All those who are considering brake upgrades

[Haveacamper]

As there have been so many people over the years expressing an interest in upgrading their brakes to Porsche brakes under the assumption it will provide incredibly better braking performance, I have recently had to work out the difference to enable me to register my recently imported camper to Australia.

As this forum has been an incredible place of support and knowledge sharing over the years I have been a member I thought that I would share with everyone my findings.

Initially, I will only deal with the front brakes – I will hopefully add the rear brakes with time.

Standard setup:
Twin pot calliper with 54mm piston
Solid disc of 280mm diameter

My upgraded Porsche Boxster S setup:
Quad pot calliper with 40mm & 36mm pistons
Vented cross drilled disc 320mm diameter

Standard non-servo Master Cylinder:
Bore = 20.64mm
Front circuit stroke = 21.00mm
Rear circuit stroke = 17.00mm

Standard Servo-assisted Master Cylinder:
Bore = 23.81mm
Front circuit stroke = 19.00mm
Rear circuit stroke = 13.00mm

Quickly, you can calculate the total piston area for both standard and Porsche setup
Standard = 4580mm^2
Porsche = 4549mm ^2

So for the same brake pressure the piston force would be almost identical (Force = Pressure X Area)

By upgrading the discs (read larger disc diameter), you do increase the distance from the wheel centre that the brake force is applied. Therefore, the brake torque (Force x perpendicular distance that the force is applied (Lever arm)) increases.

So even though the piston force is almost identical for the same line pressure, the increased disc diameter will improve the brake torque.

The Master cylinder sizing essentially translates the pedal force to line pressure. The Bore x Stroke provides the overall MC volume of a single pedal depression which obviously must be greater than the total piston volume required otherwise you “stroke” the MC. The greater the Bore, the lower the line pressure for a given pedal force. Another way to look at it…if you increase the bore and want to achieve the same braking torque, you would have to depress the pedal further.

Now if we consider replacing the non-servo MC with the Servo MC we see that the bore increases in diameter and decreases slightly in the stroke. The result is that the overall volumetric capacity goes up, but as I just noted, the line pressure for the same pedal travel would go down. THIS IS NOT CONSIDERING THE BENEFIT OF WHAT THE SERVO ASSISTANCE PROVIDES!!!

AN ASIDE: When I first “upgraded” my brakes and drove the camper to the MOT station, I did not have the servo hooked up to the engine vacuum. I was considerably underwhelmed by the braking performance (even with the rear disc brake conversion). Having now done these calculations I understand why. My overall piston area is slightly lower, I had a larger bore master cylinder providing less line pressure for the same brake pedal force!!!

Servo:
Looking in the Haynes/Bentley it would appear that the servo unit provides a ratio of 2.0:1.0 assist. Implying that even though my master cylinder on its own degrades my braking performance, the gains I would achieve from the servo assistance seriously outweighs that.

What would certainly be worth considering for those who are weighing up a brake upgrade and specifically by adding a servo, is to actually fit a remote servo which allows you to maintain the stock master cylinder. This way, you maintain a smaller MC bore (i.e higher line pressure) AND a servo factor of 2:1. The “upgrade” of Porsche brakes considering the desire for better brake torque in isolation may appear to not really be worth the financials and/or effort.

I apologise if I mis-quote here, but I think it was Graham L who said to someone on here a few years ago that an individual who wished to upgrade to Porsche brakes would be better off fitting a brake servo before outlaying for the Porsche crest!

There would be no doubt that you would be able to purchase higher performance pads (read higher friction) which will further improve the brake torque being applied with the Porsche setup. Also, the larger pad area and vented disc will greatly improve brake fade and over-heating for multiple hard braking, but my guessing is that most here are more concerned with stopping distance and braking effort for a single event.

So the purpose of this thread was really just to highlight with some real life numbers that I believe people would be better off, in the first instance , adding a servo to their braking system for an initial upgrade.

Any questions then please ask and I will try to explain. I am currently trying to sort out the rear and will add this in time.

