949 Racing - Supermiata
Using a Tire Pyrometer
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I get asked all the time "what pressures should I run?". My answer is usually "I don't know, what were your pyrometer
readings that day? You spent $1600 on tires last year but
don't own a $100 pyrometer
? Oh the shame.
One should determine optimum pressures primarily with a lap timer, driver feedback and pyrometer
. Data acquisition helps but isn't always necessary. The pressure gauge is just a meter and doesn't tell you much about how the tire is working. IOW, using timer, pyro and good driver, you could optimize pressures without a tire gauge, you just wouldn't know what they are.
There is no such thing as "ideal" or "best" pressure. The finer you are trying to hone your setup for a particular condition, the more minute the variations. I recall a story by legendary Indy driver Rick Mears (a god). He was telling a journalist about having a car that was a bit loose in qualifying on a super high speed oval. So he deliberately drives a line that will induce understeer for one lap, thus increasing the heat and making the right front tire grown in diameter by a few thousandths of an inch. Due to the ultra sensitive set up of an Indy car, that tiny stagger difference balanced the car perfectly for the next lap for a great qualifying lap. The tire cooled down after than lap but that pro driver got the job done by actively playing with tire pressures just with driving technique. Every single thing that happens at the track affects tread temps and what optimum might be for that lap. Suns comes out, wind picks up, tires going off, oil in dampers losing viscosity from heat and on and on.
For HPDE's, autocross and most club racing, getting to within .5 psi of optimum in each tire is close enough. It is common to end up with a different hot and cold pressure on each tire. Clockwise course with a few long fast right hand turns will obviously put more heat into the left side tires. So starting left side cold pressures might be below the right side cold pressure but the higher when the tires are hot. Running 2-3 qualifying or time attack laps? Your cold pressures will be different than if running a 35 minute sprint race or 3hr enduro even with the exact same setup. One driver likes the car loose and throws the car into the turn with lots of steering corrections. Another driver in the same car likes the car neutral, enters turns earlier and has fewer and smaller steering inputs. Both drivers run the same lap times. Same pressures? Of course not.
What I am trying to indicate here is that no one can tell you what optimum pressure is so don't ask. Get a pyrometer, learn how to use it and become your own expert.
measures tread temperatures. Any tire will develop it's optimum grip when the rubber compound is heated above ambient temp. Street tires like to stay fairly cool, 140-170°. UHP, EHP,MHP tires (R1R, Star Spec etc) will work best at slightly higher temps, around 150-190°. Full race tires (Hoosier, V710, NT01 etc) will want in the 170-220° range. Race tire manufacturers will always provide optimum tread temp info for a given compound, just ask them or go to their website in the competition tire section. Tires will always cool a bit after coming off track no matter how quickly you get to them so as long as your procedure remains the same, you can still get excellent data. The relative value is what matters, how the temps changed when you did this or that adjustment.
For the pyrometer
, use a probe type. The laser type only see the surface of the tire which cools too rapidly to get accurate measurements. The laser measures reflectivity so and older heat cycled tire might have a different relative temp reading than a freshly shaved race tire, even though the absolute tread temp is exactly the same.
The goal is to get all four tires with a as close to perfect heating pattern as possible. Occasionally, I briefly manage that for a given track condition. Those settings always coincide with my fastest laps. Expect it to take a few sessions of tuning if you are starting from scratch. Highly recommend keeping a notebook will all your measurements, brief driving impressions, car set up and of course, lap times.
- Without a cool down lap, get the car stopped safely but quickly in the pits
- Start with the tires on the outside of the track, example: left tires on CW track
- Start with the inside and work out; Inside, Center, Outside.
- The shoulder measurement must be in the scuffed an heated part of the tread, usually 1/2~ 1" inboard of the extreme edge of the tread. Not the shiny unused outer corner.
- Put the probe into the exact same area of the tread on each tire and push in the same amount.
- For a given car, always use the same sequence. IOW, for a Miata, I always start with the outside front, outside rear, coldside rear, coldside front. S2000's generally heat the rears more so I start with the rear.
- You should see a 5~10° drop from inside to center, then the same drop from center to outside shoulder. The total gradient that works best for radials on the loaded side of the car (outside of track) is usually 15-20° in even steps.
- The outside tires (left side on clockwise course) are the important ones. It is possible for the more lightly loaded inside course tires to display a greater temp delta of 20-30° on cars with a lot of static camber and stiffer suspensions (more than OEM specs). This is normal and usually doesn't need to be corrected.
Some basic guidelines for what the temperatures mean:
Center too hot - example: I 170° - C 180° - O 170°
Too high air pressure. This can be too high cold or "starting" pressure or excessive heat caused by sliding due to car setup or driving style.
Center too low - example: I 180° - C 160° - O 170°
Too low air pressure. This can be too low cold or "starting" pressure, car setup or driving style.
Inside high or low - example: I 190° - C 175° - O 160°
Too much toe out or negative camber can cause excessively high inside edge temps. The way to differentiate is if both LF and RF have the same delta, it's toe. If the outside course tires have a much different delta than the inside course tires, it's usually camber. Also, excessive toe out will usually show a scalloping or feathering of the tread where camber will not.
Outside high or low - example: I 170° - C 180° - O 200°
This is the pattern most commonly seen on production based cars and is usually caused by insufficient negative camber. One band-aid fix is to help make it through the day is to overinflate the tires to raise center temps. This can help reduce the workload of the overheated outside shoulder
With the pyro, you're seeing vehicle configuration, weather, setup, and driving style.
Example: Too much front sway bar/spring and not enough camber? Front tires significantly hotter than rear, outside shoulders hotter than inside.
Example: Rears like 50° hotter than fronts? Too much drifting
Example: All tires low temps? First session, cold day on old tires
Expect to see one outside tire slightly hotter than the rest if the track has more than one long duration turn in the main direction of travel. WSIR has two very high speed right hand turns on a CW track. The fastest way through in just about every rwd car is to have the tail out a bit so the left rear tire takes a beating and always runs 5-10° hotter than the rest. Buttonwillow CW13 OTOH, shows pretty even temps on a well set up rwd car because it has plenty of hard left handers too.
Buying a pyrometer
There are two basic types; Probe and laser. The laser merely reads the radiant heat off the tire surface. Unless the specific laser is calibrated for that particulat tire texture and reflectivity, the reading will not be true. The other problem with laser pyros is that they only read the surface. When you roll off track, the surface cools down quickly while the rubber and casing underneath retain their heat a bit longer. Measure the surface at trackside and it's to late. Lasers only work in real time when aimed at tire while on track and recording to an onboard data logger. For this reason, you should choose a probe type pyrometer. They're available everywhere. A quick google search will find plenty of deals.
A few shops I like to frequent for this kind of stuff