Traction in 4low?

T

Tom351

New member
Hi all, I am new to 4Runners.. I just purchased my first one- a 2015 SR5 Premium 4x4 in December. I have a good level of automotive knowledge but it is mostly related to 2wd cars as I have owned and raced a few different muscle cars over the years.

As I understand it (from several searches here)- my 4Runner has open differentials for both front and rear axles with the TRAC/A-TRAC system in place to minimize the problem of single wheel slippage. As a guy that has been around a lot of drag racing I think of an open diff as basically "1 wheel drive" (when traction becomes an issue) but I understand how the traction control on the 4Runner can manage the slip effectively even with open diffs. Here is my question and correct me if I am wrong about something:

In 4hi the 4Runner uses TRAC to manage slippage (unless I disable), but when I switch to 4lo TRAC is disabled and A-TRAC is off by default... In this "default" 4lo setting am I really only driving two open differentials..which to me effectively seems like 2 wheel drive - one front and one rear as soon as traction becomes limited.. Am I correct about this? I absolutely love my 4Runner but I am trying to determine how useful 4lo is (without using A-TRAC) in a limited traction situation.
 
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Tom351 said:
In this "default" 4lo setting am I really only driving two open differentials..which to me effectively seems like 2 wheel drive - one front and one rear as soon as traction becomes limited.. Am I correct about this?
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This is correct. The only way you're going to get true 4wd in all situations is being locked front and rear. 4LO plus ATRAC does help a lot for limited traction situations. It's not as good as being locked, but it does make a difference.

Tom351 said:
I absolutely love my 4Runner but I am trying to determine how useful 4lo is (without using A-TRAC) in a limited traction situation.
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Well, you're only going to use 4LO in off road situations. For any on-road driving 4HI plus VSC and TRC will get you what you need.
 
Often need power (higher RPM at low speed) when in 4L and don't want brakes being applied. All depends on situation and with experience you will learn which mode works best for you and particular situation.
 
4 WD alone with open axles will do very well in many situations. I started with a Chevy Silverado with open axles and if I controlled the power I was surprised how well it did even on sand dunes. But now we can all turn on ATRAC.

TRAC works well in 2 hi also. Yesterday I left a gravel area at an angle to enter a paved road. I hit the gas to get a fast start and had wheel spin. Then the right tire was on the pavement and TRAC grabbed the spinning wheel and I got a very good launch, stronger than usual from my 4Runner. Made my wife comment about it. That stuff really works.
 
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Couple of things...

1. Most 4x4 vehicles run open diffs front and rear.

2. Open diffs don't drive one wheel on each axle. They apply equal Torque across the axle, so if both wheels have 100% equal traction, both wheels move at the same speed. Equal traction is obviously not the norm in off-road situations. Once one wheels starts to spin it ends up getting most of the tq applied. Locked differentials allow for unequal tq to be applied to each wheel. What ATRAC is doing is increasing the tq on the slipping wheel by applying the brake, which transfers tq to the other wheel. It is a little jerky at times, but pretty smooth overall.

3. The aggressiveness of ATRAC is much different than TRAC... TRAC is not very effective in low speed situations.

Bottom line, ATRAC works very well and it is what you want for low speed low traction situations. The combination of the transmissions tq converter and high gearing make controlling wheel slip very hard at low speeds. 4Lo is where most technical off roading is done. In fact, I find that I almost never need 4h. 2h is typically all I ever need at speed. There are good uses for 4H, I just don't happen to encounter many myself.
 
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For me atrac is very use full when a tire is in the air going uphill on uneven terrain. When a tire is in the air it has zero traction and you go no where. When atrac is on it applies the brake to that tire and pulls you through. It is noisier than heck but works well.
 
Whiplash Willy said:
What kind of situations would you not want TRAC enabled in 4Hi or ATRAC enabled in 4Lo?
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When stopped going up a snow or mud covered hill and having to get started again. You need the tires right at max tractive effort or actually spinning. Can't have brakes being applied to keep from spinning when you need them to spin!
 
Whiplash Willy said:
What kind of situations would you not want TRAC enabled in 4Hi or ATRAC enabled in 4Lo?
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There are a couple of situations. One issues is that ATRAC can overheat the brakes in a mud situation...

I do wish the ATRAC button was a switch. I also really wish the RSCA button was a switch.
 
Whiplash Willy said:
What kind of situations would you not want TRAC enabled in 4Hi or ATRAC enabled in 4Lo?
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When you have to power out of something...like a mudhole or after being buried by snow.

TRAC/A-TRAC main issue is that it is a RE-active system...so, it is always a step behind real world conditions. TRAC (and A-TRAC) respond to wheel speed differences across the same axle. So, if one wheel spins significantly faster, then ABS pulses the brake on that spinning wheel to slow it down; thereby directing driving power to the slower wheel/tire. Torque/driving power go to the wheel with least resistance...so, by pulsing the brake on that spinning tire; it pushes some torque back to the wheel with (supposedly) traction.

A-TRAC is just like TRAC except that it is more aggressive in reacting to speed differences b/w tires (on same axle). You cannot use A-TRAC on the road because that would create issue whenever you turn a corner. TRAC allows more wheel speed differences before it intervenes...thus, more suitable for on-road use and foul weather use.

A-TRAC pulses the brakes...so, it is not a total shutdown of the slipping/spinning wheel. The torque transfer is not perfect either because of the pulsation and you lose power thru heat...thus, A-TRAC cannot function for more than 12-15 minutes of continuous use (per Toyota Land Cruiser 200 info).

