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Optimizing a vehicle for specific purposes is a great way to increase the usefulness and satisfaction from that vehicle. There are many areas that can be upgraded to suit a particular purpose, and in this edition of Shop Talk we’re going to talk about the rear-axle assembly.
Let me say up front that “rear-axle assembly” applies only to rear-wheel drive or four-wheel drive vehicles, although the things we will discuss apply equally to front-wheel drive vehicles. The proper terminology should be “final-drive assembly”, but since most gearheads refer to the final-drive assembly as the “rear end”, that’s the term we’ll use.
Two main areas of the rear-axle assembly can be upgraded for better performance – the gear ratio and the differential type. Changing the gear ratio requires changing the ring and pinion gears. Differentials are usually upgraded to limited-slip types to improve traction.
Let’s start by discussing gear swaps, and what we mean by rear-axle (or final-drive) ratio. The rear-axle assembly is a gearbox, and the rear-axle ratio compares the input and output speeds of the gearbox. In other words, if the input makes three complete revolutions and the output makes one complete revolution, the ratio of the final drive assembly is 3:1 - the driveshaft will turn three times for every one complete revolution of the rear tire. This is oversimplified since the overall tire diameter has some effect, but you get the idea.
Here’s where it can get confusing. Installing a ring and pinion with a higher numerical ratio, say 4:1 versus the 3:1 illustrated earlier, is referred to as installing a lower (or shorter or deeper) gear set. Installing a lower numerical ratio, for example 2:1, is referred to as a higher (or taller) gear set. Confused yet?
OK, enough nomenclature, let’s get into why one would want to change rear-axle ratios. The short answer is that by installing a lower rear gear (higher numerically), mechanical advantage is increased, which makes acceleration and towing heavy loads easier. The downside to this is the engine has to perform more work (in revolutions) to move the vehicle the same amount of distance versus a taller gear – this extra work usually leads to decreased fuel mileage and lower top speeds. Installing taller gears (lower numerically) typically results in better fuel mileage and higher top speed (provided the engine makes enough power to “pull” the taller gear). For street vehicles, top speeds aren’t really an issue, so you’re left with poor mileage with a deep gear or lackadaisical acceleration with a tall gear.
Another instance where a gear change can be warranted is when very large diameter tires are installed on the vehicle, like those found on hardcore off-road vehicles. Large tires help off-road vehicles negotiate rough terrain, but have the same effect as installing a very tall gear. When large tires are fitted, most off-roaders will install deep gears to compensate for them. Off-road types also like increasing the mechanical advantage by installing deeper gears – the vehicle has an easier time climbing steep hills or crawling over large rocks.
So, what all this boils down to is: If you are towing a heavy load, or are trying to maximize acceleration, installing a lower rear-gear ratio (higher numerically) will help. To increase fuel mileage or to reduce the amount of work the engine has to perform while cruising, installing a taller (lower numerically) will help. If you want your vehicle to do some of both, you’re probably better off leaving the factory ratio in it.
One other point about changing gear ratios – in modern vehicles the computer that controls virtually everything has to have accurate road speed information. Vehicle speed information is typically generated at the transmission, which is “upstream” of the rear axle. Which means, of course, that changing the rear-axle ratio results in inaccurate speed information. This information plays an important part in controlling the fuel injection, the airbags, ABS, and speed display. If the rear-axle ratio is changed (or even if wheel/tire sizes are changed) the computer has to be updated for everything to function properly.
Now that we’ve confused everyone with regard to rear-axle ratios, let’s talk about another component in the rear-axle assembly that is often upgraded – the differential. The differential is the device that transfers the power from the driveshaft to the axle shafts. Differentials come in a variety of forms, but for the purposes of our discussion we’ll concentrate on only a few types - the open differential, the limited-slip differential, and the locked differential. The difference in these types of differentials is how they distribute the power to the axle shafts. An open differential, which is found in most standard vehicles, can transfer power to either axle shaft depending upon certain conditions. This sounds good, but is actually bad if limited tire traction is available, because the open differential will send power down the path of least resistance. This is usually to the tire with the least amount of traction, which means one tire will spin (on ice, in mud, whatever) while the other just sits there and does nothing. The limited-slip differential will actually send power to both axle shafts in specific circumstances, such as when the vehicle is traveling straight. When cornering, the limited-slip unit will allow the rear wheels to spin independently of each other, reducing inside tire slippage. A locked differential is a bit of a misnomer since most lockers don’t actually lock both rear wheels together at all times. A locker will tie both rears together when traveling straight, but will usually unlock when cornering.
The pros and cons of each style are as follows: Open differentials work just fine for standard vehicles, which is why they’re the most common type of differential around. They’re relatively simple and inexpensive, so the manufacturers love them. But, if you’re trying to harness a bunch of power in a muscle car, or pulling a boat out of the lake on a wet ramp, or spend any time in the mud or snow, the downside of the open differential is readily apparent – only one wheel pulls which makes tire-spinning a way of life. Limited-slip differentials are the best compromise for street vehicles, and offer good traction for acceleration, towing or off-roading while maintaining civil street manners and drivability. Limited slips are usually optional on most vehicles, and installing a limited-slip unit is a common upgrade. The downside to limited-slip units is they’re more expensive than open differentials, they can occasionally chatter when cornering, and in extreme situations (such as very high load conditions or when subjected to lots of horsepower) they can fail to deliver power to both wheels. Lockers are popular with the off-road crowd in particular since many open differentials can be upgraded to locker status by installing a locking mechanism in the open unit itself – this saves money. Lockers also do a good job of distributing power to both wheels even in extreme situations. The downside of lockers is they make a lot of noise when cornering (a loud click, click, click). By the way, spools are used to lock the axle shafts together all the time, and are found primarily in drag cars. Spools should never be used on street vehicles!
Finally, rear-axle assembly modifications are best left to those with experience. While the act of changing rear gears and differentials is relatively simple, some special tools are needed, and more importantly, getting the gears to mesh together properly is critical to gear life and quiet operation. Gears that are improperly set up will make a very annoying whining noise at best, and at worst the gears will fail and destroy just about everything in the rear-axle assembly. Again, unless you’ve done a few, let someone else do it!
