What are the different types of rear suspension, and which ...

When it comes to what's underneath your car, it's pretty easy not to pay much attention, unless it's a traffic cone, a wombat, or the sky.

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But, much like flossing, suspension is both tricky to get enthused about and a lot more important than it's given credit for.

Even on a glass-smooth road, car suspension has its work cut out. Traversing a simple bend subjects your undercarriage to severe stresses and strains.

There are a multitude of methods that have been used to keep your wheels under control, with deeply confusing terms like Hotchkiss drive, Watt's link, swing axle, semi-trailing arm and De Dion tube (which sounds like a '60s surf band, but isn't).

As indecipherable as they seem, each is basically a way to make the rear wheels do what you want and prevent them from reacting to cornering and road surfaces in unpredictable and potentially life-threatening ways.

Most of these arcane setups are outmoded these days, thanks to the two predominant methods widely adopted for keeping your car's rear end in check: semi-dependent torsion bars and fully independent suspension systems.

But how do they work, how do they compare and should you consider one over the other when choosing your car?

First things first; what do we mean by independent rear suspension? Well, there are three basic ways to sort out a rear axle.

Dependent

Dependent suspension ' also known as a 'live' axle ' is where the wheels are linked to each other in some fashion. In effect, this means that the motion of one wheel causes the other to gambol around and generally misbehave. It's the simplest, oldest and cheapest way of doing things, found on trucks, farm equipment and the Ford Falcon GLi.

In the '94 Falcon, this caused hilarious amounts of oversteer at the slightest provocation and required a lot of Band-Aid engineering throughout successive models to patch up the live rear axle's shortcomings.

Read More: All Falcon reviews, news, price and specs

All Falcon reviews, news, price and specs

Independent

Keeping the wheels independent of each other but still connected requires more than a bit of tricky engineering

The notion of independent suspension, then, seems like a self-evidently better idea. The motion of each wheel is isolated ' independent, if you will ' from the other, free to move and react without introducing sympathetic movements on the other side of the car.

Keeping the wheels independent of each other but still connected to the body, brakes and drive shafts requires more than a bit of tricky engineering.

Engineers have tackled this problem with varying degrees of complexity, but the best known are the MacPherson Strut, double wishbone and multi-link systems.

MacPherson strut

As one of the simpler and therefore cheaper options for independent suspension, the MacPherson strut has seen widespread use since its invention back in the s.

Compared to the torsion bar system, the MacPherson strut is both large and expensive. Next to dual wishbone and multi-link systems, the MacPherson offers little in the way of control, isolation or adjustability.

Generally speaking, MacPherson struts have been relegated to the cheaper and more mainstream models, especially those designed for unstressed highway and city use. Manufacturers also tend to fit MacPherson struts to the front axle only, so we'll talk about them more another time.

Read More: What's the different types of front suspension?

What's the different types of front suspension?

Multi-link and dual wishbone

Multi-link and dual wishbone systems are the holy grail of suspension designs, allowing engineers to achieve agile handling without compromising ride quality.

Multi-link and double-wishbone independent setups are the most expensive and complex

Both multi-link and dual-wishbone setups offer true independence and unparalleled control, with the multi-link system edging ahead of the double-wishbone choice by offering greater adjustability.

Of course, nothing comes for free. Multi-link and double-wishbone independent setups are the most expensive and complex, as well as taking up more space than the simpler and slighter torsion beam.

Finding dual wishbone or multi-link suspension in your prospective new car usually means heading to the more premium end of the hatchback, wagon and sedan markets. It also means being pretty selective in the SUV and light commercial segments.

Using a dual wishbone or multi-link system isn't the be-all and end-all, however. Think of them as the best method, like a great recipe for a delicious meal. It doesn't make everyone with a frying pan Heston Blumenthal, if you catch our drift. Haval, for instance, touted a multi-link rear end at the launch of its H8, which is commendable, but you still won't confuse that Chinese machine with a Ferrari.

Semi-independent

As you might guess, semi-independent suspension is where the wheels are still connected but, thanks to inherent flexibility in the connections, can operate with a semblance of autonomy.

One such system is the De Dion tube which is hardly ever used these days ' although suspension geeks will point out that if you want to get a rare glimpse of one, you should look underneath the Smart ForTwo micro car (if you can still find one in Australia, probably best to head to Europe if you're really keen).

Torsion beam

So, that brings us, at last, to the torsion bar, also known as the twist beam, torsion beam, or any other variation on that theme. And it really is as simple, and ingenious, as that; a bar connecting both wheels that twists ever so slightly under duress, in order to give a degree of independence to each wheel's movements.

Torsion beam setups are relatively cheap and easy to manufacture

Under lateral (side-to-side) force, however, the torsion beam remains rigid, to resist body roll, which lets you corner with confidence. Simple, solid, sorted.

Torsion bars take up less vertical space than MacPherson strut setups and less horizontal space than double wishbone and multi-link systems, netting more room for humans in the cabin and their stuff in the boot.

Torsion-beam setups are relatively cheap and easy to manufacture, install and maintain, but they don't offer the engineer anywhere near the same amount of freedom to balance comfort, agility and stability as independent systems do.

