LSPV Adjustment through fluid column length

[EricE]

I have adjusted both sides of the LSPV a few times and now I have no idea where the correct setting is. So I need a way of getting back to factory settings. As you know, following the FSM adjustment requires the use of two special purpose gages (gages I don't have). So I started thinking about a way to measure the relative pressure going to each caliper.

My idea goes something like this: Attach lengths of clear tubing to the DS front caliper bleed screw and one to the rear DS caliper bleed screew. Set the tubings so that it goes up verticly from the bleed screw. Crack both bleeder screws and push the pedal to the floor once and hold. Measure the length of fluid column produced at front and back. Adjust the LSPV until the lengths of the fluid columns approach the ratio of the FSM pressure ratio recommended setting.

Does that sound like it has any merit? I suppose I need to go one step further and adjust for the relative piston sizes front and back. Does anyone know the piston size of the 2 in the front caliper and the piston in the back? With that I can use hydrolics to calculate the proper fluid volume to produce the FSM settings.

FSM settings:
Front 1138psi
Rear 889psi +-86psi

 

[EricE]

OK, I suspect nobody can make sense of my scrambled thoughts. Still, it was pointed out to me that how much fluid flow happens under no pressure might not tell me anything about what happens under pressure. So I really need to know how the LSPV does what it does before I can assess that. Anyone know how the LSPV sends less pressure out one tube than the other? Like what is going in mechanically inside so that it can do that?

In the end I’ll probably do what others have done and just take it to a parking lot and repeatedly panic brake until I can adjust the rear braking so it won’t lockup first.

 

[scottm]

Could you measure the angle of the arm relative to the valve housing, and compare it to another 80 you believe is set correctly?

 

[miked]

It sounds like your setup would measure volume, not pressure.

eta: repeated panic braking will heat up the pads and disc and make them fade. If setpoint is made while hot and faded, it seems to me that you would get an incorrect setting when cold.

 

[EricE]

Could you measure the angle of the arm relative to the valve housing, and compare it to another 80 you believe is set correctly?

I like that idea. It may not be perfect for my vehicle with the heavy drawers in back but it would be much closer than where I am now. Is there such a measurement floating around this forum somewhere for a stock tire size truck with OME medium springs/shocks?

Edit: I have spent a couple of hrs reading all the LSPV threads and don't see anyplace someone reported the angle of their LSPV arm relative to their housing. If you have a truck setup like mine and you believe your LSPV is adjusted proper (or close) then could I ask you to measure that angle and report. I'll use it as a baseline to start adjusting mine. Thanks.

 

[Bearcat80]

Grab your coveralls and head for the parking lot of a ski area. Panic braking in slick conditions will help you set up way more accurately than any other method. I found out the hard way in the snow that I adjusted mine too much, and I just dinked with it in small increments until I got a nice balanced brake application. You want the ABS to kick in evenly front and rear. Alternatively, if you are looking for a baseline, ask someone who is still stock for the length of exposed bolt on the axle end and start there.

 

[EricE]

I posted this on another thread where I was trying to figure out the reason for my poor braking. I am cross posting it here because this is sort of a follow up to my original thought way back when. Its now confirmed that I can use this to adjust relative front to back braking percent.


My LSPV was adjusted to send more fluid/presure to the rear brakes. I set it myself but I realize now I did it wrong. To correct the problem I adjusted the LSPV to push equal fluid/presure to front and back. From there I'll fine tune. I did this by the following procedure:
I opened a bleeder valve on a rear brake and connected a tube to the bleeder valve. The tube flowed into an output cup/reservoir. I then pushed the pedal 20 times to its stop. I have speed bleeders on each wheel so none of the fluid I pushed out would flow backwards when I let up on the pedal. This produced a certain volume of fluid in the output cup.
I then did the exact same thing with a front wheel. I found that the setting of the LSPV DOES affect how much fluid is pushed out front v.s. back. (I can't explain how it does this; remember, only one bleeder was open during this procedure, front or back) Before I adjusted the LSPV I had at least an inch more of fluid coming out of the rear brakes than the front. I then adjusted the LSPV and repeated the procedure until the back brake put out the same height/volume of fluid as a front brake.

