Working out spring rate in Assetto Corsa

The setup screen only gives us wheel rate, which, I believe, is a value derived from the spring rate and the length of the suspension arms combined (or something).

I normally setup my cars by installing springs based on the weight distribution of the car and it normally works fine.

There are loads of other values given in the different dev apps so does anyone know how to work out spring rate using those values? Maybe there is a formula or something?

Much appreciated.

 

I'm afraid it'd have to be done on a car-by-car basis as it depends on the geometry of the suspension - which is (more or less) unique to every car. You could figure it out yourself, knowing the wheel rate, by finding out the motion ratio of the suspension in question. This can be done if you have access to the real car or if you can find photos or drawings suitable to measure it for yourself.

Anyway, setting up a car using spring rates alone isn't going to be terribly helpful. Wheel rate is what really matters, and that's what you work with natively in AC.

 

Thanks for answering aphidgod. So what is the best way, using wheel rate to do this? Is there a method?

 

It's actually fairly simple to work it out using the wheel rate, so long as you can measure the relevant parts of the car. (Which, depending on the car in question, may or may not be simple at all.)

The motion ratio is simply this:



The image on the left is obviously a McPherson strut-type suspension. Those have a MR close to 1.0 as you can see (my WRX is about 0.97 in the front, 0.98 in the rear for example)

The image on the right isn't quite a DWB setup, but the spring arrangement is typical of many DWB suspensions which often have motion ratios much less than 1.

Once you measure D1 and D2 for the vehicle you're studying, and calculate your MR, the equation is simply: spring rate = wheel rate x (MR^2)

So, for example, a 600# spring... on the left side, you have a MR of ~0.95, so your wheel rate is 541.5#. On the right side, with a MR of ~0.5, that same 600# spring gets you a wheel rate of 150#.

 

IRL its not a problem usually for simpler builds to go by spring rates. The track guys usually have different setups figured out for a given car, depending on usage, tyres and downforce level. Whilst an EP3 Civic may run 1800lbs springs on the rear, a early gen Miata may run "only" 800lbs ones on the front. It varies a lot depending on geometry but as I said, with basic research its not an issue.

When making a mod tho you kinda have to know the wheel rates. For making setups usually you'd have a range of springs to choose from so I dont think it would matter much. Its not like starting from scratch.

 

OK, so next question then: does anyone know the D1 and D2 measurements for the cars in AC? I'm currently tuning the BMW Z4 GT3. Is it possible to get that data out of the files or something?

Seems it would be simple if we had that data.

 

For what purpose you need the spring rates? Even real life engineers calculate the wheel rates because that's the only relevant thing controlling your unsprung mass and the tyre unless you're designing the suspension and worried about structural strengths and such..

 

It's just something that's always worked for me. I install the springs relative to the weight distribution of the car. For example, I'm of the opinion that in a mid engined car the rear springs should be stiffer than the front springs because the rear supports the engine, and vice versa for a front engined car. With this wheel rate I can't really see where I am. The WR in the setup screen is just a slider. It doesn't tell you what value it is and the values aren't the same on each slider.

I'm just trying to find a starting point that's all.

Do you have an idea of how to set up WR beyond merely the feel of the car?

Thanks.

 

I get that, but if the motion ratio is not close to 1, then it may well be the case that this is not correct. By using Wheel Rate, what you say is absolutely correct, that you can install springs (according to their wheel rate) with regards to the weight distribution.

50/50 weight distribution? Sure, you could have front/rear wheel rates exactly the same and the car would be evenly sprung, even if the actual spring rate is different (not that this is recommended, but thats another matter that I'm not qualified to give advice on).

 

Can't you use the Hz to setup the car? It's displayed on the car engineer app

Sent from my Moto G (4) using Tapatalk

 

That's just it, though - wheel rate is the only way you can see where you are. The raw spring rate only tells you how stiff the coil itself is, there is no inherent relation to how stiffly sprung your car will be by using it.

As said, if you're working with a given vehicle (and thus a constant MR) it matters less - a 400# spring is twice the rate of a 200# spring whether the MR is 0.95 or 0.5. But you can't carry that number over to a different vehicle or even to the other end of the same car, since the entire calculation is moot as soon as the MR changes. It's entirely possible to have a car with a 50/50 weight balance, equal wheel rate F/R, with 200# springs at one end and 600# springs at the other.

 

OK thanks for everyone's comments.

The setup screen for the BMWZ4 GT3 gives front WR of 150-240 Nmm and a rear WR of 55-90 Nmm. How do I make those roughly even for a car with an approximate 50/50 weight bias?

Are any of the dev apps useful for this? Like Leonardo said, in the car engineer app it gives frequency.

 

Ride frequency is the best way to really judge what you're doing with the springs. It basically does all that math for you, spitting out one simple number that can be compared across different vehicles. If you're already there, and comfortable using it, you're all set.

 

Bad car for spring adjustments because it's one of the few in the game that uses the helper spring at rear. You need to adjust packer stiffness (if it's adjustable) to adjust the mainspring. Yes, REALLY confusing and I made a thread about it. It was marked as "NO ISSUE", so don't expect a change any time soon. It's just one of those things you just have to know to make sense of. Here's the thread where's my explanation of it:
https://www.assettocorsa.net/forum/index.php?threads/about-helper-spring-rates.42710/

 

You'd need to enable dev apps, it's done via an ini file, a quick Google search will take you there, or probably other guys will tell exactly how. I'm not at the pc right now, sorry

Sent from my Moto G (4) using Tapatalk

 

here you go:
C:\Program Files (x86)\Steam\steamapps\common\assettocorsa\system\cfg\assetto_corsa.ini

[AC_APPS]
ENABLE_DEV_APPS=1
SCALE_STEP=0.1

 

OK so I've just been doing some more fiddling around using the spring frequencies given in the Car Engineer app.

I first used the Car Physics app to determine the weigh distribution of the car. The line "DISTR" says 51.5, which matches closely with what BMW say the car should have. This figure only says 52 (BMW's figure) if I completely empty the car of fuel. When I accelerate forward this figure goes down, when I brake it goes up, which is as you would expect.

However after fiddling with the wheel rates I can't get the correct distribution. The closest I can get is by setting the front fully soft (150 N/mm) and the rear fully hard (90 N/mm). This gives 55/45, which is still a ways off.

So can anyone comment on this? Am I doing something wrong?

Also, I've noticed the font in the Setup Debug app is the wrong size and some of the text goes off the bottom. Is it possible to resize this?

Thanks for your help.

 

Did you read my post at all? Also what distribution you mean by this 55/45? Weight distribution? It shouldn't move that much by adjusting springs or rod lengths in their normal range.

 

The ride frequency takes into account the weight at that end of the car already, so to "match them" as you're trying to do you'll want the frequencies to be equal, not to echo the ratio of the weight distribution of the car.

 

I meant 55 front/45 rear, which is the closest I can get to BMW's own figure of 52 front/ 48 rear.

Also, do you think the presence of this 'helper spring' is spoiling my data and I won't ever get the figures I'm looking for?