Last weekend I was fast in Heats 1 and 2, in the pre final I was again one of the fastest in terms of lap times, but it was done in the middle of the race and slowly tapered off.
For the final it was decided (between myself and my faithful pit crew) to go up half a PSI all round as the sun was going down and it was getting colder, but this time my fastest lap was very early on and I got slower as the race went on, while a lot of the other competitors actually got faster and peaked with a few laps to go.
I can’t help but think the increase in pressure was a mistake, and if anything, based on the pre final should have actually gone down half a PSI for the final despite the cooler temp of the day. I guess my question is; is a track with a full days worth of laid rubber going to be affected that much by a couple of degrees drop in ambient temperature?
A few degrees of ambient temp shouldn’t make much difference to your tire pressure. Sometimes rubber can become greasy in the heat of the day and feel like you’ve lost grip, and then when it cools down a bit in the afternoon the track might actually pick up grip.
I have noticed at our track with the KA running MG Red SH2, most everyone’s (field of 12-14) lap times slow as the day goes on. Fastest laps are always in qualifying and slowest laps are in the final. Typically quali is late AM and Final is mid afternoon.
Personally, I am lowering pressures but wondering if I should be going the other way or some other change that we all seem to be missing.
That’s interesting. Heat 1 was on new tires, cooler temperature and on a track with the least amount of rubber it saw all day. This is when the competitive drivers did their fastest lap times for the meeting (I did the fastest time of my class).
Heats 2 and 3 were between 11am-2pm when temps peaked. Most drivers lap times fell off (myself included) by 2-3 tenths in this period, this was consistent across all classes.
Come final in the late afternoon, temperatures were cool again and the track at peak rubber levels, the best of them got within 1 tenth or so off their morning times whereas I actually got an additional tenth or so slower, with my times peaking early race.
I know it’s hard to know for sure without being there or knowing my set up but under these circumstances and in hindsight do you think I should have at least stayed the same tire pressure or maybe even dropped half a pound, instead of the half pound increase? Thanks for the help btw, always find your posts and videos insightful.
LeConts LH03’s (yellows), a level below the Whites in term of stickiness but still a very sticky tire. We’re in Winter at the moment but in Summer where temperatures get around the high 30 celsius, lap times for everyone fall off by literally 1.5 seconds consistently (50 second track). The extreme heat is an absolute killer for pace and lap times, but it’s all relative.
Our Final races, as the sun goes down over the horizon or when it’s night time racing, guys seem to get closer to their quali/morning times again which I can only guess is because of the drop off in ambient temp (as long as you aren’t going UP in psi, which is what I did last meeting).
Track temps are far more important than air temp. Direct overhead sunlight contributes to track temp quite a bit. So cloud cover on a 95F day may actually have a cooler track surface temp than a full sun 85F day. Often times as evening cools the track is slower to cool than the ambient air and can hold some temp into the evening. So you can’t really on what you feel in the air.
Also rubber layed down. As @tjkoyen mentioned the track typically picks up rubber throughout the day. Sometimes during peak temps that rubber layer can actually make the track greasy and loose grip instead of gain grip, then as the track cools a touch in the late afternoon and evening grip can come up more.
The last one that can be a factor as evening comes on is humidity. On extremely humid days the moisture in the air can start to make the track loose grip as the sun sets. I find this to be somewhat common in the spring and fall here in TX. Usually too hot for it to be of concern in the summer.
How much the lap times fell off helps give you a read. You also need to consider length of race when making pressure adjustments. I would not have added pressure, especially for a longer race, unless temps dropped significantly and/or dropped with an increase in clouds. I likely would have sent it as-is if the final and heats were similar lengths. If the final is longer, I would likely drop pressures by half pound.
Evening transition is one I have gotten wrong more than a few times. I was too quick to assume we needed more grip when in fact the track gained some performance and my son went out overgripped and peaked too early.
All that makes sense and is consistent with how my day evolved. After the pre final I noted how my times fell off in the last 3-4 laps of the 10 lap race, but after taking a break for an hour or so, seemingly forgot all that and before I went out for the final, thought “Hey it’s getting a bit cooler and the sun’s going down, better up those tire pressures by half a pound!”. I peaked on lap 4 while most other competitors were back to lapping close to morning qualifying times.
My experience with night races is the track always gets more free when it cools down, but that may not always be the case as I know car tracks seem to do the opposite. As far as raising pressure I think it was the right idea when the temps drop, but the fact that your race is longer I think more than offsets the temp drop.
I agree night racing (after the sun goes down) most tracks, most of the time, will free up some. Its that transition time that has always seemed hard to nail down for us. Figuring out just how much the track surface temp drops as the sun is getting lower is hard to do from the pits.