If relative pressure is supposed to be 1138 front and 889 rear then that works out to 56% front and 44% rear so a 50/50 split is pretty close. I don't think I'll mess with it further since my braking is so good now.

 

[RedTacoma]

Dirty way of making your brakes equal.
(this works best with small thin tires)
1. Disable ABS (pull the fuse works best)
2. Find patch of loose sand EDIT: has to be level)
3. Speed up and hammer the brakes until you feel one of the wheels are locking.
4. Get out and check if rear or front had locked. or if both which one was first

Goal: To have 4 wheels lockup at the same time.

Adjust valve accordingly.

 

[RedTacoma]

....

If relative pressure is supposed to be 1138 front and 889 rear then that works out to 56% front and 44% rear so a 50/50 split is pretty close. I don't think I'll mess with it further since my braking is so good now.

This is the part where I am not convinced about using gauge method. Having equal pressure does not accommodate load and weight distribution.
That's why I always used "fathers" aka "dirty Method" .
It makes big difference because it actually takes to account all the variables down to pavement. Makes a big difference in winter.

 

[Tools R Us]

Dirty way of making your brakes equal.
(this works best with small thin tires)
1. Disable ABS (pull the fuse works best)
2. Find patch of loose sand EDIT: has to be level)
3. Speed up and hammer the brakes until you feel one of the wheels are locking.
4. Get out and check if rear or front had locked. or if both which one was first

Goal: To have 4 wheels lockup at the same time.

Adjust valve accordingly.

That is also my favorite method, I use a dirt road and shoot for the fronts to lock first by a slight amount. Having the LSPV correctly adjusted makes a big difference in brake performance, on a lot of lifted rigs the rears aren't doing much.

 

[Ebag333]

Goal: To have 4 wheels lockup at the same time.

While I think your method is spot on, a word of warning....

You want the fronts to lock up just a bit before the rear.

If you set it for 50/50, there are situations where the rear could easily lock up before the front. This is a bad thing as it can cause oversteer (back end breaking loose).

If you set the bias too heavily to the front, then you can cause understeer.

With drums in the rear, oversteer isn't much of a concern (drum brakes will have a hard time overpowering disk brakes). But with disks front and rear, it can be an issue.

That's one reason why I don't think it's a good idea to replace the LSPV in an 80 with disc's with a T, since the proportioning is 50/50 then you can easily cause oversteer.

Remember when you're testing this that you will likely be travelling in a fairly straight line, under/oversteer won't have much effect here. When you need to worry about it is when you need to steer around the object in the road on a corner in slick conditions at speed. In those situations oversteer can be downright dangerous.


In the image below:

Figure A is what happens when your front tires lose grip. It's called understeer or push. Figure B is what happens when your rear tires lose grip. It's called oversteer or fishtailing.

Push/understeer feels like the steering column has suddenly transformed into rubber: You've got the wheel cranked left but the car isn't turning nearly as much as you'd like. In severe cases, it feels as if the steering wheel has snapped off: No matter how much you turn the wheel the car goes straight.

Loose/oversteer, to most highway drivers, feels much like hitting a tree or running into a ditch, because that's what their car does when it goes loose. A race driver battling a loose car feels what the outside rear tire is doing. Before the rear end steps out there's a lightness or, sometimes, shuddering in the outside rear tire: That's the tire saying, "Whoa, that's about all I can take!"

 

[RedTacoma]

You want the fronts to lock up just a bit before the rear.

If you set it for 50/50, there are situations where the rear could easily lock up before the front. This is a bad thing as it can cause oversteer (back end breaking loose).

I am gonna go ahead and disagree on that... (atleast in my opinion)
50/50 ((real life breaking test)) is after taking to account everything which includes- brake pressure, brake pads, rotors, weight, load distribution , extra friction applied to front wheels (real life test) that's why it is better then 50/50 of just brake fluid pressure.

