Suspension Dynamics Quickie

Most of you know that I'm a bit of a night owl and do most of my work in the wee hours of the morning. One morning, a buddy of mine, Tyson, posted a really good question to the FWD 4AGE & Corolla Forum that Andy Karkos and I run. Tyson said, "...We could talk about what parts make a FF car do what... Like the different settings on an adjustable rear sway..."

I appreciated his attitude. Why? In the San Francisco Region SCCA, I see more than my fair share of "young ones" who keep throwing parts at their cars in hopes of getting faster. In most cases, they do get faster, but they're not nearly as fast as they could be if they just learned how to drive their cars. Even I made that mistake. But, I learned my lesson and stuck with the same setup for most of my novice autox season.

Tyson's question required a truncated lesson in suspension dynamics--what component of the suspension affects which component of a car's handling traits?

Although my reply was dedicated to our front-engined front wheel drive (FF) cars, all of the following information can be applied to most road-going race cars (even motorcycles, to some extent). The following is my attempt to explain how a suspension system's various parts affect handling and how to effect changes for the better.

Hopefully, you can take this information and figure out and decide for yourself how you would like to set up your car instead of relying on others' setups. Just like clothes, each driver has their own preference. Supposedly, the ideal handling setup is for the car to slightly understeer on corner entry, transition to neutral as you approach the apex, then finally transition to slight oversteer on corner exit. If you think about it, this gives the driver a safety net on unfamiliar tracks.

Pop quiz hot shot: How do you achieve the aforementioned balance? Let's assume that everything else in the suspension package has been optimized, so you only have two variables to work with; spring rates and strut valving. If you've read this entire page, it should be easy to figure out. The answer's at the bottom of this page. Take your mouse cursor, then drag and highlight the blank area between the last bulleted tire pressure item and the gray separation bar.

My preference is an understeering-to-neutral balance while on the throttle, then slight oversteer off the throttle--makes for a great point-and-shoot car that can change direction easily. Nope... None of that typical FWD understeer for me, thank you!

Adjustable Struts

  • Struts usually have separate valves that control the rate at which the piston rod compresses (bump or compression stroke) and expands (rebound stroke). The really nice (read: expensive!) struts have the capability of user-adjustable valving. Some struts are 1-way adjustable (rebound-only) while others are 2-way adjustable (rebound and compression). As we get into the higher echelons of motorsports, you can control the low- and high-speed rebound and compression valving. Crazy, eh?
  • Rebound: stiffer settings will slow down the rate at which the suspension reaches its designed roll angle under load. What does that mean? Slowing down the rate equals less transient traction when changing directions (e.g. slaloms and corner entry). Since we want our FF car looser in the rear for better rotation, we stiffen up the rear rebound--the front end sticks and turns, while the rear lags a little bit and isn't allowed ideal traction for instant, thus making the rear end "loose." If the car's too loose, I usually leave the rear rebound alone, but stiffen up the fronts a little at a time until the corner entry is to my liking. Of course, how much you stiffen the rebound at one end depends on your car's current balance.
  • Compression: Many people mistakenly use compression valving to limit the roll in transients. NOPE!!! Shouldn't do that... Compression valving should only be used to fine tune the overall suspension stiffness for the particular track surface; you can use relatively stiff settings on a smooth track. Conversely, if you use the same stiff settings on a bumpy, irregular surface, your car will inherently skip and dart all over the place... Makes for one helluva nerve wracking ride! And on top of that, your lap times will suffer.
  • The ideal strut will have the proper stroke (fully extended length minus fully compressed length) range for your car's ride height. At rest, your car should be near the middle of its stroke range--the standard is to have 55% of the total stroke available for the compression travel. Since most of us will drop the car's ride height anywhere from 0.5"-2.5", this usually means that the compression stroke is decreased to the point that we only have 1"-1.5" of compression travel if we're lucky. The solution? Shorten the bodies of the strut housings by at least 2". Unfortunately, this usually means going to a race shop like Truechoice or Advance Design (of Ground Control) to get modified struts. And we all know that custom fabrication equates to hurtin' bank accounts and wallets. If you're serious about racing your AE92 GT-S, though, there's no way around this.

