AWD Braking Discussion: Ideal Brake Force Distribution

[Grench]

Looking for more bees to agitate...

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.

Thanks, I do understand the physics involved. I do understand that the net rotation axle shaft to axle shaft is flat. However, it does not equate to equal spin wheel to wheel. It is the rubber on the road that makes the difference.

With a locked CDL and no ABS there is nothing to prevent one wheel locking and forcing the other on the same axle to spin twice as fast, pushed by the other axle, breaking it free.

Example/Scenario:
Going around a corner to the right at 25 mph, braking, right side of road is sheet ice, left side of road is clean pavement. Sheet ice ends suddenly resulting in the front right having traction while the rear still does not. This is a very comon road condition anywhere that the snow is removed from a crowned road by a flat bladed plow.

With ABS:
Right front and rear (both) ABS chatter like mad, braking ability is reduced but controlled. Some traction is retained on the ice side. Rear braking suffers the most. All 4 wheels turn at or near road speed throughout.

CDL on (no ABS):
Right side wheels both lock up when brakes are applied. Left side wheels both turning at same rate, but hopefully holding the line. Linear control (keeping it straight) becomes hard as the vehicle wants to rotate left as the driver is turning it right.

When the front right hits traction, it chirps and starts to turn at road speed. This means you have two front wheels moving at 25mph. The CDL is locked so the rear axle must get a net 25mph*2 spin and it is going to happen suddenly. If the left side has perfect traction, the right will likely spin up. If the left has marginal traction it will possibly spin up beyond road speed... be sure to hold on with your thumbs out at this point because operator involvement has just become optional.

The effect of the CDL is evening up the drive shaft speeds and doing nothing at the wheels during differential side to side traction and weight distribution scenarios. The left and right sides are on open diffs and will still behave as such.

/*sarcasm filter on*/
Clearly we are all idiots. Clearly the Toyota engineers who created the FZJ80 drivetrain were all idiots. None of us know how to drive.
/*sarcasm filter off*/

 

[FirstToy]

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

No, in normal driving- not even extreme turns- but turning on a street or whatever- the steering turns into a ton of bricks. I don't see how, esp in hazardous conditions where reaction time is a factor, this is good procedure?

The 4Runners have a Torsen (supposedly the most advanced one at the time in 03- maybe still) center diff that you can lock from a switch on the dash.

 

[KliersLC]

So, can we skip ahead to the part where you tell us how to get better braking performance out of our rigs without having the center diff locked up??

 

[SUMOTOY]

Thanks, I do understand the physics involved. I do understand that the net rotation axle shaft to axle shaft is flat. However, it does not equate to equal spin wheel to wheel. It is the rubber on the road that makes the difference.

With a locked CDL and no ABS there is nothing to prevent one wheel locking and forcing the other on the same axle to spin twice as fast, pushed by the other axle, breaking it free.

First, in my explanation of IBFD I present mu as a constant. Because opening mixed mu scenarios yields a lot of confusion. Start here (your own reference to 'linear' control in a turn). The above would depend on you defining mixed mu. Without lockup, your scenario is not correct, it's only supposed. And there is no ideal brake force distribution on an ABS vehicle in mixed mu conditiions. Any steering input (radius change) negates any claim that ABS or non ABS will have 'better' control.

Scenario:
Going around a corner to the right at 25 mph, braking, right side of road is sheet ice, left side of road is clean pavement. Sheet ice ends suddenly resulting in the front right having traction while the rear still does not. This is a very comon road condition anywhere that the snow is removed from a crowned road by a flat bladed plow.

Wheelbase mixed mu? Grench, stick to the IBFD as presented in post 1. Make mu a constant x 4, and straight line applied torque (for now... we can move to chassis dynamics in turns, but mixed mu scenarios won't get us far). Constant mu is exactly how every single SAE paper I've ever read on braking dynamics, traction and turning dynamics is presented. That's the conditions under which ABS algorythums are programmed. No one can predict wheelbase/track mixed mu. If you want to start the plug and play on this in the supercray, we need things like how much braking force, turning radius, wheelbase, speed, axle shaft length, weight and distribution... Forget the CDL in your scenario, I'll just take a ride in the truck with the lower COG!

