355 Tire Pressure differences

I'm NO expert, but I take a stab. I'm sure Ferrari spent time optimizing any OEM tire that was offered as an option through dealers. The front tires are doing the steering of the car and are subjected to continuously changing geometry and load forces. Each manufacturers tires have pressure specifications which are dependent upon numerous variables, e.g., tire dimensions, aspect ratio, compounding, tread design, speed rating, etc. These variables are, in turn, dependent upon the specific car on which the tires are mounted and vary with the car's suspension type/geometry (i.e., toe, caster, camber), how the car is being driven, brake system, etc. Thus, the recommended tire pressure has been optimized for best overall performance (i.e., maximizing tread contact patch, minimizing sidewall deformation, minimizing overheating, etc.) given an F355 which has been adjusted to factory recommended specifications.

Many FChat members have altered suspension type/geometry, tire type, etc. on their cars to optimize certain characteristics. I'm sure one of the Pros will chime in here and expand on my rather poor explanation.

 

I know what exactly what you mean. I too was surprised when I was looking through the owner's manual and saw the tire pressure differences between the three factory-offered tires. Hopefully, some "tire guy" or Ferrari Tech will enlighten us both!

 

The F355 is very sensitive to small changes in tire grip. Due to the extreme mass centralization, it changes directions very rapidly, more rapidly than may drivers are aware that a car can turn. Thus, the car feels stable at a very carefully controlled set of tire grip. Tire grip is controlled primarily by tire temperature, and tire temperature is controlled by (presto) air pressure.

One can alter the grip at one end of the car or the other by changing the tire pressures. IN what follows I will be speaking about tire pressures of HOT tires, do not confuse these pressures with sitting the the garage pressures, or driving gently down the freeway tire pressures.

At the back on is striving for maximum grip, this generally occurs with tires very close to 40 PSI and very close to 190dF temperatures (180dF-200dF). The rear grip determins EVERYTHING from this mid-rear engined car, and we then set the front grip for the handling characteristics we want (understeer, neutral, oversteer).

Back at the front, we alter the front tires pressures until the car feels right. {Or as we see alter measure the tire with a pyrometer to see that the pressure is right} If the suspension is setup at the correct ride heights and at factory alignments AND the same tread ratio as the factory tread specifications, then the front tire will have a similar pressure as does the rear (also at HOT operating conditions).

Here is where it gets tricky. There are a whole bunch of ways to setup the suspension to dial in or out understeer. One can diddle with the HOT front tire pressures, or one can diddle with the rear ride heights, and occasionally one will diddle with the front toe if the tire cannot otherwise be brought up to temperature.

High speed Front braking stability is controled by the front ride height. Rear braking stability is controlled rear toe.

If you have a handling issue, the correct tool to sort it out with is a probe tip tire pyrometer. With it, you an tell if each tire is operating in the correct air pressure range, has the right tire camber, and even if the toe is set correctly. A tool such as this can be found for 10% the cost of a set of tires. I can't see why everyone who is actually interested in handling does not use it first, and use it to tell the person who aligns the car what settings to change.

 

I moved this to a new post for clarity:

Measuring tire performance with a probe tip tire pyrometer.

We are looking at tire performance at the top end of the performance chain. The closest a Ferrari tire will ever come to this is on a real race track with the driver running the living snot out of the car for at least 10 full minutes. All of the suff realted below is doen under the suposition that the car has be used correctly and hard, and is driven into the pits immediately with absolutely no cool down period. Jump right out and mesure the tires at 3 positionis on each tire {Inside edge rubber block, middle rubber block, outside edge rubber block}. Punch the pyrometer tip into the rubber so that it is measureing the tire a couple of mm below the surface.

A tire running with correct suspensionis setup and ir pressure will have a nice 10dF-20dF temperature gradient; cooler on the outside and hotter on the inside. We should be looking at temperatures in the 180dF-200dF range, where street rubber (max performance,...) gives best traction.

Air pressure: If the center is cooler than the average of the outside and inside temperatures, add air pressure. If the center is hotter than the average of the inside and outside temps, reduce air pressure.

Camber: If the inside edge is more than 20dF hotter than the outside edge, subtract negative camber. If the inside is not less than 10dF hotter than the outside edge, add negative camber.

If the front tires are more than 20dF cooler than the rear tires, and the air pressure profile is linear (center is average of edges), lower the back end to induce more understeer and work the fron tires harder. If the front tires are hotter than the rear tires, raise the rear end to reduce understeer. ** The F355 is VERY sensitive to ride height adjustments. Take a perfectly setup F355 running absolutely neutral in the oversteer/understeer department. 2 turns on the shock towers will change it from perfect ballance into an oversteering pig (up) or an understeering pig (down). This is also how one adjusts for non factory tire widths front and rear. With a little work one can get the car perfectly setup with almost any set of tires that fit on the rims.

Fronts toe: After getting the above three items sorted our FIRST:: if the front tires remain more than 20dF cooler than the rears, you might want to consider adding a touch of front toe-in to work the fronts harder. {Race cars can add to the toe-out they already posess.}

(**)Once we have the car in the right ball park (ride height wise) we can now change the understeer to oversteer relationship with small changes to air pressure. The tire pyrometer will tell you which end of the car to modify (mostly the front).

 

Sure it does.

1: the F355 has extreme mass centralization
2: this means if there is a grip difference the F355 accelerates towards the difference faster than (say) 360 or 430. This makes the car harder to "stay in front of" for the driver.
3: Ferrari knew this, and specified tire pressures to quell this tendency (to slow the car down--CoG rotationally).
4: for the range of tires in that particular year, the rears only had a 4 PSI window to achieve optimal traction.
5: for the range of tires in that particular year, the fronts needed as much as 11 PSI difference to balance the car (given the rear tires already set pressures).