50/50 (real life breaking test) may not be for all possible scenarios but I would rather have rear lock first before front steering wheels do (You do need to steer). One of my favorite games in winter is spinning truck with my hand brake on Locked rear is not bad, locked front will put the truck into the ditch very fast.

If 50/50 (real life breaking test) is set then the only way for rear to lockup is really to remove good amount of weight from the back. In my case I end up carrying extra weight in winter (tow strap, fuel, oil, shovel etc) which puts my rear to lock up less likely by a hair. (assuming I check my tire pressure frequently)

To stop the truck all 4 wheels have to share the stopping force. If some do less work then other wheels will have to pickup as much as physics allow it. That was my understanding, as to why Load Sensing Proportioning Valve is installed. LSPV changes how much pressure goes to rear when brakes are applied and that depends on the load of axle which changes with weight/ weight distribution and how hard you want to stop.
NOTE on the quote: Dont know who wrote that but I will take oversteer over the understeer anytime.

 

[Ebag333]

I am gonna go ahead and disagree on that... (atleast in my opinion)
50/50 ((real life breaking test)) is after taking to account everything which includes- brake pressure, brake pads, rotors, weight, load distribution , extra friction applied to front wheels (real life test) that's why it is better then 50/50 of just brake fluid pressure.

50/50 (real life breaking test) may not be for all possible scenarios but I would rather have rear lock first before front steering wheels do (You do need to steer). One of my favorite games in winter is spinning truck with my hand brake on Locked rear is not bad, locked front will put the truck into the ditch very fast.

If 50/50 (real life breaking test) is set then the only way for rear to lockup is really to remove good amount of weight from the back. In my case I end up carrying extra weight in winter (tow strap, fuel, oil, shovel etc) which puts my rear to lock up less likely by a hair. (assuming I check my tire pressure frequently)

To stop the truck all 4 wheels have to share the stopping force. If some do less work then other wheels will have to pickup as much as physics allow it. That was my understanding, as to why Load Sensing Proportioning Valve is installed. LSPV changes how much pressure goes to rear when brakes are applied and that depends on the load of axle which changes with weight/ weight distribution and how hard you want to stop.
NOTE on the quote: Dont know who wrote that but I will take oversteer over the understeer anytime.

For a professional driver, whether they prefer understeer, oversteer, or balanced will depend on the vehicle, conditions, track, and personal preference.

For your average Joe, understeer is safer than oversteer.

Worst case scenario, understeer means you're going off the side of the road. You're still travelling forward, the engine is between you and any potential impacts, and the seat belts and air bags are operating in the direction that they were designed for.

Worst case scenario, oversteer means that you are still going off the side of the road, but travelling sideways (or backwards!). Your seatbelt's effectiveness is drastically reduced (unless you have a 5 point harness it's designed to stop forward/back movement, not side to side), your airbags effectiveness is reduced, and you only have the door (or the roof) between you and the hard object you may hit.



Something to keep in mind is that under hard braking you will be taking a significant amount of weight off your rear axle. This will increase the tendancy of the back end to break loose or lock up, especially if you are cornering at the time.

If your brakes are already biased toward the rear, it becomes quite easy to lock them up.


I've actually personally experienced this with my truck. I had my rear brakes setup right around 50/50, which seemed and felt fine for the several weeks that I drove it. One day while headed home from work, someone pulled out in front of me and slammed on the brakes. Shouldn't have been a big problem, except it was raining and the roads were pretty slick.

I had never had the fronts or rears lock up on me on dry pavement (despite trying), but in this instance of hard braking plus the slick pavement the rear wheels broke loose. ABS kicked in to keep them from locking up, which drastically increased my stopping time, and nearly ended up in me rear ending the idjit who was in front of me. (And of course I would have been at fault.)

The front was no where near locking up, and ABS should not have kicked on. I adjusted my LSPV, and have had zero issues with the back locking up before the front whether on dry, wet, gravel, etc.

(That right there is also a perfect example of where ABS can make you take longer to stop.)




All that to say, is if you know how to drive and handle oversteer it's not always a bad thing. But not many people are experienced enough to handle it, so I wouldn't recommend as a general rule a 50/50 bias.

