Budget FWD suspension upgrade, Part 3

In this series we’re spending minimal money to upgrade a light-weight, front-wheel drive car’s suspension. The car uses MacPherson front struts and a torsion beam rear axle. Making it harder, it’s also a car for which there’s no readily available aftermarket parts. So far we’ve covered measuring the standard spring rates (see Budget FWD suspension upgrade, Part 1) and specifying the new springs (see Budget FWD suspension upgrade, Part 2).

This issue, we’ll look at roll stiffness and add some new rear dampers.

Springs

In previous parts in this series we looked how to calculate the requirements for new springs. If you read those stories, you’ll know that some simple measurements and maths allowed us to:

(1) find out the characteristics of the standard springing

(2) devise new spring specs that gave the required new ride height and spring stiffness.

We also introduced the idea of measuring natural frequencies of suspensions. This is something really easily done with a smart phone and a vibration measuring app. Measuring natural frequencies allowed us to see not only the stiffness of the springing, but also relate that to the car’s weight working through that spring.

At the end of those stories we’d got the following suspension changes in place:

Front springs:

	Old springs	New springs
Spring rate	2.0 kg/mm	2.9 kg/mm
Natural frequency	1.6Hz	1.9Hz

Rear springs:

	Old springs	New springs
Spring rate	1.4 kg/mm	2.1 kg/mm
Natural frequency	1.7Hz	1.8Hz

Note that the natural frequency shows what is actually happening on the car – it takes into account motion ratios, the variable rate of the springs, etc. Go here to see more detail on how to do these frequency measurements. An additional note: the higher the frequency, the stiffer the suspension.

Roll rates

Increasing spring stiffness of both the front and back suspensions also increases roll stiffness. That is, the stiffer front and rear springs, when compressed in roll, will better resist roll - and so roll angles at the same speed and around the same corner will be reduced.

However, cars have specific springs that come into effect only in roll (and on one-wheel bumps) – these are called anti-roll bars.

The example car that we’re using in this series (2001 Honda Insight) has a front anti-roll bar. The rear roll stiffness is supplied through twisting of the open-U of the torsion beam rear suspension.

So firstly, how stiff is the standard car in roll? The easiest way of measuring this is to measure the natural frequency in roll.

Using the smart phone and vibration measuring app, and bouncing one side of the car manually, the standard car natural frequency in roll was measured at 1.7Hz. With the front springs providing 1.6Hz, and the rear springs 1.7Hz, you can see that the front anti-roll bar and rear torsion beam added very little roll stiffness.

So there was plenty of body roll – most roll resistance was being done by the wheel springs, not the anti-roll springs.

Front/rear roll stiffness

But what about the relationship between the roll stiffnesses at each end of the car?

As the tables above show, the rear end (1.7Hz) is actually effectively stiffer than the front end (1.6Hz). This is what is expected (especially in a front-wheel drive), as the extra rear roll stiffness helps hold the front flat without lifting an inner front wheel (and so bleeding away power through wheelspin). It also allows the front tyres to do their dual torqueing/turning duties more easily.

But in the real world, driving the car hard tells you more about what front/rear roll stiffness you’d actually want to have. In short, in a front-wheel drive, if understeer dominates, stiffen the rear until throttle lift-off oversteer, or turn-in oversteer, becomes too pronounced. The example car has pronounced understeer, and so stiffer rear roll stiffness was needed.

Let’s look again at the change made to the springs:

	Old springs	New springs
Front natural frequency	1.6Hz	1.9Hz
Rear natural frequency	1.7Hz	1.8Hz

In this case, you can see that while both front and rear springs were made stiffer, the front has got up a greater amount in stiffness than the back. But hold on, we want a greater rear roll stiffness! So why stiffen the front more than the back? One reason that this was done is because if you get all the rear roll stiffness increase from just the wheel springs, the ride becomes too hard. Better to leave the rear springing (relatively) soft and then add an extra anti-roll bar, a spring that acts only in roll (and also in one-wheel bumps).

Therefore, in this case, extra rear roll stiffness needed to be provided by an additional anti-roll bar. So how do you do that with a torsion beam rear suspension?

Torsion beam roll stiffness

In plan, a torsion beam suspension is usually shaped like a slightly odd-looking ‘H’.

Here is the view looking down on a torsion beam suspension. The green rectangles are the tyres. The purple blocks are pivot points attached to the body and the black lines are the suspension members. The thick blue line is the transverse torsion beam that is welded to each of the trailing arms.

