I read something that said having a Limited Slip in a shortbed truck makes them want to swap ends too easily. True?

 

Under normal driving condition no.
On ice and snow it's a bit more likely and it requires a lot more attention to conditions,
And slower speeds. But then speed on slick surfaces will get you in trouble whether you've have limted slip or not. I would'nt recomend this setup of short wheel base and limited slip for an inexperienced driver.

Pat

 

I wouldn't recommend a pickup for an inexperienced driver in ice/snow conditions. Best thing to do is get to a parking lot and try couple of panic stops and hard starts. Other than that it's all common sense and caution.

 

As I understand it, an LS rear will engage "the other" tire when the drive wheel begins to lose traction. It seems to me that having traction is better than not, so why would it be a problem in a shortbed? Does it happen so violently that it jars the natural balance of the truck or something?

 

The problem you run into is that when the LS sends power to other wheel on slick surfaces. It has a tendence to cause the vehicle to fish tail. And being a short wheel based vehicle it is more likely to swap ends and do it in a hurry. This mostly depends on how agressive the LS is the amount of throttle pressure your right foot is producing, and the speed you are going.
My Bronco has a factory LS this is a very short wheel base vehicle. With light acceleration the LS has a fairly soft engagement, and you feel a slight bump when it kicks in. But as throttle pressure increases engagement becomes more of a jerk. This is not to much of a concern at low speed, But as speed increases you can find your self pulling vaccum on the seat cover and making them little girly sounds in the blink of an eye. As with any thing it's learning curve that can be overcome with experience.

Pat

 

Forgive me for writing a book, but I think that I'll explain the mechanics of a open diff vs. limited slip/locker/spool and the effect they have on control.

The open differential as we all should know allows the inside tire to spin at a slower speed than the outside tire during a cornering manuever. This is a very good thing. However, when we need more traction, such as in slippery conditions, the open differential does us no favors. This is where traction-aiding devices such as "limited slip" differentials, "locking" differentials, and "spools" come in. These will be reffered to as "TADs" (traction-aiding device)from now on in the text.

An open differential allows independent wheel speed from each tire on the axle. This speed may be faster or slower than the ring gear in different situations. This phenomenon allows for good tire wear, seamless transitions from straight-ahead driving to cornering, and smooth operation. However like stated above, when one tire has traction and the other has less, the tire with less traction is going to spin while the other tire will sit there uselessly. This IS A GOOD THING, believe it or not. The tire that is not spinning is giving us directional control. It's like locking up your front tires in a braking situation...when they're locked, the vehicle will not turn; it continues to go straight.

Think of spinning out with a front wheel drive car. If one front wheel spins, the vehicle will maintain directional control. However, if both tires spin, you cannot turn. The vehicle, no matter how far to the left or right the front wheels are turned, will tend to go straight or slide sideways.

In a rear wheel drive application (such as a pickup truck or RWD car), one rear tire spinning gives you forward movement while the other tire gives directional control. Adding a TAD will no doubt cause the other tire to spin as well, thus taking away from rear-axle directional control.

In a turn with an open diff, the outside tire and inside tire share the driving force to each tire. With a TAD, (especially lockers and spools), the driving force is made primarily by the inside tire. This may not make sense until you think about what's going on. The rear tires on a TAD-equipped axle both want to 'push' at exactly the same speed (ring gear speed). The inside tire wants to push at ring gear speed and the outside tire wants to slow down to ring gear speed...this is not possible to do without tire slippage....which we all know is bad for directional control.

Now, apply this lack of directional control to something such as a SWB or LWB pickup. Think of the distance between the front and rear axles (wheelbase) as a lever. Any given amount of lateral movement on one end of two different length levers is going to produce a different angle on each lever. The fact that the SWB is a shorter 'lever' means that for every foot that the rear axle moves laterally (sideways), the angle of change from the desired direction of travel is going to be greater than a LWB truck. Taken too far, the angle at which the rear of the vehicle is in relation to the direction of travel becomes too much for either the maximum steering angle of the front wheels to overcome or the speed of angle change is to fast for the driver to turn the front wheels to compensate. In simpler terms, the rear of the LWB has to travel laterally a greater distance than the SWB truck to achive the same angle of skid.

On a LWB truck, the speed at which the given angle change occurs is slower due to the farther distance from the center of rotation (front axle centerline). It is therefore less likely for a fishtail on a LWB truck to become out of control than a fishtail on a SWB vehicle.

Hope that this helps less than it confuses.

Cody

 

LMAO Pat! "Pulling vacuum on the seat cover..."

Cleatus, thanks! Great explanation! Everyone who reads this should bump Cleatus' reputation up a notch, (Click the "scales" icon adjacent to his name).

My question then is, how in a shortbed truck, can you safely add traction control devices without exacerbating the fish-tail problem??? Or, is it REALLY that much of a problem to begin with?

 

You know, it's really NOT that big of a deal as far as adding a limited slip or the like. For example, I had a shortbox 76 F150 for 9 years. It wasn't that powerful, nor did it have anything more effective than an open differential, but something about it made it completely undriveable on ice. No matter what, that stupid pickup always tried to find an excuse to aim me straight at the ditch or at the car directly behind me.

I also own a 73 F100 longbox 2wd. That vehicle is AWESOME on the ice (as far as controlability is concerned). I had a spool in it for about 2 years. It was the only vehicle I had at the time that was almost impossible to make fishtail. Now that I'm back to an open diff, it's useless in the snow, but I sure do like keeping a set of rear tires on it longer than 6 months. I am thinking of going back to a spool just because of how well it gets around in the snow.

Here's another thing to consider about locking the rear axle:
My 95 CrewCab Longbox sucks to drive on snowy forest service roads. Not because the rear end likes to come around under acceleration (that's almost impossible due to it's length), but if the road gets too hairy, it's impossible to turn it backing up in reverse. The back tires wanting to turn at the same speed keep the front tires from bringing the front of the pickup around. The rear will "drag" the front tires straight. Ask Ms. cleatus12r about that....talk about having to have your posterior end surgically removed from the seat!!


Also, I put the spool in my g/f's Gran Torino last year.....BIG MISTAKE. Ice and a little bit of throttle at 50 MPH put her right in the ditch (after I put 42 hours of bodywork and paint on it). I blame myself, but she had to get to work and she couldn't even get out of the driveway, so I put the spool in....worked well until then.
Cody


P.S. Thanks for the kind words. Do you have any idea how hard it is to type with major caffiene jitters?

 

I assume by spool you mean a locker, and that's not really a limited slip. Of course the traction on ice is probably low enough that they would behave identically.

 

Nope, I mean spool.....minispool actually. Locks the axleshaft splines together inside the carrier so there is NO DIFFERENCE IN WHEEL SPEED WHATSOEVER. It would take failure of splines, axleshaft, or spool to allow different wheelspeed.

Oh and you're right, a good (tight) limited slip WILL behave about the same as a locker on ice.

Cody