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Up to 100% of Torque to the wheels? BS

A huge Pet Peeve of mine is when marketers/salesman/ads use the term “transfers up to 100% of the torque (power, etc) to a single wheel!” when referring to brakes based traction control systems.

I realize its just me being pedantic, but no...you are NOT transferring 100% of torque to a single single wheel or axle unless you lock the diff. I know it has the “effect” of a locking differential, but it doesn’t actually work with respect to torque in the same way for one simple reason: Open differentials always, ALWAYS, split torque 50:50. (minus a small amount of internal friction)

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Any differential can only apply as much torque as the to the ground as the traction will accept. In the case of the open diff if you have one wheel that can accept 500 ft-lbs of force and one wheel in the air receiving no torque then no torque is applied to the ground and the car stays put. An open diff will only be able to use (at max) the lowest torque value that doesn’t spin the wheels. If you spin a wheel at 20 ft-lbs on ice for example, you can only apply 20+20 (for each side) and so you don’t move.

What helped me get this concept was the idea that torque is an applied force and so if there is no application there is no torque. i.e. it doesn’t matter that your engine figures say it produces 500 ft-lbs of torque at whatever rpm, if it can’t apply it to the ground via the wheels its not producing torque (which is why engine dynos have to be under load). So if you are thinking to yourself (like I was) “well where does all my engine torque go?” Well it never gets applied. It doesn’t matter how strong you are, you can’t torque a spinning bolt, for example.

If you brake that spinning wheel you are applying artificial applied torque up to the limit of the 50/50 split and the traction limit on the grounded wheel. i.e. if the good wheel can accept 500 ft-lbs before slipping and you can apply 500 ft-lbs of clamping force to the braking wheel for a total input of 1000 ft-lbs (half in the brakes, half on the ground) and after that the wheel looses traction. In this case it gets you moving like a locked diff but still only sends 50% of the available torque to any given wheel. If you had 2000 ft-lbs of engine force to apply you could still only apply 500 ft-lbs to the good wheel before it slipped and then you are back to being only apply to 1000 total ft-lbs (500 until the wheel slips and 500 to the other side). There is never a scenario with an open diff that allows you to send 100% of supplied torque to one wheel.

Man, I didn’t mean for this to be an engineering rant, more a rant on the phrase “up to 100% of torque”...but here we are.

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Now, a locked differential CAN vector torque up to 100% and there are LSD’s that lock up but if its an open diff don’t let a marketer or salesman feed you this “up to 100% of torque” BS. Its not their fault they don’t get physics but you...you don’t fall for it.

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