Every number below is off our kart (the kart's setup record, the 2026-08-15 and 2026-09-19 session entries, lesson 1 and lesson 3 answers) except the ones marked (typical), which are published figures for kart slicks, not measurements of our Evinco Blues. The pyrometer replaces them.
The one idea
A tire's grip goes up with the load pressing on it, but less than in proportion: double the load and you get less than double the grip. That single fact, called load sensitivity, is why weight transfer always costs total grip, why the outside tire does the work, and why "more pressure" and "more grip" are not the same sentence. Everything about tires is that curve plus heat.
Three facts worth knowing
- Your pressure gauge is a thermometer. Air in a closed tire obeys pressure / temperature = constant (absolute units both). On 9/19 you set 12 psi cold and read 14 psi hot. That 2 psi is an 8.1% rise in absolute pressure (26.7 over 24.7), which means an 8.1% rise in absolute temperature: about 24 C of tire-air heating, measured with a $60 gauge.
- The chalk line trick is a temperature test. People rub chalk on a new tire's shoulder and read how far it wore off. That is the same measurement a pyrometer makes across the tread, done with dust: how much of the tire is working.
- Tires are consumed, not worn. A kart slick has a fixed number of heat cycles in it (typical: race sets go off after 3-6 cycles) before the compound hardens and stops gripping, long before the rubber is gone. A tire that looks new can be dead. This is why the class rule of one set per race weekend matters and why the "fresh tires" gain on 9/19 (0.5-0.8 s) was real.
Worksheet
Units on every line.
1. Load sensitivity. Take grip = k x load^0.9 (typical exponent for a kart slick). Two rear tires at 90 lb each versus, mid-corner, one at 160 lb and one at 20 lb: same total load, 180 lb. Set k = 1 (so the answers are in "grip units", only good for comparing with each other, as in lesson 3 Q6) and compute total grip both ways. Which is bigger, and by what percent? Then compute grip per pound of load at 20, 90 and 160 lb. Which load gives the most grip per pound? If lightly loaded tires are the most efficient, why does a kart lift the inside rear anyway (lesson 3's one idea)?
2. The thermometer, exactly. Convert 12 psi cold and 14 psi hot to absolute (lesson 1 Q5: add 14.7). Take the air in the cold tire at 27 C = 300 K. What is the air temperature in the hot tire, in K and in C? How many degrees C did it rise?
3. The growth target. The chassis baseline sheet says to look for 2.5-3 psi of growth from a 10-12 psi cold start. Starting at 12 psi cold and 27 C, what air temperature does 2.5 psi of growth imply? What does 3 psi imply? So the target is a temperature window, not a pressure window: write that window in C.
4. Bakersfield asymmetry. On 8/15 you set 10 psi cold all round and read 12 left / 13 right hot. Convert both sides to air temperature (cold air 27 C). Which side was hotter, and by how many degrees C? A kart loads its outside tires in a corner: so which way did that track mostly turn, left or right?
5. The patch shrinks as it heats. Lesson 1 Q8: 90 lb on 12 psi is a 7.5 in^2 patch. At 14 psi hot, what is the patch in in^2? What percent smaller is it? So a tire that comes in hot has less rubber on the ground than the one you set up in the pits. Is a smaller hot patch good or bad for grip, and what measurement at the track would tell you?
6. Reading the pyrometer. A probe pyrometer reads the tread in three places, inside / middle / outside, immediately after a session. Suppose the right front reads 150 / 140 / 125 F. What is the difference between the inside and outside readings? What is the average of the three? Using the rule of thumb below, is this tire's problem pressure or camber (camber = the tilt of the wheel from vertical, set at the front by the kingpin pills)? (The rule of thumb: all three within 10-15 F is a working tire; a hot middle means over-inflated, two hot edges mean under-inflated, one hot edge means camber.) Then: if a REAR tire read the same 150 / 140 / 125, "adjust the camber" is not an answer, because the rear axle is one solid bar with no camber adjustment. What could make a rear tire run hot on one edge?
7. The chuck. Lesson 1 Q7 said the gauge's rated accuracy is 0.75 psi. Suppose each gauge check leaks 0.2 psi. If you read that lower pressure as a temperature (Q2's method), how many degrees C cooler does a 0.2 psi loss make a 12 psi tire look? After three checks in a row before rolling out, how many degrees cooler does the tire look on paper, when its real temperature hasn't changed at all? What habit fixes it?
8. Half a psi is how many degrees? The right rear went 12 -> 11.5 in qualifying. What is 0.5 psi as a percent of the absolute pressure, and as degrees C? Compare that to the 24 C the tire gains in a session (Q2) and to the gauge's 0.75 psi rated accuracy. Was the 0.5 psi change bigger than what the gauge can resolve, or inside it?
9. Night race. Air was 84 F when you set 12 psi in the afternoon and 70 F when the main rolled out. If the tire had cooled to air temperature before the start, what would the gauge have read at the grid? What does that say about when to set pressures for a night final?
10. The measurement. At the next track day: set 12 psi cold on all four, run the session, and within a minute read (a) hot pressure and (b) pyrometer inside / middle / outside on every tire. Compute the air temperature from (a). Compare to the tread temperature from (b). They will not match. Write down by how much and which one is higher, and bring that number back: it is the correction factor for the whole "gauge is a thermometer" idea on our tires.
Done? Q10 is the one that matters. Everything above it assumes the air in the tire and the rubber on the track are at the same temperature. They are not, and the difference is what the pyrometer is for.
Going deeper why grip is not proportional to load