 

[Cattledog]

While I think your method is spot on, a word of warning....

You want the fronts to lock up just a bit before the rear.

If you set it for 50/50, there are situations where the rear could easily lock up before the front. This is a bad thing as it can cause oversteer (back end breaking loose).

If you set the bias too heavily to the front, then you can cause understeer.

With drums in the rear, oversteer isn't much of a concern (drum brakes will have a hard time overpowering disk brakes). But with disks front and rear, it can be an issue.

That's one reason why I don't think it's a good idea to replace the LSPV in an 80 with disc's with a T, since the proportioning is 50/50 then you can easily cause oversteer.

Remember when you're testing this that you will likely be travelling in a fairly straight line, under/oversteer won't have much effect here. When you need to worry about it is when you need to steer around the object in the road on a corner in slick conditions at speed. In those situations oversteer can be downright dangerous.


In the image below:

I agree with you Ebag333. There is a reason that the factory has the pressures in the rear lower and that the rear LSPV has a slight delay built in too. It's all about controlling oversteer, including when you are coming downhill in a low traction situation i.e. mud, snow, gravel. It will suck hard to try an catch the rear end of a heavy 80 in those situations.

 

[SUMOTOY]

I agree with you Ebag333. There is a reason that the factory has the pressures in the rear lower and that the rear LSPV has a slight delay built in too. It's all about controlling oversteer, including when you are coming downhill in a low traction situation i.e. mud, snow, gravel. It will suck hard to try an catch the rear end of a heavy 80 in those situations.

I agree with Cattledog and Ebag333, on the summary... First Rule: Ideal brake force distribution follows weight distribution exactly. There are several variables that influence braking dynamics. Choosing 50/50, I don't even do that on the race cars I service. You don't even have the same caliper or rotor front and rear, because you will never need that much braking force in the rear. I do not agree with many of the followup explanations and opinions, and would rather leave the summary that the fronts lock up before the rears = always. ABS intervention is a separate set of dynamics best explored in a separate thread. FYI II, most race style adjustable proportioning valves (Tilton, Wilwood, etc) will max rear brake proportioning at 70% of front. That's because, if braking force follows weight transfer, the front brakes will *always* support more of the braking force.

Backing up to design... OEM brakes are setup to give maximum brake force distribution for a given vehicle at the GVWR. The proportioning valve increases the rear force when there is more weight in the back of a truck. In practice and design, all brakes should be setup to lock up the fronts before the rears in any and all conditions = control. The proportioning valve design then, reduces the braking force rear betwen GVWR and Curb Weight (empty). This is done so that the rears will not lockup before the fronts in any payload of the 80.

Control is the reason that the rear ABS circuit uses the "Select Low" principle (see search engine, I posted a long and nerdy on this). Basically, whichever rear tire has the least amount of traction, ABS will send the same braking force to the opposing rear tire. This is a control dynamic built into every ABS system built (with no exceptions AFAIK).

The bottom line? Rear lockup is to be avoided at all costs. This isn't about 'oversteer' or 'understeer' this is about *control* vs *loss of control*. The baseline setting of the LSPV is a pressure equation to ride height in the FSM. I've used and set many PV with this type of tool, and it works. The geomety of lift dictates if you raise the pivot point (chassis) of the LSPV 2in higher off the axle, you lift the axle attachment point 2 inches above the axle. This plate was included in my old 4R 2in lift kit.

Fine tuning LSPV without the guages can be done in the manner described, the lower the cf the better. Front ABS intervention or lockup before the rears, always.

The rock-bone stock FZJ80 tested in 93 (with ABS) got a documented braking distance of 128feet from 60-0, and the quote was "this is the best braking of any SUV tested". I haven't seen it bested yet... Compare it to many high performance automobiles, the FZJ80 has phenomenal brakes (not speaking to wear, only to performance). I put in stock pads and stock rotors and use the stock LSPV factory settings, because I don't see any change resulting in 'better' braking performance.

