Hi!
I have a set for the XRR that I really like. Well balanced, fairly grippy, good on tires. A good set. Problem is, when it get lighter fuel loads it goes all weird. I get oversteer at entry, understeer mid-corner, and slight oversteer at exit. No amount of ARB adjustment can fix this. What I was thinking was this:
Could it be that the weight of the fuel (100l / 70kg) is enough to throw the suspension frequencies off, so that my dampers mess me up? Should I go slightly softer to have lower suspension frequencies with a lighter load, and just try to adjust when the car is heavier? Could it be something else that's throwing me off? It really doesn't feel like it's just the balance of the car that's different. It's both ends of the car that's off, making me suspect the suspension. Maybe ColCob or Bob could have a say in this? What could cause this behaviour? If I knew,I'm sure I could compensate. Right now nothing seems to help. I only manage to mess up the set with a heavy load too, and not making it better when it's light. Any ideas, anyone?

i had some similar problem with an xf gtr set. but after some time i noticed it was my driving style.
the set was really stable with higher fuel loads, then, when i drove with less fuel, i thought it was twitchy and bad. but i got used to it and a lot faster. you have to turn a bit smoother because of the lower weight in the rear.

Using the replay analyzer I see that I am running about the same speeds with the light car, and throttle is about the same too. So I really think the car acts differently. I might have to adjust my driving, but the effect is so pronounced, and the set is so good with heavy fuel that I'd rather sacrifice some high-fuel grip to get better low-fuel stability. I usually lose the car when the fuel is below 50%. The tires are fine, btw. It's not excessive wear. I'm usually pretty smooth and nice to my tires anyway. I set my car up to run fairly hot in the beginning of the race, so the tires reach optimum temps when they start to wear after 10-12 laps. Then I can push harder when the fuel-load decrease.

when we did an endu race at AS Historic with the XRR, I put 1° more of downforce to the rear (and change camber and pressure too to optimize the wear) to do the race, than on the qualif. setup because of the fuel, and that was very good. That is logic that adding more than 70Kg on the rear wheels modify the handling of the car

edit : forget a word

Well, since you didn't give away any info about your setup I'll just have to guess some settings for my example.

Full fuel load:
51.2% F weight distribution
Set springs to 3Hz front and rear (that's 103.8 kN/m front and 99 kN/m rear btw)

10% fuel load:
54.4% F weight distribution
Front spring still 3Hz, Rear is now 3.2Hz

Cog for the fuel is almost perfectly over the rear wheels (actually SLIGHTLY behind them), so we can ignore weight change over the front wheels.

So basically we see if the car is setup to be neutral with a full tank, we've 0.2Hz of oversteer in the springs now (which is quite a bit). Springs have a larger affect than anti-roll bars, so you're always going to feel some looseness. However the weight distribution is now more forward, which is generally going to make the car more understeery.

Looking at dampers, assuming they were setup for neutral transient handling with a full fuel load, at 10% fuel you've added some oversteer to corner entry and some understeer to corner exit. Of course damping is heavily affected by downforce as well, but assuming downforce is setup reasonable neutral it will just drop your spring frequencies and increase the critical damping value.


So basically that ties up with the affects you've noticed. Possible solutions? Soften the rear a little to avoid the oversteer with low fuel loads, then play with anti-roll bars to try and remove understeer at high fuel loads. You can usee F11 to add it back in again during the race. You'll never get exactly the same handling though, it's something you're always going to notice, and it's a character trait of the car. Obviously you try and have the change controllable though.

Thanks a lot. Seems I was on the right track after all, then!

I always try to tune my car when I've got about a 50% fuel load. That way, any weirdness in handling caused by changing weight is averaged out, starting slightly off, getting better, then slightly off again, instead of starting perfect and then just getting worse.

Granted I've yet to have a race when I needed a full tank... but the idea is still valid.

That's true. But the set just suddenly felt right when I drove it, and I realized I had a lot of fuel in the car. And I have an endurancerace coming up, so I was experimenting with fuel loads vs laptimes. I am actually fastest by lap 3, jsut as the tires have reached working temperatures. I can push very hard right out of the pits on a full tank of fuel, and that is a neat trait in a setup. But still I lose the advantage as fuel is burned. I am gonna work out a compromise.

@bob smith.....my question is a little bit offtopic....but pls could you let me know how do you calculate the spring frequency into spring strenght? I was reading yr setup Guidance and was wondering how do you can work with Hz and kN/m same time. Are you using a separate tool or is it only simple mathematic?

thx in advance.

Ok, for the next NAL race on sunday, my LX6 car also suffers the same effect, at 100% fuel, it drives awesomely, with 75, its OK, but anything lower than 50 it drives like a bitch. I desperately need to know how to fix it becuse I can't even start to drive before Im in the sand-trap.

Quote from PudelHH :

@bob smith.....how do you calculate the spring frequency into spring strenght? I was reading yr setup Guidance and was wondering how do you can work with Hz and kN/m same time. Are you using a separate tool or is it only simple mathematic?

This question seems to be the only one people ask me whenever I get questions about my guide. I re-read the paragraph in the guide and noticed it wasn't very clear on this aspect, hence that section was altered when I updated the guide the other day. So I'm assuming you have an older revision of the guide.

Anyway, it's only simple mathematics to work it out however Colcob's Setup Analyser provides this information and much more. It is only for S1 though.

Quote from PudelHH :

@bob smith.....my question is a little bit offtopic....but pls could you let me know how do you calculate the spring frequency into spring strenght? I was reading yr setup Guidance and was wondering how do you can work with Hz and kN/m same time. Are you using a separate tool or is it only simple mathematic?

thx in advance.

not sure, but I have read that : spring stiffness = 2 * pi² * m * f²
where m is the mass on the front wheels in Kg (if you want to calculate springs for the front ) and f the spring frequency in Hz. And in general, lots of persons advise to begin with 2Hz, which does not really work with S2 :s

Flotch, we can see the spring stiffness, so there's no need to calculate it. If you write the formula to calculate frequency from stiffness and then connect it to a slider, it doesn't matter which way round you're doing it.

My "easy race" setups are between 2.1 and 2.6Hz, so it has some truth to it, but high frequencies in LFS don't have all of the downsides that they do IRL.

thanks for the answers. yesterday i read some old physic-schoolbooks and came to the same formula as flotch posted, but than i saw yr answer to atledreier and was wondering that 3 Hz are around 103 kN/m, which did not match my calculations (I calculated with total weight of the car). So it seems I have to split the weight for front and rear tyres. So I will check colcobs analyser to find out how to calculate the mass on rear and front.


btw, the guide I was looking into, was posted from you here on 15.08. (direct link).

If you want to do the maths yourself, you need to get the total mass of the car and subtract the unsprung mass to get the sprung mass. This you then multiply by the "correct" weight distribution (depends if you are looking at front or rear), then divide by two (assuming 50:50 L/R mass balance).

does anyone remember whether the mass is in the equation because of its weight or because of its inertia ?

If you want spring stiffness in kN/m, then obviously you need to use force in units of kN, not the mass.

um sorry reading it again i didnt verbalise the question well enough ...

in the equation for spring frequencies ... it the mass in it because of its weight or because of its inertia ?

It's simple harmonic motion (SHM). Obviously less mass is going to have less inertia, and the frequency will be higher given the same stiffness.

Quote from Shotglass :

um sorry reading it again i didnt verbalise the question well enough ...

in the equation for spring frequencies ... it the mass in it because of its weight or because of its inertia ?

Interesting question. If you imagine a mass attached to a spring in a zero G environment, If you push the mass to deflect the spring, it will rebound and undergo simple harmonic motion. And the mass/inertia of the object will enter into the equation. So obviously its mass does matter as the acceleration the mass undergoes as a result of the force applied by the spring is dependent upon the mass itself.

I think, although I havent done the maths to check it, that the frequency of oscillation will not alter with differing G forces, just the original deflection of the spring at rest.

I fact if you think about it, gravity cant enter into it, because g isnt in the equation.

You may sometimes see frequency equations which contain g, but they are actually just fudges which work because of the bizarre co-incidence that pi^2 = 9.87, which is close enough to g for people to think the equation works.

so in other words downforce has no effect on spring frequencies ?

Downforce effectively adds more mass to the car as it goes faster. So yes, downforce has the effect of reducing spring frequencies.

Quote from Flotch :

not sure, but I have read that : spring stiffness = 2 * pi² * m * f²
where m is the mass on the front wheels in Kg (if you want to calculate springs for the front ) and f the spring frequency in Hz. And in general, lots of persons advise to begin with 2Hz, which does not really work with S2 :s

Hey, is the formular correct? In this suspension calculator(0.6), it's (2*f*pi)²*m, which is 4 * pi² * m * f². Which one is correct? I hope it's not about for one tyre or for two tyre, because in the Gear Rate Calculator it's for front and rear side (should be for two tyre).

Quote from khtwo :

Hey, is the formular correct? In this suspension calculator(0.6), it's (2*f*pi)²*m, which is 4 * pi² * m * f². Which one is correct? I hope it's not about for one tyre or for two tyre, because in the Gear Rate Calculator it's for front and rear side (should be for two tyre).

Well, I'm biased, but I checked the math over once more to make sure I hadn't made any stupid mistakes (again), and I'm pretty confident that (f*2pi)^2*m is correct, at least judging by my source for the equation, http://hyperphysics.phy-astr.gsu.edu/hbase/shm2.html

Whether or not the fact that we're working with two tires/springs makes a difference, I don't know. I haven't thought about it before, but I don't think it makes a difference. My guess is that the slider shows the total stiffness of both springs taken together, so everything works out the same.

Quote from 5th Earth :

Well, I'm biased, but I checked the math over once more to make sure I hadn't made any stupid mistakes (again), and I'm pretty confident that (f*2pi)^2*m is correct, at least judging by my source for the equation, http://hyperphysics.phy-astr.gsu.edu/hbase/shm2.html

Whether or not the fact that we're working with two tires/springs makes a difference, I don't know. I haven't thought about it before, but I don't think it makes a difference. My guess is that the slider shows the total stiffness of both springs taken together, so everything works out the same.

In that, we can't adjust the frequency higher than 2.1 because the range of stiffness in LFS is limited to 260KN/m. Could you please try to calculate the stiffness for FZ50 GTR and see the result. If I use the result, then the FZR feels like a train.