iRacing Dynamic Weather Discussion : How weather affects racing

[Daunt]

Opening a thread to discuss impressions of dynamic weather and track temp. I'm not really a weather expert, on how it affects racing, etc, so I really would like to hear some opinions on what makes things grippier, worse for wear, etc and how tire pressures can offset this.

So please share your tips!

I've found this graph that shows the offset that cloud cover creates in track temp. So you will find it possible to have a colder track temperature than at night depending on temp / track temp.

 

[Slider]

Cold weather: more time to heat up tires = more wear?
Hot weather: more grip = less wear?

I'm not sure if that's correct.

 

[Shilka]

Wear of rubber is minimal close to the point where the friction is maximal, i.e. in the viscoelastic region. At low temperatures wear increases,but also at high temperatures. Due to the link with friction, tyres wear less in those conditions where grip is maximal, so it really depends on for which temperature the tires have been designed to have maximum grip. Any deviation from the ideal temperature, either higher or lower, will lead to less grip and more wear.

 

[ElektroVodka]

That means you need the tirecompound information used by iRacing right?

 

[Shilka]

Yeah. But keep in mind that external temperatures are not the only influence. Your tire temperature will be influenced too by tracktype, tire wear, fuel level, your style of driving etc.

Here's some general info on what to do about tire temperature deviations:

Symptom	Diagnosis
Center hotter than edges	Tire pressure too high. Reduce 1 psi for each 5' F delta.
Edges hotter than center	Tire pressure too low. Add 1 psi for each 5' F delta.
Inner edge hotter than outer	Too much negative camber.
Out edge hotter than inner	Not enough negative camber or too much toe-in.
Tire below ideal temperature range	Tire pressure too high, tire too wide, or springs/sway bars too stiff at that axle.
Tire above ideal temperature range	Tire pressure too low, tire too narrow, or springs/sway bars too soft at that axle.
Front tires hotter than rear	Car is under steering (pushing). Too much front spring/sway bar, not enough rear spring/sway bar, front pressure too low, rear pressure too high, front tires too narrow, rear tires too wide.
Rear tires hotter than front	Car is over steering (loose). Too much rear spring/sway bar, not enough front spring/sway bar, rear pressure too low, front pressure too high, rear tires too narrow, front tires too wide.

 

[Daunt]

That first post shilka:

That second post looks like gold though! I love it.

 

[Shilka]

Here's some good info about adjusting tire pressure for ambient temperature:

10 degrees F changes the tire pressure about 1 psig. If a tire starts with a certain cold pressure in the morning, the warming of the day is effectively adding pressure to the tire over the course of that day. The temperature the tire runs at will be affected by the combination of the ambient air temperature, the track surface temperature, and the amount of friction introduced (how hard you drive).

If for your first session of the day, it's overcast, the air temp is 65 and the track temp is 70, then in the afternoon the sky is clear, the air temp is 85, and the track temp is 105, there is considerably more heat to influence the temperature of the tire surface. If you drive just as hard, the tire will be hotter, and the pressure will be higher. This change from morning to afternoon is going to be much more pronounced in some climates than others. The southwestern U.S. for example can see days with a 45 degree morning and a 90 degree afternoon. This will increase the starting tire pressure by 4 psig by the end of the day. This will make a world of difference in the handling of the car during each session.

To maintain the same racing pressure settings in the afternoon as achieved in the morning, you'll have to compensate for the increased pressure due to ambient temperature. While there is probably a formula to understand the effect of the relationship of the ambient temperature and the track temperature, it will be complicated by the aerodynamics around the tires, and the heat generated by the brakes.

You can generally use the ambient air temperature and the 10 degrees per 1 psig relationship as a guide for adjusting tire pressures throughout the day. Take ambient air temperature readings at the start of each session, and use this to determine how to adjust the tire pressures. As the day warms, you'll have to drop tire pressures accordingly before each session.

 

[Slider]

 

[Daunt]

Thanks Shilka. Sounds like you know a thing or two about it! How'd you learn it?

For me, I don't really know anything in regards to tire pressure, temps, and overall weather, so it's a lot to take in. I think I need a lesson on tire pressure/temps as well to compliment this. How do pressures relate to handling characteristics? What can I expect with a lower pressure vs high?

 

[Shilka]

It is very complex, but let me try to explain it in my own words:

You have an optimum tire pressure. This is the pressure where the surface of the tire will sit flat on the track. Increase pressure, and your tire wil bulge in the middle, causing less grip because there is less contact surface. If tire pressure is too low, the middle will 'sink in', and you start driving on the outer edges of your tire, which also causes less contact surface = less grip (pic)

less grip = more wear


What the optimal pressure is for you is you'll have to figure out, for your personal driving style. If you drive 'harder', your tires will get hotter, hot air expands, so driving harder will increase your tire pressure, meaning people with a less refined/smooth drivingstyle need lower starting tire pressure then people who drive smoother. So basicly, it's not the tire having an optimum pressure by itself, it's the tire - the way you specifically drive it on a specific track at a specific ambient temperature and fuel load - that has an optimum pressure. Keep in mind that pushing harder means more wear and ending up slower then the smooth driver.

