Rear Calipers - Rennlist - Porsche Discussion Forums

Notices

Rear Calipers

Reply

Old 08-01-2015, 11:12 PM
  #1  
fasteddie313
User
Thread Starter
 
fasteddie313's Avatar
 
Join Date: Apr 2014
Location: northern michigan
Posts: 472
Default Rear Calipers

I'm kicking around what I want to do with my rear brakes. I already have 4 pot fronts that are about equal to 951 brembo fronts, 4X 38mm pistons ea.
Now I want to upgrade my rears and install an in-cabin prop valve..

As far as I know 951 brembo rears will be bolt on and therefore the most ideal option I'm currently aware of. They are "bolt on" to early 944 steel trailing arms correct? Do I need 951 rotors for them or anything?

Confirmation on that would be great as would some comment of fair market value for such 951 rear calipers.
From ebay pricing they are $400 a set and that's just not going to happen, but ebay might not be a good indicator on pricing of these things..

What should I expect to pay for, or to make a fair offer on some 951 rear brembo calipers?

If there really $400 I'll improvise something else. If not I WTB somewhere closer to earth.
Paint condition does not matter, needing rebuild components would be OK but rebuildable would be a must..

Thanks in advance for the advice guys

p.s. I got a membership so I could pester you guys for pieces, hope you don't mind..

Last edited by fasteddie313; 08-02-2015 at 03:01 PM.
fasteddie313 is offline  
Reply With Quote
Old 08-02-2015, 01:37 AM
  #2  
Humboldtgrin
Addict
Rennlist Member
 
Humboldtgrin's Avatar
 
Join Date: Jun 2014
Location: NORTHERN CALIFORNIA
Posts: 1,959
Default

86 rear 951 calipers have smaller pots then 87+ models. All turbo model rear calipers are interchangeable. And all need the same spacers to center them on the trailing arm. The 86 turbo disc are different then the 87+ due to offset. But you can use 87+ calipers that have a bigger pot and cost less to rebuild then the 86 model on an 86 rear disc setup. You can use an 86 to 89 trailing arm with an 86 hub and use 86 rotors with 87+ calipers. That's what I have. Or use 87 disc's and hub with 86 calipers(86 calipers cost a lot more to rebuild the 87+ calipers and have smaller pots). Bottom line 86 has smaller pots then 87+ calipers and all need same spacers and all use same trailing arm. Hub and brake disc are the differences in the offset. 400 is a good price for a set of 87+ calipers, they cost much less to rebuild then 86 951 model calipers. I have both.

Last edited by Humboldtgrin; 08-02-2015 at 12:56 PM.
Humboldtgrin is offline  
Reply With Quote
Old 08-02-2015, 10:29 AM
  #3  
MAGK944
Addict
Rennlist Member
 
MAGK944's Avatar
 
Join Date: Jun 2007
Location: Palm Beach, Florida
Posts: 4,905
Default

Basically you will need 86 951 rotors, 86on 951 calipers, caliper spacers and the 951 proportioning valve fitted to the reservior. Ideally you should also replace the two rear brake hardlines feeding the calipers, though if you are careful you can bend those lines to fit.

If you buy used rear calipers make sure the pistons are in good shape as they are expensive to replace. You should replace the piston seals anyway, the seals are cheap.

$400-$500 should get you there, if that's not in your budget I cannot see how adapting another 4-pot caliper to fit would be any cheaper. 951 rear rotors alone will set you back $200-$250, the 944na rotors are smaller and the wrong thickness to work with the Brembos.
MAGK944 is offline  
Reply With Quote
Old 08-02-2015, 02:33 PM
  #4  
fasteddie313
User
Thread Starter
 
fasteddie313's Avatar
 
Join Date: Apr 2014
Location: northern michigan
Posts: 472
Default

I don't really need 4 pot rears, its just one of the "easy" options but I guess quite expensive..

All I'm really looking for is bigger rear calipers period, more piston surface area, to put me rear biased enough for an adjustable proportioning valve to turn them back down and fine tune..

