[underthehood]

Does anyone know how to bypass the load sensing proportioning valve on my 93 FZJ80? That's what I call the piece attached to the rear axle that adjusts the brake fluid pressure to the rear calipers depending on how heavy the load is. It has two lines coming in and one coming out. All the other ones I have dealt with just have an in and an out. Those were easy. I need some guidance on this one since I have ABS and I don't want to mess that up. Thanks in advance.

[Ebag333]

ABS will not be messed up by bypassing the LSPV, unless you let air get into the ABS actuator.

If you have rear disk brakes, I would not bypass the LSPV. It can cause oversteer, which can be dangerous.

There are a bunch of threads about bypassing it, searching "bypass LSPV" should net you a bunch of results.

[SUMOTOY]

Originally Posted by underthehood

Does anyone know how to bypass the load sensing proportioning valve on my 93 FZJ80? That's what I call the piece attached to the rear axle that adjusts the brake fluid pressure to the rear calipers depending on how heavy the load is. It has two lines coming in and one coming out. All the other ones I have dealt with just have an in and an out. Those were easy. I need some guidance on this one since I have ABS and I don't want to mess that up. Thanks in advance.

The early 3 channel brake toyota trucks, the LSPV utilizes a different circuit than most other vehicles. In the Toyota, the LSPV has an input feed (line from Master Cyl via ABS box), an output feed (line to rear brakes) and a bypass feed (what is not proportioned) which is fed back into the front braking circuit. Most vehicles do this at the Master cylinder or within the ABS box, but the effect is the same. The purpose of this circuit is to feed any non proportioned rear brake force to the front circuit. This allows a larger variation in rear brake proportioning for a given size Master cylinder bore.

When I put a tilton adjustable valve to replace the LSPV in my (non-abs) 4R, I ran a T to the Tilton, and fed the non proportioned brake force back into the stock location of the front brake circuit.

I disagree with Ebag333, ABS will be affected with any modification of the LSPV. In F/R brake force properties of a vehicle, Ideal Brake Force Distribution follows weight transfer. The LSPV is designed to give 'near' maximum designed brake force rear at GVWR (heavy), and 'near' maximum designed brake force 'empty'. The rod travel follows rear axle loading which adds or reduces baseline brake force to the rear axle. If you remove or modify the LSPV, you will either have too much brake force rear (LSPV removed) or too much brake force front (LSPV restricted, or LSPV fixed 'light' vs GVWR). The former will cause premature ABS intervention rear, the latter will cause premature ABS function Front.

I would only consider doing this mod on a non-ABS equipped 80. Doing this with the ABS 80 can result in loss of vehicle control under threshold braking. See other recent threads on topic.

HTH

Scott J
94 FZJ80 Supercharged

[alkaline747trio]

Originally Posted by SUMOTOY

I would only consider doing this mod on a non-ABS equipped 80. Doing this with the ABS 80 can result in loss of vehicle control under threshold braking. See other recent threads on topic.

HTH

Scott J
94 FZJ80 Supercharged

I disagree with this. An ABS equipped rig modded in this manner has no possible way of handling any worse than a non-ABS rig. Maximum forces are still the same, so maximum lockup is still the same. If you don't have ABS you can lose control under threshold braking just the same as an ABS equipped 80.


I am all for this mod on SF/Drum Brake equipped 80s. It is as easy as removing the valve, the arm, and the extra line feeding back to the front brake system. On a rear disc brake 80, the bias might be too much to the rear, but I can not say either was as I have not driven a RDB 80 w/o a proportioning valve.

[96r50]

It should be noted that the contraption on the 80 series is actually a Load Sensing Proportioning & Bypass Valve (LSP&BV). The 80's braking system is divided into two separate circuits of front brakes and rear brakes so if one circuit fails the other can still stop the truck. The LSP&BV is a critical part of the safety design as the Bypass Valve allows full pressure braking to the rear brakes (not modulated pressure as it is through the regular rear LSP circuit) in the event of a front circuit failure.

If you remove the bypass line you will have very weak rear brakes in the event of a front circuit failure, as a manual proportioning valve will not allow full pressure to the rear brakes like the LSP&BV does. I would highly recommend not removing the LSP&BV.

[Ebag333]

Originally Posted by SUMOTOY

I disagree with Ebag333, ABS will be affected with any modification of the LSPV.

Scott,

I was only addressing that removal of the LSPV would not change the way that ABS actually functions. ABS will still engage the same way as it did before the removal, removing the LSPV would not prevent it from functioning or anything of that sort.

As I outlined in another thread I do believe that having the LSPV too open or even removing it can cause the ABS to prematurely engage on the rear wheels which can cause oversteer (due to the tires locking up) and greatly increase stopping distances (due to the ABS engaging on the rear long before it needs to on the front).

