Tire Pressures

"Dude! What pressure should I set my tires to?"

"More than ten, less than two-hundred."

"Hunh?"

What tire pressure you want is one of the most common questions I get trackside, right behind "What IS that thing?" in reference to my Nox.

It's a complicated issue.  It USED to be that at the track, you could get away with starting at around 28-30psi front and rear on your street tires, and if one end or the other was sliding, you'd take some pressure out until it just "felt right".

With the increase in the number of people running DOT-R tires or slicks, and the wide range of available carcasses from different manufacturers, it's just not as easy any more.  Even street tires, which were pretty much a 32psi front and 36psi rear sort of arrangement for most sporty bikes, are starting to use a different formula.  

Many street tires are starting to use much higher pressures.  The Avon Storms that I had on my ST4s until recently liked 36-38psi in the front, and 38-42 in the rear.  A set of ancient Pirelli Diablo's that I have on there now still run quite well at around 33psi front and 36-37 rear.

Some of the Dunlop race tires are using down in the 20psi range.  My Bridgestone tire guy says to set the BT-003R DOT-R's on my track bike and the GP slicks on my Nox to 28/25 front/rear when they're cold first thing in the morning.  And then I usually end up taking a little out of each throughout the day after they've been on the warmers and ambient temperature goes up, increasing tire pressure.

My Formula 7 bike uses tire pressures somewhere in the sub-20psi range, since I'm running 125 GP slicks on it, and it's a very light bike.

"That's all great, but what tire pressures should I run in MY bike?!" I hear you saying.

Before we get to that, I also want to dispell a VERY common myth about tire pressures, for both bikes and cages:

The tire pressure on the side of the tire is NOT the pressure to run the tires at.  That's the MAXIMUM ACCEPTABLE tire pressure for the tire at maximum load.  For bike tires, this will often be something higher than 50psi, but every tire is different.  

I can say that you probably DON'T want to run 50psi in your tires, no matter what.  That would cause your tires to be very "hard", and they would not deform enough to create a good contact patch.  The contact patch is the only thing keeping you hooked up with the pavement.  On a bike, this contact patch is often described as being roughly the size of a deck of cards for each tire.  As the tire sits on the ground with the weight of the bike and the rider, it will cause the tire to deform and flatten.  With too much pressure, this area will be smaller than with the correct pressure.  This contact patch changes size and location on the tire carcass as you lean the bike, and under braking and acceleration into through and out of the turn.

A secondary factor of cold tires is that they will not heat up sufficiently to provide maximum grip.  All tire compounds that you will use on a road-race track will provide more grip at a median temperature than they will at a very low temperature.  Hotter isn't necessarily better, but cold is not the most desirable of situations.  Too hot, and a tire begins to break down and feel "greasy", but more on that later.  When cold, the tires will spin up easier, slide under heavy lean, and will provide poor feeling under braking, as well as longer braking distances.  If you come in from the track, and place a bare hand on your tires, they should have about the same temperature front and rear, be slightly tacky to the touch, and very warm(think slightly cool fried pie that is ready to eat, but won't burn you).  If the air and track temps are low(in the 40's to 50's), you may not get much heat in the tires at all, and there will only be so much you can do about it without causing too much deformation of the carcass and excessive wear on the tires due to running very low pressures to get the tires to heat up.

So too HIGH of a pressure is bad, but so is too low.  If you are running low pressures(less than 20psi for common tires on big bikes), then you will get a LOT of tire carcass deformation, which will cause excessive heat in your tires, causing the tires to break down quickly, and providing very poor feedback.  You get more contact patch, but at the cost of deforming the sidewalls and center carcass so much that they heat up and break down quickly.  Just rolling the tire on the road causes the tire to flex, which creates heat.  Add in braking and acceleration forces, and a tire can be deformed quite a lot, and potentially could come off "the bead", which would cause catastrophic tire failure, and almost certainly a crash.  If the bike feels a little wobbly at the start of a track session, feels better in the middle, and then feels "slick" later in the session, it's possible that you have too little pressure in one tire or the other.  If you come in from a session, lay a bare hand on a tire, and you can't leave your hand on it comfortably, it's probably low on pressure unless the ambient temperature is in the 90's or higher.

