Keeping on the straight and narrow - Open diff vs LSD

If I kick out the tail in my Z4 in a corner it's not that hard to stay on the throttle and ride it out until it gets straight again. The Z4 has an open differential so I'm just curious if control is significantly different - harder or easier - with limited slip. The only other RWD cars in which I've had much seat time were open diff 2nd and 3rd gen Camaros and it was the same story -- fairly easy to keep them pointed in generally the right direction.

Or might the ease of control be from it not having a ton of torque?

It's not any harder to maintain control in a slide with an LSD or locker - in fact it might be easier. Drifters run something between a ridiculously tight clutch-type LSD and a spool. I think balance and suspension design make the most difference in the difficulty of holding a slide - the easiest car to slide I've driven was a Datsun rally car with a welded diff. The hardest was a Radical SR3 which hated being sideways almost as much as an F1 car.

I'd have to disagree.  How to say this?  The one wheel dragging diff helps maintain a modicum of grip the LSD doesn't.  The first time you drive a car with an LSD in the wet after having an open diff you'll really appreciate an open diff.  I went off in a ditch my first time. 

Also the limited slip promotes understeer.  That's why Lotus didn't put them in the original Elise.  But all the homies said "But I gots to have an LSD cause it's much cooler" so they finally added it.  A friend of mine spent lots of good money to add th limited slip only to take it out the following year because he was slower than the other Elises on an autocross course.  I don't know if it would have as noticeable an effect on a road course because he never did it.

While I can agree with your comments, I can’t agree with some of it. An open diff is great for keeping in control, when the vehicle is in control. When the vehicle is out of control, AND YOU KNOW HOW TO CONTROL A SLIDE, an LSD is a godsend. I can’t tell you how many cars I’ve SAVED having a way to control them when I get stupid. 

Now an open diff is not necessarily a bad thing, autocross, race track, etc. but let’s think this through- those at the front of the pack in any Motorsport, have an lsd. Not a spool, not an open diff, not some electronic diff garbage. A clutched, or mechanical, LSD. 

A mid rear engine vehicle would probably respond much differently to an LSD than a front engine RWD would.

It depends on if you're experienced with inducing, maintaining and manipulating oversteer or not. I had a mini-spool for a while in the Crown Vic and it was super easy and fun to play with. Meanwhile the vLSD in the G35, while safe and stable, isn't nearly as fun because it never actually locks.

carguy123 said:

I'd have to disagree.  How to say this?  The one wheel dragging diff helps maintain a modicum of grip the LSD doesn't.  The first time you drive a car with an LSD in the wet after having an open diff you'll really appreciate an open diff.  I went off in a ditch my first time. 

While BMW has not offered it since 1996. My old 318ti had the factory "winter package" LSD. Weaker than the ones in the M-Cars, it none the less made that car less of a handful in bad conditions. It actually made driving in the snow enjoyable.

In reply to carguy123 :

Why would an LSD induce understeer? It should be the opposite, no? At least it should cause power-on oversteer. Boot it through a corner with an open diff and all you do is light up the inside tire. With an LSD you either go or the rear washes out.

The extra traction propelling the car forward (from the LSD) can overwhelm the front tires if they're already at their limit, causing understeer.

I've found that an LSD can tend to grab harder when it gets traction, where an open differential feels smoother when you lose traction.  For a less experienced driver, the open diff may be easier to modulate in a slide (I'm assuming RWD here).

If you don't know how to drive a RWD vehicle, I'm sure an open diff does feel safer. But for the rest of us, that extra wheel pushing is not only wanted but needed. The wife got her truck stuck in the middle of our road one winter. On smooth ice. At the time it had an open diff, and it spun the left rear and made an instant divot in the icepack. Granted, her ham fisting the throttle didn't help, but that truck hasn't been stuck since we installed the LSD. IT is soooooo much better to drive now. You can actually pull out into traffic in the rain. You can get moving in ice/snow with ease. And, honestly, who doesn't enjoy hooning a 20' long truck in parking lots? 

The purpose of differential is to allow the inside and outside tires to travel at different speeds when going around a corner. The inside tire has a shorter distance to travel than the outside tire.

True limited slips attempt to equalize the two tire's rotation. The effect will be different based on how tight the diff is set up. If no other changes are made to the vehicle, other than the LSD being installed, and driven in the exact same manor, the car will understeer. 

Plus, burnouts are now equally harsh on both tires. 

An LSD does make it easier to get the tail out under power in low traction situations (instead of just spinning 1 tire with an open diff), but on the flip side of that coin, the tail is easier to control while it's sliding around with the LSD.  

Having driven the same RWD vehicle in the snow with a totally shot LSD that acted like an open diff and with a working LSD, I'll take the LSD every time.  

Appleseed said:

The purpose of differential is to allow the inside and outside tires to travel at different speeds when going around a corner. The inside tire has a shorter distance to travel than the outside tire.

True limited slips attempt to equalize the two tire's rotation. The effect will be different based on how tight the diff is set up. If no other changes are made to the vehicle, other than the LSD being installed, and driven in the exact same manor, the car will understeer. 

Plus, burnouts are now equally harsh on both tires. 

