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Strut Stuff

Making your car handle

by Julian Edgar

Click on pics to view larger images


This article was first published in 1998.

If you want your late model car to handle, you're going need to look at the struts. Most cars these days run strut suspension - if not on all four wheels, then on the front suspension. A strut is where the coil spring surrounds the damper which in turn is hidden away inside a tube. There's a heap of things that you can do to modify your struts - from just swapping springs to a complete strut re-design.

Factory Stuff

A typical MacPherson strut (sometimes called a Chapman strut when it's on the back) is pretty simple. There's a tube attached to a support that bolts to the hub assembly down the bottom. On the tube is welded a spring seat, with - wait for it! - the spring sitting on it. A damper ("strut insert") fits inside the tube, with the shiny piston rod sticking out the top and sometimes visible through the spring. Talking about the top, up there you'll find a mounting plate with a bearing in it that allows the strut to turn - if it's on a steering axle, anyway! And that's about it.

But like all things, there are some subtleties to it. Like, how far up the strut tube is the spring seat welded? And how long is the spring anyway? Is the spring a different length when it's on and off the strut? Is the strut mounting plate adjustable or is it fixed? There are lots of changes that can be made, culminating in its ultimate form in a strut fully adjustable for ride height, spring pre-load, spring rate, corner weights, and camber. All that can give an awesome improvement in handling.

Modifying Standard Struts

Tops
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If the strut is at the front of the car, it contains a bearing on which the strut pivots when you steer the wheels. There's also usually a big rubber bush and the threaded retainers that bolt to the bodywork. Replacing the rubber bush with a spherical bearing (as has been done here) will reduce slop substantially. The downside is that it will also transfer more noise to the bodywork!

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Moving the top of the strut closer or further away from the centreline of the car will change camber. If the car is prone to understeer (the front of the car sliding first), negative camber will normally reduce this. Neg camber is where the top of the wheel is closer to the centreline of the car than the bottom of the wheel. The camber adjustment slots can be seen on this fabricated race car strut top. Aftermarket camber adjustable strut tops are available for many cars.

Springs
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The most common mod made to a strut is changing the spring. Reducing the number of coils and/or increasing the thickness of the wire makes the spring stiffer. Spring stiffness (called the spring "rate") is measured in pounds per inch. This refers to how many pounds weight it takes to compress a spring by one inch. To improve handling, the spring rate is generally increased.

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The spring is pre-loaded when it's on the strut. This means that the length of the spring when it is on and off the strut is normally different - that's why spring compressors like this need to be used when removing the spring. The amount of spring pre-load and the rate of the spring will determine the ride-height. The higher the pre-load, the less the strut will compress with the car's weight on it. Specifying the right spring rate and pre-load is quite complex - there are heaps of mistakes that can be made. That's why we suggest that you buy aftermarket springs specifically made for your car.

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But if you're doing a real custom job (such as swapping the bottom half of another strut into your car to get bigger brakes), you may need to move or change the lower spring seat. The spring seat is welded to the strut, but it can be un-welded by careful grinding. This allows it to be moved up or down (changing the ride height of the car and the pre-load), or even replaced with the spring seat from another strut. This can be very useful if you are changing spring diameters. The moved (or new) spring seat can then be welded back onto the tube.

Dampers
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Another very common swap is to change the damper (shock absorber) for another, stiffer one. Over-the-shelf direct drop-in replacements are available for the struts in most cars. Putting in the new damper is as easy as pulling the strut out of the car, removing the spring, and then unscrewing the top retaining collar. The old damper can then be slid out, the new unit inserted, and then the collar screwed back on.

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But if you need to change the stroke of the damper (or you have a rare car) and there aren't any dampers available to suit, you can use a universal strut replacement damper. Instead of being held in place by the threaded collar, this design is secured into place by a new bolt inserted from the bottom of the strut. This takes away problems of getting the length of the damper body exactly right so that it suits the normal retaining collar. It also means that non-repairable struts can still be fitted with new dampers. Koni is one company making strut inserts that can be installed in this way.

Making Adjustable Struts

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But if you want the best in flexibility and design, you really need to put together a fully fabricated strut - basically, to build your own. The strut that you see here was developed by Ian Richards of Adelaide, South Australia for a unique application in a tiny four wheel drive Daihatsu Mira 660cc Turbo car. Apart from the lower suspension mount and the outside of the upper mount, all that you see here is new.

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The starting point was this seamless tube, to which the lower spring flange was welded. The factory lower mounting plate was removed from its strut (in this case a G100 Daihatsu Charade unit) and then welded to the tube. Ian used the Charade part to match the larger Charade hub and brakes being used on this car.

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Next up, a threaded collar was machined from aluminium, complete with two locking collars. Note that the left hand collar is quite thick, to get sufficient thread depth inside and so have the strength needed. This collar also acts as the lower spring seat.

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The threaded collar was slid onto the tube, the spring put into place, the damper inserted and secured by its bottom bolt, and then the strut top (which we've already seen above) screwed into place. Changes to ride height and spring pre-load are as easy as turning the threaded collars. Add to that the fact that the strut now has a Daihatsu Charade lower mount and a Daihatsu Mira upper mount!

Top Shelf Bits

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If you don't want to go to the trouble of developing your own adjustable struts, they are available in the aftermarket. Trouble is, they'll cost quite a lot and won't necessarily have your mix of requirements. These Japanese Tein coil-overs are for a R32 Nissan Skyline. Note how two different springs (with different rates!) have been used on the one assembly. Mixing and matching springs is a lot easier with adjustable platform struts, which usually have small diameter springs.

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These superb Australian Drummond Motorsports struts are designed for full-house rally cars and are used worldwide. They use external recirculation of fluid and are beautifully made. Some DMS struts are water cooled by an external pump and radiator!

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This Ford Mondeo 2 litre Super Tourer race car uses two different rate springs, a lower spring seat fixed on its thread by a grub screw, and a deep collar to keep the two springs aligned with one another. A major reason that race cars run height-adjustable struts is to allow the corner weights to be equalised. This is where the spring pre-load is adjusted so that the car pushes down on the road by an equal amount each corner. Handling improvements are the result.

Contacts:

Ian Richards (Adelaide, Australia)
+61 8 8363 5657
Drummond Motorsport (Wodonga, Australia)
+61 2 60 244734
www.dmshocks.com

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