Question regarding ATRAC

[lockedlex]

Hello,

I've got a question, probably related to ATRAC (tried a search on it but didn't address this exact issue). On a recent trip to Gold Mountain (Big Bear, CA), our friend's 07 4Runner (Limited, V8) has exhibited some unusual behavior, and we're wondering if anyone has noticed this too. This is especially applicable if you have off-roaded with a 3rd Gen w/ a Locking Rear Diff.

Basically, while taking the same line over an obstacle (say, 10" tall rocks, against one front and one rear wheel... usually diagonally from each other, going uphill) the 98 4Runner Rear locker w/ the same tires as the 07 (AT KOs), same PSI, and completely stock seems to pull right over that series of obstacles, in a relatively straight line, with some tire slipping, etc, but it generally pulls straight forward, no drama. But the 07, taking that same line, basically the same speed, seemed to more often than not slip entirely to the right or left (depending on the angle that truck is in, it could be inches or over a foot) and of course ATRAC is activating right before the sideway slipping occurs... both trucks in 4Lo/Lo.

We can't seem to make heads or tails on why this is happening. What's throwing us off then, is that spotting the truck the same way that we'd spot any other truck, the 07 ended up slipping off what it's trying to climb and subsequently hitting something in the undercarriage. I'm at a loss to why this happening and I've been spotting for 15 years... so I'm wondering if we need to do something differently for an ATRAC enabled truck.

I will say that this trail and specifically the obstacles when this sideways slipping happens, tends to be "slippery" either with powdery dirt, or slickrock type rocks, but it's not wet or icy, etc.

Has anyone else noticed this happening? On this particular trail, he probably slipped sideways on 2-3 different obstacles using the same line that my buddy's 98 took and never slipped on. I also have a 98 w/ a locking rear, but since it's not stock, I'm not including it in this comparison (that said, I did not slip sideways either).

 

[JB.]

Dumb question but was the center diff locked on the 07? Not that I have a ready explanation otherwise, but the potential for unpredictable behavior is higher without the center locked.

You said the lines were taken at the same speed which implies steady motion. I'd get out of 1st gear for that. ATRAC works best while in motion at low, steady revs. Over revving will delay and diminish ATRACs intervention and allow more spin than usual. If you let the transmission decide, you'd be amazed at how steep a hill it will crawl up without going to 1st gear.

From a standing start, ATRAC on the 4th gen will allow more wheel spin before activating than when underway. Again, low, steady revs should give the best results.

 

[ToyTech99]

My guess is that it's more a result of the tires than anything else. I don't see how ATRAC could cause a sideways slip by itself.

 

[CJ3Flyr]

Are you comparing the straight pulling '98 with the diff locked or unlocked? If the '98 was running the trail unlocked then your discovering one of the reasons driver selectable lockers are preferred for off-roading; very little sideways slip when the diffs are open.



If the '98 was locked then.... Given you have the same tires, same pressure etc. I have 2 questions: 1st, you say same tires... Are they both 70 profile or is the 4th gen a 265/65/17 and the 3rd gen a 265/75/16? IMHO that will make a difference. The taller tire, aired down, will conform to the terrain and grip better. 2nd, IIRC the 4th gen is a lot stiffer in terms of bodyroll. Is the 30mm front swaybar of the 4th gen showing it's ugly head and leaving the 4th gen spending less time with all 4wheels firmly planted?


It sounds more like a 'traction-in-general' issue than an ATRAC issue to me. The stiffer roll axis and shorter profile tires of the 4th gen (assuming my guess about tire size is correct) are the culprits. That's my guess.
:cheers:

 

[lockedlex]

Thanks so much for the replies guys.

Some answers... we have an expression we use with the 07... "are you locked/low?" meaning is the center diff locked and the tranny in 4lo first gear. That's really interesting... JB... to just leave it in 4low, but in "D" I'm assuming and letting the tranny do it's thing. The thing is... for those obstacles, we didn't take a "running start," but setup a line at the start of the obstacle and gave it gentle steady gas. So you're saying try to start a bit earlier and just give it steady continuous gas and let ATRAC do its thing... we'll give that a try next time.

