Increase Traction w/o a rear locker? How?

[KLF]

You are forgetting about heat.

No, not forgetting, I thought that been addressed in the earlier post with a friction/temperature graph for one brand of brakes.

[But I did overlook the description there of "stiction," which said essential the same thing I did about the potential detrimental effect (for left foot braking) of static friction.]

[Edit: Let's add another chart for a different brand
http://wiki.seloc.org/images/1/17/EBCcomparisonchart.gif]


In light of need to overpower the (likely greater) static friction of the stationary side, I did not see much likelihood of being able to create enough variation in kinetic friction to achieve the desired/necessary imbalance of torque.

But feel free to run us through how this would work. What's the protocol for spinning a wheel with the brake applied in order to generate the extra heat that is going to "unbalance" the respective coefficients of friction? I thought the procedure you described was:

apply hard left foot brake to stop everything - and then slowly increase throttle until the wheels began to rotate (overcoming the heavy brake resistance).

That clearly would not do it if heat variability is the critical factor.

(thinking out loud)
In any case we would need to dissipate power to generate heat, and that power is going to come from the engine via the drive train. How should the use of the throttle be balanced against application of the brakes?

 

[SnowedIn]

The imbalance in effective braking friction between either side isn't enough to keep the technique from working.

 

[KLF]

No, the imbalance in effective braking friction between either side is not enough to ALLOW the technique to work.

(If the assumption is that the technique works due to the common (and probably erroneous) wisdom that some net driving torque on the stalled wheel is being created by braking.)

 

[MOguy]

Some people have brought up using your brakes. That can be extremely effective, some vehicles more than other. It is extremely effective in the Hummer H1. Which is great be use the suspension in an H1 sucks for most off road purposes and you are almost always hanging a tire if driving an H1.

Here is a link explaining one method. Search brake modulation.

http://www.hummerknowledgebase.com/driving/btm.html

 

[Stumpalump]

I've got thousands of miles of off roading in rental cars and company cars. Mostly in the front wheel drive Dodge Caravans. I remember climbing a trail that turned into a cliff in Moab with no way to turn around. I finally found a place to do a 12 point turn and literally jumped out and crapped on the ground. My trick is to drive just hard enough to know the limits of ground clearance and suspension bump stops. Once those perimeters are second nature in you head you can push pretty hard without a stuck or damage. In other words momentum is your freind to the point you do not damage the vehicle. I'd put a front drive Caravan against most 4x4 SUV's and never hurt it. The break stomp thing works best if you are moving. Just modulate both pedals and you get a quick feel for what it's doing. Works best under very hard throttle. If modifying a rig without lockers then look into perfecting bump stops, increasing spring rates if you can and installing stiffer valving shocks. These mods will allow you to increase momentum. If you watch rigs at hard off road events and trail rides you will see a lot of fellows with sub par rigs doing amazing things just by mashing the gas pedal and climbing with momentum. Work your way up and it can become more than just bashing your way thru a tuff spot.

 

[KLF]

Some people have brought up using your brakes. That can be extremely effective, some vehicles more than other. It is extremely effective in the Hummer H1. Which is great be use the suspension in an H1 sucks for most off road purposes and you are almost always hanging a tire if driving an H1.

Here is a link explaining one method. Search brake modulation.

http://www.hummerknowledgebase.com/driving/btm.html

That is no surprise, since as mentioned in that link, Hummers use "torsen" style torque biasing differentials.
For reasons that have been discussed here, it is far less clear whether the technique can be considered universal and/or similarly effective in a vehicle with open differentials.

 

[javajoe79]

First, you cannot apply 2500 ft/lbs of torque to the differential if the greatest resisting torque is the 1100 ft/lbs provided by braking at the spinning wheel.
That is the inherent limitation of an open differential.
And, although it is oft repeated, an open differential does not "split" torque. In other words, it is not divided into fractional proportions.
The available (transferable) torque is identical at both wheels (1:1) and can be no greater than that of the wheel with the least resistance. Using your example, 1100 ft/lbs.
And with that maximum available 1100 ft/lbs, you are now trying to make the vehicle move - which requires turning the wheel with traction.
That means (1) overcoming the 1100 ft/lbs. of braking force you are applying to each wheel (oops, there nothing left) AND (in your example) also providing the 1000ft/lbs you have set as the amount needed to turn it.

Aside from the braking force, keep in mind that the stalled wheel is stalled, not because it requires some huge amount of torque to turn it, but because it is simply not getting any torque at all (if the opposite wheel is spinning).

1100 at each wheel plus whatever it takes to move the truck. The numbers may be imperfect but he's still correct. The brakes equalize the force needed to turn the wheels so they both turn. IMO The extra heat from the spinning wheel helps the brakes on that side grab even better.

 

[KLF]

1100 at each wheel plus whatever it takes to move the truck. The numbers may be imperfect but he's still correct. The brakes equalize the force needed to turn the wheels so they both turn. IMO The extra heat from the spinning wheel helps the brakes on that side grab even better.

