WRX & Sti Suspension

So you’ve got yourself a Subie (WRX / Sti) and the thrill of carving through corners has got you pumped, but now you want more! Well here is some, hopefully, useful info. The following information is provided from a non technical perspective and is based on personal experience and research. If you want to get more technical just do a search on the terms used, there’s plenty of info on the WWW.

The best approach is to start with a plan. Try and think about what you want out of the car and the type of personality you are (compulsive obsessive?). Perhaps you just want to sharpen the handling as little or eventually you want to get on the track for some weekend fun. Having some idea will help you avoid spending $$ on rework.

The Subaru Impreza – the good and the bad!  Your subie is built on a humble chassis with fairly middle of the road McPherson Strut suspension, which is basic and reasonably effective. However with the Subaru AWD system the car gets an extra helping hand which helps to stabilise it and also masks some of the inherent limitations of the suspension. And with the stiffer springs and shocks in the WRX the whole equation gets better, and with the Sti variant it goes another step with increased damping and spring rates plus some other tricky bits.

Like most typical sedans the Impreza ‘s suspension geometry is designed with safety in mind...read understeer (the car runs wide or front end pushes out at the limit of adhesion). Understeer is easier to control, usually by backing off the accelerator pedal, which is typically your first reaction when your over cook it in a corner.

So what can you do to increase thrill level next time you take your subie for a drive. Well he is a list of bang for your buck (BFYB) improvements you can make listed in order of priority and cost.

·         Correct tyre pressure

·         Good wheel alignment

·         Driver training

·         Sway bars

·         Springs

·         Shocks

·         Alignment & chassis stiffening products

You can make a noticeable difference to the handling and balance of your car (WRX or STi) with a quick visit to the local service station. Higher tyre pressure will make the car handling sharper (less deformation of the tyre sidewall) and generally improve stability. Don’t be afraid of it, the only drawback is a slightly harsher ride.

Keep in mind the front tyres will need more pressure (more weight on the front) and this will decrease your understeer. A good performance starting point is 36 psi front and 33 rear. You can go more but don’t exceed the limit of the tyre (usually printed on the sidewall). Remember, the tyre pressure will increase a few psi going from cold to hot.

A good wheel alignment is a must for good performance and longevity of tyres. This is where the whole discussion can get complex, when we start to talk about the basics like camber, caster & toe. So let’s keep it simple, but if you want to know more just do a search on the WEB.

The best approach is to find a good performance alignment shop that’s had experience with Subaru’s and ask them to set your car up for a performance set up. If you want a recommendation feel free to email or give Zashtone a call.

Generally the only adjustments you can make on a Impreza WRX / Sti is front camber and front and rear toe. The more negative camber you set up the better the car will turn in to and grip in corners. About -1 deg is the max you will get from an Impreza. Negative camber (your wheels are tilted in on a vertical axis) can wear the inside of your tyres (tyre is angled more on the inside edge). However if you are a spirited driver -1 degree front camber shouldn’t be a problem on an Impreza (if you can get that much from stock settings). If you keep toe in setting to 1mm or less this will help to alleviate tyre wear with more negative camber.

You can make rear camber adjustable with the addition of camber bolts (about $70-80 in parts)

The inherent Impreza understeer is really starting to bug you now and you want to know what else you can do. And maybe you’re pushing it a bit harder and the body roll is starting to become noticeable.

Increasing the size of Sway bars or anti roll bars could be the answer. As the term implies the bars will help to reduce body roll. The benefit of sway bars is that they effectively increase your cars spring rate without making the ride noticeably harsher (except where one side of the car is in on a bump)

A good starting point is to upgrade the rear sway bar. The cheapest option is a new rear 22mm bar (around $260 fitted) or for a little more you can go for an adjustable rear bar which gives you a little more latitude for fine tuning. The rear bar will reduce some of the body roll and help the front tyres (yes the front) bite more, hence improving front end grip and reducing understeer (yippee).

Be careful, and take it easy in getting used to driving when making such changes. Like Ying & Yang, reducing understeer means you are getting more oversteer (the tail of the car going sideways at the limit of grip). Oversteer followed by fishtailing is more difficult to recover from and needs to be treated with respect (remember the driver training).

Of course adding a front bar (costs a little more to install) will help to balance the equation and further reduce body roll. Again with adjustable bars you can tweak the balance and over / under steer behaviour.

How big can I go? You will see good results from 22mm adjustable bars front and rear. Some like to go 24mm rear and 22mm front to further neutralise the understeer characteristics. This can be fun but it also introduces other problems like rear wheel lift and sometimes unpredictable oversteer.

A 24mm bar may be exceeding the limits of your springs so you may see less rear traction due to inside rear wheel lifting and spinning. Transition through a corner can be more violent when you lift of the throttle and or have to brake. The car can snap back quickly with a surprising amount of oversteer that can be difficult to catch and you may find yourself pointing in the opposite direction L.

Now you’re starting to get really hyped and you want more (almost time to start thinking about the local MA group). Let’s look at springs

You car has coil springs on each corner. Upgrading the stiffness of springs will reduce body roll, improve control and general performance of the car. There are a broad range of aftermarket springs available. Most of them are a progressive rate springs, ie softer in the intial part of the spring/coil action to help maintain ride quality. Springs take more work to install so choose carefully as it’s an expense process to have to repeat.

