Rigidity is an important part of your cars handling. When you ask it to change directions any play in the suspension or chassis will be taken up before the car adjusts its course. Conversely if the car hits a bump or bit of road camber then the play in the chassis and suspension will be taken up before this interaction is communicated to the driver. As a result soft suspension and a chassis that lacks rigidity will dampen the cars responsiveness as well as the feedback it gives to the driver. By stiffening the chassis you also prevent the suspension geometry from changing under load (as the chassis flexes) and improve the overall cornering and grip characteristics of the car.
To improve the chassis stiffness/rigidity we can add brace bars or a full cage between key load bearing points of the car's chassis. Roll cages in race cars serve a second function beyond that of safety as they dramatically improve the rigidity of the car but on the flipside are not practical for everyday road use. Not only do they impair access in and out of the car but, they pose a serious threat to the wellbeing of your skull in the event of an accident, and as such should be used in conjunction with a helmet! They also weigh a fair amount and so if you are striving to up the power to weight ratio then a full cage will not help. The road going compromise is to fit a/several strut brace/s. These tend to be rigid bits of metal that span the gaps between key chassis and suspension pick up points. There are many different designs available to suit different cars but there are three common types: front upper, front lower, and rear.
A front upper strut brace (Eibach £90, Autocavan £50, Sparco £70, omp etc.) spans and braces the tops of the two front suspension turrets. They also have a cosmetic benefit in that they are available in a range of finishes (polished, anodised etc.) and so add the finishing touch to any designer engine bay. Rear strut braces span and brace the tops of the rear suspension turrets, and some also include a diagonal which bolts down to the rear seat belt mounts (as these are reinforced) and this design is preferable. Again these can serve a cosmetic function if you have opted for the stripped out racer look. The front lower brace goes between the front lower wishbone pick up points and unless you exhibit the underside of your car with mirrors then they serve no cosmetic benefit.
The Mk 1 cries out for a front lower brace to be fitted and it will improve handling and responsiveness no end. It is a well renowned Achilles heel and so much so that VW fitted a front lower strut brace as standard to the 16v Scirrocco (Mk 1 chassis) and to the very late Mk 1 cabs. The brace is a simple affair and so weighs in at about £50. The front upper brace has less effect on the Mk 1 and will probably only serve as a cosmetic addition, unless the rest of the chassis is extremely well sorted. The same goes for the rear brace.
The Mk2 benefits immensely from a front upper brace especially if you are running very stiff suspension. A front lower brace will do very little as the Mk 2 already has a fairly beefy front subframe. Ditto for the rear brace, which tends to be a cosmetic mod only. Obviously if the car has seriously sorted suspension then it will benefit from any of these braces just as a race car benefits from a full cage, but the benefits will be less noticeable than the key mods highlighted above.
The Mk3 and 4 are stiffer cars out of the box and so don't have any particular weak spots (more metal, more weight, more rigidity). As a result strut braces have less of a noticeable effect but are a worthy addition if you have already fitted some sports suspension. On a general note it is not worth fitting a strut brace until you have uprated the suspension as any gain will be lost in the soft suspension.
Fitting wise each brace is fairly simple to fit. The lower brace bolts between the forward facing bolts that secure the wishbones to the sub-frame. Remove these bolts having supported the weight of the car on a jack/s and then replace the bolts securing the brace in the process. Do not tighten the bolts until you have the car off the jack/s and allowed the suspension to settle. Front upper braces usually bolt to the top of the suspension turrets. Lay the brace in place and mark where you need to drill the holes.
Check the clearance on the bonnet before drilling as you may find that the brace needs moving slightly one way or another. Try not to line the brace up so that you end up drilling through the spot welds that go round the top of the turret. Carefully drill the holes and rustproof the bare metal of the hole bores. This next part tends to be a two- man job unless you have fairly long arms. Jack the car up until the suspension/wheel drops low enough that you can get your arm into the wheel arch (make sure the car is properly supported), some cars will require removal of the wheel and possibly the wheel arch liner (this was not necessary on my Mk2).