6: if you slow down the car (like for example in a 360 with less mass centralization) this can be achieved with less variance in tire pressures.

7: if Ferrari stated anything about ride height adjustments the average consumer would get lost much faster than they could get found. So Ferrari, in its infinte wisdom, says nothing about ride heights other than use the factory numbers.

8: BTW in 95 (my car's year) the fronts only had a 7 PSI range.

 

There are differences in sidewall construction and belts in the tire as the tires migrate from small 18'ers (fronts) to larger 18"ers (rears). One manufacture might target the naturally oversteering 911 and alter the construction of the tires to not oversteer so much (M). Another might alter the sidewalls so as to optimize a Corvette (G). Still another might descide the optimal target car for its tires is the BMW M3 (P). You will see these designations if you puruse TireRack.

Thus, the tires were designed with a kind of target car in mind. The designer and manufacture end up balancing that weight distribution with that power and with that kind of suspensions (which are genrally fixed in ride height). Ferrari makes to few cars to com onto the tire manufactures radar screen to end up with a targeted application. {Porshce, BMW, Viper, Merc do}

Luckily Ferraris come with adjustible suspensions--and if you know how, you can make the car pretty much put up with any reasonable difference in footprints with a little ride height adjustment, air pressure and small tweeks in camber and toe. Given that you aren't going to adjust the ride heights, then air pressure is the first and most reasonable option for an interested customer to diddle with.

 

"...If the front tires are more than 20dF cooler than the rear tires, and the air pressure profile is linear (center is average of edges), lower the back end to induce more understeer and work the fron tires harder. If the front tires are hotter than the rear tires, raise the rear end to reduce understeer"

Mitch: I agree with this but have been reading up a lot on this subject and the general concensus seems to be saying the opposite. That is one increases air pressure at the rear to increase grip and INCREASE understeer: Pg 19 How to Make Your car Handle by Fred Puhn - HP Books published in 1981. He even says that if you put say 50 lbs in a small sedan fron tires, the car would oversteer. I agree with you and all my experience in SCCA, Koni etc says the same but guys like Puhn dont't get to publish without some expertise!

 

The Fred Puhn book (which I read decades ago) is directed at your typical front heavy cars that have suspension geometry setup to understeer moderatly-to-heavily at the limit. Note, this kind of understeer is inherent in the (front) suspension geometry as explained in "Race car vehicular dynamics" Milliken and Milliken. Only excessively hard springs avoid entering the area of terminal understeer.

Here, we are dealing with a car setup (if correctly) to neither understeer nor oversteer and with the tread widths almost equal to the weight on the tires (same weight per sq-in of tire contact). The understeer is present in pure latteral acceleration, but add power than the US bocomes neutral and fades into oversteer. Back down and the car tends to also oversteer. This leads to the observatioin that an F355 at the chassis limit will oversteer if you add OR subtract power to the optimal speed in a corner. The Carrosel at TWS (T10) is a good example of this. You can even use this to determine the optimal sped in that corner.

So, what Fed says is absolutely the correct thing for a (say) Mustang. The 50 PSIs was chosen so that the contact print of the rear tire would loose its inside and outside edges. Thus with less rubber on the road, it would have less grip and thus the understeer of the car would "go away".

Over in the Carrol Smith "Tune to <pick one>" series of books, we see that car development starts with whatever axel has the least traction (also in Van Valkenburg). We work on this end until its traction comes up and surpasses the other, switch axels and continue until the tires are working as well as the suspension can allow.

Then, modern tires have changes the game (somewhat). The wide tires are (now) more sensitive to having too much pressure, whereas the old 205/70-SR16 were not. And in addition, Freds book (if I remember correctly) is targeted towards autocross where the tires have to be at pressure when you launch; this is not what happens in a Ferrari HPDE today, and so the advice with still correct for its target is not correct for cars such as F355s.

Modern tires have a fairly narrow window of acceptable camber, and acceptable air pressure AND a narrow range of optimal temperatures. Air pressure adjusts tire contact patch area and the amount of work the tire can deliver. The work delivered ends up as temperature which we read with a pyrometer (above).

Next time you are at the track, put 50 PSI in your rear tires and then come back in after a few squirly laps and drop the HOT pressures back to 40 PSI and see that Fred is not correct for F355s (and many other modern cars).

 

Anyone have a recommended tire pressure for my 1999 F1 Spider?

I have Goodyear Eagle F1 Assymetrics and I LOVE THEM!

 

If you have been following this thread, you now know the answer depnds on how your car is setup and how you drive it, and what the numbers are on the tire.

For example, My F355 is perfectly happy* with 32 PSI all around (cold), If I raise the ride height of my F355 by 1 turn on the rear spring perch, I need 33.5 PSI at the back to make up for the oversteer imbalance of the chassis setup. For race track use I start with 34 PSI all around (again cold) and aim for 40 PSI at the end of a session.

(*) Bridgestone S03s 225/40 ZR18 front, 275/40-ZR18s rear, ride height set down so that these tires are at the factory recommended ride heights for 265/40-ZR18s. I am still experimenting wih my new RE-050PPs.

By perfectly happy, I mean: These tires last 5 race weekends on the track and another 6K road miles or 12K road miles with no track miles. And independent of whether at road race tracks or not; all 4 tires go slick within 100 miles of each other and go slick across the tread evenly (on a good day within 5 laps of each other). {Car was last ride height adjusted, cornerweighted, and aligned in 2003--4 sets of tires ago; although it gets checked regularly, it needed no adjustments}