E30 Zone

I've spent the past week or so reading posts on this forum (some very good info from Geoff Bob and Theo) regarding master cylinder sizing when using an aftermarket pedalbox. Although, I've read that much now that im still not 100% with what I should really go for.

Some basic info:

- Front brakes - WMS 4pots (1.38" piston) with DS3000 pads (will be changing to a different pad once worn out).
- Rear brakes - Standard calipers with DS2500 pads.
- Standard 15" BBS wheels, usually R888/equiv or Slicks
- GAZ Golds all round
- I also have a hydraulic handbrake in line on the rear line, using a 0.625" Wilwood cylinder.
- Car has an SR20 fitted, from what I've read the standard Nissan slave cylinder is 0.625".
- Track only.

Now the pedalbox has a pedal ratio of 6:1, an adjustable bias balance bar, requires separate front/rear cylinders and a clutch cylinder.

I've previously been running a Tilton lever style proportioning valve in the rear line with the hydraulic handbrake, but I'm unsure whether it will still be required with having a balance bar in the pedal box also?

I'm after a short pedal feel, but after research still dont know what combination will give me that especially with the different front and rear calipers.

Would a 0.750" for the front, 0.687" for the rear and a 0.625" for the clutch suffice? Or could anyone suggest a setup?

Any help would be appreciated.

From experience (im old fashioned, simple and bad at maths) you'll need a 6.25 for the clutch. 7.00 for the rear and 6.25 for the front. Although having used a similar setup to this in the past, i felt it could have really done with a 0.58 front which is available from wilwood. Pedal box setup and master cylinder sizing is as much to do with preference and feel as it is to do with the math and physics to make it work to a decent standard.

You shouldnt really need the lever in the line if things are set up right but wouldnt do any harm to have it in the line

Thanks for the input. Yeah after reading I realise its down to preference, should have really explained myself better - I was more trying to find out what feel a certain size gives etc...

F0xy,

I’m assuming you are using stock discs on both front and rear?

The 3/4”a M/C you propose for the front will do fine. I would, however, recommend that you reduce the rear M/C by one size from 11/16”a to 5/8”a.

Take note that the leverage lost when the servo is deleted is regained (at least in part) by:
a) increasing the pedal ratio,
b) fitting callipers with bigger/more pistons, and
c) fitting larger diameter discs.

In your case you will be at a disadvantage since (despite the increased ratio of the pedal) your callipers and discs are little or no bigger than stock. This means that you will need to put significantly more effort into pushing your brake pedal down than say someone with larger callipers and discs. I typically work towards a pedal force of 35 to 40 kg to reach the threshold of traction (the point at which your wheels lock). In your case you are looking at 55 kg. Typical calliper and disc sizes would be at least 4x40mm pistons on a 280mm disc on the front, with 2x35mm pistons on a 260mm disc on the rear. Anything upwards from there on is a potential solution.

See the graph below for how your brakes will typically perform. I’m assuming a coefficient of friction between pads and disc of 0.5 at working temperature. I sincerely recommend that you use pads of identical compound on both front and rear or your brakes will not function as indicated. I’m also assuming identical tyre size front and rear. Knee pressure of the brake proportioning valve (installed in line with the rear callipers) is set to 600 psi.



See here for an explanation of how a brake proportioning valve works. Take note that it does not serve the same purpose as the bias bar, but functions more specifically to shift the brake bias forwards when the rear line pressure exceeds the ”aknee”a pressure to which the valve is adjusted. Without the prop valve your rear line pressure (and hence brake torque) will rise too high as the cars weight shifts forwards, thus allowing the rear wheels to break traction sooner than would occur with the prop valve.

Thanks for the input Geoff... Fronts are VW Corrado discs if i remember correctly, approximately 280mm yeah. Rears are standard. Wheels are the same all round, with 205/50/15 tyres. Im not sure if any bigger can be had under the standard 15" BBS that I ideally want to retain. I guess I could go twin-pot on the back, but it wont be for a while yet.

