This page is... UNDER CONSTRUCTION
WARNING: I am not an expert on cars, automotive engineering, VW Beetles, beach buggies or anything else discussed in these pages. What I have written is my current understanding of the issues involved in building Tina's buggy. These opinions are based only on my own research in books and on the web. They, therefore, have no basis in fact, may well be wrong and potentially downright dangerous if taken to be gospel truth. If you intend to use any of this information for any purpose other than pure entertainment, then please get its validity confirmed by someone who knows what they're talking about! You have been warned...
This page contains the following Sections:
A car brake system consists of a brake pedal, master cylinder, rigid and flexible brake lines, front and rear wheel cylinders/calipers, brake shoes/pads and brake drums/disks. When the brake pedal is depressed, the master cylinder pressurises the brake fluid in the entire system to a uniform pressure. This pressure, in turn, applies a force to the wheel cylinders/calipers, which presses the brake shoes/pads against the rotating drums/disks to provide the stopping force. Nice and simple :-)
On a 'normal' road car, with standard wheels and tyres, the front brakes do most of the work, because when the brakes are applied, weight transfers to the front tyres and the rear tyres are unloaded. Since the amount of traction at a wheel is proportional to the load it exerts on the ground the front wheels are far more capable of stopping the car than the rear wheels. Therefore, the braking bias on most road cars assigns about 60% of the braking effort to the front wheels. This is done by making the piston diameter of the front wheel cylinders/calipers larger than that of the rear cylinders/calipers. In addition to this, the front brakes usually also have a larger brake friction area than the rear wheels to reduce wear, aid cooling and minimise brake fade.
A beach buggy, is different in one important way to the above general description of a 'normal' road car; The weight distribution is biased much more to the rear of the car due to the light body and rear mounted engine and gearbox; The rear tyres are also usually bigger and provide much better grip than the front tyres.
In order to overcome the difficulties of using the 'normal' brake set-up in a beach buggy, it is necessary to change the braking bias so that more of the braking effort is applied to the rear wheels than the front wheels. If this is done, however, additional consideration is needed to improve the performance of the rear brakes, as they weren't designed for this primary braking role. This will involve maximising the size of the brake friction area and the diameter of the brake.
Disk brakes have several advantages over drum brakes; They brake more progressively than drums; They are less prone to brake fade than drums, due to better cooling; They don't trap dirt inside, which minimises brake wear; They dry out much quicker after getting wet, which improves braking performance; They are self adjusting.
Both drum and disk brake set-ups weigh about the same, so there is little advantage either way in terms of unsprung weight.
The disadvantages of disk brakes on a beach buggy are that they are: Over-kill for the very light weight of the car; Difficult, and therefore expensive, to get with a 5 bolt 205mm pcd pattern, as custom parts must be used because VW never made a 5 bolt front disk brake; Difficult to adjust the front/rear braking bias; Too biased to the front of the car.
Whilst disk brakes have many desirable features for an off-road beach buggy, the low speed, light weight and rear weight bias of the buggy mean that a drum brake set-up will be more than adequate and a hell of a lot cheaper than disks.
I will, therefore, use drum brakes all round on Tina's buggy.
There are two options for the front drum brakes on Tina's buggy, which will have ball-joint front suspension and a 5 bolt 205mm pcd wheel attachment:
The more straightforward of the two options appears to be the first, as it uses only unmodified stock VW components from a single year and model. There is also no real need to improve the performance of the front brakes on an off-road beach buggy, as they are already adequate and I need to shift the braking bias to the rear brakes.
I will, therefore, use 1966-67 Beetle front drum brakes on Tina's buggy.
All external front brake components will be painted black to maximise cooling.
Note: All ball-joint Beetles use a front wheel outer bearing size of 17.45mm (ID) but the inner bearing size changed from 27mm to 29mm (ID) in May 1968.
Having selected the type of front brakes to use, I need to consider how to shift the braking bias from the front to the rear of the car. Most Beetles use 22mm front and 17mm rear brake cylinders to give approximately 60% braking effort at the front and 40% at the rear. In a beach buggy, this needs to be reversed so that approximately 40% of the braking effort is at the front and 60% is at the rear. This can be achieved most easily by using a 17mm brake cylinder at the front and a 22mm cylinder at the rear.
There are two types of 17mm REAR drum brake cylinders that will fit in the FRONT drum brakes from a 1966-67 Beetle:
The 68-or-later Beetle option appears to be a no-brainer, especially as the 57-or-earlier cylinders are over 4 times more expensive than the 68-or-later ones!
I will, therefore, use 1968 or later Beetle REAR brake cylinders in the FRONT brakes on Tina's buggy.
As the brake bias will be shifted to the rear in Tina's buggy, I need to consider how to improve the performance of the rear brakes, as they are not usually required to play such a large part in stopping the car. This will involve maximising the friction area of the shoes, maximising the diameter of the drums and maximising cooling.
Pre-1968 Beetles had 30mm wide brake shoes, giving a lining area of 40 square inches, whereas the 1968-or-later Beetles had improved 40mm wide brake shoes, giving a lining area of 56 square inches.
If I was to use the rear drums to match the front drums off the 1966-67 Beetle specified above, to get the correct 5 bolt 205mm pcd bolt pattern, that would give me very poor performance from the rear brakes, as they would be the earlier 30mm wide type. This is no good, especially as I don't know if they can be made to fit to the IRS rear suspension on Tina's buggy.
