Tire Tech: Tuning for your Tires

September 14, 2009 | Performance Tests | Track Tests | Andrew Wojteczko | Comments (4)

Getting the most from your tires is what chassis and suspension tuning is all about. To maximize grip we need to achieve ideal temperatures across each tire, ideal pressures when up to temperature, minimize load transfer, and minimize contact patch load variation. In part 1 of Tire Tech we will cover the basics of pressures and temperatures.

Temperatures

All tires have a temperature range within which they will generate maximum grip. Below this point the tire will begin to slide and break away prematurely. Above this point and the tire will once again begin to slide prematurely, but can also begin to blister and suffer permanent damage. So how do you know what is too cold or too hot? Most tire manufacturers will provide a range of temperatures to target, which is a great starting point. For example, BFGoodrich recommends an operating range of 160-220F for their g-Force R1 competition tire (more on these later).

Measuring tire temperatures requires the use of a pyrometer. There are two different styles available, the probe type and the infrared type. Infrared pyrometers measure the surface temperature of the tire, whereas probe type pyrometers measure the core temperature. Infrared pyrometers are great for realtime tire temperature monitoring where it is impossible to use a probe type. Because surface temperatures change very quickly, unless you are monitoring and recording the values in realtime, by the time the vehicle rolls down pit lane the surface temperatures will not be of any use. This is where the probe type pyrometer is the tool of choice, as the tire temperature is far more stable below the surface.

There are many different probe type tire pyrometers available from a range of manufacturers. You can expect to pay between $100 and $400 depending on which model you choose. Some of the more expensive versions feature memory functions that allow you to view your recordings after taking the measurements. You can save some money by using a pen and paper to record the temperature readings as you go, or better yet have a helper record the temperature readings while taking the measurements. We have had good success with Intercomp digitial tire pyrometers as well as the OTC 700 Multimeter with an Intercomp type k thermo- couple tire pyrometer probe attached, as pictured here.

If you have an older pyrometer and are unsure of the calibration, or want to make sure your new unit is within spec, simply boil water, insert the probe and ensure the meter reads 212F. You can also insert the probe into a bag of icy water, where it should see 32F. If your meter is not reading correctly, contact the manufacturer to have the unit calibrated.

To carry out a tire temperature measurement, have the driver complete enough laps to get the tire fully up to temperature. This duration is dependent on the tire you are using and the weather conditions. If using a very soft Time Attack specific compound on a hot sunny day, you can expect to reach operating temperature on your second lap. If using an endurance racing compound on a damp cool day, it may take 10-15 laps to reach operating temperature. Carrying out multiple tests in different conditions and getting to know your tire of choice will help you judge how much track time is necessary before the tire is ready to measure.

Have the driver maintain speed on the in-lap so that the tires are not allowed to cool before entering pit lane. Be sure to obey pit speed limits and have the driver bring the car to a stop where you are located with the pyrometer in-hand and a helper with a clipboard ready to record your readings.

The pyrometer has what is known as thermal inertia, or a resistance to change in temperature. If you simply insert a probe at room temperature into the tire it will take an extended period to reach the actual tire temperature, allowing the tires to cool in the meantime reducing the validity of your measurements. While the driver is on track you can help limit this by holding the probe of pyrometer between your fingertips so the probe is starting at your body temperature.

Once the car is stopped in pit lane, have the driver steer the front wheels hard to one side to improve your access to the tire surface, then begin your measurements. Insert the probe of the pyrometer at a slight angle, approximately 3mm deep into the tire surface. Do not jam the probe too deep into the tire, which can result in damage to the probe and the tire. Because the probe is still below the temperature of the tire, it will reduce the tire temperature as the probe/tire reach equilibrium. To prevent this from skewing your readings, on the first tire you measure, insert the probe in the following sequence:

1. Inside, stabilize, remove
2. Inside in a new location, stabilize, remove
3. Inside in a new location, record
4. Middle, record
5. Outside, record

Then, proceed to the next tire and repeat steps 3-5.

