :lol: :lol: )
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PostPost by: Tahoe » Sat Jan 22, 2011 11:13 pm

types26/36/74 wrote:Do you have a servo? If I remember correctly a servo assisted car has a .7 master cyl and a non servo car has a 5/8 cylinder, unless you are one of those people who don't like servos why not fit one.....that should help with the pedal feel.
(This should kick off the old chesnut......."take off servo" debate.... :lol: :lol: )


Well,I would have to add 2 servos because of the dual circuit. I prefer to not have a servo. I've sourced some +2 front brakes, complete with axle shafts, brackets, rotors and calipers, etc. Haven't bought them yet but will most likely buy them if the price is right. I've also noticed some GT6 brake kits advertised lately, but that would still leave me short the axles and caliper brackets, so buying everything in one lot seems to make more sense.
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PostPost by: bcmc33 » Sun Jan 23, 2011 12:44 am

Tahoe wrote:Just posted in things in common and identified my Master Cylinder as a Datsun dual master with a .75 bore. If I go with +2 front brakes will this be a good combo? Pedal is hard now which is understandable with a large bore and original brakes. I know the pedal pressure should decrease with the +2 calipers, but will it decrease enough, or should I source a slightly smaller bore, say .7? I've read most of the old threads on this subject, but I'm not getting a definitive answer if this is agood combo, so I posted again.

A 3/4" bore M/Cyl will work OK but it will need a lot more pedal pressure than the ideal 5/8" M/Cyl to achieve good braking performance.
With a non servo system the 5/8" will be best, especially when changing to +2 calipers.
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PostPost by: cabc26b » Sun Jan 23, 2011 4:38 pm

Brian ,

Lotus used a master in .7 w/o servo ford also used a .7 and .75 non servo on the Cortina.( both single and tandem)

BCMC33,

What constitutes a "lot more pressure" ? Are there some maths you have that express this at the pedal ?

Also I thought that if you went up in caliper piston size you needed to displace more fluid to move the piston the same distance so, an increase in the master cylinder is required ( you don"t want to bottom out) . you also have or can play with pedal ratio , pad's etc.

I have never done the whole exercise and so thought maybe you have run the math on the whole system to calculate brake torque ( and here , I think, you are capped to a large extent by limitations in the diameter of the disk.)


Tahoe - I once ran a system close to what you are proposing ( I say close b/c I can't recall the diameter of the tandem set up , but it was not .625 ) I'll see if I have a part number for the tandem and report back.

George
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PostPost by: mikealdren » Sun Jan 23, 2011 5:53 pm

The force applied through the pedal is converted to pressure in the fluid. For a given force on the pedal, the smaller the piston in the master cylinder, the higher the pressure. The smaller piston requires greater pedal travel for the same fluid displacement.

Similarly, at the other end, a small piston produces less force on the pad. A larger piston produces more force but requires more fluid displacement.

The force exerted is in proportion to the areas of the pistons (and hence the square of the radii). Decreasing a piston from .75" to .70" reduces the radius from .375 to .35 and decreases the load required by about 13% (if my maths is right) for the same fluid pressure.

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