NAXJA Forums -::- North American XJ Association  

Go Back   NAXJA Forums -::- North American XJ Association > NAXJA Unibody Jeep Technical Forums > Jeep Cherokee XJ (1984 - 2001) > Modified Tech Discussion
HOME Member FAQ Sponsor Info Rules Bylaws E-Mail

Modified Tech Discussion Forum for Tech related discussion for Modified XJ's and MJ's.

Reply
 
Thread Tools Display Modes
  #1  
Old February 10th, 2019, 12:14
solomon7 solomon7 is offline
NAXJA Forum User
 
Join Date: Aug 2008
Location: Portland OR
Posts: 462
ZJ disc conversion solution!

I bought an 8.25 years ago that had had a disc conversion done.
For years I've had weak brakes and long pedal travel even after the swap.
I expected better brakes when I swapped but nothing really changed.
Fast forward quite a bit. About 65000 miles.
I've replaced my front pads twice and haven't had to do the rears once. Weird.
So I bust out the IR gun, do a drive with the pedal down a bit to ride the brakes and heat 'em up.
I stop and temp all rotors. Fronts are 160F. Rear? 55F. Ambient temp. Those ****ers arent' even touching!

I decided to switch the guts of the prop valve from a ZJ into my XJ. I lost the ZJ cap so due to the ZJ rod being shorter and producing a leak under pressure, I ONLY PUT IN THE ZJ SPRING. Kept the XJ rod, o ring and cap.

OMG NIGHT AND DAY DIFFERENCE! I used to push the pedal down about 4 inches before I even felt braking happen. NOW it's within an inch.
Best part is now, after riding the brakes again around the block, Fronts are 160F.
Rears are 110!
It's like a totally different vehicle now and for the first time, I can lock up my wheels at speed. No more nose dives!
JUST PUT IN THE ZJ SPRING YA'LL. IT works!
Reply With Quote
  #2  
Old February 10th, 2019, 12:26
Greenz's Avatar
Greenz Greenz is offline
NAXJA Member
 
Join Date: Nov 2005
Location: Oregon
Posts: 333
Re: ZJ disc conversion solution!

Great info, thanks!
Reply With Quote
  #3  
Old February 10th, 2019, 19:57
lazyxj's Avatar
lazyxj lazyxj is offline
NAXJA Member #1310
 
Join Date: Mar 2004
Location: Los Angeles
Posts: 2,342
Re: ZJ disc conversion solution!

Some old information on the subject or proportioning valves that is still applicable to this modification.
http://gojeep.willyshotrod.com/HowtoPvalve.htm
http://gojeep.willyshotrod.com/HowtoZJvalve.htm
__________________
Both XJs are still Real XJs http://www.naxja.org/forum/showthrea...hlight=real+XJ
01 Sport, 4.6L Russ Pottenger 4.6, AW4, 4" lift, 33 X 10.50 KM2, 4.56-8.25 TruTrac, D-30-E-Locker with alloy shafts, Hidden M8000 winch, Vanco front and ZJ rear disc brakes, Frame stiffeners, Lots of armor, Towing baseplate. http://www.flickr.com/photos/lwade/c...7603770130683/
1997 Country, Mostly stock, 2" lift, Engine mods, NP242, Towing baseplate
Reply With Quote
  #4  
Old February 10th, 2019, 20:48
RCP Phx RCP Phx is online now
NAXJA Forum User
 
Join Date: Mar 2003
Location: Phoenix,Az
Posts: 12,136
Re: ZJ disc conversion solution!

That's always been the "recommend" for rear disc's. I ran into the inverse situation with WJ front's and KJ rear's, since mine was a 2k the proportioning was great but the pedal travel was unacceptable so I ended up with a 1-1/8" master cylinder.
Reply With Quote
  #5  
Old February 12th, 2019, 01:50
Jim Malcolm Jim Malcolm is offline
NAXJA Forum User
 
Join Date: Aug 2014
Location: PA
Posts: 102
Re: ZJ disc conversion solution!

