Physics Behind Gear vs. Clutch Limited Slip

[LateOnTheBrakes]

I am a mechanical engineer and have read every article on the different styles of limited slip differentials but I am still somewhat confused. I have an '09 and I'm trying to decide which differential is for me. Although I'm sure either style (truetrac/gear/torsen or posi-traction/clutch) would be more than sufficient, I want to understand the difference between them. It is my understanding that the gear style is a torque multiplier meaning if one tire is on ice and able to put down no torque, there will be no torque at the wheel with traction. In the same situation with the clutch style, what happens at the wheel with traction?

For the sake of argument...

If I am at part throttle putting 100 ft*lbs to the differential and I was using a 5:1 gear style differential: If I light up one tire on ice or oil, am I to understand the other tire will receive no torque and coast?

If I am at part throttle putting 100ft*lbs to the differential and have a clutch style differential that requires 40 ft/lbs to overcome the clutch pack: If I light up one tire on ice or oil, does the tire with traction get 40 ft*lbs, 60 ft*lbs of torque, or does the preload on the clutch pack locks the wheels together and both wheels get an even split while one tire can put down the torque and one spins helplessly?

I want the limited slip because I tow my race bikes fairly often, I hate lighting up the inside tire, I do not want to get stuck spinning one wheel if it snows, and I want to be able to kick out the rear of the truck out for quick u-turns. I do not mind replacing the clutches periodically if it is the better route. Do I want the truetrac unit, or do I want the posi-traction unit shimmed for more aggressive preload?

Also, I am definitely not getting a detroit locker, and please refrain from blinding saying posi or truetrac with no supporting rationale.

 

[Gadget]

I know the TruTrac type requires resistance on both wheels for it to work. The advantage of it is that it allows for differential wheel speed while at the same time equally distributing the torque. For example when you are going around a turn and getting on it, both tire get equal torque, but the outside one is turning faster since it is covering a greater distance. In the same condition the clutch type will tighten up and try to make both tires turn the same speed and can cause you to spin out easier and make the inside tire squeak like crazy.

There are several different clutch types out there and the way they work is a little different. The one type I know more about works off of differential wheel speed. They try to set those up so that when you are in a tight turn the tires turn different speeds since they are traveling different distances. Now this differential rotation is the set point of the LSD unit. If the differential of the wheel speeds exceeds that point a ratchet engages and tightens the clutch pack and as it tightens it will try to make the wheels turn the same speed. I am not totally sure but I think this type still requires resistance on both tires for it to work right.

I drove a government car for a while with this type LSD and I hated it. It caused the rear end to break and spin around way to easy without warning. Now in the snow with chains on it it was absolutely unstoppable. It had traction like a bulldozer.

There are other types of clutch types that keep tension on the clutches all the time so they are always kind of half on and somehow get tighter when the wheel speeds differ.

Lockers are different. Most will engage suddenly and solidly lock the wheels so they have to turn the same speed. These are good for off road use and can be scary on road. The sudden lock up can make steering hard some times. These do not require both wheels to be on the ground or have resistance. They are very handy when you are lifting one wheel off the ground and the one on the ground and push you out. I do not like auto lockers at all. On my off road truck I use ARB air lockers and can instantaneous engage and disengage with the flip of a switch.

My favorite for on road use is the gear type LSD. I think they have the best overall road manors and are very smooth in operation and are less likely to put you in a spin unexpectedly. I also think they are faster around turns since both tires are getting equal torque but can turn at different speeds so you are planting the most power you can with both tires.

G

 

[Super Werty]

Wow, good info right there. This should be a sticky:top:

 

[WCD]

It would be hard to measure the torque split with the clutch LSD that we have. You want some slippage around corners, but also for both sides to receive ample torque when you step on it. It might be 60/40 or 70/30, but I'm not sure. And that number will change as the clutch pack wears down.

A helical gear LSD will have a tough time if one side has road contact but the other side doesn't (like one wheel in snow and the other on the road). But if you have chains then that won't be a problem.

The ratcheting clutch diff Gadget mentioned sounds like the GM Gov-Lock. They don't make one for Toyota and I'm glad for that. The Gov-Lock has a centrifugal weight that "locks" the other wheel after you reach a certain speed. That right there is the problem with it. Imagine sending all your power to one wheel as you take off, and then once you hit about 10-15 mph, that weight spins up and abruptly locks the other side in. The G/L has so many small parts that they can explode when this happens, and when they do they take out everything else in the diff.

My vote is a Detroit True Trac. As long as you have chains for when it snows you won't have a problem.