    Spring Rates

  • This is what mechanically suspends your car and affects how the weight is distributed fore-aft, side-to-side, and diagonally.
  • Figure out your car's front-rear weight distribution so you can match the front and rear spring frequencies, taking the mass at either end into account. All this does is "suspend" each end equally under static conditions given the different mass at each end.
  • The AE92 GT-S approximately has a 60/40 front/rear distribution. Taking that into account, let's calculate the spring rates for a neutral handling balance.
  • Let's arbitrarily choose a 400 lbs/in front spring. It's stiff enough for autox use to keep the weight transfer to a minimum, but still comfortable (I'm a bit sadistic, though!).

     

    Front spring rate divided by front weight ratio = front spring frequency, so:

    400 lbs/in / 0.60 = 666.67 = front spring frequency

     

    Front spring frequency multiplied by rear weight ratio = rear spring rate, so:

    666.67 * 0.40 = 266.67 lbs/in = rear spring rate

     

  • So for a neutral balance, our "matched" springs are F: 400# and R: 266.67#. To shift the balance, all we need to do is increase the spring rate at the end that we want to lose traction at first. For more understeer, increase the front rates; for more oversteer, increase the rear rates. You can also drop the rates at either end using the same concept and still achieve the same results, albeit with a softer ride. I'd use 50 lbs/in increments because 25 lbs/in increments barely make a difference on our mass production-based (read: torsionally- and longitudinally-flexy) cars.
  • Starting with the "neutral" spring rates above, we can change the rates and affect the car's steady state balance... Understeer rates would be F: 450# and R: 266.67# (or F: 400# and R: 200#). Oversteer rates would be F: 400# and R: 300# (or F: 350# and R: 266.67#).
  • Don't go nuts making the rear spring rates so stiff for oversteer. In a tight course layout like your typical SCCA Solo 2 event, a really loose car will be a handful and kill your times because you'll be trying to catch the rear end so much.
  • Eibach's "ERS" springs are pretty much the standard springs in motorsports. They come in different free lengths, two outside diameters, and spring rates in 25-lb increments. Check out the 2.5" O.D. spring catalog to see what your choices are.

    • Adjustable Anti-Swaybars

  • Use bars to fine tune the car's balance--overall/general balance should be accomplished using frequency-matched spring rates for the particular F/R weight distribution.
  • Mounting holes towards the ends of the bars will make the bar's rate on the soft side.
  • Mounting holes away from the ends stiffen the bar's rate.
  • A stiffer bar will usually cause the car to react the same as stiffening the rates at one end; so, stiffening the rear will cause it to lose traction first which means the rear end will be loose.
  • If inside rear wheel still lifts off the ground a lot, you need to stiffen up front bar to plant the inside rear wheel. Stiffening the rear bar will actually exacerbate the problem.

    • Alignment

  • Toe settings can affect the car's turn-in response and steady state balance.
  • On our FF cars, we can run some slight toe out in front to "pitch" the car into turns. Be forewarned that running some toe out up front will cause the car to wander much more in the straightaways, especially if there are ruts in the surface.
  • A very small amount of toe out in the rear can help bring the rear end around and let you hold a tighter line. Don't go overboard with the rear toe out as it will easily cause the car to feel nervous.
  • Unfortunately, caster on the AE92 model (and most other Toyotas) is non-adjustable. You can buy some Whiteline bushings to allow for some caster adjustment, though. Running as much positive caster as possible will cause the suspension to gain more negative camber in a turn.

    • Camber

  • We want the tire's entire tread face to load equally under lateral acceleration (cornering). Using race and street tires with less than -1.75° up front (even that's a compromise I won't make) will cause the outside shoulders to prematurely wear out. Have you guys seen the pics of my poor Dunlops and Bridgestones after a full season of autox'ing and a full day of autox school??? -2.0° up front is a decent compromise between street and autox use. That said I'm currently running -2.5° up front and haven't noticed accelerated tire wear on the street.
  • Check for proper camber by using a tire pyrometer or something even more fancy--the palm of your hand! After a hot lap, pull into your pit area under moderate braking and immediately check the tire temps across the tread face. Divide each section of the face into thirds, going across.

