As mentioned before, grip drop off with pressure is linear, which doesn't make sense. Pump enough air into the tyre and it won't work at all? Hmm.

Quote from cpachmann :

Well..i think you cant generalise that, as there is so a hugh variety of different cars, tracks, strategies, wheterconditions where tyre pressure does a hugh difference in laptimes. I´m pretty shure that a f1 cars tyre pressure is above of an f3000´s tyre presure couse of the different forces on the tyres, as an example.

I remember the tire suppliers complaining that they were running the tires too low at 15 psi in the late '90s at Spa, which seems about the minimum for VF1 sets. Remember though an F1 car is unusual because they run high profile tires with most of the suspension work being done by the super stiff sidewall of the tire. Why they are designed like this I don't know. I've picked up one of the magnesium rims used on the 2005 Renault and it is incredibly light, with a fully inflated tire on it though it was actually heavier (although a lot larger obviously) than the wheels we use on the small single seaters (the other thing worth doing if you can is to feel just how light the kevlar suspension components and the brake discs are).

From my own experience with single seaters running about 17/18 psi on cross ply slicks and about 22 psi is about right for radials. It is very dependant on wheather conditions and temperature. These cars are running for about 20 minutes at a time with enough camber to get heat into them so we don't put them too low in order to save tire costs (we aim to get a set to last a season, whereas the fast guys will go through a set a meeting). In my limited GT experiences about 25 psi seemed the order of the day with masses of camber to last 2 hours.

Regarding changing pressure for optimum contact patch...When autocrossing, whether using road or racing tyres, we put chalk at the edge of the tyre down onto the sidewall and then set the tyre pressure low enough that we are going to the edge of the tyre but not onto the sidewall. In my experience, that's how most people set their pressures in autox, and this was also recommended by the designer of the r-compounds I use. A bit different from circuit racing (?) as you're not concerned about rolling resistance, just contact patch and temperature, often on camber limited cars.

My biggest issues is that the street tires used on the cars are more like cheep economy tires in reguards to size while profiding the grip levels of high performance tires. Combined this makes the tires feel weird. I don't know about the rest of you but safety is the bigest concern for me when running my car in a performance driving manner. Low profile, stiff sidewall soft compound tires are the way to go. You would never find me pushing my car to the edge if it had tires like a XRT on it. It would be unsafe as the sidewall flex woudl be unacceptable.

Very few cars in LFS are running low profile tires. The ones that are have better handeling characteristics then the ones with higher profile tires. This is most notible in the LX's they have masive sidewall flex at pressures needed to get the tires up close to temp. In the real world you would never run the pressures that low, it's just dangerous. On my street car I have to run the tires up near 40psi to keep the tires from rolling on to the sidewalls. These are 140mph rated 185/50 15s tires on a car that weighs 1100kg or so. On the XRT which weights more and has 75% more power I have to run pressues below 30psi to get the temps right, but sidewal flex is ugly to the poing of pulling the bead off the rim, visualy.

I think part of the problem is in the LFS tires construction, specs and the sizes used, the combination just is not realistic.

Quote from Gimpster :

My biggest issues is that the street tires used on the cars are more like cheep economy tires in reguards to size while profiding the grip levels of high performance tires. Combined this makes the tires feel weird. I don't know about the rest of you but safety is the bigest concern for me when running my car in a performance driving manner. Low profile, stiff sidewall soft compound tires are the way to go. You would never find me pushing my car to the edge if it had tires like a XRT on it. It would be unsafe as the sidewall flex woudl be unacceptable.

What makes it unsafe? My rx7 has 60 profile tyres on it (similar to the xrt I guess) and it never has any problems on the limit of adhesion with high grip, performance tyres. Performance tyres are available in higher profiles.

Quote from Blowtus :

on the limit of adhesion with high grip.

That sounds like an oxymoron to me

I don't know alot about tyres but I think 60's are pretty low profile

And I think Gimpster is talking about safety in terms of running low psi in road tyres very scary stuff I can only agree with him

I use the 10% less than maximum (find the max psi written on tyre and pump up to 10% less so if max is 40psi I run about 36) but thats to get better economy


SD.

Quote from SparkyDave :

That sounds like an oxymoron to me

I don't know alot about tyres but I think 60's are pretty low profile

Your comprehension skills are lacking if it sounds like an oxymoron
60's are not low profile in the slightest, I wouldn't have mentioned it if they were...

Quote from Blowtus :

What makes it unsafe? My rx7 has 60 profile tyres on it (similar to the xrt I guess) and it never has any problems on the limit of adhesion with high grip, performance tyres. Performance tyres are available in higher profiles.

The damgerous part is in running that tire at the same pressures that you would need to in the XRT. The tire size is not the issue, just the fact that the pressures needed to get propper heating are unrealistic for a soft sidewall tire, which in your real car would be dangerous and destructive to the tire.

I think anything under a 65 series tire is fine for performance driving given that the sidewall is stiff enough to not fold under corner loads when run at pressures required to achieve sufficent heating.

In LFS the tires sidewalls are not up to the task and will fold or distort greatly when run at pressures to allow for propper heating. Thats the main issue.

Quote from frokki :

What's the main reason why unrealistically low or high tyre pressures actually work in LFS?

With flatened tyres you have same acceleration as guy with tyres pumped to 3bars

Im really, really tired at the moment, and therefore my brain functions and english skills are lower than a zombie's, but didn't you just somewhat repeat my question instead of answering it?