 

[LateOnTheBrakes]

It seems the truetrac would be the best for accelerating out of turns without the fear of the rear coming out unexpectedly as opposed to the clutch style that fight wheel slip more with more throttle. The truetrac sounds like the preferred route to go, except when I want to spin up both tires together.

Gadget - With the truetrac installed how does it act if you launch in a straight line hard enough to break a tire loose? How about if you crank the steering wheel hard to one side, rev it up, and dump the clutch? Does it act like an open differential?

 

[Gadget]

No it acts like an LSD and you will be spinning both tires equally.

There is a vid somewhere of me doing some doughnuts. Both tires where smoking pretty well.

G

 

[BlackCat]

... my head hurts ! ...

 

[darthyota]

and here i thought that the true trac would still provide some go if say one tire had no traction but not to the same extent that a full locker or a spool would

 

[XRunnerDevil07]

I like where this thread is going..:top: Completely informative but I understand it too!

 

[LateOnTheBrakes]

The way I understand it, the truetrac is a torque multiplier. If you light one tire up, but the spinning tire is able to still transmit 20 ft*lbs and for argument sake you have a 5:1 ratio multiplier, you could in theory transmit up to 100 ft*lbs to the other wheel. However if you are on ice or one wheel is off the ground you would transmit none to the other wheel as anything times zero is still zero. You could overcome that by applying the brake and it would transmit power to the wheel with traction though.

 

[ntinhri]

Just don't weld the diff! If I had the $$$ I'd have a tru trac! Great for auto cross, road coarses, drag strip, and the street! Ive seen gadgets truck in action. He is able to plant power when the rest of us slip, slide, and spin.

 

[XRunnerDevil07]

The way I understand it, the truetrac is a torque multiplier. If you light one tire up, but the spinning tire is able to still transmit 20 ft*lbs and for argument sake you have a 5:1 ratio multiplier, you could in theory transmit up to 100 ft*lbs to the other wheel. However if you are on ice or one wheel is off the ground you would transmit none to the other wheel as anything times zero is still zero. You could overcome that by applying the brake and it would transmit power to the wheel with traction though.

Makes sense to me yes in theory on that but my question is, if you apply the brake wouldn't it cause the vehicle to "overshoot"the amount of torque expected? That part i'm lost on.

 

[LateOnTheBrakes]

What do you mean by "overshoot" the amount of torque expected?

 

[LateOnTheBrakes]

[FONT=&quot]After researching as best I can I believe I have compiled what I believe to be the pros and cons of each style differential. If I missed something please let me know.[/FONT]

[FONT=&quot][/FONT]
[FONT=&quot]Truetrac[/FONT]

[FONT=&quot][/FONT]
[FONT=&quot]Pros[/FONT]
[FONT=&quot]-[FONT=&quot] [/FONT][/FONT][FONT=&quot]No wear items to replace[/FONT]
[FONT=&quot]-[FONT=&quot] [/FONT][/FONT][FONT=&quot]Seamless, quiet operation[/FONT]
[FONT=&quot]-[FONT=&quot] [/FONT][/FONT][FONT=&quot]Transmits more torque to wheel with traction[/FONT]
[FONT=&quot]-[FONT=&quot] [/FONT][/FONT][FONT=&quot]Good street and track manners[/FONT]
[FONT=&quot]Cons[/FONT]
[FONT=&quot]-[FONT=&quot] [/FONT][/FONT][FONT=&quot]Behaves similar to open differential in snow, ice, oil, and other low traction situations[/FONT]
[FONT=&quot]-[FONT=&quot] [/FONT][/FONT][FONT=&quot]More expensive[/FONT]
[FONT=&quot]-[FONT=&quot] [/FONT][/FONT][FONT=&quot]Non-serviceable[/FONT]
[FONT=&quot] [/FONT]
[FONT=&quot]Posi-traction[/FONT]

[FONT=&quot][/FONT]
[FONT=&quot]Pros[/FONT]
[FONT=&quot]-[FONT=&quot] [/FONT][/FONT][FONT=&quot]TRD backed unit available, warranted, and can be serviced by toyota[/FONT]
[FONT=&quot]-[FONT=&quot] [/FONT][/FONT][FONT=&quot]Can be shimmed for more aggressive engagement[/FONT]
[FONT=&quot]-[FONT=&quot] [/FONT][/FONT][FONT=&quot]Provides torque to wheel with traction when one wheel spins on ice/oil or is off the ground[/FONT]
[FONT=&quot]-[FONT=&quot] [/FONT][/FONT][FONT=&quot]Less expensive[/FONT]
[FONT=&quot]-[FONT=&quot] [/FONT][/FONT][FONT=&quot]More likely to fishtail during cornering on the throttle[/FONT]