Because torsion bars offer less isolation than independent systems, they can also allow more noise, vibration and harshness to filter into the cabin.

Now, in a well-soundproofed car that isn't driven in anger, these issues might never become apparent, let alone cause any grief.

Even in snappy, small hot hatches like the Polo GTI and Fiesta ST, the torsion beam is still more than capable.

It's only when demands for comfort, poise and stability outstrip what engineers can achieve with the torsion beam that independent systems really come into their own.

So, while independent suspension can be the best way to look after what's going on under your car, it's not always necessary, and you should seriously consider whether it's worth spending the extra money for a vehicle that's fitted with it.

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Still have lingering questions about rear suspension? Let us know in the comments below.

Well, cars like the Renault Megane RS and the FK2 Honda Civic Type R use torsion beam suspension setups. Even the Mazda 3 switched to torsion beam from multi-link, so they might not be so inferior after all.

If you drive an econobox, it is most likely using a torsion beam (or twist beam) setup. If you have a 'premium 'or 'performance' car, it is most likely using a multi-link suspension setup. So, torsion beams are clearly the inferior suspension, right?

Just like every other technology, the torsion beam suspension has its own set of pros and cons.

Con - Less comfortable 

Unlike multi-link systems, the left and right wheel are not independent of each other.

The most obvious drawback of the torsion beam setup is the dependence of the left and right wheels on each other. This is because the torsion beam is welded to the trailing arms on both left and right side.

The solid axle with leaf springs can be found in trucks like the Toyota .

Compared to a solid axle (e.g., in a Toyota Hilux), a torsion beam is able to flex and reduce the transmitted force from the right to the left (or vice versa). Hence, torsion beams are a sort of "semi-dependent" suspension.

However, fully-independent suspension setups (multi-link) are much better when it comes to isolating road undulations.

The forces are still transmitted, hence reducing the comfort levels of the rear passengers.

The main benefit of independent suspension is the isolation of undulations.

The ride comfort of torsion beam setups can be improved with the design of the beam itself (thickness, welding points, shape, pivot points). More and more modern cars are adopting torsion beam designs with minimal compromise on ride comfort, but there will always be a limit for the torsion beam design.

Con - Limited design and modification potential

The toe and camber are pretty much fixed on a torsion beam, unless you use custom washers to adjust the wheel angles (tedious).

The torsion beam suspension has limitations and when it comes to modification. Of course, this only concerns those who enjoy fiddling with their cars. With a torsion beam design, you are stuck with one camber curve. It's rigid nature also restricts adjustment of the toe angle.

The camber curve shows how the camber changes with the vertical wheel movement. With a torsion beam, you can't change the camber curve.

However, if designed with handling in mind from the beginning, torsion beam setups should not be an issue for those who desire spirited driving.

Hot hatches like Renault Megane RS and FK2 Honda Civic Type R have proven that torsion beam suspensions are worthy of 'sports car' performance too. 

Pro ' Cost

For manufacturers, the torsion beam setup saves cost since it consists of less parts and is easier to assemble. It might actually cost more in R&D since handling, comfort, and reliability (fatigue strength), all need to be optimised for a single torsion beam piece.

As a car owner, the torsion beam is cheaper to maintain since there are only two bushings that mount the trailing arm to the chassis.

Pro - Space

The torsion beam setup allows for a large boot space in the Proton Persona

Generally, torsion beam suspensions allow for more space in the cabin and boot. They don't require sub-frames and anti-roll bars. That's why they are so popular in compact saloons and hatchbacks.

Some cars are switching back to torsion beam

Besides the usual econoboxes, some notable cars that use the torsion beam setup include the Mazda 3 and the upcoming Toyota Corolla Cross.

The case of the Mazda 3 is in an interesting one since it even uses a torsion beam setup on the AWD Mazda 3. With its varying thickness of the torsion beam, the left and right side feel almost independent of each other. The comfort is comparable to that of a multi-link setup. 

Another interesting case is the torsion beam setup of the Toyota Corolla Cross. Despite sharing the same TNGA-C platform as the Toyota Corolla and Toyota CH-R, the Corolla Cross uses a torsion beam setup instead of a double wishbone setup.

It seems the torsion beam technology has improved the years. Gone are the days of the full-on double-wishbone setup on all four corners of the first-gen Honda CR-V.

Are all multi-link suspensions inherently superior?

Just because a car uses a multi-link setup, it doesn't make the car superior in terms of handling and comfort. We've tried some cars with multi-link/double wishbone setups and some of them didn't live up to expectation in terms of comfort.

And when it comes to performance, hot hatches like the Megan RS and FK2 Type R show that the torsion beam setup is very capable if done right.

Besides the design, the tuning of the suspension setup plays a big role.

Summary

Obvious benefits of the torsion beam are the low cost and compact design. 

Its obvious flaw is the limited isolation of road undulations. Like it or not, there is still a physical bridge between the left and right side.

There are also limitations in terms of handling, but the modern-day car buyer puts more priority on costs and fuel efficiency. Just like many other automotive technologies, the torsion beam technology has also improved and is sufficient for the average daily driver.