High-Performance Differentials, Axles and Drivelines
Over the years I've gained my fair share of knowledge on the subject of axles and I've successfully diagnosed, repaired, rebuilt, restored, modified, and customized a number of rear ends, myself.
I can tell you from extensive research that source material on differentials, axles, and drivelines isn't readily available.
There are a few academic texts intended for the professional mechanic-in-training and some factory sources, but not even the factory vehicle repair manuals I've read say much on the subject. The factory literature that is available — which is often supplied as a supplement to the factory vehicle repair manual — isn't just dry, it's simply not arranged in a manner that provides a clear, concise, approachable introduction to rear end repair — and this is a complicated subject that needs a clear understanding before taking a deep dive into the more technical aspects.
Enter Joseph Palazzolo and his book, High-Performance Differentials, Axles and Drivelines, published by CarTech.
This isn't the first enthusiast-oriented book on the subject of differentials, but it is the first one I know of produced by a major publisher and easy to find on a bookstore shelf or online at major book retailers.
I read a lot of enthusiast-oriented literature — along with an array of academic books, professional and factory manuals, and documentation — and one of my favorite types is the enthusiast-oriented book written by industry insiders. Why? They usually provide an approachable glimpse into the subject, but with a level of expertise that provides aspects such as real engineering know-how that you often don't find in material written by enthusiasts. As an example, Joseph Palazzolo is a drivetrain engineer and an ASE Master Mechanic from the heart of American car country, Michigan, who doesn't just describe what an axle is and then rattles off bunch of procedures, he looks at the engineering behind the components — which, yes, includes the math.
I like that.
This book doesn't require you to understand the complex math behind things like friction surfaces and differential spring rates, but it does provide them for those of us who enjoy and can use that sort of thing — while giving a thorough and accessible look at how these components work and how to work on them for the lay person.
It also does a good job of covering almost every Muscle Car rear end out there and just about every factory differential used in them or aftermarket unit available for them — as well as ring and pinion gears, axle housings, axles, and drivelines.
That's a tall order for only 144 pages and somehow, the author manages to pull it off well.
Read on for a chapter by chapter breakdown of this solid, enjoyable book:
Chapter 1: Rear Axle Fundamentals
This chapter is an introduction, but to more than just the way a rear end's internals work. It starts with the driveshaft, then moves on to the rear end and the hypoid gears. While it explains the way the gears work, it also — and far more interesting in my opinion — explains the way the lubrication system works inside the housing. In so doing, Palazzolo addresses not just the gear oil and the way it circulates, but why it circulates and the types of loads and stressors seen by the various components.
For me, the most unexpected part of this chapter involves the author's foray into housing loads. This is the kind of information I'd expect to see in a suspension book, but it's an integral part of the way a rear end is designed, so it makes perfect sense to include it, here, too. Why do these forces matter? Well, other than underscoring the need for robustness in rear end housing engineering, explaining these forces illustrates how and why traction loss occurs with solid axle designs and through that understanding — with enough depth and breadth of knowledge surrounding the subject — you can begin to get an idea of what can be done to combat that loss of traction — either through driving or through the use of limited-slip differentials.
The last portion of Chapter 1 focuses on how to find a good shop for axle repair — something that I can tell you from experience, isn't easy to do. The advice is solid. Do yourself a favor and listen closely — or read carefully. It may save you from having a crappy rear end experience.
Chapter 2: Factory Rear Axle Housing
In the Bench Racing 2/9/20 posting entitled "Car Butts," I explain the differences between banjo and Salisbury axles. Chapter 2 covers that information in more depth and continues to cover axle housings in even greater breadth. Not only does it explain the differences between banjo and Salisbury axles, but it digs much deeper into ICAs. It also covers axle retention methods, axle tube construction methods, and the major performance axle housings from every American car company — AMC, Chrysler, Ford, and GM.
Sadly, it doesn't cover the 7 1/4" Salisbury axle used in the Mustang that I previously mentioned in Car Butts. But trust me, that axle was never worth much — it had issues surviving behind the poorly running bone-stock 200 I6 in my Mom's car. It's one thing to thrash the differential of a Ford 8" trying to channel the grunt of a 351W like I did in the Original 351. It's quite another to eat the side gears with an engine that struggles to spin a single 195/75R14 tire in the rain. In defense of the 7 1/4", my Mother's car does have somewhere north of 300,000 miles on the odometer and it's only needed one rear end swap. However, a high-performance axle design, it is not.