Most aftermarket springs will lower the ride height of the car to a degree. Don’t get carried away here, going to low can completely compromise the handling performance of your car and not to mention the really harsh ride. Impreza’s WRX / Sti have very limited suspension travel and reducing this means you are losing the benefit of your springs and shocks as your car will be riding on stiff rubber bump stops. That’s bad for your handling, really bad for your ride quality and it will shorten the life of the suspension and other part of the car.

Up to 25mm drop is the max you want to go and even that needs some modification of bump stops to recover lost travel. You may have noticed that Impreza’s have a bit of a saggy back end look, this is due to the front / back difference in gap between wheel arch and tyre. Some springs adjust the drop to compensate for this. Spacers are also available for the back if it really bugs you, but don’t go overboard as you will screw the suspension geometry.

The best advice is to stick to reputable brands and ensure that the spring rate is matched to your shock absorber. Stiffer is not better, poorly matched springs will stuff your ride quality and potentially give your unpredictable handling. As an example, Sti springs are too stiff for WRX shocks.

Things to be aware off when using lowering springs! You may need to fit a camber pin to the rear suspension as the rear may go too negative and cause premature tyre wear. There is a little scope for fiddling with Impeza WRX / Sti rear camber but it’s not adjustable. A set of camber pins will allow you to adjust and correct the camber.

On the front end the lower springs will affect the ‘roll centre’ of your suspension geometry (complex techy stuff). You can correct this with a roll centre kit and it has other positive effects on your handling.

 

You want to go to a stiffer spring and or more performance. Your shocks are stuffed or you’re thinking, if I’m going to change the springs I may as well go the whole hog.

The big debate you will find here is between the strut spring set up vs. Coilovers. There is no right answer it all depends on your objectives.

Coilovers offer an alternative to the typical spring strut set up with the added benefit of being height adjustable.

Some pros and cons:

Pros:

·         There are a broad range of coilover options in the market

·         Ride height is adjustable (could also be illegal on the road)

·         Most offer adjustable damping

·         Some come with adjustable camber caster tops

·         Cheaper than spring strut set up (debatable if you compare quality)

·         You can slam your car

Cons:

·         Require a lot more fiddling to set up, more complex configuration

·         You may be buying a lot of adjustability you don’t need or want

·         They can be noisy due to their inherent design (particularly the cheaper ones)

·         Ride quality can be an problem (particularly with cheaper units)

·         May require more maintenance, depending on use

·         You can slam your car

 

There are a range of strut shock set ups for the WRX and less so for the STi with the inverted strut design. Like coilovers you can get adjustable struts which a usually come with a simpler single knob mechanism that adjusts both damping and rebound.

The benefit of adjustable shocks is greater scope for tuning and matching with springs. Also allows for a stiffer set up for track use or spirited driving.

Again stick to quality brands and spend as much as you can afford. Good quality shocks with matched springs can significantly improve handling while still offering a compliant ride. If you’re going to skimp then you’re better off sticking with the OEM gear.

It’s not uncommon to see enthusiasts replace newly upgraded shocks or coilovers because they’re not happy with the compromise in ride quality. Ie. the trade of in daily driving ability isn’t worth the performance improvement. The trick is getting gear that complements your car and properly matched springs and shocks.

There are a range of alignment products available to help you get more caster and camber and help correct steering geometry (see Roll Centre Kit mentioned earlier). We have touched on camber and another key area is caster. More +ve Caster helps to provide more dynamic  –ve camber (while your turning the wheel in a corner)

Some of the other more popular products include:

·         adjustable strut tops allowing easy adjustment of camber and caster

·         A range of stiffer bushings which help to keep the steering and chassis in alignment under cornering forces

·         Anti Lift Kits provide more caster and other things

Bracing is another popular and big discussion, but in reality, like some of the bushing products, the benefits are mainly felt on the race track. Your whole car is an extension of your suspension. Under severe cornering forces the chassis starts to deflect and twist and all the good static alignment settings will alter in use and affect the ability of your tyres and suspension to do their job.  Bracing can help to alleviate this but comes at a price!!

This is a good time to talk about NVH which is a key design criteria car manufacturers aim for when building cars. Most of like quite, comfy and smooth cars...don’t we ??

Unfortunately just about all these handling improvements we discussed with have a negative impact on NVH. Some maybe barely noticeable ( a good wheel alignment) while others will be severe ( xtra heavy duty shocks/springs and bushings)

This is a choice you need to make, how much of a compromise will you (and wife , kids & dog) be happy with for the extra benefits in handling performance. When you’re considering these options don’t be afraid to ask, how much extra NVH will they create. Better still try and go for a ride in a car with these options, as smooth to some may be a nightmare ride for others.

Yes tyres, this is where the rubber meets the road so to speak and where all that good suspension work can be made good or wasted depending on the quality of your tyres.

This is a huge topic so do plenty of research and really buy the best you can afford. And if you can’t afford good tyres then forget about doing work on your suspension until you can.