AWD Braking Discussion: Ideal Brake Force Distribution (2 Viewers)

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Ideal Brake Force Distribution - Definition and self test

Background. It is accepted awd theory - and physics - that a locked center differential (and torsen center - not applicable to 80) under braking will result in Ideal Brake Force Distribution. This means that the brake loading (up to lockup) will follow *exactly* weight distribution. Specifically, the extra braking force of the front brakes (defined as 'overrun brake torque) is transmitted through the locked driveshafts to the rear tires.

Defining Ideal Brake Force Distribution: Let's say under maximum braking, the weight distribution of the 80 is 78% on the front axle /22% on the rear axle. On a 4700lb 80 that means that the front axle loading is 3666lbs and the rear axle loading is 22%. In 'ideal brake force distribution' theory, you would then want exactly 78% of the braking force available to the front axle and 22% to the rear axle.

In Brake Proportioning systems found on the 80, you have a variable proportioning valve for rear braking. This helps in maintaining a 'better' (vs fixed) brake force distribution thru varying payload weights and distribution. As the rear of the truck settles from weight, so does the increase in rear brake force. These proportioning systems are a compromise of best estimates and hardware dependent. The closest any of these systems get to 'ideal brake force distribution' is only during ABS *activation*. However, how close they get to ideal depends on several factors, and no 'extra' brake force (over run brake torque) can be transfered to the 'other' axle.

Bad pads, weak springs, changing shocks, changing COG, changing payload all effect how close to the factory design brake force distribution gets. How far you stray from the factory design, is directly correlated to how far you will stray from ideal brake force distribution.

To test or feel how close *your* specific truck brakes/hardware is to ideal brake force distribution is easy if you have a center locker (CDL switch) installed. Lock CDL and brake in a straight line, this yields the ideal brake forced distribution given what you have on your truck at the time you lock the CDL. Now, unlock the CDL, and try the same brake test. Notice how much more the chassis wants to dive, and how much more brake effort and distance is required? My conclusion is that the brake force distribution with the stock proportioning valve is massively biased to the front brakes on the 80. This also follows accepted application of brake hardware in production vehicles.

In terms of chassis dynamics, the less you upset the chassis weight distribution, the better the performance you will have under braking and handling. IME with the 80 chassis (TJM, airbags, Blizzacks, otherwise stock chassis), the comparison of ideal (CDL locked) vs stock (CDL unlocked) leads me to conclude that the 80 is massively overbraked in the front compared to the rear. I suspect this is a combination of inherently innacurate rear prop valve, and the algorythums in the ABS computer.

Next up in Part II: ABS in the 80 and Select Low Principle


HTH

Scott Justusson
 
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Accepted theory by whom?

The brake force isn't being distributed, the wheel torque is being evenly transmitted to the most effective brakes with the CDL engaged. The rear brakes don't add any more or less stopping power then they normally would. Your rear brakes could be completely disconnected and the front brakes won't lock independant of the rear with the differential locked.

-Spike
 
Accepted theory by whom?
Spike, it's not just theory, it's physics. It's also in just about every single SAE paper on awd braking dynamics. This is part of 101, because it's the most basic part of understanding CDL operation. In a locked diff any applied torque (acceleration) and any applied reverse torque (braking) follow exactly weight distribution up to wheel spin or lockup respectively.

The brake force isn't being distributed, the wheel torque is being evenly transmitted to the most effective brakes with the CDL engaged. The rear brakes don't add any more or less stopping power then they normally would. Your rear brakes could be completely disconnected and the front brakes won't lock independant of the rear with the differential locked.
-Spike

You contradict yourself. Yes, if the rear brakes were disconnected, the fronts won't lock independent of the rear. However in a straight line, your brake force distribution of the braking *capacity* follows exactly weight distribution in a locked center diff - always. Add the rear brakes back into the equation, the available braking *capacity* increases, the F/R brake distribution hasn't changed.

