Marlin in Minn

The rear end is posi-track which to me means that both rear wheels should
be pulling equally. But, the rear end swings to the right from low speeds or
from a standing start...

Does anybody know what actually causes this?

95BLKVette

T :conehead O :conehead R :conehead Q :conehead U :conehead E :p:

Nathan Plemons

Actually it's your posi that does it. If you didn't have a posi you would just light up one tire. Usually mine kicks right only when I actually break the tires loose. Or for instance if you're in the rain and you punch it, tends to go to the right. Something about the direction of the rotation, blah, blah. I wish I knew the physics behind it but I don't.

All I know is that without a posi you will just spin one wheel all day long and on a GM with a Posi you will move, just sideways :)

Ever notice how ever car that pulls the wheels off the ground has one wheel higher than the other? Ever notice how it's always the same wheel. Except, and I've never really paid much attention to this one, a friend of mine tells me that mustangs will pull the opposite tire off the ground first. Like I said, I've never noticed, could be true who knows. This same friend tells me that Mustang's actually spin the opposite direction of GM's, i.e. if our motor turns clockwise theirs goes counter, and hence the reason they pull the other wheel off the ground. He could have also just inhaled too many fumes from the track.

Can anybody prove or disprove this?

merrick

One of Newton's laws. For every action, there is an equal and opposite reaction (or something like that). The action is torque rotation in one direction and the car rolls in the other direction

jfb

Muskrats and other cars with solid rear axles spin one wheel even with positraction because driveshaft torque which is also applied to the rear axle tries to lift one rear wheel and push down on the other. The wheel being lifted spins and loses traction and the assymmetrical accelerating force on the rear wheels causes the car to turn away from the wheel with traction.
Since 1963, all vettes have independant suspension and the differential is bolted to the frame, so driveshaft torque does not get applied to the rear axles. Since your rear tires do have the EXACT coefficient of friction nor the EXACT same weight on them, the wheel with the combination of less weight and coefficient of friction will spin first, lose traction , you will have assymmetrical thrust on the rear wheels which causes the car to turn away from the wheel with the most traction. The biggest reason the rear swings out is due to the weak wheel losing traction first and the other wheel begins to spin which reduces its tractive force with the pavement and it then cannot resist sideways movement very much and away she goes.

kabnvtt

Gyroscopic Procession. :cheers:

sinistervette

My vette is the first IRS car I've ever driven, all of my drag cars, stock cars, and street cars have all had solid rear axles. If the solid rear axle is equiped with a true posi(locker/gear type posi/spool/etc..), not a limited slip differential, both tires will spin unless something breaks. The statement above about driveshaft rotation would be better stated as, the direction of rotation of the pinion gear actually makes it climb the ring gear therefore "lifting" the side of a solid axle that the pinion is on in relationship to the ring gear. Simply stated, if the ring gear is located on the passengers side of the ring gear, it will "lift" the passenger side of the axle allowing passengers side tire to loose traction and spin first. This is as long as both rear tires are close in weight, grip, and both on the same type of surface. If you have a true posi, when accelerating if your car tends to turn left,back end goes out to the right, you have more bite with the right rear than you do with your left rear. If you have ever broken a rear axle shaft, or want to experiment and remove 1 of your half shafts(I don't know if you can do this, I'm using it for example) you will know what I'm saying. I've broken several axle shafts in spool equiped cars, and if the right rear broke, the car would turn hard to the right and vice versa. If you could pull a half shaft and go into a open area, lets say you pull the right one now going straight you punch it, the car will want to turn right as the rear would come out around to the left. Now try and spin a doughnut. You will be able to spin like crazy to the right but not the left. Talk to a circle track racer, and see what tire has the most weight on it, 95% of them will say left rear. It is not only so when they go into the corner that the weight transfers to the outside equalizing traction, but also when they accelerate out of the corner the left rear will actually "tighten" the car up by having the most weight/traction. Also keep in mind that almost every road we drive on is "crowned" higher in the middle, so as we spin both rear tires they will actually slide down the crown. Try pulling a holeshot sometime on the opposite side of the road! "PLEASE MAKE SURE YOU ARE A PROFESSIONAL DRIVER ON A CLOSED COURSE"! :lol: :lol: Have any of you noticed a difference in spinning your tires on a road versus at the dragstrip? Most of the time dragstrips are either flat or have a slight crown. Hopefully this made things a little clearer........like mud! :yesnod: :lol: :cheers:

tkrussell

Correct Spelling is "Precession".

