Increase Traction w/o a rear locker? How?

[Pntyrmvr]

Sounds like none of you have been 16 years old, stuck in too much fresh snow without snow tires, alone in your Dad's Cordoba with a curfew deadline looming.

You put the floor mats under the rear tires and shoot them off impatiently. As the perspiration builds you note one tire is spinning and that if both would spin you could be home free.

You then click the parking brake on one notch a time until miraculously the damn car moves forward off the freshly created ice patch.

The lesson in torque, and patience lives with you for years.

The melted rubber on the edge of the floor mats and the cords hanging off the Firestone 721's, alas still get you busted. But you are home on time.


"Talk is cheap. Whiskey costs money."

 

[proper4wd]

We are in agreement on how a differential works and the forces involved. Also that traction control is effective.

However, real world experiments of left foot braking deliver a much different result than your theorized outcome.

How do you explain that?

 

[KLF]

We are in agreement on how a differential works and the forces involved. Also that traction control is effective.

However, real world experiments of left foot braking deliver a much different result than your theorized outcome.

How do you explain that?

Um, how about "I don't believe everything I read on the web?"

Seriously though, that's partially what I'm looking to hear.
Have any anecdotes and/or details you would like to share?

If it really does work well, there certainly should be a physical explanation (that stands up to scrutiny).
The classic explanation does NOT stand up to scrutiny.
I can see how having a torque biased differential might lead to some beneficial effect.
As could a vehicle with ABS brakes (even without traction control)

My limited experimentation with left footed braking has NOT shown results different from my theory.
If there is a flaw in the theory I'd like to know. If the technique could be useful for my purposes I'd like to understand it sufficiently to be able to apply it.
And if the positive "real world" results are largely placebo and/or highly situational, I'd like to know that too.

 

[proper4wd]

I have experimented this with many different vehicles and I can conclusively say that vehicles with bigger brakes and higher torque engines left foot brake much more effectively.

Many times I would drive my 2006 F350 diesel to the point it was completely cross axled and spinning opposite tires - apply hard left foot brake to stop everything - and then slowly increase throttle until the wheels began to rotate (overcoming the heavy brake resistance). Both the airborne wheels and the on-ground wheels would turn at the same rate and the vehicle would creep out of the cross axle.

This was as effective as engaging f/r differential locks in other vehicles I've owned. You should go try it, rather than just..... writing on the internet (which is just as bad as reading on the internet).

 

[KLF]

I have experimented this with many different vehicles and I can conclusively say that vehicles with bigger brakes and higher torque engines left foot brake much more effectively.

Many times I would drive my 2006 F350 diesel to the point it was completely cross axled and spinning opposite tires - apply hard left foot brake to stop everything - and then slowly increase throttle until the wheels began to rotate (overcoming the heavy brake resistance). Both the airborne wheels and the on-ground wheels would turn at the same rate and the vehicle would creep out of the cross axle.

This was as effective as engaging f/r differential locks in other vehicles I've owned. You should go try it, rather than just..... writing on the internet (which is just as bad as reading on the internet).

And your F350 had two open differentials? No differential in the transfer case? ABS? 4 wheel? (Wasn't that standard on most vehicles by then?) Are you talking about level (but irregular) ground or climbing a grade? How do you know the wheels were all turning at the same speed? Got any videos?

By your description it is hard brake followed by throttle modulation. As opposed to throttle followed by brake modulation? And no manual pulsing of the brakes? I have seen that described and demonstrated, especially using an e-brake.

I will play with it some more when I get a chance. As I have described, it has not shown its benefit in any situation I have been been able to try thus far (since becoming aware of the concept not too long ago).
I do wish I could go back in time and see if it could have made a difference in one situation on a short climb in Canyonlands I have done often (usually with little difficulty, often with somebody else driving the vehicle). But that day, the surface had badly deteriorated and was wet, and after a couple of unsuccessful attempts I took a pull to be eased over the top. In that same part of the world, I have also ceased moving by having two wheels teetering off the ground where the sandstone surface would provide unlimited traction when all wheels are on the ground. But those situations are also commonly on steep climbs where a mere "nudge" that would work on flat ground would be insufficient.

I would still like to understand WHY the technique works, and how effective it is likely to be. If it is the factor that is actually creating the benefits commonly cited.

