Would someone please explain to me the reason this is true:
LOWER pressure in the REAR and HIGHER in the FRONT induces OVERSTEER
HIGHER pressure in the rear and LOWER in the FRONT induces UNDERSTEER
From my own reasoning, I would assume that lowering the pressure in the rear while keeping the fronts higher would induce UNDERSTEER because of the increased contact patch of the flatter tire. But, I've read, from a few different sources, that it is the other way around. Why is this?
Because the lower pressure 'end' has no sidewall strength (due to low pressures), and hence moves around a lot, rolls up, and generally has little grip.
Small differences (+-2psi) can be used to adjust the balance, but only very fine tuning. Bit changes won't produce predictable results.
I see. Thank you. So, if I had my fronts at 40 psi and rears at, say, 38, I would have a bit of oversteer, but any more deviation would cause unknown and possibly unwanted results?
During my time as a banger racer, we used to reduce pressure in tyres to force the car to behave in different ways, dependent on the track we were on. But yes, too much adjustment and it becomes unpredictable. It has to be.. Delicate
It depends a lot on the weight distribution of the car and if the wheels are powered or not. Generally I run higher pressure at the rear regardless on RWD cars, but more-so on the FZ50 then the XR GT.
Tyres are very complicated things, since they deform they act like springs, the resistance of a rubber gives the tyres it's own damping, contact patches change both size and shape. All these affect grip under varying conditions.
Hmm ok I was playing with this today upon discovering that I couldn't actually stop my GTT from oversteering. At first I thought it was engine braking causing problems like the FZ50, but locking the coast side of the diff didn't really seem to help much.
Then I tried maxing out tyre pressure (55psi instead of 29) and, voila, understeer. The rears were at 34psi both times. Any less than about 40psi and the problem starts to come back (which is too soon, which points to other problems).
Wondering why I'd not noticed this problem on the FZ50, I think it comes back to what I was saying about weight distribution. On RWD cars you generally need more pressure at the rear to maintain optimum temps at both ends of the car. On a car with 50/50 weight dist this is going to give some oversteer since (generally) this is going to give less grip at the rear.
On the FZ50 this is no problem since there's much more weight at the rear anyway, so it has way more grip than the front, but on front heavy cars the reverse is true so it becomes difficult to keep all the tyres at their optimum temperatures while maintaining a neutral balance.
I've read (although I've no hard data) that the tyre in LFS are not particularly load sensitive which has both plus and minus points. Considering the very small differences in cornering speeds between the road cars (although some weigh twice as much as others) this seems to be accurate.
I think making them more load sensitive would help with acceleration in the RWD cars, since you'd lose grip from the front and gain at the rear the harder you pressed the throttle, which must be of some benefit.
Lastly, I've noticed that, on the whole, cars with slicks need plenty of oversteer to get the front end to turn in (when cars like that should have loads of front end bite), while cars with road tyres tend to be quite loose. Assuming that there's just different parameters being fed into the tyre physics (that seperates the handling of road tyres from slicks), perhaps some jiggery pokery can swap these things around a bit while retaining the differences in handling characteristics that are currently correct?
Actually, making them more load sensitive would reduce the RWD grip under acceleration. You'd get more tendency toward power-on oversteer and power-off understeer.
On tire pressures and under/oversteer: This is a confusing area until you see some graphs and try to code the effects into a sim
Basically, when you increase tire pressure you're doing two things:
1. Increasing the stiffness of the tire, i.e., cornering stiffness. This means that if the force peaked at 12 degrees slip originally, the force might peak at only 10 degrees once you cranked up the air pressure. Imagine your rear tires peak at 12 degrees. Assuming the maximum grip is the same at both ends of the car and doesn't bother to change with tire pressure, cranking up the front pressures can induce oversteer. If you're in a turn and the front tires are maxed at 10 degrees and the rears are maxed at 12, you have an oversteer situation by definition, even if all the tires have exactly the same amount of grip they had before the pressure change. This car will wallow all over the place and want to turn in really quick and not stop rotating. Spin city... I've read that slalom and autocross racers will frequently crank up their front pressures to the max to take advantage of this, even if they lower the total amount of available force at that end, it now oversteers.
Decreasing front pressures to get the fronts to peak at 13 degrees will make the car nice and stable. Your max cornering speed is the same, but now the car is understeering ever so slightly and it's more stable on the straights as well. It took me awhile to figure this out. My own sim car would wander all over and was a trick to drive when I first got it going. Eventually it dawned on me that increasing the rear stiffnesses above the fronts would stabilize it and get rid of the oversteer. Perhaps this is one reason many racing cars use wider and probably stiffer tires at the rear.