Or to your example, a locked center diff with the rear brakes disconnected, still has rear braking through the driveshafts the front brakes are just slowing 4 tires instead of two. If you accept in a straight line that an *open diff* under acceleration always allocates torque 50/50, then you accept in a *locked center diff* that under acceleration torque allocation always follows weight distribution. That is the definition of a locked center diff. By extension, there is no difference between applied torque (acceleration) and reverse torque (braking).

ST
 
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I guess I just don't care yet, as I have no plans to lock my CDL prior to braking.
 
I guess I just don't care yet, as I have no plans to lock my CDL prior to braking.

It might be interesting to compare your lifted truck to your non lifted truck with CDL on/off under braking. How have you changed your rear brake regulation with the lift kit? What effect does raising the COG have on braking and pitch?

ST
 
ST--

I'm waiting for part II as I am one of those people who have ripped out the ABS stuff on their 80 series and I think it was the best thing I could have done for driving this truck the way I do. I like the feel much better, the control I now have over the vehicle and its movements in braking maneuvers.

-o-
 
Wow... Another beehive? Wheer's my stick/

To start with...
:shotts: :shotts: :shotts:
At some point this becomes chat as the technical portion already has N^X threads, doesn't it?

Background. It is accepted awd theory - and physics - that a locked center differential (and torsen center - not applicable to 80) under braking will result in Ideal Brake Force Distribution. This means that the brake loading (up to lockup) will follow *exactly* weight distribution. Specifically, the extra braking force of the front brakes (defined as 'overrun brake torque) is transmitted through the locked driveshafts to the rear tires.

So long as you assume that the traction available at all wheels is the same AND that no wheel is locked/skidding AND that the opperator is able to modulate the brake pressure perfectly to the limits of the current road conditions.

Too many assumptions for my taste.

In terms of chassis dynamics, the less you upset the chassis weight distribution, the better the performance you will have under braking and handling. IME with the 80 chassis (TJM, airbags, Blizzacks, otherwise stock chassis), the comparison of ideal (CDL locked) vs stock (CDL unlocked) leads me to conclude that the 80 is massively overbraked in the front compared to the rear. I suspect this is a combination of inherently innacurate rear prop valve, and the algorythums in the ABS computer.

Again, I think you're assuming track conditions with race drivers. The majority of us would like to stay well within the bounds of the traction coeficient when driving over to Fred's place for pizza and beer. Under heavy braking under nearly any road conditions the ABS does one major job. It keeps the rear end of the truck in the rear.

I agree that under repeated runs on repeated conditions that trained drivers can outbrake the ABS by a bit. However, for most drivers most of the time in random conditions where they are only going to be emergency braking in emergencies, the unlocked / ABS on is a better system.

There are no do-overs in on-road emergency maneuvers and drivers are very often inatentive in mundane conditions and panic like little girls when another vehicle does a four wheel power drift around the corner in front of them.

From your posts it sounds like you are endorsing driving at the 'limits of cohesion' with the CDL locked in daily driving. If I did this, the local constabulary would be on a first name basis.

Save the performance driving for the track so our insurance rates don't go up as fast please.
 
Physics 101?

...
So long as you assume that the traction available at all wheels is the same AND that no wheel is locked/skidding AND that the opperator is able to modulate the brake pressure perfectly to the limits of the current road conditions.
Too many assumptions for my taste.

Grench, I deal with these responses all the time. This is a basic misunderstanding of how a locked center diff works, in ANY AWD chassis. ANY brake force, *ANY* brake force applied will follow the weight distribution in a locked center diff. Make bigger brakes, don't push the pedal as hard, the weight distribution (front dive under braking) might change, but any brake force applied follows that weight distribution with a locked diff. Let's assume no mixed mu conditions because ABS doesn't handle mixed mu any better, and we aren't there yet. KISS: Make 3 assumptions: No locked wheel, 4 wheels same mu, straight line. The operator doesn't need to modulate pressure perfectly, this holds true with ANY brake pressure applied up to wheel lock.

