Hi everyone.

A quick question on the response of the system.

Situation: If you're driving home shortly after a BIG snowfall...the truck pulls off the plowed street into the unplowed driveway and gets bogged down in the deep snow...you put it into 4-wheel drive. The snow is deep enough to offer a challenge and the rear limited slip delivers even power to both back wheels. The snow is deep enough that the back wheels spin and grab...spin and grab, together. However, the truck still moves ahead spinning and grabing up the driveway with even torque on the back wheels. (We won't discuss the front wheels yet.)

Same situation with advance trac...you enter the deep snow in the driveway and one rear tire starts to spin. Here is the million dollar question: Does the system quickly break the spinning wheel and transfer 100% power to the opposite wheel? If so, wouldn't the opposite wheel break free because it's getting all the torque? So, now the system senses the other spinning wheel, breaks it, and throws power back to the first side.

Left side spin...stop! Right side spin...stop! Left side spin...stop! Right side spin...stop!... My thoughts are that a limited slip will push both rear wheels evenly ahead and not suffer from this transferring effect.

Or??? Does the advance trac "slowly" apply break to the left spinning wheel and slowly transfer power to the right. If the transfer of power were slow, the rear wheels would at some point balance the torque and push evenly. Can the advance trac system strike a balance in torque between the left and right wheels, or is there a slam to the right, pause, slam to the left, pause, slam to the right kind of thing?

After all the theory, what really happens in this situation? Can advance trac gently slow the spinning wheel and proportionately transfer power to the other wheel striking and holding a torque balance between the two. It's this "balance", much like a limited slip, that we are looking for in this situation.

Has anyone tested the advance trac in a similar deep snow situation?

 

I haven't tested Advance Trac in that exact situation. I have, however, tested other manufacturer's similar traction control systems, and tested Ford/Lincoln/Mercury's previous stabs at brake/engine based traction control, and my experience was that the computer wasn't fast enough..and wasn't good enough with the brakes...so it was the spin-stop-spin-stop situation. The only relief I got, in that situation, was to hammer it and--even with the engine power cutoff--overpower the system so I had more torque at the wheels than the amount of braking power it generated...thus putting power out through both wheels. It was kind of like using a few thousand dollars of equipment to replace the old 'gently pull the parking brake' trick. I imagine with the new system on which you can disable the engine power reduction, it is that much easier to overpower the brakes and smoke them into oblivion.

I think Jeep's system, or a L/S differential makes far more sense for traction control...try to increase the power going out to the wheels, rather than trying to reduce the power at certain wheels. Until the systems get faster and smarter (and start monitoring more parameters), I think the LS route will be the way to go. The brake based systems make perfect sense for stability control, though.

 

CMCSL - the phenomenon you describe is a common problem with traction control systems, but Ford put a lot of development into refining both the traction control and stability control systems. The left-right-left shifting of torque (sort of a waddle effect) won't be found on the Ford SUV AdvanceTrac systems. Basically, it is possible to transfer torque slowly and smoothly by gradually applying the brake. The goal is not to STOP the spinning wheel, but simply to reduce it's speed to match the other wheels. If this is done gradually, then the opposite tire will not see a sudden increase in torque causing it to break free.

FarmLaw -- In general, I agree with most of your points. However, the AdvanceTrac system will definately out climb a standard limited slip axle, because a limited slip can only transfer a relatively small amount of torque to the opposite tire. If you lift two diagonal tires with a limited slip, the vehicle will NOT climb assuming you're on a significant incline. A more common real world example would be a steep incline with the right side tire path covered with ice and left side tire path with grip (split mu). A standard limited slip axle will spin the tires on the side with ice, and will transfer as much torque as it can to the side with traction, but there will not be enough torque transfer to move the vehicle up the incline (greater than about 15% grade).
Your points about the system response also have some merit. The system responds quickly, within milliseconds, and the system reacts to very small amounts of wheelslip. However, the wheels must slip before the system intervenes. True, the vehicle may tend to "rock" when diagonal tires are off the ground, however, learning to control the throttle to gradually apply torque will reduce the lurching and allow the vehicle to climb out of the rut. You mention having spinning tires and chunking off rubber. This can depend upon driving style (throttle control, and how erratically it is applied). In the same maneuver with limited slip, the tires off the ground will be spinning like mad... think about what will happen when those tires come back into contact with the ground. That's how axles get broken.