My MC is not stepped like yours, mine has the same size pistons for both front and back, the same size as 951 front brake MC pistons for me both front and rear so that helps me get some more rear bias right there..

I have 36mm single piston rears now and can change the cylinder assemblies/barrels in my rear calipers to single 48mm pistons gaining 12mm diameter on each side and putting my rear bias right in the center of a bias valves adjustable range (as would 951 brembo rear = 2X 20mm + 2X 30mm). It would mean turning them down around 25%.

Brembos would of course be better but I can pull the 48mm single piston rears off for $40-$100 easy and not need new rotors, just bolt on, same lines/pads and everything. I think I will go this route, I don't really need massive breaks, my car is pretty light..

I also have a 54mm single piston rear caliper option for free (because I already have them) but that (on paper) would be too much rear bias pushing the limits of a prop valves range.. In surface area calculations a 54mm single is near as makes no difference to 2X 38mm pistons (as in my 4 pot front) so it would put me at 1:1 ratio and thats too much rear..

I think my 4 pot fronts, 48mm rear singles, F/R line split, and in cabin prop valve on my rears, combined with great brake pads will be plenty for my needs..
I'm thinking EBC Yellowstuff pads all around, eventually.. And eventually braided lines from paragon as all my brakes will still use OEM lines. I'm doing this one stage at a time..

Stage 1: 4X38mm 4 pot fronts.. DONE
Stage 2: bigger rears, line split, prop valve (maybe with gauges).. IN PROGRESS
Stage 3: Sport pads, braided lines..

That's enough.
Maybe..

Stage 4: Adjustable leverage pedal arm (I'll make that), possibly delete booster and/or play with different MC sizes, firewall reinforcement..

Stage 5: Lightweight calipers, just big enough.. $$$$$$ Prolly never..

This could all change as I discover more options and information..

Last edited by fasteddie313; 08-09-2015 at 08:15 PM.
fasteddie313 is offline  
Reply With Quote
Old 08-05-2015, 07:41 PM
  #5  
odurandina
Slayer of Economic Optimism

Rennlist Member
 
odurandina's Avatar
 
Join Date: Oct 2009
Location: one thousand, five hundred miles north of Ft. Lauderdale for the summer.
Posts: 27,213
Default

getting too much rear brake bite might not be so good....

just go with the late 944T or 968 rear calipers....

then NOT the ripoff $500+ full rebuild kits available at the various parts houses, but
just these....
http://www.partsgeek.com/catalog/199...epair_kit.html
odurandina is offline  
Reply With Quote
Old 08-08-2015, 09:21 PM
  #6  
George D
Super User
 
George D's Avatar
 
Join Date: Aug 2006
Location: Tucson and Greer Arizona
Posts: 2,659
Default

I use stock 951 front calipers for my rear brakes painted red to match the fronts. Here's a pic. Easy mod, and very effective setup.

You'll have to update a few things from a late 87 or later 951. Best way to accomplish your goals, IMHO.

G

Originally Posted by fasteddie313 View Post
I don't really need 4 pot rears, its just one of the "easy" options but I guess quite expensive..

All I'm really looking for is bigger rear calipers period, more piston surface area, to put me rear biased enough for an adjustable proportioning valve to turn them back down and fine tune..

My MC is not stepped like yours, mine has the same size pistons for both front and back, the same size as 951 front brake MC pistons for me both front and rear so that helps me get some more rear bias right there..

I have 36mm single piston rears now and can change the cylinder assemblies/barrels in my rear calipers to single 48mm pistons gaining 12mm diameter on each side and putting my rear bias right in the center of a bias valves adjustable range (as would 951 brembo rear = 2X 20mm + 2X 30mm). It would mean turning them down around 25%.

Brembos would of course be better but I can pull the 48mm single piston rears off for $40-$100 easy and not need new rotors, just bolt on, same lines/pads and everything. I think I will go this route, I don't really need massive breaks, my car is pretty light..