Again, I think you're assuming track conditions with race drivers. The majority of us would like to stay well within the bounds of the traction coeficient when driving over to Fred's place for pizza and beer. Under heavy braking under nearly any road conditions the ABS does one major job. It keeps the rear end of the truck in the rear.

*ACTIVATED* ABS tries to emulate ideal brake force distribution. I am speaking to straight line braking, locked center vs unlocked center, NO impending lockup. Reread my post, this has nothing to do with racing. In fact, don't even touch the brakes, engine braking torque allocation follows F/R weight distribution in a locked center diff. Engine on-throttle torque during acceleration does too!

I agree that under repeated runs on repeated conditions that trained drivers can outbrake the ABS by a bit. However, for most drivers most of the time in random conditions where they are only going to be emergency braking in emergencies, the unlocked / ABS on is a better system.

I don't think it has anything to do with 'outbraking' ABS, I make no claims of that (part VII?). In this session, I only speak to ideal brake force distribution via a locked center, vs programming close to ideal brake force distribution with ABS. The falacy of believing that ABS is ideal brake force distribution is exactly that ABS must be activated to get even close, then you have a hardware problem because you have isolated total brake capacity (locked diff) to individual axle brake capacity (unlocked diff). In a locked center you have ideal brake force distribution from the time your brake pads hit the rotors up to wheel lockup.

There are no do-overs in on-road emergency maneuvers and drivers are very often inatentive in mundane conditions and panic like little girls when another vehicle does a four wheel power drift around the corner in front of them.

I see that as a driver training problem. Not solved by ABS, solved by driver education and skillset. In a controlled environment (like Steamboat for instance) you can do-over as many on-road emergency manuevers as you please, only hurting ego when you have exceeded the laws of physics. I advocate and teach the first time at the limit of adhesion shouldn't be on-road. When it counts, have the training already in place. ABS is NOT an emergency maneuver saver, it's and electronic attempting to put Ideal Brake Force Distribution in a vehicle without it.

From your posts it sounds like you are endorsing driving at the 'limits of cohesion' with the CDL locked in daily driving. If I did this, the local constabulary would be on a first name basis.
Save the performance driving for the track so our insurance rates don't go up as fast please.

??? No Grench, I drive normally 90+% of the time (ok a bit less with the SC installed;). Again, this is a misunderstanding of the concept of Ideal Brake Force Distribution. This has nothing to do with 'how' one drives, this has to do with putting your foot on the brake with the CDL locked, on dry pavement in a straight line. Baby steps. We need to get agreement on some physics properties of awd torque allocation here. I'm happy to move on to more complicated AWD chassis dynamics, but grasping the concept of CDL torque and reverse torque allocation in a straight line on dry pavement is as basic as it gets. Don't jump ahead here if you don't understand IBFD.

I can try to explain IBFD better, I can't change physics? It's clear that you disagree that IDBFD is a physical property of a locked center diff. I can't address that part easily, except to understand that maybe I should back up and deal with Ideal Axle Torque Distribution during acceration with a locked diff first.

ST
 
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...KISS: Make 3 assumptions: No locked wheel, 4 wheels same mu, straight line. The operator doesn't need to modulate pressure perfectly, this holds true with ANY brake pressure applied up to wheel lock...

...??? No Grench, I drive normally 90+% of the time (ok a bit less with the SC installed;). Again, this is a misunderstanding of the concept of Ideal Brake Force Distribution. This has nothing to do with 'how' one drives, this has to do with putting your foot on the brake with the CDL locked, on dry pavement in a straight line. Baby steps. We need to get agreement on some physics properties of awd torque allocation here. I'm happy to move on to more complicated AWD chassis dynamics, but grasping the concept of CDL torque and reverse torque allocation in a straight line on dry pavement is as basic as it gets. Don't jump ahead here if you don't understand IBFD.

