The LFS Suspension Calc is a small and simple spreadsheet that does a few slightly complicated equations regarding spring rates and damping. It's a fast and relatively easy way to determine how stiff to make your springs, and how much damping to use for a given stiffness.

This spreadsheet, although technically usable on its own, does require the knowledge of the exact forces on the front and rear of the car. This can be determined with difficulty by yourself, or it can be determined easily and quickly by using Bob Smith's LFS GRC2, which includes a downforce calculator.

The first version of this spreadsheet, 0.5, is fundamentally flawed. It will give a decent approximation of the right values, but it is, in a word, wrong. Version 0.6 (hopefully) fixes these flaws, mostly by making the talented Bob Smith do the real work. This version is also a little easier to use.

Have you seen Bob Smith's work on this subject?

If you need a hand with any aero information I'd be willing to tell you whatever you need. LFS GRC has the downforce pretty much sorted (if I say so myself), although obviously it doesn't have anything to do with suspension, so you can't see your spring rates drop.

However, Colcob already made a very good suspension and damping calculator for S1 and we are working on an improved S2 compatible version already.

I see you mention both his tool and my guide in the readme. Aside from the downforce side of things I can't see what this adds? And you could hack that into Colcob's anaylser just by adding the downforce onto the car weight in the data sheet.

Personally, when the S2 version of Colcob's analyzer comes out, I'll definitely be using that. The issue is not just that the S1 version doesn't have downforce support, as mentioned, but also that it doesn't have physical data for the S2-only cars (weight, weight distribution, and fuel loads). Getting the correct values out of it would involve inputing just as much data as you do in my version, except you have to hunt a lot farther to find the appropriate fields. The main virtue of my version, once you know how it works, is it is simple, fast, and flexible to new cars. It could be a lot faster, given the amount of manual calculation that is involved, but I intend to improve that eventually, provided Colcob's S2 analyzer doesn't come out soon.

The issue with downforce, if you're curious, is that the undertray and body downforce are applied unevenly to the front and rear of the car. I'm making an educated guess, judging from the forces view in LFS, that they are applied to the CG of the car, which means they affect the front and rear in proportions equal to the weight distribution of the car. A 60/40 distribution means body downforce is also split 60/40. I include this in my calculations.

This is all well and good--except my spreadsheet doesn't include the "dry" weight distribution, only the distribution including driver and fuel. While driver and fuel will affect the distribution of physical weight in the car, they will NOT affect the distribution of downforce. Which means that, if the distribution with driver and fuel is 55/45, but the "dry" distribution is 60/40, then downforce from the undertray and body will be split 55/45 instead of 60/40, as would be correct.

I call this a minor issue because the difference in distribution is usually only a few percentage points, and so has little end effect on the spring rates. But it's there, and I felt obligated to mention it. (in addition, front/rear wing forces are simplified to acting directly and solely on the front/rear wheels, but I don't have data to do them properly, as you have in the GRC)

God I really ought to get a move on shouldnt I.

I've updated the calc. After looking at Bob Smith's GRC2, I've realized that my calculations for downforce aren't just slightly wrong, but terribly, terribly wrong. I've changed the spreadsheet accordingly, and I'm now advocating using the GRC2 to derive force data, so I'll limit myself to the equations I'm pretty sure I understand. The spreadsheet should be much more accurate now, though of course, there's no accounting for taste--just because the spreadsheet gives a numerical answer down to 3 decimal points doesn't mean it's "right".

Hi, 5th World, is this calculator suitable for FZ50 GTR?

When I put in the force, the stiffness of the sprint seems huge and exceed the
range offered by LFS? (I have divided it by 1000, the result is 244 for front and 166 for rear?)

Have I did something wrong?

And in Gear Ratio Calculator, the force does not include the driver and gas, right?

Quote from khtwo :

And in Gear Ratio Calculator, the force does not include the driver and gas, right?

GRC uses the mass of the car with the driver in and half a tank of fuel (since that's most useful). Obviously downforce is unaffected by such things.

Quote from Bob Smith :

GRC uses the mass of the car with the driver in and half a tank of fuel (since that's most useful). Obviously downforce is unaffected by such things.

Hi, Bob. Thanks for your advancd setup guide. Very good.
Do you think a stiffness of 244 in front and 166 in rear is normal for a FZ50 GTR?

Quote from khtwo :

Hi, 5th World, is this calculator suitable for FZ50 GTR?

When I put in the force, the stiffness of the sprint seems huge and exceed the
range offered by LFS? (I have divided it by 1000, the result is 244 for front and 166 for rear?)

Have I did something wrong?

And in Gear Ratio Calculator, the force does not include the driver and gas, right?

The force given in the GRC includes both a driver and a 50% full gas tank.

At what speed are you measuring the force, and what are your wing angles? That will make a big difference what sort of values you are getting. And what are you using as optimum frequency?