Imagine both wheels pass over a single upwards bump – like a speed hump. Both wheels move upwards as the trailing arms rotate around their forward pivots. As the wheels move upwards, so does the torsion beam that links them.

Now imagine that only one wheel passes over an upwards bump. This time, only one trailing arm pivots around its forward point as the single wheel rises. But what happens to the torsion beam? If one wheel rises and the other does not, the torsion beam must bend (to accommodate the different heights to which the trailing arms have risen) and it must also twist (because a turning moment occurs).

The torsion beam is able to accommodate these movements because it is appropriately shaped from the correct steel grade to allow it to flex in both bending and torsion.

So what’s this got to do with body roll? Lots!

The more than you can configure a suspension system so that the left and right wheels always move up/down by the same amount, the less body roll that will occur when cornering.

As its name suggests, in the torsion beam design the axle beam acts like a torsion bar. In most cases, the torsion beam is made from an open ‘U’ or ‘V’ section of rolled steel. Such a design has little torsional stiffness – although since these beams are made from thick gauge material, they still provide some torsional resistance.

Many car designers leave the set-up like this but others add a hollow or solid steel bar that runs inside the torsion beam, linking the two trailing arms and giving the beam greater torsional resistance. In this type of suspension system, this round bar or tube is the anti-roll bar. In this diagram it’s coloured green (and here is called a stabiliser bar.)

To improve the stiffness of a torsion beam rear suspension, it’s therefore normal to add an internal member that resists twisting.

Front anti-roll bar? Most front-wheel drives benefit far more from an increase in rear roll stiffness through the fitting of a new rear anti-roll bar, with the front roll stiffness increase provided only by the stiffer springs. Increasing front roll stiffness with a larger anti-roll bar can cause issues when the front inside wheel is unloaded (the anti-roll bar tries to lift the inner wheel) and so causes inner wheel spin under power.

Doing it

In our example car it was decided to improve rear roll stiffness by adding an internal bar to the torsion beam. The starting point was the hollow steel bar used for this function in the rear axle of a Toyota Corolla (early Nineties model). The bar was shortened and new mounts welded to it. It was then mounted within the open ‘U’ of the torsion beam.

With the new stiffer springs and the added anti-roll bar, the roll natural frequency rose to 2Hz - compare that with the standard car’s 1.7Hz. Most of this additional stiffness came from the fitting of the rear ‘bar – the front/rear stiffness becoming more biased to the rear.

Note that the size of the bar was largely a guess: it’s difficult to calculate the required rear roll stiffness, and how much extra stiffness the bar will actually provide. Looking at what manufacturers have done on similar weight cars, and then adding a bit, gives a good starting point. However, plan to potentially trial different rear bars until one is found that best suits.

New rear dampers

New rear dampers were sourced from Gaz in the UK. Gaz has chosen to develop new dampers for the Insight – the only company in the world making these! The Gaz dampers are non gas-pressurised, single point adjustable (ie adjustable for bump and rebound with the one knob). The dampers are set at about the mid position in their adjustment.

Test drive notes So what were the results of these spring changes, the additional of the rear anti-roll bar and new rear dampers? Here are my test drive notes. Much reduced understeer, puts power down better on corner exits, quicker transient steering response, rear ride quality a bit firmer (i.e. one wheel bumps are stiffer). Higher speed turn-in and steering precision at speed absolutely transformed - road feels much wider. Can now pick cornering line to inches not feet. Large bump absorption at speed when cornering is transformed - feels like it has an extra 75mm of bump travel, not 25mm. Car maintains line with much more precision. Stiffer springs in this situation feel far more supple - obviously not hitting outside front bump stop as it did standard. Testing on slippery surfaces at relatively low speeds: slow speed turn-in and on-power - understeer; sudden throttle lift - oversteer. Higher speed turn-in on slippery surfaces - four wheel drift, balanced then with throttle. Rear damping now feels almost perfect – a touch less high speed bump damping would be nice when knob is set for correct rebound damping. Front dampers (only about 100,000km on them) feel very good with stiffer springs – no need to change.

Conclusion

With the fitting of the new springs, rear anti-roll bar and rear dampers, the car is absolutely transformed in its ride and handling…. and all at a very low cost. If you want to save money and end up with a car that does what you want it to do, look at doing your own suspension development.

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