TheSamba.com :: View topic

Wed Apr 07, 2021 10:31 pm

You made me really curious. I asked Google and he told me the rear Box are 28mm +30mm, just as the 944 turbo rear calipers. But this gives a total area of 4540sqmm, which is more than the 54mm area (2289sqmm), so even more front bias. 43mm single front pistons would be 1451 sqmm (do they have two pistons?). Maybe I do not understand well how the area is computed, I see in the other thread I divided by the pump piston size (why did I do that? It seems so long ago). Also, in my calculation I did not take into consideration the pad area, just the piston sizes (on my brakes, the pads are the same front and rear).

Wed Apr 07, 2021 10:44 pm

Edit: I found why I did the calculation that way: hydraulic leverage.

"Hydraulic leverage ratios are simply the ratio of the slave to master cylinder area, which is the ratio of the diameters squared. So, for a T1 front disc w/ T1 master cylinder, that's (40/19.06)^2 = 4.404:1. For a 944 NA rear disc w/ T1 master cylinder, that's (36/19.06)^2 = 3.567:1. For the F/R ratio, just divide those two numbers.

Note that technically-speaking, brake bias is determined by braking _power_, not force (as the above relates to). The difference is the multiplication by the geometric average rotor radius and the wheel speed. But, since these figures are more-or-less the same for front and rear on the cases you described, we can neglect these terms."

Wed Apr 07, 2021 11:11 pm

The way you calculate piston area for a set of calipers is literally just the area of the piston(s) on one side. Obviously there are more factors involved such as rotor diameter and pad area, but this, generally, allows you to compare apples to apples, so to speak. That's why single piston sliding calipers are used on some higher performance cars and work effectively.

So I did a few quik calculations. BTW, I had remembered incorrectly, Boxster calipers are slightly more that a 41mm piston caliper, NOT 43mm:

41mm....1320.3mm^2
Boxster(28mm and 30mm)....1322.6mm^2
42mm....1385.4mm^2
54mm....2290.2mm^2

H2OSB

Wed Apr 07, 2021 11:30 pm

Sometime back I tried to reconcile MC effectiveness relative to caliper piston size by either dividing or multiplying the piston area by both MC piston size and MC piston area and I could never get it to make sense. Of course I'm not an automotive engineer, and there are probably people reading this who understand how it works, implicitly.

I was prompted to try to figure it out after reading the 17mm Porsche 914 MC gives a firmer but less effective pedal than the 19mm MC all the high performance 914 guys switch to. I know the 944 MC has two circuits with a 23mm piston in one and a 18mm piston in the other but I don't know which piston goes to the front or rear circuit.

Anyway, I've decided I don't want to reinvent the wheel and will just use the anecdotal information gleaned from experienced guys Wally and Steve Carter(among others) to base my decisions on.

H2OSB

Thu Apr 08, 2021 6:49 am

We are getting technical here, I like that. I hope Dodgy will not mind this hijack.
I made a mistake earlier, the 4540sqmm is for the FRONT 4-pot calipers, 36+40mm. For the rear calipers I get 2643. Why would you take only half of it?

Thu Apr 08, 2021 8:01 am

Basically because we know a single piston sliding caliper functions as if it were two opposed pistons. Technically, you certainly could count the area of both sides, but you'd always need to double the area of a single piston and that could become confusing. Opposed pistons in a caliper will always be the mirror image of the other, so it's always just 2x the area of the single side. I cannot imagine where having the doulble area value would change the outcome of calculations when comparing one caliper to another

H2OSB

Thu Apr 08, 2021 8:50 am

tzepesh wrote: We are getting technical here, I like that. I hope Dodgy will not mind this hijack.
Ha ha! No worries, I like technical and am enjoying following along 😉

For a single-piston caliper, the distance travelled by the piston (swept volume) is doubled.
So when the piston area is used for volume calculation (pedal travel) it will be the same as considering two opposed pistons of the same diameter.

I think the forces acting on the rotor are also the same for both caliper types:
- with opposed-piston calipers the reaction forces from each piston are 'lost' on the caliper body / axle
- with single piston capilers the reaction force is passed to the opposing pad, the caliper frame is isolated from the axle

Fri Apr 09, 2021 12:13 am

Understood, thank you for the explanation! Yes, for the F/R ratio calculation it does not have an impact. Maybe just if somebody has sliding caliper on one side and multiple piston caliper on the other.

Fri Apr 09, 2021 12:37 am

tzepesh wrote: Understood, thank you for the explanation! Yes, for the F/R ratio calculation it does not have an impact. Maybe just if somebody has sliding caliper on one side and multiple piston caliper on the other.
Wow that would be a scary scrapyard build! :shock:

Fri Apr 09, 2021 5:43 am

Don't get so scared, I was not referring to left and right, but to front and rear :))

Fri Apr 09, 2021 5:49 am

tzepesh wrote: Don't get so scared, I was not referring to left and right, but to front and rear :))

Lol. That's exactly what I have, but I'm with Dodgy, it sounded like you meant L/R. The Boxster calipers are 4 piston opposed and the 944 rear is a single 36mm sliding type.

