“Watercooler” talk at the track this past weekend made me come away with a question as inevitably on the race weekend you’re going to get the “What pressure did you go out at” question.
I know the ultimate answer here is figure out a methodology that works for you and stick with it and take great notes, but curious to hear thoughts here.
One way is to measure pressures in the pits, with the karts under the tent in the shade. As close to ambient temp as possible.
Another is leaving the karts in the sun all day and set them in the pits, kart in the sun where tires would be warmer than ambient.
Some racers wait until the very last minute on the grid to set the final cold pressures in the sun right before the start of the session.
So which one of the above do you consider the “standard”? is there even a better way that I’m missing? If you had 3 people set pressures at 20 cold and they all used different methodologies above the hot pressures are going to be 3 different results.
I see a lot of folks at the club use the last minute option. I usually keep under the tent and set the pressures before I walk to grid. (206) not sure what the best way is either. I’ve kinda done all 3 without noticing much, interested to see what others have done.
My initial set is in the sun, then I start adjusting when I come off the track based on the run so I can see pressure gains/drops from ambient to end of race. I kinda of just know where stuff needs to be at different times of day at the home track now based on weather during the day using this method.
As long as you’re doing it the same everytime, all those methods work.
Andy, I had the same question as there was a lot of chatter in tech after yesterday’s final (I was hearing as much as 4lb delta from where I set my “cold” pressure). Personally, I’ve always set the pressures under the tent as close to ambient as possible after they’ve had a chance to normalize from the previous session.
Seeing a couple of the guys set pressures on the grid with the sun beating on the tires for 10+ minutes made me second guess if there was a better way to do it…
The sun does increase the pressure as it heats up the tyre when exposed to direct sun light and I’ve seen it increase 3-4 psi. I usually let the kart bake in the sun 2-3 session before we head out just to normalize the tyre temp, and then set just before. in my mind this is a simple way to control that element and removes the variable if its sunny or not.
I’ve don’t it various ways over the years, and what matters most is having a consistent and repeatable process. I used to always set mine after having the kart in the sun for a while, but this was when we were club racing, and had our karts usually sitting outside the garage.
After working and racing with some large North American teams, I switched to setting cold pressures under the tent after the tires have come down to ambient temp, as it was best practice in those settings. This also seems a bit more repeatable, so I’ve stuck with it going forward.
Also, don’t get hung up if you’re 1-2 psi different from your competitors…it’s easy to tune yourself out of the window by following what others are doing, when instead managing your own program and trusting your gut usually produces the best results.
This was in my head to say similar also. Trouble I see with setting them cold under sun soaked condition is the suns UV power (which heats the rubber) varies in strength on any given day. So your results will vary as much. Ambient temperature under shade is much less variable so yeah, agree more consistent results are attainable
After over-analyzing this for months as I am wont to do, my current procedure is to set in shade at ambient temp, taking into account how strong I think sun’s influence will be on track temp. Then I put an umbrella over the fuel tank and push kart out into sun to let the tires warm up.
I also think this brings in another question, which is what you do for subsequent sessions when feeling the need to change pressures. For instance, you set your cold pressures at 15 for session 1 of the day. You come off track and hot pressures are 18, and you decide for session 2 you need to reduce the cold pressures to 14 (longer session, kart came in too early, etc.). When preparing for session 2 an hour later you realize that the tires, even while sitting under the tent out of the sun, have not returned to 15lbs “cold” and are still at 16lbs. Do you:
A) Bleed 1lb from each tire because they were originally at 15lbs cold?
OR
B) Bleed 2lbs from each tire to get them to 14lbs at that moment in time?
This is something I’ve always struggled with but I imagine it could be the same answer that as long as you have a consistent process you’ll get consistent results.
Option B. This is a great real world example of how atmospheric conditions can affect tire pressures throughout the day, but ultimately one needs to adjust to the target pressure (in this case session A -2psi), and do so via the same procedure, regardless of relative changes due to atmospheric conditions.
Thanks @Andy_Kutscher@Muskabeatz and I’m glad I added that question because I’ve always done option A.
Yesterday, I wanted to keep the tire pressures the same from the prefinal to final. Before the final they were still “slightly” above the pre-prefinal pressure but I didn’t touch them because I figured the tires hadn’t fully come down from the prefinal…which in hindsight seems dumb because there was roughly a 3-hour window between the two sessions and the atmospheric conditions had clearly changed as you mentioned, Evan. Seems that I need to rethink some of my methodologies for 2024.
