A common question is ‘What bore size master cylinders should I use? In theory, it is possible to determine the correct size of the master cylinder (piston diameter) by calculating pedal ratio, pedal travel and caliper piston diameters but it’s often easier to ‘suck it and see’ by choosing a .750" master cylinder to begin with and working from there. Many factors will determine the optimum master cylinder size such as weight of the vehicle, tyre diameter, brake pedal length, weight distribution and servo or non-servo assistance. Brake pedal ratio is often the easiest parameter to change. Increasing the pedal length is, in effect, increasing the leverage but the pay-off is less fluid movement for a given pedal travel. If the pedal is 12" long from the fulcrum to the foot pad and the cylinder push rod is 3 inches from the fulcrum then the pedal has a 3 to 1 ratio. Increasing the ratio to say 4 to 1 will give more leverage but with longer pedal travel. For a clutch the same rules apply to the release fork. A higher ratio will make the clutch action easier but there will be less travel of the release bearing. Determine the distance the release bearing has to travel to fully disengage the clutch (usually about 10mm) and work backwards from there. A release fork with 2 to 1 ratio will require 20mm travel of the slave cylinder. Brake hydraulics is really all about fluid movement so, to give you an idea of the effect of changing master cylinder sizes, here’s a chart of how much fluid each size master cylinder moves with each 1cm travel of the push rod. You can see that a 1" cylinder will move 2 ½ times more fluid than a .625" cylinder. But the pedal will require 2 ½ times more force to get the same braking performance. However, the pedal travel will be 2 ½ times less. Of course, if you want to increase braking power without changing the pedal or cylinder you could just add a servo.
|CYLINDER PISTON DIAMETER||FLUID MOVED FOR 1cm OF PISTON TRAVEL|
|0.625" (5⁄8")||1.98 cc|
|0.750" (3⁄4")||2.85 cc|
|0.825" (7⁄8")||3.45 cc|
Our single fluid reservoir #FLRES1 is a great favourite with racers and rally car builders but, without a fluid level warning device, it’s not IVA compliant as a brake reservoir on new car builds. All is not lost however. It can be modified with our #FLCAP. You can probably use a similar procedure to that outlined below to modify other non-float caps.
It is not possible to use the rubber diaphragm with this modification.
Most production cars feature a tandem brake master cylinder that presets the front to back brake balance in accordance with front to back weight distribution. However, although many specialist vehicles use donor braking components, the weight distribution can be vastly different from the donor car creating potentially lethal unbalanced braking characteristics. One solution is to install a twin master cylinder system - one for the front brakes, one for the rear and an adjustable balance bar to control the proportion of front and rear braking pressure between them. For correct and safe operation it is essential to understand some basic principles in the design of this type of dual brake circuit brake pedal box and how to set it up.
The balance bar is designed to rock around it’s spherical pivot as braking pressure is applied. Each end of the balance bar passes through a cylindrical threaded pivot pin upon which a threaded clevis is free to rotate. The master cylinder pushrods are each connected to a clevis. By screwing the balance bar clockwise or anticlockwise its pivot point will move left or right in the retaining tube in the brake pedal. More braking force will be applied to the master cylinder closest to the pivot point and less to the one furthest away.
A very common mistake is to lock the position of the balance bar by tightening the locknuts directly on the clevises effectively locking their floating action. THIS IS INCORRECT AND DANGEROUS. If the balance bar is to be locked after adjustment, all that is necessary is a single locknut tightened against a sliding sleeve to lock the thread against one pivot pin, thus allowing the clevises to swivel freely and maintain alignment with the master cylinder push rods and pistons. The sleeve must have a wall thickness thin enough to allow clearance in the clevis. Also, to maintain sufficient braking pressure on one cylinder should the other fail completely you must minimise the clearance between the two clevises and the steel tube in the brake pedal so that the balance bar rocking movement is limited to the angle necessary for the desired front to back balance, and no more.
NOTE: IVA compliance requires that, after the brake bias has been set to an appropriate and acceptable ratio, the adjustment be rendered immovable. We understand that drilling and pinning is no longer acceptable and that the locking nut must now be welded to the threaded shaft.
HOW DO YOU MEASURE TUBES AND HOSES? To the experienced car builder that may seem like a daft question but you may be surprised at how much confusion this can cause a beginner. Lets start with the basics - apart from the length, which is obvious, every hose or tube has three dimensions: 1. Outside diameter (O.D.) 2. Inside diameter (I.D.) 3. Wall thickness - the thickness of the metal or rubber tube wall. You can measure the length easily enough with a tape measure but for the other three measurements the best tool is a Digital or Vernier Caliper. But what dimension do you ask for when you need to order some hose or tubing - Inside or Outside diameter?
HERE’S A SIMPLE RULE TO REMEMBER.... Everything soft like Rubber, PVC, Silicone, Neoprene, Polythene is always denoted using it’s Inside Diameter (I.D.) and everything hard like Steel, Aluminium, Copper, Brass, Nylon etc. is always denoted using it’s Outside Diameter (O.D.) Asking for a rubber hose with an O.D. of 43 mm is meaningless - unless you know the wall thickness - you can then calculate the I.D. In fact, if you know any two out of I.D., O.D. or wall thickness, you can easily calculate the third. Absolute measuring accuracy is not always important. For instance, 38mm rubber hose is, as far as cooling system plumbing goes, the same as 1½" hose. The size difference is unimportant. However, 5/16" copper tube looks alarmingly similar in size to 8mm copper tube. But if you’re using compression fittings and olives on, for instance, a fuel system using the correct size components is essential for safe, tight joints.
