Race-kart rubber + rentals

I am very confused with one thing after 3 sessions I did today with rentals. Look at the lap times:

Look at the fastest laps of 1st and 3rd session. Before both session, sport karts were on track. I couldn´t believe the first timed lap, it felt alright but it really wasn´t good in either session, I switched karts, missed one or two apexes, not the best possible lines because I can´t possibly adapt to a kart from the first lap with it, yet I posted the session best in first lap in 1st and 3rd session, only because of? Rubber laid down by sport karts? Is this possible? Second session was just after the first one, with no break in between, so lap times have progressed normally (except L6, traffic) as I can see, so I only see rubber as a factor.

Has this ever happened to you guys? I was fairly confused, although I still am to some degree. Why, how does the rubber wanish all of a sudden after a lap? Don´t get me wrong, 3rd session was horrible considering traffic, there was a guy doing 1:20 - 1:30 laptimes vs mine 46s, so maybe not all the potential was extracted from a 12 lap run but still there is a constant 0.2s or 0.3s gap from 1st lap and all others.

I undertsand that rubber increases traction dramatically, but how does the laid rubber “last” one lap and then drop off? I have never heard of this.

Any thoughts?

Maybe it has more to do with track temperature and less to do with actual rubber? Rentals will scrub rubber off the track, but not sure how quickly that process would happen in this case.

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It can increase grip, but the rubber laid down needs to work well with the tires on the kart. Sometimes the rubber compounds are not “compatible” and you don’t get the grip increase you might expect.

Let’s talk temps.

@tjkoyen Is it possible that the tires were slightly overinflated so that “cold” lap was the correct pressure? As it comes in his time dropped off?

It’s possible, but the times do come back at the end.

I’m with TJ on this, probably track temperature still being hot on the first lap and then it cools down. Race karts put a lot of friction force onto the pavement, which heats up the surface a lot more than what you’ll be able to do in a rental. That hea lasts for a very short time, which is why the first lap is so quick.

Rubber laid down on a track really depends on which manufacturer is being used on each kart, or car. Different brands will work more or less effectively with each other, and it’s entirely dependent on which manufacturers are being used. I don’t know why it works that way, just that it does.

It most likely does (track t°) in overall grip levels but I am confused as second session looked normally on the graph, the lap time progress, only sessions after race karts look weird at the beginning.

This and the rubber manufacturer seem like the only logical explanation to me. This is very interesting. Do you think that extra heat in corners can last like 5mins, because that is at least how long it took for racers to pit and them to release us onto the track? It sounds like potential solution/answer, but as you said, it doesn´t last long. I guess because it was (guess) 30-40°C track t°, maybe the heat was kept in the asphalt longer, thus still being there exactly in opening lap.

I would figure track temperature has a semi-lasting effect for 5 to 10 minutes, depending on the ambient temp, sunshine, and wind. The class of kart will change how much heat gets put in, too. A TaG or Shifter will add a lot more heat, which will then last longer, compared to say a cadet or slower senior kart.

Hey also ambient stuff like sun going down and track staying warm while air cools can have an effect.

Also their timing system might have some weird thing going on. The one at NJMP ignores everyone that comes after the leader finishes, which is problematic for places 2-infinity.

Alright, thank you for your replies, as far as I understood, its a mix between rubber type, tyre temp and the type of race kart that was driven before the session.

That or their timing system is bananas. It’s very odd being a second faster on the outlap.

The last time I drove rental karts at Bushnell, my faster laps happened definitely after a few laps, I tried driving as fast as could in the first lap or two, and every time I did that I spun out, and I have a feeling it was because the tires were cold because after the first spin I wouldn’t spin again and was braking at around the same point. I’m not a great driver (yet :wink:) but I noticed that I had to take at least the first lap to take it easy and wait for the tires to heat up a bit before going hard.

It could be because of the tire pressures in the case you mention, but could it also be because of the surface you’re driving on? I’m guessing also that the tires being really old could play a factor since they’re already wore out, unless they change the tires periodically. As for tire pressure, I expect them to inflate them more because they’re not so much interested in getting the rental karts to go as fast as possible, but have the tires last as long as possible.

I’m not 100% sure how rental karts handle compared to race karts, don’t the rental karts get more grip after a few laps? Does hotter track temps increase or decrease grip and does rubber on the track increase grip as well?

Rental compounds do indeed take a bit to get to temp like regular kart (racing) tires. My experience is with lecont whites for comparison.

It takes a rental kart 1.5 - 2 laps of the rental track to be ready to go. I think the LeConts a would take bit longer, another lap or two to get ideal.

The difference seems to be that once rental tires are at temp, the grip is consistent from beginning to end, pretty much. The race tires take longer to get to temp and fall off rapidly, in comparison.

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Are the lap times consistent in the rental kart because of the tire pressures and worn out tires? I’m assuming that the race tire falls off faster because of the lower tire pressures and you’re pushing the tires as much as you can

I think it’s the rubber actually. The hard compound seems indifferent to wear until cords show up, basically. Tire pressure is a variable we have no control over… that being said, the karts I have been in have varied pressures due to, well, rentals, and the grip seems about the same. I have had a few karts where one tire is much more worn or perhaps improperly inflated and you do notice it.

With the leconts quali was faster than pre which was faster than race, generally. Rental tires don’t do that.

And yes rental tires can be very consistent. I have races where I have multiple laps within hundredths.

So basically the rubber causes the quicker tire wear because it heats up the track more right? How come the rental tires stay so consistent, what causes the slower tire wear?

I do t really know but I imagine it has to do with the hardness of the rubber. I am guessing the wear life of the rentals is 1000+ laps as compared to 100 laps maybe for a race tire.

I am also guessing that the harder the rubber is, the less of an effect heat and heat cycling has on it.

Well I think its simply because the tyres are much harder, produce less heat. The degradation process is much slower/longer thus you don’t see the big drops in laptimes over a day. Also, I think it has more to do with rentals, than their tyres.

Rentals are generally worn out karts, they can literally change behaviour mid lap, so it always has a different behaviour in certain situations. Race karts are tuned to perfection and kart is consistent all the time. So when you are not feeling well in kart, your laptimes will reflect your imputs more precisely, and if the imputs are off or are inconsistent, your laptimes are all over the place.

Not to mention the obvious, race karts are harder to keep at a constant but fast level, because they have much more HP that you need to control. On top of that, race tyres are much softer and degrade quicker, so the drop is expected and evident, whereas you can pull time in rentals if you know how to reduce its weakness as much as possible, comparing to others who maybe dont recognize the weakness or simply dont have skill, knowledge or even idea how to adapt to it.