 

[IdaSHO]

I've used the technique extensively up here in north Idaho.

Once upon a time I drove truck, for a large retail home center.
So I was expected to deliver materials to jobsites, regardless of road conditions, or even no road


Trucks were straight trucks, 2 ton and better, all 2wd (4wd would have been nice :Wow1
And during the winter it seemed that at least one delivery a week Id find a rutted, snow covered, or muddy incline to a home or jobsite that I simply could not get up.
Regardless of how I treated the throttle, even fully chained up, one set of duals would always start spinning, then I was done. Too much weight, not enough traction, movement stopped.


Overheard the brake pedal trick from another driver, and started using it.
And it works. Pure and simple.


I'm not sure how it works, but it does.

My simple theory is that it doesn't necessarily redirect power to the non-spinning wheel, but instead slows the spinning wheel to the point of regaining traction.

After-all, traction is at its peak when the wheel speed matches the road speed.

 

[IdaSHO]

Not if its been posted yet, but this video shows it in action pretty well.

Starting around :53

https://www.youtube.com/watch?v=L85t5WLCK84

 

[KLF]

Not if its been posted yet, but this video shows it in action pretty well.

Starting around :53

https://www.youtube.com/watch?v=L85t5WLCK84

That's one of a handful of online videos I am aware of. None of them suggest much more than just barely being able to get a vehicle moving on flat ground. Elsewhere on this site - see "VOTD: Left-Foot Braking" which includes a link to the same video.

That video is not exactly a ringing endorsement for its overall effectiveness, especially as it might be used off road. Envision the video with the addition of a wheel-chock sized rock placed in front of one of the stationary wheels. I seriously doubt the vehicle would move.

I agree with your assessment that any positive results may be mostly from minimizing the spin of the wheel without traction. It would seem the same thing could be accomplished with judicious use of the throttle, although perhaps use of the brake pedal allows better modulation.

Otherwise, the anecdotal information to be found, including that in this thread, runs the gamut.
There are clearly vehicle factors that could contribute to it working, (ABS, torque biasing diff, etc.). But that is far from establishing that it is universally effective. And the most commonly cited basis for it working (torque transfer to a wheel with traction) does not appear to be accurate.

My curiosity relates to the type of off-road situation where progress has been stalled by wheel slippage and some significant additional propulsion will be necessary to get the vehicle moving (e.g., up a climb, rocks to get over, or both). I have yet to find anything which would lead me to believe that left foot braking an open-differential 4x4 will accomplish this in any meaningful fashion.

 

[proper4wd]

Dude, don't take this the wrong way - but you have many many accounts in this thread of people who know that it works, based on real world experience. You refute it based on theoretical opinion. Whats wrong with this picture?

 

[KLF]

Dude, don't take this the wrong way - but you have many many accounts in this thread of people who know that it works, based on real world experience. You refute it based on theoretical opinion. Whats wrong with this picture?

I am curious, did you read what I wrote? You comment would suggest you did not.

For the record, I did NOT say "It doesn't work."
I pointed out the likely limitations of the technique based upon my experience AND many accounts in this thread and elsewhere along with theoretical opinion (interesting term - I prefer "basic physics")
In doing that I tried to reconcile all the available details and noted factors (equipment and situational) which could account for some degree of success. Analysis and understanding is not just a matter of tallying the "it works" votes and the "doesn't work" votes. Even many of the "it works" folks acknowledge that it's, at best, a last resort and maybe just a little better than nothing.
Don't take this the wrong way, but I don't think you have any interest in analysis, understanding or, for that matter, reasonable exchange of information.

In any case, MG first raised the issue in this thread many years ago in post #27 "but equal braking on an open diff has me baffled."
The discussion has gone round in circles, but a physical explanation (which should be pretty straightforward) still remains a mystery.

 

[KLF]

If there is a perfect 1:1 torque bias and the brakes apply equal force to each wheel there can be no increased traction from brake application. But perhaps 1:1 bias is not perfect, or the brakes do not actually apply equal force to each wheel? There is some friction within the differential, and the momentum of the spider gears would have some effect, but I doubt these effects are perceptible. I therefore conjecture that if a traction increase is observed with brake application it is due to the brakes themselves.