Thankfully, the Type 3 (Squareback, Notchback or Fastback) had 45mm wide brake shoes, giving a lining area of 69 square inches, with slightly larger drums at 248x46mm, rather than 230x40mm on 1968-or-later Beetles. The Type 3 already uses a 22mm rear brake cylinder as standard, which will give me the rear brake bias I want. Also, if the rear brake drum from a 65-or-earlier Type 3 is used, this will give me the correct 5 bolt 205mm pcd pattern. Note: The 65-or-earlier Type 3 drum can be used with the brake shoes and cylinders from any year of Type 3. Finally, the 65-or-earlier Type 3 rear drum brake assembly will fit any 68-or-later swing-axle or IRS Beetle. Hoorah!
I will, therefore, use 1965 or earlier complete Type 3 rear brakes on Tina's buggy.
All external rear brake components will be painted black to maximise cooling.
Note: Using the larger Type 3 brakes on the rear will increase the buggy's track by about an inch but this shouldn't be a problem, as the Doon has generous rear arches and I don't want 'monster' wide rear wheels.
Master cylinders come in two basic types, single and dual circuit.
In a single circuit system, the master cylinder has a single piston. This single piston pressurises the brake fluid in a single brake pipe circuit and applies the pressure to all of the four wheel cylinders.
In a dual circuit system, the master cylinder has two pistons. One of the pistons pressurises the brake fluid in the brake pipe circuit which goes to the front wheel cylinders and the other piston pressurises the brake fluid in the brake pipe circuit which goes to the rear wheel cylinders. The front and rear brake pipe circuits are completely separate from each other.
Prior to 1967, all Beetles used a single circuit, 17mm bore, master cylinder. However, in 1967 the dual circuit, 19mm bore, master cylinder was introduced on some models. The dual circuit master cylinder was introduced to comply with safety regulations which required that a leak in the system would not affect all of the brakes simultaneously. This is a good thing :-)
As I have retained the stock brake wheel cylinder sizes but have just swapped them front and rear, there should be no need to change the bore of the master cylinder from the standard 19mm of the dual-circuit system. This is also beneficial, as the standard pedal effort and travel will be retained.
I will, therefore, use a 1967 or later, dual circuit, 19mm bore, master cylinder on Tina's buggy.
There are two different types of residual pressure valves which are used in both front and rear disk and drum brake systems as follows:
Note: Residual pressure valves should be fitted as close to the master cylinder as possible.
VW originally fitted 10 psi residual pressure valves to all 3 outlets (2 front and 1 rear) on drum/drum dual circuit master cylinders. When disk/drum master cylinders were introduced, the two front residual pressure valves were replaced with 2 psi units and the single rear 10 psi unit was retained. VW then dispensed with residual pressure valves altogether on later drum/drum master cylinders. The VW residual pressure valves were not integral with the master cylinder but were screw-in units, fitted between the master cylinder outlets and the brake lines.
Click here to see a picture of the stock VW residual pressure valves on a '71/'72 US Super Beetle with drum brakes all round.
Replacement Type 1 dual circuit master cylinders available now do not have any residual pressure valves fitted and the VW type screw-in units don't appear to be available any more.
Luckily, there are several specialist brake component companies that still manufacture 10 and 2 psi residual pressure valves, as pictured above, which fit anywhere in the brake lines.
I will, therefore, fit 10 psi residual pressure valves in both the front and rear brake circuits on Tina's buggy.
There are two different types of proportioning valves which can be used with either disk or drum brakes, both of which are used in the rear brake circuit, as follows:
VW never fitted either type of proportioning valve to the Beetle. However, since I have significantly changed the stock braking bias towards the rear of the car and I don't really know whether the new ratio will be exactly right or not, I may have to fit an adjustable proportioning valve to the rear brake circuit on Tina's buggy, if necessary, to enable me to compensate for any rear wheel lock-up problems that I might have created.
Metering valves (also referred to as 'hold-off' valves) are used in the front (disk) circuit of a disk/drum brake system to prevent the pressure rising in the front disc brake circuit until the pressure at the rear drums has risen sufficiently to overcome the brake shoe return springs. This helps prevent front wheel lock-up by ensuring that the rear brakes are always actuated first.
As I don't intend to use front disk brakes on Tina's buggy, and I've shifted the braking bias to the rear, I don't think I'll suffer from front wheel lock-up and so I shouldn't need one of these valves. However, if I subsequently change the front brakes from drums to disks, it may become necessary to fit one.
A hydraulic Park-Lock can be fitted in the rear brake circuit to give far better holding power on steep slopes than the normal Beetle hand brake. This is achieved by maintaining the line pressure in the rear brake circuit to hold the rear brakes on. The Park-Lock is a simple hydraulic valve and no additional electrical or mechanical linkage is required. To actuate the Park-Lock, press and hold the foot brake, push the Park-Lock button down, release the foot brake and it's locked. To release the Park-Lock, press the foot brake again and it opens automatically. The only problem with this type of device seems to be that the pressure required to release the lock must be greater than the pressure when it was locked. So, if the brakes were applied particularly hard when it was locked, then it can be almost impossible to release it, especially if the temperature (and therefore pressure) has risen significantly since it was locked. If this happens, the pressure must be reduced by opening a bleed valve before the lock can be released.
A hydraulic Park-Lock cannot legally be used as a hand brake (aka emergency or parking brake), as it is not independent of the primary brake circuit, although practically it fulfills the same purpose.
Since the Beetle hand brake was pretty good; the buggy is much lighter than a Beetle and I have also upgraded the rear drum brakes to bigger Type 3 units, I don't think I'll need a hydraulic Park-Lock on Tina's buggy.
In a dual circuit braking system, the brake fluid reservoir must be separate (or at lease divided) for each of the circuits, so that a fluid leak in one of the circuits cannot drain the fluid from the reservoir for both circuits.
The SVA test requires that a brake fluid level warning light be fitted to the reservoir so that the driver is alerted to a low brake fluid condition.
