Spring rates (discovery)

Hey all. Back from an apparent ban. First off, if l come across as being a know-it-all, l apologize. I do not know everything. On the contrary, l’m not totally confident about any of my techniques. They work. The math is correct. There just isn’t enough reliable information out there on what to base the math on, and there’s some important specs about the cars missing that we would need to properly set up suspension, so we’re stuck trying to find out what to base our math on.

That being said, this has been tested on rally/off road suspension, but l’m sure it will translate to race suspension. The percentage will just change. I figured it out while tuning GRC’S. I was running a certain percentage of my total weight for my spring rates, but as soon as l started tuning a non-grc rally car, my range either wouldn’t be on my scale, or way on the higher end. So, l’d end up going center rate on my non-grc’s, but was just kind of guessing on my GRC cars. I started checking the center rates against my total weight but was all over the place, percentage wise. Or so it seemed. I was taking my percentage from the total weight, but l started working with my individual front and back weight/loads separately(total weight times your front weight percentage, repeat for the rear), and noticed that the center rate on non-grc rally cars were at 31 pct of their respective weight distributions.

So, your total weight times your front distribution percentage gives you the static load on your front. From there, if you go 31 pct, you’ll arrive at the center of your scale. When l reference center scale, l’m talking about weight distribution, with center of the distribution being center of the scale.

I’ll use my VW grc stats as an example since l have most of the specs memorized. As l said, l haven’t done it with race suspension, but l’m pretty confident, you’ll find the same thing going on, just a different percentage…or maybe the same, but it won’t be random, and gives you a reliable baseline. You won’t arrive at center scale with the grc because it runs all the way to like 90 pct of your load up front, and right at 100 rear, but non grc suspension is probably half of that…give or take. This method just gives you a consistent way to get the same load support on the same type of vehicles. For me, it’s 30 pct on rally cars, and 20 on passenger trucks…for now. 33, 25, and 20 will give you a better understanding of spring support and travel, if that information is relevant to your tuning methodology.

VW GRC beetle
Total weight: 2550
Weight distribution: 54% front

2550 x .54 = 1377 - front load
1377 x .31 = 426.87 -front spring rate.

2550 x .46 = 1173 - rear load
1173 x .31= 363.63 - rear spring rate

You can repeat this for the rear, as l’ve shown here, but l prefer dividing by 1.1739 which you get by dividing 54 by 46. I use this for all my weight distributing. You get 363.46 rear. A fractional variance, so you decide which to use, but my preferred method works when distributing anything, really. From alignment to diff bias, which imo isn’t linear. So, it’s worth learning and using.

Common rates rounded:

54/46 - 1.174
56/44 - 1.272
58/42 - 1.383
60/40 - 1.5

Well, that’s it. I don’t see the significance of .31 so, l dropped it to .30, and ended up at .20 on my Raptors, but the point is, if you work with front load/back load instead of total weight alone, you will find a consistent feel, and if you get real sick with it, you can figure your spring travel from here. That’s for another post that l’m not entirely prepared for. I’ve personally gone in circles with frequency, spring travel, wheel travel and all that, and have decided that forza simply didn’t take it that far. As l mentioned, there’s some crucial info missing that you need to get these figures. A-arm and hub measurements to name a couple. Not to mention that l can’t read equations or formulas, but haven’t been motivated to learn them because l realize that the info isn’t available. Hope this is useful to someone.


Update: l finally settled on setting my rate at half my weight distribution numbers. 30 pct, 20 pct for 60/40, 27/23, for my 54/46…etc. Just happens to be what my differential settings are, as well. Puts 25 pct as center. It works extremely well.

Not being a know-it-all, and if you think this post is moronic, it’s on you. It works. Your way probably does too. Don’t assume you have it all figured out, and anybody who does it different is tripping. The difference is, until l get confirmation that my tunes are perfect, and my techniques work on any drivetrain or engine placement, l’m going to continue to experiment and evolve. I won’t be posting anything on here unless l’m positive that it’s correct. That being said. This is preference. Suggestions. If you’re trying to get your springs right, and you can’t figure it out, this is what it’s for. It’s not a declaration that this is perfect, and this is how it’s done. It’s a “hey, try this. It works for me.” It feels like it’s right. But this is just a very driveable configuration, and fairly simple to try. That’s the only endorsement l’ll give. I have nothing to gain from this. I don’t get off on likes and such. I’m trying to help the community. Save them the time, confusion, and frustration l went through learning this stuff.

End of rant. I’ve already spent more time defending myself than l should have. Hope this helps someone. The info, not the explaining myself part.


For previous versions of Forza springs were per corner not per axel, so a 2000 pound 50/50 car would start at 500f/500r, is Horizon 4 different? High spring rates can be made to work; requires stiff shocks and ARB’s, but you may not get the benefit of moving suspension parts when cornering and braking, and of course bumpy roads can be a nightmare…

I find that I use total combined spring setting at somewhere between 30% and 50% of the car’s weight (in US settings), depending on the height of the car and some other characteristics. How I get there is by driving it around and feeling how the car reacts. My goal is a stable, controllable tune.

It’s not just about keeping the car off the bump stops. Cars with too-soft springs tend to wallow around drunkenly. Brake and too much weight unloads from the rear, causing instability. Accelerate and your front end will wash out. Corner performance feels like you’re driving with slinkies at all four corners. Still, a lot of tuners adopt ridiculously soft springs. Cars with too-stiff springs jump over bumps and a tire that isn’t touching the road isn’t turning, accelerating, or decelerating - and that’s bad.

Rear-engined Porsches are a great example to tune. They tend to need stiffer springs than their FR & MR counterparts, because all that weight is over and behind the rear axle. When they brake, it’s really easy to overload the front and unload the rear, leading to oversteer. When they accelerate, it’s similarly easy to unload the front and overload the rear, leading to understeer. Stiffer springs keep the weight balanced more toward center, mitigating the extremes inherent in the design. Ohlins recommends 685/1256 rates for their 991 GT3RS clubsport kit, for a combined 1941 on a car that weighs just over 3100lbs. My rates aren’t THAT high on my GT3 RS, but they’re a lot closer to that 50% mark than the 30% end of the range.

(FF cars with forward-mounted engines are in a similar position to the rear-engined Porsches, but I haven’t tuned as many of those)

CoffeeHawk00: FH4 uses per corner numbers with equal springs on either side of the axle. You still don’t just divide the car’s weight by four to get your base, pre-distribution number, though, because you’re looking at pounds-per-inch ratings on the springs, aero, surface conditions, and unsprung weight to account for. Pounds-per-inch is key, because the more travel allowed by the suspension, the softer you can go. Aero adds sprung weight to the front and rear of the car that varies by speed. Surface conditions also factor in: the more and bigger bumps/dips you face, the more suspension travel you’ll need and that will determine whether you can have soft springs that keep the tires in contact with the surface or stiff springs that lose contact with the road (a bad thing). Cars have a certain amount of unsprung weight that isn’t carried on the springs. This includes tires, wheels, axles, brakes, shocks, the springs themselves, and that sort of thing.