Pressures

Optimizing tire pressures is another important parameter in achieving maximum grip from your tires. As with temperatures, tires have a range of pressures that will produce maximum grip. To determine your ideal pressure, once again turn to the tire manufacturers recommendations for a starting point. BFGoodrich recommends between 34-43PSI hot pressure for their g-Force R1, depending on your vehicle’s weight. To download BFG’s ‘Care & Feeding Guide’ for the R1, go to: http://www.tirerack.com/images/tires/tiretech/bfg_Gforce/bfGoodrich_gForce_R1_care.pdf

Changes in pressure have an effect on the vertical stiffness or spring rate of the tire. This is why heavier vehicles require higher pressures to maintain a constant level of tire deflection/compliance. The BFG R1, for example, is a fairly stiff tire with vertical stiffness ranging from 2300 lb/inch at 36psi to 2700 lb/inch at 43 psi. One thing to consider with front heavy FWD vehicles or rear heavy rear/mid engine cars is that symmetrical hot pressures will not produce maximum grip at both ends. To get the most grip, the heavier end of the vehicle will require higher hot pressures than the lighter end.

To measure pressures, you will need an accurate tire pressure gauge. There are many gauges available over a wide price range. A good quality tire pressure gauge is something you will use on a regular basis and can last for a very long time if cared for, so it is worth spending a little extra. By choosing a racing specific gauge such as this Rebco model ($29.95 MSRP) you can easily read the high resolution dial, and can quickly bleed off excess pressure with the integrated relief valve.

Tire pressures are commonly referred to as either hot or cold pressures. Your cold pressures are your starting point and are set when the tires are “cold”. Hot pressures are the pressures you record once the tires are up to operating temperature after a number of laps around the track. Note that while recording your tire temperatures is also a perfect opportunity to record your hot pressures. Hot pressures will be higher than your cold pressures because the air used to inflate the tire expands as it is heated. The difference between hot and cold pressures can be minimized by inflating the tires with nitrogen rather than air, which expands less as it is heated. You can expect to see approximately 8-10PSI growth with air pressure and about 5-8 PSI with Nitrogen.

Interpreting the data

Now that you have collected temperature and pressure readings, you need to know target values and what changes to make to reach these targets. Based on testing, the majority of performance/racing tires will build maximum grip with approximately 20F higher temperatures on the inside shoulder versus the outside shoulder. Based on BFG’s recommendations for the R1, a target temperature readout will look like this:

But where do the pressures come into play? Tire pressures have an effect on the shape of the tire. As pressure increases, the center of the tire will increase in diameter, resulting in the center of the tire building more heat. Consequently, as pressure is reduced the center of the tire will relax, causing outer temperatures to rise and middle to fall.

Recommended setup changes

When making changes it is best to make one change and then retest so you can properly evaluate the impact your change has had. For pressure changes, try working in 2psi increments to start, then less and less as you get closer to your target temperatures. Also, with some basic data acquisition with lap time and accelerometer functions, you should be able to see an increase in cornering/braking/acceleration forces and reduced lap times as you get closer and closer to your target temperature pattern. Be sure to do as much setup development within as close a succession as possible to ensure that ambient changes over the course of the day don’t skew the results of your changes.

BFGoodrich G-Force R1 Tire impressions

The opportunity to compete at a world renowned circuit such as VIR with its high speed, flowing layout, and challenging technical sections made the 16-hour road trip to Virginia one we couldn’t pass up. This also served as the perfect venue to test out the BFGoodrich g-Force R1 tire on our lightly modified Honda S2000. Having learned the basics of the track using IRacing’s simulation of VIR (which is very accurate), I was able to get up to speed quickly and begin setting the car up to suit the BFG tires.