I also have a 54mm single piston rear caliper option for free (because I already have them) but that (on paper) would be too much rear bias pushing the limits of a prop valves range.. In surface area calculations a 54mm single is near as makes no difference to 2X 38mm pistons (as in my 4 pot front) so it would put me at 1:1 ratio and thats too much rear..

I think my 4 pot fronts, 48mm rear singles, F/R line split, and in cabin prop valve on my rears, combined with great brake pads will be plenty for my needs..
I'm thinking EBC Yellowstuff pads all around, eventually.. And eventually braided lines from paragon as all my brakes will still use OEM lines. I'm doing this one stage at a time..

Stage 1: 4X48mm 4 pot fronts.. DONE
Stage 2: bigger rears, line split, prop valve (maybe with gauges).. IN PROGRESS
Stage 3: Sport pads, braided lines..

That's enough.
Maybe..

Stage 4: Adjustable leverage pedal arm (I'll make that), possibly delete booster and/or play with different MC sizes, firewall reinforcement..

Stage 5: Lightweight calipers, just big enough.. $$$$$$ Prolly never..

This could all change as I discover more options and information..
Attached Images   
George D is offline  
Reply With Quote
Old 08-08-2015, 09:25 PM
  #7  
George D
Super User
 
George D's Avatar
 
Join Date: Aug 2006
Location: Tucson and Greer Arizona
Posts: 2,659
Default

Just got back from driving in the rain...guess I should go clean the wheels

G
George D is offline  
Reply With Quote
Old 08-08-2015, 11:38 PM
  #8  
mikey_audiogeek
Addict
Rennlist Member
 
mikey_audiogeek's Avatar
 
Join Date: Oct 2004
Location: Northland, New Zealand
Posts: 1,543
Default

Originally Posted by fasteddie313 View Post
My MC is not stepped like yours, mine has the same size pistons for both front and back, the same size as 951 front brake MC pistons for me both front and rear so that helps me get some more rear bias right there..

The stepped tandem MC does not affect the bias. Stepped tandem MC's generate the same pressure in both circuits.

The stepped MC is to match the VOLUME requirements of the different circuits.

This diagram shows the pressure relationship in more detail. Note that the diameter where the rear piston is acted on by the front circuit pressure and the diameter where the rear piston generates pressure in its own circuit are identical.

http://www.bmwtips.com/tipsntricks/B...s/image004.jpg

So you have exactly the same bias as if you were running a 944 or 951 MC.

Cheers,
Mike
mikey_audiogeek is offline  
Reply With Quote
Old 08-09-2015, 11:38 AM
  #9  
fasteddie313
User
Thread Starter
 
fasteddie313's Avatar
 
Join Date: Apr 2014
Location: northern michigan
Posts: 472
Default

Originally Posted by mikey_audiogeek View Post
The stepped tandem MC does not affect the bias. Stepped tandem MC's generate the same pressure in both circuits.

The stepped MC is to match the VOLUME requirements of the different circuits.

This diagram shows the pressure relationship in more detail. Note that the diameter where the rear piston is acted on by the front circuit pressure and the diameter where the rear piston generates pressure in its own circuit are identical.



Ok, I did not realize this..

So if I understand this correctly the main master cylinder piston for the front brakes side is responsible for generating the pressure for both circuits. The smaller rear brakes circuit piston is not directly attached to the brake pedal but rather is acted upon by the pressure generated by the main front brakes master piston.

So the master cylinder really only has one master piston for the fronts and a separate (secondary intermediate semi sprung floating slave piston?) that transfers the same pressure through to the rears.

What is the point of the secondary piston? Is it's job only to separate the hydraulic circuits in order maintain one side or the other if one circuit fails/leaks for safety purposes?

I can also see how this would automatically adjust for the volume requirements of each side with the second piston transferring the same pressure but it is free to move at whatever distance it takes to bring the rears up to the same pressure as the fronts therefore automatically compensating for the different volume requirements between the fronts and rears by having a non-fixed stroke.

So would this work/act exactly the same as having both front and rear circuits on the same master piston/cylinder but serves the purpose of separating the hydraulic circuits for safety in case of failure on one side or the other?