[FONT=&quot][/FONT]
[FONT=&quot]Cons[/FONT]
[FONT=&quot]-[FONT=&quot] [/FONT][/FONT][FONT=&quot]Clutches may wear and require replacement[/FONT]
[FONT=&quot]-[FONT=&quot] [/FONT][/FONT][FONT=&quot]Clutches slip/wear when outside wheel travels faster than inside wheel (cornering)[/FONT]
[FONT=&quot]-[FONT=&quot] [/FONT][/FONT][FONT=&quot]Noisy operation[/FONT]
[FONT=&quot]-[FONT=&quot] [/FONT][/FONT][FONT=&quot]Puts extra stress on drivetrain in tight turns (u-turns)[/FONT]
[FONT=&quot]-[FONT=&quot] [/FONT][/FONT][FONT=&quot]More likely to fishtail while cornering on the throttle[/FONT]
[FONT=&quot]*Note – I placed the tendency to kick the rear-end out as both a pro and con because I believe that is a matter of personal preference.[/FONT]

 

[revobreaker]

Wow thats some good reading.

Would someone car to explain how the 09 rear works, in the different "modes" that you can select with the traction control button? I know its based on the brakes, but I dont have a full understanding.

And how will 09 react with a real LSD in there or a tru track, will the computer freak out and start pulling brakes or anything?

 

[LateOnTheBrakes]

For the '09 you have three modes, default when you turn the truck on (no lights on dash), Auto LSD, and Off. You always have an open differential.

Default Mode: is an open differential, traction control will utilize the rear wheel speed (ABS) sensors to sense slip, the brakes will kick in to stop the slip. Vehicle Stability Control (VCS) is active so if you try to fishtail the truck, it will utilize all four wheel speed sensors and brake individual wheels as necessary to straighten the truck out.

Auto LSD: traction control is disabled in a sense, and the vehicle stability control is off until you go 30mph or faster. When one of the rear wheel speed sensors recognizes one wheel is spinning faster than the other, it applies the brake to the spinning wheel. By doing this it tries to let the other wheel to catch up until both wheels are spinning the same speed. This is supposed to simulate the effect of a limited slip differential.

Off: traction control and vehicle stability control are completely disabled at all speeds. If you spin one tire if will spin uncontrollably (open differential). If you fishtail it will not assist you in straightening out the vehicle.

You may want to review the thread I created on to clarify the traction control, auto lsd, and stability management as well.

http://www.xr-underground.com/vb3/showthread.php?t=29273

I also wondered what would happen if you installed a mechanical differential on an '09. I was afraid the electronics would fight the mechanical linkage. If I put a mechanical limited slip on my truck I will be turning everything off every time I get in the truck unless it is raining or another limited traction situation. If that is the case I would take it easy trying not to spin a tire and utilizing the vehicle stability control to keep me from killing myself. My thread on the electronics fighting the mechanical linkage is as follows:

http://www.xr-underground.com/vb3/showthread.php?t=30881

 

[X-roller]

Ok heres the deal....I understand what you are saying and geared is the route you would wanna go. Seems like what you are looking for is more of a "torque-sensitive" differential and well if one tire does break loose or lifts up off the ground the then diff will just act like an open diff. Depending the clutch type LSD whether it is 1 - way, 1.5 way, or 2 - way LSD. You may get the results you are looking for but mainly if you are really looking for straight line go geared. Drift cars like to use a more aggressive 2-way LSD differential because it is a lot easier to break them loose that way and keep them loose. A 1 - way LSD is strongly throttle dependent and will give you more control once you let the throttle go. Basically if you are lighting up the tires and let go of the throttle you will practically gain traction a lot quicker than a more aggressive 2-way LSD which mainly keeps the tires spinning until both your tires start to slow down. Only issue with Clutch-type LSD is just that...clutches are worn out a lot quicker and have to be replaced periodically depending on how hard you drive on them. Hope that helps.

 

[revobreaker]

Yeah I have read those, it makes more sense and is easier to read/follow when its laid out like you just did.

Thanks

 

[LateOnTheBrakes]

The truetrac sounds pretty interesting. But the clutch style is the supported format and it's the only type I've ever driven. And I put a ton of miles on my previous vehicles without ever having to replace the clutches. I am don't do autocross, I do not drag race. I just want the truck to stop lighting up the inside tire through corners, and stop spinning one tire if I leave a light aggressively. I've also been towing up hill in the rain when a tire spins, the traction control kicks in, and completely ruined my forward momentum.

 

[X-roller]

Trutrac is your diff. :top: 1-way LSD can act like a geared one but a trutrac will honestly just last longer. Trutrac takes more wear on the gear components than it would on the clutches....clutch-type is vice versa.