Traction control+Limited-slip differential?

[UNC017]

Originally Posted By: QuadDriver
( I got news for ya, the LSD does not 'send power to the wheel with most traction', it sends power to BOTH wheels, and the wheel with traction can do something about it)


That's a good point. The other mis-quote I often hear is that "Limited Slip sends the same torque to both wheels." Wrong. An OPEN differential sends exactly the same torque to both wheels- and unfortunately that torque is the torque it takes to make the first wheel slip. Limited slip actually sends MORE torque to the wheel with traction than to the one that's slipping.

Think of it this way: your right wheel is on ice, and will slip when the torque on the right axle hits 20 foot-pounds. Your left wheel is on dry pavement, and won't slip until the torque reaches 150 foot-pounds. OK, with an open differential, the SAME torque goes to both rear wheels, and the right one slips at 20 ft-lbs, so the left wheel (the one with traction) also only gets 20 ft-lbs, and the car doesn't move.

Now put in a limited slip differential with a clutch pack that slips at 100 foot-lbs torque differential between the two wheels. Apply torque slowly. At 20 ft-lb, the right wheel starts slipping. But the clutches in the LSD keep transferring 100 foot-lb MORE torque than the slipping wheel to the left wheel, so you can get up to 120 foot-lb to the wheel with traction and the car moves.

All the numbers above are just for the thought experiment, not real torque numbers.

 

[The_Jeff]

My 2000 buick century has some new for its year ABS/trac system. The previous system was the ABS-VI that was in practically every GM car from the 90s. They were gloating that this new one can actually apply brakes, something ABS-VI couldn't pull off. First gen traction control used to just cut ignition timing; throttle by wire adds a nicer tool for that.

It's pretty smooth and not too obtrusive. On snow having one wheel grab then the other doesn't make the car dart/ torque steer objectively, or any more than I'm used to with open diffs.

I can only disable it with manual 2nd/1st gear, but that's not too much of a problem.

 

[unDummy]

Most stock limited slips are over rated. Many of them are useless without enough "slip limiting" because of the weak preload. LSD should not be confused with real torque biasing(quaife/truetrac/torsen), lockers(whether electric, mechanical, hydraulic, or air), and viscous couplings.

Traction control is also limited to the programming and braking power. I wish that TC came with a GAIN knob instead of an on/off button, so that you can power yourself out of a sticky situation while still sending some of the power to the wheel with most traction.

LSD doesn't increase the torque to the wheel with most grip. LSD doesn't send more torque to the tire that is spinning. You do both by stepping on the gas pedal some more.

LSD's are actually 'locked' up to the pre-load amount sending equal power to both wheels.. Once you exceed that 'preload' amount, it slips away. This preload is the grip of the clutch frictions, assisted by the springs squeezing them, and crippled by your LSD additive dose. Once you exceed that preload, that is it. There is no fancy power movement to the tire with traction.

A simple example is my Tundra, with factory LSD. At very light throttle, with 1 tire on ice and other on dry pavement, I can pull away without spinning the tire on ice. The LSD is LOCKED because I haven't exceeded its lb/ft torque static friction rating. If I add more power, exceed its rating, the tire on ice spins. I still accelerate away using the tire on dry pavement, but I'm now relying on kinetic friction(which is a lower torque rating) of clutches + LSD additive. All power exceeding the LSD's limit goes to the tire on ice. Like threshold braking, an LSD is pretty weak so I make every attempt at thresh-hold acceleration to maximize the LSD's preload to reduce wheel spin. An aftermarket diff, for my vehicle, has 3x the pre-load with a considerable higher torque rating in both static/kinetic situations which basically means that I can use 3x the gas pedal compared to the stock LSD.

Once an LSD slips, its capability drops. So, if you add traction control, it will brake the tire on ice(up to the brake shoe/pad limit). Power has to go somewhere and your tire on pavement sees more. If you have way too much wheel spin, the TC will then castrate engine output. With an open diff, the TC reacts more quickly since there is no LSD. If it is programmed well, you should do fine. Most aren't and is why you will benefit from the addition of an LSD with the TC.

