michaelgroves
Explorer
My view on these things is that every phenomenon - everything that works - has a theory behind it; a logical explanation. I like to understand the theory, because it allows me to exploit the phenomenon better. So if I believed that this technique worked, I would still be trying to understand why.
In this particular case, even though it's a long-standing legend, I'm not atall sure it does work - how would one know if one's success was attributable to BTM or not? (Or if one's failure was despite BTM, for that matter). It certainly doesn't work well enough to be obvious when it is working.
I've yet to hear a convincing theory of how it works, and I have tried experimenting myself quite extensively, both while driving and in contrived situations.
Sorry, I thought I had made clear why I don't consider this a valid explanation. I do understand that braking the wheel increases the torque (on the shaft), and therefore increases the torque on the opposite side.
But the vehicle requires a force on the tyres to make it move. (Let's say it needs a total of 2000Nm to make it climb a particular slope). Only the friction between the ground and the tyres can generate that force - none of the friction between the brake discs and pads counts towards the forward movement of the truck. So let's say that one tyre has enough traction to generate 300Nm, and the other has enough traction to generate 3000Nm. Obviously, since the one tyre starts to slip at 300Nm, the stationary wheel is limited to another 300Nm, giving only 600 out of the necessary 2000 - a shortfall of 1400Nm.
So, if you apply a braking force to each side - say an additional 1000Nm on each side. That will raise the total torque on the "weak" side to 1300Nm, and of course the other side also can now generate 1300Nm, giving a total of 2600Nm, as measured on the shafts.
Enough torque? No. Because instead of requiring 2000Nm to move, we now need an additional 2000Nm to overcome the 1000Nm of braking force applied to each side. So we have 2600 out of a required total of 4000 - still a shorfall of 1400Nm!
By all means plug your own numbers in here - I don't know how hard you'd want to work the brakes! But however much you brake one side, you brake the other by precisely the same amount, and so you're back to square one.
How does pressing the brake pedal prevent a tyre from losing traction? Traction is friction between tyre and ground, and is independent of any braking you do.
My suspicion is that "it works" simply because it prevents people from spinning the tyre too wildly. A fast-spinning tyre has less traction than a slowly turning tyre, and the best traction is just before it breaks loose. But no braking is necessary to achieve that.
I once did see it work very convincingly - a guy in a 2WD pickup was showing me how well it worked, and it was impressive. Only thing was, it turned out he had a LSD in there, and didn't know it!
Here's one of the simple experiments I did, with a series Land Rover, in 2WD, and then the same with a Disco 1 (centre diff unlocked):
I jacked up one of the back wheels, using a trolley jack with casters, on a smooth concrete floor. Then I just put the car in various gears, and experimented with different brake/throttle techniques.
Result?
The Series truck didn't move at all. If there had been any positive effect from the BTM, it was completely offset by the front wheels being braked at the same time.
The Discovery jerked a little bit back and forth on the castered jack. I doubt it would have fallen off its own bottle jack, except perhaps on some of the more extreme wheel-spinning/slamming-on-the-brake attempts. (Incidentally, I also tried this with a front and a rear wheel up on castered jacks, and the centre diff both locked and unlocked).
This was more or less in line with what I would have predicted. Certainly there was no significant or systematic increase in driving force induced by BTM, else the car would have been driven forwards quite easily. I attribute the rocking/jerking to uneven braking forces and some internal friction in the diffs.
The good thing about this experiment is that it is easily repeatable. If anyone is intersted, by all means try it for yourself.
One final point: I have heard a different explantion for why BTM "works" woth open diffs, which I'm afraid I also don't subscribe to, and which (if true) should also have resulted in the car moving during my experiment. I'll leave it unless someone else raises it, as it is something of a red herring.
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