Our S2000 currently weighs in at 2900 lbs with driver and features neutral front to rear weight distribution. Based on BFG’s recommendations we targeted a hot pressure of 37 PSI all around. Because VIR is predominately right hand corners, we expected to work the front left tire a little harder and started with the following cold pressures:

Cold Tire Pressures

Hot Tire Pressures and Temperatures After 1st 3-Lap Session

Pressures:

Temperatures:

Based on the pressure data, we increased front right tire pressure 2.5 PSI, and rear tire pressure by 1 PSI on both sides. It became clear that achieving optimum tire temperatures (targeting 200°F on the inside) would be very difficult with the 3 lap format Redline was using for the Time Attack.

To help generate heat in the tire more quickly, the following techniques were employed:

1. Increased canister pressure in the Moton dampers to 300 PSI to increase spring rate. The increased spring rate causes the tire to slip more over surface irregularities, helping to build heat.
2. Raised our starting tire pressures to more quickly reach target hot pressures, improving stability and allowing the tire to work well even when not fully up to temperature. Note that Nitrogen would have allowed the higher starting pressures while still maintaining target hot pressures even as temperatures reached their target range.
3. Alignment. Based on the temperatures, I would have increased negative camber from -2.1 to -3.5 on the front left and from -2.1 to -3 on the front right. I would have also increased rear camber from -2.5 to -3 on the left rear and from -2.5 to -2.8 on the right rear. Unfortunately camber was already maxed out within the OEM adjustment limits so this was not an option. The car was already experiencing a slight oversteer balance in certain corners and a slight understeer balance in others depending on inclination and surface, so I opted to not add any additional toe at either end which would have made the car more nervous and less predictable.
4. Use warm-up lap to drive very aggressively with high slip angles, and aggressive throttle/brake applications to achieve high longitudinal slip percentages. This would help build more heat in the tire.
5. Finally, leave the tire in the sun prior to the session to allow it to warm up before it even gets on track. Better yet, with the use of tire warmers, the tire can be at maximum grip entering the 1st corner.

Based on our VBox DriftBox GPS-based data acquisition system, we were able to generate a maximum of 1.7g lateral acceleration and sustain 1.5g. This is very impressive for a lightly modified S2000 and is a testament to the grip that the R1 tire provides. More impressively, these results were over a 3-lap Time Attack session where the tires weren’t even up to their target operating temperature. Where the R1 tire truly excels is during extended sessions where it will reward the driver with lap after lap of consistent grip and predictable handling. By producing a tire that generates consistent grip, it allows a driver to build confidence in the vehicle and continue to push the limits and improve lap times, unlike a tire that quickly falls off and is unable to repeat braking points and cornering force from lap to lap.

Eager to further test the capabilities of the BFG g-Force R1 we took advantage of an opportunity to test the R1 back to back with another r-compound track tire at Mosport International Raceway. We used our Speed World Challenge TC spec Lexus IS300 as the test vehicle. To ensure a fair comparison a new set of r-compound tires, size 235/40/17 were tested over a 20-minute session. Immediately after this session the g-Force R1’s size 235/40/17 were installed and the fuel load was returned to the starting amount. The R1’s were then tested over a 20-minute session. Ambient temperatures rose slightly between the two tests and average intake air temperatures were recorded to be 1-degree C higher than the first session. Despite this slight temperature increase, the results shown in the MoTeC data were very impressive. The R1 was able to carry approximately 10 km/hr higher cornering speeds and resulted in a lap time of 1:30.3 vs. 1:32.2 with the other r-compound tire. The g-Force R1 was also able to generate an enormous 2.3g lateral acceleration at the bottom of the infamous Mosport Corner #2, a very impressive feat.

Click on the movie to view it now

Tires are an incredibly important part of the overall performance package of any vehicle, and as such they warrant careful testing and evaluation over mutliple days at multiple tracks before you can hope to begin to optimize your car’s setup for maximum tire performance. We certainly intend to continue our testing and tuning of the BFGoodrich g-Force R1’s, given the tremenedous potential they’ve shown so far under limited testing and less than ideal conditions. So be sure to check tune2win.com soon for our next tire tech story and video, where we’ll bring you more insight into these tires and many of the other top performing options for the track and for the street.

September 14, 2009 | Performance Tests | Track Tests | Andrew Wojteczko | Comments (4)