FM3 and Tire Pressure.

[Choccy]

Weight distribution and overall handling balance:
- running cold(during the first lap), minor changes in tire pressure aren't very noticeable, but steering inputs and handling feels sluggish
- running hot, very minor changes in tire pressure result in significant handling changes
- at race pace and tire temperatures, tire pressure can vary from 0.5 to 1.0 psi(depending on how close you drive to the limit of grip), changes that can impact the handling of the car and your perception of how fast you can negotiate a turn from entry to exit
- to compensate, you need to ensure any tire pressure settings and pressure ratios from front to rear match the weight distribution of your car

As an example, I ran my 73 Porsche 911 at Sedona for five laps with cold tire pressure at 23.0 / 23.5 front to rear. On the first lap, the tire pressure ratio was 23.0 / 23.5 front to rear. By the second lap, the psi difference between front and rear had decreased in some turns to 0.4 psi. By the third lap, the difference had decreased to 0.3 psi. While the car maintained comfortable levels of grip at all times, it developed an increasing tendency to understeer as the difference in psi front to rear decreased. Equal tire pressure front to rear is effective in cars with near or perfect 50/50 weight distribution, but this is not desirable for most Porsches. To compensate, I increased the cold tire pressure of the rear tires to 24.0 psi. This had the overall effect of neutralizing the perceived handling tendencies of the car; at race start, the tendency was slight oversteer which gradually developed into neutral handling by mid-race.

Suspension considerations:
- increasing tire pressure stiffens the tire carcass and removes flexibility in the tire, a desirable result to a point
- finding the optimal tire pressure has a dual effect of maximizing the tire profile(contact patch) with the road surface AND isolating the suspension(you can more readily feel the movement of the body of the car as it dives under braking, lurches under acceleration and rolls from side to side in a turn(s))
- excessive tire pressure makes the tire carcass too rigid; in a turn, the tire profile will be concentrated on the outside edge(temperature will be significantly high, optimal grip will be compromised)
- the optimal tire pressure isolates the spring-like flex of the tire(you DO NOT want to eliminate tire flex completely) from the spring effect of the suspension

Taking a set:
- it becomes very noticeable when the car settles into equilibrium in the turn
- all four wheels feel equally planted in spite of the fact the car is negotiating a turn
- you can feel the steering become heavier(even using a control pad), very minor steering inputs translate into significant shifts of weight

*** You will only observe these subtle handling changes if your driving style and steering inputs are smooth and consistent! ***

In application:

Car: 1973 Porsche 911. [C425] 2,116 lbs weight, 44% front weight distribution. STREET - Ignition, Pistons, Oil, Clutch, Transmission and Tires. SPORT - Air Filter, Intake Manifold, Exhaust, Cams, Valves and Flywheel, Brakes, Springs and Chassis Reinforcement, Driveline and Differential. Compomotive MO5s.
Tune: Tire Pressure 23.0 front, 24.0 rear, Differential 10%.
Sample Venue: Sedona Club Circuit. Five laps.

The tire pressure formula discussed in a previous thread has proven very effective as a baseline measure. Since that time, I've revised all of my tire pressure tuning using that formula and results have been positive in terms of off-the-line starts, threshold braking and negotiating turns at speed. Truth be told, I now apply the formula, add two psi to the front and rear tires, then fine tune by no more than 1.0 psi(front, rear or both)to yield the best handling balance for each car.

Admittedly, I haven't focused my tire pressure testing on tires that are wider then OEM spec. That will follow shortly.

[feuerdog]

I've been playing a little with higher pressures myself.....

Optimal tire pressures are altered more appropriately by workload, not just specifically weight distribution.

How much work a tire can perform is altered by static weight, aerodynamic weight, and tire size. It is also affected by driving aggression/consistency, track surface conditions, alignments, and pressure.

After experimenting with higher pressures(30~32psi) for several builds(using my stiffer newer calcs), I noticed only a change in response and feel and not a distinct performance advantage.

With a softer suspension setting, the tires loaded differenetly under weight transfer, and a performance advantage was noticed,....but is probably more attributed to the suspension not the tires.

That said, with a stiffer suspension(45'ish% of vehicle weight/distribution*calcs) the cars are responding/performing best with my default 29psi settings.

Do lower pressures improve grip? Yes.
Do lower pressures improve response? No.

Again, in my opinion, and based on my calculations, building and driving techniques, and desired feel, pressures of 28~29psi result in the best trade-off of grip and responsiveness.

If a build performed better with a 26psi setting, then i'd be more likely to add tire width or compound and then run a higher pressure to get the same level of performance from it.

In the end, to each thier own.