This. That's why I think the best differential setup for racing is a helical LSD or a lightly set 1-way clutch-type LSD. You want just enough lockup to keep 1-wheel-peeling in check, and only when you're on the gas.

Lockup on coast or decel is simply bad for handling - your differential is resisting the car's rotation for no reason. The closest thing to a reason might be an attempt to mitigate a suspension problem with a drivetrain solution.

Toebra said:

A mid rear engine vehicle would probably respond much differently to an LSD than a front engine RWD would.

Yep...with an open diff in MR, you're probably boned when you are in too deep and the arse kicks out.  With an LSD, you're also probably boned, but at a much higher rate of speed.  

That said, an MR car has more inherent grip on corner exit before then need for an LSD creeps in.

My MR or RR experience is with MR2s and Porsches.  I'd take an LSD every time, but the need isn't nearly as pronounced as in an FR car. 

And LSD definitely induces understeer on the gas or trail-braking, depending on how it's set up.   If we're just talking about hanging the arse out / hooning, I find that open diffs suck equally for both putting down power and keeping a power slide under control.  The only good thing there is the ability to induce power-on oversteer at lower speeds or HP.

My mid rear experience is in a 914, so this power on oversteer thing is not something familiar to me, even on tall, 165 tires.

Yeah, it's really easy to control open diff sliding because only one rear wheel is spinning. The other is rolling along the road, providing a very stable side to side force on the rear of your car. You might even find it difficult to kick the rear out with an open diff, because if one wheel is already turning faster than the other all the power of your clutch kick will just go to that one wheel.

If you are skidding (like locked handbrake) it's a different story but an LSD has no effect on that.

When you initiate power oversteer, the vehicle with a limited slip will throw the back out farther and faster. It is easier to lose control with an LSD, assuming you have the power to start the slide in the first place. Since you have more traction, the threshold is higher, but once you're past it the back end steps out fast and hard compared to open diff. I still think it's a good thing for enthusiasts. But, considering how powerful modern pickup trucks are (for example) it's probably a good thing they have traction and stability control built in. Modern power levels combined with limited slips and a lack of nannies would have a lot of people putting these trucks in ditches upside down. 

How does an LSD perform on an underpowered RWD car? Should be perfect, no? Adds some traction out of the corners but there's still not enough power to get in trouble so I wouldn't get any of the bad effects? Sorry to hijack, just thinking about my 3250lbs, 85rwhp car.

In reply to Vigo :

I disagree. It takes some serious negligence to get a 20' long vehicle into a ditch. 

EDIT: never mind... I just reread my statement and I remember the common driver. You are correct. 

buzzboy said:

How does an LSD perform on an underpowered RWD car? Should be perfect, no? Adds some traction out of the corners but there's still not enough power to get in trouble so I wouldn't get any of the bad effects? Sorry to hijack, just thinking about my 3250lbs, 85rwhp car.

I loved it on my 100whp 3700lb truck! 

Yeah, people manage to put pickups in the ditch in the snow all the time.  Although in theory, they should be one of the easiest vehicles to control the rear end on when sliding, as the rear end is fairly light and they've got a long wheelbase.  

In reply to rslifkin :

oh yeah.... it's like drifting but in slow motion. 

Autocrossing and on track, a proper LSD is always going to be faster.  You simply get more grip on corner exit and the speed at the end of the straight will be faster.  You do need to possibly alter your throttle pickup points and braking, but no way an open diff is as good.  Mid engine cars are a bit different in that they have more weight over the drive wheels and do not benefit as much until hp overrides tire grip.  Hoelscher's DSP championship winning X1/9 was incredible without one, but my MR2 Turbo needed one desperately for instance. FWD cars are different in that they transfer weight off the drive wheels under acceleration and need all the help they can get.   My non LSD Cooper S for instance will spin its tires through two gears, and any throttle change while cornering simply lights up the front tires.  It loses a lot of time simply trying to get grip.  

How does an LSD perform on an underpowered RWD car? Should be perfect, no? Adds some traction out of the corners but there's still not enough power to get in trouble so I wouldn't get any of the bad effects? Sorry to hijack, just thinking about my 3250lbs, 85rwhp car.

I really don't think it will do much of anything for you. Even my FWD cars with that low of power wouldn't benefit much.  Limited slips are generally fairly expensive mods and if you don't have the power to spin the tires often and severely i think that money would usually be better spent on other improvements.  

Vigo said:

How does an LSD perform on an underpowered RWD car? Should be perfect, no? Adds some traction out of the corners but there's still not enough power to get in trouble so I wouldn't get any of the bad effects? Sorry to hijack, just thinking about my 3250lbs, 85rwhp car.

I really don't think it will do much of anything for you. Even my FWD cars with that low of power wouldn't benefit much.  Limited slips are generally fairly expensive mods and if you don't have the power to spin the tires often and severely i think that money would usually be better spent on other improvements.  

I doubt it would help you too much. I know in my old 318ti, with all of 140hp at the crank, I could feel the LSD work it's magic on most acceleration, if I was coming out of a corner and got on it, you could feel the rear end squat and "dig in" as it accelerated