converting to dual M/C's - size recommendation needed

[Bill Verburg]

Originally Posted by martin_gibson

Thanks for all the feedback - there's clearly a little more to this than I originally anticipated when I decided to mate the 930' calipers to a twin MC.

Anyway, I'm still a little confused as to why the 22mm+17m f/r (or is it 17mm+22mm f/r?) combination used by the 73 RSR doesn't work well for a 930 set-up which has (I understand) a similiar caliper configuration - am I missing something?

Martin

The 2 things that have to be balanced are f/r bias and hydraulic amplification of leg effort.

bias is the result of the cumulative effects of
f/r line pressure
f/r caliper piston sizes
f/r rotor diameter
f/r pad friction
f/r pad height

hydraulic amplification is the slave to master ratio.

for a car w/ 930 calipers the total slave area is 14727.79mm^2(9072.92front/5654.87rear)

the f/r ratio gives you f/r hyd bias which in this case is 1.6 right about the historic sweet spot for all 911 & 930 that don't use ABS or p/v.

if a 23.8mm dual circuit DoT m/c is used the slave to master ratio is 16.539. the lower this # the higher & harder the pedal is and the easier it is to modulate w/ this set up and the mechanical amplification from the pedal set up every lb of leg efffort is multiplied by ~100(stock non boosted pedal ratio is 5.8, 5.8x16.539 =~100). ~16.1 is the lowest ever used by Porsche in any car, most street cars are ~20 w/ the single exception of '07RSR wich has 14.74 but uses the massive 380/355 rotors.

any way back to the point, a sporting'race pedal ratio is in the 16s, street in the 20s, lower is higher/harder higher has more travel and needs less effort.

unboosted I'd aim for ~16.5min.

now back to the bias. again historically 1.6 is the magic 911/930 factory #, you can use more front(up to 2.061 on 993tt) or less 1.57 on 993RS, 1.504 on '07RSR, 1.554 on '02Cup for examples

given the nature of brake difficulties, over heating fronts locking rears being of most concern, every effort needs to be taken to remove the load from the front and place it in back, especially where the big rear rotors can be used. To do this the car needs to be as low as possible, as stiff as possible and have a very effective lsd, if these criteria are met the ratios in the 1.5 to 1.6 range work great because you are taking as much load off the front as can be managed and the rear load is not so great that rear lockup is an issue(abs and p/v help too).

Most set the car up the easy way using more front, in the 1.7 to 1.8 range, this is easy and safe as the fronts will lock before the rears but has the negative effect of placing a greater load on the front. As long as the rotors are properly sized this is fine. But in the event of marginal front rotor temperature management schemes can and will be sub optimal.

There has been a lot of research on brake bias, the latest says that of the three possible situations, ie optimum, too much front or too much rear, optimum is best followed by too much rear followed by too much front

[martin_gibson]

Originally Posted by Bill Verburg

for a car w/ 930 calipers the total slave area is 14727.79mm^2(9072.92front/5654.87rear) the f/r ratio gives you f/r hyd bias which in this case is 1.6 right about the historic sweet spot for all 911 & 930 that don't use ABS or p/v.

Bill,

This thread has really turned into a braking master class - much appreciated

So having concluded that the 930 set-up has natural bias and that a pair of twin
(2x) 5/8" (16mm) cylinders would maintain that historic sweetspot of 1.6 with the bias bar in the neutral position, I am still confused as to why Porsche selected a 22mm/17mm arrangement for the 73 RSR or is my assumption of caliper equivalency (RSR caliper = 930 caliper) unfounded?

I am also interested to know what calipers were used on the SC/RS which Steve suggested had a a 17.78mm/15.87mm MC ratio.

Thx

Martin

[Bill Verburg]

Originally Posted by martin_gibson

Bill,

...... I am still confused as to why Porsche selected a 22mm/17mm arrangement for the 73 RSR or is my assumption of caliper equivalency (RSR caliper = 930 caliper) unfounded?

I am also interested to know what calipers were used on the SC/RS which Steve suggested had a a 17.78mm/15.87mm MC ratio.

Thx

Martin

That was why I requested Steve to recheck the info he provided, the #s just don't make sense unless the RSR used much bigger pistons than the 930/917 calipers. I don't know.

The SC/RS did use the stock 930 38/38 font and 30/30 rear which works fine w/ the m/c's Steve mentioned but does move bias way forward. The balance bar can move it further forward or to the back

Here is a typical balance bars effect, the vertical bar is considered neutral for this car

[Quicksilver]

I'm wondering what kind of pedal ratios the RS/RSR combo had. That would throw the numbers out the window.

