Another term we have thrown around a lot that bears some explanation is Hotchkiss drive. This is a suspension layout very common on front-engine/rear-drive cars and trucks from the 1920s until the late 1970s and still used on many pick-up trucks and SUVs. In this installment of Ate Up With Motor, we’ll look at in more detail.
Many rear-drive cars and trucks use a live axle at the rear — that is, the rear axle incorporates the differential and halfshafts into a single rigid unit that moves up and down with the rear wheels. Live axles are cheap and rugged and ensure that the camber of both rear wheels remains constant as the wheels move through their suspension travel. The main drawback of live axles is that they are heavy. Since their that mass is part of the vehicle’s unsprung weight, that’s detrimental to ride and handling.
A live axle must be located — that is, its movements in each plane must be limited so that the rear wheels remain firmly on the ground as much as possible — and it requires some means of transmitting any torque applied to the axle (whether by the engine or the brakes) to the body/frame.
There are two basic methods for transmitting axle torque:
- Closed driveshaft (torque tube): The first option is to enclose the driveshaft in a torque tube, connected rigidly to the axle housing and linked to the transmission via a single universal joint that allows the axle to move relative to the transmission without affecting the shaft’s rotation. Drive forces from the wheels are transmitted through the rigid torque tube to the transmission mount. Torque tubes were fairly common until the early 1960s (they were used by, among others, American Motors, Buick, and Chevrolet), but they fell out of favor because the torque tube adds significantly to unsprung weight.
- Open driveshaft: The second option is to leave the driveshaft open and use a universal joint on each end, allowing the axle to move relative to the driveshaft. Since the linkage between the axle and the driveshaft is now flexible, the driveshaft itself cannot transmit acceleration or braking torque. Instead, that chore must be performed by trailing arms (also known as radius rods) that connect the axle to the body/frame.
When the engine applies torque to the axle, that force tends to cause windup — that is, causing the axle assembly to twist along with the drive wheels. Severe axle windup can cause the entire axle to hop up and down on the rear springs, an effect known as axle tramp. Once the vehicle is actually accelerating, front-to-rear weight transfer also tends to cause the axle to squat. Similarly, when the brakes are applied, the rear axle tries to rise as weight shifts forward onto the front springs (an effect called brake dive), sometimes causing the rear wheels to break traction with the ground and hop. Meanwhile, in turns or on uneven pavement, lateral forces attempt to displace the axle assembly to one side. For the axle to do its job properly, it must be connected to the body with locating members — control arms or links — whose leverage resists all these forces.
Automakers have developed a variety of methods for locating a live axle. One approach, popular at GM for many years, uses four trailing arms, two above the axle and two below it, with the upper arms angled inward so that they resist lateral displacement of the axle. A three-link variation of that approach, used by Alfa Romeo for many years, uses a single triangular or T-shaped upper arm attached to the differential housing to perform the same function. Another common three-link layout, used by Buick in the 1960s, also uses two lower control arms and a single upper arm mounted next to the differential, but adds a lateral track bar (a Panhard rod) or parallelogram linkage (Watt’s linkage) to limit lateral motion.
Both the three-link and four-link layouts are reasonably effective, but the control arms and track bars make the rear suspension more complex and thus more expensive to build. A simpler alternative is Hotchkiss drive. In a Hotchkiss layout, the axle is suspended by a pair of longitudinally mounted semi-elliptical leaf springs that serve to locate the axle as well as support the weight of the body. Wheel forces are transmitted to the frame and/or body through the leading (front) portion of each spring, which also serves to resist squat and axle tramp. The trailing (rear) portion of each spring acts as a leading arm, resisting wheel hop under braking. The springs also resist any lateral axle motions. By making the springs perform multiple duties in this way, Hotchkiss drive is very simple and thus very cheap. Since it has few parts, it’s also very sturdy, which is useful for heavy-duty vehicles like trucks.