Had a good read about the knee valve and it makes sense, I guess its a good idea to leave it in then. I will read over a couple of your other posts in related threads but I think I will go with the smaller rear m/c.

Ferodo claim a coefficient of friction for the DS3000s of approximately 0.65, and approximately 0.55 for the DS2500s on the rear (both at working temp).

f0xy wrote: Fronts are VW Corrado discs if i remember correctly, approximately 280mm yeah.

Sorry, I assumed you had stock discs as you didn’t state otherwise. Give me a chance to recalculate before you change anything. Could be that your original rear M/C will be just fine after all. FYI, since you have an SR20DET under the bonnet I have assumed the same weight distribution as I did for Theo’s car: 1160kg with a 55% forward bias. It is usual, however, to weigh the car before performing such calculations.

f0xy wrote:Wheels are the same all round, with 205/50/15 tyres

Exactly as I assumed in the calculation.

f0xy wrote:Had a good read about the knee valve and it makes sense, I guess its a good idea to leave it in then. I will read over a couple of your other posts in related threads but I think I will go with the smaller rear m/c.

It is a good idea to leave it, but it is more important to set it up correctly. Read the data sheet for your prop valve (should have been in the box with the unit or can usually be found online) telling you what setting gives you what knee pressure.

f0xy wrote:Ferodo claim a coefficient of friction for the DS3000s of approximately 0.65, and approximately 0.55 for the DS2500s on the rear (both at working temp).

I usually assume a value of 0.5 for DS2500 pads, which I think you indicated is the pad you will be using once your DS3000’s on the front are done. Personally, I use DS3000 pads all round for my track only car. Just say the word and I can adjust the value as you see fit.

Incidentally, my brake setup was very similar to yours until I fitted two pot callipers to the rear with 280mm BMW Z1 discs. My four pots on the front (over the same 280mm Corrado disc) do, however, have larger pistons. Before you consider fitting two-pot callipers to the rear I would suggest fitting larger piston callipers to the front. Talk to Keri about this. It sounds to me as if you have the 4-pot WMS callipers that he sells for use with the standard servo assisted tandem M/C. If you are fitting / have fitted a pedal box you should consider larger. Maybe Keri could do you a part exchange deal or at least point you towards someone who wants WMS 4-pots that are compatible with the stock tandem M/C.

For what it's worth here are the "static, at caliper/disc" numbers we use:

Standard:


1.25/1.25" WMS 280mm front - the default piston size for RHD cars with OEM master cylinder due to the E30's tendancy to a soft pedal:


Optional 1.38/1.38" WMS 280mm front numbers:

Thanks Keri,

It appears that F0xy has your 1.38"/1.38" calliper (last of the three) with a piston area of 38.6cm^2, which is only slightly larger than the OEM 36.2cm^2. No doubt suitable for use with the stock tandem M/C, but he may need to look at something a bit bigger for use with a non-assisted pedal box.

Regards,
Geoff

I suppose it depends how small the master cylinder can be if you're binning the servo.

We do have 1.62/1.75" staggered-bore versions of the T20 caliper as well...totally usless on anything other than a custom setup of course!

As long as parts are mint/unused (and we have stock!) we can sometimes swap from one size to another for the cost of shipping?

What is Theo using on the front of his? The calipers have only done about 20 laps of Oulton if that, the thing is any bigger caliper wont fit under the stock 15" I'm guessing?

keri-WMS wrote:I suppose it depends how small the master cylinder can be if you're binning the servo.

Unfortunately, that's the deciding factor once the pedal ratio has been fixed. 5/8" is the smallest M/C that I am aware of, and there is already one proposed for use in the rear line. Reduce the size of the front M/C and you have to reduce the rear along with or upset the bias (beyond what can be compensated for with the bias bar), which isn't possible if its already a 5/8".

keri-WMS wrote:We do have 1.62/1.75" staggered-bore versions of the T20 caliper as well...totally usless on anything other than a custom setup of course!

Now that's more like it! Useless with servo asisted brakes but ideal for use with a pedal box. I have 38mm/41mm (1.5"/1.6") staggered bore callipers on my own car. I had thought that WMS might doing something silmilar.