[Bill Verburg]

Originally Posted by Quicksilver

I'm wondering what kind of pedal ratios the RS/RSR combo had. That would throw the numbers out the window.

The pedal ratio will be about the same as for other non assisted 911 that I posted above, ~5.8

[martin_gibson]

Originally Posted by Bill Verburg

That was why I requested Steve to recheck the info he provided, the #s just don't make sense unless the RSR used much bigger pistons than the 930/917 calipers. I don't know.

I guess we're waiting on Steve W then

Thx

Martin

[martin_gibson]

Originally Posted by Bill Verburg

That was why I requested Steve to recheck the info he provided, the #s just don't make sense unless the RSR used much bigger pistons than the 930/917 calipers. I don't know.

The SC/RS did use the stock 930 38/38 font and 30/30 rear which works fine w/ the m/c's Steve mentioned but does move bias way forward. The balance bar can move it further forward or to the back

There is a set of 917 / 73RSR calipers for sale on ElevenParts with the attached text:

917 brake calipers, 2 front with 38 mm piston, 2 rear with 42 mm piston. Also used on factory RSR cars

Martin

[Bill Verburg]

Originally Posted by martin_gibson

There is a set of 917 / 73RSR calipers for sale on ElevenParts with the attached text:

917 brake calipers, 2 front with 38 mm piston, 2 rear with 42 mm piston. Also used on factory RSR cars

Martin

If thats the case then the #s Steve posted are credible. An RSR w/ 8x38 front & 8x42 rear would be horribly rear biased w/ staggered twin m/c and balance bar, hyd boost would be right in the ballpark @ 16.6

even if the info on that site is backward ie correct is 8x42 front & 8x38rear, the car would have horrible bias w/o help at the m/cs

[martin_gibson]

Originally Posted by Bill Verburg

If thats the case then the #s Steve posted are credible. An RSR w/ 8x38 front & 8x42 rear would be horribly rear biased w/ staggered twin m/c and balance bar, hyd boost would be right in the ballpark @ 16.6....even if the info on that site is backward ie correct is 8x42 front & 8x38rear, the car would have horrible bias w/o help at the m/cs

Thanks for that Bill

[abisel]

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[chris_seven]

There does seem to be a great deal of confusion about piston sizes.

The RSR (Both 2.8 and 3.0) seem to have used 43mm dia. pistons in the front calipers and 38mm in the rears.

The part numbers of these pistons is as follows:

43mm dia 917.351.706.00

38mm dia 917.352.710.00

The master cylinders were 17mm front and 22mm rear with the following part numbers:

17mm dia 910.355.013.00

22mm dia 917.355.014.00

According to the Workshop Manual the standard pedal ratio declared by Porsche is 5.4:1

Just out of interest the RSR seems to have used a single fluid reservoir Part No. 906.355.013.00

The 934 used the same piston sizes and master cylinder sizes but quite different part numbers:

43mm piston 917.351.704.00
38mm piston 917.352.708.00

17mm Master 911.355.019.00
22mm Master 911.355.021.00

The 934 used 2 x Master Cylinder reservoirs Part No. 917.355.012.00

A couple of other bits of info about the 934: It used an early gas pedal 901.423.010.00

The Bias Brake pedal was 930.423.056.00 and the clutch pedal was 930.423.077.00

The cylinder sizes used on the SCRS seem to indicate that they were AP Racing Parts with the 15.875mm being 5/8" and the 17.78mm being a 0.7".

Just for information the CP2623 range of cylinders starts at 14mm and goes up to 25.4mm in very small steps.

They are also available with 160mm pushrods.

To give and equivalent pedal to the RSR the SCRS should really use a 15mm dia front Cylinder and a 17.78mm dia rear. Both these sizes are available from AP.

The alternative would be to increase pedal ratio to about 6.2:1 which is just about manageable.

It is also interesteing to see just why there are some problems when these pedal boxes are used with A Type, M Type or S Calipers.

The description of the The calipers on Elevenparts website just has to be a mistake.

Also the Girling Calipers with the 38mm pistons shown in another part of the site are interesting as I think some of the late 917s used these calipers and something similar may still be available.

[martin_gibson]

Just got my dual MC pedalbox back from Chris Flavell (Fenn Lane Motorsport, UK) which utilises a 17mm front & 20mm rear AP racing cylinders matched to my 930 calipers.

Results:

I'm very pleased

For road use, I now have a nice firm pedal without too much travel and can readily modulate/control the braking load, whilst with the bar in the neutral position there is small (but controllable) front bias which will lock up the front tyres first in damp conditions.

Will be interested to see how the 930 calipers compare to my previous set of Bremteks next time I do a track-day.

In the meantime, Chris is making me up a remote 'bias' adjuster so I don't damage my nails

Martin