Brake circuit design (without math)

Warning! Don’t touch your brakes unless you know what you are doing. As with other safety relevant components in your car (steering, suspension a.s.o.) your life depends on it. What I describe in the following is about my car. You may find information suitable for your application, but I can in no way be made responsible for anything happening to you,  your car or anybody (anything included) getting in your way as you drive, brake or trying to.

Components list
Front:  HiSpec 4 piston aluminium brake calipers, M16 bolt pattern, ventilated discs 245x20mm
Rear: standard Ford 9” drums
Pedalbox: highly modified OBP
MC’s: Girling type (see text for diameters)
Fluid reservoirs: Aluminium with level sensor
Fluid: ATE DOT4
Tubing: Cunifer
Flaring-tools: K-tools

My car had hanging pedals from stock. I went to floor mounted pedals during the rebuild and redesigned most of the braking system. Although you could do this with mathematics, starting from caliper diameters over pedal ratios and needed volume, I had some of the components already in place and had to adapt everything around. Anyway, the brake pedal now operates two separate master cylinders for the front and rear brakes, I stayed with the cable operated clutch. The pedalbox has been purchased from rallynuts.com

                                                

This particular model has a square recess where a chassis rail is present in most kitcars. Thus, it can easily be adapted. Although it is intended to be floor-mount, I welded brackets on to the chassis for a secure fixing. The kit comprises a bias bar, brake fluid reservoirs, hoses and two master cylinders (MC):
- diameter 0.700 intended for rear brake circuit
- diameter 0.625 intended for front brake circuit

As my particular set-up showed, those values are not applicable for me. Both MC’s are too spongy for the front circuit and pedal travel therefore is too long. The pedal even hits the bulkhead. The smaller MC is all too big for the rear circuit which results in a very hard pedal. Further on, I will check whether I will insert a pressure relieve valve to accommodate with the rear brakes.

Assembly  
For some reason, and I can only guess it is because of safety matters, manufacturers and resellers of brake components don’t give a lot of information about the assembly and set-up of their products. Maybe they prefer professionals to do those installations to prevent us amateurs fiddling around. On the other hand, the more good information I have, the better and safer my modifications will be.

The first problem I encountered was how to assemble the bias bar. It is not obvious, at least not for me, how to do it, unless somebody tells you. The clevis is mounted first, and then the master cylinder’s pushrod is bolt on. This rod can rotate, if you have enough space, you can rotate the whole MC to bolt it to the clevis (prior to plumbing). If you mount the clevis to the MC’s pushrod prior to fit it to the bias bar, it is impossible to mount them both. As soon as you screw in the second clevis, the first one is screwed out. Sounds funny? It is!

Above diagram shows how to align everything to start, further settings will show alterations.

After that, you have to be familiar with the function of that bias bar. I found a very good explanation in a UK forum http://www.locostbuilders.co.uk/viewthread.php?tid=171423 . Hopefully this thread will be reachable with above URL forever.

This diagram explains how it works, but … you must know that while pushing the brake pedal, you start braking on one MC, which then becomes the pivot point together with the big internal olive of the bias bar. Only then force is applied to the second MC and the braking process goes on. Now you can set up the system to suit your driving or legal regulation i.e. the front brakes should lock before the rears. In above you can see that the front MC pushrod is not as deeply bolt into the clevis as the rear MC.

Design
The front brakes need more fluid for operation since the brake pistons are of a larger diameter. This means that the front MC needs more travel for a given MC diameter. There are some possibilities to achieve this:
- set the bias bar askew as of start of pedal movement (see above 'no pedal effort'-position)
- shorten one of the pushrods (not recommended)
- put a spacer under one of the MC’s to put it away from the bias bar (see below)

To start the design for my car, I had the following set-up with the new pedalbox:

The approximate pedal travel of 124mm must suffice to activate the front brake until block and even leave some place before hitting the bulkhead, which leads to problem number 1:
- the 0.625 MC intended for front brakes needs too much travel and the pedal hits the bulkhead
- the 0.700 MC intended for rear brakes used up front for trial wasn’t big enough either

 
Solutions
- order a 0.750 MC and try (order placed March 5, 2013, arrived March 12,2013)
- alter the pedal ratio or bring it nearer to the driver, both need major mechanical alterations (will do if no success with above solution)

Possibilities to reduce distance between pedal and bias bar / MC and bias bar:
 - Put in washer when mounting MC
 - Shorten pushrod
 - Screw pushrod further into clevis
 - Manufacture specific clevis

Further design is only feasible once problem no.1 (pedal hitting the bulkhead) has been solved.

Some TEE (trial and error engineering) to follow...
2013 03 18  

Yesterday, we put in the 0.750 for the front brake circuit and the 0.625 for the rear. The pivot is set completely to the left side (front brake). Problem no.1 is solved, the pedal not hitting the bulkhead anymore. After bleeding both circuits, it is not obvious that braking is ok because pedal travel is reduced by the whole setup and I don’t know if the front brakes get activated enough for a full stop or block.
Next thing to do: try to further reduce pushrod length off the rear MC (bolt it further into clevis). I have to design a little tool to grip the pin holding the protective sleeve and turn the pushrod. I will turn it into the clevis as far as possible without blocking the bias bar movement (tonight). Next option will be to recess the rear MC mounting. Pedal travel is around 100mm for now. With a ratio of 5:1, this gives 20mm piston travel (max is 25mm). 

I measured the piston travel from rest to fully applied and found 25mm (1” … ?).
2013 03 20  

Thoughts …
The pushrod of the rear MC was already bolt in at its max, no further setting possible.
Next weekend, I want to assemble the car for roll out and do some testing outside. Therefore I need to mount the scuttle and plumb the water circuit ready, fasten the suspension bolts once the car rests on its wheels, and maybe some other smaller jobs.
2013 03 23  

Short roll out with 0.750 front and 0.625 rear, I can block the front wheels but haven’t seen if the rears block first, second or if they block at all. Next time I will have Max or Pit (or both) to watch the wheels. Ideally, it is possible to do the fine setting. Pedal feel is quite ok and I look forward to the final setup. 
2013 09 20 (engine rebuild in between)  

Pedal feel is quite hard for full stop, but ok for normal operation.
Car passed MoT (SNCT-Sandweiler). Note that for some technical controls (depending on countries), the bias bar has to be blocked and dash mounted knobs permitting bias adjust on the fly are prohibited.
In conclusion
Although my approach was far from being scientific, I managed to design the brake circuit to operate the already existing braking gear. Selection of MC’s was by trial and error (and some swearing), but one should be able to resell what isn’t needed or keep it for other projects. At the time of construction, universal Girling style MC’s cost around 25.-£. Click labels at the end of this post to see more about braking on this blog.

Cheers.