2. Increasing tire pressure can either raise or lower the over all grip. LFS at the moment seems to produce peak grip at rather low pressures (which is realistic). But because of #1, if you decrease the rear pressures to increase their grip, you can still wind up with an oversteering car if the rear cornering stiffness is lower than the front. I.e., you might be at 15 degree peaks at the rear and 12 up front. The car oversteers like crazy, but in all actuality it has more grip at the rear than it had before
So, as others have said, it's probably best to play very gently with tire pressures in little increments rather than trying to make huge jumps because of the two combating effects. It's very likely that you may start out lowering the rear pressures a bit and find you're getting more understeer because of the increased grip, but the moment those rear tires peak at a slip angle greater than the fronts, the situation will reverse itself. At that point, lowering air pressure further will cause oversteer, even though the rear grip is higher. Wierd, huh?
Thanks Todd / JTW for that explanation. For years I have been taking a few PSI out of the rears in games like GPL and NR2003 and assuming that this was helping me to tame the oversteering beast. This thread was therefore really starting to confuse me, but you have set me straight. Much appreciated.
funny this thread poping up as just last night i was tweaking my WH set for the FO8 i thought about raising the pressure in left front i raised it by couple of psi and OMG! it was a completly different car 1 tire with raised pressure opened up a new bag of problems, it was wild understeering in the long sweeping bends and oversteering in the slow tight turns, i quickly droped the pressure again...
but with tire pressures on the whole i tend to dial it back out with role bars anyway but you know what its like change one thing and boom it opens other doors
In my limited experience, Westhill FO8 is the hardest to setup. So many corners are flat or very very near flat. You can't have much understeer, but any oversteer tends to snap the car round, especially at T2 as it goes over the crest.
Maybe it's just my limited setup ability, but thats by far my worse setup track.
months and i mean months i have been working on it, actualy i chose the track to start with when s2 was released as i thought it looked one of the easyest ones, how wrong can you be!...
i had the car setup severel times running decent lap times but even held back and did 1.28 the right front tire was burning up after 4 laps i tried all sorts to get it to saty cool but still offer the amount of grip needed for t2, its only been the last 2 weeks or so i have FINALY cracked it 10 laps now and the front right starts to get worm but i lay of and a couple of laps at say the mid 1.27 and im good to go again, im getting 1.26 prety consistant now and considering its only a few seconds of the WR and and thats my race pace its actualy very competative, all im worried about now is heavy fuel loads and what efect thats going to have in the longer races, but come 20 laps the tires are little warm and thin on the old rubber so i probably wont be carrying more than 30% fuel anyways....
but your right its a bugger of a track to setup for..... pretty pleased now though
Set-up's frutrate me, as I have very limited knowledge of how things work when it comes to some of the setting's avaliable and people are generally nice enough to share their sets unless they are team ones, which is fair enough.
Should we not separately define sliding and gripping oversteer/understeer? Let's say the front's peak grip slip angle is 12 degrees, a lower rear grip peaking at 11 degrees vs. a higher rear grip peaking at 15 degrees. Without sliding, the former gives the same amount of grip at a lower slip angle, hence "gripping understeery". But the former has less max grip so is easier to slide, which makes it "sliding oversteery".
Understeer/oversteer is defined by the difference in slip angles, regardless of much grip is available at these slip angles. So, let's go through your examples:
12 deg front, 11 deg rear. By definition this is understeer. However, you said the rear has less grip (and these are both peak values). So if we were to move time forward a bit, the rear would begin to move faster than the front, i.e. oversteer. But at this point of course the rear slip angle is now going to be higher.
12 deg front, 15 degree rear. By definition this is oversteer. However since again these are both peak values, this is going to give you the fastest cornering speeds. Again though, move time forward, and the front end is going to wash out a bit since it doesn't have as much grip.
I'm a bit confused myself on how it all works, but basically slip angles show the current oversteer/understeer, while the difference in traction shows how they are going to change. At least that's how I see it.
I don't know anything about setups (which is, why I'm very happy that people are sharing ), but I'm finding this thread very interesting to read. And a little confusing. This has helped me to understand what exactly slip angle is. Might be of use to others .
Hmm... I think I was using oversteer/understeer to describe a car's characteristics, not a car's situation (as originally defined). I know what you mean perfectly, though.
Let me try it again:
A "gripping oversteery" car has the rear at a higher slip angle before any tyre slides. A "sliding oversteery" car has the rear break traction to slide sooner than the front.
So a car can be both "gripping oversteery" and "sliding understeery", which I *guess* is the characteristic of lower rear tyre pressure.
It's weird, it sounds completely "wrong" yet kinda makes sense.
The bit about weight transfer and slip angles is interesting, would actually explain more why softer setups are easier to drive. Assuming this is all modelled in LFS of course (which seems likely). I need to tweak my guide anyway because I still haven't corrected the error about suspension frequencies and downforce, I'll make a few tweaks in other places as well.
hehehe, it is almost like we have good oversteer and bad oversteer, just like we have good cholesterol and bad cholesterol.
I liked that linked article on slip angle. It makes a lot of sense. Too bad they use the term "slip" angle. It would be better if it were termed "twist" angle. I bet the term comes from the old bias ply tires used in the 60's.
Anyway, it is easier to understand now how a lower tire pressure in the rear would lead to more twist and a larger "slip" angle that would be interpreted as an overteer by the driver.