Does my master that is not "stepped' have the same sort of floating piston for the rears but happens to be the same diameter of the main piston but its size is basically of no consequence?

I was told and believed that Porsche moved to a stepped master in the later cars to compensate for the different bias requirement for the heavier car and to accommodate for the front/rear brake split of the later cars as opposed to the OEM cross/X/opposing corner split of the older cars like mine..

So I can throw all my bias calculations for the newer cars out the window because I was calculating for the different master piston sizes front/rear and that simply is not the case..
fasteddie313 is offline  
Reply With Quote
Old 08-09-2015, 11:57 AM
  #10  
fasteddie313
User
Thread Starter
 
fasteddie313's Avatar
 
Join Date: Apr 2014
Location: northern michigan
Posts: 472
Default

odurandina - thanks for the tip

George D - That is awesome, but my brake aspirations are not quite THAT high, for now..
fasteddie313 is offline  
Reply With Quote
Old 08-09-2015, 12:37 PM
  #11  
Humboldtgrin
Addict
Rennlist Member
 
Humboldtgrin's Avatar
 
Join Date: Jun 2014
Location: NORTHERN CALIFORNIA
Posts: 1,959
Default

We all wish we had George D's cars setup! Every aspect, inside and out! Just viewing the photos, his car is awesome!!!
Originally Posted by fasteddie313 View Post
odurandina - thanks for the tip

George D - That is awesome, but my brake aspirations are not quite THAT high, for now..
Humboldtgrin is offline  
Reply With Quote
Old 08-09-2015, 03:55 PM
  #12  
George D
Super User
 
George D's Avatar
 
Join Date: Aug 2006
Location: Tucson and Greer Arizona
Posts: 2,659
Default

Originally Posted by Humboldtgrin View Post
We all wish we had George D's cars setup! Every aspect, inside and out! Just viewing the photos, his car is awesome!!!
OT: Thanks, it's been a journey, and this is my third 951. I've owned this one for about 12 years. Favorite sports car. Every other one I've sold, I've missed. To me, it's like that "girlfriend" you never loved, but can't forget. With the help of many, I built this one specifically for me to enjoy. It's been extremely reliable now that the bugs have been sorted. Amazing performance, and it's street legal. Here's a vid while tuning at 12 psi to 6K rpm with the new TIAL/Garrett turbo at 12 PSI. Car's a beast at 21 psi, but you get used to the power - still puts a smile on my face every time I drive it!

G
Attached Files
File Type: mov
IMG_0634.MOV (3.27 MB, 53 views)
George D is offline  
Reply With Quote
Old 08-09-2015, 07:21 PM
  #13  
mikey_audiogeek
Addict
Rennlist Member
 
mikey_audiogeek's Avatar
 
Join Date: Oct 2004
Location: Northland, New Zealand
Posts: 1,543
Default

Originally Posted by fasteddie313 View Post


Ok, I did not realize this..

So if I understand this correctly the main master cylinder piston for the front brakes side is responsible for generating the pressure for both circuits. The smaller rear brakes circuit piston is not directly attached to the brake pedal but rather is acted upon by the pressure generated by the main front brakes master piston.

So the master cylinder really only has one master piston for the fronts and a separate (secondary intermediate semi sprung floating slave piston?) that transfers the same pressure through to the rears.

What is the point of the secondary piston? Is it's job only to separate the hydraulic circuits in order maintain one side or the other if one circuit fails/leaks for safety purposes?

I can also see how this would automatically adjust for the volume requirements of each side with the second piston transferring the same pressure but it is free to move at whatever distance it takes to bring the rears up to the same pressure as the fronts therefore automatically compensating for the different volume requirements between the fronts and rears by having a non-fixed stroke.

So would this work/act exactly the same as having both front and rear circuits on the same master piston/cylinder but serves the purpose of separating the hydraulic circuits for safety in case of failure on one side or the other?

Does my master that is not "stepped' have the same sort of floating piston for the rears but happens to be the same diameter of the main piston but its size is basically of no consequence?

