FAO GeoffBob

[Demlotcrew]

Geoff,

Can you work out for me the best MC for the following calliper configuration.

Front 6 pot.

27.00mm
31.8mm
38.1mm

Rear 4 pot.

38.1mm
41.3mm

Also what about the braking effort by both?

Front's are 330mm with a pad depth of 51.5 and area of 77.20cm

Rear's are 304mm with a a pad depth of 50.3 and area of 57.4cm

Much appreciated

Andrew

[GeoffBob]

No problem, will do.

Would make my job easier if you had corner weights for me, otherwise total weight with rough estimation of weight distribution will do for a start.

I assume this is for a dual M/C setup with pedal-box? Will you be fitting a brake proportioning valve. It’s recommended, otherwise your brake bias will be static (typically set around 2:1) regardless of dynamic weight distribution (dependant upon how hard you are braking). So much better to have the bias dynamically adjusting according to pedal force.

Hard or soft pedal desired? With callipers that size you can easily achieve a fairly soft road pedal (more braking torque for less pedal force), but won't be as easy to modulate on track with large pedal displacement (commensurate to a soft pedal).

[Demlotcrew]

Thanks Geoff

I don't have any weights I'm afraid, the car should quite light, no idea on how much!

So as not to get bogged down can you use Theo's figures?

Pedal to be medium and ideally numbers for both single and dual mc's

Andrew

[GeoffBob]

OK, will do.

Keep in mind that a tandem M/C with servo will only work if your chosen set of discs and callipers already results in close to a 2:1 front:rear bias since (by definition) a single bore tandem M/C delivers equal fluid pressure to the front and rear. With a dual M/C setup this can be compensated for by adjusting the bias bar or changing the size of one or the other M/C.

I should have a few results/options posted up for you by tomorrow.

Geoff

[Demlotcrew]

Thanks Geoff,

Looking forward to the results

[GeoffBob]

Andrew,

Assuming equal fluid pressure applied to the front and rear callipers, then your cars front:rear brake bias will be roughly 52:48. That is to say you have nearly equal braking torque at your front and rear wheels. Given that figure corresponds roughly to your cars weight distribution (say around 55:45 front:rear with your 4-pot S14) then you'd have near perfect brake force distribution if your cars dynamic weight distribution were identical to its static weight distribution. Unfortunately, this is not the case. Weight always redistributes forwards while braking. This shift in weight can be reduced by lowering the cars COG or lengthening its wheelbase, although not usually an option.

I can only imagine that if you were to employ a single bore tandem M/C with a setup such as this that there would be a very definite tendency for the rear to break traction (lock up) under hard braking, since, the harder you decelerate the more weight is shifted forwards reducing traction over the rear wheels, allowing the rear to break loose while the front continues to grip. To avoid this situation we must reduce either the hydraulic pressure applied to the rear as the weight shifts forwards, or reduce the size of the rear calliper and/or disc until a static brake bias of 66:33 (2:1) is realised.

More to follow in due course ”¦

[Demlotcrew]

Geoff, thanks for that.

At least your maths matches my actual setup and both front/rear ratio in hardware terms are spot on with the cars weight distribution.

I'm running a screw type bias valve inline to the rear, I hope this will be enough for me to keep my tandem MC and not lock rears before the fronts, but this is where I need you to conclude if this actually work?

Andrew

[GeoffBob]

It will certainly help, but you'll have to set it to a fairly low knee pressure for it to be effective with rear callipers and discs that big. I'll do a calculation assuming a single bore tandem M/C, but with a brake proportioning valve in line with the rears. I suspect, however, that the model may indicate that you'll be better off with two different sized M/C's.

