Davidmgbv8
Supporter
I just got my AP calipers from Southern GT. I am going to floor mount my pedals. Does anyone have a recommended master cylinder size for the front and rears of the SGT?
Thanks
Thanks
Brake master cylinder sizing.
- Thread starter JP Verweij
- Start date
Neil
Supporter
Use the calculator that was posted here. It is accurate and will save you time and money by your not having to experiment with different sized master cylinders.
There is a book in the Competition Car book series that is just on brakes. In that book it suggested aiming for 75lbs of pedal push. I tried that using a set of scales up against a wall like you said to see how I liked it. I liked it so then fed that into the calculations. This was not in a 40 replica but another race car I built. When it hit the tracks the brake pedal pressure felt good. the calculations took into account the distances between the pedal hinge point, the point you pushed it with your foot and the point where the master cylinder bar connected. Raising or lowering the floor or alternatively moving the pedal plate up or down the pedal changes this ratio and so makes things adjustable. I guess the maths is the same whether you have floor mounted or bulkhead mounted pedals. Its just leverage and packaging convenience.
As to SGT cylinder sizes, they must have done enough cars now to have a standard size recommendation?
As to SGT cylinder sizes, they must have done enough cars now to have a standard size recommendation?
Howard Jones
Supporter
Frank, I don't suggest you would use a piece of wood for a project of yours. I intended to warn off those of us who might be less experienced. Clearly, experience is something you don't lack
I have floor mount peddles and placing a "shim" under the heel increases the peddle ratio as it effectively lengthens the long arm of the peddle from the fulcrum to the point where your foot presses on the peddle.
You're correct for a drop-down mount pedal, the effect is exactly the opposite. That's a good idea, lowering the floor to increase the pedal ratio.
I have floor mount peddles and placing a "shim" under the heel increases the peddle ratio as it effectively lengthens the long arm of the peddle from the fulcrum to the point where your foot presses on the peddle.
You're correct for a drop-down mount pedal, the effect is exactly the opposite. That's a good idea, lowering the floor to increase the pedal ratio.
That only works if your not out of stroke of the master, which seems to be my case.. more piston area than bmc volume.
Most bmc's have just 2.9cm stroke. Pedal ratio wont change that.
Most bias brake boxes come with an intruction manual for a front engined car. Thats where I went wrong. I did it all by the book, it just wasn't the GT40 book, more likely the Escort mk2 & Capri rally book.
Most bmc's have just 2.9cm stroke. Pedal ratio wont change that.
Most bias brake boxes come with an intruction manual for a front engined car. Thats where I went wrong. I did it all by the book, it just wasn't the GT40 book, more likely the Escort mk2 & Capri rally book.
Terry Oxandale
Skinny Man
Yep, two different animals in terms of weight distribution, etc. AND, you are also utilizing components of that same configuration (front engine, front weight, large front brake pieces compared with the rear....on a mid-engined car.
Ron McCall
Supporter
Pretty cool contraption. I have used these in the last few race cars that I have built.
It's nice to know instantly where the bias is adjusted with just a glance.
Ron
Terry Oxandale
Skinny Man
I guess I need to study this more Ron. To me, it nothing more than the usual bias adjustment knob, only with a scale added to the base-plate. Looking at the parts in the photos, it's hard to see it being anything other than that?
Neil
Supporter
I think the knob is a multi-turn type, geared to the rear scale.
Ron McCall
Supporter
Thanks for the calculator links in this thread, have been using them on my car. One thing it showed me was that i am seriously underbraked at the rear of my car compared to the front. And there I was thinkning my car stopped pretty jolly well! In putting the data into the calculator the one item I was unable to properly input or even knowledgeably guess at was the height of CG. I left it at the suggested 15 inches but suspect that is slightly high for a GT40 replica. Mine is a GTD with a Southern GT lowered engine chassis upgrade. Does anyone have a more likely figure to use please?
As I am on standard GTD rear brakes on GTD steel uprights I suspect I will end up having to upgrade my brakes at the rear to compliment the front. Currently only just getting about 20% rear braking so plenty of improvement can be had from what I have taken from this discussion thread! The issue I will get is that I like Alcon brakes and have them up front in the form of 4 pots with 44.5mm and 41.3mm dia pistons. These are the smallest Alcon calipers out there as far as I can tell, certainly their current range seems to only include bigger/more powerful brakes than this but I have seen a supply of these brakes so can get another set for the rear. Handbrake is solvable as I have a new set of AP dedicated handbrake calipers on the shelf somewhere which are quite rare I beleive nowadays. But to fit both new calipers and the handbrake calipers will require new uprights as the GTD steel set up only has one pair of caliper mounting holes. I can see a good number of domino effect side issues cropping up before I get to the end of this journey!
And the reason for doing all this? My car has sat for too many years at the back of my garage untouched but this year it will be 30 years since I road registered it so that must be cause sufficeint to tickle it back into life!
As I am on standard GTD rear brakes on GTD steel uprights I suspect I will end up having to upgrade my brakes at the rear to compliment the front. Currently only just getting about 20% rear braking so plenty of improvement can be had from what I have taken from this discussion thread! The issue I will get is that I like Alcon brakes and have them up front in the form of 4 pots with 44.5mm and 41.3mm dia pistons. These are the smallest Alcon calipers out there as far as I can tell, certainly their current range seems to only include bigger/more powerful brakes than this but I have seen a supply of these brakes so can get another set for the rear. Handbrake is solvable as I have a new set of AP dedicated handbrake calipers on the shelf somewhere which are quite rare I beleive nowadays. But to fit both new calipers and the handbrake calipers will require new uprights as the GTD steel set up only has one pair of caliper mounting holes. I can see a good number of domino effect side issues cropping up before I get to the end of this journey!
And the reason for doing all this? My car has sat for too many years at the back of my garage untouched but this year it will be 30 years since I road registered it so that must be cause sufficeint to tickle it back into life!
Aha found some smaller 2 pot Alcon calipers with 44.5mm pistons. If using these on 11 inch discs then I could get a 65/35% set up which if fine tuned a bit with bmc sizing OR balance bar adjustment can easily move % towards 60/40% or 70/30% depending on what I want the pedal to feel like ie hard short travel or softer longer travel. Likely to be a compromise between the two in the end I guess as not planning to redesign every part of the system from scratch.
As you say Howard, friction coefficient from softer or harder pads makes a big difference in results.
Also from when I was running Avon slicks on the car when in competition, I recall never locking a wheel up in dry conditions. Ever. But I could when on road tyres. Playing with the deceleration G figure shows the braking efficiency of my system moving from well above 100% with 0.6g estimated deceleration for road tyres to well under 100% with 1g estimated deceleration for slick tyres. Can't see me running slicks again so will work with road tyre estimated figure!
As you say Howard, friction coefficient from softer or harder pads makes a big difference in results.
Also from when I was running Avon slicks on the car when in competition, I recall never locking a wheel up in dry conditions. Ever. But I could when on road tyres. Playing with the deceleration G figure shows the braking efficiency of my system moving from well above 100% with 0.6g estimated deceleration for road tyres to well under 100% with 1g estimated deceleration for slick tyres. Can't see me running slicks again so will work with road tyre estimated figure!
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