The drawback of Hotchkiss drive is that while the springs can perform all these various functions, they don’t necessarily do them well. The flexibility of a leaf spring limits its usefulness as a control link; if the spring isn’t very rigid, it will move or deform in response to the various forces on the axle rather than resisting them. Making the springs stiffer makes them more effective in controlling axle movement, but it also makes the ride firmer, sometimes uncomfortably so. The more powerful the engine, the greater the problem. Torquey engines like the big-block V8s of the muscle car era can exert so much force on the axle that the only ways to adequately limit axle movement are to (a) make the springs brutally stiff or (b) add auxiliary control arms (popularly known as “traction bars”) — and/or a Panhard rod and/or Watt’s linkage — to help control the axle, which is anathema to the whole rationale for using Hotchkiss drive in the first place.
One stopgap method, which Chrysler and some Studebakers used for many years, is to change the position of the axle on the springs. The spring rate of a leaf spring is proportional to its length. If you move the axle forward toward the leading ends of the springs, the front section of the spring will be shorter and thus stiffer, allowing it to better control axle tramp without making the ride harsher. The drawback is doing so effectively softens the rear portions of the springs (the sections aft of the axle assembly), making them less able to resist brake dive or wheel hop on deceleration. Powerful Chrysler cars of the sixties had good axle control on acceleration, but were prone to violent wheel hop on hard braking, occasionally with harrowing results.
Another stopgap, employed at various points by Chevrolet and Ford, among others, is to “stagger” the rear shock absorbers, mounting one ahead of the axle, the other behind it. In this way, the shock absorbers are made to perform double duty, resisting axle tramp. Staggered shocks can be reasonably effective for street cars, but may not be adequate for really high-powered applications like drag racing.
Whence the name “Hotchkiss drive?” The layout was first adopted in 1905 by the French firm of Hotchkiss et Cie. Few things in the automotive world are ever really new, however, and similar layouts had previously been used by other automakers, including Cleveland’s Peerless Motor Company. Nevertheless, the name has stuck, even if it’s not entirely accurate.
There’s so much tuning that can be done with leaf spring suspensions: adjusting the front-to-back angle of the springs, the spring width, assorted spring pack dimensions, spring-under Vs spring-over, lateral links, etc, etc.
…but by the time you do all of that, you can throw together a much better 5-link with 4 roughly parallel arms and a lateral link for the same price.
The new Dodge 1500s ditched leaves for coils, as it’s pretty easy to pull off for light-duty use. Of course, everything old is new again, as 60s Chevys had (significantly more crude) coils as well.
We’ll see if the idea sticks this time…
[quote]There’s so much tuning that can be done with leaf spring suspensions[/quote]
Oh, absolutely, and this wasn’t intended to be a treatise on the application of leaf springs, just of Hotchkiss. There are plenty of leaf-spring suspensions that are [i]not[/i] Hotchkiss, but it was so common it bore some explanation.
The main advantage of coils is ride quality. The inter-leaf friction of leaf springs helps to keep the axle located, but it also transmits more harshness, unless the leaves are isolated with very soft rubber bushings on the spring shackles. A coil does not, so a coil spring will provide a softer ride than leaves of the same overall spring rate. That was why Chevrolet and eventually Ford went to rear coils for their passenger cars.
Interestingly, Ford’s initial efforts at rear coils (in ’58) were disastrous. The big Lincolns and the Thunderbird initially had coils and trailing links, but they had horrendous problems with axle hop. They finally gave up and went back to leaves for 1960. And many European manufacturers who used rear coils took a belt-and-suspenders approach to axle location. We’ll be talking in a few weeks about the Rover P6, which had an insanely complicated De Dion set-up, and Alfa and Volvo had very involved locating members. The problem, of course, is that it was expensive…
Hotchkiss drive technology is still in use and it’s good compared to the recent inventions by looking at cost, time spent during mantainance and durability.
I understand the locating aspect of the leaf springs, but why wouldn’t the addition of a panhard bar give hotchkiss designers more latitude in other aspects of the leaf spring selection and arranging? As far as the unsprung weight issue, some aftermarket 9″ Ford live axles intended for race use have nearly miraculous weight reduction courtesy of advanced design to give strength substituting for simple dumb mass. Couldn’t that help extend Hotchkiss drive into future car designs?
