Brake proportion question

[Eddie Greenlee]

97 TJ
With a Dana 30 up front and a
Ford 8.8 in the rear.
What should the breaking percentage be?
Ex.( The front breaks 80% more than the rear brakes ? Maybe they are 50/50 ?

 

[JustDandee]

Question are you using an adjustable proportioning valve? What brakes do you have the axle drum/disc? One reason I ask is I that put an 8.8 with the discs into an old Mustang I had- got a life lesson taught that the you want the residual valves to be set up for the disc brakes not the drum brakes. The front brakes are doing the majority of stopping so you’re going to be front biased on the split - spitballing 60-70% front. If you have an adjustable I usually am road testing and trying to dial that in - as much rear brake I can get with out the back locking up first. Disclosure I know enough to be dangerous there are some more experience folks here- just wanted to call out the residual valve pressure differs between drum/disc.

 

[Eddie Greenlee]

Rear 8.8 disk. Sorry to say don’t know anything about the adjustable proportioning valve.

 

[mrblaine]

97 TJ
With a Dana 35 up front and a
Ford 8.8 in the rear.
What should the breaking percentage be?
Ex.( The front breaks 80% more than the rear brakes ? Maybe they are 50/50 ?

You would be lucky to get an 80/20 split. Higher CoG, flexible suspension means the weight transfer onto the front axle under a high energy braking event puts the front bits doing the vast majority of the work. There is a reason the OEM came up with lots of janky adjustable proportioning valve solutions over the years before ABS to try and stop pick-ups from locking up the rear brakes when the bed is empty. We fight the same problems.

 

[TheBoogieman]

97 TJ
With a Dana 35 up front

 

[Jerry Bransford]

97 TJ
With a Dana 35 up front and a
Ford 8.8 in the rear.
What should the breaking percentage be?
Ex.( The front breaks 80% more than the rear brakes ? Maybe they are 50/50 ?

You have a Dana 30 up front. Braking is usually set to around 70/30 via the proportioning valve with 70% for the front. 50:50 would not be appropriate, the rear brakes would lock up way too easily under hard braking with a 50:50 ratio which can throw a vehicle out of control. The harder you brake the more the weight transfers forward onto the front brakes away from the rear brakes. Without much weight on the rear tires during hard braking due to that weight transfer it doesn't take much to lock up the rear brakes which is why there are brake proportioning valves... to limit the braking power given to the rear.

 

[mrblaine]

You have a Dana 30 up front. Braking is usually set to around 70/30 via the proportioning valve with 70% for the front. 50:50 would not be appropriate, the rear brakes would lock up way too easily under hard braking with a 50:50 ratio which can throw a vehicle out of control. The harder you brake the more the weight transfers forward onto the front brakes away from the rear brakes. Without much weight on the rear tires during hard braking due to that weight transfer it doesn't take much to lock up the rear brakes which is why there are brake proportioning valves... to limit the braking power given to the rear.

I don't believe we can get a TJ that good. One of the better biased cars on the road before ABS was the mid engine Porsche 911 with the higher weight on the rear axle due to where the engine sits. It was around 75/25. There is no way a lifted TJ gets any where close to that.

 

[Eddie Greenlee]

Sorry y’all. Guess it’s a little early 🤦 Dana 30 up front

 

[tnjeepguy]

So what proportioning valve should he use and how to set it up ??

 

[Eddie Greenlee]

You have a Dana 30 up front. Braking is usually set to around 70/30 via the proportioning valve with 70% for the front. 50:50 would not be appropriate, the rear brakes would lock up way too easily under hard braking with a 50:50 ratio which can throw a vehicle out of control. The harder you brake the more the weight transfers forward onto the front brakes away from the rear brakes. Without much weight on the rear tires during hard braking due to that weight transfer it doesn't take much to lock up the rear brakes which is why there are brake proportioning valves... to limit the braking power given to the rear.

So where is the proportioning valve?on my tj

 

[Eddie Greenlee]

After looking up the valve. It looks like I best leave it alone at OEM specs

 

[Jerry Bransford]

So where is the proportioning valve?on my tj

Item 6...

 

[BlueC]

After looking up the valve. It looks like I best leave it alone at OEM specs

I'd leave it as is, unless you're locking up the rear.

Here are some online calculators for someone who has the time and interest. I have no idea of their accuracy.

Bias Calculator

Brake Bias Calculator | Master Cylinder Calculator

Calculator for Automotive and Racing Brake Systems, Master Cylinders, Brake Bias, Piston Area, CG Height, Weight Transfer, Pedal Stroke and Maximum Deceleration

brakepower.com

 

[tnjeepguy]

Will the newer prop valve from the rear disk TJ's fit the older drum TJ's and are they materially different ? What about ABS ??
Used about 50, new 250+ ??

 

[Jerry Bransford]

Will the newer prop valve from the rear disk TJ's fit the older drum TJ's and are they materially different ? What about ABS ??
Used about 50, new 250+ ??

The factory installed the same proportioning valve for both drum and disk brakes, ABS or no ABS.

 

[Rickyd]

97 TJ
With a Dana 30 up front and a
Ford 8.8 in the rear.
What should the breaking percentage be?
Ex.( The front breaks 80% more than the rear brakes ? Maybe they are 50/50 ?

What problem are you trying to solve?
The 8.8 uses the same rear disc brakes as the tj. And the tj uses the same proportioning valve for drum or disc.
You don't need aftermarket doodads.
That is unless bigger tires are keeping it from stopping well. In which case you should check out black magic brakes

 

[JustDandee]

After looking up the valve. It looks like I best leave it alone at OEM specs

Sounds likely going to be a none issue to use the OEM. When I drove that Mustang on the street I had zero issues- it was only on the track that the rears would act up/engage harder than the front. According to Wildwood brakes the residual pressure on a wheel cylinder and a caliper are different(but this maybe more sensitive for that application). Since others have ran it on the street, I would do that. If you see an issues as I described then you may know where to start.

 

[macleanflood]

If you do anything to your brakes install a line lock. Great for front digs and super nice to lock the brakes on hard when winching. You'll also need a D300 or Atlas for those front digs.

-Mac