Q & A: When Moving to a New Platform, Where Do You Start With Suspension Tuning?

Another Q & A, this one from Rad Tatis: "When moving to a new platform, where do you start with suspension tuning? Shock tuning? Sway bars? Alignment? Tire pressures?"

This is a long one. There are a ton of opinions on this, but I can share what's worked for us. I'm not going to go too far deep into the "how" on each one, but I think I can offer a quick overview. I'm also going to write it for the average person in a smaller home garage with a limited amount of engineering background and time.

Start With a Blank Slate

First, and this is a tough one for many, I pretend nobody else has ever owned this car before. I want to throw away preconceived notions, tribal logic, and other people's conclusions that may or may not be backed in science and as accurate as they claim. Every car we've set up has done this. It prevents you from going down a rabbit hole where you follow a superior driver's path in a drastically inferior car, and fight your own feelings about the car because they can outdrive the competition even though their car is worse. So whether it's some obscure platform, or a Miata or a BRZ, I look at this the same.

Step 1: Read the Rulebook

Make sure you understand what you can and can't do within your ruleset. Don't pick and choose based on what "everyone does", pick and choose based on what the rules allow you to do. There are a lot of pieces here that the folks who simply copy others miss out on. There will probably be parts of it you don't immediately understand, or have to google and research a bit to grasp. There will be parts of it you'll cycle back to when you have a better understanding. This part is cyclical, and I like to do it pretty often. It also will help you from mistakenly installing a part "the collective" thinks is legal, but is explicitly forbidden by the rules.

Step 2: Motion Ratios and Corner Weights

I want to know motion ratios and corner weights for the car. Even better if you can get toe + camber curves for each axle. How to measure these exceeds the scope of the article, but they don't need to be absolutely exact. Your corner weights will vary as prep continues (think about something like installing a 2 lb battery to replace a 64 lb battery on a BMW), but getting it in the ballpark is key here. That said, getting the motion ratios correct will impact every downstream decision, so being pretty close is really key.

Step 3: Check the Differential

Am I likely to be hampered by the differential in the car? If so, this is something I really like to tackle early, as patch fixing around a too tight (less common) or too open (more common) diff will result in a lot of errors with trickle down effects as you continue. A great idea for a guest article from an excellent diff builder.

Step 4: Spring Rates

Figure out what spring rates I want to run. I get this from the motion ratio + camber curves. The further divergent the camber curves are F/R, the more control I'll need here, and the less variable geometry I want. I target a 10-15% stiffer rear wheel frequency than front. How stiff you go in total is a bit more of a question mark depending on the chassis itself, how small of a ride height range you want to keep the car, whether or not it's predominantly street driven or only occasionally, but I'd probably start, for 200TW classes, somewhere in the 2.3-2.7 Hz range up front. You can use other "levers" to impact the balance of the car, but this will broadly be good at control.

Step 5: Sway Bars

For many cars, this will require much more of a front bar contribution than will occur on typical "forum/internet" setups (or much less or no rear bar). You can spend the time modeling it, or you can make a reasonable guess and be willing to change.

Step 6: Dampers

Select and install your dampers. Find a shock builder, not a reseller, pass on the information you've acquired above and the intended use case, and ensure whatever you choose passes the sniff tests we've gone through in our variety of articles on damping. Penske resellers are exempt here, as Penske builds every set of shocks in-house, to customer spec. So the reseller is functioning as a go-between, not as someone selling you a "one size fits all" product. Make sure you're comfortable working with them, and make sure that there's enough flexibility in the hardware to suit your specific demands. If it's a one-size fits all valving and dimension, and they can't explain why they've chosen to build the damper how it is for you, move on. Set your dampers front/rear to their initial targets.

Step 7: Ride Height, Rake, and Alignment

Set ride height, rake, alignment. I like to start with a square ride height if I don't have a science-based reason to not start that way. Camber is going to be dependent on the platform itself and the camber curves you've come across. If you've gotten that part fairly close, the front camber will typically end up about a degree more than the rear. How much you run is going to depend upon the particular tire. I've yet to come across a 200TW tire / platform combination that works best with less than 3 degrees up front, closer to 4 is more typical. I like to begin with a baseline of a small amount of toe out, or none up front, with a bit in (1/8") in the rear for most platforms, though some cars can have very unusual toe curves in the ride height I want to run that would change that.

Step 8: Tire Pressure Split

The split should be broadly set, assuming you're just chasing optimal grip on the axle, based around corner weight. So a car that has 10% heavier front corners than rear? 10% less tire pressure in the back. Set tire pressure based around grip, not feel. You have a lot of other alternatives to change how your car feels.

Step 9: Test and Iterate

I almost always want to find a test day where I can take a lot of runs, or do a lot of laps, and make a lot of changes early on. I start off with the shocks set very softly to ensure they aren't contributing heavily to what I infer about the overall balance of the car. Get broad tire pressure testing done early (article to come!). Through this initial pass, you'll have a good # of runs in the car, and hopefully a laundry list of things you want the car to do better. Go attack each of them one by one, knowing that they all interplay with each other. I try to be as prepared as I can be for every test day and give myself the facilities to make changes as rapidly as possible in this environment, with the intent of both knowing the degree of impact each change has, and working towards a car that will need few if any changes mid-event in the future.

Great question, Rad. Each one of these points could probably spin off a few more articles of its own, so if there's anything you want to see further dove into, let us know!