The actual friction coefficient of pad on rotor is probably not invariant to surface velocity, and it is surely not invariant to temperature.
...
These factors may be why some report success with braking open differentials while others have no success at all.

I agree with these potential variables.
But to build upon that a bit - let's also note that, all else being equal, a coefficient of static friction is generally higher than the coefficient of kinetic friction.
https://en.wikipedia.org/wiki/Friction#Static_friction

So, it would be expected that, with brakes applied equally to a spinning wheel and a stationary wheel, the braking torque on a stationary wheel (static) would be greater that on the spinning wheel (kinetic).
If that is an accurate assessment, application of the brakes in such a scenario is actually counterproductive to getting more net driving torque to the stationary wheel.

 

[IdaSHO]

As stated earlier.

I think it has little to do with transfer of power to non-spinning wheel.

My simple theory is that it doesn't necessarily redirect power to the non-spinning wheel, but instead slows the spinning wheel to the point of regaining traction.

After-all, traction is at its peak when the wheel speed matches the road speed.

 

[proper4wd]

I agree with these potential variables.
But to build upon that a bit - let's also note that, all else being equal, a coefficient of static friction is generally higher than the coefficient of kinetic friction.
https://en.wikipedia.org/wiki/Friction#Static_friction

So, it would be expected that, with brakes applied equally to a spinning wheel and a stationary wheel, the braking torque on a stationary wheel (static) would be greater that on the spinning wheel (kinetic).
If that is an accurate assessment, application of the brakes in such a scenario is actually counterproductive to getting more net driving torque to the stationary wheel.

You are forgetting about heat.

 

[INSAYN]

A trick that I had used to free my old '85 Toyota with open diffs when I had one rear and opposite front stuffed up in the wheel well and aimed head first towards a deep ravine. My only option was to back up, but my two unweighted tires would just spin. No winch, no hi-lift jack, no other vehicles around, no other people with me to jump up and down on bed, etc.
Walking out was an option, but I did have recovery strap under the seat. I wrapped the strap around the rear tire that was unloaded and looped the strap through itself and pulled it tight against the tire near the ground. From there I had to attach the other end to something, but no trees within reach for me to do this. I ended up wrapping some of the excess strap around the rear bumper making sure to over lap it so it will tighten down. The final end was locked down by closing the tailgate on it and slipping a hammer handle through the loop so it can't slip back out.

Once everything was in place, I just eased the throttle as I backed up. The stuffed rear tire now had full grip, as the free hanging tire was tied down and unable to rotate.
The truck was able to grip quite well with all the pressure on the stuffed tire and pull it up enough for the front axle to back off the huge rock that was stuffing the opposite front tire in that wheel well. Essentially had 3 wheel drive after about a two feet of travel.

I would imagine if I had to do this all over again only needing to go forward, I would attach the strap to a rock slider if that was available, or attach the strap to the frame or front axle somewhere.

 

[KLF]

A trick that I had used to free my old '85 Toyota with open diffs when I had one rear and opposite front stuffed up in the wheel well and aimed head first towards a deep ravine. My only option was to back up, but my two unweighted tires would just spin. No winch, no hi-lift jack, no other vehicles around, no other people with me to jump up and down on bed, etc.
Walking out was an option, but I did have recovery strap under the seat. I wrapped the strap around the rear tire that was unloaded and looped the strap through itself and pulled it tight against the tire near the ground. From there I had to attach the other end to something, but no trees within reach for me to do this. I ended up wrapping some of the excess strap around the rear bumper making sure to over lap it so it will tighten down. The final end was locked down by closing the tailgate on it and slipping a hammer handle through the loop so it can't slip back out.

Once everything was in place, I just eased the throttle as I backed up. The stuffed rear tire now had full grip, as the free hanging tire was tied down and unable to rotate.
The truck was able to grip quite well with all the pressure on the stuffed tire and pull it up enough for the front axle to back off the huge rock that was stuffing the opposite front tire in that wheel well. Essentially had 3 wheel drive after about a two feet of travel.

I would imagine if I had to do this all over again only needing to go forward, I would attach the strap to a rock slider if that was available, or attach the strap to the frame or front axle somewhere.

Call that "Cave man's traction control."