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Suspension for Street, Autocross and Track Days

Suspension upgrades and settings are a big discussion topic anytime performance-oriented 944/924S people get together.  How can I make my car handle better on the track?  How can I tune my suspension to make it work better?  Let’s look at the various components individually: Ride Height, Shock Choice, Spring Choice, Sway Bar Choice, and Overall Setup/Alignment.

Ride Height

kIq9yGhyRdSznSH%zV9bhwStock ride height in the U.S. was determined by bumper height regulations back in the day.  Many car makers from outside of the U.S. found that in order to make the minimum bumper height rules, they simply raised ride height.  That also means that lowering your car down to a reasonable height is probably pretty easy – just reversing what the manufacturers did when new.

On the 944, the front ride height is set by the length (height?) of the spring itself.  Unless you change the springs, changing ride height in the front just isn’t possible.  Do not cut the spring to lower the car!  This can cause many more problems, and of course it is not reversible.  That said, the stock spring rates are made for a boulevard cruiser, so you are not going to get the handling you want by just lowering the car a little bit.  More on spring choices later.

There is some adjustment in the rear trailing arm plates that you can use to lower the car in the rear.  Using all of the adjustment to lower the rear of the car can help get it down to a reasonable point in a performance-minded street car, but if you want to put it on the ground, some serious and difficult modifications are needed – including changing the rear torsion bars.

Where to set your ride height is the bigger question.  If you’re racing, check the rules and make your ride height as low as the rules allow.  The lower, the better when it comes to handling, but there is a point where you will compromise the street-ability of the car.  Also consider ride height in relation to loading a track car on a trailer – you may create more problems than you solve.

Also, set ride height first before doing anything else.  Changing ride height changes everything else, so set your ride height prior to any alignments.

Shock Choice

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Shock Tower Brace helps to stabilize the tops of the struts

Most performance shocks for our cars are either Koni or Bilstein.  There is a reason why the various racing bodies restrict shock choices to one or both of these brands – they are reasonable in price and offer great performance.

The Koni Sport adjustable shocks are by far the more popular.  There is a single adjustment in these shocks – “rebound adjustable.”  That means that you can adjust the rate at which the shock gets longer – extends – either softer or harder.  The rears are difficult to adjust, but the fronts use a little knob on the top of the shock for adjustment – about two and a quarter turns lock to lock.

Spring Choice

Some feel that putting the highest spring rate on the car is the best idea – make is so that the suspension cannot go down when your big buddy sits on the fender.  Spring rate is defined as the amount of force (weight) that is required to compress the spring one inch.  Admittedly, the stock spring rates are not very good for spirited track driving.  The earliest 924 had a front spring rate of just 140 pounds.  the 944 started at 160 pounds and went up from there as the cars got heavier.  Most of the 944/951/968 front spring rates were under 175 pounds.  Bottom line – get new adjustable coil-overs for the front.

co-cv88-tomFor a street/autocross/track car, keep the spring rate well under 300 pounds – 250 seems to be a good start.  Over 300 pounds will create a car that will not transition well in the turns – will not “roll over” at all, causing very surprising attitudinal changes as the weight fights to transfer side-to-side.  Remember that with the right adjustable sway bars, you can make changes to help the springs stiffen up in the turns and resist roll. Make your choice adjustable so that you can more easily change the ride height.

As for the rear, you car probably has stock torsion bars.  Rear torsion bars started with a diameter of 22mm and went up to 26.8mm in some cars.  Most torsion bars are solid, but some are tubular (hollow).  Stock spring rates for the torsion bars range from 97 lbs to 175 pounds.  Changing the rear torsion bars requires that you drop the rear suspension out of the car to get access to the bars.  If that is going to happen, get plastic or delrin bushings for the suspension mounts at the same time.  You can also change the ride height at this point.  Rear coil-overs are also an alternative, but are expensive and require a lot of other modifications.  If you are racing, make sure that rear coil overs are legal.

Sway Bar Choice

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Weltmeister Front Sway Bar

 

The least expensive and most popular choice for sway bars is to go to the 944T (951) sway bars front and rear.  The 25.5mm front sway bar and the 16mm rear sway bar are a popular option.  Remember that the bushings for the sway bar will need to match the sway bar you choose – always use new bushings.

Another alternative that is popular – but expensive – is to go to Weltmeister adjustable sway bars.  This allows you to adjust the amount of force that the sway bar will exert during cornering.  Front and rear and cost upwards of a thousand dollars, but is the choice for racers.

Overall Setup/Alignment

So once you have set the ride height, chosen and installed the springs, torsion bars, shocks and sway bars that you want, where do you go from here?

Setup is a function of expected use and personal preferences.  Some will use their cars only on the track while others will drive them every day.  So let’s look at setups for street cars, autocross cars and dedicated track cars.

alignment-screen.jpgStreet cars should use the factory settings for the alignment, although a little more front camber may be desirable.  If the front wheels are set about a half-degree more negative camber, there will be little effect on street driving or tire wear but better outside front tire “bite” when turning during spirited driving.  Many alignment shops do not have the tools needed to change much in the rear, so going with factory settings in this case is probably the best idea.

For those who drive on the street and also do some autocross, the setup can be a little more aggressive, but be careful not to sacrifice daily driving for sixty seconds on the autocross track!  With adjustable camber plates, adjustable shocks and adjustable sway bars, you can make some small changes at the track for the day, then revert back to more normal settings for the rest of you street driving.  With that said, you can stiffen the front shocks, stiffen the sway bar a little and also put in a half-degree more of negative camber on the front for the autocross day.  Again, these adjustments are related to your driving style, other equipment on the car and many other factors.  

For dedicated track cars, getting the alignment right is critical.  “Those who know” will tell us that you should have a pretty aggressive setup for the track, allowing small adjustments with the front shocks and maybe some camber adjustments to account for track changes during the day.  For the rear, a tiny bit of “toe out” will help stabilize the rear of the car.  Put as much negative camber in as you can – normally 2 1/2 to 3 degrees if you can get that much.  For the front, the same camber is desirable with no toe in at all – if you have to, a tiny bit of toe in is all you should need.  Use factory settings for caster.

Of course, these are all starting points, adjusting as needed to make the car suit your driving style, overall weight, tire type, etc.  Remember too to only adjust one thing at a time so that you know what effect that one change had on the car.  Multiple changes at the same time leads to confusion and normally disappointment.

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Nort Northam (left) and Aaron Jones (right) pulling out for the parade lap at the OctoberFast PCA Race Weekend at Daytona International Speedway, 10/2018.

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Driving, How-To

Kevin Duffy, Author and Chief Geek View All

Kevin Duffy is an Associate Professor of Criminal Justice at Daytona State College in Florida and a dedicated car guy. He divides his time between teaching criminal justice topics in the online environment and working on/driving cars, particularly Porsches. Kevin is one of the principals in InspiringLifeOver50.com.

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