To find out what your ideal tire pressure is for your driving style, you will have to make heaps of test runs. You'll have to increase/decrease tire pressure while making consistent runs, and find the sweet spot where it feels you'll have the most grip. Keep in mind that you'll need to drive like at least 3-4 consistent laps before judging grip, as you start on cold(er) tires in the first lap(s). You will be doing this at a certain ambient temperature of course. So once you've established your optimum tire pressure for a certain ambient temperature during your test runs, you can now adjust for same track/different temperature by using the 10F difference = 1 psig rule. If you drive the same track at lower temperature, increase tire pressure, is it hotter, decrease pressure.

EDIT: the main problem with tire pressure, is that you want an optimal pressure - with warm tires, tires that are at operating temperature-. But you can only set the -cold- starting pressure.

How much hotter (and hotter means more pressure) then the starting temperature your tires will get completely depends on things like the track (more bends, less straights gets hotter then few bends/many straights), your driving style (driving harder gets hotter tires then driving smooth), ambient road temperature (hotter outside makes hotter tires) and fuel load (more load increases pressure)

So the only way to get your ideal (starting) tire pressure is by testing.

 

[Daunt]

Keep in mind that you'll need to drive like at least 3-4 consistent laps

Welp looks like I'll never know the ideal tire pressure.

Thanks a lot Shilka. I'll try applying this to next week's race.

 

[Noodleguitar]

With regards to handling, it's not always a manner of deviation from optimal pressure = less grip. At least in iRacing, higher pressures help with high loads, so in sharp and demanding corners you'll be better off with the pressures somewhat higher. On the other hand, lower pressures will give you more grip in slower corners, where there is less load on the tire.

That said, you probably want to be on the edge of grip in every single corner and thus want to tune the pressure for high loads only.

-------

I think the main factor in weather is the amount of clouds. The graph shows a minor difference between clear and partly cloudy but there is a huge gap going towards mostly cloudy and overcast. I assume the cloudiness only affects the temperature.

Colder weather means more overall grip and like Shilka said you want to run higher pressures in that case. This is also what I found while testing tire pressures this week at Zolder with different weather scenarios.

Colder (grippier) weather also usually means that the outlap is very deadly. I guess this is due to the fact that the difference between cold tire temperatures and operating temperatures is higher but I'm not sure.

Finally the humidity is still involved somehow but I have no idea what exactly it affects.

 

[Shilka]

Well, there are more factors indeed, the things I said are very simplified, and most stuff I said is about tirepressure, i.o.w. tire shape.

The other factor involved when it comes to tire temperatures is the actual rubber compound, which has it's own optimal temperature. this would be lower for road tires, and higher for racing tires ofcourse, but it also has a fall off at both sides of the range.
Cold rubber becomes harder, less grippy and more prone to wear because of the loss of flexibility combined with the loss of grip. ( compare it to a sander on hard wood will take material off, but the same sander on your softer hand won't, at least, not as fast) Hot rubber becomes 'greasy', in other words it becomes so soft it loses structural integrity, and instead of material 'sanding off', it gets rubbed off.

Of course you want the optimal temperature for tire shape and rubber compound to be the same, but as said before, track etc is also of influence. You can however assume that driving 'hard' is not advisable, as the choice of compound is usually done in such a way that it assumes you will drive on the edge of grip as noodle said, but not over the edge of grip (driving hard).

Same goes for braking. Hot brakes will up your tire temperature, so eventhough sharp braking and sudden release of brakes may win time in the short run, the upped tire temperatures will lead to more tire wear in the long run, so it's better to do less sharp braking and more gradual release of the brakes.

Rough driving style is bad for tires, though you can compensate high tire temperatures with lower starting pressure when it comes to tire shape, but it won't help for your rubber compound optimum temperature.

Humidity is a tough one, but in general it magnifies temperature effects. Humid cool air will cool stuff faster, but humid warm air will warm things faster then the not humid variants.

Then tire profile depth complicates things even more. The deeper the profile, the more negative the hot effect becomes. As the rubber becomes softer, the rubber 'blocks (iow the area between the profile) bends more, so a bending block on the road is laterally offset compared to the next block that will touch the road, causing the tires to 'sidestep' even when still having full grip. That's why, next to contact surfacearea, you don't want to use rain tires on the dry.