Line the brace up with the holes and put the bolts through the holes using a washer to bear against the brace. Reach up into the wheel arch and put the nuts on the ends of all the bolts, again using washers on each. You can then tighten them down using a ring spanner on the bolt heads and a socket on the nuts. You will probably need an extension piece on the socket.
Once tight on both turrets the car can be put back on the ground and you can admire your handiwork. Check the clearance on the bonnet again. You may find a small point of contact, which is nothing to worry about, but if the brace impairs closing the bonnet then you should remove the brace and start again. Some braces are shaped in such a way that you need to move some of the ancillaries in the engine bay in order to get the correct clearance (such as the expansion tank). This is something to check before you buy.
The rear braces are secured in one of two ways. Either they bolt directly onto the inner arches or they have a U-Shaped clamp, which goes round the top of the strut and can be tightened into place. This later method requires no drilling and so is slightly easier. If the brace bolts straight onto the inner arches then the procedure is similar to that of fitting a front brace. Jack the car up and remove the two rear wheels. Depending on access and the exact dimensions of the brace ends you may have to remove the rear suspension as well. Remove the parcel shelf and carpet from the boot and offer the brace up between the suspension turrets.
Mark holes (usually four at each end of the brace) on the metal work and drill out to the correct size (determined by the diameter of bolt you are using, which should be the largest you can fit through the holes in the brace). Due to limited access in the wheel arch you will have to drill from the boot outwards taking care not to go to far through the other side as you might catch the drill bit on the suspension components. Also take great care in the rear right arch as the fuel filler neck lives here.
Thoroughly rust proof the holes (Waxoyl is ideal) and bolt the brace in place using a washer on either side of the cars metal work. Check that the bolts do not interfere with the movement of the rear suspension (i.e. catch on the rear springs) and then assuming all is ok the wheels can be replaced and the car lowered to the ground. You may want to bolt the brace over the carpet that covers the rear turrets in which case this must be put back in place before bolting the brace in place. If you do not do this you will have to cut a neat slit into the carpet in order to fit it back in place around the strut brace.
There are numerous different designs of brace on the market and some are better than others. Obviously if you are fitting the brace for cosmetic purposes then pick which ever one looks the best, otherwise just consider some of the basic laws of physics. As far as front upper braces go the stiffest tend to be the ones that triangulate onto the suspension turrets but these also give more clearance headaches than other designs. Remember that the brace is only as strong as it's weakest part so despite a chunky looking main bar if this bar tapers down to nothing at each end and the welds are puny then it will not be very strong. Braces, which are adjustable in length, are useful as they allow a degree of leeway when securing, in order to clear any ancillaries or wires that may sit around the turret tops. Some braces (as mine is) can be detached from their mounts which makes servicing and access to the back of the engine bay far easier than it would be with the brace in place. Another key consideration is the mounting method. Four bolts on each end can spread the load far better than three or two and on the whole the ring style mounting braces tend to be the most secure.
Front lower braces do not vary much in design other than to be adjustable in length that may help compensate for any slightly out of line bodywork. They also can vary in their strength (due to dimensions and material). Rear braces as already mentioned can bolt to the metalwork or clamp to the strut tops. The former is a fairly permanent mod (as it leaves holes in your car) whereas the latter is reversible. The strongest design of rear brace triangulates to the boot floor. Before rushing out to buy a rear brace do check your boot for clearance around any subs you might have in the boot as these may well foul on most conventional braces.
Another consideration is that of material choice from the weight and finish point of view. Whilst polished braces look nice, if you are never going to keep them polished then they will tarnish and you may be better with an anodised or painted finish. If you have stripped the car down to save weight or are looking for that extra 10th down the strip than a chunky steel brace won't help matters. You would be better opting for a lightweight alloy brace which whilst being slightly more expensive will not weigh so much.