[GeoffBob]

Results are in:

With a 1" single bore tandem M/C, a 4:1 ratio pedal, and no inline brake proportioning valve you can definitely expect the rear wheels to brake traction ahead of the front. The figure below shows that, as the weight of your car shifts forwards under the action of increased pedal force, the front brakes are only using up ~60% of the traction available at the tyre contact patch when the rear locks up. Nonetheless, this will still result in ~1.0g of deceleration on a smooth tarmac surface with semi-slick tyres (both brakes and tyres at working temperature). Of greater concern, however, is that the model indicates that you only require 25kg of pedal force to reach the threshold of traction. For good reason I have not included a servo in the simulation (no two servo’s are ever the same), so with the servo connected I can only imagine how sensitive your brakes must be. For a start you could try them with the vacuum hose to the booster disconnected. It won’t change the displacement of the pedal required to achieve the same braking torque at the wheels, but it will make the pedal more apparent underfoot. And lets face it, 25kg is hardly lifting weights, so it will still be a workable pedal (discounting the action of the spring inside the servo housing). More importantly, the brake torque will be a linear function of the pedal force. I’d suggest trying it next time you are at the track.



The figure below shows the effect of installing an inline prop valve. As I suspected, it is necessary to adjust the prop valve to its lowest setting (wound all the way in) at 200psi in order for there to be any visible effect. Reducing the line pressure to the rear callipers reduces the braking torque at the rear wheels, allowing the fronts to work all the way through to ~80% of the available traction. The net result is now 1.1g of deceleration. The price you pay, however, for this reduction in the rear line pressure is an increase in the pedal force, with ~35kg required to reach the threshold of traction at the rear wheels. This I do not consider to be a bad thing since I would consider 35kg (still without a servo) to be an ideal pedal weight.



Finally, the figure below shows the effect of installing a pedal box with side-by-side dual M/C’s operated from a 6.25:1 ratio pedal. A 3/4”a (19.05mm) front M/C and 7/8”a (22.23mm) rear M/C result in just over 30kg of pedal force required to brake traction simultaneously at the front and rear wheels, achieving a best possible deceleration of 1.2g. I would consider this to be a medium weight pedal, but you could reduce it further by decreasing the size of the M/C’s. Note how the rear M/C is larger than the front M/C in order to reduce the rear line pressure, thus reducing the rear brake torque. The bias bar is adjusted slightly off centre, 52% towards the front. Due to the size of your chosen discs and callipers your whole brake system operates at only a fraction of the pressure of other hydraulic brake systems, and thus we still cannot get away from the fact that the brake proportioning valve must be adjusted to its lowest setting. Most prop valves will not reduce the knee pressure any lower than around 180 to 200 psi. Adjusting the prop valve upwards will only see the rear break traction ahead of the front. If you wish to make greater use of the valve then I would suggest (ironically) fitting smaller brakes to the rear. As a rough guide, assuming you have front and rear discs of similar dimensions, aim for a rear calliper with half the total piston area of the front.



Andrew, I’ll leave it to you to decide if you wish to pursue that last 0.2g of deceleration. In all honesty there isn’t much difference between the second and third graph. You can make up half the missing deceleration by simply installing your prop valve and winding it all the way in (as per the second graph). That would certainly require the minimum of effort.

HTH

Geoff

[Demlotcrew]

Thanks Geoff much appreciated!

I already have the rear bias valve but the brakes are very sensitive and at times I don't have confidence in them (when wet mostly).

My options as i see it

1. Keep as is.
2. Disconnect the servo.
3. Replace 25mm MC with a stock E30 tandem MC which is 22.2mm and 17.46mm (which is close).
4. Split MC's in a pedal box.

I have just had a look and a pedal box plus two AP Short Master cylinders are not that much money so i might actually go ahead and get that, but i think i will miss the servo too much, I hope this is not a mistake, but i also don't want to keep struggling on with over sensitive brakes in the wet.

Thanks again! I owe you a beer

Andrew

[GeoffBob]

Thanks Geoff much appreciated!

You’re welcome Andrew.

