| SPRING TUNING FOR FORZA MOTORSPORT |
| I know it gives you fits. I know it's a source of confusion as you shake you head and make a distressed post on the forums.... |
| So the question of the ages is this: Where the hell do we set our spring rates to start? Is stiffer better? After all, a stiff car is a responsive car. Is softer better? After all, a soft suspension is more compliant, and allows for adequate wheel travel, which is what we use our springs for in the first place. |
| I've done a lot of research, and talked to a lot of car people. What conclusions was I able to draw from what I've learned? Nothing really. Just to give you an example I have an S2000, and have other autocross friends with S2000s. Take myself and two others who both have prepped cars for STR-Class. My springs are 725-610 ....another friend is running springs 650-500....yet another was running 850-700 and said he wanted to go stiffer next year. Go figure.... |
| Let's have a look at what our resident expert tells us: |
| "I wish that there were hard and fast rules for the determination of optimum (spring) rates. To my knowledge, there are none. Optimum rates vary with gross vehicle weight, power to weight ratio, aerodynamic downforce generation, tire width, track characteristics, driver preference and technique and, quite probably, the phase of the moon. My basic system is to run the softest rear springs that will keep the car off the track - and maintain some semblance of camber control - at the ride height that I want to run. I then balance the understeer/oversteer with the front springs - and equal rate front and rear bars. I try to do this in long, medium speed corners so as to simulate steady state conditions at low enough road speeds so that aerodynamic downforce doesn't confuse the issue (say 60 mph).....This gives the basic front and rear ride (spring) rates." |
| Carroll Smith - 'Tune To Win' - pg.73 |
| We got a lot of insight from this information. First of all, he points us in the direction of as soft a rear spring rate as he can get away with. I think this is especially so with RWD cars, (which is what he was dealing with). Remember: |
| suspension travel = tire compliance = more grip. |
| The next thing to note is he's tinkering with front spring rates in his initial set-up to balance oversteer / understeer. The last thing to note is he takes into consideration dynamic camber change when setting his rates. |
I'm going to interject right here. If you're looking for a system because you're new to tuning, (I don't really have one), but if you need a baseline, read the above quote. This is basically how I tuned my Lotus Exige, the A600 machine that's running low 1:32s in Rivals Mode and has two fast friends (Don Ente and PpR Impala) in the top 50 with that tune. Here's what you do: |
| 1- Take the total default sway bar (sway bar #1 + sway bar #2 then divide by 2), so let's say you had default ARBs at 15.5 and 24.5, now your sway bars should be at 20-20. Set that up. What Carroll Smith is saying is if we run equal roll stiffnes with the ARBs, it'll help us figure out what the right balance is for the springs. |
| 2- Take your total default spring rate (front springs + rear springs) and remember that number. Let's say default springs were 580-620. Your total spring is 1200. Leave the springs alone for now. |
| 3- You can slam the ride height (if the track is flat and permits it - if not, start with the default RH for tracks like Infineon and Sebring) and test drive the car. Use the telemetry to figure out how soft you can go (or want to go) on the rear springs. If you're seeing red in telemetry, stiffen up. If you get a little red in a fast straight over a bump it's not as big of a deal, but if the car is unloding in the corners it's a problem. As you test, decide how you want to set the front springs based off the feedback you're getting from the car. If the car is loose, add front spring, if the car it tight, take spring away. Take into consideration the weight distribution (but don't get married to it - lap times are the ONLY THING that ultimately matter). Say the car is 50-50 for example. If you've got the rear spring down at 450 in/lbs and the default front spring was 850 in/lb, it's most likely going to be very pushy, so the weight distribution is a clue that we need to soften the front springs. |
| Take a look at the graphics below so we know what we're looking at: |
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| Allrighty, so what does our other expert have to say: |
| "Since most race cars can adjust the spring load to set the correct ride height, choosing the best spring rate is usually of the most concern to racers. We have discussed reasons why more spring rate might be desired, but how do you know if you have too high a spring rate? In general, it is best to run as soft a spring rate as possible. The traction between a car's tires and the road is the only source for developing cornering power. If the tires are not in contact with the road, they develop zero cornering power. Soft spring rates allow the tires to better follow road bumps and irregularities so the tires stay in contact with the road a higher percentage of the time. Higher spring rates can also limit suspension in both the bump and rebound directions. If your car is not getting full suspension travel, it might not be allowing the tires to follow the road as much as possible." |
| Herb Adams - 'Chassis Engineering' - pg.31 |
| We're getting more reinforcement here for a softer spring rate. Notice the emphasis being placed on tire compliance and suspension travel, once again. He doesn't even mention oversteer/understeer or body roll. Not to say they're not to be considered in our set-up (after all we're adjusting the front springs for balance as we test), it's just that we're tuning springs right now so the main focus is keeping the tires on the ground. |