I can try to explain IBFD better, I can't change physics? It's clear that you disagree that IDBFD is a physical property of a locked center diff. I can't address that part easily, except to understand that maybe I should back up and deal with Ideal Axle Torque Distribution during acceration with a locked diff first.

ST

So, this is another exercise in futility. I plan on being in situations where I don't brake in a straight line, and don't have a perfectly dry and uniform surface to drive on. Frankly, I attempt to drive well within the limits of adhesion, and I would venture to guess that most others do as well. In fact, I have never lost traction on the road, in my 80 except on black ice, and playing in snow.
Your arrogance, and apparent belief that you are the only one who understands the physics is just annoying, and in keeping with your web identity. It is a clever ploy to insinuate that all others who do not adhere to your application of your theory, have poor driver training and/or skillset. I hope most of us can see through your subtle but frequent insults.
 
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So, this is another exercise in futility. I plan on being in situations where I don't brake in a straight line, and don't have a perfectly dry and uniform surface to drive on. ....

I am happy to address CDL chassis dynamics while turning. Given the posts so far in this thread, that appears... premature IMO. Please tolerate me sticking with the thread and the folks that are having a tougher time grasping the concept of IBFD in a straight line. As I'm sure you know, IBFD applies to *any* constant surface up to lockup, not just "dry". I've tried several times over the years to speak to mixed mu chassis dynamics, it's quite a pandora's box. More education comes from understanding chassis dynamics from a given surface (x4) than working thru mixed mu scenarios. I can direct you to dozens of SAE articles on chassis dynamics that also avoid pandora's box.

If we speak to "normal" on road driving, mu as a constant is the best baseline. Even the best ABS algorythums out there use this assumption in programming.


ST
 
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I am happy to address CDL chassis dynamics while turning. Given the posts so far in this thread, that appears... premature IMO. Please tolerate me sticking with the thread and the folks that are having a tougher time grasping the concept of IBFD in a straight line.

Thanks

ST

I really don't care if you ever address CDL chassis dynamics while turning, as it will have no effect on me. It is tough to tolerate your arrogance and insults, but I suppose I have no choice. :rolleyes:
 
I should state that the opinions here-in represented by SUMOTOY are his opinions and do not represent the position of the forum Please consider this when evaluating SUMOTOYs opinions as they apply to your vehicle. This forum and the moderator do not recommend driving with the CDL engaged and the ABS off in normal driving conditions. Not saying he is wrong, just that it is his opinion.
 

I have to agree, as this is not 80 specific, it is the same for any PT4wd vehicle that is in 4wd. I'm sure our brothers driving 40s will appreciate being told they are idiots for not driving around town in 4wd.
 
I am happy to address CDL chassis dynamics while turning. Given the posts so far in this thread, that appears... premature IMO. Please tolerate me sticking with the thread and the folks that are having a tougher time grasping the concept of IBFD in a straight line. As I'm sure you know, IBFD applies to *any* constant surface up to lockup, not just "dry". I've tried several times over the years to speak to mixed mu chassis dynamics, it's quite a pandora's box. More education comes from understanding chassis dynamics from a given surface (x4) than working thru mixed mu scenarios. I can direct you to dozens of SAE articles on chassis dynamics that also avoid pandora's box.

If we speak to "normal" on road driving, mu as a constant is the best baseline. Even the best ABS algorythums out there use this assumption in programming.


ST

Well crap, you added to your post after I responded to it. I know that "IBFD" applies to *any* constant surface, which is precisely why I wrote:
"I plan on being in situations where I don't brake in a straight line, and don't have a perfectly dry and uniform surface to drive on."
I really don't know what "mu" is, (here's your chance to tout my ignorance), but if you think that in normal driving, it is best to assume a uniform surface, you must live where there is no precipitation and the roads are repaved daily.
 
I think it is kind of lame that we have someone with obvious experience and theoretical knowledge about braking systems, 4WD systems, and chassis knowledge, and we are giving him a hard time because we don't do our own work and research what he is arguing. Or, we think that he is advocating/telling us to drive with the CDL locked. I don't think he is doing that at all.

