Tire pressure?

[FireFly]

If you were to do a track day and leave the tire pressure at street settings instead of evening them out and letting air out, would you be more likely to take a spill on the track?? How would the rear respond with higher tire pressure on street tires at the track? Also assume that the suspension is set up for every day street riding. What is the most common cause of a highSide? ANyone have any first hand experience with this> Thanks

[OreoGaborio]

A to Q #1 - Yes... how much more likely it is you'll spill however, depends 100% on the rider & their abilities

A to Q #2 - The rear would respond same way as the front... higher pressures = less contact patch = less traction.

A to Q #3 - Simplest way of saying it, the number 1 cause of a highside is a loss of traction in the rear, followed by a sudden regaining of traction in the rear.

A to Q #4 - Yeah... plenty of us... Nazo, Degsy, Sedition.... the list goes on.




I think what you're tryin to ask is "why are track pressures lower than street pressures?" There's a few reasons, here's my take on it... hopfully others will chime in & support or correct me.

1) Lower pressure means larger contact patch, which means more grip. Obvious.

2) Street pressures are higher because you're not using all that traction, so you increase the pressure, REDUCING the contact patch, REDUCING wear and INCREASING the life of the tire.

3) On the track tires get much hotter. After a spirited street ride your tires may reach 100 degrees depending on ambient temp... On the track they'll get up to 150 or more. That meansthe air inside gets warmer as well & expands, increasing pressure. So you set your temps lower when they're cold so they're not rock hard when they're at race temp.

One thing iIm gonna experiment with this race season is setting my temps cold and checking to see where they're at when i come off the track & they're still hot just to see how much it changes. This will give me one more bit of info when I'm setting pressures.



Bottom line (and an elaboration to my answer to your first question) is this: don't worry too much about tire pressures when you do your first trackday. Set em to what the tire guy at the track suggests & put it out of your mind. Just concentrate on turning the track into your favorite set of twisites & ride em at the same pace you do on the street. Even if you leave your tire pressures alone & go out there at 35/40, it doesn't mean you're gonna crash. Hasn't caused you to crash on the street, has it?

[FireFly]

Thanks for the info pete. I should have figured out the contact patch reason, just didn;t think of it. I want to drag a knee like a 13yr old wants to feel boobies. I'm just concerned about holding traction while doing it. I know this is dumb/dangerous but I've come close on exit ramps where theres never any traffic at the end of a parkway in Ny. I wasn;t willing to push the envelope in those conditions. I'm curious to know how big of a difference there is in the way the bike handles in that small margin of space where your close to dragging knee to actually doin it.

[OreoGaborio]

yeah, I'd wait for a trackday before draggin a knee, Mr. Sete the G.

As far as handling characteristics near the "knee dragging" point... it feels perfectly normal... you're leaning, turning harder, leaning more, turning harder & then it's.... "oh... that's interesting... my knee slider is on the ground............... now my elbow, my hip, my ass, my legs, my bike." jk.

As long as you're smooth & are on good clean pavment & everything (including the pavment) is up to temp, it's not like the bike will feel any different as you get closer to planting your knee, but yes... I'd much rather have appropriate track pressures in my tires before I try & plant a knee down, track OR street, just to increase traction & get that (rather large) extra margin of safety.

PS... I have yet to put a knee down on the street. Not saying it's "UBER DANGEROUS", but unless your legs are 4" long & it's super easy to plant a knee, it's just not necessary to push it that hard.

[SVRACER01]

Originally posted by OreoGaborio

A to Q #2 - The rear would respond same way as the front... higher pressures = less contact patch = less traction.


1) Lower pressure means larger contact patch, which means more grip. Obvious.

2) Street pressures are higher because you're not using all that traction, so you increase the pressure, REDUCING the contact patch, REDUCING wear and INCREASING the life of the tire.

3) On the track tires get much hotter. After a spirited street ride your tires may reach 100 degrees depending on ambient temp... On the track they'll get up to 150 or more. That meansthe air inside gets warmer as well & expands, increasing pressure. So you set your temps lower when they're cold so they're not rock hard when they're at race temp.

thats pretty close pete.

setting the tire at a lower press will increase flex in the tire thus creating more heat. if you tires are at the right temp before you go on the track then they should be around the press they would be at after street riding at street temps.

IIRC lower press for colder tracks thus getting tire hotter faster. higher press for hotter tracks . (higher press of course only being a couple PSI)

then again if you arent riding hard enough with pess. at say 26PSI then the bike might handle funny.... like you dont have enough air in them or something

[SVRACER01]

Originally posted by OreoGaborio

As far as handling characteristics near the "knee dragging" point... it feels perfectly normal... you're leaning, turning harder, leaning more, turning harder & then it's.... "oh... that's interesting... my knee slider is on the ground............... now my elbow, MY HEAD JUST HIT THAT CONE!!

[OreoGaborio]

that too

[FireFly]

Pete do you know how the weight of the rider affects the handleing and stability of a bike , leaning in the turns, ie. ass off the seat/knee out on a 380lb bike with a 230lb rider?

[OreoGaborio]

The more ya weigh the more your weight effects the performance, both handling wise and acceleration wise. Think about those little 125cc GP bikes. They'll always have the most cornerspeed of all the GP bikes, even more than Rossi. It's because they weigh so little that there's less momentum trying to pull them to the outside of the turn which means they require less of a turning force to change their direction as compared to a heavier bike/rider combination.