F0xy, if I can recommed that you seriously consider bigger front callipers before looking to your your rear callipers. Unless you increase the size of the front calliper pistons you are going to need the legs of Garth to press your brake pedal.

If it helps, think of it this way: Your rear callipers and discs are OEM. Your front callipers are only slightly bigger than OEM and you have a slighly bigger disc. Your front brakes are thus only slightly stronger than OEM. If you delete your servo, your only mechanism to regain some of the lost leverage will be your slightly taller pedal ratio. You will finish up with very heavy brakes (as indicated in the above graph). If, however, you are a 6ft rugby player then this is probably a non-issue. Just a suggestion.

f0xy wrote:What is Theo using on the front of his? The calipers have only done about 20 laps of Oulton if that, the thing is any bigger caliper wont fit under the stock 15" I'm guessing?

Not necessarily a bigger calliper, just bigger diameter pistons. Mine are under 15" rims with same 280mm diameter disc.

Trouble is if I'm swapping bits I need to be able to re-sell them as brand new.

The T20 has several piston sizes but the caliper doesn't get any bigger on the outside.

Theo's also on 1.38/1.38's, more recently he also got the rear 2-pot kit. He's got big 1.75" 2-pots though, the new rear ones are 1.38" 2-pot.

If Theo had a setup worked out before changing to 2-pot rears, that would be what you need - if it worked well etc of course!

I just had a look back at the calculations I did for Theo and you are correct Keri, he does indeed have the same front calliper (35mm pistons) and a 280mm disc, and yet his pedal clocks in nearly 20kg lighter compared to F0xy’s setup ??

Major difference appears to be the much bigger rear calliper, which facilitates a larger rear M/C, which allows both front and rear M/C’s to be further downsized to regain the lost leverage due to the deleted servo. See below the result that I got for Theo. He had, if I recall correctly, a sliding rear calliper with 36mm piston on a 280mm disc, with which I recommended using two 5/8”a M/C’s (front + rear) and the knee pressure set to 500psi (as per the graph below).



F0xy, this of course puts to bed my recommendation of fitting a larger front calliper in favour of a larger rear calliper since a larger rear clearly facilitates the use of a larger rear M/C, thus allowing both front and rear M/C's to be down-sized together to regain some more of the lost leverage. This was not something I considered as a possible solution ”“ My sincerest apologies.

As Keri said, if the setup I originally proposed to Theo worked (we’ll have to wait to see what Theo has to say about that) then you might consider copying what he had.

Ahhhhh..... I forgot Theo had 280mm (Z1?) rear brakes before going WMS 2-pot.

One thing you can do to increase the rears while staying with E30 rear bits is find some Touring calipers. Saloon rears have a 32.0mm piston, Touring rears have a 35.0mm piston. Not common knowledge!

Also while the newer WMS 2-pot rear has 1.38" pistons (=35.05mm, so about the same as Touring rears), is normally has the 1.75" piston so that's an easy option.

keri-WMS wrote:Ahhhhh..... I forgot Theo had 280mm (Z1?) rear brakes before going WMS 2-pot.

I'm not sure that it was a Z1 disc Keri. Hasn't Theo gone the 5-stud conversion route? Been such a long time I honestly can't remember. I have 280mm Z1 discs on the rear of my car with 2-pot 35mm callipers and I am very happy with them.

Oh, and I like your touring calliper suggestion. I'll add that to the mix when I get to my computer at home.

I'm pretty sure it was Z1 - and that he's on 4-stud.

I can't get to 280mm with the WMS 2-pot sadly as it's lug-mount, 290mm is the minimum and even that is tight (but not an issue really as it still goes in the BBS)!