Sorry if it's me who is acting all funny, I promise I'll go to bed right away ->

EDIT: I slept a night, but still have no clue why you quoted me

Quote from pasibrzuch :

With flatened tyres you have same acceleration as guy with tyres pumped to 3bars

No you don't. There are a number of cars where signifigantly quicker times could be achieved if there were no upper limit to tyre pressure.

Can you give some examples with telemetry included?

It's due to physics and I tested it personaly. There is no way to achevie better acceleration with 1,5 bar vs 3,0 bar, if grip problems are excluded (eg: higher speeds or low powered car).

Um you can see that there is a difference in acceleration from low to high pressures easily in LFS (rolling risistance is influenced by tyre pressures). E.g. run high pressures arround BL1 in XFG and you get higher speeds at the end of the straight...

Also look at the drag strip, for example using minimum pressures on all tyres in the RB4 gives a higher ET despire the increase in traction over medium-low pressures. I'm sure I made this same example earlier in the thread too.

Well, I don't see any difference really . Sometimes, if I look at G-force meter there is diff by 0,01 (!) G, but only for 0,5 second. Notice that this is difference between minimun and maximum pressure, which is pretty wide in LFS.

Easiest way to see the difference is taking own car and feel it by own body.

Even in this shitty program 'Vroom vroom', where they're tuning astra to achieve better drag time, BIGGEST difference gave them pumping tyres. Not sporty exhaust, not alu rims, not sporty filter.

Please put theory away and try it by yourself IR.

I'm here in the name of Superb Acceleration with High Pressure!

If you want I can get you use some data from RAF outputs in LFS that will prove with numbers how much rolling resistance changes with tyre pressue. However I've a report to write tonight so you'll have to wait until tomorrow (unless someone else wants to do it).

One reason it will never make a big change is that at motorway speeds and above, rolling resistance accounts for less than 10% of the total resistance on the vehicle, the rest being aero drag.

So I still prefer to go 220km/h than 200km/h

Bob I can see with my eyes that in LFS difference is very, very little. I would prefer to see differences from real car.

So you have some data on real tyres for us to compare against?

No

Too bad I must do it as one alien in this case... anyone can do some tests with his car?

All I can do is video footage, but not soon.

edit:
I forgot about this. When you have low pressure in one tyre you can clearly feel it just in normal cruising or braking. Car goes to left or right. It's because that one tyre is slowing down in one side hardly enough to change direction of car. In LFS all four tyres may have 3 times less pressure and it will not slow you any little bit. I think now you know why I'm shouting all the time.

So what's this? He's saying big pressure in should mean much faster acceleration and that the difference is not big enough in LFS?

Hmm.. Lots of interesting posts lately around here (no I'm not being sarcastic)

Quote from pasibrzuch :

forgot about this. When you have low pressure in one tyre you can clearly feel it just in normal cruising or braking. Car goes to left or right. It's because that one tyre is slowing down in one side hardly enough to change direction of car.

Problem with comparing RL feel to LFS feel is IRL your seatofpants meter is far more sensitive than the LFS G Force meter. IRL when you can feel a difference it can be infact quite a small difference... that's why it is so much easier to catch the tail end going out IRL than it is in LFS.

I can't give scientific proof either, I can only comment on the basis of alot of driving experience feel. The difference between 26PSI and 40PSI is not huge for top end speed but I can feel a difference in car responsiveness and rolling resistence. If you underinflat the tyres significantly the difference becomes much more obvious... So I think the problem in LFS is that at extremly low pressures rolling resistance is not effected nearly enough. As Bob said I suspect in LFS rolling resistance vs tyre pressure is too linear at the moment.

Quote from pasibrzuch :

I forgot about this. When you have low pressure in one tyre you can clearly feel it just in normal cruising or braking. Car goes to left or right. It's because that one tyre is slowing down in one side hardly enough to change direction of car. In LFS all four tyres may have 3 times less pressure and it will not slow you any little bit. I think now you know why I'm shouting all the time.

Take the fox for a drive on westhill with moderate pressures, then crank them up and see how much faster you go.

Tyre pressure really affects top speed and acceleration in the MRT. It has such low power that tyre resistance really slows you down.

With any simulation, there will generally be a point where the model you are using breaks down. I think that happens with LFS with lower tire pressures.

There are many systems in the world that depending on external conditions will change their behavior dramatically. A simple example is water. It is a liquid. Make it cold enough, it changes from a liquid to a solid. If you water simulator does not take that change into account, you will get some odd results.

I think that LFS models tires pretty darned well, but in certain cases, the model breaks down. I doubt there is much in the system about deformation due to low pressures, material flex, bead adhesion, etc. Plus, tires change how they react dramatically with pressure. It would be hard to completely model all of that realistically.

My suggestion for LFS is to limit the usable tire pressure range for each tire type. Then work on the heating algorithms a bit to tweak them in a bit better. That way, you wouldn't be able to use an unrealistic tire pressure to gain advantage and it would be the same for everybody. (Nobody IRL drives a street car on the track with only 22psi in the tires)

You also wouldn't have to try to model super low tire pressures in the system completely because you either have a reasonable tire pressure, or the tire is flat.

Maybe just tweaking the temperature and sidewall behavior would be enough, but limiting the tire pressure range would be effective too.