When in doubt, and your tire manufacturer won't tell you what pressures to run with your tire, with your bike, for the given conditions, follow this process for setting your pressures for track days:

Start at 30psi front and rear first thing in the morning, unless you are running Dunlop DOT-R's, in which case, start at 25psi front and rear.  I HIGHLY recommend the purchase of a good tire gauge.  Almost nothing will have a greater effect on the performance of your bike than correct tire pressure, so it's a very good investment.  The stick style and the 99 cent ones from the auto store are wildly imprecise and inaccurate.  A good tire gauge with a large face, in the 0-60psi range, a flexible hose, and an air bleed button will run between 40 and 60 dollars and last for a long time.  Prefer oil-filled ones.  Most tire vendors at track events will have these available for sale, or are often available at shops that carry racing supplies for cars or go-karts.  The type with a small ball on the end that is perpendicular to the hose is ideal, as the angled heads with the long stick are difficult to use with most motorcycle wheels, especially if they have spokes or use tubes.

Go out, for the session, and come back in.  Jump off the bike and take a glove off as soon as you park.  Feel the front tire.  Then feel the rear.  Are they the same temperature?  If so, then ask yourself how the bike felt?  Were you pushing going into the corner? Were you spinning up the rear coming out?  Were either of these due to poor form(Leaning in on the brakes and pushing the front or opening the throttle before the bike is straight up and down)?  Did it feel like the bike was controllable but wanted to slide when leaned over fully? Did it feel like the tires were gripping really well at the beginning but not the end? or vice versa?

If both tires are too cold, and the ambient temperature is not very cold, then try taking a psi out of both tires.  

If both tires are VERY hot, the ambient temperature is not very high(95+) and there is graining on both tires(looks like little rolls of rubber generally running along the direction of drive for the tire, especially in the drive line, then try adding a psi to both.  The drive line is usually an inch in from the sidewall on 180-190mm wide tires on the rear and a half inch on 120mm tires on the front. Smaller tires, such as 160mm rears on SV650's and motards, will be proportional to these numbers.  That is, a 160mm tire will have a drive line that is approximately 3/4 of an inch in from the edge. If either end feels like it is considerably hotter or colder than the other, I would say take a psi out of the colder tire, since 30psi is towards the high end, esp. for a rear tire that is cold and feels too slippery on a big bike.  

If they feel about right to you, or you don't know what to look for/feel for, and the temperatures are medium to hot for both air and track, the tires are showing a little wear, but not excessive esp. in the drive line, then I suggest you play a little.  Take a little out of the rear, like 2psi, and then do a session and see how it feels.  Writing things down here, including ambient temperatures and "feels", will help out a lot.  Even notes like "cloudy", "full sun", "squirmy", "planted", "flat-tracker", and "cold as hell" will give you little clues in the future as to where to start, since they're all in your own words.  A pyrometer is killer for this purpose, but really overkill for the average track-day attendee.  If you're REALLY curious, ask someone with a BIG trailer.  They probably have one somewhere, and can tell you what the track temp is.  Just knowing it's really cold or really hot is usually good enough, since it's rare that air-temp and track temp are not correlated.

Take notes on how the bike felt with the -2psi.  Feel the tires when you get off.  Do they feel noticeably different now?  With 2psi, if you are pushing at all, you should notice a difference.  Even on a round-robin, you should notice a difference in the temperatures, since a track session is still twenty minutes of riding time, with probably more sustained corner speed than you typically get on the street.  By the way, corner-speed is the primary producer of heat in a motorcycle tire.  It causes F=m*a amount of force, just like everything else.  So going down the road, you produce 400lbs of bike + 170lbs of rider worth of force.  Acceleration is effectively nothing special except gravity, so you end up with a given force.  Now lean the bike over at 75mph.  Now, your acceleration around the corner is being added to the gravitational force and multiplied by the combined weight of you and the bike.  It's not hard to see how that adds up quickly to tire wear and more heat.

Now try adding in three psi(so that you are 1psi above where you started before taking out 2psi).  This is a small amount, but on the next session, should be an obvious change from where you were.  If you were close to begin with, the bike should now want to push more than it did initially, and the tires should be noticeably cooler than the session previous where you took out 2psi.  

You should now have a distinct idea of how the bike is going to behave as you change tire pressure by small amounts.  Never make drastic changes to tire pressure between sessions, since the different behaviors can put you on your butt if you're not ready for them.

All contents Copyright Dash Zero Systems 2008