Open diffs and brakes for "torque vectoring"

"You can tell a Finn but you can't tell him much" (youcantellafinn)
02/09/2017 at 14:51 • Filed to: tech, differentials, lsd

TL;DR Brake based torque vectoring doesn’t actually vector torque, it just absorbs the torque going to the low traction tire that couldn’t be used anyway which allows the high traction tire to use all the torque that it gets.

Engineering Explained posted a video talking about the McClaren P1's open diff and explaining how the brakes are used to mimic an LSD / torque vectoring differential. HHFP posted an awesome explanation of !!!error: Indecipherable SUB-paragraph formatting!!! a while back that bears reviewing. If you have a handle on that post my following ramblings might make sense.

We know that an open differential always has a 50/50 torque split. If you put 200 lb-ft of torque into the differential you get 100 lb-ft to each tire. Things go south when one tire can only put down 10 lb-ft. Once that happens each tire only gets 10 lb-ft since there is a 50/50 torque split and we can only use 20 lb-ft of our torque. If we engage the brakes on the side that can only absorb 10 lb-ft we can dissipate the excess torque as heat. Assuming the brakes are capable of absorbing all the excess torque we can still put 200 lb-ft into the diff. Both tires still get 100 lb-ft of torque because of the 50/50 torque split, but the low traction side has 90 lb-ft of torque absorbed by the brakes while only 10 lb-ft of torque gets transferred to the ground.

For some reason the EE video finally made it click in my head that the brakes are absorbing the torque that goes to the low traction tire, they don’t vector torque to the other side.

DISCUSSION (25)

HammerheadFistpunch > You can tell a Finn but you can't tell him much
02/09/2017 at 14:56

Bingo! Thats why brake based traction control logic doesn’t make sense for small, low torque engines in “off roaders” *Cough* Renegade.

benjrblant > HammerheadFistpunch
02/09/2017 at 14:57

Don’t most Land Rovers have this too?

Satoshi "Zipang" Katsura > You can tell a Finn but you can't tell him much
02/09/2017 at 15:02

Good tech, but why not use a type of diff that does the job - like Honda’s SH-AWD?

RamblinRover Luxury-Yacht > HammerheadFistpunch
02/09/2017 at 15:02

Particularly when the control logic is based on disparate wheel movement and you’re in a situation where you need to make use of any friction you can get - static or otherwise. Like, say for instance, ice.

*engine applies 100lb-ft, split 50/50
*slipping wheel wastes 20/50
*brakes apply, taking away 30, leaving 20 instead of 30
*advance up hill comes to a halt

*swearing commences.

RamblinRover Luxury-Yacht > benjrblant
02/09/2017 at 15:06

http://youwheel.com/home/2016/10/31/land-rover-awd-system-dissected-range-rover-range-rover-sport/

You can tell a Finn but you can't tell him much > RamblinRover Luxury-Yacht
02/09/2017 at 15:12

Momentum is the answer. See what happens when you don’t have enough momentum?

Kinja'd!!!

You get stuck.

RamblinRover Luxury-Yacht > benjrblant
02/09/2017 at 15:12

Adding to previous post, the Discovery Sport has two different systems both featuring *real* torque vectoring, and the LR4, while featuring brakes-based traction control in normal driving, does have locking diffs.

Jake - Has Bad Luck So You Don't Have To > You can tell a Finn but you can't tell him much
02/09/2017 at 15:13

what on earth is that from?

RamblinRover Luxury-Yacht > You can tell a Finn but you can't tell him much
02/09/2017 at 15:15

Non traction-based propulsion can also help.

Kinja'd!!!

Nonster > Satoshi "Zipang" Katsura
02/09/2017 at 15:21

I can’t be sure of McLaren’s logic, but one potential reason for going with an open diff is to improve the car’s turn-in. In a full traction scenario a diff’s job is to allow the inner wheel to turn more slowly than the outer wheel. A limited slip diff actually slightly resists this and can make the car want to push/understeer more than it would with an open diff. In reality the effect of this is probably pretty negligible in most cases.

Additionally an open diff is simpler and lighter than an LSD. It’s probably mostly software thats needed to operate the torque vectoring so it’s not adding any additional weight.

RamblinRover Luxury-Yacht > Nonster
02/09/2017 at 15:23

All this makes sense. If they want to make a rally or drag race version they would have to rethink some of this, but as is, not so much.

HammerheadFistpunch > Satoshi "Zipang" Katsura
02/09/2017 at 15:26

Shawd/vtm4 is not designed to handle 100% torque but the idea of a diff overdrive has been used successfully on lots of high power cars

You can tell a Finn but you can't tell him much > Jake - Has Bad Luck So You Don't Have To
02/09/2017 at 15:46

I have no idea, but if you find the source I’d love to see it. Found it through GIS. I think I searched for “stuck offroading” or something like that and filtered for GIF’s.