The DEBATABLE side of the equation says that if you weigh more as a rider you can actually work less than a lighter rider... In theory a light rider has to put more effort into getting a bike turned because he/she can't shift as much weight as a heavier rider. However if you think of it as a balancing act, just because a rider weighs more doesn't mean it's center of gravity changes more than a lighter rider's....

damnit, now ya got me thinkin physics & i'm about to get off work so i can't continue writing down my thoughts... to be continued

[OreoGaborio]

DISCLAIMER: I am NOT Keith Code, this is just shit i like to think about.... these are my own THEORIES and are just that... theories. they might be right or wrong, I have no idea, but i feel like thinkin it through & typing it all out ... please correct me if i'm wrong... Hopfully Tony, Degs, Paul & other experts will chime in on this topic.

With that said, Ok, answer to your question parts 2-12


lets take a bike... saaayyyy an '05 F4i

Put a 250lb rider on the bike & because of their weight their CG plays a bigger role in the handling of the bike as compared to a 150lb rider.

During turn-in, the heavier rider's weigh works for them because their CG plays a greater role in changing the orientation of the bike... ie, they works less to lean it over because they has more weight to use.

while leaned over their weight effects two things... lean angle and centrifugal force... their weight helps keep the bike leaned over, but the centrifugal force is greater because of their weight, trying to stand the bike up... so it kinda cancels out...

but... becaue the centrifugal force is greater, the centripetal force (inward force) must be greater to keep them turning, so it puts more pressure on the tires at a given speed to push you in the direction that you're turning... that means a heavier rider can only cary so much corner speed before the tires reach their limit.

A lighter rider on the same bike has to work harder to initiate the turn & lean off the bike more because their CG plays a smaller roll in the bike's orientation. However centrifugal force is less because of their weight. This means they can carry more cornerspeed, but they've got to lean off the bike more in order to have any effect on the bike's cornering ability.

[skiierx]

Pete,

I think you are making him think to much here. For his first trackday he should not fool with his air pressure, set them at the recommended pressure as you have many more things to deal with. As he gets used to riding on a track then he should start dialing in the tire pressure to suit the track for that days temps. Tony has the trackday set up perfectly for first timers, meaning the riders meeting, tech, following the leader and informational seminars throughout the day. Once he learns how to be comfortable out on the track then he can work on his techniques, bike setup and so on. Once he getsa faster then the inevitable will happen...knee contact! After awhile you will not worry about your knee but more about learning what your bike is really capable of doing (most of us, including myself, will never learn what our bikes are really capable of handling)!

Good luck and go out and enjoy your first trackday!

[oreo_n2]

Damint pete, you're bringing back bad memories of free body diagrams and centripetal vs centrifugal force.

you just made my head hurt.

effer!

[OreoGaborio]

Originally posted by skiierx
Pete,

I think you are making him think to much here.

Correction... he's making ME think too much Yah, yer right Jamie... he asked a bunch of questions, I answered all of em to the best of my knowledge & then at the end of my first post I said "BOTTOM LINE.... don't worry bout your pressures" etc. So ya, I agree w/ you Jamie... it's not something to worry too much about when you first go out there. You're not pushing much harder than you would on the street & we don't crash there cuz of high tire pressures, now do we? So yer right... don't worry bout it. Just make sure they're not too low & you'll be fine.

Brent....

[FireFly]

Wouldn't centrifugal force be pushing outwards? Thats a concern being a heavier rider. I"M Guessing here but, wouldn't the added weight in the turns , givin the centrifugal force, cause the tires to give out easier leading to a lowside? OR would the added weight counteract loss of traction because of the added pressure to the pavement?Also , take into account how much the heavy rider is leaning off the bike Ie. weight displacement. for example; Ass completely off the seat with the thigh over the seat instead of half a butt cheek. WIll this make the bike more stable or less stable (without the knee down, acting as a 3rd contact point) ? Sorry for the added headache dude, but if you feel like it feel free to respond

[OreoGaborio]

Ya, i said Centrifugal force is one that feels like it's pulling you to the outside, Centripetal force is basically the inward force that you create to turn.

Body position doesn't really have a huge impact on stability, just turning forces. You can hang off like a mad man or none at all, if you're not near the limits of traction it's not gonna make a difference on whether or not you crash.

The reason why you hang off is so you move the center of gravity of the bike & rider to the inside, creating a turning force... this releives the necessity to lean the bike over so far which means the bike will be standing up a little more, keeping a good contact patch on the pavment, maximizing your traction.

btw.... Sign up for the March 25th Tony's Trackday, will ya, sucka foo??

[Awesome]

The Physics of Racing,
Part 4: There Is No Such Thing as Centrifugal Force

http://www.miata.net/sport/Physics/0...ntrifugal.html

info is nice and everything works in theory. Theory is not going to get you around the track, thinking about this sort of thing might not be the worst idea, but just remember you can justify anything to yourself logically, that doesn't make any of it right or worth a damn.

Now to the original Q'a

Tire pressures depend on conditions, riding ability, riding intensity and what tires. i think it may be Michelins that have a recommended 22 PSI in the rear or something like that.

High pressures, less heat in the tire less carcas flex. When the tires are up to temp on the track or trying to get there the tires pressure and heat are proportional..or is it inversly proportional....either way, when a tire is hotter the PSI is rising more, and equalling out to a colder tire with a higher set PSI. Makes the contact patch pretty close. That's why PSI is measured at cold and at hot to get it to the right number and get the right tire temps.

Higher temps on the track keep you from geting the traction you need because you're not getting the heat from carcass flex and optomizing the rubber, as well as the contact patch being smaller, of course depending how far off your pressures are.

Highside = crashing the hurty way, happens because you puss out of the gas when spinning up the back tire.

[OreoGaborio]

Originally posted by hessogood
The Physics of Racing,
Part 4: There Is No Such Thing as Centrifugal Force

Indeed... but i wasn't about to get into that