Tire pressure for optimum cold traction and general handling and dynamics questions


(Ji Simmons) #1

2005 TK Krypton shifter with new MG YZ (greens) tires on OTK magnesium wheels (not sure if that matters). Rear width is 55" (1397mm) and front is 53" (1346mm) both measured at the outside edge of the rim. Oval front torsion bar installed oriented with the oval vertically, and no rear torsion bar. Front and rear ride height are both set as high as they can go, caster is set to max, and camber appears to be neutral from eyeballing it. No idea on toe. I bought this kart from a gentleman with about 18 years of karting experience and decided to not mess with any of the chassis tuning settings as he probably knows better than I. As this is is my first kart, and only the 2nd kart I’ve ever been in aside from indoor rentals in the past 15 years, I’m not even sure of what a good handling kart should feel like. I try to avoid comparing it to what a good handling car is because apples to oranges. Lastly, due to lack of an available track, I’ve nobody locally to turn laps with. I’m quite literally on my own and am trying to avoid the echo chamber of my own hubris.

My goal with this kart is to autocross it. As such, my runs will be under a minute and won’t afford the tires time to get up to the temps you guys see at the track. Warmers are out of the question, so I need to maximize cold grip.

My initial session on the greens was on a partly cloudy day with air temps in the mid 70s. The pavement was cool to luke warm to the touch. I set the pressures to 14 psi f\r, however I was experiencing a bit of understeer at turn in. Or at least more than I thought there should or would be. The kart balance was fairly neutral off throttle and brake, but as I rolled on the throttle approaching mid turn, it didn’t take much encouragement to transition to oversteer. I was driving aggressively, or as aggressively as I dared given the proximity to curbs and buildings, and the tire temps never got hotter than warm to the touch, even after about five minutes of lapping a lopsided figure 8 course. The fronts stayed colder than the rears.

My questions are,

Would I lower or raise cold pressure to maximize grip. Everything I’ve learned from bikes and cars says lower pressure generates more heat, but I’ve read somewhere that the opposite is true for karts. That makes no sense to me, but I’m suspending conventional wisdom and keeping an open mind.

Are my track widths within reason for the intended use of the kart? I intend to make single large adjustments of track width and camber\caster and take notes, but a baseline would be nice to have for next time.

Am I driving it wrong? 22 years of driving fast is telling me to do all my braking before turn in, then start rolling on the throttle as I approach the apex and using throttle modulation for the 2nd half of the turn to balance under and oversteer. However, I understand that the kart needs to unload the inside rear and that the rear behaves more like a motorcycle in regards to vertical CG and the vector of load between it and the contact patch, and more like a car up front in terms of weight transfer to the outside tire due to lateral acceleration and the jacking effect from the inside front. Is my application of throttle negating the jacking effect and thusly, loading the rear up? It seems odd that I should be almost coasting through the majority of the turn but again, I’m suspending conventional wisdom and am open to what you guys say.

Lastly, tire wear. For autocross cars, it’s always been my general rule of thumb for pressures that the tire should be rolling over on the shoulder just enough to wear the tread up to the edge of the block. Looking at my kart tires, the tires aren’t rolling over on their shoulders at all. Can I apply this method to karts to dial in pressures, or is this a different animal in that regard? I’ve attached a couple pics of the tires to show what I’m referring to.


(TJ Koyen) #2

Lots to answer here, but you’ve done a good job outlining what you’re doing and you’re asking good questions. I’ll try and answer the best I can in order.

The complicated answer is “it depends”. The tire works best within a certain temperature window. If you’re under or over that temperature window, you might feel similar handling ailments. Typically when we are racing, we have a window of tire pressures that we know work for a set compound. For example, an MG Yellow (slightly harder than your Greens) is usually between 7.5-10 psi depending on track temperature. Working within that window, we usually go down on pressures to keep the kart from overheating the tires. However, lowering tire pressure to a point can also help the kart take a better “set” in the corner. If I feel like the rear of the kart isn’t digging enough in a corner, I might lower the pressures .5 psi in the rear only, to just slightly shift the balance. Now all that being said, based on the photo you posted or your tires, it looks like you are barely working the tire. Which is expected on such a short run on a track that doesn’t have a bunch of rubber laid down. If your tires look like that (sanded smooth), it might be worth trying some high pressures to see if you can generate some heat and get them to start working some more. When we race on freshly paved tracks, that’s how our tires look, and we end up running significantly higher pressures than normal. Regarding wear over the shoulder, we also look at that to some extent, but don’t rely on it for too much other than peripheral information. The stopwatch will determine if you’re using enough of the tire.