Mu is a constant x 4 in Ideal Brake Force Distribution. I can speak to chassis dynamics, but mixed mu really is a single matrix that doesn't speak to Ideal Brake Force Distribution in ABS *or* non ABS equipped vehicles.

/*sarcasm filter on*/
Clearly we are all idiots. Clearly the Toyota engineers who created the FZJ80 drivetrain were all idiots. None of us know how to drive.
/*sarcasm filter off*/

Ideal Brake Force Distribution has little to do with drivetrain 'engineers', only the presence of a CDL in awd.

ST

 

[SUMOTOY]

...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.

Again, a basic misunderstanding of IBFD. I assume if you are on snow, or wet pavement or dry, gravel or sand, you have a uniform surface. Specifically mu is a constant to either an ABS vehicle or a non ABS vehicle. I understand the want and need to propose split mu conditions, but the matrix of that in terms of 'control' regarding CDL activation is only a guess, and changes with every single difference between trucks and driver. And a lifted truck (COG) will have less control in any split mu condition in terms of 'safety' than the CDL mode.

I see agreement with post 1 as written. I believe if anyone wants to speak of mixed mu conditions in wheelbase/track, it steps out of bounds of post 1, and changes physics to a set of variables without any definitive conclusion whatsoever.

ST

 

[firetruck41]

Again, a basic misunderstanding of IBFD. I assume if you are on snow, or wet pavement or dry, gravel or sand, you have a uniform surface. Specifically mu is a constant to either an ABS vehicle or a non ABS vehicle. I understand the want and need to propose split mu conditions, but the matrix of that in terms of 'control' regarding CDL activation is only a guess, and changes with every single difference between trucks and driver. And a lifted truck (COG) will have less control in any split mu condition in terms of 'safety' than the CDL mode.

I see agreement with post 1 as written. I believe if anyone wants to speak of mixed mu conditions in wheelbase/track, it steps out of bounds of post 1, and changes physics to a set of variables without any definitive conclusion whatsoever.

ST

I do not misunderstand the theory, I only misunderstood that you were trying to write something that was related to driving on a road. With your last post, I see you are only concerned about driving in labratory conditions. I don't drive in a labaratory, so it doesn't apply to me, I hope others, who do not drive only in labratories, can see that your thread is not intended for real world application.

 

[SUMOTOY]

I do not misunderstand the theory, I only misunderstood that you were trying to write something that was related to driving on a road. With your last post, I see you are only concerned about driving in labratory conditions. I don't drive in a labaratory, so it doesn't apply to me, I hope others, who do not drive only in labratories, can see that your thread is not intended for real world application.

IBFD has nothing to do with laboratories, it's applied physics. It's the design and target of every single ABS algorythum ever programmed. Select Low Rear brake force during ABS activation is the reality of how that doesn't work. Post 1 IBFD applies to every single center locked AWD vehicle under braking in constant mu, where we all spend *most* of our time driving.

It is exactly real world, and anyone with CDL can test it out, and compare IBFD to what Mr. T delivered in the real world. It can also help identify how real world chassis modifications compare and affect chassis dynamics and identify possible brake/chassis modifications for better vehicle control.

I will address some of Mr. T's real world hardware and it's real world effects in some further discussions. Specifically looking at the stock rear brake proportioning prior to ABS activation or impending lockup.

ST

 

[EricG]

Ok, I think people are missing the point here. As I understand it, SUMOTOY is trying to get at whether the brake proportioning valve is set up properly by comparing the braking performance with CDL locked and CDL unlocked. Presumanbly wiith the goal of adjusting the brake proportioning valve to provide a better brake force distribution with the CDL unlocked?

