Locker vs Open

[michaelgroves]

In the case you mention, when the vehicle rocked back and forth, the explanation is simple:

As the wheel in the air accelerates, it's rotational inertia does present some torsional resistance on the axle, which does get passed through onto the other wheel which would then receive some torque which is what rocked the vehicle.

Rob, yes, that makes sense. And decelleration through engine braking would have the same effect, I guess (but backwards).

 

[michaelgroves]

Interesting debate. Michael, retry your experiment but instead use a vehicle that has the parking brakes on the wheels rather than the driveshaft, as on your Rover. Instead of applying the service brakes, slowly apply the parking brake as you gradually apply power. Let us know the result.

Pete

Hi Pete,

I've done that (or something similar) on a few occasions! Of course, if a rear wheel were jacked up, applying a brake to the rear only (such as a transmission parking brake) would cause the vehicle to move forward. The same principle as "fiddle brakes" on an axle.

Equally obviously, the parking brake would have no effect if one of the front wheels were spinning, since the rears would be stationary anyway.

And if one wheel on each axle were jacked up, then applying the parking brake would make the front wheel spin faster, but not move the car. (That's a prediction, not an experimental result).

All those assume an unlocked centre diff, of course.


EDIT: Oops, I just re-read your post - duh!
Do you mean a full-time 4x4 (unlocked centre), with one rear wheel and one front wheel jacked up, and a parking brake applied to the wheels rather than the rear transmission? If so, I don't have one of those beasts to try it on. However, the only functional difference I can see, would be that with a parking brake that acts on the rear wheels individually, there is some scope for uneven braking (which would work positively or negatively, depending on which wheel got more braking effort).

If anyone else has a suitable vehicle, I'd be interested to hear the results of such an experiment.

 

[michaelgroves]

Yes, left foot braking can provide some traction. I have used it with positive (though marginal) effect on the Discovery (5-speed), and with greater effect on a Grand Cherokee with an automatic. In the most simplified example, the effect is torque equalization, similar to, but less effective than traction control, where a spinning tire can be arrested completely by the brakes, and no brake pressure is applied to the stationary tire. Applying brake force begins to equalize the resistance to both axles of the differential, and if the grade or obstacle does not have greater resistance, you can get some forward motion.

It can, and does work, but to very limited effect based on my experience.

Hi Scott.

I am still having difficulty reconciling "it does work for me" with any workable theory of how it might work. Perhaps more damningly, reconciling it with my controlled experiment.

You explanation seems to be the same as Thom/muskyman's, and fails to explain in principle how the increase in torque due to braking on the spinning halfshaft exceeds the additional torque required on the other halfshaft due to braking.

Any explanation would also have to explain why it would work in a complicated situation in the field, but not in a controlled experiment. (Or point out the flaw in the controlled experiment, of course).

I'd be interested in your further comment.

M...

P.S. The only GC I've driven was also an auto, but it had a rear LSD, in which case all bets are off! (I'm assuming your GC didn't have a LSD).

 

[michaelgroves]

Michael,
Do you have the LT-230? If so, try this and you'll see what Thom is saying.

-With the center diff unlocked, find a grassy hill steep enough that you'll loose traction.
-Drive onto that hill and stop half way
-With no foot on the brake, try to start moving forward again (hopefully the hill you've chosen is steep enough, or slick enough, that you cannot make it without locking the CDL)
-Once your tires break traction, start to apply the brake gently while continuing to press the accelerator. See what happens.

Hi Hank,

Yes I have an LT230. I have tried many times to do what you are suggesting, trying various combinations of brake and throttle. Of course, it could be that I am not getting quite the right tecnique, or that the slope is too steep or slick (just as locking the CDL sometimes isn't enough either).

It was those problems that made me think of a more controlled experiment. Can you think of any reason why the experiment you've suggested should "work", and the one I tried "failed"? Because on the face of it, it sounds like no matter how it works, it should work equally well the way I set it up.

 

[Scott Brady]

Michael,

The GC did was open differential. It also works quite well with my brother-in-laws TJ automatic. The automatics have the best effect.

I completely understand your reservation in believing it. A 70 year old guy showed my the technique nearly 15 years ago, much to my suprise.

Hey, this is a great excuse for you to visit Prescott. I will arrange the open diff. Jeep, a fun trail in Sedona and a BBQ at the office

 

[michaelgroves]

Hey, this is a great excuse for you to visit Prescott. I will arrange the open diff. Jeep, a fun trail in Sedona and a BBQ at the office

I'd love to! I might well hold you to that, it's time for a littel adventure soon...

 

[michaelgroves]

Here's a video of me going up Dishpan Springs trail near Big Bear, in a DII.

http://www.youtube.com/watch?v=ApZ2KKQFsR0

Nice video, btw. The whole drive was efficient and deceptively unspectacular - it made a very technical section look easy. A textbook example of a good driver/spotter team working well.