NOTE: Rubber hoses are not always perfectly round and can be flattened slightly in storage. Whilst this does not affect the performance of the hose it can make them difficult to measure accurately. Simply take the dimension at the widest point and again at the thinnest point and average the two.
WHAT IS P.C.D.? This means 'Pitch Circle Diameter' and is the diameter of a circle drawn through the centre of a set of holes on a circle. So, four equally spaced 12mm holes on a 98mm PDC could refer to the size and spacing of your wheel bolts, for example.
ENGINE TURNING is the name given to a regular pattern of abraded circles on a metal panel. This impressive decorative effect is surprisingly easy to achieve using basic home workshop equipment. The key – Scotchbrite. No, not the wimpy pad that you use for scouring the Sunday roast dish. There are much tougher versions available. Check out your DIY Superstore. It is available in sheets like wet & dry paper, in rolls 25mm wide or in discs, glued to a hard backing pad for use in your small angle grinder. At a push, try your industrial cleaning supplies merchant – the coarsest grade floor polisher scrubbers are a little on the big side but should be a cost-effective way of buying the stuff. The tool – A valve. Yes, from a cylinder head. We use mostly 25mm or 1 inch. Be sure that the valve has a flat head –dished valves are a no-no. Cut a disc from your Scotchbrite the same size as your valve head. A wad-punch (sharp, tubular cutter for making holes in leather, gaskets etc.) is ideal but scissors are OK. ‘Superglue’ the disc to the valve, allow to cure and fit in a drill chuck on a pillar drill. Don’t attempt to engine turn with a hand drill – it won’t work.
Mount the valve tightly in the drill chuck and set the speed at slow. Use plenty of light oil (Duck Oil or WD40) and, applying even pressure each time, gradually move across the panel overlapping the last turn by about a third. You can clamp a piece of timber to your drill platform as a guide-bar to ensure parallel lines. Start either at the top or the bottom and work from left to right. Overlap each circle by approximately one third. At the end of the first row move back and start a second row, placing the circle between the two above and overlapping them by one third. And so on. Always keep the panel flat on the drill base and try to avoid forming small sections of circles at the edges or the holes in your panel. Be careful, this can tear your Scotchbrite disc clean off the valve. Whenever you use a new piece of Scotchbrite always ‘break it in’ on a piece of scrap. This will help ensure that your circles maintain a uniform texture. As always, it makes sense to practice on a piece of waste material, perfecting your technique before finally creating your masterpiece.
CORRECTING HOSE SIZE MISMATCHES Mainstream car and motorcycle manufacturers have a habit of making hoses and hose outlets in some weird and wonderful sizes like 23mm or 41mm. And sometimes there is no exact hose size equivalent when you come to re-plumb your cooling system. OK, you can stretch a 32mm rubber hose over a 34mm outlet and you can squeeze a 25mm hose down to 22mm with a good hose clip but it’s better to aim for a good fit - and here’s one way to achieve it.
A bicycle inner tube is really just a very thin-walled hose with it’s ends joined together. You can get four different sizes for about a tenner on ebay that will suit any hose size on your system. Cut a 30mm section from one and you have a wide ring that you can slip over your outlet to increase it’s diameter by about 2mm. You can even build up the thickness with a couple of layers if necessary. In the picture above a short section of inner tube has been used to increase the diameter of one end of a 25mm hose joiner to 27mm.
WELDING UNIVERSAL JOINTS It's sometimes necessary to weld a Universal Joint to a shaft for steering columns or gear linkages - a perfectly acceptable practice. The problem is, most UJ's have rubber or plastic seals that keep out the dust and grit and keep in the grease. Excessive welding heat, just a couple of inches away will melt the seals and destroy the component. The solution? - Immerse most of the UJ in a bucket of water whilst you weld the above the water level. The water will soak away heat and protect the UJ's seals. In the picture we're supporting the UJ and shaft with mole grips just above the water level. Of course water and mains electricity don't mix, so take the necessary precautions if you're trying this one. But if you have the savvy to build a car you'll know that already.
CUTTING U CHANNEL RUBBER TRIM Our rubber ‘U’ channel is pretty flexible stuff and will form nicely around gentle curves. But if the required radius is too small the trim may pucker as it tries to form itself to the curve. One method of ensuring a neat fit is to cut the arms of the ‘U’, either along the whole length or just where it goes around the tight corners. In other words, cut the tall ‘U’ down to a short ‘U’. We use this technique to edge our stainless pedestal mirror with a short length or our #TRMU2, increasing it’s edge radius to make it IVAOK.
Here’s our method:
Find a scrap piece of aluminium sheet, preferably the same thickness as the gap in the ‘U’ channel - in this case 2mm. Push the length of trim fully onto the edge of the sheet. Then, with a straight edge or steel rule and a sharp knife, cut down the length of the trim legs either on both sides or one side as required.
If you’re bonding the trim to your component don’t forget to wash the ‘U’ channel out with solvent to remove the wax residue from the manufacturing process.