| SPRING TUNING FOR FORZA MOTORSPORT 3 |
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| I'm going out on a limb here, not knowing what the new FM3 has in store in the way of 'consequences'. We had the telemetry function in FM2 which was essentially useless when it came to tuning our springs. The trend in the last version was to set spring rates very, very low. When you ran telemetry on suspension travel, the meter would flash red constantly as the car bottomed out over bumps, and was dead red, using up more than the allowable limits of bump travel as the car flew through fast corners. |
| The problem was, the lack of suspension stiffness didn't translate to negative consequences on the road. Despite the mushy spring rates the tires never unloaded, tire compliance was never lost and the car put up fast lap times. |
| So where do we go from here. My guess is stick to what the real world tells us to do and keep our fingers crossed until Forza Motorsport 3 hits the store shelves. |
| 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 people both at autocross events, and at the track. What conclusions was I able to draw from what I've learned? Nothing really conclusive in the way of an iron clad, fool-proof formula. |
| Let's have a look at what the experts tell 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 get a lot of insight from this information. First of all, he leans in the direction of as soft a spring rate as he can get away with, remember: |
| suspension travel = tire compliance = more grip. |
| The next thing to note is he does tinker with 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. |
| Moving on: |
| "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, they're just not as important as tire compliance. Body roll and balance are better and easier controlled with our anti-roll bars. |
| 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 emphasis placed on the importance of tire compliance. |
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| SO, WHERE DO WE GO FROM HERE? |
| In the absence of an actual copy of Forza Motorsport 3, it's hard to say what set-up will yield the fastest lap times. But if we're going to use Carroll Smith's basic method for setting spring rates, we want to set a rate just stiff enough to keep the car off the ground. |
| If the suspension tuning options from FM2 are any indication of what we might expect from FM3, dynamic camber change under the brakes and under power won't be a consideration, unless they build something into the telemetry for us to monitor it. I'm assuming we'll only need to use static camber settings to optimize our tire compliance as best we can. Considering we're building race cars from stock cars, and suspension geometry is most likely non-adjustable, dynamic camber change won't be an issue. If I had to venture a guess, there will be greater importance placed on keeping the chassis off the ground in FM3. |
| Since a lower ride 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. If we're running a car on a fast track like Nurburgring with a lot of downforce, we'll have to stiffen our rates to account for this. |
| Initial rates could be set (at least the way I did it in FM2) in accordance with the car's weight ratio. So on a car with a 50/50 weight bias, I would set front and rear springs at the same rate. On a mid-engine rear wheel drive car with a 40/60 weight bias, I'd set up the car (hypothetically) with springs of 200/300 lb-in front and rear respectively. |
| Like Carroll Smith said, set your initial anti-roll settings equally. I'd probably set both up in the middle. Also, if we're going to set the rear springs first (as he suggested) as soft as we can get away with, our adjustments will be as follows: |
| If the car oversteers - stiffen the front springs. If the car understeers - soften the front springs. |
| 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. I'd also like to add a caveat to the mix. Carroll Smith built ME-RWD (acronym) cars, which is why I believe he set his rear rates first. Oversteer (especially on corner exit) is characteristic of the RWD (acronym) 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 |
| In our case, we have front wheel drive and all wheel drive cars to add to the mix. That said, the exact method for establishing balance will most likely have to be tweaked in accordance with our chosen drive type and engine location. |
| Once we set up (through testing) our rates for oversteer / understeer balance, any track to track changes in springs should be made while maintaining the front to rear rate ratio. |
| If we decide to run extra downforce from our initial set-up, and we need to stiffen the springs to keep the car off the ground, we want to raise both front and rear in proportion to one another. |
| Let's say we have spring rates of 200 lb/in - front and 300 lb/in - rear. |
| One method to try would be to make a percentage change - let's say 10%. |
| This would result in new rates of: 220 / 330. |
| Or we could try keeping the spread between the two rates constant, in this case the variance in rates is 100 lb/in. |
| Using this method, if we raise the front rates 20 lb/in to 220 then we'll raise the rear rates the same, which would be 320 lb/in. |
| Whichever method is more effective won't be known until the game is in-hand. But in either scenario we're on the right track to a successful suspension spring tune. |