I don't see anything reccomending driving with CDL locked/ABS off on a regular basis - just for the purpose of this test.

I'll admit, the OP comes off as a bit preachy - the delivery could use some work. But you guys could relax a bit also.

 

[firetruck41]

IBFD has nothing to do with laboratories, it's applied physics. It's the design and target of every single ABS algorythum ever programmed.

If IBFD has nothing to do with labratory conditions (my term for your perfectly uniform surface), and it has nothing to do with split mu conditions, then exactly what kind of conditions does it have to do with.

...Post 1 IBFD applies to every single center locked AWD vehicle under braking in constant mu, where we all spend *most* of our time driving.

I don't spend enough of my time at "constant mu" to ignore the rest of my driving:
-driving on a paved road the has some gravel on it
-driving on a road with a puddle on one side of it
-driving on a road the is partially dry and partially wet
-driving on a road that is half of lane of concrete and half of a lane of pavement
-driving on a dry road with some patches of ice
-driving on a road with slushy patches
-driving on a road with some wet pavement and some water over ice
- etc...
But maybe I'm the only one that encounters, other than "constant mu".

It is exactly real world, and anyone with CDL can test it out, and compare IBFD to what Mr. T delivered in the real world. It can also help identify how real world chassis modifications compare and affect chassis dynamics and identify possible brake/chassis modifications for better vehicle control.

I will address some of Mr. T's real world hardware and it's real world effects in some further discussions. Specifically looking at the stock rear brake proportioning prior to ABS activation or impending lockup.

ST

How does "exactly real world", square up with your other statements below?

First, in my explanation of IBFD I present mu as a constant...

KISS: Make 3 assumptions: No locked wheel, 4 wheels same mu, straight line...

As I'm sure you know, IBFD applies to *any* constant surface up to lockup, not just "dry"...

 

[FirstToy]

No, in normal driving- not even extreme turns- but turning on a street or whatever- the steering turns into a ton of bricks. I don't see how, esp in hazardous conditions where reaction time is a factor, this is good procedure?

can someone answer this for me then... in plain english... maybe just one or two brief sentences?

 

[firetruck41]

can someone answer this for me then... in plain english... maybe just one or two brief sentences?

Umm, I don't think it is normally a good procedure.

That was only one sentence.

 

[Grench]

More bees anyone?

So, if you can't dazzle them with brilliance...

Without lockup, your scenario is not correct, it's only supposed. And there is no ideal brake force distribution on an ABS vehicle in mixed mu conditiions. Any steering input (radius change) negates any claim that ABS or non ABS will have 'better' control.

So, the CDL model requires an equal traction scenario and disallows lockup? On what planet? You're missing the A#1 reason for ABS. Control while braking. You can still steer while braking with ABS. No, you can't defy physics and do a 90* turn. You are still limited to the road surface.

Note: Motorcycle ABS doesn't work this way as the net vector with lean is not calculable by the in-production computers. Brake, then turn a bike per NSF guidelines.

Wheelbase mixed mu? Grench, stick to the IBFD as presented in post 1.

Yep. Real world sucks. It is where I drive. And with a, "mixed mu," ABS rules. I wish it were independent rear as well, but hey, live with what we get.

No one can predict wheelbase/track mixed mu. If you want to start the plug and play on this in the supercray, we need things like how much braking force, turning radius, wheelbase, speed, axle shaft length, weight and distribution...

You may not be able to predict it to 10 decimal points, but I'll bet you can do it to 2 (depending on units) AND you wouldn't even have to do that to calculate the difference between CDL and ABS and none under panic brake application in a curve on mixed surfaces. Nasty scenario, but realistic.