Stability control, to be most effective, needs all wheels to be independent. You can't "tank steer" a vehicle if the diff's are locked.

BTW, an open diff is an LSD with gear friction as the LS device. You can use super thick 250wt gear oil and get a hint of "slip limiting", maybe a couple ft/lbs which may or may not get you moving. The thicker you go, until you use an almost solid fluid, the more preload that open diff has.

A torsen diff has torque limitations and preload too. This is why you sometimes have to tap the brake pedal to assist it. In theory, it can possibly confuse traction control. The TCS brakes the wheel on ice. The torsen sees that TCS action as traction and then sends more power to the iced wheel compounding the slip issue. But, lucky for you, your brakes and torsen have limitations usually far below your engine's output, and each others capability.

The easiest way to explain a viscous diff is to call it a torque converter with a really really low stall speed. Its weakness is the fluid shearing or whipping into foam, which increases the stall speed, rendering it useless. Should be safe with traction control.

A locker is great. But, now your 'tire traction level' becomes the limiter, and the only fuse is how good your foot is on the gas pedal.

 

[SteveSRT8]

That's why the factory locker is so nice in my Silverado! No surprises, dead predictable every time.

Quaiffe is a nice unit but many owners with my car type have blown them to kingdom come and NO WARRANTY due to the halfshafts breaking. I hate weasel words in a warranty!

Newer Cars with the Getrag LSD units are experiencing preload issues and clutch failures.

Too many people here are making broad assumptions about systems that actually vary quite a bit by mfgr.

 

[rpn453]

Originally Posted By: SteveSRT8
Too many people here are making broad assumptions about systems that actually vary quite a bit by mfgr.


Yeah, I'm guilty of that. I didn't know that most factory LSDs are as simple as the system unDummy described.

Good explanation of a lot of details, unDummy.

 

[Jim_Beverlin]

I have a LSD on my Ford Explorer Sport Trac and traction control on a Jeep Liberty. In snow and in 2WD the Ford just keeps trudging along in a straight path. The Jeep can become a handful moving from side to side. Particularly in heavy wet snow. You usually end up turning the TC off and engaging 4WD. This winter in central MI we have had more snow than usual. With the Ford having a LSD and Nokian winter tires I have used 4WD about 50% less than I normally would. Would not have pickup without a LSD.

 

[rslifkin]

^ I agree, RWD / 4WD with an LSD (at least in the rear with 4WD, front is nice too though) is the only way to do it in the snow. It's a little easier to get sideways around a corner, but it's so much better getting moving on patchy stuff, particularly in intersections.

 

[rshunter]

Originally Posted By: rslifkin
It's a little easier to get sideways around a corner...

...and that's the only way to do it!

 

[Craig in Canada]

Traction control's primary purpose is to eliminate wheel spin even if that means it completely cuts off your power delivery. More complex stability control systems start to intervene for over/under steer too - generally using the BRAKES. An LSD main purpose is to get power to the ground. One problem with LSD cars is that when you do break the powered axle free you've got nothing left. If you induce some power oversteer in a RWD LSD equipped car the rear end is going to come around on you because all the rear grip is "used up". In an open diff car you'll start to spin the inside rear but the outside rear still has plenty of grip left so the car doesn't spin. LSD+stability control should save your butt from power oversteer but it can't rewrite the laws of physics.

I had a Porsche 968 (front engine, rear wheel drive) with a clutch-type LSD and no traction control. The LSD was the single greatest feature of the car, in a way. Even in driving rain I could really honk out of corners etc... with a little bit of clunking from the inside rear starting to spin but then the LSD stopping it. My open diff RWD BMW with traction control and similar capability tires would just start to blink the traction light and cut power in the same situation. Of course, if you pushed it too far past the threshold on the LSD car you'd be drifting since both rears would light up. If you're trained and ready for it - fine - but if it catches you off guard it wouldn't be fun.

 

[rslifkin]

^ That's exactly how my Jeep behaves in 2wd. It requires knowing how to drive it, but once you get used to that, it's no big deal at all, and the ability to rip on the throttle out of corners and such is nice.