I still maintain that Ford's AdvanceTrac system will definately outclimb any limited slip axle, simply due to it's ability to transfer more torque left to right and front to rear. Keep in mind that most vehicles do not have a limited slip on the front axle. I can go into much greater detail on all this if desired.

Now if you want a TRUE off road machine, and you plan to lift tires regularly off the ground, then you probably want a Detroit Locker axle both front and rear. With the locked transfer case, and locked front and rear axles, this will make all 4 tires turn the same speed. This truly puts 100% of the torque to any tire with traction.

FarmLaw -- I would agree that traction control systems in the recent past are as you describe. However, do you know how many parameters are monitored by the newer AdvanceTrac system? It's actually quite sophistocated. Also, no worries about "smoking the brakes into oblivion". There are safeguards against this built into the system, even when engine torque reduction is disabled... this is covered in the owner's manual.

 

FarmLaw-
"I haven't tested Advance Trac in that exact situation. I have, however, tested other manufacturer's similar traction control systems, and tested Ford/Lincoln/Mercury's previous stabs at brake/engine based traction control, and my experience was that the computer wasn't fast enough..and wasn't good enough with the brakes...so it was the spin-stop-spin-stop situation."

VS:

Ford Boy 72
"- the phenomenon you describe is a common problem with traction control systems, but Ford put a lot of development into refining both the traction control and stability control systems. The left-right-left shifting of torque (sort of a waddle effect) won't be found on the Ford SUV AdvanceTrac systems. Basically, it is possible to transfer torque slowly and smoothly by gradually applying the brake. The goal is not to STOP the spinning wheel, but simply to reduce it's speed to match the other wheels. If this is done gradually, then the opposite tire will not see a sudden increase in torque causing it to break free."

Great discussion! Now we get down to the heart of the matter.

According to Ford Boy 72, the Advance Trac system does transfer power smoothly and gradually from the spinning wheel to the opposite wheel. The "magic bullet" is to prevent the opposite wheel from breaking free in the transfer of power thereby preventing the waddle effect from getting started. We need a quick clarification on the application of the break. Do we need to gradually apply the brakes manually to create a slow transfer of power or does the Advance Trac system do this automatically? FarmLaw has tested other systems and reports that the system, without driver intervention, cannot transfer power gradually...so it was the spin-stop-spin-stop situation."

The difinitive question:

Given the same situation, without driver intervention on the brakes, is the transfer of power gradual?

Cast your votes...Yes/No, in the deep snow.

 

You don't need (or want) to manually apply the brakes with your foot on the brake pedal for the system to operate smoothly. The system is designed to work when you are accelerating on the throttle (off the brake).
The AdvanceTrac system has been tested in deep snow (any many other road surfaces!) with favorable results.

Here's another thought to ponder... you get maximum traction when the tires are not overspinning. If you have lim slip and you're spinning both rear wheels, you are not operating at the peak traction level of the tires. Since AdvanceTrac monitors and controls wheelslip at all 4 tires, it constantly gives you the most available traction at each tire simultaneously. Therefore, AdvanceTrac is preferred over a limited slip in deep snow, or any other surface where traction is limited.

 

Actually, Ford Boy, you get maximum tractive effort from tires on soft surfaces when they are spinning ever so slightly. The range of maximal efficiency varies from as low as 2-3% up to 15%. I'm not certain what the story would be on dry pavement, but on rocks, dirt, mud, snow, etc., the maximum force that any tire can generate is achieved with some slip. For those of you who farm out there, you know that you ballast (add weight) to a tractor to achieve this optimal amount of slippage...add to much weight and eliminate the slip and your efficiency actually decreases!