Since this book is all about high-performance rear ends, here's what's covered in this chapter: the AMC 20, the Chrysler 8 3/4", the Dana 60, the Ford 8", 9" and 8.8" designs, and the GM 10-Bolt and 12-Bolt axles. But that's not all, it also looks at a unique design not used on a factory car, the Quick Change rear end — which allows for quick gear ratio swaps through a pair of interchangeable spur gears on the back of the housing that can be changed without the need for time-intensive backlash adjustment and pattern checking done with ring and pinion gears.
The rear ends that I find oddly missing are the Chrysler 8 1/4" and 9 1/4" — as in, they aren't even mentioned at all. Even the Dana 35 and 44 — and the crappy Ford 7.5" — are mentioned.
Chapter 3: Rebuilding the Rear Axle
If you're reading this review to find out whether or not this book will teach you how to re-do your rear, then this is the chapter for you.
The focus of Chapter 3 is the Ford 8.8", but, before you freak out and shut down your browser window in exasperation because the rebuild section doesn't use your rear end as an example, bear with me. The Ford 8.8" really is an inbetweener. It's built very, very similar to other Salisbury axle designs (which includes those of you with GM 10 and 12-Bolts, Chrysler 8 1/4" and 9 1/4", Dana 35s, 44s and 60s — and even those of you with AMC 20s). But what about you banjo-boys (and girls and everyone else)? I've wrenched on both types and they're all pretty similar — the major differences are the axle retention method, the fact that you get to drop the pumpkin out and work on it separately from the housing, and the addition of a pinion cartridge — that is the way the pinion is held in and removed from the pumpkin in Ford 8" and 9" axles; it kinda has its own little housing. Trust me, it's easier to take the information from a Salisbury rebuild and apply it to a banjo, then to go the other way.
This chapter doesn't cover gear set up or differential rebuilding — those are covered in later chapters — but it does cover diagnosis of common driveline-related problems in some pretty thorough detail, vehicle preparation, disassembly of an axle housing, cleaning and inspection of the housing, and reassembly — which is why it's not really important which axle is being used as an example: This stuff is fairly similar in all axles and virtually identical for all Salisbury designs. Plus, the author takes the time to note some of the different procedures for different types of axles — so that helps, too.
There is one point I disagree with the author on: in the section entitled "Vehicle Preparation," he says to lift the car by the rear end center section. This is a big no-no for a few reasons, (one he clearly states):
If you have a Salisbury axle in your car, you may dislodge the tubes in the housing enough to either crack or break the plug welds and to cause leaking around the tube-to-center section boss. If you are going to lift a car from the rear end, first be sure it is a factory recommended lift point, then only do so from the outer ends of the tubes near the suspension mount points — again, only if it’s a factory recommended lift point. Those points are intended to handle vehicle weight because they are already doing so via the wheels through the axles and the suspension mounts.
If you have a banjo housing, you can dent the housing by lifting from the center point — or crack one of the welds. If you forgo lifting at the housing and instead use the pumpkin, you can cause a leak at the sealing surface between the housing and the pumpkin.
Back to the Salisbury axle. You run the risk of damaging the rear cover — which Palazzolo mentions in the book.
In any case, the center of the rear end isn't the most stable lifting location because the car is teetering on a single, relatively small point — and that point is rounded, not flat — and the car can slip off the jack and damage all sorts of things. The results could be — but may not be limited to — gouging the housing/center section, denting the floor pan, crimping brake or fuel lines, etc. In my experience, its best to just lift from the factory lift points and play it safe. If one of the factory lift points happens to be the center section, I'd still avoid it — just sayin'.
Chapter 4: Factory Limited-Slip Differentials
I really enjoyed this chapter.
Unlike a lot of information about axle repair out there for both the enthusiast and professional mechanic alike, Chapter 4 digs a bit into the engineering behind the differential — and it's a fascinating subject.
At least to me.