I've checked out some of the fluid dynamics stuff that ST posted in previous threads and most of the stuff he explained about cooling was pretty spot on with technical documents you can find at the SAE website and other sources. It was funny because everyone gave him a hard time when in fact most of what he was arguing is true.

Albeit, most of the stuff was a little above my own knowledge, but when you take the time to confirm stuff he is saying, he is right most of the time with what engineers have documented.

I'm just saying, why don't we all put aside our preconceived/accepted notions of how to drive our rigs and hear what someone else has to say. Dismissal of a position you find disagreeable is easy; accepting a different position based on knowledge you don't have and don't want to investigate yourself makes you lazy.

-o-
 
This is very confusing.
First of all, this seems like the same topic as that other thread so now it's spread out amongst 2 diff. threads- seemingly repeating the same thing.

In my 2003 4Runner (Torsen center diff) stated not to lock the CDL in high-traction areas. Just offroad only. I locked it on road and you cannot friggin' steer w/ it locked!

In the 80, I haven't locked it on road because of the simple fact I want to be able to steer.
How do you get around this fact??
 
This is very confusing.
First of all, this seems like the same topic as that other thread so now it's spread out amongst 2 diff. threads- seemingly repeating the same thing.

I hope to confuse less. In the AWD class 101 thread I jumped into my advocacy that CDL isn't unsafe in terms of vehicle dynamics. What I identified in that opinion, is that this forum might not have some basic understanding of how CDL works, from a physics standpoint. That appears correct. I started a new thread on it to get rid of opinion and just put forth physics. Still controversial? I'm confused now.

In my 2003 4Runner (Torsen center diff) stated not to lock the CDL in high-traction areas. Just offroad only. I locked it on road and you cannot friggin' steer w/ it locked!

In the 80, I haven't locked it on road because of the simple fact I want to be able to steer.
How do you get around this fact??

Wheelbase, track, COG, driveshaft height and angle all effect how well a locked center diff 'steers'. I don't get around the fact that a locked CDL 80 steer in normal on pavement city driving. In extreme tight turns, it can bind to the point of shoving tires.

Does a 2003 4R really have a center torsen AND a locker center?

ST
 
I think it is kind of lame that we have someone with obvious experience and theoretical knowledge about braking systems, 4WD systems, and chassis knowledge, and we are giving him a hard time because we don't do our own work and research what he is arguing. Or, we think that he is advocating/telling us to drive with the CDL locked. I don't think he is doing that at all.
From what I gather from his posts, if you want maximum performance when braking you should assume you are on a uniform surface and leave your CDL locked. If you do not subscribe to his method, you have a driver training/education problem. So he doesn't come right out and say to drive with CDL locked, just that you are an idiot if you don't

I've checked out some of the fluid dynamics stuff that ST posted in previous threads and most of the stuff he explained about cooling was pretty spot on with technical documents you can find at the SAE website and other sources. It was funny because everyone gave him a hard time when in fact most of what he was arguing is true.

Albeit, most of the stuff was a little above my own knowledge, but when you take the time to confirm stuff he is saying, he is right most of the time with what engineers have documented.
He is great at theory, I just hope people don't take his application/methods w/out a grain of salt, as he tends to post in such a way that the uninitiated might take as gospel.

I'm just saying, why don't we all put aside our preconceived/accepted notions of how to drive our rigs and hear what someone else has to say. Dismissal of a position you find disagreeable is easy; accepting a different position based on knowledge you don't have and don't want to investigate yourself makes you lazy.

-o-

I don't plan to dismiss my "preconceived/accepted notions", otherwise known as 17 years of driving experience, on his say so, and I don't think anyone should. His assertion that others that disagree with his applications, are lazy or uninformed, are just rude.
 
...what he said !.....

Beno has it spot on!:cheers:
 

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