[SUMOTOY]

Originally Posted by Ebag333

Scott,
I was only addressing that removal of the LSPV would not change the way that ABS actually functions. ABS will still engage the same way as it did before the removal, removing the LSPV would not prevent it from functioning or anything of that sort.

As I outlined in another thread I do believe that having the LSPV too open or even removing it can cause the ABS to prematurely engage on the rear wheels which can cause oversteer (due to the tires locking up) and greatly increase stopping distances (due to the ABS engaging on the rear long before it needs to on the front).

Actually, ABS does change it's function based on LSPV removal or modification. I equate ABS 'function' as increasing stopping distance, premature lockup (front or rear), and Select Low Principle applied to the rear brake ABS algorithums.

ABS engagement does not cause oversteer, in fact, abs algorithum during engagement is designed to always present understeer (Search "Select Low Principle" - presented by me on this forum). Threshold braking prior to ABS engagement can cause loss of car control due to dynamic chassis 'oversteer' (too much brake force rear) or 'understeer' (too much bypassed rear brake force to fronts). Once ABS is engaged, understeer is the designed result of ABS application. If Ideal Brake Force follows weight transfer, and Select Low Principle is applied to the rear brakes under ABS intervention, how can ABS engagement cause 'oversteer'?

Scott J
94 FZJ80 Supercharged

[alkaline747trio]

Originally Posted by 96r50

It should be noted that the contraption on the 80 series is actually a Load Sensing Proportioning & Bypass Valve (LSP&BV). The 80's braking system is divided into two separate circuits of front brakes and rear brakes so if one circuit fails the other can still stop the truck. The LSP&BV is a critical part of the safety design as the Bypass Valve allows full pressure braking to the rear brakes (not modulated pressure as it is through the regular rear LSP circuit) in the event of a front circuit failure.

If you remove the bypass line you will have very weak rear brakes in the event of a front circuit failure, as a manual proportioning valve will not allow full pressure to the rear brakes like the LSP&BV does. I would highly recommend not removing the LSP&BV.

This is only assuming that one adds a proportioning valve back to the system. I would much rather have two seperate, non-connected in a brake failure situation; without a valve at all.

[SUMOTOY]

Originally Posted by alkaline747trio

I disagree with this. An ABS equipped rig modded in this manner has no possible way of handling any worse than a non-ABS rig. Maximum forces are still the same, so maximum lockup is still the same. If you don't have ABS you can lose control under threshold braking just the same as an ABS equipped 80.

I am all for this mod on SF/Drum Brake equipped 80s. It is as easy as removing the valve, the arm, and the extra line feeding back to the front brake system. On a rear disc brake 80, the bias might be too much to the rear, but I can not say either was as I have not driven a RDB 80 w/o a proportioning valve.

Contradiction paragraph 1 to paragraph 2. This has nothing to do with disc or drum - this is strictly ABS vs non-ABS strategy. The ABS rig will brake with more steering control under ABS activated conditions. The ABS rig will always send the lowest tractive braking force of the rear wheels to both rear brakes, where-as the non-ABS rig will always send *equal* braking force to the rear brakes (regardless of LSPV setting). That defines understeer vs oversteer under impending lockup.

It is quite possible to use a proportioning valve in a non ABS vehicle, because it only has to lock the fronts before the rears. In the ABS vehicle, the LSPV is key to how the Select Low Principle algorithums are applied to the rear brakes. There are drawbacks to removing LSPV in either case, but the setting on the non-ABS vehicle has no electronic intervention that would change chassis dynamics in threshold> braking dynamics. One could argue that 'understeer' is a safer vehicle control dynamic under braking, so ABS is 'safer'.

However, if LSPV is set wrong, you could have an oversteer dynamic, which the non-abs vehicle will always exibit up to and including lockup. The ABS vehicle, will have that same oversteer dynamic, until impending lockup (ABS intervention), when the ABS is designed to change that 'oversteer' to 'understeer'. That 'at the limit' change in chassis dynamic usually doesn't correlate to good vehicle control.

Scott J
94 FZJ80 Supercharged

[Tapage]

Since I like to lock the rear before the front tires in my 2X80 and 60 ... but all are non ABS TLC .. what should happen if I modify the LSPV to loc the rears before fronts in a ABS 80 .. ?

[96r50]

Originally Posted by alkaline747trio

This is only assuming that one adds a proportioning valve back to the system. I would much rather have two seperate, non-connected in a brake failure situation; without a valve at all.

