BRAKES
by: D.J. Punshon
from: Fortification - January 1997

Fortification is the Club Magazine of the GT40 Enthusiasts Club
Copyright: GT40 Enthusiasts Club 1997

Return to Robin Sundt's Family Home Page
Return to Robin Sundt's GT40 Home Page


BRAKES.

Having spent many years building, modifying and generally fiddling with a GTD 40, I've come across tips and solutions to many problems, as most people have. However, as all cars are different, I would like to share my experiences.
I'm using a 351 Cleveland engine, which, as KEN SAUNDERS has mentioned in a previous article, needs a lot of cutting, grinding and welding of the chassis to fit properly (especially if it is fitted as low as possible), but the biggest problem, apart from HUDDART ENGINES, has been the inlet manifold, as no-one makes one for downdraught DELORTOS. I've had to make my own, so if you are going the same route, either use WEBBER IDA'S, for which a manifold is available from Fast Freddys, or be prepared to fabricate one.
The Cleveland's heads are completely different to the 302/351 Windsor engines (excluding BOSS versions), but they can, with some small mods, be fitted to the small blocks as they are a very high flowing, big valve head, remembering of course the different inlet and exhaust manifolds needed.
With powerful engines come the need for better brakes, The standard Granada Scorpio based brakes are OK for the road, but are no use for any type of track work, Many people have changed to AP or similar discs and calipers which are a tremendous improvement. However they are expensive and require a lot of maintenance.
Personally, I don't like using servos, I've used them in the past, but a properly designed system without them can have the same braking effort with more feel (important for those "OH SHIT" moments). Whether you use the latest dinner plate discs and multi piston racing calipers or mass produced items, a properly thought out system is needed to bring out their best.
The standard GTD set for instance, uses a brake pedal ratio of around 2.5/3.0:1 because of the twin servo assistance. However, if servos are not used, the pedal ratio of at least 5:1 is needed, so a lot of cutting and welding is required, if a balance bar is to be used (very useful if tyre or pad changes are made later), the procedure is made more complicated. Another method is to use an adjustable proportioning valve incorporated into the rear brake system, you can even combine the two, depending on what the car will be used for. Its possible to calculate the brake torque, pedal effort etc, needed or given for any particular system using simple mathematics. As an example, I will use the calculations for my braking system:-

Front Discs = 10.75" dia ventilated. (Jaguar XJS).
Front Calipers = 4 pot, 44mm dia pistons. (Rover SD1).
Rear Discs = solid 11" dia. (Jaguar E type front).
Rear Calipers = 4 pot, 38mm dia pistons. (Austin Princess).

The following information and calculations were obtained with reference to the Brake Handbook by Fred Puhn:

GTD 40 I = wheelbase (95").
Rft = Rolling Radius of Front Tyre (12").
Rrt = Rolling Radius of Rear Tyre (12").
FF = Vertical Friction Force On Both Front Tyres (1328 lbs).
FR = Vertical Friction Force On Both Rear Tyres (1172 lbs).
Weight Of Car = 2500 lbs.

The following information is difficult to obtain so a typical or approximate value must be used:

N = Coefficient of Friction of Tyres = .75
dm = Maximum Deceleration = .75 G
Ycg = Height of Centre of Gravity = 21"
N1 = Coefficient of Friction of Pads = .3
Front Brake Torque = (N x FF x Rft)/2 = 5976 "/ lbs

Rear Rrake Torque = (N x FR x Rrt)/2 = 5274 "/lbs

Front Caliper Piston Area = 8.547 sq"
Rear Caliper Piston Area = 7.065 sq"

Front Max Hyd Pressure = (Brake Torque)/(N1 x Total Caliper Piston Area x Effective Brake Radius) = 460.5 psi

Rear Max Hyd Pressure = 553 psi

Now, the maximum brake torque both front and rear is known, also the hydraulic pressure is known and the caliper must be able to operate safely at these pressures.
Next, we have to determine:
1). The pedal ratio; and,
2). The master cylinder size and pressure.

I'm using a pedal ratio of 5:1, its a typical starting ratio and because of the pedal box design and footwell constraints it was the maximum practical ratio possible.
Note, if a balance bar is used the pedal effort is effectively halved to each master cylinder.

Pedal Pressure (typ) = 75 lbs

Force on Cylinder Piston = (75 x 5)/2 = 187.5 lbs

Front, Master Cylinder Piston area = 187.5/438.5 = 0.427 sq"

Nearest Available Cylinder Diameter= .70" to .75"

Rear Master Cylinder Piston Area = 0.30 sq"

Nearest Available Cylinder Diameter = 0.625"

I am using a .70" front cylinder, a .625" rear cylinder and a .625" cylinder for the hydraulic handbrake (I also have a separate mechanical handbrake system using cables and Willwood mechanical calipers).
To help keep the cockpit as cool as possible and also to improve aerodynamics, I have installed the radiator further forward and panelled behind it to form a smooth air flow path from the radiator to the twin nostril outlets.
To improve access to the rear of the dashboard if needed, I cut the dash into top and bottom halves. I fitted brackets to the rear side of the mouldings so the pieces can be screwed together. The bottom half is painted and the top half is trimmed in leather cloth.
All I have to do now is to fit the sunroof, furry dice, towbar and the car is complete, sort of.

Dave Punshon

[Ed. After having typed this in, I rang Dave who explained to me how to find out the Brake Pedal Pressure needed for the above calculations, without going to the expense of an inline pressure gauge.
1). Put a brick on the brake pedal.
2). Place your bathroom scales on the brick.
3). Stick your foot on the scale and press, then read off the pedal pressure.
[Its easy when you know how!]


Email to Robin Sundt:

Mail email to sundt@pncl.co.uk

Return to Robin Sundt's Family Home Page
Return to Robin Sundt's GT40 Home Page