Track Tac

How do you know when tire pressure is correct?

paulkish

paulkish

old fart
Yes it's obvious this would be about slip ratio's. ... :)

Slip ratio is based on revolutions of a tire per distance traveled. Acceleration based on slip ratio states, maximum acceleration is at maximum wind up of a tire prior to slipping.

That's slip ratio in a nutshell. My theory of how stagger on a solid axle is used based on tire surface speed, boils down to adjusting slip ratio's on the fly by controlling the retention and movement of weight. In particular the retention and movement of weight is applied to get different acceleration relationships between the two rear tires, during deceleration, rolling the corner, the start of acceleration and final acceleration.

Without considering lateral forces and the need to not slip laterally, grip comes into play by setting where maximum acceleration is able to occur, within the limits of your tires ability to wind up. In general reduce grip and slip ratio will increase. There is a range of slip ratio's for every tire, in which you can directly adjust the point of maximum acceleration by altering grip.

I'll assume everyone followed me on the above and move on. ... :)

Ok then how does the above relate to air pressure? What changing air pressure does is increase or decrease the range of tire wind up, where grip is able to set slip ratio.

Only considering longitudinal direction and not including lateral forces, assuming you have hp available, when you change air pressure in a rear tire you change when maximum acceleration will occur. If you want more positive or negative acceleration, the amount of tire wind up must be increased.

All that is easy if we had rear tires which were able to accelerate independently. But since the two rear tires are connected via a solid axle, the range of where maximum acceleration and slip can occur for each tire is also dependent on the other tire. Changing the range of operation at one tire, even without consideration of applied weight, directly effects the range of operation of the other tire.

... so, when you make an air pressure adjustment on a rear tire it alters when maximum acceleration will occur at that tire and without even considering changes in loading, it directly changes when maximum acceleration will occur at the other tire. ... just started writing because I felt like it and it seemed interesting and never thought I'd end up in the process, at that last sentence. Guess the only thing this all means is it is IMHO and ain't necessairly right anyway. ... :)

Got up late and I think after that I do need another cup of coffee.

I wonder when I read what I just wrote, if it will make any sense? proly not
 
Last edited:
I posted and looked at the physical size of what I just wrote. There ain't no way I want to wade though all that. ... :(
 
awwwww, second cup of coffee, fixed a couple of things in the first post and it sort of reads ok, for me anyway. ... :)

When I read through what I wrote, it presented to me that air pressure is correct when you can get needed positive and negative acceleration from each rear tire, all around the track. If that is true, then all on track problems caused by the back, can be fixed by changing the slip ratio and in turn acceleration, at one tire or both tires.

Ok now that that is said, let's bring lateral acceleration back into the picture. If all on track problems can be fixed by adjusting slip ratio, then you can never have too much lateral grip. You can have too little, but never too much, because too much or too little lateral grip is not a consideration with a staggered solid axle. The only considerations needed and the only thing needing adjusting when using a staggered solid axle, is positive and negative acceleration at each of the rear tires when what your racing is decelerating, rolling the corner, at the start of acceleration and during final acceleration.

maybe????? ... :)

edit: ain't this fun? ... :)
 
Just thought of a question.

Is there an on board way to get real time slip ratio readings, for each rear tire?
 
paulkish said:
Yes it's obvious this would be about slip ratio's. ... :)

Slip ratio is based on revolutions of a tire per distance traveled. Acceleration based on slip ratio states, maximum acceleration is at maximum wind up of a tire prior to slipping.

That's slip ratio in a nutshell. My theory of how stagger on a solid axle is used based on tire surface speed, boils down to adjusting slip ratio's on the fly by controlling the retention and movement of weight. In particular the retention and movement of weight is applied to get different acceleration relationships between the two rear tires, during deceleration, rolling the corner, the start of acceleration and final acceleration.

Without considering lateral forces and the need to not slip laterally, grip comes into play by setting where maximum acceleration is able to occur, within the limits of your tires ability to wind up. In general reduce grip and slip ratio will increase. There is a range of slip ratio's for every tire, in which you can directly adjust the point of maximum acceleration by altering grip.

I'll assume everyone followed me on the above and move on. ... :)

Ok then how does the above relate to air pressure? What changing air pressure does is increase or decrease the range of tire wind up, where grip is able to set slip ratio.