Tires are an interesting point... the 98 is running 265/70/16 and the 07 has got to be whatever the stock 18s size is, though they're both running AT KOs. Hadn't thought about the "conformity" abilities of a "taller sidewall" tire. Yes maybe the 16s do give more "grip" than the 18s can... interesting point...

 

[lockedlex]

oh, I should mention why we didn't give it a very big running start, on any of these obstacles, is because it was basically rockcrawling... and there was a tendency for the suspension to rebound and the 07 to bounce badly if we took it too fast or did a running start.

But maybe the trick is to start a few inches back and keep giving it gas, hoping that it doesn't bounce around too much.

EDIT: Oh and yes, the 98s were always rear locked.

 

[CJ3Flyr]

It sounds like JB's suggestion of gear selection may be a big help. Based on what you just described I might select 2 (or 3 if going a little faster). If you do that rather than D it'll accomplish what what JB suggested while crawling and give you decent engine braking still. The other thing it'll help with is to prevent the undesired lunge if it decided it wanted to shift into 3rd.

:cheers:

 

[JB.]

Thanks so much for the replies guys.

Some answers... we have an expression we use with the 07... "are you locked/low?" meaning is the center diff locked and the tranny in 4lo first gear. That's really interesting... JB... to just leave it in 4low, but in "D" I'm assuming and letting the tranny do it's thing. The thing is... for those obstacles, we didn't take a "running start," but setup a line at the start of the obstacle and gave it gentle steady gas. So you're saying try to start a bit earlier and just give it steady continuous gas and let ATRAC do its thing... we'll give that a try next time.

I should have been more specific. For steady uphill you can go D if you want. It will only use 4 gears in low anyway. I usually go 2nd or sometimes 3rd for the reasons CJ3Flyr mentioned.

I realize you don't always have the option to keep moving and sometimes the tradeoff is not worth it, but if you can, it should help. It's the standing starts from a difficult position that are the most problematic. You can get 1 or 2 wheel spin revolutions before the 4th gen ATRAC kicks in. If motion stops or is close to stopping when ATRAC is active that's a little different. Steady gas and let ATRAC work.

You're not going to match the performance of having the rear locker, but if you get the brew right, you might come closer than you think.

 

[lockedlex]

So my 07 buddy has a theory. Basically it goes something like this: Under certain circumstances, when diagonally opposed tires (FR, RL for instance) are up against an obstacle (like rocks) which causes those tires to spin and subsquently not go anywhere, ATRAC brakes those wheels (as it should) and power is transferred to the opposing wheel. However, the time difference between the OTHER front and rear wheels getting the power, be it milliseconds, is enough to result in a "pivot point" (since one axle/tire is now turning even a tiny bit faster than the other, AND the opposite tire is now braked, acting like a pivot (diagonally between opposite tires front and rear)--and of course since there is no tire braking in a non ATRAC truck it doesn't experience this) resulting in one of those wheels pushing the truck off its line and slipping sideways--this is especially probable when the truck is already at an angle due to the unevenness of the terrain--more angle, more sideways slip.

It's an interesting theory, but then the type of trail that we were doing is probably done by hundreds of others, so I'd expect that to be a common discussion with ATRAC (which if course is not the case).

I suppose arguably, if the tires had more grip in the first place, there'd be less chance of it losing traction and the truck slipping sideways--so maybe higher profile tires might be better.

I actually like the concept behind ATRAC, it was indeed remarkable seeing it trying to go up over those obstacles and seeing all 4 wheels spinning (well, braking, spinnng, braking, spinning but doing it so fast that it was like all 4 were spinning), it was just disturbing to see it do that (not to mention all the brake smoke), then all of a sudden slip to one side. The other 98 Runner would be spinning as well (rears locked and both spinning, one front tire spinning), but would eventually just climb over the obstacle--pretty much straight on.

JB, I like the system, my only initial concern for it ages ago was what happens when you need to power through something like Sand/Mud (if you floor it, I understand that ATRAC is effectively turned off and it leaves you with open diffs...but then wouldn't that now be the problem, you now have open diffs). I didn't expect to see something like this.