Going back to it being a reactive system...it does not see your terrain. Therefore, it reacts to slippage. An axle locker, on the hand, is PRO-active because you can lock the axle BEFORE going up a rockledge (for example). And torque is equally divided no matter the condition. No reacting to slippage needed.

And because it is a reactive system, and because you cannot individual control how it acts over an axle, the system may react wrongly, such as when you're buried in mud/snow. It will transfer power back and forth while it tries to sense slippage via ABS pulsation...but at the same time causing you to lose momentum (due to braking of tires). In some situation, you just want to punch it (in a controlled manner) and let the tires spin so that you can build momentum getting out. TRAC/A-TRAC may potentially prevent this.

In addition, you guys need to disable VSC while off-roading (via locking center diff or LO range?)...because in addition to above, VSC also cuts power to engine and thus momentum and power...again, bad for off-roading.
 
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Thai said:
In addition, you guys need to disable VSC while off-roading (via locking center diff or LO range?)...because in addition to above, VSC also cuts power to engine and thus momentum and power...again, bad for off-roading.
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Thanks for the great info guys! Thai brings something up I have always questioned. In 2Hi or 4Hi, you can't just disable VSC. You can either only disable TRAC, via the TRAC button, or you can disable TRAC and VSC by being stopped, and pressing and holding the TRAC button for several seconds, to disable both.

Wouldn't there be situations where you would want to keep TRAC enabled, but disable VSC? I would think there would be times you would want TRAC enabled to control wheelspin, but VSC disabled so it doesn't cut engine power.
 
Whiplash Willy said:
Thanks for the great info guys! Thai brings something up I have always questioned. In 2Hi or 4Hi, you can't just disable VSC. You can either only disable TRAC, via the TRAC button, or you can disable TRAC and VSC by being stopped, and pressing and holding the TRAC button for several seconds, to disable both.

Wouldn't there be situations where you would want to keep TRAC enabled, but disable VSC? I would think there would be times you would want TRAC enabled to control wheelspin, but VSC disabled so it doesn't cut engine power.
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There might be times but I have not run into that specific situation. If I want my wheels to spin I want the power too. If I don't want my wheels to spin I want every resource available to keep them from spinning and that includes the throttle r e t a r d.
 
Whiplash Willy said:
Wouldn't there be situations where you would want to keep TRAC enabled, but disable VSC? I would think there would be times you would want TRAC enabled to control wheelspin, but VSC disabled so it doesn't cut engine power.
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TRAC is for on-road use...and i cannot think of many situations where i want VSC off but TRAC ON. If you are buried in snow (on-road use), then you want BOTH disabled to power yourself out. VSC/TRAC OFF is also used if you do racetrack runs i guess or into doing donuts. Otherwise, for on-road use, you want BOTH ON all the time IMHO.

For off-road use, then yes, i would press and hold the button to turn OFF both. Then click on A-TRAC. Then have fun!
 
Thai said:
TRAC is for on-road use...and i cannot think of many situations where i want VSC off but TRAC ON. If you are buried in snow (on-road use), then you want BOTH disabled to power yourself out. VSC/TRAC OFF is also used if you do racetrack runs i guess or into doing donuts. Otherwise, for on-road use, you want BOTH ON all the time IMHO.

For off-road use, then yes, i would press and hold the button to turn OFF both. Then click on A-TRAC. Then have fun!
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It goes off automatically in 4LO, which is the only mode where ATRAC can be turned on.
 
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Thai said:
TRAC is for on-road use...and i cannot think of many situations where i want VSC off but TRAC ON. If you are buried in snow (on-road use), then you want BOTH disabled to power yourself out. VSC/TRAC OFF is also used if you do racetrack runs i guess or into doing donuts. Otherwise, for on-road use, you want BOTH ON all the time IMHO.

For off-road use, then yes, i would press and hold the button to turn OFF both. Then click on A-TRAC. Then have fun!
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Yes, out in the desert, high speed, sandy washes or dunes. In 4Hi for much more positive front steering control but all traction control OFF. Traction control takes all the fun out of offroading fast and is even a little dangerous in it's unpredictability, kicking it at the wrong time as your "drifting" around a corner and possibly sending you off track into the cactus!

I've also had it get me stuck going up sandy hills as half way up it kicks in and starts messing with momentum. So in short, offroad I always turn traction off now as soon as I switch into 4HI. And sadly need to redo that every time you swtich back to 4HI from 4LO too :(
 
2014 Warthog said:
Yes, out in the desert, high speed, sandy washes or dunes. In 4Hi for much more positive front steering control but all traction control OFF. Traction control takes all the fun out of offroading fast and is even a little dangerous in it's unpredictability, kicking it at the wrong time as your "drifting" around a corner and possibly sending you off track into the cactus!

I've also had it get me stuck going up sandy hills as half way up it kicks in and starts messing with momentum. So in short, offroad I always turn traction off now as soon as I switch into 4HI. And sadly need to redo that every time you swtich back to 4HI from 4LO too :(
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Running down the beach, sliding sideways in the sand, power wheeling came to and end! Darn truck keeps trying to correct for the slide! Needs to be one more function to VSC, A-Trac, Multi Terrain Select, Locking Diff . . . a "fun/safety" button. One button that turns the "I'm smarter than the driver" electronics off!
 
Mooney said:
Running down the beach, sliding sideways in the sand, power wheeling came to and end! Darn truck keeps trying to correct for the slide! Needs to be one more function to VSC, A-Trac, Multi Terrain Select, Locking Diff . . . a "fun/safety" button. One button that turns the "I'm smarter than the driver" electronics off!
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Toyota lawyers will hinder that...and lets face it, in today's society, there will be some dumba$$ that will sue Toyota for that one button....
 
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