It’s certainly possible to provide some other means for lateral location (Panhard rod or Watt’s linkage), just as it’s possible to add trailing links to augment fore-aft location. The downsides of doing that are that it costs more and you can end up with some weird geometry issues, causing the additional links to bind under certain conditions. At that point, it becomes tempting to substitute coils or torsion bars so that you can avoid those geometric problems and eliminate inter-leaf friction for a smoother ride.
The main advantage of Hotchkiss drive is that it’s simple and inexpensive, so the more you add to it, the less sense it makes except for specific applications, like racing, which has a narrower set of design priorities than street cars do. If you’re not concerned about ride quality and you know exactly how you need the axle to behave in all of the circumstances it’s likely to experience (as in drag racing, for example), Hotchkiss may be a perfectly viable option. General passenger car duty may be less severe than racing, but is a much trickier design challenge because there are so many different priorities to balance, including cost. (I assume ultra-lightweight racing axles aren’t cheap!)
“Anything you can do with coil springs you can also do with leaf springs, except better.” So said an old mechanic to me once…. a long tome ago (like 1959).
So, let’s review some advantages of leaf springs:
1) Longevity (one simple motion does not greatly distort, work, or twist the metal matrix internally);
2) Less expensive per pound;
3) Provide TWO attachment points to the frame to distribute the load, acceleration, and braking forces;
4) When properly sized, adequately locates the rear axle laterally and longitudinally without complexity;
5) Can be easily and inexpensively replaced, and can be custom made;
6) Interleaf friction provides an automatic, built-in dampening effect like that from shock absorbers;
7) The leaf-pack can be assembled in various options, including top “feather” leaves for a good ride;
8) The leaf-pack can be made with graduated multiple spring “rates” to handle increasingly heavier loads;
9) The pack itself can be easily under-laid with “helper” or auxiliary springs for overload situations.
Disadvantages?
1) Will be heavier on average than coils springs for the same nominal spring rate and capability;
2) For off-road use, leaf springs may not provide as large an articulation as coil springs;
3) Leaf packs do take up more space, and require a longer longitudinal length for mounting.
So, for small cars, coils make more sense; for large trucks, leaf packs make more sense. Anything in between is anyone’s guess…
==================
A major disadvantage you don’t mention, which was a major consideration in the widespread adoption of coil springs, is that leaf springs have inter-leaf friction, which adds ride harshness even with relatively soft spring rates. With Hotchkiss drive, as the article says, axle location also becomes directly proportional to spring stiffness, which forces compromises between location and ride stiffness — a compromise may be adequate by some standards, but it’s still a compromise. Variable-rate multi-leaf springs or the use of auxiliary locating members impose compromises of their own, which aren’t necessarily desirable. (Transverse leaf springs are a somewhat different story in the latter regard if they’re not providing wheel location or transmitting acceleration or braking torque.)
Obviously, for trucks, leaf springs make a fair bit of sense, which is why they’ve been used for well over a century. For cars, where ride quality takes precedence over load-carrying ability, there are reasons coils largely supplanted leaf springs decades ago.
In the modern era Hotchkiss drive is pretty much confined to trucks and heavy commercial vehicles, although many buses use air bag suspension systems instead.
Front wheel drive is the norm today, rear wheel drive passenger cars are mostly high end vehicles with complex and sophisticated rear suspensions. Buyers of such cars expect both good road manners and a smooth ride and pay the premium prices that it costs to build them.
Independent rear suspension is universal for small cars, except possibly Lada vehicles still being produced in Russia, and possibly Hindustan cars in India, however these are anachronisms built from cast off European designs over half a century old now.
My point being, Hotchkiss drive is largely irrelevant to cars built much after the 1980s.
Having said that, thanks for another informative article Aaron. Just because something has become outdated doesn’t mean it isn’t interesting to learn more about it. For those like me who are interested in the collector car hobby understanding the principles of old fashioned designs will help when restoring or repairing older cars.
My nephew has a degree in mechanical engineering, he was interested to learn how a points ignition system worked, he grasped the principle readily, but found it curious the I and other mechanics back in the day spent so much time setting up and adjusting them to get engines to run smoothly and efficiently. He was born after points and carburettors had ceased to be fitted to cars sold in the Western World.