The other thing is, OK, so let's say he's going up these rocks, keeps it in 2nd or 3rd gear, but keeps the RPMs under 3000 (or whatever that magic number is in order to keep ATRAC activated), then why would the truck ever up shift from 1st and if it did, wouldn't that mean that he's now got less "climbing power" over those rocks? So wouldn't it be better to leave it in 1st? I should have specified too, that by uphill, I didn't mean climbing a long hill going up... but more like having large rocks in front of you, then trying to climb over those, while on an "incline").

EDIT: Thanks again for the input. I really want to get to the bottom of this (or at least try some things that might stop it from happening) since it's causing some frustrations among the spotter vs. the driver vs. the truck.

 

[JB.]

A lot in your post. I might come back to the pivot point theory if I find some extra brain cells lying around.

JB, I like the system, my only initial concern for it ages ago was what happens when you need to power through something like Sand/Mud (if you floor it, I understand that ATRAC is effectively turned off and it leaves you with open diffs...but then wouldn't that now be the problem, you now have open diffs).

Yep.

The other 98 Runner would be spinning as well (rears locked and both spinning, one front tire spinning), but would eventually just climb over the obstacle--pretty much straight on.

Three wheels spinning with two lockers? It sounds to me like the problem is simply that the trail is a little beyond the capabilites of the 07 as it stands.

The other thing is, OK, so let's say he's going up these rocks, keeps it in 2nd or 3rd gear, but keeps the RPMs under 3000 (or whatever that magic number is in order to keep ATRAC activated)

I've heard 2000. Even that sounds a little high to me. 3000 is too high if you want ATRAC fully effective.

then why would the truck ever up shift from 1st

Don't know. Just goin' by my experience as something I'd try. The V8 goes to 2-lo very easily and will climb a telephone pole in that gear.

and if it did, wouldn't that mean that he's now got less "climbing power" over those rocks?

Sure, but all he needs is enough. With ATRAC involved, sometimes more is not better. This stuff might be completely useless to you with the terrain you're tackling, but it's an easy thing to try.

EDIT: Thanks again for the input. I really want to get to the bottom of this (or at least try some things that might stop it from happening) since it's causing some frustrations among the spotter vs. the driver vs. the truck.

Please keep us posted on what you find out. Even if what you find out is that I'm full of !*@#! :lookout:

 

[Thai]

So my 07 buddy has a theory. Basically it goes something like this: Under certain circumstances, when diagonally opposed tires (FR, RL for instance) are up against an obstacle (like rocks) which causes those tires to spin and subsquently not go anywhere, ATRAC brakes those wheels (as it should) and power is transferred to the opposing wheel. However, the time difference between the OTHER front and rear wheels getting the power, be it milliseconds, is enough to result in a "pivot point" (since one axle/tire is now turning even a tiny bit faster than the other, AND the opposite tire is now braked, acting like a pivot (diagonally between opposite tires front and rear)--and of course since there is no tire braking in a non ATRAC truck it doesn't experience this) resulting in one of those wheels pushing the truck off its line and slipping sideways--this is especially probable when the truck is already at an angle due to the unevenness of the terrain--more angle, more sideways slip.

I suppose arguably, if the tires had more grip in the first place, there'd be less chance of it losing traction and the truck slipping sideways--so maybe higher profile tires might be better.

Your friend's pivot theory is correct. This is the problem with ATRAC. The transfer of the torque from wheel to wheel (on the same axle) is NOT instantaneous...there is some wheelspin involved. Climbing up the same hill, my G500 will go sideways with 4-ETS (Mercedes' ATRAC...less aggressive than Toyota's btw) and will go straight up with locker(s). This is why Toyota is now introducing "A-TRAC" in the FJ Cruiser that works only in 4-LO and CRAWL in the upcoming 5th gen 4runner (and Land Cruiser/LX). These two systems, A-TRAC and CRAWL, are much more aggressive at clamping down on the spinning wheel, allowing for quicker transfer of power from wheel to wheel...and thus, reducing the sideway (or pivot) effect.

But, with aggressive intervention of the brakes, there is a price to be paid...potential brake overheating. This is why CRAWL is only intended for less than 15 minutes (i think it is around 12 minutes or so) of constant operation. This is also why A-TRAC in the FJ is ONLY operational in 4-LO.