What sorts of engineering information does it explore? Side gear preloads, wheel speeds, and clutch friction to name a few. While I find gear sets in transmissions interesting, as well, once you get past the puzzle of power channeling through the gears and shafts, the mechanics are fairly simple, but with differentials, there's a subtlety and finesse to power distribution that I find enthralling. It can completely change the way a car drives at the ragged edge — which just so happens to be the place I most like to drive. No, not in a ham-fisted, beat-on-it, drive-it-like-I-stole-it sort of way, but rather in a controlled, precise, smooth way. Think turning a bucking bronco into a thoroughbred racehorse by channeling its random, thrashing aggression into smooth, focused, thundering speed.
Proper differential condition and set-up can play a big role in that experience.
Unlike the housing, the differential is really where manufacturer-specific knowledge matters. Housings and directional change for the power going through the powertrain is pretty generic, but there are a number of ways that manufacturers have come up with to channel power to both tires. Thankfully, factory differentials from older Muscle Cars keep it relatively simple.
While High-Performance Differentials, Axles and Drivelines continues to focus on the Ford 8.8" with its Traction-Lok limited-slip differential, after disassembling, inspecting, and rebuilding, it continues on to cover GM Positraction and Chrysler Sure-Grip differentials as well. Along the way it provides a satisfying amount of information on clutch plates, reaction plates, cones, differential springs, side and pinion gears, and a surprisingly in-depth look at friction modifier.
The only place it's light, is on Ford 8" and 9" differentials and the Chrysler Sure-Grip. There's a good reason for being light on the Sure-Grip, though. There were two styles of Sure-Grip produced, the Dana plate-style Power-Lok — which is built like the GM Positraction, which was already covered — and the later Borg-Warner cone-style Spin-Resistant — which can't be rebuilt. However, there was no reason provided for ignoring the Ford 8" and 9" differentials, only that they were intentionally disregarded — which given the ubiquitous nature of that design, seems like an odd omission.
The lack of details on the early Ford differentials aside, when you combine all of the information provided on the many internal differential components, its exploration of the engineering behind differentials, and the rebuild process, Chapter 4 provides a very thorough and engaging overview of factory high performance diffs.
Chapter 5: Aftermarket Differentials
Now things get complicated. You see, there are quite a few aftermarket differentials and some of them use designs that are wildly different than the old tried and true Muscle Car differentials we're used to in our older factory performance cars. As mentioned in the book, there are four basic types of differentials made: open, limited-slip, locker, and spool. Chapter 5 covers all of them, plus four additional power distribution technologies also found in limited-slip designs: eLSD, helical, viscous control, and torque-vectoring.
This chapter has a lot to consume. Although Palazzolo does his best to put the concepts in the simplest terms possible, there is some heavy hitting engineering in differentials today and the plethora of devices available is extraordinary — not all of which are available for aftermarket performance enthusiasts.
Chapter 5 builds on the technical concepts laid out in previous chapters by further explaining how plate and cone style differentials function both from technical and behavioral perspectives. He also explains the differences between passive and active differentials, the approaches taken by speed and torque sensing differentials, and the effects of differential preload.
After getting into the nitty-gritty of limited-slip technology, he digs into open differentials and their opposite, spools and mini-spools. Then he explores the numerous locking differential options that have evolved since the days of the ratcheting Detroit Locker used in the 1965 Shelby GT-350 and numerous aftermarket applications.
After the more standard Muscle Car fare we've been used to in the past, he takes a look at the helical differential, which, in case you were wondering, is a factory option on cars like the 2013 and later Mustangs, as well as GM F-Bodies — so it is a valuable technology to understand for those of you with later Muscle Cars.
Chapter 5 concludes with a look at viscous control technologies and torque vectoring designs, which are, quite frankly, getting outside the realm of the average Muscle Car enthusiast — but are none-the-less fascinating and worth reading about.
Chapter 6: Ring and Pinion Gears
Chapter 6 of High-Performance Differentials, Axles and Drivelines finally gets into the subject a lot of people are going to pick up this book for: gear swaps.
I can tell you from personal experience, there's an art and a science to swapping gears and it's easy to make mistakes: reading gear patterns and making certain you're getting a clean reading during the gear backlash check isn't like setting torque on a bolt with a torque wrench. What's worse, as Joseph Palazzolo notes, failing to do so correctly can lead to the catastrophic failure of a ring and pinion set.