Hotchkiss drive has disappeared from modern cars as suspension designs have become more sophisticated. However Hotchkiss drive is still the standard for pick up trucks. I don’t know about the US, but elsewhere 4 door / dual cab compact pickups like the Ford Ranger and Toyota Hilux are refined enough for many to use as cars. Seems the market for simple suspensions is not dead yet.
If you look at the comparative North American sales of passenger cars and pickup trucks, it’s clear that a lot of people in the U.S. do indeed purchase trucks as substitutes for cars. The U.S. market for compact (i.e., ordinary-size) trucks like the Ranger and Hilux is dying because there’s not enough of a price savings to convince buyers a full-size truck isn’t a better deal, particularly with shockingly low fuel prices of late, but the point remains.
Of course, buyer expectations are also a consideration. People don’t typically go to a pickup truck expecting brilliant handling and a cushy ride, and thus may be impressed by the same characteristics that would be derided as old-fashioned and crude in a car. (This is not to say current trucks are crude, because most really are not, but there are questions of degree and design priority.)
Chrysler got some good results refining the Hotchkiss system over the decades, with variable-rate springs with some leaves shorter than others (variable rate being the big advantage of their torsion bars, too) and with additional resistance to axle dislocation during cornering provided by shock absorbers angled inward at the tops. I’ve found helper shocks do interesting things when placed at that angle. Coil-overs seem to defeat the anti-roll bar by providing extra support so far from the outer edges of the car. Air shocks, on the other hand, raise rates and help locate the axle without adding harshness.
I think the real reason for the end of Hotchkiss rear ends is the popularity of independent rear suspension. With a live axle, both springs work together to resist lateral forces; CV joints take that advantage away. And, of course, leaf springs are very poor at preventing camber changes.
Hotchkiss works. It’s the live axle that went out of style.
Given how many tens of millions of cars and trucks have used Hotchkiss drive, it would be silly to say it didn’t work. Engineering isn’t a question of absolutes — existing technology doesn’t cease to be useful just because somebody came up with a newer idea, especially if it takes a while to get the new technology to work as advertised or the newer technology costs more. However, older technology does sometimes eventually come to the limits of its adaptability to newer demands and expectations, which happened to solid front axles and carburetors and has gradually happened to Hotchkiss drive.
Quite a lot of modern small cars still have beam axles in back (albeit generally not live axles), including most A- and B-segment cars and some low-end versions of bigger cars. Almost all use coil springs, not leaf springs, which is suggestive. Cost is a big factor, which is part of why A- and B-segment cars generally don’t have independent rear suspension. However, buyer expectations for ride and handling are much higher than they were 30 or 40 years ago. The big preoccupation of modern suspension engineering is what I suppose you could call planar independence — differing degrees of stiffness in different planes — which is easier to achieve with coils and control arms, even with a beam axle. Plus, coils avoid the harshness you get from inter-leaf friction, which is helpful when you’re dealing with a relatively inexpensive vehicle with a relatively low sprung/unsprung weight ratio and fairly short wheelbase.
Torsion bars are not intrinsically variable-rate; they’re height-adjustable, but that’s not the same thing. The rate of a torsion bar depends on its diameter and the length of the lever arm, so I suppose you could make a variable-rate torsion bar by providing some mechanism to dynamically alter the lever-arm length. (There are adjustable anti-roll bars and some high-end vehicles have systems that can selectively disengage the anti-roll bars.) Variable-rate springs aren’t always desirable, though, because they can give you some twitchy handling dynamics when pressed.
The drawback is doing so effectively softens the rear portions of the springs (the sections aft of the axle assembly), making them able to resist brake dive or wheel hop on deceleration.
I assume “able” was meant to be “unable”. Am I right?
Stellar writing as always. I hope you survive Covid 19 and are able to continue writing. Have you thought of You tube?
Oops! Yes, that should be “less able” — I fixed the text now. Thanks for the correction!
Los Angeles is now on a shelter-in-place order for the COVID-19 outbreak, so just about everything except grocery stores, pharmacies, and gas stations is shut down. It’s pretty alarming.
Oops.
I should have written “less able”.