The Ramblings and Rantings - A Life Unreal, Well, Mostly...

Having posted recently about the availability and cost involved in a number of radial master cylinder options, we have been chatting to quite a few people about how the choice is made. As Scottie would say, "Ye cannae beat the laws a' physics Jim", so hopefully taking a peep at the immutable laws behind what happens when you squeeze the lever may be useful.

First of all, the choice between the type of leverage employed. The axial type of master cylinder is based on a first order lever, like pliers. A radial master cylinder is based on a second order lever, like nutcrackers. To compare the ease by which effort can be applied using each sort, crack a nut with a pair of nutcrackers, then try cracking the same type of nut with a pair of pliers. No contest! So, it is easy to see that with a radial pump it is easier to apply load to the piston and get a bigger return in terms of braking effort to the force applied by your fingers.

From a manufacturing point of view, it is also easy to adjust the load ratio of a radial type by moving the load point towards or away from the fulcrum. This is why you will see in some cases, the 'size' of a radial master cylinder quoted as 17 x 18 or 17 x 20. The first figure is the diameter of the piston and the second is an expression of the movement ratio of the lever, giving a longer or shorter stroke to adjust the volume of fluid displaced.

Whichever type of pump you have, the key to it all is in the above sentence; volume of fluid displaced. It is this factor that decides everything to do with your braking effort and feel at the lever. When you get to 'feel' the hard and fast rules begin to blur a little as some riders prefer a firmer feel and some prefer a little more movement. Hopefully the chart below will help with this aspect of choice.

In considering which pump goes with what caliper set up, I have considered some of the more common installations, from single caliper with one large piston up to two calipers with six small pistons each. Changes in piston diameter have a huge effect on overall area and thus the swept volume of fluid that must be moved to bring the pads into contact and start the braking effort.

Consider a pizza restaurant. You order a 12inch pizza. The waiter brings you two six inch pizzas, explaining that they have run out of 12 inch bases. Has he robbed you? Yes, of course he has. Taking a simple version of Pi at 3.142, we divide our 12 inches by 2 = 6 inches. Then square the result = 36. Multiply by our simple Pi and we get 113.112 in² of pizza. Taking a 6 inch pizza, our radius is 3, squared that is 9 and multiplied by Pi we have 28.278 in² per pizza. OK, so the nice waiter gave you two of them, so that is 56.556 in², so quite a big difference eh?

Converting that to metal, and taking a benchmark of fluid volume for 1mm of combined pad movement for example, our single piston single caliper at say 48mm diameter, you will need to move 1.809792 ml of fluid (((24²³ which we divide by 1000 to convert to ml. Working that back the other way, take the area of the master cylinder piston and divide it into the required volume, which will give the required piston travel to displace that amount of fluid. As an example, if we take a 13mm master cylinder, the piston area is ~ 132.75mm², which translates to ~13.633mm of piston travel. At the other end of the scale, if we applied a 20mm piston master cylinder to this caliper, the required movement would only be 5.76mm.

That is piston travel though, so the length of our brake lever and distances between the load (master cylinder piston), fulcrum (lever pivot) and effort (your fingers) are the factors that turn piston travel into real live lever movement. Taking my own bikes as examples here, I have one radial master cylinder with a lever length of 180mm and fulcrum to load length of 24mm, giving a movement ratio of 7.5:1. Taking the movement of the piston and lever as isoceles triangles of different height, our 13.66mm piston movement equates to 33° vertex angle, while at the other end of the scale, 5.76mm gives us a vertex angle of 13.78°.

Meanwhile back in the real world… applying those angles to the triangle described by the end of the lever, we have movement of 102.2mm at one end of the scale and 41.7mm at the other. Don't forget though, that is at the very tip of the lever, which is very rarely where you will find your finger, so measuring to where you prefer to apply force is handy. Measuring this on my own bike to a point where the slack is just taken up gives me a reading of 70mm, which is the feel that I like.

