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 Picking the right rear 
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Joined: Fri Jul 11, 2003 11:53 am
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Location: Saratoga Springs NY
Post Picking the right rear

By Gerry Proctor

First, this isn’t a guide that will tell you what rearend you should use. The purpose of this tech article is to help
you make a good choice since a fully dressed rear can easily cost over $2,000.00 and it is your money.

I’ll start with an overview of the different rearend types and how they may play into your overall
plans, then we’ll take a look at rear gear ratios.


The most common rearend is the open differential. The open differential is the most sensible choice for a car
that is mostly street driven and isn’t too compromised by the fact that your burnouts leave only one tire mark.
Interestingly if you had to buy a new open differential, they aren’t much cheaper than any of the traction differentials.
The pro of the open differential is that it is about the safest of any automotive rearend.
By design, an open differential makes sure that each wheel is doing equal work.
This is very stable on take-offs and during cornering since there won’t be any side-slip. The con of the open
differential is that when one wheel is on a different friction surface, it may –probably will- loose traction.
Now, I’m going to write something that seems outrageous, but it’s true. Recall from a couple sentences past
that I wrote the wheels do equal work? That sounds like a limited slip differential, doesn’t it?
Here’s how that comes to be. If you have one wheel on ice, the other on dry pavement and you apply power, the
wheel on ice just spins. The other wheel does nothing. Well, that’s equal work! Or in this case neither wheel is moving the car. The same thing happens in a break-stand burnout. You are purposely causing one wheel to loose traction and do no work (other than melt rubber). If you have good traction and don’t overpower the tires, there’s no reason that a car with an open differential will be at a disadvantage.


The limited slip differential is one of the best choices for a person looking for more driving traction in a street or street/strip car that isn’t going to use slicks. It offers nearly all of the benefits of the open differential as far as streetability. Limited slip differentials use clutches in the carrier to improve the power distribution to each axle. The way a limited slip works is that when you apply power, the pinion gears (some call them spider gears) push the axle side gears toward the side of the carrier (or into the direction of the side bearings). This force compresses the clutch pack(s) that are placed between the side gear and the carrier and, as long as there is no significant friction difference between the tires, will drive the axles equally. Since this process uses friction materials, it’s possible for these friction materials, or packs, to slip when enough force is applied. This occurs when cornering with a limited slip. The force of the inner wheel needing to turn slower causes the clutch pack to slip. Limited slips do have some quirky behaviors. Since the axles are allowed to do differential work, it’s common for a driver to be on unequal traction surfaces (like one wheel in mud, the other on dry pavement) and get stuck because only one wheel is driving. This is because there is very little axle torque in this situation and the clutch packs are not applying because the pinion gears aren’t forcing the side gears to compress the clutch pack(s). The solution to this situation is to release the throttle, slightly apply the emergency brake, and then ease into the throttle. The application of the emergency brake puts equal resistance on the axles and compresses the clutch pack(s). You may be wondering why I use the "(s)" in clutch pack. This is because the Ford Traction-Loc uses only one clutch pack as opposed to an Eaton-style that uses a clutch pack for each axle. There is no dispute that this makes the Ford carrier a less than perfect design when compared to the Eaton, but in the real world it doesn’t show. It’s likely you’d only notice the difference under high-traction situations like using slicks at the drag strip. Limited slips are generally regarded as the way to go for an upgrade rear for street cars because of their relatively safe behavior. They offer improved traction potential and don’t necessarily require any special driving considerations.


The Detroit Locker is a completely different carrier design. Instead of using pinion gears, the locker uses toothed collars on the pinion shaft and axle side gears. Springs force the two into engagement and only opposing axle torque, such as when going around a corner under coast conditions, can release the two. The axles become, for practical purposes, welded together. Lockers take some different driving techniques on the street. For instance, if you’re going to make a turn from a stopped position, the axles begin locked. If you apply too much power, the axles stay locked and you will squeal the inside tire since it is being forced to turn at the same speed as the faster-turning outside tire. It doesn’t take much force to un-lock the axles. The inner springs are very light and are designed to release at the slightest opposing axle torque to allow differential action in cornering. As long as you don’t overpower the axles in a turn, the axles will release. A locker is also very prone to side-slip anytime you loose traction since both wheels loose traction equally. Your car will follow the road crown or whatever else influences the car’s mass–inertia for instance. Unless you are driving on ice, most of these behaviors are manageable. Now, a locker is about the ultimate in traction that you can drive on the street since the axles positively lock together. Slicks on the strip? No problem. Unless you break it, you’re guaranteed a two-wheel burnout and full power to both tires on the launch.