NaFe

 

[NickJ]

Very interesting, thanks for sharing 8)

 

[StuF]

Cheers mate - what an informative post!

 

[mike202]

That's a great post. It confirms what I had thought from all the anecdotal evidence read on various forums.

Unless your drums/ disks brakes are shot, get a servo first if you want to improve braking ability.

 

[StuF]

Just need someone to make fully self adjusting drums now!
Its my most painful pre-mot job

 

[Haveacamper]

Thanks guys. Just wanted to give people a "real-life" comparison.

I have since noted that what I have labelled Brake Torque Nm is in fact the Braking Force at the tyre contact patch.

I am trying to work up drum brake to disc brake "upgrade" I have made, but currently my calcs are suggesting that my drums are producing a fair bit more braking force than the discs which I really can't believe....

 

[ozziedog]

Thanks guys. Just wanted to give people a "real-life" comparison.

I have since noted that what I have labelled Brake Torque Nm is in fact the Braking Force at the tyre contact patch.

I am trying to work up drum brake to disc brake "upgrade" I have made, but currently my calcs are suggesting that my drums are producing a fair bit more braking force than the discs which I really can't believe....

Ozziedog,,,,,, Yeeaaahhhhh!! For the drums :mrgreen:

 

[Haveacamper]

So embarrassingly, I think my original calculations where I worked out that the drum brakes performed better than my rear disc conversion in terms of braking force was actually correct! And as I mentioned in the discussion in the front, it all comes down to the bore size of the master cylinder as to the pressure applied to the brake piston in both cases….

Again, let me re-iterate that I am only considering the like for like braking force based on a given foot pedal force. I am not considering the general improvements you get through disc design over drum – i.e. heat dissipation, repetitive braking etc. as the calculations are different for drum that disc I will try to explain as best as I can.

Standard setup:
Drum brake piston = 22.2mm
Drum Diameter = 252mm
Drum piston area = 387mm^2

My rear Disc Conversion:
Calliper piston = 36mm
Solid disc diameter = 262mm
Calliper Piston area = 1018mm^2

Standard non-servo Master Cylinder:
Bore = 20.64mm
Rear Circuit Stroke = 17.00mm

Standard servo-assisted Master Cylinder:
Bore = 23.81mm
Rear Circuit Stroke = 13.00mm

So drum brake force:
Many books publish this equation but I found this in Fundamentals of Vehicle Dynamics [Thomas D. Gillespie]

You can see that by taking moments about each of the brake shoe pivot points you get:

Moments about Pivot a = e*Pa + n*mu*Na-m*Na = 0 [1]
Moments about Pivot b = e*Pa + n*mu*Nb +m*Nb = 0

And we know the friction force is the coefficient of friction (mu) and the opposing normal force (Na and Nb) so we know
Fa = mu*Na and Fb = mu*Nb [2]

So substituting equations [2] into equations [1] and rearranging for Fa and Fb provides us with the Friction force for the leading and tailing brake shoe:

We can work out the piston force (Pa) through our choice of master cylinder and brake pedal ratio. And we can take approximate measurement for the distances between the pivot point and necessary points. I worked out the following approximate points:

e = 200 mm
n =90 mm
m =105 mm
mu = 0.35


So considering the same brake pedal force as the front, 30kg we get:

Pedal Force = 30kg
Master cylinder Force = 30kg x 9.81 x pedal ratio = 615N
THIS IS WHERE I HAVE DISCOVERED THE MASTER CYLINDER BORE SIZE THE CRITICAL DOWNFALL
Master cylinder pressure = MC force / MC piston area = 615N / 335 mm^2 = 1.84 MPa <----REMEMBER THIS NUMBER!!!