Quote:
Originally Posted by RCP Phx View Post
That's always been the "recommend" for rear disc's. I ran into the inverse situation with WJ front's and KJ rear's, since mine was a 2k the proportioning was great but the pedal travel was unacceptable so I ended up with a 1-1/8" master cylinder.
It's not "recommended", it's an Engineering fact. Rather than swap in a <$5 part, or free depending on your honesty, you band-aid it with something that is "law of physics" guaranteed to reduce your ability to stop. By increasing the master cylinder diameter from 25mm stock on a WJ to 1-1/16", you've reduced your ability to stop by 14%.

Simple pressure x area calculation. Ignore the numbers for a minute and just do the math with the variables. Pressure is the force produced by the brake booster divided by the area of the master cylinder. Area is that of the brake calipers. The ratio of performance is (Force/ MC Area) * Brake Area of one MC size divided by the other. Brake area is the same, so it reduces to (BB Force/MC1 Area) / (BB Force/MC2 Area). Brake Booster force cancels out, so it doesn't matter how hard you push. Flip it around and you end up with MC2 Area / MC1 Area. MC2 is 1-1/16" (0.88 in2), MC1 is 25mm (0.76 in2). 0.88/0.76 = 0.86. With the 1-1/16" MC, you are generating 86% of the pressure if it was a 25mm bore. Braking performance is system pressure x pad contact area x coefficient of friction. Two of the 3 stay essentially the same since it's not that much of a pressure drop. It's not much of a stretch to say that 14% decrease in pressure = 14% decrease in braking performance. Trend would be towards further reduction with regards to affect on COF and pad area.

Proportioning Valve is a generic term. When you replace the spring and plunger with one from a ZJ, you're actually replacing the metering valve, not the proportioning valve.

https://www.freeasestudyguides.com/b...ing-valve.html

What does the metering valve do? It prevents the front brakes from being applied until the slack is taken up in the rear drum brakes so that the brakes are applied evenly. Once the rear drum brakes build up some pressure (~100 psi), the metering valve shifts and fluid is allowed to pass to the front calipers. What did you say your issue was again? Swap in a ZJ spring and plunger, put a 1" master cylinder on it and I bet you have better brakes than you do now. I'd offer to let you drive my 94. Everyone who has comments on how touchy my brakes are. Hitting my brakes is a recipe for eating steering wheel. You don't hit my brakes, you ease them with your big toe.
Reply With Quote
  #6  
Old February 12th, 2019, 07:00
RCP Phx RCP Phx is online now
NAXJA Forum User
 
Join Date: Mar 2003
Location: Phoenix,Az
Posts: 12,136
Re: ZJ disc conversion solution!

In my case(I did do all the calcs) I tried the factory 1", then a 1-1/16", and finally settled on 1-1/8". For the last bit of travel recovery I shimmed the rear axles. Now I have basically zero pedal travel and more stopping power than most could use.
Reply With Quote
  #7  
Old February 12th, 2019, 13:15
Jim Malcolm Jim Malcolm is offline
NAXJA Forum User
 
Join Date: Aug 2014
Location: PA
Posts: 102
Re: ZJ disc conversion solution!

So you admit to knowingly distributing definitively false information? And not the first time you've been corrected either. Fortunately God is definitely a gear-head so there is certainly a special place in hell for people like you.
Reply With Quote
Reply


Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)
 
Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump

Similar Threads
Thread Thread Starter Forum Replies Last Post
KJ disc conversion Whitetail Modified Tech Discussion 16 December 25th, 2010 15:42
d60 and disc conversion? projectTK Modified Tech Discussion 2 April 5th, 2009 10:43
Rear Disc Brake Hardware (from ZJ for drum-disc conversion) LocoJeeper NAC Classifieds 7 September 16th, 2008 13:03
disc conversion badboys Modified Tech Discussion 11 April 12th, 2008 17:49
ZJ Disc Swap - Parking brake cable solution korda Modified Tech Discussion 1 February 26th, 2004 19:55


All times are GMT -7. The time now is 20:31.


Powered by vBulletin® Version 3.8.9
Copyright ©2000 - 2019, vBulletin Solutions, Inc.
NAXJA and NAXJA logo's Copyright NAXJA. All content/images Copyright NAXJA 1999-2014