[Choccy]

[A600] Corvette C6 Z06, 515 hp / 479 lb/ft, 3,142 lbs, 50% weight dist.
SPORT Pistons, Brakes, Springs, Clutch, Driveline and Differential. Nothing else.
Venue: Catalunya Grand Prix Circuit

- default cold psi is 30.0 psi front and rear
- 3142 / 100 = 31.4, round to 31.5
- increase the cold psi to 31.5 front and rear, run two laps to establish race temps, monitor the surface temps across the tire profile
- increase the cold psi by 0.5 front and rear, run two laps, monitor the surface temps across the profile and the "hot" psi between front to rear
- continue increasing cold psi by 0.5 psi increments front and rear until the cold tire psi is 33.5 front and rear
- you should see the tire profile temps increasing at equal rates from outside, center and inside edges(not by the same amount, just at roughly the same rate)
- tire responsiveness might feel rigid at 33.5(FM3 only permits 0.5 increments, using the tire pressure calc requires some rounding effort, so 33.5 is technically "just a bit" too high for this car's weight), if you feel the tires are too rigid, relieve psi by 0.5 front and rear
- in steady-state cornering, the ability of the car to "stick" the turn will become more apparent as you increase the psi up to the 33.0/33.5 threshold
- threshold braking will be very easy to determine
- when increasing the cold psi by 0.5 increments, test and observe by making changes to front or rear instead of both ends at the same time for this 50% weight distributed car, run your laps and observe how the car responds under progressive acceleration through turns

I can't emphasize enough, you won't notice a difference if you throw the car around violently. Your inputs should match what driving instructors try to educate during HPDE events. Be smooth! You would be surprised how much faster you can go by slowing down your inputs. Andretti(and others) was right about that for certain.

[feuerdog]

I absolutely agree on the smoothness part,.....unfortunately i'm not that good.

Especially in the close competition I have running MP races with Exodus racing,....the slight improvement in grip of a lower pressure allows me to make more mistakes, drive a hair more aggressively, and recover from incidents jsut a little closer to my comfort level.

I ran 490hp A-class Challenger around Silverstone for 20 laps last night in a race event.
275+ Slick tires on 19" wheels, starting pressures were 29psi.

After 2 laps tire temps were hovering around 180~200.
After 8 laps tires temps were around 190~220.
Pressures stayed in the 32~35psi range.
All tire temps remained within approx. 10 degrees from inner-to-outer across each tire.

The sustained nature of the final hairpin usually cooked the front left tire, but the ndersteer was needed to keep this torque monsters rear end in check. I could spin the rear tires on command in the first 4 gears while cornering if I wasn't careful.

Temps over 220, and pressures over 36psi feel too sloppy too me, so I tune according to those limits.

[Choccy]

I've been playing a little with higher pressures myself.....

Optimal tire pressures are altered more appropriately by workload, not just specifically weight distribution.

How much work a tire can perform is altered by static weight, aerodynamic weight, and tire size. It is also affected by driving aggression/consistency, track surface conditions, alignments, and pressure.

After experimenting with higher pressures(30~32psi) for several builds(using my stiffer newer calcs), I noticed only a change in response and feel and not a distinct performance advantage.

With a softer suspension setting, the tires loaded differenetly under weight transfer, and a performance advantage was noticed,....but is probably more attributed to the suspension not the tires.

That said, with a stiffer suspension(45'ish% of vehicle weight/distribution*calcs) the cars are responding/performing best with my default 29psi settings.

Do lower pressures improve grip? Yes.
Do lower pressures improve response? No.

Again, in my opinion, and based on my calculations, building and driving techniques, and desired feel, pressures of 28~29psi result in the best trade-off of grip and responsiveness.

If a build performed better with a 26psi setting, then i'd be more likely to add tire width or compound and then run a higher pressure to get the same level of performance from it.

In the end, to each thier own.

Of all my builds tested thoroughly to-date, none have bolt-on aero parts and none have Race suspension added. I wanted to be confident about that tire pressure calculation first and foremost. Street or Sport suspension is all I've used so far(with the exception of cars that have Race suspension as standard fitment). I wanted to isolate the subject matter, tire pressure. If the clock is the best indicator of how well a build works, then based on times from various tracks, I'd tend to agree that the tire pressure calc is an effective tool to determine optimal setup, for OEM tire widths at least.

The build and test plan I used had one goal, to find optimal tire pressures based on the most critical factor, the car's weight. Aero parts generate downforce, adding to the car's weight. Rollcages for added rigidity add to the car's weight. Ultimately, you are impacting how much the car weighs, in the dynamic or the static sense.

