INDEPENDENT THINKING, INDEPENDENT SUSPENSION
Beam axles have long been the mark of a serious off-road vehicle, but there’s now just a handful of examples left on the market. What does that tell us?
Before we get too far into the pros and cons of suspension design we’d best recap on just what is meant by ‘beam axle’ and ‘independent suspension’.
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A beam axle looks like the left:
It is a single cylinder connecting two wheels on an axle, and it’s also known as solid or live-axle suspension. Both wheels are on the same axle, so what happens to one wheel affects the other.
Independent suspension looks like the right:
There’s two separate axles which can move independently from each other, hence the name. The term ‘IFS’ stands for Independent Front Suspension, and this usually implies a rear beam axle. The term ‘IRS’ stands for Independent Rear Suspension; and while technically you could create a car with IRS and a beam front end, that doesn’t make any engineering sense. So, all IRS vehicles are also IFS.
Today, almost all non-off-road passenger vehicles are what’s known as ‘fully independent’, or both IFS and IRS, and so are many serious 4WDs such as all current Land Rovers and Range Rovers, Pajeros, Jeep Cherokee Trailhawks, Jeep Grand Cherokees and Y62 Nissan Patrols. There are increasingly few vehicles with live axles front and rear, as the Defender and GU Patrol have just been discontinued. This leaves IFS (with the implication of a beam rear axle) as the most common configuration – and that design is used by even the newest vehicles such as Everest, Pajero Sport and Fortuner (joining stalwarts such as Prado, LC200, MU-X and all the utes). Incidentally, most of the utes use leaf springs in the rear instead of coils like the wagons; but the type of spring doesn’t matter – a live axle is still a live axle.
So let’s get into the pros and cons. Independent suspension is lighter than a live-axle system. Yet in the 4X4 world very few people care about weight, and indeed there’s a misguided view that heavier is better. Still, light weight pays off everywhere – off-road performance, on-road handling, fuel consumption, and wear and tear on the vehicle. Independent suspension also offers better handling because movement on each wheel doesn’t affect the other as much, and there’s the flexibility to change things like camber and toe – that’s the alignment of the wheel relative to the chassis – including changing angles when the wheel moves up and down. That’s why it is used on road cars, race cars, SUVs, off-road buggies and rally cars.
Independent suspension means ground clearance is improved over live axles – let’s look at Pajero with 265/65/18 tyres and 235mm, and Pajero Sport with the same diameter tyres with 218mm… a significant difference of 17mm. If we put a 50mm lift into each, then you’ll get even greater clearance under the Pajero, but a live-axle vehicle will have the same actual ground clearance as it is limited by its differential housing.
The reason is that while an independently sprung 4X4 has two differentials, the same as a live-axled vehicle, they are tucked up out of the way flush with the chassis. That means the underbody of the vehicle is nice and flat with no differential housing to drag and catch. As an example, the fully-independent Y62 Patrol offers 283mm of clearance with 31.5in diameter tyres; and to get that sort of air under the diff housing of a live-axled GU Patrol you’d need 36in diameter tyres.
So why bother with live axles? The main reason is easy modification with big suspension lifts and tyres. Put a big suspension lift in an independent vehicle and the control arms start to point downwards, reducing track (width between centre of wheels) – so you need greater offset wheels (another set of potential problems), wheel alignment specs are exceeded, CV joints operate at angles that mean failure is more likely; and in general, a properly designed big lift is a major engineering job – and by ‘big’ we mean more than 50mm.
Another advantage of beam axles is that they generally have better flex than independent, and can certainly be more easily engineered to provide yet more flex. That’s why you see live-axles on giant-tyred rock buggies and extreme machines. And to some extent, when one wheel on a live axle moves up the other one is forced down… although that’s dependent on design, such as how far inboard the springs are. If the springs are very close to the wheel this effect is minimised, but ride is improved as the two wheels on the axle can move more independently; and vice-versa.
Live-axled suspension vehicles also don’t lose ground clearance between the wheels on an axle when suspension compresses, because they can’t. In contrast, when an IFS vehicle descends a hill the suspension compresses, reducing clearance. Same deal when an IRS goes uphill. This was very apparent on a recent Pajero Sport (IFS) test, where the car didn’t scrape on its way up, but did on the way down even with the same line. However, limited travel suspension can, in some instances, be an advantage. Ever see a vehicle with live axles get really cross-axled? Notice how the chassis gets a lot closer to the ground? An independent vehicle wouldn’t have the flex, and because of that fact it wouldn’t lower its chassis as far. As with most things 4WD, in some circumstances a particular design wins; in others it loses. But with off-roading, what stops you more often is running out of clearance… not lifting a wheel.
Let’s now get into the way of the future. Back in the day, independent suspension didn’t flex very much (ever seen an NA Pajero off-road?) and if a wheel was in the air or even had little weight on it, the open differentials of the day would uselessly spin the wheel. Yes, there were cross-axle differential locks but they tended not to be standard, and they were expensive.
So the logic was that keeping all four wheels firmly on the ground was the way to go – because lifting a wheel was the end of traction. That principle still holds today, as four wheels on the ground is better than two or three – not just for traction, but for stability.
What’s changed over the last 20 years or so is the advent of electronic brake traction control. This system notices when a wheel spins and brakes just that wheel, which has the effect of increasing torque to the other wheel which has better traction. Now, controlled progress can still be made when wheels are lifted, and independent suspension flexes better than ever. Have a look at the image above.
The Defender wins, but look at the difference between the two independent suspension Discovery 3s. One has a cross-linked air suspension system, the other has plain old coils. The difference in flex is quite clear. Toyota has something similar called KDSS (Kinetic Dynamic Suspension System) and Nissan has its HBMC (Hydraulic Body Motion Control) system, all designed to get more flex out of independent suspension. There’s no such development work going on for live axles.
There’s a couple of commonly cited (but incorrect) factors against independent suspension that haven’t been mentioned. The first is it’s weak, and the second –related – is that it can’t carry a load. The latter is also (equally incorrectly) a view held about coil springs.
The truth is that the relative strength of either live axles or independent suspension is more in the engineering than the base design.
The likes of Pajeros and Discoveries are not known for snapping suspension components; and take a look above.
It’s a Hawkei military vehicle. Seats six, 200kW diesel engine, gross vehicle mass of 10 tonnes, and look at that lovely ground clearance thanks to front and rear independent suspension!
It’s easy to confuse cause with effect; load-carrying vehicles typically use live axles and light-duty SUVs are independent; therefore live axles must be heavy-duty. However the real reason is that such heavy-duty vehicles are often based on old designs, live axles are simpler and cheaper to build, and the advantages of independent suspension – handling and lighter weight – don’t make much difference with heavy vehicles.
So in future, you can forget all about live axles. Independent is the way forwards; just as leaf springs die out, all diesels are intercooled and turbocharged, rims get larger and manual gearboxes disappear. Perhaps the very last 4X4 to run live axles will be the Jeep Wrangler JL, due next year (we think).
You may or may not like the future, but it’s coming your way all the same. And it’s independently sprung.