Braking force? Assume pedal pressure at 110% of whatever translates to traction available.
Turning radius? > 40' and < 120' doesn't matter.
Wheelbase? FZJ80 - not that it matters for this example. It would matter if we were going from one vehicle to another. For this example, just use 1. Why? Because our Unit is one FZJ80 wheelbase.
Speed? Lets use 50mph, although by itself it doesn't matter much as long as it is faster than the lateral adhesion of a spinning/stopped tire on iced concrete. 50mph will work.
Weight? 1 FZJ80. Units go away this way. Like it?
Distribution? Well, that is going to be 60% + a near linear delta dependent on the cohesion available.
Cohesion available? This is where it gets EASY. Wet Ice = 0 (near enough) and dry pavement = 1. Again, we're free to make our own units since we aren't going from vehicle to vehicle.

An OVERLY simplified equation for the braking forces available would be: CoWheel1 + CoWheel2 + CoWheel3 + CoWheel4 where CoWheelX is the net friction coefficient present at any 1 wheel. It is after all, what is available at any single tire that matters.

So, we have 3 scenarios. ABS, CDL and none. None is our baseline. I'm assuming the "mixed mu" scenario I listed before. Right side has ice, left side has pavement. CoWheel1 = Left Front, CoWheel2 = Right Front, CoWheel3 = Left Rear, CoWheel4 = Right Rear.

Since I have given myself permission to make my own units, 1 is the maximum theoretical traction available whether linear or horizontally applied. 0 is the traction available from a locked tire on ice. A locked tire on concrete is going to be somewhere around 0.20 on this scale. Close enough. Yeah, I'm ignoring a lot. The rest doesn't matter in this 110% example.

At 110% brake pedal pressure for an "equal mu" under ideal conditions applied to above mixed scenario assuming 25% of available traction is horizontally consumed (curve). If the maximum is proclaimed as 4 on ideal conditions, our 1/2 ice road example has a maximum of 2.

None CoWheel1 = 0.15 linear, 0.05 horizontal
None CoWheel2 = 0 linear, 0 horizontal
None CoWheel3 = 0.15 linear, 0.05 horizontal
None CoWheel4 = 0 linear, near 0 horizontal
NoneTraction = 0.15 + 0.05 + 0 + 0 + 0.15 + 0.05 + 0 + 0 = 0.40.

CDL CoWheel1 = 0.15 linear, 0.05 horizontal
CDL CoWheel2 = 0 linear, 0 horizontal
CDL CoWheel3 = 0.15 linear, 0.05 horizontal
CDL CoWheel4 = 0 linear, 0 horizontal
CDLTraction = 0.15 + 0.05 + 0 + 0 + 0.15 + 0.05 + 0 + 0 = 0.40.

ABS CoWheel1 = 0.74 linear, 0.25 horizontal
ABS CoWheel2 = 0.01 linear, 0.01 horizontal
ABS CoWheel3 = 0.01 linear, 0.25 horizontal
ABS CoWheel4 = 0.01 linear, 0.01 horizontal
ABSTraction = 0.74 + 0.25 + 0.01 + 0.01 + 0.01 + 0.25 + 0.01 + 0.01 = 1.29.

OK, so the 110% pedal (braking force at the user) of net available traction (tires on road) scenario is an easy worst case non-ideal uncontrolled situation. Yep. It sure is. Its a crude approximation, but in this scenario, I predict ABS will have about a 3X likelihood of keeping the vehicle on the road in it's lane. 4 wheel independent ABS would be better, but we don't have that.

Forget the CDL in your scenario, I'll just take a ride in the truck with the lower COG!

If it were only so simple as to switch vehicle dynamics when road conditions change. Lets see... ABS does that for one of our variables.

Ideal Brake Force Distribution has little to do with drivetrain 'engineers', only the presence of a CDL in awd.

Which does nothing for lateral (horizontal) stability under real world worst case differential road conditions.

Thanks, I'll take the computer's ability to modulate braking force in unpredictable scenarios in order to keep my tires turning. Turning tires stick laterally. Now, on an ice track? Hell ya, CDL in with the skinny pedal down and an ambulance on speed dial!

 

[-Spike-]

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.

