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-   -   Pro's and Con's of Positraction? (http://www.hotrodders.com/forum/pros-cons-positraction-47464.html)

Malibu73 09-10-2004 12:19 AM

Pro's and Con's of Positraction?
 
Hey guys, I've always wondered what the pros and cons were for positraction, other than making both your rear wheels spin making it look cooler when you burn rubber, what other advantages does it have?

dh79 09-10-2004 01:09 AM

To put it simply: the whole point of positraction is to apply the power evenly between the two wheels during acceleration. Without it, there is the tendancy to have all the power go to only one wheel. This one wheel will spin and the car will not go anywhere fast. If the power is divided between the two wheels, there is twice as much traction so the car will accelerate faster.

Are there any disadvantages? Not really. A posi unit costs more than an open unit. Also, since the posi tries to keep the rear wheels locked together, this may make the car susceptable to fishtailing in the rain/snow. But that is not really an issue. (in fact some of us think it's fun). All race cars have posi or another more aggressive system which performs the same task (spool, locker).

k2mooch 09-10-2004 12:20 PM

I was wondring about the same thing about normal driving performance. Never thought about the clutches keeping them locked enough to fishtail in poor traction. AWESOME!!!!!! :thumbup:

I can't wait.






MoocH

Ghetto Jet 09-10-2004 05:59 PM

Well driving in the snow can be difficult with a posi, and if that rear ever goes while your moving down the road hang on.

flyfisher 09-10-2004 06:26 PM

I love posi in the snow. I have a jeep without posi and a jimmy with posi. I drove around for a month not knowing 4WD wasn't engaged in the jimmy and didn't really notice it because when the posi kicked in I just drove. The Jeep doesn't get half the traction of the Jimmy, and I beleive it's mainly because of the posi. I guess it's time for an air locker on the Jeep.

A couple of down falls of posi are the cost of repair, the greater chance of things breaking (this hasn't happened to me in the past 465 000 kms. change the oil and add a little friction modifier every 80 000 kms, and that's all the maintenance it gets). There are a few posi systems (I think three? ratchet, solid, and clutch or limited slip) and every one has it's application.

I heard a story about Mustangs with posi's going off the road because they would mash the pedal making a left hand turn, the posi would kick in and although the front wheels were turned the car would go straight. Makes sense, since the torque from the rear wheels would lift the car enough to cancel the friction on the front tires for steering, and posi's want to travel in a straight line, but I haven't seen it myself, so take it with a grain of salt. Another thing about posi's is that you shouldn't be making turns when they're engaged. They are locked so they want to travel straight forward any turning would stress the components. Busted the posi on the tractor this way.

BOBCRMAN@aol.com 09-10-2004 06:54 PM

In a nose heavy car on icy streets, a posi will pull you to the lowest part of the street. You will follow the curb and gutter till you get to an intersection then after the intersecyion its back to the gutter again. The rear slides to the lowest point or till there is traction enuff to push the front wheels. One morning in January many years ago I experienced this strange happening. Three day old big block Nova with factory tires, on city streets, in a Michigan ice storm. Walking would have been faster.

poncho62 09-10-2004 06:58 PM

A "posi" slides around a bit more in wet weather...............other than that, no complaints .

trees 09-10-2004 07:14 PM

Try driving a "all wheel drive" on a slick side hill and enjoy the slide to the bottom. A one legger will walk right along. A trade off is to have posi on the rear only and then you can get srtaight up and down if you have maneuvering room. Basically, pos on two axles has its place in flat sand, snow, ice and mud, but get in the mountains/hills and you may wish for some one leggers. My 36 pickup with lots of ponys under the hood and a posi under a feather weight rear end demands a very light foot in the rain and just a 90 degree turn at a street corner can wake me up quick if I am not paying attention!!

Malibu73 09-10-2004 08:19 PM

Well, I was wondering what the benefits and disadvantages would be in a street car like my chevy malibu. I would be interested in putting it only on the rear wheels.

I'm not so clear on how the standard wheel drive works either. Is the car driven by only the right rear tire? I'm a little confused because up at my friends house we were burning out in his pontiac Lemans SS and the right rear tire would make all the smoke and be spinning like crazy and the left rear wouldn't.

But when we were in the gravel and he did a quick tap on the gas, the left tire spun too. :confused:

dmc12mk3 09-10-2004 10:13 PM

howstuffworks.com

no kidding, its a nifty resource.

Differentials are needed because it makes sense to have at least two drive wheels. The trouble comes in when you have to apply power to both wheels, but they have to spin at different speeds (when you go around a curve, the inside wheel does not turn as much as the outside). You could lock the two wheels together where they always spin at the same speed, but that makes it hard to turn as one of your wheels will not be rolling along the pavement very well.

