Auto LSD versus Rear Locking Differential

I was reading through the manual for my 2021 Tacoma TRD Offroad, and came across a section explaining the use of the "VSC off" button to engage the Auto LSD. It seems that this Auto LSD system served to electronically brake a spinning wheel such that more power is applied to the wheel with traction in slippery conditions. I might have this backwards though - is this what the VSC does?

How is this any different from what a rear locking differential does? I do not understand why a 4x4 truck would need to have both options - is the only difference that the Auto LSD is for 2WD and 4H modes (normal driving on, say, icy roads), whereas the rear locking differential is only for 4L mode (getting out of a mud hole)?

I'd appreciate any clarification on what the Auto LSD actually does (and how that differs from the normal VSC) and when to use it.

 

Go watch a video on Ford focus ST lap times.

That also has a brake-induced computerized LSD.

It's also why I own a brz with a mechanical LSD, and not a focus ST or RS.

If the point of LSD is to better apply forward motion, applying a friction material results in a negative forward motion until enough forward motion is applied to overcome the net loss.

Is it going to work? Sure. Same as me slapping you if my intent is to get you to chase me around the play ground. But my intent is to play, your intent is to beat my ass senseless.

 

Auto LSD works great in situations where you want to be in 2wd but it’s slippery. I used it in early winter/late spring when intersections would still have icy patches but not enough to want to be in 4hi. It allows the truck to prevent wheel spin and keep decent traction and control in those conditions.

 

A modern electronically controlled LSD slows the spinning tire by the use of the brakes but doesn't lock the two wheels together, they still rotate independently depending on the traction each tire has. A true locking differential locks both wheels/tires together so they rotate at exactly the same speed no matter the traction differential each side has.

 

Vehicle Stability Control (VSC) is a system designed to help maintain control of the vehicle in slippery conditions, it does this by comparing driver input (steering angle) to actual vehicle direction (wheel speeds, yaw, & lateral acceleration). When these don't match it uses individual braking and engine torque management to attempt to correct it.

Traction Control uses the wheel speed sensors and engine torque management to limit wheel spin in slippery conditions.

Auto LSD uses the service brakes to slow a slipping wheel so available torque is transferred to the other wheel that has traction.

The rear locker fully locks the rear end so both rear wheels are forced to turn the same speed for maximum traction.

 

Single hit or 4-way hit.

I accidentally did a 4-way hit of LSD once back in the 70's.
Never been the same.

Ohhhh you're talking Limited Slip Differential function.... sorry I'll go away now.

 

or in other words, the truck pulls a very large "jesus take the wheel" moment, right as the fun is about to start.

 

AKA, Simulated LSD!

 

https://www.tacomaworld.com/threads/3rd-gen-traction-control-modes-explained.530695/

 

When I first got the truck, that ensuing winter I went to a local snow filled parking lot. Driving forward I tried to loose control of the truck. I was very impressed with what VSC was able to do. Never had a vehicle that modern before that could do that.

Now, turning it off for certain situations is where you need to think about it and experiment. Off road, turning off VSC and going to LSD was impressive too. The places the truck would go just in 2WD and LSD enabled was noticeable.

If you just want to have fun and do mindless doughnuts, hard press the VSC button for 5 seconds then everything is turned off.

 

This was posted on this site a while ago, though I cannot remember by whom. Credit goes to that member.

 

If you follow the link I posted spreadsheet is given credit to @BigWhiteTRD

 

Could you elaborate on how the VSC was able to prevent you from losing traction on the snow?

I would have thought you should turn the VSC off and enable Auto LSD in conditions where the wheels would be spinning?

I don’t think I quite understand exactly what the VSC does internally - why can’t it also apply braking to a single wheel for stability control like the LSD is supposed to do?

 

Here's everything on how the systems work.

 

I'd love to talk to a driveline engineer and ask them why they don't make helical LSD lockers, as in lockers that are LSDs when not locked. It's either a cost thing or an engineering one. I'm curious which.

 

Thanks - according to these documents, the Auto LSD does the following:

***

Briefly pressing the VSC OFF switch in normal mode enters TRAC-OFF mode, allowing the Auto LSD to operate. The Auto LSD achieves a function equivalent to that of an LSD system through the use of the TRAC. The Auto LSD applies brake hydraulic pressure to a slipping wheel to reduce the difference in rotation speed between the left and right drive wheels.This causes driving torque to be transmitted to the wheel opposite the slipping wheel.

***

This makes it seem like pressing the VSC OFF switch just allows the Auto LSD to override the TRAC and use it to brake the slipping wheel. The VSC does the following:

***

The VSC system helps prevent the vehicle from slipping sideways as a result of strong front wheel skid or strong rearwheel skid during cornering.

***


It seems that this means that the VSC itself is not incompatible with the Auto LSD working at the same time and that pressing the VSC OFF button only effects TRAC while leaving the VSC on - why do they call the button VSC off then?

More importantly, why wouldn't you always drive with that button pressed? Seems like you would always want to override the TRAC to brake the slipping wheel in all conditions.

 

He said control, not traction. But either way it will do its best to limit excessive wheel spin and side slip which keeps you in the tractive window.

Not exactly. VSC includes standard traction control along with stability control. This is true traction control that uses engine cut and individual wheel braking to limit wheel slip to the reference speed of the vehicle helping maintain traction and control.

AutoLSD mimics a limited slip differential and only uses individual wheel braking (no engine cut) to limit slip across the axle and not relative to vehicle speed like traction control.

It can and does.

 

Auto LSD and traction control in general need you to lose traction before they can react to the situation. If you try to climb a steep hill using them you will instantly realize why an actual locker is often preferred as you will lose forward momentum as the wheel slips before the computer trys to help.

Auto LSD, etc also require a decent amount of throttle to operate. When people break drive line components it is often because the wheels were slipping then suddenly grab with a good deal of throttle being applied. With a locker you can instead take things nice and easy to avoid this situation.

 

The way I understand is that with (normal setting) everything on, the VSC and LSD functions are both active. With a single press of the VSC off button, the VSC functions are disabled, leaving only LSD functions active. Then as mentioned above if you want to do donuts in the snow, long press of the button turns it all off. If I am mistaken please correct me!

Obviously the default is all on because you want those safety systems available if you need it. My understanding is where you would want to turn VSC off is if you are in slippery conditions, say icy roads, and you want the simulated LSD help without the throttle reducing of VSC so forward momentum doesn't get killed. As confusing as it can be, I love that the option is given to the driver to turn it all off if desired.

 

I backed into a grass covered irrigation ditch with my 22 2wd sr5. The rear passenger side wheel was at full droop and probably 3 inches from the nearest traction while resting on the frame. With VSC-off the truck drove out of the ditch like it had a front locker.