By Rob Schermerhorn
This is but a very brief introduction and limited advice to a very complex subject.
Disclaimer: The following information is used at your own risk, for off-road use only! Please follow manufacturer specifications for tire pressures and wheel alignment.
On to the recommendations:
Here’s the scoop on optimizing your track set-up on a car where the only adjustment you will or can make is tire pressure:
One option is you can ask me as I’ve tracked just about every modern model Ferrari. For example, on a Ferrari 550, I suggest 34 psi front and 32 psi rear cold tire pressure with street tires (14.7 psi = 1 bar). This is a safe, conservative place to start cold tire pressures (Cold means 100 °F or less). This will yield a nice balance with general understeer, but not too much. It is also on the higher side, which allows you to set them once in the morning and then bleed pressure off if necessary, which is faster than pumping them up at the track.
If you have way too much understeer, the car doesn’t want to point into the corner on turn-in, start dropping front pressure in three psi (0.2 bar) steps per track session.
Is Lower Pressure Better?
Remember, after your first session, you’ll be dealing with hot pressures. Hot may be 38 psi front and 35 psi rear. Generally, reducing the tire pressure on one axle will yield improved grip on that end of the car, but you can go too low, which is not safe. I would not go below 30 psi HOT in any one tire. I would definitely lower any tire coming in above 40 psi hot, due to crowning the contact patch and reducing grip. Try to keep the difference side to side within one psi, which is to say, don’t run 38 Right Front (RF) and 35 LF, unless you’re running Lime Rock and the car feels great (all but one turn is right).
Again, in general, to a point of diminishing returns, lowering tire pressure increases grip on that end of the car. But don’t go below 30 psi hot, you will begin to increase tire wear while not gaining grip and reduce the heat capacity of the tire (important for safety). Conversely, raising tire pressure may improve grip because the tire requires increased “support” from the increased internal air pressure to maintain complete contact with the road. If you autocross a Ford Mustang with OEM 14 inch wheels and tires, you can find running as much as 50 psi in the front tires works best just for this reason.
For example, let’s say hot tire pressures are now 42 psi front and 34 psi rear. The car understeers at corner entry under braking, and understeers at corner exit under power. Try lowering front pressures to 39 psi hot (don’t wait 30 minutes to make this change as the tire will cool a bit, this throws off your data) and go out for the next session.
OR, method two is to start with Ferrari recommended pressures listed in your manual and doorjamb. Now make adjustments in three-psi (0.2 bar) increments until you’re happy. It’s okay to raise pressures to help balance the car, and this technique includes sacrificing some grip to improve balance. This may require a source of compressed air, which is why I go to the track with Nitrogen cylinders. I even have a nice small aluminum one that fits into many cars.
Log the Data
Use a nice air pressure gauge, and start a track day log book, noting cold starting pressures, all changes when and why, then note what the tire cools down to at the end of the day so you know where to start next time (memory never works here). You can add checklists to your track day log, too. Makes a nice owner’s history to go with the car.
I don’t recommend running less than 30 psi cold on the street (unless your owner’s manual states otherwise), you are always best off on the street following factory recommendations.
Next upgrade is dedicated track-day rubber:
The Hoosier DOT legal race tire is great place to start (or Kumho, Yokohama, BFG, etc. DOT-legal race tires) and is about the closest one can come to a proper slick race tire. If you’ve never driven a race tire before, you’re in for a treat. Grip is phenomenal. For track day use, you should be able to run six or more events, dependent upon your alignment, driving style and number of spins. Their performance will drop a bit after three or four “heat cycles”, but for track days, that doesn’t matter to you.
More grip equals increased roll in corners, equals probable increase in static camber settings to get the ultimate from the tire. My recommendation, however, is to just bolt them on with factory alignment settings and have fun.
If I assume you also drive on the street with normal road tires, an aggressive race-type alignment will certainly increase tire wear on your street tires. However, if you drive 80% or more on the track, do one event and take notes of how the tires feel and their tread wear. Use this information to determine if a change in alignment is necessary, or even slightly desirable. Ideally, one wants to take other data like carcass temperatures, too. This is where consulting with an experienced race engineer pays off.
I recommend a cold tire pressure of 36 psi front, and 34 psi rear as a place to start with DOT race tires on modern Ferrari’s. Note the pressure gain after your first session, now you can adjust the feel of the car by adding or reducing air (probably reducing).
For most cars, I suspect you will end up running around 36 to 42 psi hot, and probably have less than 3 psi difference front to rear.
DOT legal race rubber is different, higher grip and different construction requires a more sophisticated tuning method. Though you can still go by your “ass-o-meter” here too, just doesn’t work as well.
Race slicks are a different breed altogether, requiring a more disciplined approach to get the ultimate tune.
Next step in improvements requires more chassis-specific information, but in general, modern Ferrari’s respond very well to increasing the roll-couple distribution forward.
Some background on Ferrari suspensions:
Every modern Ferrari road car I have measured or found the workshop manual specifications for have relatively soft wheel rates. The difference between wheel rate and spring rate is the wheel rate (also in lb/in) is what the driver feels and the tires deal with while driving. Wheel rate takes the geometry of the suspension into the equation (motion ratio) and makes it easy to compare different cars. A better comparison is ride frequency, but I’ll present one thing at a time.
Motion ratio is simple to find. Jack the spindle up one inch and measure how far the lower spring perch has moved relative to the upper spring perch. On a Ferrari 348 rear suspension, the spring perch will move an average of 0.85 inches for every one-inch the wheel moves over the range of full droop to full bump. So, the motion ratio for 348 rear is 0.85 (you may see equations that use the inverse of this number, Carroll Smith’s equations would come up with 1.18 for the ratio).
With this knowledge, one discovers that many Ferrari’s are designed with wheel rates as low as 100 LBf/in, which is the Testarossa rear double shock suspension and 308 series. So, your 200 LBf/in spring in the 308 drops with the motion ratio to 98 LBf/in. The equation is WR = SR(MR)2,WR is wheel rate in LBf/in, SR is the spring constant (also known as k) in LBf/in, and MR is motion ratio where MR is stated as shock position/wheel position.
Why are Ferrari’s so “soft”?
This term is relative, IMO. Ferrari’s are no softer than most other road going cars, even other sporting automobiles. Suspension design is all about compromise with a road car. The environments change, the market is worldwide. Ferrari determined that this is the best solution, and I agree. Most Ferrari’s are comfortable, even on long drives (and I’ve driven them cross-country), and sporting enough to be better in many ways than the competition. Ferrari improves on the average sporting car with a bit more suspension damping. As an owner, overall you are satisfied. But this compromise in design opens the door for improvements if you (the owner/driver) have interests outside Ferrari’s average design parameters, like track events or actual competition on the racetrack.
For reference, the front wheel rate of a F355 Challenge car is 1,078 LBf/in with a 2200 LBf/in spring! This is very uncomfortable on the street, plus this system utilizes a tender spring to take up slack when the suspension goes full droop, and comes crashing down on this tender spring with every slight roadway undulation. But, the on-track race car’s vehicle dynamics are superior to the road car’s.
The F355 Challenge factory settings confirm that the factory knows increasing front roll stiffness increases grip and drivability on the racetrack with race tires.
So now we are into changing springs, and increasing spring rates involves increasing damper (shocks) forces. Here is where consulting with an experienced team or engineer will pay off by shortening or eliminating your development time.
Feel free to email me with details of your goals for your project.
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