Dandy. I doubt there's a problem, but if there is, it's not related to the drive axle. First let me explain the operation of it. There's a left axle shaft and a right one. Each wheel is bolted to one of them. It's because of the gear arrangement in the assembly called the "differential" that the two wheels will rotate in opposite directions. Then, that entire assembly is placed inside the axle housing where it is free to be turned by the engine / transmission / drive shaft. Since the entire assembly is turning, you could expect both wheels to turn in the same direction and at the same speed. The clinker though is that any insignificant imbalance in friction will cause one wheel to stand still and the other one will turn. Brake drag, bearing tightness, a mosquito sitting on the brake drum or rotor, anything can cause one wheel to spin easier, so that's what will happen. When you see this on a hoist, if you hold a piece of wood against the spinning tire, as it goes slower, the other one will start turning and go faster. THAT is the observation that convinces many people something is wrong when only one wheel spins.
The magic happens when both tires are on the ground. One tire could propel the car forward, OR, that axle shaft would find it much easier to just turn the other shaft. The problem is though, that other shaft is also hooked to a wheel and tire that wants to propel the car forward. Now it's not so easy for one shaft to spin the other one. Instead, they both turn equally and provide equal power to move the car.
That magic falls apart when one tire can't get the same amount of traction, as on ice or in mud. The drive shaft is trying to rotate the entire differential assembly, and it doesn't know or care which axle shaft is turning. There's no reason for the side with traction to move the heavy car when it's much easier to just spin the tire that's on ice. That's another thing people see and assume there's something wrong in the axle assembly.
To further complicate this sad story, on older versions of these axles, the optional "locking" versions simply had a small set of clutch plates between the two axle shafts. During normal driving both shafts turn at the same speed and in the same direction, so those clutch plates do absolutely nothing. It's when one tire is spinning on ice that those clutch plates try to make both shafts turn together. That's not 100 percent foolproof, but it forces the tire with some traction to do some of the pulling while the other one just spins.
When you had an older version of Chrysler's Sure-Grip axle, it was impossible to turn the wheels in opposite directions by hand. All you could do is turn both of them the same way when the transmission was out of "park". Today there are some different designs out there that require the axle shafts to be moving at different speeds, (one tire spinning), to cause the clutch to engage. For that reason, on a hoist, the axle will appear to be the non-locking, or standard axle. People who know they have a locking axle will assume something is broken inside it.
There were a few 4wd trucks in the '60s that used locking axles on the front, but today that will never be done. A left and right wheel will turn at different speeds when going around a corner, and in a rear axle, the clutch plates will just slip a little to allow that, or at the worst, on icy roads one tire will skid a little, but that will be hard to notice. It's a different story on the front. You don't want to do anything to cause the slightest loss of traction on the tires that are steering the vehicle. That makes for a really miserable and unstable vehicle to drive. On those older trucks, typical speeds on the highway in those days were a lot lower than today, and those trucks were built for utility, not comfort.
Now, to address the observation of a pull during hard acceleration, there are a number of variables involved in that. I used to work with a fellow who ran a dragster, and he had the same problem. The goal is to get the car to stay perfectly straight. He used to joke that there were 99 things he had to check or adjust to achieve that goal. 3/4 of those things were related to excessive horsepower, after all, he WAS racing Chrysler products! The point is, every variable has to be perfectly in balance for the car to stay straight. Spinning or skidding tires have no traction so they're free to move sideways. Which way they move is influenced by how much snow is plowed and piled up in front of one tire, which way the road surface leans, (usually to the right so rain runs off), the friction of a small spot of patching material on the road surface, weight distribution in the car, and dozens more things like that.
The goal of every race car driver is to never ever spin or slide the tires. They should be constantly sticking to the road surface. I'll never admit that when I was a youngster and felt the need to show off my car's power, the goal was to show how little traction my tires had and make skid marks to impress my friends. The real heroes were those who could fishtail and make "S"-bends in their skid marks. Now, ... To say your friend's car has a problem because it also doesn't stay in a straight line under hard acceleration does not necessarily mean it has a problem. If he is spinning the tires, as too many Chrysler owners like to do, all bets are off as to which way the car will go. It's if the car has a hard pull one way under hard, but "normal" acceleration that doesn't involved spinning tires that we have to be concerned.
With front-wheel-drive cars, "torque steer" was a serious problem. We had a Chevy product in the early '80s that you could drive for dozens of miles on the highway without touching the steering wheel. All steering was done by how hard you pressed the accelerator pedal. The more hair-raising version of that was when the car would pull hard one way under acceleration, you'd counter-steer with the steering wheel, then the car would dart the way you were counter-steering when you stopped accelerating. Chrysler was the first to find a solution for that, and now almost no manufacturer has that problem, ... Unless there is a suspension problem on the car. Parts called "control arms" hold axle housings in place but let them move up and down on bumpy roads. Those arms are pretty strong, but for comfort, they're always attached through rubber bushings to isolate road shock from the passengers. Those bushings can deteriorate and prevent the arms from holding the axle in its proper alignment and position. The three main symptoms are a clunking or rattling noise on bumpy roads, a steering wheel that shifts position, meaning one time it's straight when you're driving straight ahead, and the next time it's off-center a little, and a feeling of uncertainty which way the car is going to go when you hit bumps or go around corners.
Control arm bushings used to commonly last the life of the vehicle, but for the last 10 - 15 years it seems everyone is having common problems. I suspect it might be due to the use of softer rubber that flexes more, to address the complaints of more-demanding owners who consider comfort more important that safety or reliability. If you suspect a problem with the car, the steering and suspension systems should be inspected at a tire and alignment shop. This may be changing, but in the past, we never paid much attention to bushings unless the owner's complaints or descriptions of the symptoms led us to believe they could be involved. We're more accustomed to finding worn ball joints and tie rod ends. The next thing we do is "read" the tire wear patterns. When there is a complaint related to handling, there is usually a tell-tale clue in the tire wear patterns to tell us where to stat looking.
To condense all this great and wondrous information into something usable, consider how the car is being driven. If it is common and normal driving and a handling problem exists, have the car inspected at a tire and alignment shop. If there is no observable handling problem when driven the way my mother would drive the car, it's more likely the cause is the person whose foot is in the shoe on the accelerator pedal.
Friday, October 31st, 2014 AT 10:58 PM