I dislike the inherent inefficiency of brake-based traction control systems. Why buildup all of the driveline tension, heat, pressure, etc., that such systems develop? Why burn fuel to heat the brake pads? I agree that it is better to burn fuel and heat the brakes and produce some forward motion as compared to open differentials, but I again think, as a pragmatic response to wheelspin, it would be better to evenly modulate the power to all four wheels than to try to shift a bunch of power around with the brakes.

I will defer to your advance trac experiences...as I stated, I have none with the newest variant of Ford's traction control. However, I will note that, universally, every magazine that has reviewed the system in an off-road context (at least that I've been privy to) has had nothing good to say about Advance Trac. The old complaints still surface (chunking tires, jerky motion, inability to react fast enough, etc.). I believe Four Wheeler came right out and said that they'd take a limited slip over Advance Trac anyday--and I would be unwilling to chalk their experience up to driver error.

 

I don't see why mud-dirt would make a difference, but as far as dry pavement goes, or wet pavement for that matter the co-efficient of friction is reduced slightly when something is sliding - an easy real life example:

Imagine a heavy cardboard box on a concrete floor... It's slightly harder to start moving, than it is to keep it moving. I really don't see why it would be different under different surfaces - perhaps it's less inneficient due to the weight difference?

I guess the "proven" example is ABS - tries to stop your wheels from slipping because you get more traction with your wheels not sliding.

 

I haven't seen that 4 Wheeler article. Not arguing it, just haven't seen it. I would suspect they meant to say they would take a locking axle (true locking axle, not just a limited slip). Those true off roaders who like to crawl straight up walls and over gnarly obstacles would love nothing more than lockers all around. Again, when comparing AdvanceTrac to a standard limited slip, the AdvanceTrac system is definately superior. I speak from first hand experience with both systems.

Your points on tire traction are correct. I was keeping it simple when stating that overspinning tires have less traction than tires that are not overspinning (which is still true, by the way). When I say overspinning, I mean well over 15% slip. If we're talking about breaking tires loose in deep snow, we're talking about 100% slip or greater. In fact, a tire will not generate any forces if there is no slip. To accelerate the vehicle at all, there must be some amount of slip at the tire to generate the longitudinal force. And you're right that the peak force is generated at around 10% (give or take) slip, depending on the characteristics of the tire and the road surface. This principle is true for dry pavement, wet pavement, snow, ice, etc. Any given traction control system will try to maintain a small positive amount of wheelslip at the tire to achieve maximum forward traction, just like any given ABS system will try to maintain a small negative amount of wheelslip at the tire to achieve maximum braking traction.

Your points on loading up the driveline by using engine power at one end and brake torque at the other end are well taken. Keep in mind that in most real world situations, the engine torque reduction is also in effect to reduce the loading on the driveline and to give a more refined feeling. Another major factor to consider is the overall cost of the system. Every vehicle already has brakes and most have the ability to control the engine torque, so why not take advantage of that ability to provide a good traction control system without adding a lot of extra cost (more hardware). When you start out with an ABS system, you don't have to add much hardware to have full control over the brake system.
There are more fancy systems as you suggest which try to divert engine torque to the wheels which can best benefit from the torque, but this requires active differentials all around... pretty costly for today's market. You'll see this type of thing on rally cars, and maybe the more exotic sports cars with AWD. Perhaps after a few more years this technology, like all new technology, will work it's way into mainstream.

 

SouthPark -- Tires behave differently than other sliding objects. Typically, static friction is greater than sliding friction (your box on the floor example). For a tire to produce a force, the tread must deform. This is true for most road surfaces. When the blocks of tread in contact with the pavement deform, they move closer to the next blocks of tread that are about to come into contact with the road (assuming the vehicle is accelerating, in this example). This means that under normal acceleration, the driven tires (those powered by the engine) actually turn a little bit faster than the undriven tires (the front tires, if you're in rear drive mode). How much faster they are turning is the amount of slip at the tire.