Only considering longitudinal direction and not including lateral forces, assuming you have hp available, when you change air pressure in a rear tire you change when maximum acceleration will occur. If you want more positive or negative acceleration, the amount of tire wind up must be increased.

All that is easy if we had rear tires which were able to accelerate independently. But since the two rear tires are connected via a solid axle, the range of where maximum acceleration and slip can occur for each tire is also dependent on the other tire. Changing the range of operation at one tire, even without consideration of applied weight, directly effects the range of operation of the other tire.

... so, when you make an air pressure adjustment on a rear tire it alters when maximum acceleration will occur at that tire and without even considering changes in loading, it directly changes when maximum acceleration will occur at the other tire. ... just started writing because I felt like it and it seemed interesting and never thought I'd end up in the process, at that last sentence. Guess the only thing this all means is it is IMHO and ain't necessairly right anyway. ... :)

Got up late and I think after that I do need another cup of coffee.

I wonder when I read what I just wrote, if it will make any sense? proly not
Click to expand...

When you win the pressure is right......
 
oldman50 said:
than the posts
Click to expand...

Time to try to fix it, thank you for the help.

paul

_________________

A few other rules I have. ... :)

Never bet on an inside straight.
No set, no bet.
Only play suited aces and faces, pairs and cheap cards.
First you have to have the cards.
Slow play puny pairs, flushes and open ended straights.
Look both ways before crossing straights and flushes.
Look out for trips.
If they bet like a duck, walk like a duck and talk like a duck, duck.
Never go all in with a king or less than two pair.
Pick every lock, but only open yours.
 
Last edited:
As long as the tires do not go flat while racing ..........you set your pressure (5 psi) after they heat up what happens ??? and ever time you get a caution they cool down something else happens ??............???? you has no control over that bro....
 
Tire pressures affect the sidewall stiffness (lateral tire response to load), the circumference, the growth as pressure rises due to heating (friction with track), and also the effective spring rate of the tire adds to the spring of the chassis while acting as a suspension system. And racers also use pressure to affect stagger (though that's less than optimum method.) So.....after saying all that, the optimum pressure is the one that lets you balance all those factors to produce the fastest lap time. It would be very hard to calculate, but seems to be experimentally somewhere between 4-5 psi lefts and 5-6 psi rights in the dirtphalt Southeast. :)
 
One helpful guide can be tire temperatures (taken with a probe type tire thermometer, not an infrared/laser thermometer) Take the outside, middle and inside temperatures and average them, repeating this operation for each tire. If your middle tire temperature is higher than the average tire temperature for that tire, you have too much air pressure for track conditions. If your middle tire temperature is lower than the average tire temperature, you have too little air pressure. We don't get too upset with small deviations (especially if the driver likes the setup as is), but if it seems a bit large, it's one more tool to help evaluate setup.
 
Wow. I didn't know it was this complex. I run enough air to keep the kart on the edge of braking loose.
 
I can't say that I'm ever sure that our air pressures are perfectly correct. Our best indication comes from driver feedback - whether he likes the "feel" or not. If the track has been fairly consistent during a race we might tell by how much the pressure increased by the end of the race - generally if it picked up around a quarter pound (for karts). But don't take that number as gospel because there are lots of variables that effect it.

Slip ratio does have something to do with air pressure, but it's more of a result rather than a cause. Because the air supports the tire, it affects how much grip the tire can generate. Heat from friction against the track, as well as friction generated within the tire carcass itself, is a self-affecting system – the heat causes the air pressure to rise, and the rising air pressure can alter the grip and heat generated later. It’s what’s happening when you lose the handle after some laps because your tires overheated.

“If all on track problems can be fixed by adjusting slip ratio, then you can never have too much lateral grip.” I’m not sure about this. First, it’s difficult to adjust slip ratio – especially on dirt, and if you get it right for a few laps it can change (and go wrong) in another few laps. You can’t really adjust it while you’re driving, except by adjusting your driving line to find places with better grip and / or cooling your tires by driving through wetter stuff. Second, you can have too much lateral grip if it binds up your chassis. You get “locked down” which may handle great but it’s not fast. So we try to approach (or even slightly exceed) the limits set by the slip ratio that is available at the time.
 
I often wonder when I look at tire manufacturers recommended pressure range and then look at kart racers actual pressures. Roll resistance anyone....
 
You must log in or register to reply here.
Back
Top