Set my cold starting pressures for session one of the day. I keep a log of prior session’s cold and hot pressures and the atmospheric conditions. This way I can quickly look up where I set cold pressures when conditions were similar in the past. It will not be 100% accurate but gets me into the window. I record the cold pressures I set on my session log.
Measure temps as soon as I can coming off the track. This is much easier on practice days without scale lines but even on race days, I do my best to measure them as soon as I can. I record the hot pressures on my session log.
I will then qualitatively think back on the prior session and how grip felt. I will also look at my tires to see if they are getting overheated. I will then decide on how much I want to change the hot pressure of the tires to get them to where I want them to be.
Prior to the next session, I measure the tire pressures again in the pits and check the ambient conditions. Generally, there is enough time between sessions (for me) that the tires have cooled and are lower pressure than where they were when I finished the prior session. However, they are also usually still warmer than when the cold psi was set prior to the first session of the day. That has no impact on my adjustment. I will just add or subtract the adjustment amount to whatever psi the tires are sitting at prior to the next session. However, I will factor in changing conditions prior to making the adjustment if they have changed significantly. For example, let’s say I determined I need +1 psi higher hot temps but session one was early in the morning and was cloudy. Prior to session two, it is now fully sunny and 10F hotter. Well, in that situation, I might choose to add just 0.5 psi instead of the 1 psi to account for the higher track temps in the upcoming session.
Generally, I would come off the track after session two with hot pressures pretty darn close to where I want them. But, if not, I’ll just repeat the process again and adjust for the next session.
I’ve found that it is too hard to measure hot pressures quick enough coming off the track on a race day to use the method of directly bringing the tires up/down to the target hot pressure after each session. If the tire has cooled before you measure its “hot pressure”, then the adjustment made directly to that “hot pressure” may be inaccurate once you get the tire actually hot again in the next session. That method also does not account for the track temperatures rising between each session.
This might not be the most optimal way to do things but it has worked totally fine for me.
I use a similar method as Spencer, but I also think you have to take into account how your transport your kart to the track. I use an open trailer and it takes me almost an hour to get to the track. The tires have been soaking up what ever level of sun they were exposed to. My initial setting is almost always the the same cold pressure.
I leave my tire guage on the kart stand so I can measure the pressures as soon as possible after exiting the track. Our scale lines are short, so little time is lost and the duration is fairly consistent. Based on how the kart felt through various corners and what my hot temps are, I make adjustments.
For example, I take into consideration how long it took for the tires to come in. How long were they in the working window. Did one or more tires fall off sooner or take longer to come in than the others. I compare the results of the track handling data to the tire pressures and make adjustments to hot pressure as soon as possible. If one tire didn’t come in compared to the rest and it has lower pressure, then I may bump the pressure up in that tire. If another tire fell off faster than the rest and is at a higher pressure, I may drop the pressure in that tire. For the most consistency, I have found that all four tires will be within about a 1/4 -1/2 psi of each other when set correctly after coming off track.
Then I let them drop back to ambient temp, usually under a canopy. Before the next session, I check the pressures again. Always lower that hot temp, but not always the same as previous low temp. Then I decide what the next session will ask of them. Is it a short qualifying, a medium length heat or a long main. What changes has the track gone through? Has the sun warmed the track? Has the track taken rubber? Is night falling? Is there condensation?
For a qualifying session on clear, sunny day I would generally bump the pressures slightly by 1 psi all the way around. Keep in mind, it does not matter if they are in the sun or not because you are only adjusting what they will come up to when hot and how fast or slow they come up. Where they dropped to when they cooled off is a factor of environment and the number is just the number. Hot is what you are targeting to. For the heat race I will usually take out that extra psi I added for qually. Sometimes more, sometimes less depending on how they felt in the previous session. Measure again after coming off the track. Usually they are right where I want them. Sometimes I have to equalize them a hair or two. Let them cool and measure again. Now for the main, I want them to start coming in later so as to not fall off too soon. I usually drop them another 1/2 psi or more depending on how warm the track is. Lastly I measure them again after the final. Most times the tires are right where I want them and within a 1/2 - 1/4 psi of each other. Spring and Fall are the hardest to judge adjustments due to the larger temps swings from dawn to dark, but experience helps with that.
I guess the big take away is that it does not matter the conditions the cold tires are in, if you know what pressure you want them to reach on track and adjust them then.