We know that each drum brake has two pistons which produce a force of:
Force = Pressure x Area = 1.84 x 387 = 712N piston force
So using this along with the drum dimensions we can calculate the leading and trailing drum braking force:
Leading = 681 N
Trailing = 364 N
Total Drum = 1045 N
Rear Axle (2 drums) = 2090N

This then translates to a braking force on the ground as the simple ration between drum radius and rolling radius of the standard tyre:

Brake force at Ground for 30kg pedal force = 2090 x (126/269) = 978.6 N

OK so now for the “upgraded” Disc brake. Again we start with the master cylinder (which is now the servo assisted master cylinder:
Pedal Force = 30kg
Master cylinder Force = 30kg x 9.81 x pedal ratio = 615N
The servo master cylinder has much bigger bore so the:
Master cylinder pressure = MC force / MC piston area = 615N / 445 mm^2 = 1.38 MPa <---COMPARED TO THE 1.84 WE GOT WITH THE STANDARD!!!

So we are producing a lot less pressure at the calliper piston for the same pedal force
Brake force at the pad = MC pressure x total piston area x pad friction = 1.38 x (1018 x 2) x 0.38 = 1069 N
Brake force for both wheels = 1069 x 2 = 2138 N

Again, this then translates to a braking force on the ground as the simple ration between drum radius and rolling radius of the standard tyre:

Brake force at Ground for 30kg pedal force = 2138 x (106/262) = 865 N

For a 30kg brake pedal force:
Drum brakes generates 978.6N
My Disc setup generates 865 N

WHY IS THIS?????
Well the crucks of it comes down to the master cylinder bore size. My disc conversion is running a slightly higher mu than standard, a slightly favourable rolling radius, but although the disc is bigger than the drum, the effective radius of it (i.e. where the centre of the pad assumed results in a smaller radius. Having said that, if I were tohave kept the standard sized master cylinder I would have been generating a braking force of 1151 N. A 18% improvement over stock drums and a whopping 33% more than I am currently – not taking into account the servo assist factor of 2.0:1.

So again, it would appear that based on achieveable braking force alone, you would be better off maintaining your stock master cylinder with its favourable bore diameter and fitting a remote servo. This would give you the double benefit of greater non-servo assisted braking which is then further improved by the servo factor.

So what I have learned….Master cylinder bore diameter is paramount. Just because the brakes are red in colour, does not mean that they are infinitely better than stock haha. With my little bit of increased knowledge I will be looking in the future to better optimise my setup

I hope this serves as a good real-life working example of where to channel your priorities. I do have much better brakes over all, but considering the braking force alone, the majority of my improvement is coming from the installation of a servo. You can double up with this win, by maintaining the original smaller bore sized master cylinder and install a remote servo.

After this I feel like I may sound like I’m affiliated with a remote servo company – I am not 

 

[moler]

That's some magnificent work there, thanks for sharing

 

[dubdubz]

thanks for this - I cannot pretend to understand all the maths!

so getting cross drilled discs - and decent pads like greenstuff on standard bay calipers - is likely to be as good as the porker brakes!

 

[Haveacamper]

thanks for this - I cannot pretend to understand all the maths!

so getting cross drilled discs - and decent pads like greenstuff on standard bay calipers - is likely to be as good as the porker brakes!

Performance pads would assist in terms of a higher friction (also would wear more quickly). I believe cross drilled discs assist with enabling gases that are generated between pads and disc surface to escape, allowing a more consistent braking force to be maintaned between the pad and disc. also a level of heat dissipation maybe.

I think the overall to take away is that the preference should be to:
a) install remote servo to maintain the preferential smaller bore diameter of the stock master cylinder
b) upgrade to servo with matching master cylinder. you lose line pressure, but not as much as what you gain from the assist from the servo.
c) change brakes for larger piston and disc diameter. but before you do, pay attention to what your current brake piston area is!
i) The front stock piston diameter of 54mm is pretty generous as seen compared to the 4-pot boxster
ii) even though the rear drum piston is fairly small, the drum is fairly generous, so get a disc with an effective radius you have to go quite large.

What I have shown is a little skewed as I have shown the the non-servo master cylinder vs with servo master cylinder (without the servo assist).

 

[lhu128l]

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[lhu128l]

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