This is why with vehicles equipped with ATRAC, you need to preserve or max out your articulation and air down...similar to what you would do if you have open diffs. This includes avoiding lift kits that decrease wheel travel. Airing down is a must anyway for everyone who off-roads. But this also means that you want to avoid low-profile tires, as you stated above.

The torque transfer via ATRAC is never 100%. That is, some of the power is lost to heat via braking action. Therefore, the wheel that is getting the power (via ATRAC clamping down on the opposing slipping wheel) does not get all of the engine power routed to it. It only gets a small percentage of it...and it gets it in a pulsatile manner due to the nature of ABS. It works, but it is a rather inefficient process if you think about it. This is why the Hummer H2 brochure states that a Hummer can go up a 45 degree incline with all 4 wheels turning, BUT with ATRAC (their version of it) working on only one tire, it can only go up a max 10 degree incline. (Toyota has never gone into this much detail about its system.) In contrast, if the H2 had full lockers (front, center, rear), then it should (in theory, if traction permits) be able to go up a 45 degree with just one tire having traction since there is no heat loss with a fully locked vehicle.

 

[lockedlex]

Your friend's pivot theory is correct. This is the problem with ATRAC. The transfer of the torque from wheel to wheel (on the same axle) is NOT instantaneous...there is some wheelspin involved. Climbing up the same hill, my G500 will go sideways with 4-ETS (Mercedes' ATRAC...less aggressive than Toyota's btw) and will go straight up with locker(s). This is why Toyota is now introducing "A-TRAC" in the FJ Cruiser that works only in 4-LO and CRAWL in the upcoming 5th gen 4runner (and Land Cruiser/LX). These two systems, A-TRAC and CRAWL, are much more aggressive at clamping down on the spinning wheel, allowing for quicker transfer of power from wheel to wheel...and thus, reducing the sideway (or pivot) effect.
.

JB/Thai, thanks for the input. I appreciate the insight. So basically, if the "ATRAC" isn't "aggressive" enough (with the least amount of delay in applying the brakes/transfer of power) the side slipping under these circumstances can be expected. Wow, that adds a variable to spotting I hadn't considered. Got to keep an eye out for that and avoid lines (if possible) that will cause him to slip onto something nasty. At least though, now we know if it does happen, that it's not really the driver or spotter's fault.

Considering this is all "programming" though, I don't see why Toyota can't bump up the algorithms to allow for an "all-out super agressive mode" (but maybe time limit it or something like that). I can imagine a 2-5 minute window where you enter this mode and the brakes and power distribution does its thing really fast to get you over something... then you're back to normal ATRAC--and it could just be an "upload" to the ATRAC ECU.

My 07 buddy has a (non spacer) 2 inch lift kit, sliders, bumper/winch, and full underbody skid plates, I'm not sure how this affects his articulation and subsequently his traction, but he does seem to have some nice wheel travel. I suppose the focus now is to airdown more (what's safe for 265 AT KOs running on 18" with this kind of weight?) and spot him in such a way that there's not too much "tire up in the air" problem. I suppose we were relying on ATRAC as a miracle solution for front/rear lockers (and for a while it actually looked like all 4 wheels are spinning, but it's those millisec differences that I suppose makes all the difference), so it does seem like a true locker still is the best overall advantage?

 

[Thai]

lockedlex,

Yes, Toyota has something like what you want...A-TRAC on the FJ (and on 2009 4runner Trail Edition) is just that...a super aggressive 4-wheel traction control for off-roading. It is operational ONLY in 4-LO. I suppose the new CRAWL (on 5th gen 4runners and Land Cruisers/LX570) will be just as effective.

The ATRAC on the 4runners (your friend's) is for all surfaces/all-season and for both 4-HI and 4-LO. Because of this generality, it cannot be too aggressive...it is a compromise if you will. Basically, with the FJ, Toyota has added an additional programming to allow for more manually-activated aggressive traction control on top of the regular traction control.

With low profile BFG AT KO, i would imagine that you can still air them down. I would imagine that something like 18 psi is fine. (Disclaimer: notice i use, "would imagine" several times!)

And yes, a locker is the best tool for forward movement when off-roading. But, it does have its disadvantages (straight only, binding, etc.).