I already have the rear bias valve but the brakes are very sensitive and at times I don't have confidence in them (when wet mostly).

The way I see it is you have this problem because the force required to operate your brake pedal is much too little. You will thus barely feel the pedal underfoot, and you will thus struggle to modulate your brake torque, particularly in the wet.

This problem has come about largely due to the fact that such large callipers require very little fluid pressure to bite hard enough on your discs to lock your wheels. Your situation is exacerbated by the fact that you have a servo fitted to callipers that (other than on a very heavy vehicle) will not require any form of assistance. I honestly don’t think you would miss it if you were to lose the servo, not with callipers and discs as big as you have. To the contrary, without a servo you will be able to feel the pedal underfoot, and hence you will have greater control of your brakes.

If I could ask you to try something for me Andrew: Wind the valve all the way in, or at least just short of all the way in. Then disconnect the vacuum hose from the servo. I am sure I don’t need to mention that the hose from the intake manifold should be blocked to stop it sucking in air. Do this as an experiment, not as a solution. If you are happier with how it feels then it will give you an idea of the direction you need to follow. Please try this and let me know what you think. While the pedal will feel harder, you will have more control.

[Demlotcrew]

Thanks Geoff,

The Irony is that there were a lot of reasons to go for larger disks, the main one was because of the design of Hartge Classics 16" the spokes bend inboard as they approach the hub, thus making larger callipers easier to fit when the disk is larger. Other reasons were durability, at £230+VAT per disk with a wear rate of 0.5mm per side and pads costing over £260 (just for the fronts), they will last and will take much more abuse as seen at Rockingham last year.



I will of course try what you have suggested and report back, but it wont be for a long while as the car is currently off the road with me working on it, if i dont have the pedal box in by then.

To simplify things im just going to go for one of these (less the MC's). - Thats assuming the pedal OE ratio will be ok.

http://www.ebay.co.uk/itm/BMW-E30-Brake ... 539wt_1199

Thanks again!

Andrew

[GeoffBob]

... a stock E30 tandem MC which is 22.2mm and 17.46mm

Crap! I am not sure why but I had in mind that the stock E30 325is M/C was a single bore 15/16" (23.8mm) unit. I've just done a search and I am clearly wrong. Sorry about that.

Take note that fitting a smaller M/C than you have at the moment will only exacerbate your problem. The smaller the piston in the M/C the greater the fluid pressure it produces for a given pedal force. I shall, however, run the numbers to see if you could use the OEM M/C without a servo.

[Demlotcrew]

Geoff, currently I have a E32 735iL 25.4mm MC.

E30's came with (depending on engine and spec).

22.2/17.46mm
20.64/20.64mm (two port)
20.64/20.64mm (three port)

Andrew

[GeoffBob]

Thanks Andrew, it would appear I was getting confused with the E30 M3 M/C which has (I believe?) a single bore 23.8mm unit.

[GeoffBob]

Andrew,

It would seem that you could indeed achieve very good results with the OEM 325is M/C (as per your cylinder measurements above). The graph below indicates that your front wheels would break traction very slightly ahead of your rears at a pedal force of 21kg, provided you set your prop valve down to its minimum setting of 200psi.

Note, however, that you would absolutely have to disconnect your booster else you would finish up with brakes even more sensitive (and thus prone to locking up) than you have at the moment. Given the size of your discs and callipers I really don’t think that you would miss that servo, not one little bit. I would go so far as too say that 20kg on the pedal to reach the threshold of traction constitutes a very light pedal.

I would honestly suggest that you at least try this option with the servo disconnected. If you like it (and I believe that you will), then lose the servo altogether. Please note that I honestly would not suggest that you drive your car on the road (I assume your car is road legal) without servo assisted brakes if it weren’t for the fact that you have such large discs and callipers.

[GeoffBob]

[Demlotcrew]

Thanks Andrew, it would appear I was getting confused with the E30 M3 M/C which has (I believe?) a single bore 23.8mm unit.