I was told and believed that Porsche moved to a stepped master in the later cars to compensate for the different bias requirement for the heavier car and to accommodate for the front/rear brake split of the later cars as opposed to the OEM cross/X/opposing corner split of the older cars like mine..

So I can throw all my bias calculations for the newer cars out the window because I was calculating for the different master piston sizes front/rear and that simply is not the case..
Yep you got it.

The reason for the two pistons is to split the circuits. In normal operation the rear piston does nothing. If the front circuit leaks then the front piston MECHANICALLY drives the rear pison and partial braking is maintained.
If the rear circuit leaks then the rear piston hits the end stop and the front circuit is still effective.

The staggered bore is to make sure that this only happens when required...which depends on volume/compliance of the individual circuits (which is affected by piston size = bias).

So a particular bias requires a particular staggered bore - but staggered bore doesn't change the bias.

Cheers,
Mike
mikey_audiogeek is offline  
Reply With Quote
Old 08-09-2015, 08:32 PM
  #14  
fasteddie313
User
Thread Starter
 
fasteddie313's Avatar
 
Join Date: Apr 2014
Location: northern michigan
Posts: 472
Default

So the staggered bore is to tune to the failure so it performs well in that case. And it is tuned to the particular set of calipers that it is supposed to come with due to there difference in volume requirements.

931 X split, the master pistons are the same because the 2 circuits are the same.
951 F/R split, the masters are different, smaller rear calipers, smaller rear MC piston..

Doesnt matter at all in a performance perspective, only in failure mode..

So all this means is that I need even bigger rear calipers than I thought, because I do not have a rear bias advantage because of my bigger rear MC piston, I was wrong again trying to understand brakes

But, that means that the big back brakes that I have, that I thought were TOO big, might not be too big after all.. I think they fit but haven't tried...

I've been wrong on this a lot. First for calculating both sides of opposing caliper pistons. Secondly for calculating different bias ratios because of the different MC piston sizes..

What do YOU think..

MC=23mm diameter piston
Fronts 4X 38mm opposing pistons each - done, installed, broke in, feel good

Rear options
36mm single piston - on car now Feels a bit front biased
48mm single piston - $50-$100 I might be able to squeeze in the budget before too long
54mm single piston - I have and think they will fit easily
87+ 951/968 rear brembos - $400-$500, I can't even consider this right now because my semi-custom fuel injection project is my budget priority now and will be for a while, this option means do nothing more with the brakes for a long time.

What would you choose and why?
Any other rear calipers I should/could consider for early 944 steel trailing arms setup?

Remember, the goal is to go a little too big in the rear so I can have my nice in cabin prop valve to play with, but not so big as to exceed the limits of a valve, I want to be somewhere in the middle of its range.. I think, unless I'm wrong here too.

Thanks for the info and helping me learn this..
fasteddie313 is offline  
Reply With Quote
Old 08-09-2015, 09:20 PM
  #15  
mikey_audiogeek
Addict
Rennlist Member
 
mikey_audiogeek's Avatar
 
Join Date: Oct 2004
Location: Northland, New Zealand
Posts: 1,543
Default

Hi fasteddie,

I have learned a lot recently in researching my Lexus brake upgrade...

I am not familiar with the early trailing arm setup and do not know what would fit here. others should chime in here!

I would be reluctant to mix fixed and floating calipers, just doesn't seem like a good idea to me.
So I would fit the 951 rears. If I needed any bigger, there are some 993 calipers with 34/30 pistons instead of 30/28.

http://vdubengineering.com/technical...e-conversions/

Make sure you get rid of any stock bias valve before you fit a cockpit adjustable bias valve. Bias valves in series are a BAD idea!

Cheers,
Mike
mikey_audiogeek is offline  
Reply With Quote

Thread Tools
Search this Thread
Quick Reply: Rear Calipers


Contact Us Advertising Cookie Policy Privacy Statement Terms of Service

We are a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for us to earn fees by linking to Amazon.com and affiliated sites.
 
  • Ask a Question
    Get answers from community experts
Question Title:
Description:
Your question will be posted in: