does it affect aerodynamic drag in lfs?

last night i tried someone else's FXR/AS3 set. the set came from someone who is very fast, and it was very different than mine in every way other than steering. it was a bit wobbly in the corners for my taste, but it had 2-3 km/h more straight line speed than my set, even with me taking the corner exits slower than i normally would. i began to compare settings, looking for differences that would affect top speed.

total body and wing drag was nearly the same, so my next guess was gear ratios. i applied his ratios to my car, and it turned out that mine were actually better, so that theory was out too.

next i looked at the suspension, and noticed that he had 10mm of rake, while i had none. i lifted my rear end by 5mm as an experiment, and voila! 2 laps to warm the tires, then i put the hammer down and destroyed my pb by 0.3 seconds, with top speeds 3-4 km/h higher than i'd seen before.

so now i'm a believer in rake, but my question is, did the time come off because the aerodynamics are different with rake, or because the handling is better? because the downforce screen doesn't show any changes in drag when you adjust the suspension ride height.

It'll be handling. The aero in LFS is quite simplified still (although it's pretty good in most cases). A better handling car will enable more speed through and out of corners, and every mph you gain out of a corner you keep all the way down the straight (okay, so it's not quite like that, but you do keep an advantage).

that's what i suspected, but it's still surprising that such a small change (which didn't seem to affect the balance of the car much at all) could have such a big affect on lap time. my old pb had stood for several hundred laps, and i wondered if i would ever match it again, let alone beat it easily (with 7 laps worth of fuel in the car, no less).

The aero figures in the garage don't take rake into account. The lift and drag generated by the wing when on track does take the direction of airflow over the wing into account. So lifting you rear end will actually increase drag, since you are actually very slightly increasing the angle of attack. At least that's my understanding.

I do find it unusual that raising the ride height of the car has increased the speed though, kind of contrary to usual thinking.

i noticed it the most on the flat-out twisty section of sector 2. i would normally hit 248 or so coming down the hill, lose some speed on the right hander at the bottom, and crest the hill before the hairpin at about 242. with the rear end lifted 5mm i was basically able to carry all that speed through the corner and crest the hill at 247.

maybe the explanation has to do with less tire scrubbing around that corner?

try if you can get the same effect by increasing downforce a little on your original setup

There is that (less tire scrubbing) and then there is (probably) the fact that the suspension has more travel distance available to cover using the same stiffness as before (before being your original setup, which as you said has the same stiffness) - which means that due to forces from the wing (probably) the rear is forced more of a distance downwards (bigger distance/same stiffness) so the angle of attack changes more and you have less aero friction as you accelerate than with a shorter distance suspension, which happens until the forces balance out (friction causes the car to not accelerate any more and no speed flunctuation means the wing's angle of attack stays stable).

Just a thought, after a couple of J&Bs.

In any case, some telemetry analysis could shed some light into this.

Quote from yoink :

mmmm yes, the operative word being "probably"

Actually, the keyword is "J&B"...

By raising the rear you are getting a little more travel out of the rear suspension and in this case it seems to have improved the handling.

And the AoA was bigger, so he got a little bit more downforce, enabling him to go through the corners faster...

It's also worth remembering that changing the ride height will affect the live camber settings. Chances are you reduced negative camber slightly when you increased rear ride height. This may have resulted in better rear end lateral grip or traction.

Quote from Gentlefoot :

It's also worth remembering that changing the ride height will affect the live camber settings. Chances are you reduced negative camber slightly when you increased rear ride height. This may have resulted in better rear end lateral grip or traction.

you may have it here. i did do the test of going back to my old (flat) setup, adding more rear downforce, and it didn't help. i'm thinking, more downforce = more camber = less contact patch. so maybe i could make the flat set work if i played with my camber a bit, but meanwhile i went back to the new raked set and knocked another 0.2 seconds off my pb.

Quote from evilgeek :

you may have it here. i did do the test of going back to my old (flat) setup, adding more rear downforce, and it didn't help. i'm thinking, more downforce = more camber = less contact patch. so maybe i could make the flat set work if i played with my camber a bit, but meanwhile i went back to the new raked set and knocked another 0.2 seconds off my pb.

Nice work.

Yeah - lots of things have an effect on live camber. Ride height, spring rates, rollbar rates, damper rates and downforce. Even how much fuel you have on board may make a difference. I always make a note of my live camber before I make a change and then try to compensate. Although it's worth remembering that with a really stiff set up you will need less neg camber because the ride height will change less when loaded up in corners.