Master cylinder upgrade

[Demlotcrew]

we worked out that for the E36 M3 front calipers and E36 318ti rear calipers I'm fitting, with a stock master cylinder, the rears would barely be gripping the discs and all a proportioning valve could do was reduce the fluid pressure in the rear circuit.

Absolutely, the M3 front pads have a much larger surface area and the disks are larger in diameter.

Andrew

[GeoffBob]

As I see it, there are really only two good reasons for fitting a pedal box with dual M/C's, neither of which will result in improved braking unless setup correctly.

1) To facilitate the appropriate distribution of fluid pressure to the front and rear callipers specifically where non-standard callipers and/or discs have been fitted. That is to say, aftermarket brakes have been fitted that are incompatible with the stock tandem M/C (assuming of course there is no other tandem M/C that is suitable).

2) Where the brake servo has been deleted and it is required that some of the lost leverage be regained.

In some cases (as Keri has more than adequately demonstrated) it is possible to change callipers and discs without a net change in fluid pressure required to achieve the same braking torque as before, and thus the existing tandem M/C may be retained without compromising stability (by upsetting the front/rear bias). With many brake conversions, however, this is not the case. A dual M/C setup provides the opportunity to rectify this situation when non-standard callipers and discs are fitted.

It is worth noting (at least I think so) that the invention of the brake servo has probably contributed more to vehicle safety than any other component, bar (possibly) the headlamp, mirrors and the grooved pneumatic tyre, so you have to have a very good reason to want to remove it, even from a race car. That reason is linearity of brake response. Due to the way in which a brake servo works the gain (multiplication) in brake force provided by a typical vacuum operated servo is not constant across the range of force applied by the driver to the pedal. That is to say, brake torque at the wheels becomes a non-linear function of the force applied to the pedal. This is not an issue for the average road user, but on the track it is highly undesirable and warrants, under certain circumstances, doing away with the servo.

Of course there are other advantages to having dual M/C’s, one of which is the ability to adjust brake bias while out on the track as a function of tyre wear and temperature (both of which influence available traction). This is not to say, however, that everyone should rush out and fit adjustable pedal boxes to their track cars. There is still plenty of room on the track for servo assisted tandem M/C’s, provided they do an effective job of operating the callipers to which they are attached.

[Theo]

Theo, I have the Video of DaveG driving your car and complaining about the brakes, I also recall you complaining about them being too heavy at Rockingham, not you?

Andrew

Is that the video on Vimeo? I don't remember Dave complaining about them, though he did drive when it had the dreaded EBC yellows so I wouldn't be surprised. They gave a bizarre feeling almost as if something was flexing, I thought maybe the calipers were flexing but this theory was dispelled when I fitted PF01's.

I had/don't have any complaints about feel - I lreally like how the slightest application on the pedal will result in a measureable braking effect, this wasn't the case when I ran servo assisted brakes.

Hi Theo, quick question please

What is your opinion on having a suitable servo and master cylinder fabricated in on the drivers side of the car? For example, using the brake and clutch pedals from an s14.

Thanks.

Theoretically it's a good idea, but in pracitce it would be extremely difficult due to the strange shape of the bulkhead.

[Demlotcrew]

One of the areas that all of the calculations dont take in to account is the total pedal travel in relation to the throttle, too far down and heel toe gets tricky, too high up and the brakes are too sensitive.

One needs to have a decent amount of brake force and the pedal to be level with the accelerator.

Andrew

[Motorhole]

One of the areas that all of the calculations dont take in to account is the total pedal travel in relation to the throttle, too far down and heel toe gets tricky, too high up and the brakes are too sensitive.

One needs to have a decent amount of brake force and the pedal to be level with the accelerator.

Andrew

That's a very good point Andrew, something I've not yet considered. From the calcs though we should be able to work out the amount of movement in the master cylinder to apply a given braking force, and therefore work out the pedal travel taking into account the pivot point. Then measure this in relation to the accelerator at it's rest position.

Hopefully it's good news!

[GeoffBob]

From the calcs though we should be able to work out the amount of movement in the master cylinder to apply a given braking force, and therefore work out the pedal travel taking into account the pivot point.

This depends upon the bulk modulus of the working fluid and the length of the lines between the M/C's and the callipers. Other contributing factors are whether your lines include any rubber hoses that may tend to expand under pressure (hence the advantage of rigid and semi-rigid lines) and any slack (backlash) that may need to be taken up in the pedal and calliper before the pad engages the disc.

In principle, there would be zero pedal travel if the working fluid were incompressible and there were no backlash in any of the components. That is to say, the fluid would function as a mechanism for the transfer of force between the pedal and the calliper (unlike a clutch pedal that functions to transfer displacement). Imagine, if you will, pushing against a large static object (a wall, for example) with a rigid steel bar. The bar functions to transfer the force you apply to it to the wall. You can apply any measure of force you like to the rigid bar but the position of the bar remains constant so long as the wall stays in place. The moral of the story here is think force, not displacement.

As it is, even with a compressible working fluid, pedal travel with a set of non-assisted dual M/C's is significantly less than with a servo assisted pedal. This, however, does not reduce the "feel" of the brake since the feel has little to do with displacement and everything to do with the force that you apply. Your experience or sensation of the pedal under foot is in the force that you to apply to the pedal through your leg. The fact that your brake torque is proportional to the force applied to the pedal is the great advantage of non-assisted brakes (well, that and the fact that their performance is independent of the engine).

Finally, brake pedal height on a pedal box is adjustable and is always set up during initial testing to facilitate a comfortable heel-toe manoeuvre.

[Mikey_Boy]

Geoff - beautifully put as always!

Just one word here if you want to experience a truly great assisted brake set up...

Porsche

In my experiences, pedal boxes are the perfect solution, but boy do they need time setting up well. Probably because I am lazy and never going to be at Senna levels of commitment, I am happy to enjoy the compromise of a servo, slightly mis-sized M/C allied to my 8 pot calipers.

Depends on how much you want that extra 10%! For trackdays, not me... Racing? Whole other ball game!

Great thread!

[GeoffBob]

Thanks Mike. Good to see that both yourself and Theo are still around. Hadn't run into either of you for a while.

[keri-WMS]

In some cases (as Keri has more than adequately demonstrated) it is possible to change callipers and discs without a net change in fluid pressure required to achieve the same braking torque as before, and thus the existing tandem M/C may be retained without compromising stability (by upsetting the front/rear bias).

Ta Geoff!

Just a few things come to mind from what I read here:

There are pads available to fit the WMS T20 caliper ranging from £27.00>£160+vat/set...tends to muddy the waters especially as people have their favorites but cost really is a factor too.

Larger pad areas can absorb more heat, BUT the downsides are less brake torque for a given clamping load, higher unsprung weight (heavier caliper needed too) and higher cost. So for pure performance, a smaller pad is always better as long as it can withstand the heat....and many can. Large pads are a cheap and heavy fix, which is why OEM manufacturers tend to use them on heavy cars.

[Hoobs]

I don't remember Dave complaining about them, though he did drive when it had the dreaded EBC yellows so I wouldn't be surprised. They gave a bizarre feeling almost as if something was flexing, I thought maybe the calipers were flexing but this theory was dispelled when I fitted PF01's.

Interesting thread. Anyone else had problems with EBC yellows as I run them in the Wilwoods on my road e21? Once they get warm though, or under heavy braking, they just feel weird. I get the same flex feel or as if something just isn't right with the brakes. I've stripped and checked them twice though and found nothing wrong.

On the track e30 I'm running Wilwood 4 pots on a basically stock 325 brake set-up. What's the next logical upgrade?

[keri-WMS]

Depends how big your wheels are? Ferodo might be worth a stab over EBC.

[GeoffBob]

Once they get warm though, or under heavy braking, they just feel weird. I get the same flex feel or as if something just isn't right with the brakes. I've stripped and checked them twice though and found nothing wrong.

I would hazard a guess that the "yellow" compound is an excellent conductor of heat (high thermal conductivity) with a high total heat capacity, and thus does an annoyingly good job of conducting heat away from the disc to the calliper, causing the temperature of the fluid in the calliper to rise much more aggressively, and hence the spongy feel.

[Demlotcrew]

Mark, get some of these on your callipers and monitor the temps.

http://www.ebay.co.uk/itm/270926119535? ... 285wt_1197

^ They are one time use only.

Andrew