Hi Geoff,

Yes that is the E30 M3 Stock size tandem MC.

You can find technical details on many MC's on the ATE site.

http://www.ate.de/generator/www/com/en/ ... ng_en.html

On the left you will see the menu, click on "Online Catalogue - Parts"

A new window will open up, click on "Vehicles" at the top and then start your search.

Andrew

[Demlotcrew]

Geoff, why would i have trouble with the pedal box from ebay with the same size dual MC's as that of the stock E30 MC?

What i mean is, if i remove the Servo and use the Stock 22.2/17.46mm would that not have the same pedal ratio as the ebay pedal box using two MC's?

I hope they sell it without the MC's

Also what would the small MC's do to the pedal travel?

Andrew

[GeoffBob]

Geoff, why would i have trouble with the pedal box from ebay with the same size dual MC's as that of the stock E30 MC?

What i mean is, if i remove the Servo and use the Stock 22.2/17.46mm would that not have the same pedal ratio as the ebay pedal box using two MC's?

Now that is indeed a very good question Andrew. The short answer is no, and here’s why:

When you place two single M/C’s in parallel (next to each other) connected via the adjustable bias bar, the force applied to the pedal is divided between the two M/Cs. For example, if the bias bar is set to 50:50, and you apply 10kg to a 1:1 ratio pedal (for example), then 5kg will be applied to each M/C.

However, when you connect two M/C’s in series (so that one pushes on the other) as is the case with a tandem M/C, then the force applied to the pedal is applied directly to both M/C’s. For example, if you apply 10kg to the same 1:1 ratio pedal then that 10kg will be applied to both M/C’s.

What this means is that (assuming same sized M/C’s) you will require twice the pedal force with parallel connected M/C’s than will be required with tandem connected M/C’s to achieve the same hydraulic line pressure. To put it another way, dual M/C’s require twice as much pedal force to operate compared to the same M/C’s connected in tandem.

There are two ways around this problem for a given set of callipers and discs:
1) Increase the pedal ratio (although going from 4:1 to 8:1 wouldn’t be practical)
2) Decrease the bore size of the M/C’s to raise back up the line pressure for a given pedal force.

I hope they sell it without the MC's

I believe that the M/C’s are usually sold separately according to your requirements.

Also what would the small MC's do to the pedal travel?

Reducing M/C size always increases pedal travel. However, it is very important to realise that while changing from tandem to dual M/C’s doubles the required pedal force, it also halves the pedal travel. And thus, any reduction in the size of the M/C’s in order to return the pedal force to normal will simply return the pedal travel back to what it was before.

My only concern in reducing the size of M/C’s is that they might hold insufficient fluid to operate the callipers. You will notice, however, that M/C’s intended for motorsport applications hold significantly more fluid between the piston and the bore than a tandem M/C from a road car. I would go so far as to say that a Tilton 75 series single M/C is nearly as long as any OEM tandem M/C. At least then you get a friendly warning if your fluid starts to boil in the calliper, instead of instantly losing your brakes (as can be the case).

[keri-WMS]

In my past experience it tends to be the pedal hitting the floor if the the fluid boils rather than the piston bottoming out in the master cylinder? That's in un-stripped cars with carpets and the usual rubbish getting in the way though.

EDIT (second point deleted) - I just repeated Geoff!

[Duke137]

Sorry to thread hijack but Keri are you getting your PM's ??

Cheers

[keri-WMS]

Yup, I read them by email and only look at the 'zone ones when I reply so I can see easily which ones are in need of a reply! I'll PM you an update now..

[Demlotcrew]

Yup, I read them by email and only look at the 'zone ones when I reply so I can see easily which ones are in need of a reply!

I do that too, but i go one step further, i forget to reply

[keri-WMS]

I've got a spreadsheet of hundreds and hundreds of PMs to keep them in one place! #nightmare#