Spools are not a good thing for cars that are used on the street. There are some people who do use a spool on the street and all I have to say about that is just because you can do something doesn’t make it a good idea. A spool offers no differential action under any circumstances. The axles engage the carrier, which in a spool, is just a solid chunk of metal. It has no pinion gears or side gears. Spools are a drag strip only proposition. In fact, when you’re looking at spools in your catalogs, it usually is written in bold letters - sometimes bold and italic that these are "for race use only." Heed the warning. You’re not being cheated out of anything or deceived. For a car to corner safely, the outer wheel needs to turn faster than the inner wheel. This is because it has to travel a farther distance. This is the action that a differential allows…one wheel to rotate at a different speed than the other wheel. If there were no differential, both wheels are forced to rotate at the same speed. This causes one tire to skid or hop along the pavement and if the traction won’t allow the wheel to skid, the axle will twist and untwist just like a torsion bar. This is very stressful on the axles and can ultimately lead to failure of the axle…as in snap! And since you have no differential action with a spool, even following bends in the road can pitch the rearend into side-slip. Have you ever been to the strip and watched a car with big slicks and a spool turn sharp into the water? The inside slick is usually skipping and bouncing along.

Let’s go over some gearing so we can work this into the equation. From a force perspective, rear gears or the ring and pinion, multiply torque input from the driveshaft. It’s a straight mathematical proposition. If you have 100 lb/ft of torque going to the pinion and you have a 3.0:1 gear ratio, you have 300 lb/ft of torque being applied to the axle. From a motion perspective, if you have a 3.0:1 ratio, the pinion turns three times for every rotation of the ring gear. So altering the ratio impacts the axle torque (the potential to accelerate a mass) and the axle rotation speed. In combination, these two events are how much and how fast work can be done. Picking the right ratio for a street-driven car is like picking the right camshaft. The smart money is always on going a little more conservative. You have to be honest with yourself in clearly defining your objectives or your expectations. For a starter, most street performance ratios will be in the 3.00:1 to 4.10:1 range. Your package and how you intend to use it will drive the decision for a suitable ratio. And I’m not proposing that ratios outside that range don’t work.


The first thing you need to know is what ratio you now have. It seems obvious, but thinking you have a 3.0 is a lot different from knowing you have a 3.0. After all, how happy would you be if you thought you were replacing your 3.0 with a 3.25 only to discover that you had a 3.25? Next, you need to clearly define why you want to change the ratio and what you expect to get from the change. Some people will accept a slower acceleration rate because they may need a lower cruise rpm and better fuel economy and opt for a numerically lower ratio. There is no question about it, a numerically higher ratio will improve your acceleration rate in the first 60 feet, but you drive your car more than 60 feet so take that into consideration when you’re thinking drag strip gearing.

Unless you’re using an overdrive transmission with a steep first gear, a ring and pinion selection is going to be a compromise. You just have to decide where you’re going to give a little to get a little. If you’re thinking more ratio to enhance your acceleration, you’re going to have a higher cruise rpm and use more fuel. Less ratio will lower your cruise rpm, but hurt acceleration. You also need to have a ratio that compliments your engine (remember, it’s a package) and intended use. If your engine is basically stock, you don’t want to drift too far either way from your factory ratio. Going too high will leave you with an engine that passes through its useful power range too fast and going too low may leave you with a very sluggish car that has trouble getting over the top of hills. If you have a modified engine that likes to rev and can make power all the way to 6,000 rpm and above, you certainly want to consider a steeper rear gear. In fact, you may not be able to drive a car like this unless you run a steeper ratio.

There is an algebraic formula that allows you to calculate the effects of ratio changes but, through technology, there are plenty of these gear ratio calculators available on the Internet. Just enter "gear ratio calculator" in your favorite search engine. Most of these handle the calculation algebraically so you can play around with the variables and get a very good idea of how changes in tires, transmissions, and rear gears are going to affect your car. Most of all, pay attention to the cruise rpm. If you use your car on the highway, this is going to be a very big deal.


Finally, there are abundant resources for learning about driveline technology. Many of the resellers, builders, and manufacturers, such as Tractech, Eaton, Richmond, Strange, Mark Williams, Currie, Moser and others offer technical information and help on their products…if you ask for it. The library is an excellent source for research and publishing by subject matter experts (you’d be amazed what you can learn in a real book). Unfortunately, your local community library probably won’t have much in-depth material so you’re going to have to find these resources in the library of a VoTech or industrial college.

Thu Apr 28, 2005 11:24 am
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