Scott J
94 FZJ80 Supercharged

 

[turbocruiser]

I too agree with Ebag; you don't want your rears locking up first or even at exactly "the same time" as there will be so many situations where this can cause complete loss of control if not an avoidable sideways slip. You also don't want to take away the "LS", aka Load Sensing, as in putting in the tee because there are so many situations when the weight differences between front and rear require active and proper proportioning ability. Anyways just another "vote" here I guess.

 

[RedTacoma]

People are welcome to set their brake bias to whatever they want. I set my brakes to 4 wheels locking at the same time when hard braking.

I know this is as close to a new car factory setting as possible. Actually factory settings to my knowlege are set at 55 MPH or 100 km/h with fully loaded vehicle.
This can easily be checked. Simply remove ABS fuse on your brand new car and hard brake on loose surface like sand.

I never had any problems with my setup and my brake pads and rotors wear evenly on front and rear.

Ebag333
I dont know how did you set your 50/50 so I can not comment on that. The only time I had problem was when my rear hit the bump on ice while going trough the turn and breaking. Yep I made awesome 180 on ice in front of the snow plow. However my rolling front provided pivot point that kept me on the road through all that. I always laugh when I remember the face of the snow plow guy looking at me waving to him. But that had nothing to do with my brake setup it was multiple problems compounding at the same time
1. Too much speed
2. Lightweight pickup with locked rear and manual tranny
3. Ice on the road
4. Turn
5. Hard Breaking to avoid plow blocking the road.

So yes there are very rare circumstances that can force your rear to lock but as I said before I take that over the front locking anytime due to the fact that during your rear spinning you can still control the vehicle. When your front skids you can start reading the book because best driving skills on the planet will not save you.

 

[Ebag333]

I know this is as close to a new car factory setting as possible

Someone above posted the factory settings. It isn't 50/50, nor is it very close to 50/50.

 

[RedTacoma]

Someone above posted the factory settings. It isn't 50/50, nor is it very close to 50/50.

I think you need to define for me then what do you mean by 50/50.
EDIT: I have a feeling that you talking about the pressure as in psi.

 

[Ebag333]

I think you need to define for me then what do you mean by 50/50.
EDIT: I have a feeling that you talking about the pressure as in psi.

The front calipers have more stopping power than the rear calipers. Which means that in order for you to lock up your rears at the same time as the fronts, you need more pressure going to the rear than the front.

Of course that's offset by weight transfer from the rear to the front when stopping, but 80's are actually very balanced weight wise (OEM) front and rear.

So PSI wise, when you have your rears locking up as quickly as your fronts I would suspect you are very near 50/50 PSI wise, either your LSPV is pretty much wide open or you've T'd off the brake lines.

EricE posted up earlier that the OEM spec is supposed to be 1138 front and 889 rear, which is then further reduced by the LSPV.

Considering the more powerful calipers in the front and the significantly reduced pressure to the rear, and you are no where near 50/50, either from a pressure perspective, braking power, or which calipers lock up first.

OEM these are designed with a buffer in them so the fronts lock up quite a bit before the rears, and the truck is setup exactly that way while my truck has less bias toward the front (though still biased toward the front). Driving both regularly (and having performed hard stops and panic maneuvers in both) gives me a very good perspective on it.

Understeer isn't much fun, and I've experienced it in the truck. But oversteer is far less fun.

The fact is that recommending that someone setup their truck in a configuration that can introduce oversteer in an already dangerous situation is just foolish. If someone knows how to handle their vehicle and wants to set it up that way, more power to them. But most people don't, and making a general statement that it's good to setup their vehicle that way could lead to some bad situations.

Yes, it's not likely that several factors come together where oversteer could happen and injury or death could result. But I experience situations like that at least once a year or so, so they're not all THAT uncommon thanks to the idjits we have on our roads.

Slight oversteer may be "better" than slight understeer, but understeer is safer, especially for those who do not know how to handle it. Handling understeer is fairly intuitive, the hardest part is to stop the brakes from locking up and ABS will assist a novice driver there. Handling oversteer is not intuitive, in fact every single thing you WANT to do in an oversteer situation is the exact thing you shouldn't do.