Lockers don’t send 50/50 torque. Proof of a common misconception.

Firestarter
Firestarter
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Published 2021-03-03
I have seen many comments and videos stating that locked differentials send 50/50 torque to both wheels, and that open differentials will send torque to one wheel. This is backwards, as demonstrated by my video. Open differentials send equal torque while locked differentials can vary torque from 0-100% depending on the condition. Voltage was locked, so difference in motor rpm is due to load. This RC has a lockable differential, and I utilized a torque bit driver to measure the amount of torque at the wheel's hub which simulates how it would transfer torque to the ground. When unlocked, I used my hand to slow down and stall the spinning wheel to vary the torque delivered to the measured wheel. Conversely, when locked I allowed that wheel to freely spin and transfer zero torque. ** When I talk about torque, I am talking about force the wheels can generate, not the overall torque that accounts for driveline dynamics. Torque was measured in lb-in.
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Results:
The open differential behaved with a 50/50 split, always sending equal amounts of torque to both wheels. The more torque the spinning wheel experienced the more torque the stalled wheel with the more traction received. When the spinning wheel developed zero torque, so did the stalled wheel. Both wheels always received the same amount of torque.
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The locked differential distributed torque dynamically. It sent 0 to 100% of available driveline torque to the measured wheel. You'll also notice that the locked differential developed more torque at the wheel, and this is due to the torque being sent to only one wheel as opposed to the open differential that split it evenly resulting in lesser torque being measured at the one wheel. The more resistance the torque bit driver placed on the locked diff's wheel the more torque distributed to it.
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Why does this matter:
A brake based traction control system can send torque to both wheels, and do so with some intelligence. BUT. A locker will instantaneously send torque to the wheels with the most traction, AND can develop more torque at those wheels.
All Comments (4)
  • Thanks for video! I've been "fighting" people who don't understand torque distribution for many years. It's really hard. :-)
  • @mdocod
    Also... A traction control system is reactive in nature. It can only redirect torque after the threshold of traction is lost on 1 side. With the locker engaged, the full threshold of traction available from the 2 wheels on that axle can be exploited. Another misconception is that a locked center diff (4WD mode for many vehicles) does not improve stopping power on slick surfaces. In fact, it can improve stopping performance on slick surfaces because it allows the driver to exploit both axles to their threshold of traction. With an open center or 2WD mode, one end or the other almost always brakes traction first, leaving available grip on the table.
  • @renners9636
    Most have it Conceptually backwards, the statement a locker sends 50/50 torque torque to the wheels is doomed from the start, you can say it send 50/50 speed/rpm, but not torque because you have not set the conditions of operation, you need more information to qualify the statement. The amount of torque a wheel experiences, is limited by its its ability the create tractive effort, at any time a locked rear diff has a known torque available, and it has to go somewhere, this being a force at the wheels perimeter. If the car were on jack stands there would be very little torque able to be devolved at the rear axles, there is a potential for torque when resistance is applied, but you cannot send torque where it won’t be resisted, you cannot send torque where there is no traction, outside of while the wheel accelerates. A locked diff does not send its Torque, its torque builds in response to resistance, if one wheel has all the resistance/traction it takes all the torque. To the 50/50 tq crowd you just ask them would the car have only have half power of one rear tire had perfect traction and the other zero? If not why, because obviously the one tractive wheel is able to use all the torque available. Above statements not to be applied to Torsen, other, etc.