No.

One spinning wheel (we'll let it be in the air) that requires essentially 0 torque to turn. One wheel with traction, but essentially 0 torque to drive it, since torque is ALWAYS equal (1:1) through an open differential and cannot exceed that of the wheel with least resistance.

The brakes equalize the force needed to turn the wheels so they both turn.

is an erroneous statement.
You are adding a bunch of brake torque and and an equal bunch of drive torque to each wheel. Those forces ARE equalized. But the spinning wheel can never receive more torque than that necessary to overcome the brakes. Once you overcome the brakes that wheel spins because there is nothing else to resist the drive torque.
And while the stalled wheel has increased brake torque and increased drive torque, those are equal and opposite. There is still no NET drive torque to the wheel with traction - which will still remain stationary.

 

[javajoe79]

That isn't the case. How about you go out and scientifically disprove us all instead of attempting to shoot down every explanation.

 

[javajoe79]

At this point you're saying a well known method doesn't work. I don't think you have enough experience to get it. Otherwise you would say "yeah! That definitely works but I can't explain it"

 

[KLF]

At this point you're saying a well known method doesn't work. I don't think you have enough experience to get it. Otherwise you would say "yeah! That definitely works but I can't explain it"

No. You're getting pissed off only because you believe it works. Not because you understand how it works.
Back up just a bit.

What I have been saying is that if it works, the reason it works is likely not that which is so often repeated.
And, by the way, I don't know how "well known" the technique is, it's actually pretty hard to find information about it as I have been trying to do.
But there seems to be about a 50:50 split between those who have found it useful and those who haven't.

To the extent that it is regularly repeated common knowledge, well.....
See the following light perspective on "common knowledge"
http://www.nbc.com/saturday-night-live/video/common-knowledge/n9612?snl=1

Simplified bottom line:
(1) The technique works for the physical reason commonly cited (not likely), or
(2) The technique works, but for a reason different from the one commonly cited, or
(3) The technique does not work.

Now there are a number of subtle variations to the above
(1) The technique works regardless of how it is applied, anywhere and everywhere (not likely), or
(2) The technique works, but requires a very specific protocol, (So how do we apply it - and WHY does that work? What were the unsuccessful folks doing wrong), or
(3) The technique works, but only in limited situations (also explained by controlling the spinning wheel) , or
(4) The technique works, but is requires a very specific vehicle setup (Hummers qualify as per earlier link, Is ABS a factor?)
(5) The technique does not work.

So which of those is accurate?
I would like to know the physics behind it working, if there is any. Scientifically. I'll trust Newton's laws over any 10 other people you can choose.

Are you familiar with the term "confirmation bias?" Look it up.

I'd love to have a discussion with those who can provide experience, discuss the physics, or both.

Some factors have been discussed here that would help flesh out physical reasons for some folks claiming successful results.
These include (with my comments)
(1) control of the spinning wheel to help it regain traction (certainly could work, is subtle)
(2) difference in brake coefficient of friction between HOT spinning wheel and COLD stationary wheel (the effect is subtle at best, especially for "daily driver" compounds as opposed to race compounds, and probably most important - the difference between STATIC and KINETIC friction likely works to offset any difference from temperature variation)
(3) Torque biasing differentials- Yes the physics absolutely works when they are present! (Some Hummers had them and some Jeeps had air lockable diffs with torque-biasing function when unlocked diffs http://www.fourwheeler.com/how-to/transmission-drivetrain/154-0910-jeep-locker-overview/

(4) ABS Brakes - the physics can definitely work when present! (I do not know which vehicles have this and which don't)
OK, I have hinted at this but let me spell it out in detail:

Both ABS and traction control provide automated operation (both on and off) of the brakes, but are intended to accomplish almost opposite functions.
ABS came first and was simply intended to prevent wheel lockup and undesirable skidding under hard braking by controlled (modulated/pulsed) release of the brake on a given wheel.
Traction control works by engaging a wheel's brake to prevent spin of that wheel so that there is still torque driving the opposite wheel
On a vehicle with traction control, left-footed braking would be, at best, redundant since the traction control system does exactly what would be hoped for by the left foot braking (which probably does it much better).
Interestingly, on a vehicle with ABS, left footed braking would presumably cause the brake (and braking torque) on the stalled wheel to be released in a controlled fashion thereby allowing that wheel to be driven by the torque developed as a result of the braking resistance maintained on the spinning wheel. So it could actually result in a beneficial result by “biasing” the braking in a manner very similar to traction control. Both create unbalanced braking - traction control by braking one wheel, ABS by releasing the brake on one wheel.
This would have no effect if the emergency/parking brake is applied.

Now if you believe it works and don't care how, the least you can do is help fill in some of the blanks with the specific details of a vehicle, details of a situation, etc. But also beware of "confirmation bias" and potentially inaccurate conclusions like "both wheels started turning" when you have no idea what the wheels were turning. I know I can't see my wheels when I'm in the driver's seat and often cannot tell exactly what any one wheel is doing.