In our theoretical caliper range, for the sake of simplicity, I have just considered the setups of 1 caliper, 1 piston (48mm), 1 caliper, 2 pistons (48mm), 1 caliper, 4 pistons (30/34mm), 1 caliper, 6 pistons (35mm) and the two caliper equivalents. Assuming the lever mechanical advantage of my own (Frendo) radial pump, it seems that the mechanical advantage calculation of 7.5:1 holds fairly true, so the assumption is made for the sake of simplicity that each mm of piston travel needs 7.5mm of lever travel. The table below is my best guide to travel with common setups, but takes no account of feel, which is personal. My best advice there if you are undecided is to talk to fellow riders, ascertain what setups they have and ask for a grope to see if you like it.

Of course, then we have to throw a few curve balls into the mix like the likelihood of needing a bigger initial movement with calipers made to wider tolerances, or the single piston design where you are moving the body, all of which are going to have an effect on travel to the first bite and the amount of foce then required for full pad contact. Add to that the theoretical size may not be a commercially available piston diameter and you still need to do a bit of head scratching when starting with a blank sheet of paper.

Still, I hope that this information, intended purely as a rough guide and written from a personal point of view helps. Please feel free to call or email if you have any questions and we will always do our best to point you in the direction that suits you best.

The columns labeled 'firm', 'medium' and 'soft' relate to the pump diameter that will give that type of feel. For these purposes, firm is approximately 40mm end lever travel, medium is 70mm and soft is 100mm. The bottom two rows are for calipers with dual piston sizes, in this case 30/34mm.

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With the world supply chain in turmoil at the moment, the last factor can often be significant in making your choice. Below is a table of the most popular the manufacturers whose products we can supply, indicating what sizes are available, the retail cost and availability. So, if you know what size you need, take a look. If you are still deciding on a size, scroll down a bit where you may find the information useful.

For more information regarding the choice of master cylinders, there will be another article soon, but in the meanwhile, if you are looking for something specific, this table shouold help.

Piston Dia.	Manufacturer/Model	Price	Availability
12mm	Magura HC1	£372.40	Back order, 20 weeks
	Magura HC3	£809.71	Back order, 20 weeks
15mm	Magura HC1	£372.40	Back order, 20 weeks
	Magura HC3	£809.71	Back order, 20 weeks
16mm	Accossato	£295.26	Folding lever in stock, fixed lever, 2nd week December
	Brembo	£219.87	Back order, no date
	Hel	£239	Back order, no date
17mm	Frando	£233.34	Available now
18mm	Magura HC1	£372.40	Back order, 20 weeks
	Magura HC3	£809.71	Back order, 20 weeks
19mm	Accossato 19 x 18	£281.21	Available now, folding lever, fixed lever, 2nd week December
	Accossato 19 x 20	£301.87	Available now
	Brembo 19 x 18	£219.87	Back order, no date
	Brembo 19 x 20	£219.87	Back order, no date
	Frando	£233.34	Available now
	Hel	£239	Available now, limited stock

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This season, we are changing the brakes, well the master cylinders at least. Frando make radial pumps in 15, 17 and 19mm piston sizes. Their products are all high quality, forged aluminium bodies and levers with a great range of span adjustment.

We will be using the 15mm pump on our wet bike with a single blue spot caliper and for the dry, with two Triumph TT 600 Nissin four pot calipers, we will be using the 19mm.

Frando has been making high quality brake system components in Taiwan since 1993 and has gained a reputation for producing highly functional items with a pleasing finish. The products are well supported here in the UK with ready availability of spares like levers, reservoirs and mounting kits in case of mishap, and overhaul kits to keep your pump working perfectly for many years to come.

In addition to the brake master cylinders, there is also a range of matching clutch cylinders in 14, 15 and 17mm, so if your bike has a hydraulic clutch too, then you can treat it to a matching pair.

Take a look at them in our online shop here: Frando Master Cylinders There are great deals for YPM members too so make sure we know you are one of us!

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