That isn't all the pitfalls you'll find in setting up your own gears, but Chapter 6 does a heck of a job putting together a solid procedure for getting it done — and that's not all. It also digs into how hypoid ring and pinion gears are engineered and manufactured, and why that matters for gear set-up and break-in.
Be forewarned, however, for those of you without a solid background in engineering and/or metallurgy, some of the info is going to require some research to understand — such as some of the lesser-explained elements of the heat treating process and the importance of details of the different metal alloys used to make gear sets.
Personally, I found Chapter 6 fascinating and I picked up a number of things from the chapter, even though I already have quite a bit of knowledge and some experience with rear end gear swaps — I would seriously recommend this book for this chapter alone. It's not short, but it packs a lot of valuable information, tips, and tricks for getting gears set-up properly — so much so that even though the chapter is the longest in the book, it's still jammed tight.
Chapter 7: Axle Shafts
In my experience, axle shafts are one of the most overlooked parts of any type of performance car. I don't blame anyone, they seem rather innocuous. Right up until one gives out and shoots your wheel and tire into your quarter panel or you replace an axle bearing and the rumbling noise you're trying to fix won't go away.
In truth, axle shafts are highly engineered lumps of metal and in Chapter 7, Palazzolo takes you on a tour of how they're engineered and why that engineering is important. That tour includes axle shaft function, materials and manufacturing, spline design, bearing surface inspection and diagnosis, and flange and lug design, engineering, and use.
I don't know that there was anything in here I wasn't aware of in general, but, I picked up a few details I didn't know in almost every section. For such a short chapter, it covered a lot of ground.
Chapter 8: Driveshafts, Universal Joints and Pinion Angle
This chapter is of immediate use to me.
Okay, sort of. I can't work on any of my Car Projects, but the 351 has had a vibration problem on the freeway since I bought it. I've even replaced the driveshaft and it didn't resolve the problem. However, I'm fairly certain it's still driveshaft-related because the shaft I replaced it with was a used one from a Mustang junkyard and I think it may have issues as well.
Why is that anecdote relevant to this review? In Chapter 8 of High-Performance Differentials, Axles and Drivelines, the author explores vibration issues and much more.
For me, it helped to reinforce my understanding of those issues and will hopefully help me deal with the problems I'm having with the 351.
Do you know there are multiple sources for driveshaft vibration? It's true. In Chapter 8, Joseph Palazzolo explains the three main sources, which are critical speed, transverse, and torsional. At this point, I've mostly ruled out critical speed and torsion as sources for the 351's problems — but I won't know for certain until I'm able to work with the 351 again. For clarity, let me explain what those are: Critical speed is about the rotational speed of the driveshaft and its ability to resist flexing. Transverse vibrations occur because the shaft is out of balance. Torsional vibrations happen when the u-joints aren't phased correctly with each other to cancel out the wobbling forces they produce when turning.
Further understanding of those concepts and more awaits you in the final chapter of the book as it delves into driveshaft design, engineering and materials, u-joints and how to remove and replace them, pinion flange balance, and even proper u-joint lubrication.
Is there anything wrong with High-Performance Differentials, Axles and Drivelines?
So many typos that as a writer it almost hurt me inside to read it, but the information was excellent and the writing style was very approachable and clear, so it made it up for the editorial mistakes.
Will this book provide you with all you need to know to be a differential, axle, and driveline guru? No.
The one very clear take-away from this book is that rear ends are highly complex devices requiring quite a bit of knowledge and experience to build or repair effectively.
Will this book give you what you need to get you comfortable with the subject of differentials, axles, and drivelines? You bet. In fact, it'll get you on the right track straight away and remove a lot of guessing, clear up a lot of concepts that are poorly explained in a lot of literature out there — as well as give you the confidence that with enough knowledge and experience, you can effectively repair, rebuild, and modify your own rear end.
In fact, Joseph Palazollo does so well explaining the subject in High-Performance Differentials, Axles and Drivelines, that I really wish I had this book back when I started ripping into rear ends — it would have saved me a lot of headaches.
I'd recommend this book for anyone — novice and professional alike. If you're looking for a primer before you get in too deep and can't figure out which end is up — or, in this case, to the rear — then this book is for you.
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