244 is within the allowed range--maximum stiffness is 260. I know that 166 and 244 is a wide difference, but bear in mind the FZ50 GTR has really unbalanced weight distribution--more than 60% of the weight is on the back, so if you use the same optimum frequency in the front and back, the front springs will be much stiffer than the rear ones. For this reason, for the FZ the "0.75 max frequecncy difference" rule doesn't apply as strictly to the FZ as to other cars.

Is this a bad thing or not? I'm not sure, really. Try it and see what the car handles like--my guess is it will handle reasonably well, since stiffer springs in the front will tend to cause it to understeer, and the FZ normally has huge oversteer.

As for 244 and 166 being high, yes those are high values, but if you are running a lot of downforce at high speed, I could see getting numbers that high. On the other hand, I would ask you if you really need that much downforce, and is your test speed really representative of your average speed in corners, which is where having the proper spring rates is most important. For example, on my oval setups, even though I can hit 283 km/hr (~79 m/s) in the straights, I test at 270 km/hr (75 m/s) because that's closer to the speed I actually carry in the corners (and incidentally, my wings are usually set around 4-7 degrees). On a tight circuit, say AS_cadet, I might set my test speed as low 25-30 m/s.

By the way, Bob, in your new edition of advanced setup guide, page 26, there is
"the “aero distribution” from the screenshot above should be equal to the “force distribution” when at zero speed." Should the "at zero speed" be "at all speed"?
Because at zero speed, there is no aero distribution.

Quote from 5th Earth :

The force given in the GRC includes both a driver and a 50% full gas tank.

At what speed are you measuring the force, and what are your wing angles? That will make a big difference what sort of values you are getting. And what are you using as optimum frequency?

244 is within the allowed range--maximum stiffness is 260. I know that 166 and 244 is a wide difference, but bear in mind the FZ50 GTR has really unbalanced weight distribution--more than 60% of the weight is on the back, so if you use the same optimum frequency in the front and back, the front springs will be much stiffer than the rear ones. For this reason, for the FZ the "0.75 max frequecncy difference" rule doesn't apply as strictly to the FZ as to other cars.

Is this a bad thing or not? I'm not sure, really. Try it and see what the car handles like--my guess is it will handle reasonably well, since stiffer springs in the front will tend to cause it to understeer, and the FZ normally has huge oversteer.

As for 244 and 166 being high, yes those are high values, but if you are running a lot of downforce at high speed, I could see getting numbers that high. On the other hand, I would ask you if you really need that much downforce, and is your test speed really representative of your average speed in corners, which is where having the proper spring rates is most important. For example, on my oval setups, even though I can hit 283 km/hr (~79 m/s) in the straights, I test at 270 km/hr (75 m/s) because that's closer to the speed I actually carry in the corners (and incidentally, my wings are usually set around 4-7 degrees). On a tight circuit, say AS_cadet, I might set my test speed as low 25-30 m/s.

Thanks for your reply.
If I use Front wing angle of 10 and rear of 20 at speed of 144km/h
Optimum F use 2 HZ, then the Spring rate is 284356.2424(out of range) for front and 164797.7116 for rear. Does that mean we can not use Frequency higher than 1.9HZ on this car?

I don't know if I'm customed to the default suspension set of LFS, the new setup calculated from the calculator feels much different to the default setup.

Quote from khtwo :

Thanks for your reply.
If I use Front wing angle of 10 and rear of 20 at speed of 144km/h
Optimum F use 2 HZ, then the Spring rate is 284356.2424(out of range) for front and 164797.7116 for rear. Does that mean we can not use Frequency higher than 1.9HZ on this car?

With those downforce and speed settings, apparently yes. You've done nothing wrong that I can see, we appear to be looking at a limitation of LFS. The FZR is just too nose-light (now that I think about it, my previous post was inaccurate--high downforce and speed would have made the springs softer, so clearly that wasn't the problem).

Granted, if you speed up a bit then the spring rate falls to usable levels, but I can certainly see a situation where 40m/s is plenty fast.

My only suggestion is, either change the optimum frequency to a lower number, or manually plug in 260 for spring rate and work from there.

Well, it seems I have worked correctly on calculation and next step is to change my drive style. Thanks!

That's an interesting setup for the FZR, how did you end up with those figures? I get 122kN/m rear and 80kN/m front for a 3/3.1Hz setup (to give a little understeer) when parked. At 150mph with wingle angles of 7 and 9 degrees that produces ~-6800N of lift, that'll reduce spring frequencies to about 2.4Hz.

Quote from khtwo :

By the way, Bob, in your new edition of advanced setup guide, page 26, there is
"the “aero distribution” from the screenshot above should be equal to the “force distribution” when at zero speed." Should the "at zero speed" be "at all speed"?
Because at zero speed, there is no aero distribution.

Nearly the right thread... so close... but I'll let you off.

Well, maybe it's not worded as well as it could be. If the force distribution is equal to the weight distribution of the car then aero will not change the balance of the car at speed. But the weight distribution isn't really shown on the aero tab. The trouble with making it equal to the force distribution (at say 150mph) is that the force distribution takes aero into account, so you'd be trying to make the aero equal to a number that changed every time you changed the aero. Hence I say at zero speed, when the force distribution will be the static weight distribution. So, yes, I should really have said minimum speed (1m/s), so that the force distribution can be calculated.

Hi, Bob, I'd like to present the process of my calculation:

(1) I use GRC2.2's aerodynamics and set speed at 144km/h, front wing 7 degree, rear wing 9 degrees. Then in "Force Distribution info"
the Front wheel is FF=5383N and Rear wheel is FR=8893N
(2) in Suspension Calc 0.6, I put in the result get from GRC2.2. It calculates the Equivalent Mass of
front MF=FF*0.101971621298=548.91, Rear MR=FR*0.101971621298=906.83
(3) I put in optimum frequency of front OF=2 and rear OR=2, Calc0.6 calculates that adjusted Frequency of
front AF=OF/(MF/1000)=3.6435, of rear AR=OR/(MR/1000)=2.2054
(4) Then Calc0.6 calculates the Sprint rate of
front SRF = ((AF*2*PI())^2)*MF = 287684 (N/m)
rear SRR = ((AR*2*PI())^2)*MR = 174137 (N/m)
Then the rest is depend on this calculation result

Could you please check and see what's wrong with my calculation?

I found that the less equivalent mass, the higher adjusted frequency(stiffer sprint)? Is it correct?

There is no such thing as equivalent mass. Force and mass are two different things so you cant just take the forces and divide by 9.8 to give you a mass. The inertia of the car (ie. its resistance to being accelerated) remains the samee regardless of downforce.

The mass remains unchanged, the suspension frequency remains unchanged. All that changes with downforce is the amplitude of the oscillation.

Quote from colcob :

There is no such thing as equivalent mass. Force and mass are two different things so you cant just take the forces and divide by 9.8 to give you a mass. The inertia of the car (ie. its resistance to being accelerated) remains the samee regardless of downforce.

The mass remains unchanged, the suspension frequency remains unchanged. All that changes with downforce is the amplitude of the oscillation.

Could you please provide a correct formular to calculate the sprint rate? Thanks.

OK so using these equations:

Deformation = Force / Stiffness
Frequency* = 0.5 / Deformation ^ 0.5

*which was simplified from: Frequency = 0.5 / (Force / Stiffness) ^ 0.5

Obviously increased force will increase deformation but will also reduce the frequency. How do I need to alter these equation so that downforce only affects deformation?

Quote from Bob Smith :

OK so using these equations:

Deformation = Force / Stiffness
Frequency* = 0.5 / Deformation ^ 0.5

*which was simplified from: Frequency = 0.5 / (Force / Stiffness) ^ 0.5

Obviously increased force will increase deformation but will also reduce the frequency. How do I need to alter these equation so that downforce only affects deformation?

Hi, bob, by comparing your result and the equations, I guess that you divide the force by 2 to make the calculation(So it's the force for one wheel). Is that what you did?

Quote from Bob Smith :

OK so using these equations:

Deformation = Force / Stiffness
Frequency* = 0.5 / Deformation ^ 0.5

*which was simplified from: Frequency = 0.5 / (Force / Stiffness) ^ 0.5

Obviously increased force will increase deformation but will also reduce the frequency. How do I need to alter these equation so that downforce only affects deformation?

Dont do it Bob!! The deformation version of the frequency equation is a fudge based on the coincience that pi^2 = approx 9.8. I know thats the version I used in the excel analyser, but hey, we know better now.

Frequency = (1/(2*pi))*sqrt(spring rate/mass)

There is a version which includes static deflection, but you have to include g, or it doesnt work.

Frequency = (1/(2*pi))*sqrt(g/deflection)

Hehe. I knicked it from your spreadsheet so I don't blame myself.

Anyway, so you're saying the deflection equation was correct by the frequency equation wasn't? Just double checking. I couldn't understand the "swap g for pi^2" argument since I couldn't see either in mine.... but now I'm with you.

I read that equation as:
0.5 * Pi * (Stiffness/Mass) ^ 0.5

(that way seems clearer to me)

As for the other one, weren't people saying in the other thread that g doesn't affect frequency? Or is that g just cancelling out the one used in the deflection formula? In which case the speed would still affect frequency (on cars with downforce) since the force would then be converted to mass (which seems horribly wrong).

Ha, yeah my fault.

Quote :

I read that equation as:
0.5 * Pi * (Stiffness/Mass) ^ 0.5

(that way seems clearer to me)

You're right, its much clearer. Still wrong though.
0.5*pi is not the same as 1/(2*pi) . Thats just algebra.


Yeah, in the version of the frequency equation that does include g, it would be cancelled out by the static deflection if you resolved out the full equation, because static deflection = (mass*g)/stiffness.

So the g/deflection part resolves back to stiffness/mass, which is where we were in the first place.

The pi - rootG confusion goes like this;

Our wrong equation was this: