This may seem like a very ignorant question, but what does "limited slip" mean? After much investigation, I have determined that the rear axle on my 1988 F250 is a 3.55 limited slip...any help here would be appreciated.

 

I don't know the technical explanation but basically the internal mechanism allows one wheel to turn so far before locking the differential and providing power to both wheels. This allows you to make turns without wheel hop but if one wheel starts spinning (on snow, ice, mud, etc), the differential locks and acts like a posi. A good thing on a rear differential (driving on pavement) but most people I talk to don't like it on the front.

 

 

Limited slip does not lock the wheels together, it uses clutch discs under spring pressure to try to keep both wheels turning the same under fairly equal traction conditions. If one wheel has considerably less traction, the clutches do slip and cannot provide power to the wheel with more traction. For example, one wheel on grass and one on pavement, a LS axle will still allow the tire in grass to spin while the tire with traction will not get power. Both tires on pavement will result in equal power to both wheels.

A locker in contrast will allow equal power no matter the conditions but has quirks that can make it harder to live with as a daily driver.

 

Technically correct information there and a good explanation of the function. However the version of the Ford Trac-Loc limited slip found in 1/2 ton trucks can be adjusted to provide near locker performance pretty easily, it just involves restacking the clutch packs and shimming them for more preload on the spring and spider gears.

 

True, I have done that on Mustangs. Still will slip under extreme traction difference.

 

There are two types of limited slips: clutch-type and gear-type. The gear-types (TrueTrac and Torsen are the only two I believe) aren't factory on trucks of this vintage, so I'll only speak to the clutch-types (which are factory installed on many trucks). But first you need to understand how open diffs work, so I'll start there.

An open diff puts equal torque to both tires and freely allows the tires to turn different speeds (one going faster than the ring gear, the other going the same amount slower than the ring gear). This always allows the tires to turn different speeds to let you go around a corner without scuffing a tire and always drives both tires. BUT, since it always sends the same torque to both tires, if it takes very little torque to spin one tire (for instance, if it's on ice), it will only send that same little torque to the tire that has traction. If that's not enough to push the truck ahead, then you sit there with one tire not turning at all and the other spinning at twice the speed of the ring gear. Since power is proportional to torque times speed, a tire that isn't turning is getting zero power (any amount of torque times zero speed equals zero power).

A clutch-type limited slip diff works exactly the same as an open diff except that there is a friction clutch between the two sides, so now it doesn't allow the tires to FREELY turn different speeds. They still can turn different speeds, just not freely. So when you go around a corner it takes some amount of torque to make this clutch slip. Also, if one tire wants to spin it doesn't send equal torque to both sides. Instead the tire with traction will get the same torque as the slipping tire plus the torque it takes to slip the friction clutch.

How well a clutch-type limited slip works is dependent on how tight the clutch is set up. If it's set up very loose (or if it's old and worn out) it doesn't take much torque to slip it and it doesn't give much more torque to the tire with traction, so it works almost the same as an open diff. If it's set up very tight then it helps with traction a lot, but it also takes a lot of torque to slip it when going around corners on pavement, so it wears the tires worse. If it was set up infinitely tight (not actually possible) it would never slip and it would function exactly like a spool.

All of that said, the clutches in your '88 are likely so worn out that it probably works just like an open diff (I know the limited slip in my '97 is that worn out).