So what I used to have, like several years ago, were these same calipers, run off a 22mm m/cyl + 16v servo. What I now have are 2 x 0.625 balance bar master cylinders. I'm trying to work how how these compare, is this maths below right, or am I out by a factor of 2 on the tandem cylinder?
You tandem cylinder calculation is correct for your overall area of master cylinder. But I think your problem lies in that you rear calipers will have much smaller pistons than your fronts. So with the same size cylinder on both systems your rears pistons are moving much further than your fronts for every mm of pedal travel. The balance bar will obvioussly help sort this out a bit but I think you would get better feel with a more matched cylinder to caliper ratio.
Thanks. So the old set up is very comparable to the current, both on m/cyl area and clearly pot area (barring the absence of a servo and the OEM pedal leverage ratio vs the current pedal box ratio). What I'm finding is I don't have enough retardation for a given amount of (reasonable?) foot pressure. The backs aren't nipping up early and I'm quite happy with the balance I can get. The rear is on drums still, again, next step of understanding. Two small pistons pushing per side. Bore I've yet to measure, but nothing like a caliper piston. The pedal box is shortly to go in to be modified for more leverage ratio, but I am also considering a 14mm front cylinder (0.551 in). This is in theory to increase the line pressure in the front system. Very much thinking in progress!
This looks to be a very handy article on what you are doing. http://www.stoptech.com/tech_info/PedalSetup-DualMaster-Guide.pdf Off to find out how you work out the rear drum stuff. edit and more handy formulas. Starting to consider other problems like line expansion http://www.engineeringinspiration.co.uk/brakecalcs.html
Thanks to, http://www.vwspares.co.uk/g1brakes.php You either have 14.29mm or 17.46mm, rear cylinders, depending on whether the car came with a brake pressure regulator or not.
Thanks. So multipling by two pistons per wheel, the ratio looks a lot less than fronts would be (pressure less), but volume of movement higher due to the distance drums travel (hence pedal travel, but we know that about drums anyway). Reading the pdf now. Line expansion, flex in calipers etc, I'm going to ignore, as these are all like-for-like components, as this install was used on another car previously (much as I realise flex bleeds away pedal effort, on an unquantifiable basis).
14mm master cylinder is very small. How is the pedal travel at the moment? What calipers do you have also?
So you have alot of braking there to stop it providing everything is working OK. Is it the pedal effort your not happy with?
Not a huge amount of confidence inspiring braking at higher speeds - braking early / braking twice. Low speed it can just be mashed, but I don't want to be stamping hard at high speed. Just entirely lacks the adjustabilty I want from the ball of the foot.
Bingo: That's my next job - checking OE pedal ratio versus the pedal box. The increased leverage has to be where it's all gained back, as the servo has gone. Either that, or pedal boxes are only for use by highly talented animals! A little bit much snow on the ground to get a Mk1 pedal measurement today though, unless I can persuade the Danster to nip out to his shed!
What size of rear wheel cylinders are you using? Is there the chance these have been replaced with the smaller type during a refurb perhaps? The small diameter rear wheels cylinders do not use a load valve at all in oem format because they are not able to exert enough force to lock the rears. I used the larger diameter wheel cylinders and a click bias valve (so I know what position I need wet or dry) to limit rear braking effort depending on conditions. I have never been a fan of the bias pedal boxes myself as the pivot points vary depending on the actuating arm's adjustment which means that the maths to work out exactly what you need is constantly changing. I use a single Girling cylinder (single circuit) with no servo for my car. I had a friend that had problems with a bias box and braking problems, and after an awful lot of time and effort it eventually came down to the fact that his pads had "gone off".
I don't know which of the two rear cylinder sizes it's got, but have some spares which'll give an indication. This current set up has a bias adjuster aswell on it. Interestingly when I got the car, it had an OE pedal box on it, pressing on a single unservoed master cyl through the bulkhead, and a different set of 4-pot Dyanlites (dimensions of both not to hand). This set up was never staying on, but similarly I had no confidence in it whatsoever!
Increased leverage or harder pedal pressing. A favourite on most cars is to use smaller rear wheel cylinders so the rears can't be locked easily. Not needed so much with a pedal box. Whats the rest of your front brake set up Chris? 280's?
Note, drum brakes also do not entirely rely on the pressure force exerted on them. The leading shoe has a self applying effect also. Maths for drums soon get long winded.
Yep, self servo on the leading shoe. Try the old minis with the TLS system. Just something else to add to the equation. But my point stands that the small cylinders will not lock the rears in most cases. Can you lock the rears at all Chris? In the dry a lot of retardation force can be had from maximising the rear braking effort. I could feel my car sink down when the brakes were applied hard, as opposed to standing on it's nose with the weight transfer. EDIT. I will go drag a pedal box into the warmth in the next couple of minutes. Go work out what you want measured.
I struggle to lock the fronts unless animaling them! The back's aren't an issue at all, but the bias adjuster on the rear is very restrictive - modified Mini single line unit, so 0 - 100%. I've not even got into the realms of getting the rears too nip first, but I had it set up like that accidentally on a car previously, so I know exactly where you're coming from. I'm not too worried about the rears at this stage - and I feel that the issue isn't the rears, as winding the bar forwards doesn't fix the fronts. Cheers for the shed pilgrimage. Measure from pedal pad centre
Pedal pivot point too clevis pin hole for master cylinder pushrod is 50mm. Pedal pivot point too centre of brake pedal rubber pad where you push with your foot is 250mm. Obviously on a std mk1 you have all the 90deg pivots and linkages to work out too. Minus 8 out there so stick another beer for me on your tab please!
Hmmm, so 5.0:1 standard. Damn, I was hoping for less! Agree ref the pivots. Could go either way I suppose, so will check that too. I've got 5.4:1 or thereabouts currently. Therein lies the problem I think, because there is not enough compensation in the 5.4 increase edit: for the lack of servo. Bit more maths to do though. Will also check Autocavan's leverage, to see if that had any effect. That was the last install I used on these. One less beer owed from my Newky crate, many thanks!
When you say "Increased leverage or harder pedal pressing", in which context? It's got 280 fronts and the Dynalites as mentioned.