Here's what I get with the Corrado 280mm disc with 35mm/35mm 4-pot callipers on the front, and the OEM 260mm disc with 35mm 2-pot or sliding calliper on the rear. Front and rear M/C's are set at 7/10" and 5/8" respectively, with the bias bar just off-centre at 52% towards the rear. I've assumed a friction coefficient of 0.65 for DS3000 pads. Note how the pedal force required to reach the threshold of traction has reduced nicely to 35kg. The prop valve is set to 400psi. Now that's more like it

Very very interesting, and thanks again Geoff/Keri for doing the leg work. I was about to reply saying I'm not anywhere near a 6ft rugby player, then scrolled down to find the rest of the replies!

I think I may get on the lookout for some touring rear calipers then (should have kept them when I broke my touring a while ago!!) as it seems to be a solution that could work. Would a pad with a higher coefficient value require less force at the pedal also?

Like I mentioned its got DS3000s in the front at the moment, but once they are worn I will change for something else - if something with a higher value would work better I will go in that direction also...

Hi Jord,

Have you abandoned the idea of having a servo and mc setup on the RHS of the car? Say using pedals from an e36 for example.

Cheers

It seems far simpler (fabrication wise) to fit a pedal box over a servo/MC on that side. I cut the battery tray out the other night:



Even though it looks like there is room, once the standard pedal box is bolted up inside the servo would be touching the inner arch in the 'correct' position - so it would need moving in slightly taking the clevis further away from the pedal. Then I've got the issue of the clutch master/slave mismatch situation which I wanted to resolve, especially as im changing it for a paddle before it goes back in...

All in all I would rather have control over the cylinders for front/rear/clutch, incase I change calipers in the future and end up changing cylinders to match etc..

You have the advantage (as do Theo and I) of having the exhaust manifold on the left. You could thus, after some construction work, install your pedal box on the right with M/C's protruding through the fire wall if you wanted to. Would be worth the effort imo to get away from the bar that connects right to left.

Theo's setup:


My own:



Since you have an SR20DET I'd suggest copying what Theo did. Have you chosen/bought a pedal box yet?

I have not purchased one yet, I'm still deciding between floor mounted or overhung. I have the seat very low and far back so either would work. My previous car had a floor mounted box which I did prefer, but I'm still undecided with the E30.

I've also sourced some touring rear calipers so will swap them over at some point..

Just to bump this thread up - I was having a discussion the other night and we were comparing what would be needed for the same setup to work with a stock (well 'normal') single master cylinder. Is there any way of working out what the minimum size MC would be to give 'normal travel' for the calipers on the car? 23mm?

1.38" fronts
35mm touring rear calipers
same bias/knee valve

I'm still going with the pedal box and separate MC's, but it was just something that was discussed a few evenings ago.

A tandem M/C is simply the front and rear M/C's connected sequentially rather than next to each other. You would calculate the bore diameter of the front and rear cylinders of a tandem M/C no different to calculating the sizes of the M/C's in a dual M/C setup (taking cognisance of the fact that the pedal force is applied equally to both cylinders). If you arrive at different bore sizes required to operate the front and rear callipers then you would require a "staggered bore" M/C. The chances that you will find one that meets your exact requirements will be nigh on impossible. You'd be browsing catalogues for days trying to find something that matched and would no doubt have to settle for a single bore tandem M/C that offered the best compromise. A far from ideal solution!

Single bore servo assisted brakes are a convenient and cost effective solution, but require that both the front and rear callipers be chosen simultaneously to match the M/C. A dual M/C setup is the only solution that allows you (within reason) to choose your front and rear callipers independent of each other.

Thanks for the explanation, I have seen people using the staggered bore items before but not so common as you say.

The only thing I dont really fully understand is the matched calipers if using a single more M/C. With the fronts being 4pot 35mm, and the rears being (sliding) 1pot 35mm, the rears are technically half of the fronts? What would this result in if for example a single bore 22mm M/C was used?

Another part of the discussion was is it actually possible to work out how much force the servo assists with (obviously dependant on size) regards to the force needed at the pedal?

Even with a tandem M/C the bore size must be matched to the calliper and disc to achieve the desired front:rear bias. If a single bore tandem M/C is to be employed then the callipers and discs must be sized intelligently in order to achieve the desired front:rear brake bias, as can reasonably be expected to be the case with the OEM components.

