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Budget FWD suspension upgrade, Part 2

This issue, specifying the new springs

by Julian Edgar

Click on pics to view larger images


Last issue in Budget FWD suspension upgrade, Part 1 we measured the spring rates in the standard car. This was able to be done in four different ways: on the car, off the car, by inputting the springs specs into an on-line calculator, or – most trickily – by using a smart phone to measure the natural frequency of the suspension at the front and the back.

Now it’s time to work out what the specs should be of the new springs.

New springs

  • Front/rear balance

Almost always when modifying springing in cars, stiffer springs are selected. These will give less body roll (and pitch and squat) and generally improve handling. However, the amount that you go up in stiffness may not be the same for the front and the rear.

For example, in the front-wheel drive car being used as the example here, the rear stiffness (as indicated by the frequency measurement) is greater than front stiffness. (A higher frequency = a stiffer suspension.)

Natural frequency

Front

1.6Hz

Back

1.7Hz

Making the rear even stiffer compared to the front is likely to reduce understeer - for example, increasing rear spring stiffness by say 60 per cent and front stiffness by 40 per cent.

So the first decision is: will you increase the stiffness equally front and back, or will one end be made proportionally stiffer? Note that if you intend adding a much stiffer anti-roll bar (to be covered later in this series) you may choose to leave that end of the car a little softer than you otherwise would. This is because in single wheel bumps, the anti-roll bar adds to the effective spring stiffness, and going stiff with both an anti-roll bar and rear springs may give an unduly harsh ride.

  • Picking the ride height

The next issue is working out the ride height to aim for. Most people go lower with their new springs, so reducing the height of the centre of gravity – and so reducing roll. However, going lower means that there will be less bump travel – that is, the bump stops will be impacted earlier.

If you drive on only smooth roads, this will not be an issue. However, if you drive on rough roads, this will potentially be a huge issue – the effective spring rate (stiffness) will go up very quickly as the bump stop is compressed (see below for more on bump stops).

So why is this an issue?

When the car is cornering, the outside springs will be more compressed than the inner springs. If a bump is then met by the outside wheels, the bump travel may be reduced to nearly nothing. The bump stop is fully compressed, the spring rate rises dramatically - and the car bounces off the road.

By all means select springs that lower the car – but only if sufficient bump travel remains.

In the example being used here, the standard car was close to the front bump stops at normal ride height. At the rear, adding even a small load could cause the rear bump stops to be impacted. That is, the standard car was too low – even with no modifications at all! (This was done by the manufacturer presumably to improve aerodynamic drag.) Lowering this car further would never, in any real road circumstance, improve handling. So in this case, stiffer springs were specified that actually increased ride height by 25mm (1 inch).

So how do you work out how much bump (and droop) travel you have as standard?

1. Jack the car up so that the wheels are at full droop. Measure the distance from the bump stop to the part that impacts it at full suspension compression. (With a strut you may need to remove the dust cover or feel the position of the bump stop through it.) This measurement is the maximum suspension travel before the bump stop is impacted.

2. Place the car back on its wheels, go for a short drive to settle the suspension, and then measure how much of that travel has been ‘used up’ with the car sitting statically, its weight on the springs.

For example:

  • Maximum wheel travel until bump stop impacted: 140mm

  • Static wheel deflection: 70mm

Therefore 70mm of the 140mm possible travel is used up by the car’s normal weight. That means this suspension has 70mm of bump and 70mm of droop capability. If you lowered the car by say 50mm, you’d have left only 20mm of bump capability! Note also that if you load up the car (eg four people), you’ll probably use this travel up before a bump is even met…

Making this all a little better is that when you fit stiffer springs, the size of the bump needed to use up travel is greater than it was for the standard springs. In other words, the springs will compress less for a given bump, so you can get away with less bump travel than with the normal suspension.

So how much of the travel should there be for bump? Typically 50 per cent of total wheel travel should be for bump.

  • Picking springs to give correct ride height

So you’ve worked out what ride height you want and how much stiffer you want the springing. Now how do you specify a spring to achieve this?

The spring rate (stiffness) depends on wire thickness, number of free coils and diameter. The diameter will normally be fixed (it will be the same as the standard springs) so that leaves wire thickness and number of coils. Spring stiffness increases rapidly with small changes in wire thickness, and increases more gradually with a reduced number of coils.

By far the cheapest way of obtaining new springs is to source the standard springs from another, heavier and/or sportier car. I’ll cover doing that in a moment. However, what free length of spring do you want?

Here’s where you need the measurements we took last issue of the standard car. For the front springs of our example car:

Weight acting through spring

255kg

Standard spring rate

2 kg/mm

Standard spring free length

280mm

So the actual compression of the standard spring is found this way: (weight acting through spring) divided by (standard spring rate), that is, 255kg divided by 2 = 128mm.

