Posted: Wed Feb 28, 2007 12:25 am
Try again,
Right, have now had a chance to look through the material that Willie sent me. It is a series of calculations produced by the Design Manager of the Brakes Division of Automotive Products, so should be a bit of an authority on the subject? The calculations allow you to find the front to rear brake balance for any scenario of disc or drum, the effect of a servo and the pedal pressure required for any of the scenarios. Also takes into account the ride height, passengers, luggage etc.
Have put all the formulae into Excel, and created a model that can work out the effects of changing one or more factors instantly!
A number of things have become very clear:
The original 7” all-round drum set-up was very biased towards the rear – under heavy (sic) braking the rears could lock up first. The bias is also double what the current vehicle design regulations allow. Pedal pressures were very high, even for the late 50’s and early 60’s.
Retaining the early larger rear brake cylinders will produce a similar result no matter what front brakes are fitted – 7/8/9” drums or discs (Midget or Marina).
The move to 8” fronts and the smaller 3/4” cylinders provided a much better balance to the system, and is the best of the drum brakes (fitting the Wolesley 9” drums is not so effective!!). Yes, larger diameter drums, but much smaller wheel cylinder bores, so not as much braking effort for the same line pressure. From the data I have, for a given pedal pressure you get more braking effort out of the Minor 8” than the Wolesley 9”. Can anyone confirm the Wolesley wheel cylinder as 0.8” diameter? Also does anyone know the Riley cylinder bore size (Girling)? The drums are the same diameter as the Wolesley, but wider shoes I believe?
Fitting either Midget or Marina discs increases the rear bias by a small amount (Marina are worse, despite being a larger disc (smaller caliper pistons) and both need more pedal pressure to get the same results as the 8” drums. The difference is that they work harder longer.
As far as fitting a servo is concerned, if the rear cylinders are the later smaller ones, then as the system is reasonably balanced, the servo should be plumbed into all four wheels – if not, the system will be much too biased to the front, and could be unstable under heavy braking, although better than locking up the rears early.
If the early, larger rear cylinders are retained, there is an argument to servo the fronts only, to produce a better system balance, but if this is done, fitting the smaller rear cylinders later does not look like a good idea.
Fittings discs makes no difference to the decision, as it is primarily dependent on which rear cylinders are fitted more than anything else.
Without a servo the pedal pressures, with the possible exception of the standard 8” front set-up, are very much higher than a current designed car. Fitting discs makes this worse, so a servo should be considered very seriously in this case.
If there are some figures available for the Ford discs, caliper piston diameter and distance from the centreline of the stub axle to the centreline of the caliper piston, I can work out how they compare to Midget and Marina. From the size of piston and disc it could redress the balance nicely?
All the comments about rear bias refer to a lightly loaded vehicle, just the driver and a full fuel tank, as this is when it is likely to cause a problem. As the number of passengers and/or luggage increases, so the need for rear braking effort increases, and the inherent rear bias is no longer an issue. What then becomes an issue is the sheer pedal pressure required to make it stop!
Running the model for a fully loaded car with 4 people and luggage gives a brake pedal pressure of over 150lb for the original 7” all-round system, and that to achieve the design standard of 0.87g. By comparison, today’s everyday car would need half, or maybe a third of the effort and produce close to 1.00g deceleration in good conditions – we drive around in the middle of them as well, Mmmm??
As always it is still an individual decision about what is best from them, but I hope this helps to explain something of what lies behind the design of the Minor braking system. If anyone would like the Excel model please send me your email address.
I run a 2.2:1 boost servo on front and rear, and the ‘feel’ is good, but definitely not ‘over light’. Lightly laden, even with the smaller rear cylinders, the system is still a bit too biased to the rear, so fitting a pressure limiting valve in the line to the rear brakes removes the problem. Would be nice to have one that is load sensitive – has anyone done it? I have fitted a Mini one, and converted it into adjustable (again please email me if you would like the details – it’s a very easy mod to do).