Take the OEM components on a 325is as an example:
0.033m diameter pistons on the rear compared to 0.048m diameter pistons on the front. The rear callipers thus have a piston working area of 0.000855m^2, and the fronts 0.001809m^2. Both are single pot sliding callipers on near enough identical diameter discs.

Now, the OEM tandem M/C will result in equal fluid pressure applied to the front and rear hydraulic lines when you put your foot on the pedal. This has to be the case since a) the OEM M/C is single bore and b) the force applied to the pedal is applied (taking the pedal ratio into account of course) to both sections of the M/C equally, since they are connected in tandem. And so, with equal fluid pressure arriving at both the front and rear callipers, the only means (ignoring any proportioning valve for the moment) to adjust the front/rear brake bias is to adjust the size of the callipers and or discs.

Now, while the OEM front and rear discs have near enough the same diameter (0.258m rear, 0.260m front), the rear employs a slightly narrower pad, and thus has a slightly larger mean working radius (0.112m rear, 0.105m front). Since brake torque is proportional to the force applied by the callipers multiplied by the mean radius of the disc, we have:

Rear brake torque [Nm] = 2*P* k*0.000855m^2*0.112m=P*k/5221

where P is the fluid pressure (in Pascals or N/m^2) coming from the M/C and k is the coefficient of friction of the pad. Take note that the factor of 2 appears since the calliper bites down on both sides of the disc. Similarly,

Front brake torque [Nm] = 2*P* k*0.001809m^2*0.105m=P*k/2632

As stated, P is the same at both the front and rear callipers, and we assume that k is the same at the front and rear since we are using pads of the same friction material.

Dividing the front brake torque by the rear brake torque we get 5221/2632 = 1.98. That is to say, the front to rear brake bias is as good as 2:1.

Now, what your question basically boils down to is how will the stock front:rear bias of 2:1 be affected should you use your proposed callipers and discs.

To answer that: The working area of a 35mm diameter piston rear touring calliper is 0.000962m^2, while the mean radius of the rear disc is, as before, 0.112m. The working area of your 4-pot 35mm diameter piston front calliper is 0.001924m^2 (twice that of the rear), while the mean radius of the 280mm diameter front disc is 0.115m. And thus:

Rear brake torque [Nm] = 2*P* k*0.000962m^2*0.112m=P*k/4641

Front brake torque [Nm] = 2*P* k*0.001924m^2*0.115m=P*k/2259

Dividing the front brake torque by the rear brake torque we get 4641/2259 = 2.05. That is to say, the front to rear brake bias is once again (as per the OEM setup) as good as 2:1.

Your proposed brake setup is, therefore, 100% compatible with a single bore tandem M/C provided the tandem M/C is servo assisted when operated from a stock pedal. Since, however, you have chosen callipers and discs that exert a 15% greater wheel torque for a given fluid pressure you will find the brakes to be sharper (greater brake torque for a given pedal force), but at the expense of increased pedal travel compared to the OEM brakes. To regain the stock feel (if you so desire) you will need to increase the working area of the 23.8mm (15/16”a) stock M/C by roughly 15% (in order to reduce the fluid pressure by 15%). This means increasing the diameter by ~7%. A 25.4mm (1.00”a) diameter single bore M/C (such as from the M3) should do you fine.

EDIT: If you know the transfer function of your chosen servo (force multiplication as a function of pedal displacement) then it is quite easy to determine the front and rear wheel torque as a function of your pedal force. Since, however, I can only guess at the transfer function of various boosters, I cannot plot for you such a graph.

I am planning a very similar conversion to Foxy on my forest rally car in order to try and get proper feel through the brake pedal. My plan was a Wilwood pedal box with 6.25:1 ratio, and in view of GeoffBob's observations above, 4 pot calipers (with 44.5mm diameter pistons) on the front together with 280mm discs.
Guidance regarding suitable master cylinders for front and back would be appreciated. I could easily change to Touring rear sliding calipers if that helped matters.
Thanks