 

[MOguy]

I do not know the physics behind it. I know using the brakes works in the H1 because I have done it. I know it worked in my old GMC S15 because I had done it. I know it worked in my wrangler because I had done it. I now have ARBs so I have not tried it in a long time.

In the H1 it works flawlessly and consistently. It wasn't as effective in the other vehicles but it has gotten me out where not using that technique didn't.

 

[KLF]

I do not know the physics behind it. I know using the brakes works in the H1 because I have done it. I know it worked in my old GMC S15 because I had done it. I know it worked in my wrangler because I had done it. I now have ARBs so I have not tried it in a long time.

In the H1 it works flawlessly and consistently. It wasn't as effective in the other vehicles but it has gotten me out where not using that technique didn't.

Yes, the Hummer has torque biasing differentials. The physics there is pretty clear in support of why the technique works in a very positive manner.
Do you know what differentials were in the Wrangler and S15? Did they have ABS?
Again, those are factors that may help provide a sound physical explanation.

And can you describe the types of situations where it worked with those other vehicles?
I will note that descriptions of success seem to run the range from achieving near tank-like capability
(See the previously cited link: http://www.hummerknowledgebase.com/driving/btm.html - "I have been able to calmly walk up virtually every obstacle I have ever done at Moab") to just barely getting a vehicle moving again when stuck in slippery conditions, e.g. a wheel on ice.

The first type of scenario is explainable by knowing that the vehicle characteristics provide for a meaningful "redirection" of usable torque.
The second type is more akin to merely using that portion of an electronic traction control system that cuts throttle to help prevent overpowering and spinning a wheel that has poor traction.

 

[MOguy]

Yes, the Hummer has torque biasing differentials. The physics there is pretty clear in support of why the technique works in a very positive manner.
Do you know what differentials were in the Wrangler and S15? Did they have ABS? Open diffs no ABS
Again, those are factors that may help provide a sound physical explanation.

And can you describe the types of situations where it worked with those other vehicles? slick rocks and KY mud
I will note that descriptions of success seem to run the range from achieving near tank-like capability
(See the previously cited link: http://www.hummerknowledgebase.com/driving/btm.html - "I have been able to calmly walk up virtually every obstacle I have ever done at Moab") to just barely getting a vehicle moving again when stuck in slippery conditions, e.g. a wheel on ice.

The first type of scenario is explainable by knowing that the vehicle characteristics provide for a meaningful "redirection" of usable torque.
The second type is more akin to merely using that portion of an electronic traction control system that cuts throttle to help prevent overpowering and spinning a wheel that has poor traction.

Answers in Bold

except with the H1, there it works incredible well esp. when hanging tires or tittering side to side.

 

[javajoe79]

I'm not mad, I am calling you out. Disprove all the reasons it might work, or stop shooting them ALL down. Describing why you think someone's theory isn't correct, does not disprove anything. You have to provide real world data or your argument is just a theory too. So basically, just saying NO here is why I think so, is not conducive to good discussion. That is you trying to say NO I am right and you are wrong. Yes confirmation bias is real but again you only have your theory, just as we have ours.

Now I will continue with my proposal.

The reason it works with a torque biasing diff is because it enables the exact purpose of said diff, creating thrust and binding within a set of gears so as to "lock" them together.. So let's stop mentioning those as that is not what we are talking about.

Stop with the ABS and traction control talk because that is not what we are talking about.

We are talking about a normal old open differential and stepping on the brake and gas at the same time.

I will also say that we aren't increasing traction, we are taking advantage of already available traction. To simplify it, increasing traction means a change of tire, a change of surface, a change of contact patch and or the weight pressing the two together. Also maybe tire tread temperature but that's not too likely here.

As I've said, the brakes equalize the torque load at each wheel and help to enable momentary transmission of torque to the tire with grip. Usually the one tire that is spinning has to stop first so in the process of stopping that tire you put some extra heat into that brake. Once BOTH tires are stopped, that brake now holds a little bit firmer than the other side. That and the binding effect of the spider gears under load, combined or acting alone, helps to send some torque to the stationary tire. Even for just a second here and there, can help you out of a situation.

I think you are discounting brake temp and binding of the spider gears too much. Touching on a torsen style diff, it works by taking advantage of thrust and binding from the gears. It is built to create thrust and binding on purpose but any gear creates thrust in some direction when it is loaded and the more you load it the more thrust it creates. The more thrust it creates, the more binding you get. If one were to create a test rig with an open differential axle you could measure that binding by loading the axle like normal though the pinion. With both tires having full traction but not loaded enough to spin them, if you had a way to rotate that whole assembly so that one tire rotates forward and one rotates backwards while maintaining the load applied to the pinion and measure the force required to do that, you would see just how much the binding in the gears "locks" the two axle shafts together. This is essentially no different than a standard clutch type LSD. It is a resistance to the axle shafts spinning at different speeds.