So you want equal pressure running to both front and rear brakes, with no proportioning valve? That might be fine on a small, lighter, lower sedan but on a big, tall, heavy truck it is a bad idea IMO. It might be do-able on the older 80s with rear drum brakes, but with four wheels discs I think it would be a disaster looking for a place to happen. YMMV

[underthehood]

This has become a very informative thread in respects to braking with or without the ABS engaging. I appreciate all the feedback. I am mostly concerned with my j-lift and it has obviously affected the LSPV. I read of how some people relocate the bracket on the differential with a piece of metal. Seems easy enough. I do have rear disc brakes and after reading all the feedback I think I will just relocate the bracket. Thanks for the replies.

[Ebag333]

Originally Posted by Tapage

Since I like to lock the rear before the front tires in my 2X80 and 60 ... but all are non ABS TLC .. what should happen if I modify the LSPV to loc the rears before fronts in a ABS 80 .. ?

If you lock up the rears before the front, the ABS will prevent the rears from (fully) locking up.

ABS is not designed to decrease stopping distances (common misconception), but is designed to prevent the tires from locking up, which can cause an oversteer or understeer situation.

Taking ABS out of the equation for a moment, when you lock up the rears before the front you are introducing oversteer. ABS may not be able to eliminate oversteer in a bad enough situation, though it can help limit it.

Since I've already answered this in another thread, I'm just going to quote out of it.

The whole thread is worth reading, however.

http://forum.ih8mud.com/80-series-te...ml#post4924185

Originally Posted by Ebag333

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:

Originally Posted by Ebag333

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.

Originally Posted by Ebag333

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.

[Tapage]

Ebag .. brutal post .. thanks !

been read all I still thinking for me .. it's more valuable oversteer ( locking my rears segs before fronts .. ) I'm still under the impresion that in a gravel road for example, if you loc your fronts before rears in a turn you will end out of the road ..

But .. loing your rears .. you are still in control of the vehicle .. you have steering coz your front tires are still turning .. those seconds IMOP make the diference in my case.

I can release the brakes .. unloc the rear tires, push the gas and correct the course ..

[Ebag333]

Originally Posted by Tapage

Ebag .. brutal post .. thanks !

been read all I still thinking for me .. it's more valuable oversteer ( locking my rears segs before fronts .. ) I'm still under the impresion that in a gravel road for example, if you loc your fronts before rears in a turn you will end out of the road ..

But .. loing your rears .. you are still in control of the vehicle .. you have steering coz your front tires are still turning .. those seconds IMOP make the diference in my case.

I can release the brakes .. unloc the rear tires, push the gas and correct the course ..

It's obviously up to you how you setup your rig. I've experienced both oversteer and understeer in the 80, and I would probably personally prefer a bit of oversteer than understeer for myself. But oversteer can go south in a hurry real quick, understeer isn't quite as dangerous.

I don't know if you have been in any over/understeer situations, or had any experience/training in handling them. I've had the advantage of having parents who grew up learning how to drive in ice (not snow, ice ) and so they taught me how to deal with under/oversteer from the start.

Your suggested method of handling oversteer might or might not work, you really don't give enough details to say one way or the other. But if you "push the gas" that can actually cause you to oversteer the opposite way.

"Correct the course" is not intuitive with when oversteering, most people either turn with an oversteer (which of course causes it to be worse) or go the opposite direction way too far, which can cause you to oversteer the opposite way or lose control completely. While you want to turn into it, again, moderation is key.

The surface really doesn't matter for how you handle oversteer or understeer, but it does effect how much of it you can get away with. I've had my truck in 4 wheel slides on gravel before (in that circumstance, just slightly wanted to oversteer), and I wouldn't set my truck up to be biased towards the rear. Had I not been biased towards the front, that situation would have resulted in my back end kicking out and instead of being a nice 4 wheel slide, it would have been end over end.

The theory that you maintain more control in an oversteer than understeer is true...right up to the point where you lose control completely. With understeer (especially in a rig with ABS), you might lose some control but you will always maintain a certain amount of control over the situation. With oversteer, if you are experienced enough you can maintain full control through the whole situation, but if you lose control even for just an instant, you're screwed, it's more of an all or nothing situation.

Something else to consider is that 80's are pretty top heavy, especially when lifted. If you oversteer and spin out in a low slung sports car, you might be embarressed but that's about it. If you spin out in an 80....well....there's probably more than a fair chance you're going to end up on your side or top. Kinda hard to end up upside down in an understeer situation.

[Tapage]

I got your point and it's complete valid .. I don't have more experience driving more than usual pasionate off roader .. 10 years fighting with all kinds of mud down here .. somethimes it's pretty funny a downhill in reverse .. just to see if you can barely do it slow ..