My D, C, B, A and S class builds yielded consistency with psi numbers even when final curb weights differed by a thousand pounds. The tire pressure calculation, loosely applied by tire engineers, holds true. If I were to post an example of a single car per class up to S that I tested, the results would be consistent.

Minor adjustments to psi can and should be made per venue. A track where sustained, steady-state cornering is prominent, benefits from slightly lower(0.5 to 1.0 psi) tire pressures as the heat generated by friction while holding the car through the turn will magnify the tire surface temperature and increase the tire psi. Moreso than a circuit where shallow radius turns are separated by long straights.

Choosing tire pressure based on "work" is the same as determining optimal tire pressure per venue. I remember someone from fm.net dismissing my view that setting pressures per track was perhaps obsessive. But if you want an optimal setup, you have to go the extra mile.

[barumba]

Have a look at this. It is not a tuning calculator, per se, but a "What adjustments to make next based on driver feedback" tool.
I have been trying to enter values from in-game tunes, then respond to the driver input, and see what the recommended changes do. Some fields are not applicable to the in-game set-up, such as tread depth and slow and fast shocks. I have been experimenting with a "default" setting for those in order to negate their affect.
Interesting, a bit of a diversion, if not practicle...
http://www.mediafire.com/download.php?1ijdzvyymit

[Bob Boberton]

Of all my builds tested thoroughly to-date, none have bolt-on aero parts and none have Race suspension added. I wanted to be confident about that tire pressure calculation first and foremost. Street or Sport suspension is all I've used so far(with the exception of cars that have Race suspension as standard fitment). I wanted to isolate the subject matter, tire pressure. If the clock is the best indicator of how well a build works, then based on times from various tracks, I'd tend to agree that the tire pressure calc is an effective tool to determine optimal setup, for OEM tire widths at least.

Choccy, the above quote sounds exactly like the majority of my setups. Unless the car already comes equipped with a race suspension, I hardly ever install it (I play mostly with lower class cars). This means that you have to deal with a lot of positive camber that you can't dial out and you are always having to deal with the hot outside-cool inside tires.

I too wanted see what optimal tire pressures would be using lap times as validation, not accepted real world values. My first test was with a stock Hyundai Genesis on Nurburgring GP Short. I would set the tire pressure and run 10 laps. I calculated average lap times, extracted the best lap time and at what lap the best lap time was achieved. As the times showed, I acheived my best lap time, my best average lap, and the quickest best lap with a cold psi setting of 37.5. It actually managed to level the tire temps across the whole width of the tire. Just to make sure I wasn't getting those lap times due to having driven so many laps of the track, I went back and lowered the cold pressure setting to 28 and then 30 psi. I couldn't come close to my 37.5 psi times.

Of course, installing street and sport suspension with accompanying ARBs stiffens the suspension and reduces roll, and lower temperatures start to work better due to less positive camber in the turns. Using the Sport suspension with Sport ARBs almost entirely negates the effect of positive camber and a cold PSI setting between 30-32 psi seems to work best for me.

It is so hard to determine a universal setting because every single component change impacts all the other components of the car. That's the hardest part of learning this whole tuning business for me. It's not just understanding what every individual component does by itself. That's easy enough to understand. It's the relationship between the components that makes my head hurt. Add to that trying to determine whether a given problem is a driver problem or a setup problem. I'm afraid that requires a little more seat time than my non-pixelated life will allow. 

[Choccy]

It is so hard to determine a universal setting because every single component change impacts all the other components of the car. That's the hardest part of learning this whole tuning business for me. It's not just understanding what every individual component does by itself. That's easy enough to understand. It's the relationship between the components that makes my head hurt. Add to that trying to determine whether a given problem is a driver problem or a setup problem. I'm afraid that requires a little more seat time than my non-pixelated life will allow. 

Aye, that's what makes tuning so intriguing. That's why since FM3's release, I've limited my builds thus far to the basics, tire pressure and differential adjustments. And also emphasizes what gives the Forza series its legs.

I tried the same tire pressure calc on my C class Porsche 911(930)T(weighing ~2900 lbs), tested at Catalunya and applied minor psi adjustments to fine tune handling balance. I then ran the 31 laps at Silverstone C class endurance race.

After five laps, grip was as good as it was going to get, but the differences in weight distribution and handling characteristics made driving this car at speed wildly different compared to my C class '73 911 Carrera RS.

And there were still times when I felt I could and should have optimised my tire psi for the Silverstone venue. I couldn't push the car as hard in certain areas compared to Catalunya where inclines mid-turn can work to your benefit by augmenting front tire grip.

At some point soon, I'll graduate to adding Race ARB and suspension.