Right, have now had a chance to look through the material that Willie sent me. It is a series of calculations produced by the Design Manager of the Brakes Division of Automotive Products, so should be a bit of an authority on the subject? The calculations allow you to find the front to rear brake balance for any scenario of disc or drum, the effect of a servo and the pedal pressure required for any of the scenarios. Also takes into account the ride height, passengers, luggage etc.
Have put all the formulae into Excel, and created a model that can work out the effects of changing one or more factors instantly!
A number of things have become very clear:
The original 7” all-round drum set-up was very biased towards the rear – under heavy (sic) braking the rears could lock up first. The bias is also double what the current vehicle design regulations allow. Pedal pressures were very high, even for the late 50’s and early 60’s.
Retaining the early larger rear brake cylinders will produce a similar result no matter what front brakes are fitted – 7/8/9” drums or discs (Midget or Marina).
The move to 8” fronts and the smaller 3/4” cylinders provided a much better balance to the system, and is the best of the drum brakes (fitting the Wolesley 9” drums is not so effective!!). Yes, larger diameter drums, but much smaller wheel cylinder bores, so not as much braking effort for the same line pressure. From the data I have, for a given pedal pressure you get more braking effort out of the Minor 8” than the Wolesley 9”. Can anyone confirm the Wolesley wheel cylinder as 0.8” diameter? Also does anyone know the Riley cylinder bore size (Girling)? The drums are the same diameter as the Wolesley, but wider shoes I believe?
Fitting either Midget or Marina discs increases the rear bias by a small amount (Marina are worse, despite being a larger disc (smaller caliper pistons) and both need more pedal pressure to get the same results as the 8” drums. The difference is that they work harder longer.
As far as fitting a servo is concerned, if the rear cylinders are the later smaller ones, then as the system is reasonably balanced, the servo should be plumbed into all four wheels – if not, the system will be much too biased to the front, and could be unstable under heavy braking, although better than locking up the rears early.
If the early, larger rear cylinders are retained, there is an argument to servo the fronts only, to produce a better system balance, but if this is done, fitting the smaller rear cylinders later does not look like a good idea.
Fittings discs makes no difference to the decision, as it is primarily dependent on which rear cylinders are fitted more than anything else.
Without a servo the pedal pressures, with the possible exception of the standard 8” front set-up, are very much higher than a current designed car. Fitting discs makes this worse, so a servo should be considered very seriously in this case.
If there are some figures available for the Ford discs, caliper piston diameter and distance from the centreline of the stub axle to the centreline of the caliper piston, I can work out how they compare to Midget and Marina. From the size of piston and disc it could redress the balance nicely?
All the comments about rear bias refer to a lightly loaded vehicle, just the driver and a full fuel tank, as this is when it is likely to cause a problem. As the number of passengers and/or luggage increases, so the need for rear braking effort increases, and the inherent rear bias is no longer an issue. What then becomes an issue is the sheer pedal pressure required to make it stop!
Running the model for a fully loaded car with 4 people and luggage gives a brake pedal pressure of over 150lb for the original 7” all-round system, and that to achieve the design standard of 0.87g. By comparison, today’s everyday car would need half, or maybe a third of the effort and produce close to 1.00g deceleration in good conditions – we drive around in the middle of them as well, Mmmm??
As always it is still an individual decision about what is best from them, but I hope this helps to explain something of what lies behind the design of the Minor braking system. If anyone would like the Excel model please send me your email address.
I run a 2.2:1 boost servo on front and rear, and the ‘feel’ is good, but definitely not ‘over light’. Lightly laden, even with the smaller rear cylinders, the system is still a bit too biased to the rear, so fitting a pressure limiting valve in the line to the rear brakes removes the problem. Would be nice to have one that is load sensitive – has anyone done it? I have fitted a Mini one, and converted it into adjustable (again please email me if you would like the details – it’s a very easy mod to do).
I really appreciate you keeping this thread on track 