 

[ntsqd]

Somewhere I have a dvd of me going up that section of Dishpan in the VW Bug based dune buggy. It was quite the contrary experience. You can't do damage to the trail if you're not in contact with it! :Wow1:

What I have found in using this braking technique in said dune buggy is that it is good to a certain point, beyond which it degrades the situation. We called it the "Poor Man's Posi" in my buggying days. Key is that we used only the parking brake and not the service brakes for this technique. Recall that this is an open differential, rear drive only vehicle. Some of the guys had sand buggy steering brakes, which were more effective since the spinning wheel could be selectively braked, but those were expensive to a poor college student!
One could effect a compromise with the hand brake up to a point. I can't exactly define where that point is, but experimentally I found that if you can feel the brake's effect as a brake you're probably on the edge if not a little past it. When the terrain got to the point where the trick no longer was working it was time for something else as adjusting tension on the brake lever, in either direction, rarely had a positive effect. That is to say that the point of diminishing returns was quite noticeable and the degradation slope was rather steep.

I think that the key is not to try to transfer all of the available torque to the tire with traction, but rather to attempt to transfer the barest minimum necessary to overcome the obstacle. More or less transfer at that point both work against you.

 

[muskyman]

gone

 

[michaelgroves]

for a single tire to slip with a open diff that tire is getting more rolling resistance than it can over come and a lack of traction to overcome that rolling resistance.

I just don't get what that whole sentence means....

That rolling resistance can come from a number of places. In the original video the rolling resistance is coming from either a rock or a root that looks to be contacting the tire above the tires centerline, this is very common on the trail. Another very common place for rolling resistance to be applied to a wheel is at it center point or the spindle, see wheels are connected to the chassis so forces acting on the chassis will affect a wheel as well.

...which may be why the rest of this paragraph doesn't clarify things for me either.

I raised two basic objections to the most common explanation of open-diff BTM. One was an objection in principle - any torque added by equal-side braking adds nothing to the force that the tyres exert on the ground (because it's precisely offset by the braking resistance itself). I illustrated this by a quantified example.

The other was an objection in practice - I outlined a test that had the fewest number of variables I could think of, and described the results.

If my objection in principle is invalid, then some other force must be in play that I haven't taken into account. How does open-diff BTM generate additional force under the tyres? If there's link between that question and the statement that sometimes rolling resistance is applied above the tyre's centreline, it's not an obvious link to me.

If my objection in practice (the experiment) is invalid, then presumably it's to do with the BTM technique I was using (easy to resolve - someone else do the experiment, and report the results), or it's to do with some terrain feature that is fundamentally different to the control environment I described. However, no-one's yet suggested that the technique is limited to some specific terrain or situations. Indeed, Hank says it will work on a flat slippery slope, and others have used clambering over rocks as exemplars of BTM.

The next force to consider is a spinning wheel. You must understand a spinning wheel is STORED ENERGY when you apply the brakes some of the energy is turned into heat but most is transfered back to the differential and across to the other wheel. the torque can not go into the driveshaft because it is still under load and putting torque into the differential through the ring and pinion from the application of the throttle.

because the energy is only being stored on one side of the differential the effect is only going to help until both the wheels are once again turning at the same speed or one tire begins to spin again and store energy.

The momentum from a spinning wheel was the other explanation I referred to in my previous post. It's seductive at first glance. However, the problem with this is two-fold:

Firstly, when the brakes are used to slow or stop a wheel from spinning, no momentum from the wheel is transferred back into the driveline. The brakes are anchored to the fixed part of the truck, and absorb all the energy. That's simple physics. Energy from upstream in the driveline (the weight of the flywheel, propshafts etc. will be distributed equally between the two halfshafts, of course, but subject to the same rules as if it were being generated by the engine, rather than residual kinetic energy. That is, it keeps the same torque on each shaft. (And since the braking force on both shafts is equal, the "traction torque" is still limited to the spinning wheel's traction.)

Secondly, if this were an explanation, then it would apply in my experimental scenario, and sudden braking should cause the car to lurch forward.

The converse of this inertia/momentum explanation is a little more persuasive. If you suddenly accelerate rather than brake, then, as Rob pointed out in an earlier post, the inertia of the lifted wheel would generate a torque on both axles, and therefore try to move the car forwards. But not by much, and it has little to do with BTM...


An interesting thing about this discussion is that one would have imagined that such a technically knowledgeable group would quickly have come up with the right explanation. Had I been arguing that BTM doesn't work in the case of a LSD, for example, it would have been easy for someone to explain how the mechanism of a limited slip diff multiplies torque, and that therefore my chain of reasoning (and my numbers) were invalid. Or had it been an argument that keeping low engine revs are never good for traction, someone would explain how static friction is greater than dynamic friction. My point is that every phenomenon that exists, does have an explanation...

 

[michaelgroves]

LoL

yeah your right and the millions of times people have stepped on the brake to cheat a diff thats all fantasy.

nice RobinHood

http://www.badassoftheweek.com/groves.html

This isn't about whether it works - we can all hold our opinions about that until we know how it works.

Aaaanyway... I'm finding you a wee bit tedious now, especially in the light of your comments on the other thread. I think your attitude might be more appropriate on other Forums...

 

[muskyman]

gone

 

[michaelgroves]

did you watch the videos I posted?

I guess not...

you can say what you like, the technique works.

*sigh*

Did you do the experiment I described?

I guess not...

So under what circumstances does the technique work, and how (or why)?

 

[Rovernut]

driving skills will often get you through places that lockers would mess you up.

And, driving skills with lockers will take you places you wouldn't believe.:ylsmoke:

 

[muskyman]

gone