Most cars have an open differential. This does not mean that only one tire drives the car. What this means is that as long as the resistance to rotate is the same for each tire, both tires will push the car equally. This is how a car with an open diff can spin two tires when there is very little resistance (wet pavement, gravel, etc..). Usually its not this way though as one tire generally gets loaded more than the other so the one without the load lets loose and keeps getting more and more power applied to it. As soon as one tire gets less resistance to spin than the other - the one with the less resistance starts getting more power applied to it. In a turn, the inside tire has the least resistance to spin. Open diffs are very noticeable here too as the inside tire will spin if you power on too quickly.

A limited slip differential is just what it says. It will limit the slip between the two tires. There are several ways to do this. The most common for imports is the Helical Gear type LSD. This is usually referred to as its marketed name TorSen (Torque Sensing). These LSD's require no rebuild and last the entire life of the car always with the ability to transfer roughly 40 percent of the power to the wheel that is getting the grip even if the other one is completely loose. The draw back is that they consume more energy to work so you available torque to the road is limited through more parasitic drag from the drivetrain. Positraction is a brand name of GM's common clutch type limited slip differential. The most common for muscle cars on the street is this clutch type. There are a series of clutches and springs inside the differential that 'mask' the resistance difference between the two tires. This allows some slip so that the tires can go at their own pace around a turn, but when you go to launch it keeps power applied to both wheels at all times. The drawback here is that in a turn the application of torque either from pressing the gas pedal or lifting the gas pedal will cause your car to steer on its own. Apply throttle usually tosses the rear to the outside, lifting will tend to make the car plow. Torsen's are king here because they behave like open differentials when you lift in a turn which is exactly what you want when you are lifting in a turn.

Then you have ratcheting LSD's. These are less common, they use a weighted ratcheting mechanism to keep one tire from spinning free of the other but allow them to rotate at different through a turn. The drawback here is all the noise they make when you go through a turn. I'm not sure, but IIRC detroit lockers may be a brandname for this type of limited slip differential.

Up next are locking differentials. These are open differentials that allow you to use air or electricity to lock the two axles together. These are common on 4x4's where on the street you don't need the tires locked together and an open will get you along a little more efficiently. When you get ready to launch, drive through some deep mud, or up a big rock (my favorite) you just hit a button and the differential goes from being completely open to being 100 percent locked together. This means that both tires will always spin at the same speed.

Then you have your spools or mini spools. They are simply devices that slip over the two axle ends in the differential and lock them together. Common for dragsters as they don't have to turn - worthless for the street because you'll be dragging a tire across the pavement whenever you turn. You can accomplish the spool type differential by welding the spider gears in an open differential together.

I think that covers the more common lsd's available for automobiles. I'm pretty sure there are some more exotic setups but these are the only ones that come to my mind.

jimfulco 09-12-2004 12:38 AM

Posi's are fine in turns if you know how much gas your car can handle when accelerating around the turn. It just takes practice. Hint: pivot your right foot on its outside edge & control the pedal with the inside.

As far as snow, it's a rarity around here, but I've never had any trouble getting around when it does get icy. Again, practice until you know how your car acts under the conditions. It does help to plan your route, though, and also to wait until the amateurs get off the streets.:D

Awosy666 09-13-2004 11:02 AM

if you want to have power even on the rear wheels go posi. but i would recomend a "limited slip" rearend. this way you have the power but at the same time you don't have to worry too much about the wet weather making your ride dance arould the roads

brainsboy 09-13-2004 12:47 PM

I didnt read all the posts as one guy almost wrote a book here, but one thing I wanted to add is that with the detroit lockers you can use a heavier oil which helps eliminate some of the sound. The problem is that anything that has clutches is not going to last too long.

Ben

68NovaSS 09-13-2004 12:59 PM

The new style Detroit Lockers, called SofLok are much quieter than the old ones. I think all you can buy, new off the shelf, is the soflok nowadays, but they do still make some noise in the turns. Not a bad sound to a hard core gear head!

I pulled next to a very mean Chevelle once when I first got married and rolled the window down to savor the idle, my wife's comment was "wow, that sounds terrible!". After a few hot rods, she sings a different tune.

BTW, I wouldn't be without a posi as a minimum in the winter here, but would think twice before hammering a DL in the rain.

Larry

68Velle 09-23-2004 08:48 PM

One thing to remember is to have a spare the same size as your normal tires. If you put on a donut, the rear will be opened up like you were driving around a corner and burn itself out. This also might be possible if one side gets low on air, maybe?


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