H2OSB

Wed Apr 14, 2021 8:02 am

I'm on my lunch break at work and was thinking about brake caliper piston area and the desire to make sure I'm not sharing incorrect information. I found a site online called brakepower.com that I've seen used by many for their brake set up designing over the years.

They have a calculator that generates a value for total brake effectiveness per axle. It show the math which is the area of the pistons on one side of a caliper x the number of those pistons in the caliper x the number of fhose calipers per axle. In the case of a sliding caliper (they call it a floating caliper) you treat the equation as if there were two opposed pistons. So, based on this, I feel confident using just the area of all the pistons on one side of a caliper to directly compare to another. Oddly, they make no mention of pad size as a variable.

H2OSB

Wed Apr 14, 2021 11:01 pm

That is a good site, I used it to confirm my own calc (see post 7 April)

Pad area does not directly influence brake performance. Only the contact force, friction coefficient and disc / wheel diameters.

Larger pistons generate higher brake force for same hydraulic pressure, and larger pads suit the packaging. Multiple-piston calipers are just another way to increase piston area, without the penalty of increased radial dimension. (Plus they make a nice marketing gimmick)

I suppose larger pads might have a longer service life?
Not sure if they have a penalty regarding cooling - especially for multiple-piston calipers, where less of the disc face is exposed to air?

Thu Apr 15, 2021 5:12 am

When the brakes are applied, kinetic energy of the car converts to heat. Initially all the heat goes into the pads and disk rotor before it’s carried away by air convection. Bigger pads and rotor heat up less and cool faster - less fade during a high speed stop.

Thu Apr 15, 2021 8:52 am

Hmmm, that makes sense.

Thu Apr 15, 2021 10:26 am

One of the factors in choosing my brake set up was actually thermal efficiency. Now, I'm not saying I think that makes the brakes any better. The regular disc set up I had on my 1973 were superb, and autocrossing (the ONLY racing I do) is probably never going to heat a 4 wheel disc brake set up ON A BEETLE up to the point of fade, but since I was starting from scratch on the car and I would someday like to do a bucket list performance drive on Leguna Seca, i thought it was a good idea. You never know, and I'd hate to later WISH I'd gone with vented rotors.

H2OSB

Sat Nov 06, 2021 2:30 pm

Been a long time since last update, progress is slow but there is some...

Now the floorpan is the right way up again I'm tackling the brake pipe runs so I can drill all the holes for routing & clips before painting the top half of the pan.

Top tip - I found window sash cord was really good for planning the initial routing and measuring lengths before cutting & bending pipe:

Um... there are a lot more pipes than a standard ACVW install!
This routing around the framehead is to maintain clearance around the steering rack.
Where the pipes sit against each other I've added heat shrink tubing for potection.

Don't worry, the rusty master cylinder will get swapped out!
Looks like this maze of pipes could be a nightmare to bleed... but I guess it is not so different to comtemporary vehicles, so it can be done.
This is the routing round the back to the rear calipers:

And this is the final method I chose for the handbrake cables. These are shortened ACVW cables with a porsche end fitted. At the entry to the trailing arm I made up a plate which is tacked onto the elbow, held in place with a small bolt into the arm. Much neater and more secure than previous methods.

All this really is me avoiding the fact that I am getting closer to having to deal with paintwork on the body...

Sat Nov 06, 2021 7:53 pm

Dodgy, your gorgeous semi-trailing arms REALLY makes me want to explore using my set on my car. IIRC, your parts are sourced from a 951? How will the trailing arms effect axle length?

Seeing cool stuff other people do REALLY makes me want to as well, but I must force myself to stick to my plan and try to get my car on the road.

H2OSB

Sun Nov 07, 2021 2:30 am

Well it took a fair bit of work to get the arms looking good, both were sandblasted and painted. But it is worth the effort.

The arms are off early ish 944, not sure exact year / model. Somehow I ended up with two sets... These ones have the bump stop feature, which I think was deleted later. But the ABS sensor hole needed to be machined out.

AFAIK there is no difference in arm width, but there is differences in the hub flange. You would need the early 'flat' type. The axle fits ok with 80mm? inner and 90mm? outer CV joint, just needed a 2mm spacer ring making to position the Porsche CV properly as it is a bit thinner than the VW one.

Mon Nov 08, 2021 9:59 am

H2OSB, it's really easy with the 951 arms. You just use the 951 axles as well and swap the gearbox flanges to type 181 type.
Dodgy, really nice work there. Are you using Porsche ABS unit?