Wednesday, July 24, 2013

Just a few little psi

Those of you who have read this blog will know, I can't stop tinkering with stuff.

About 3 years ago, I bought the Kawasaki ZRX1200R as the basis for a future project.  I figured, I wasn't ready for the project, but I could ride the bike in the meantime.  What I discovered was that the ZRX series had a reputation for being a poor handling bike and, upon riding it for a while, I had to agree.  Going in a straight line, everything was fine, but go over a dip in the road while cornering, you better hang on.  And don't even think about squeezing on the throttle in a turn.

The guy I bought the bike from told me I should make sure to run the factory tire pressures at 36 front/36 rear.  So that's what I did.

Trying to improve the handling, I messed with every suspension setting I could think of.  I managed to move the handling in the right direction, but she was still scary and I still wasn't confident.

One morning, on a pure lark, I decided to blow the tires up to 40 psi.  WOW!  What a difference.  Suddenly, you could feel the front end and the rear would follow the front.  When I got home, I googled ZRX tire pressures and found that lots of other people had discovered that the factory specs were just wrong.  They said that the right specs were 36/42.  I gave those a try, along with 40/42 which I settled on as best for me.  Now I can go through a dip with confidence.  Now I can squeeze that throttle on at the apex and track the bike smoothly out of the corner.  I had a little surprise when an oncoming car crossed into my lane on a corner.  Grabbing the front brake to slow and tighten my line caused a minor steering reaction, but well within control.

How amazing it is that a few little psi could change the character of a bike.

That got me to thinking.  What had probably been happening is that the contact patch of the tire in a corner was moving laterally when the vertical load changed going through a dip.  There just wasn't enough tire pressure to keep the tire's shape under different loading.

Of course, the current style is big, fat, wide tires on bikes.  I guess people think they look good.  But doesn't the fat tire cause the contact patch to move laterally when the bike leans?  Wouldn't the bikes handling be more consistent if the contact patch didn't move laterally as far in roll?  It always has seemed that skinny tires are the best handling on a bike.  I can't say it is the whole reason, but I can say that the KLR has skinny tires and keeps up with just about any sport bike ridden at the same skill level.  So keep your fat tire style.  I prefer skinny any day.

P.S.  Note to self.  Always Google any question that comes to mind.  There is often useful knowledge there and the factory spec isn't always right.


  1. Interesting post. I'm going to have to experiment a bit with tire pressures now to see if I can improve the handling in corners 'cause I have one of those fat tires on the back as well.

  2. MotoGP tires are

    Front tyre size: 125/600R16.5
    Rear tyre size: 190/650R16.5

    Mighty fat. Of course they are brought into submission and 60° lean angles by a million dollar bike and million dollar rider so if you want to run fat tires on the Kaw you might have to change a few more parts.

    I'm inclined to think that your Kaw hails from a transition era when tires on the street were starting to fatten up but the basic bike was from the skinny tire era. I think your best solution, Jac, is to sell the Kaw and buy a Hayabusa or a Norton Manx.

  3. Doug,

    I'm not saying I'm anything like a MotoGP rider. My definition of good handling is predictable and confident.

    The other interesting thing that I have learned about motorcycle tires is that, short of sliding, they have corner grip proportional to the pressure in the contact patch. In overly simplified terms, that means that a skinny tire corners as well as a fat tire up to the point where you are sliding the tire. Since I don't slide and ride on the street, all I am giving up on with skinny tires is looks.

    And yes, a Manx would be cool. I'm more partial to and AJS or Matchless. A Velocette even better or maybe a Scott Flying Squirrel. Forget about the Hayabusa. Not going to happen.

  4. Jac,

    Wouldn't friction (traction) be increased as the size of the contact patch increased with tire size? I defer to you on all things regarding handling, of course.

    Not a MotoGP ride? Me either. The only way my knee touches the ground is when I'm checking tire pressures before a ride.

  5. Doug,

    We are grossly simplifying things here. There are lots of other important variables, but....

    Let's just say that, in normal riding, contact patch doesn't really change anything. A tire with a smaller contact patch will be made with stiffer rubber that can handle higher pressure and the two tires will have about the same grip, even in a hard brake stop.

    When having softer rubber and a larger contact patch comes in handy is when you are on a race track and have the rubber up to racing temperature. Under those conditions, you get the best grip out of the softer rubber which needs lower pressure and a larger contact patch to work at its best. At least, that's my understanding.

  6. Jac,

    This is the Internet. Gross oversimplification is standard operating procedure! It's the part of the discussion that comes just before comparing the other person to Nazis or communists.

    Anyway, I don't quite grasp your explanation but I know you well enough to believe you.