As best I remember my physics, here goes:
Any rapidly rotating object, such as a gyroscope, tire, or flywheel, possesses angular momentum. The ability to twist, or cause angular momentum is called torque. Greater torque= greater twisting effect.

Precession acts at a 90 degree angle to the rotation, so when both rear tires lose traction during a burnout, the car precesses (yaws) to the right. On a car without positraction, one tire still has traction and so the car cannot yaw.

This is related, but not the same as the "equal and opposite reaction", which is why the engine or body rolls to the right when you blip the throttle. Again, caused by torque, but a different effect.

Hope this helps!

vader86

I'm a physics major, and the reason you slide sideways is called

TORQUE, its your friend :D

Marlin in Minn

Thanks for the replies..

So if I'm understanding this correctly, if the engine and drive train rotated
the opposite direction, then the rear of the car would swing to the left when you spin the tires??

Is there any way to compensate for this to help the car pull straighter
under hard acceleration?

vader86

Well the wheels are gonna rotate in the same direction if youre moving forward, so youll always swing right.

JCAIRE2

Ugh, physics :smash: :smash:

J :seeya

GRX

Hopefully, no one will bring up the ugly discussion of 'beam plates' for the 'c' beam....ooops! :blueangel:


[Modified by GRX, 5:48 PM 9/23/2002]

Quiktrp94

If you are pulling to the right without spinning your tires then I would check your rear bushings. Mine did the same thing and when I jacked up the back I could move the wheels around. Changed the bushings and she pulls straight as an arrow. Hope this helps. Robert :flag

BBA

Thats the best response yet.

blk90rdstr

Actaully, streets are crowned to allow for drainage this can direct the direction of the rear once traction is gone. I have launched in a parking lot that was flat and got an occasional leftways launch. I hope thats not defying physics, just a observation. :)

Quiktrp94

Why is everyone assuming that he is referring to the wheels breaking free? Mine did the same thing under hard accel. The way I read the first post he is talking about that and not burning out. Most of you guys only think about spinning tires and 1/4 miles. Some people want good performance and mods without tearing things up and saying OOOPS. IMHO.
:boxing

Nathan Plemons

Because even the best posi unit can't compensate for the conditions of the road under the tires. Even if the posi puts the exact same force on both wheels, both wheels won't have the exact same forces to contend with, thus one of them will slip before the other. You might not SEE the wheelspin, but it will be there. A lot of this sort of thing is best explained using the drag strip and heavy wheelspin because they are the easiest to observe in a controlled environment.

The 1/4 mile is obviously not the only performance measure out there, but it's a good common reference. If I say that my car runs 12.8 in the 1/4 and then I go on to say that your car goes from 0-60 in 4.3 seconds, who has the faster car? How can you know? What if my car runs that same 0-60 in the same time, but your car only runs 14.0 in the quarter. What then? Well maybe we can see that my car makes more power in a different powerband.

1/4 mile times and drag racing references are not the end all be all, but they provide a means of presenting information that most people can easily understand and recognize. But if you don't like that sort of measure than let's do it a different way. The car kicks to the right because the coeffiecient of spam between the tires and the ground is traveling at the speed of blue.

Quiktrp94

My God you guys just don't read what other people write do you? You're still talking about spinning wheels and 1/4 miles and 0 to 60 . All he wanted to know is why the rear end moves to the right under ACCELERATION!!!! This would have NOTHING to do with WHO IS FASTER. Sometimes you can't see the obvious. We know that a lot of you have built up and then destroyed your motors in the quest for MORE POWER but not everyone is interested in "shaving a tenth" and just needs a question answered without alot of BS. Read then type, OK. Thanks. :sleep:

BBA

Hey man...I'm with ya!