    If the outside temp is hotter than the inside, you need more camber.

    If the inside temp is hotter than the outside, back off the camber.

  • You may need to slightly tweak the camber away from ideal in order to shift the car's steady state balance. This is at best a compromise and should be avoided if possible.

    • Tire Pressures

  • Prob'ly one of the more important variables.
  • Tire pressures are important because this is what actually supports the tire carcass!
  • Tires are an integral part of the suspension. The sidewall construction/design can and will affect the suspension because it acts as an soft extension of the springs.
  • Even through you may have picked the perfect mechanical suspension setup to get your target balance, running the wrong pressures won't allow you to achieve the balance under track conditions.
  • Ideal tire pressure can be confirmed by using a tire pyrometer or your hand, again. Temps should be fairly equal across the face.

    If outside and inside portions are hotter than the middle, you need to increase tire pressure.

    If middle is hotter than outside and inside, drop the tire pressure.

  • Use 2 psi increments.
  • The basic rule is "the more air pressure a tire has, the more load it can take...up to a point." WTF does that mean?.. There's a point where you find the ideal pressure, then continuing to go above this point will just cause the carcass to stiffen up too much. This will cause the tire's footprint to skip and bounce across the track surface, rather than conform to it. Drivers in stock classes (who are limited to stock suspension components) sometimes mess with their tire pressures for this very reason. A friend running an Integra LS used to run 52 psi in back and 46 psi in front--his car understeered like a dog before, but after the tire pressure changes, it was loose as hell!

    •  

    ANSWER: Whichever spring rates you chose, set it up so that the balance is tipped on the oversteer side. For example, using the spring rates in my example above, let's use F: 400 lbs/in and R: 300-325 lbs/in. Then, adjust the rebound valving so that you achieve an understeering balance for the transients/corner entry. That means you'll have to tighten up the front rebound more relative to the rear rebound.

    Think about that setup for a little bit. The spring rates alone will cause a decent amount of steady state oversteer. You use the struts' rebound valving to keep the oversteer under check in the transients (slaloms) and corner entry. As the car progresses from corner entry to holding a steady line through the apex and out to the corner exit, the rebound valving eventually "let's" the suspension revert to its naturally-oversteering state. Make sense?

    So, did you come to the same conclusion? If so, congrats!

     

     

    Now, you're prob'ly thinking, "Hey! This idiot said that this was just gonna be a 'quickie suspension dynamics lesson,' so why is this so friggin' long?.." Well, believe it or not, there's TONS more stuff that affects your car's handling. I didn't even touch on roll centers, center of gravity (height of, and fore-aft location), corner weighting, polar moments, actual spring rates at the wheel, staggering, different suspension designs (struts, unequal upper and lower arms, live axles, etc.) aero packages, etc. You can see why big race teams spend hundreds of thousands of dollars just on suspension engineering and development. And that also explains why your typical Penske racing shock costs $600+ a corner.

    Good or bad, a suspension package can add or subtract hundredths, if not tenths, of a second to your lap times. On the other hand, good or bad driving can add or subtract whole seconds to your lap times...

    But, for our purposes, this is almost purely an academic excercise. Everyone should all be working on their driving skills for now before they start dumping money into go-fast parts. Once a driver gets to the level where they can tell what a car's doing and how to change it for the better, only then would I suggest spending the money on the nice parts.

    I'd highly recommend buying How to Make Your Car Handle by Fred Puhn (I think that's his last name) and browsing through it. It has that early Porsche 911 on the cover. It's pretty much a bible for people looking to set up their car's suspension and it includes no-brainer passages and explanations for people like you and me. There's also nice, technical stuff for you engineering types.

    I sincerely hope that you guys got a little something out of this page. "Thanks!" go to Tyson for starting this thread.

    Once you get a feel for what's been said on this page, be sure to read some autox setup tips for the AE92 GT-S.

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    fsp14@ae92gts.com