Tire pressure basics

[Rick Hannah]

Okay, I guess it is my turn to start posting novice questions. Believe me, I have a thousand of them. We're MRP newbies and my two boys are hooked. I'm feeling the "crew chief dad" pressure and I want to do the best job I can to help my sons Steve (age 13) and Stuart (age 10) succeed, don't we all?

My older son Steve, who started his first JR Sportsman race this past Sunday, is having some real issues getting his kart to turn on cold tires. This appeared to be exagerated by cool conditions and the long straight going into the (first lap only) turn 2. In both the heat and the final races he entered turn 2 on cold tires following closely behind the first three karts and they all just turned in and left him in the dust, literally. His kart just pushed right through the corner and into the dirt. I felt bad as his crew chief because I may have had the tire pressures screwed up. I have been told to go for 14 psi hot and whatever it takes to get there just do it. His cold psi was about 12 to 12.5 to achieve this. Okay, my basic question is.........do you run lower pressures on cold tracks or vice versa? I knew I probably screwed up when I saw a couple of other teams checking pressures just before our final and their target appeared to be around 8 or 9 psi??

I know there are a ton of things to consider, and I know driving style and experience is 95% of the battle. I need to start somewhere!

Rick

[Garry Lobaugh]

I wouldn't beat myself up too bad Rick, the driver still has to drive the kart.
Since I was not at the event I cannot say what happened, but what might have happened is the driver forgot to relax and let off the gas just before the corner and then got on the gas after the corner. This would create a loose condition and a spin. As you know we want to do the braking before the corner and turning in under power.

To answer your question: cold weather go up in temp. Hot weather go down. For a jr or Jr. Sportsman I would have been running the 12 or maybe a bit higher since things were cool.

I'd suggest at the conclusion of your next practice day to grab a green flag and have both boys do a practice start. That will help settle the jitters and keep them on track. Glad to see you asking the questions.

[Rick Hannah]

To answer your question: cold weather go up in temp. Hot weather go down.

You mean pressure or do I need to take tire temperatures?

Thanks

[Garry Lobaugh]

Pressures.

[Marc Miller]

My older son Steve, who started his first JR Sportsman race this past Sunday, is having some real issues getting his kart to turn on cold tires. This appeared to be exagerated by cool conditions and the long straight going into the (first lap only) turn 2. In both the heat and the final races he entered turn 2 on cold tires following closely behind the first three karts and they all just turned in and left him in the dust, literally. His kart just pushed right through the corner and into the dirt. I felt bad as his crew chief because I may have had the tire pressures screwed up. I have been told to go for 14 psi hot and whatever it takes to get there just do it. His cold psi was about 12 to 12.5 to achieve this. Okay, my basic question is.........do you run lower pressures on cold tracks or vice versa? I knew I probably screwed up when I saw a couple of other teams checking pressures just before our final and their target appeared to be around 8 or 9 psi??

I know there are a ton of things to consider, and I know driving style and experience is 95% of the battle. I need to start somewhere!

Rick

Hi Rick-

I can shed some light on this most likely. Garry is right, in colder weather and morning sessions that are short, we shoot for 13-14 lbs of air pressure hot. To achieve this, you might have to start a little higher on pressures to get them up to temp faster.

There are a couple of reasons that Steve could have had problems in the first turn. 1. If the tires have several heat cycles on them - while they may show decent grip at optimum tire temps - it might take a little time to get them there resulting in lowered performance. A solution would be to have newer scuffs or a newer set handy so when you are losing "out-lap" performance you aren't a chassis problem or driver problem that in reality, is caused by a tire issue. 2. If the tires are in good shape and only have a few heat cycles on them - then Steve would benefit from learning how to get his tires warm on the warm-up lap. If you have ever watched an F1 race, you have seen drivers come out of the pit, go deep into Turn 1 and turn the wheel hard while the front slides wide.... this is an attempt to built tire temp. On every outlap, I usually give myself room and try to get up to speed to raise tire temps and pressures prior to the green. I also avoid going "off-line" as much as possible to keep the tires clean of debris and rocks which could also build up on the tire (including old rubber) giving you a lack of grip.

As far as pressures go.... once you get a new set to work with and have them scuffed in after a good practice session, check the pressures (IMMEDIATELY AFTER he comes off track.... as soon as he comes into pit exit and stops). Are they all 4 where they need to be hot? If one is a little low (let's say LR for MRP track), add a little to that tire.... if one is a little high, remove some air from that tire when they are hot.... then let them cool. Check the pressures prior to your next session and that is your target pressure/baseline pressure for the session. Don't bleed them down! When the tire cools it will go back to your baseline pressures you started with unless you have a tire leak. If the tire is still warm 30 minutes after a session. it has not cooled down enough to show those numbers, so if you bleed the pressures down - you effectively are lowering the pressures MORE than what you started with in the first place and that will show lower pressures after the next session leaving you scratching your head :P

Hope that helps.

PS - It is pretty common for me to have a different starting pressure in each tire depending on the track and how hard each tire is worked.

[Rick Hannah]

Marc,

Thank you for taking the time to post this reply!

Steve is still on his original set of tires so I'd estimate about 4 hours of practice time prior to Sunday. I was noticing a tremendous amount of rubber build up on the tires after qualifying but didn't have a clue about what I could do about it, short of buying another set. I hoped it would just scrub off on the pace lap. I was surprised to see that only the Rotax classes get a full pace lap and all the others only walk around the inner oval prior to taking the green flag. Not much chance here to build heat and scrub off any debris. Good thing though, everyone is in the same boat so we just have to learn the magic. I always told both my boys to go easy on the first practice out laps to make sure their tires were ready but this has proven to not be good preparation for the actual race. Next practice, as Garry recommended, I am going to have both my boys run a pace lap around the inner oval and then wave the green and see what happens! Should be a good learning experience.

I am feeling better about the tire pressure tricks. I think I am in the ball park to provide decent handling karts for them to continue to learn in. I'm staring at this nitrogen tank at my workplace and wondering if I need to go there ;-)

I also had a late night chat with Steve over braking points on track C. Speed entering turn 4 seems much higher on the first lap than it does on the 2nd lap due to the deletion of turns 2 and 3. We think he lifted at the same point on lap 1 as he had used for all the other laps so he probably carried too much speed going in just as Garry mentioned earlier. Is turn 4 off camber?

I never realized that this sport was going to be so much fun, and so much work. We are glad we are finally here! Now if I would have just started this 3 years ago..........