| Moving on: |
| "Maintaining tyre compliance with the road is a major consideration. In theory, we want to run the lowest rate springs, consistent with a stable chassis platform, and reasonable response for the driver. How much of the roll resistance will be allocated to anti-roll bars? How much to shock absorbers? What are the downsides of too much anti-roll bar and/or too much shock?" |
| smithees-racetech.com.au/theory/summary.html |
| Okay, do we see a pattern developing here? Smithees Race Car Technologies of Australia is a great website to check out. It was recommended to me by a fellow autocrosser and there's enough valuable information on their site to keep your 'tuning-minded' brain busy for a while. That said, we have yet another reference in favor of a softer ride, with even more emphasis placed on the importance of tire compliance. |
| SO, WHERE DO WE GO FROM HERE? |
| Since a lower ride height makes for a better handling car, and in the absence of an existing roll center, setting the ride height (as Carroll Smith does) where you want it, then setting your spring rates just stiff enough to keep the car off the ground is a good place to start. Since the in-game tutorial from the developers says a lower ride height makes for a better handling car, I slam the RH right away if I can, and tune around that. That's obviously track dependant, I'm not slamming the ride height at Infineon, or other wavy tracks where we need a lot of suspension travel, while at a place like Catalunya I'll take my chances. And again, don't ignore the telemetry; if you're hitting the bump stops and seeing a lot of red, raise your RH or stiffen things up. When you hit the bump stops (especially in the corners) the car handles like shit. This isn't Forza 2. |
| Keep in mind that we could normally soften the rear springs to dial out oversteer, but we already have the rear springs as soft as we can go, so in this instance we'll have to raise the front spring rate if we're loose. I'd also like to add a caveat to the mix. Carroll Smith built ME-RWD cars, which is why I believe he set his rear rates first. Oversteer (especially on corner exit) is characteristic of the RWD cars he worked on, and I'm guessing this is why his first order of business was to get his rear rates as low as possible. You need to be able to put the power down. |
| So maybe you're thinking I left you hanging a few paragraphs ago. We're balancing the car with springs (see #3 above), but what about the sway bars? Aren't the sway bars our best tool for balancing oversteer/understeer? |
| Yeah, I know it's complicated. It's hard to write seperate sections to tune each component on an individual basis, because of the intense interrelation of all our tuning components. In order to get things sorted out, you need to test drive using telemetry, and check the contact patches on the tire temperatures. Your tires will ultimately tell the story. Read the section on camber if you haven't already. |
| If you've got badly uneven tire temperatures, you have body roll, and this is the time to increase the roll stiffness with the ARBs. Also keep in mind, one distinct benefit of stiff ARBs and the increse in tire loading is it heats the tires up nicely which is good. We want bright green rubber. Also, if you like your spring rates, the car is compliant, and the grip is good, this should be the time to tighten things up by incresing ARB stiffness. What I'm trying to say is if we're going the route of a softly sprung car (like Carroll Smith and Herb Adams say) you have to assume that the car is going to be rolling more as we reduce rates. If we soften the suspension for tire compliance we need to increase roll stiffness to keep the car responsive. |
| Look, it's a feel thing. Using the above example, let's try to put some numbers to it. We started with: |
| 1200 in/lbs of total spring |
| 40 (whatever of total ARB) - 20 / 20 |
| These were essentially the total default values. So let's say you're tweaking your springs as you test, using the suspension screen in telemetry, and feeling around for oversteer/understeer balance in the process. And after a while you settle on spring rates of 425-375. That's 800 in/lb of total spring, so your total spring is reduced by 400 in/lb or 33.33%. Common sense would dictate that we need to ramp up the ARB stiffness to compensate and it's likely that the telemetry screen will tell you what you need to do. So you decide to keep the ARBs even (cuz you want to keep things simple like Carroll says); you ramp it up to 25-25....and test, and check telemetry for body roll, and you want those contact patches bright green, so you ramp it up to 30-30, and so on.... |
| A lot of it is seat of your pants feel. How stiff on the ARBs? Read that section. We want transitional response, a car that corners crisply, and reacts quickly to steering inputs without sliding around. |
| But the basic idea is this - if you take away spring, add ARB. |
| Once you sort out a tune you like, run some laps and set a time that you think is on par with your skill level. Once you estabish a benchmark for performance, start racing your ghost. Tweak something, race your ghost, if you can't beat it, start over, make another tweak, and race your ghost again. Rinse and repeat and try different things until you can beat it. Every now and then you'll make an adjustment that'll gain you five tenths or more....you'll crush your ghost and it'll feel really good. And not only goes is feel great when you beat it, but you learn something as well. |