Most kart racing orgs mandate a maximum of 55 1/8" (1400mm) for rear track width, so you are pretty much where most people run the rear. Your front is pretty close as well, if not a little wider than baseline. I would recommend against making big adjustments, especially on a Tony Kart. It’s very easy to go too far and end up with a totally different handling issue. We always make track width adjustments in 5mm increments.

In a kart, you want to be coasting as little as possible. The kart should always have some sort of load going through it, whether it’s braking or accelerating. As you noted, the inside rear needs to be unloaded to get the kart to roll freely through the corner. If you’re driving a shifter with a bunch of power, you can drive it a little differently, but generally braking hard initially, then trailing off the brakes as you approach apex and rolling onto the throttle simultaneously will keep the kart loaded and rotating. You don’t want to straight-line brake and then coast to apex. Shifter driving is a little more point and shoot than single-gear. You can dive into the apex on a tighter line, rotate the kart more aggressively at apex, and then hammer it straight off the corner. The biggest thing to remember is that every input you give the kart will result in some very direct load to the chassis, without springs to absorb anything. Silky smooth is key, with your hands and feet. And always try to make one input per corner. One brake application, one turn-in, one throttle application.

Just a quick note on your setup here. With the front bar set to it’s stiff position, wide front, high ride height and max caster, you’ve got a lot of grip dialed into the kart. It’s set up for a lot of weight jacking, which is probably going to be necessary on a low-grip surface with minimal laps. Just wanted to note that so you kind of know where your set up is at in terms of balance compared to a baseline/neutral setup.


(Ji Simmons) #3

TJ, thank you for taking the time to type up such a detailed answer. It answers a lot of my questions, especially the last part about the kart already being dialed in for a lot of grip. That will help me focus my effort and avoid chasing ghosts. Next time out, I’ll try narrowing the front and playing with front pressure to see what it does. Unfortunately for me, the front track width is adjusted by 5mm thick spacers, so I’m forced to make 10mm changes at a time. I’ve looked for thinner and thicker spacers like the ones I have, but as I’m finding with other parts for a kart this old, they just no longer exist.

I’m going to read through this again several more times, but initially my main question is about the pressure vs. temp relationship. It’s been my understanding for quite some time that lower pressures make more heat due to an increase in carcass flex. But you say to try increasing pressure to increase heat. I absolutely believe you, but can you explain why this is?


(Dom Callan) #4

I can relate to this.

I’m not an expert and I don’t drive shifter. That being said, the temps we run vary from maybe 10 to 11 psi. (Dry). I’m interested as well.


(Dom Callan) #5

Good question. It is true. Why would greater pressure = more rapid temp buildup? Is compressed air more “frictiony” than less compressed?
Or is it that the tire is more stretched and less energy is wasted in the rubber deforming?


(TJ Koyen) #6

A kart tire probably plays a slightly different role in the handling of the vehicle compared to a car tire. With a higher pressure, you are effectively increasing the spring-rate of the kart since there’s no suspension to deal with. Besides this, you are ballooning the tire up more and reducing the contact patch, giving the tire less area to deal with heat. What you typically find is that with a higher pressure, the kart starts to feel like it sits ‘on top’ of the track’s rubber, rather than biting in and digging. This makes the kart start to skate around and slide, which increases heat as well. So higher pressures seem to have a compounding effect of less contact patch and more susceptibility to sliding.

Kart tires have changed quite a bit, even in the 17 years I’ve been racing. Previous tires we ran had a much softer sidewall, so even if the tread compound was a similar durometer, the tire deformed more, dug harder while cornering, and felt softer. It had more play before it gave up traction and started sliding because the softer sidewall could deform and absorb some of the lateral forces. Within the past 7-8 years it seems that most tires we run on have a much stiffer sidewall, which makes it slightly easier to drive ‘over’ the tire.


(Alan Domme) #7

To relate what TJ said about sidewalls to cars- car tire sidewalls generate more heat at lower pressures because of sidewall flex. Since karts sidewalls are so short and stiff, they don’t flex enough to generate heat at low pressures like a car does.


(Ji Simmons) #8

I appreciate the replies, guys. A lot of this is making sense now that it’s been explained. It certainly is different from anything else I’ve experienced. If I have time this weekend, I’ll be trying out changes in pressure both up and down from 14 PSI and in track width, independently of each other, to see how that changes things.