The free spring is 280mm long and is compressed by 128mm = 152mm compressed spring length with weight of the car on it. (Note: spring preload makes no difference to this calculation.)

If we wanted to lower the car by 25mm, we’d want a compressed length not of 152mm, but of 127mm. (This assumes that spring travel = wheel travel.)

Let’s say we go up in spring rate from 2 kg/mm to 2.9 kg/mm. Now the weight of the car will compress that spring by (255kg divided by 2.9) = 88mm. We want a ride height that corresponds to a compressed spring length of 127mm. We just found out that with the new spring rate and the mass acting through it, it will compress by 88mm. Therefore, 127 plus 88 = a free spring length of 215mm, some 65mm shorter than standard.

To summarise this example:

Old spring

New spring

Rate

2 kg/mm

2.9 kg/mm

Free length

280mm

215mm

Result: 45 per cent stiffer spring, suspension lowered by 25mm.

Note: You’d need to ensure that a 65mm shorter spring will remain captive in the suspension at full droop.

Bump stops

In most modern cars the bump stops are an integral part of the springing. That is, they are designed to be impacted reasonably often and act as rising rate springs. When modifying suspension, ensure that the bump stops are as progressive as possible (e.g. special foam, or shaped rubber) and use them as part of the springing system.

Click for larger image

For example, the bump rubbers on the left replaced the standard items on the right to give a much more progressive stop. Good bump rubbers can be bought from wreckers for peanuts – have a look at what is available before buying new.

Sourcing springs

When you know the required spring rate, diameter and free length, you have a few options.

As described above, scouring a wrecking yard for springs with these specs is by far the cheapest. You probably won’t be able to measure the rate easily, so look for springs with the same diameter and number of coils as your existing springs, but shorter in length (to match your requirements) and with slightly thicker wire.

You can also get the springs custom made to your specs, or if you have a car where off-the-shelf springs are available, you can see if a company already makes springs of the specs you want.

Example car – 2001 Honda insight

Front springs, standard Macpherson strut

Spring rate

2.0 kg/mm

Free length

280mm

Weight acting through spring

255kg

Compressed length with weight of car

152mm

Natural frequency

1.6Hz

Front spring requirement:

  • 45 per cent stiffer – therefore, 2.9 kg/mm

  • 25mm increase in ride height (car was too near bump stops in standard form)

Car weight acting through spring divided by spring rate = 255 divided by 2.9 = 88mm spring compression.

New free length calculation:

Required compressed length = 152 plus 25mm (ride height increase) = 177mm.

88 (spring compression with static load) plus 177 (required compressed spring length) = 265mm required free spring length. That is, a free length 15mm shorter than standard.

New spring actually used:

Toyota Corolla rear spring, 1.5 coils removed and end heated, closed up and ground flat.

New front springs:

Spring rate

2.9 kg/mm

Free length

265mm

Weight acting through spring

255kg

Compressed length with weight of car

177mm

Natural frequency

1.9Hz

Click for larger image

This pic shows the new front springs.

Rear springs, torsion beam rear axle

I won’t show all the calculations, but the new rear spring requirements were done in a similar manner.

Standard:

Spring rate

1.4 kg/mm

Natural frequency

1.7Hz

Modified (+25mm ride height):

Spring rate

2.1 kg/mm

Natural frequency

1.8Hz

New spring actually used:

Standard Daewoo Matiz rear springs

Click for larger image

This pic shows the new rear springs.

Test drive notes

So what were the results of these spring changes?

  • Ride is much 'busier', as you'd expect with stiffer springs and still standard dampers.

  • Big bumps absorbed much better - previous suspension was compressing bump stops a lot. Even when lifted 25mm, travel from static deflection to beginning of bump stop is only about 20mm at front. Standard Insight = use of progressive bump stop to give rapidly rising rate front spring.

  • Small bumps a little firmer - but much less so than you'd expect with an increase in spring rates of around 50 per cent.

  • Feels to be less body roll (even with higher centre of gravity through increased ride height), changes direction more quickly, but handling still very understeery (cf greater increase in effective spring rate at front). Will need rear anti roll bar fitted.

  • Definitely needs upgraded rear damping (that is coming).

Notes

1. Roads are far from smooth - poor secondary bitumen roads with plenty of bumps.

2. Slightly greater front mass than normal with added alternator, water/air intercooler, turbo

Conclusion

If you have ever got new springs custom made for your car without giving exact spring specifications, or have just slotted-in some springs that looked like they might be good, you almost invariably will have had problems with incorrect ride heights, or incorrect ride heights and spring rates.

The way to avoid that – and also potentially save a lot of money – is to do some measurements and calculations first. It takes a few minutes to get your head around the calculations but the results can be great – and cheap too.

Next issue: improving rear roll stiffness and new rear dampers

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