Thanks Romer, that will keep me, at least, from arguing with this bananahead. I've never spoken with anyone who comes across as conceited and presumptuous, but I could get over that. What I can't stand is the thought that someone not forewarned might take his gospel as absolute truth. I'm sure he has some good information, but in my opinion some of it is dangerously wrong, and the rest of it is difficult to swallow due to his holier-than-thou professorial tone. Perhaps a disclaimer could be added to his signature line? No, I guess not. Anyway, thanks for this one.

-Spike

 

[Buckru]

Sooooo, What the f**ck is mu?

Buck

 

[reffug]

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.

X2.........................

I sometimes am extremelly envious of your self control Rome.

 

[CJF]

mu (greek letter) = coefficient of friction (commonly, and definitely in this thread).



So... as long as I'm here:

WHO HERE OTHER THAN SUMOTOY HAS TRIED DRIVING THEIR 80 AROUND TOWN WITH THE CENTER DIFF. LOCKED??

From a summary reading of this and related threads, all I've seen is people saying, "Well, I haven't tried it, but that's stupid." I'm not saying it's NOT stupid; I'm just saying it'd be nice if someone else gave it a try to see whether his handling claims are BS or not, rather than just claiming they are. (Yes, I'm volunteering to be a guinea pig if needed...)

Curtis

 

[firetruck41]

mu (greek letter) = coefficient of friction (commonly, and definitely in this thread).



So... as long as I'm here:

WHO HERE OTHER THAN SUMOTOY HAS TRIED DRIVING THEIR 80 AROUND TOWN WITH THE CENTER DIFF. LOCKED??

I HAVE!

I don't drive at or beyond the "limits of adhesion", so I can't speak to ultimate performance. However the vibrations when turning, tires scrubbing (increased tire wear) and steering effort are enough for me not to do it regularly.

 

[turbocruiser]

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.

X2.........................

I sometimes am extremelly envious of your self control Rome.

X3 - I hopefully smell a ban brewing!!! Sooner or later someone is gonna go with this dangerously dumb advise and really screw something up. I suggest at minimum Sumo provide his own disclaimers like "Just My Humble Opinion JMHO" or "Just My Humble Experience JMHE" or even a simple "Your Mileage May Vary YMMV" instead of all the BTDT, BTST, BTBS - oops. I have never heard anyone around here come across so self-righteous, so self-absorbed, so self-assured and so absolutely unable to admit any mistakes although many abound. Ohh well, if he won't provide the proper disclaimers at least Rome's always around for us! Thanks Romer!!!

mu (greek letter) = coefficient of friction (commonly, and definitely in this thread).



So... as long as I'm here:

WHO HERE OTHER THAN SUMOTOY HAS TRIED DRIVING THEIR 80 AROUND TOWN WITH THE CENTER DIFF. LOCKED??

From a summary reading of this and related threads, all I've seen is people saying, "Well, I haven't tried it, but that's stupid." I'm not saying it's NOT stupid; I'm just saying it'd be nice if someone else gave it a try to see whether his handling claims are BS or not, rather than just claiming they are. (Yes, I'm volunteering to be a guinea pig if needed...)

Curtis

I guess you missed parts 98, 99, and 100 before he got to 101! But I have tried the testing in several different situations and so have others here and we've all determined that there is dangerous amounts of binding and windup even though "windup" apparently does not exist (well neither does the perfect turbo, or the perfect suspension or whatever else he suggests is not perfect but has no alternative for either!). Sooner or later he'll shutup and you know when that will be - EXACTLY when everyone stops listening and/or when everyone stops arguing!

 

[CJF]

Sooner or later he'll shutup and you know when that will be - EXACTLY when everyone stops listening and/or when everyone stops arguing!

Bit early for that; Landtank's not even here yet. (Or has Rick decided he's done with him these days?)

 

[turbocruiser]

Bit early for that; Landtank's not even here yet. (Or has Rick decided he's done with him these days?)

I think that happened back in Cooling 102 or perhaps Cooling 103!