As copied from badweatherbikers board.
http://www.badweatherbikers.com/buell/board.html
Hot Damn!!!
Now were talking about pressures and contact areas and dynamic forces. Right up my engineering alley. If you like,
I'll give you all a sixth order differential equation, complete with diagrams, that you can solve in the general form to
determine the contact area for any tire.
Now for my 2 cents.
Aaron,
I read the article on the contact patch and it is accurate for a STATIC situation. The thing to remember is that the
Load varies with dynamic forces associated with the motorcycle. The contact patch area will always equal the total
wheel load divided by the air pressure in the tire. The load varies with acceleration from cornering, braking and
getting on the gas. The will constantly change the area, and the shape, of the contact patch during riding. Otherwise
you couldn't do a stoppie or whellie. (Friction my man, friction.)
Toyracer,
The pressures inside the tire can change with heat and to some extent with the reduction or increase, in most cases
this is a small change and typically not worth counting.
Different size rims determine the basic shape of the contact patch and this determines the tire and motorcycles
response to rider input(acceleration forces). Regardless of the rim size and the tire width, the static contact area for
all tires with the same bike and rider, in a static condition (at a stop light) and same air pressure will be the same.
When a dragster is starting the terrific acceleartion forces generate additional load on the tires, mostly from friction
between thr tire and the pavement, not centripetal acceleration, the tires are barely spinning at the line compared to
the 200 plus at the end. The acceleration load increases the tire contact area.
Your comments about grooving the tire are well taken, however, on dirt you are also changing the frictional
coefficient of the motorcycle tire. I expect that the grooving, along with slightly higher contact pressures (please
note that I said higher pressures, not area) allow you to "bite" the dirt better. You generate a confining pressure in
the dirt increasing the dirts shear strength. Although I know nothing about Pro Dirt, I make a living with soil (dirt). I
expect that you had to use different compounds, grooving, etc, more due to the different soil types and
environmental conditions at each track rather than the bike or riders. Just as in Road Racing you will change
compounds and tires based on the air temperature and track surface.
Rocketman,
You hit on a hidden truth here. The contact patch is theortical, in reality, the actual contact pressure with a grooved
tire is HIGHER than the internal air pressure because the physical contact area is smaller and the tire carcass (that
part above the grooves)is the actual portion where theoritical contact area is located. This is required for static
equilibrium of the tires. If the contact pressure was equal on a grooved surface the bike would have to accelerate
down (towards the center of the earth as my physic professor used to say).
My understanding of roadrace tires is that the compound (chemical composistion of the rubber) and the heat
response of the polymer chains are what really effects the tires. The principal thing with the grooving is that you
have increased the shear stress in the rubber, making the tire exert more pressure for pavement contact surface
area, and are breaking more polymer chains. Remember we are talking about friction here, which means heat. A
smaller area (grooved) has to dissipate hear faster than a larger contact area (slick).
The article that Aaron pointed to is accurate that most riders won't generate the heat necessary to get the most from a
DOT Racing Tire. The compound used in these tires also doesn't respond well to multiple heating cycles. If you
can't get that tire on a track you aren't getting your money's worth from it.
Wow, I like a good technical argument. Bring it on.
Arvel
[This message has been edited by jimjib (edited 20 July 2000).]
http://www.badweatherbikers.com/buell/board.html
Hot Damn!!!
Now were talking about pressures and contact areas and dynamic forces. Right up my engineering alley. If you like,
I'll give you all a sixth order differential equation, complete with diagrams, that you can solve in the general form to
determine the contact area for any tire.
Now for my 2 cents.
Aaron,
I read the article on the contact patch and it is accurate for a STATIC situation. The thing to remember is that the
Load varies with dynamic forces associated with the motorcycle. The contact patch area will always equal the total
wheel load divided by the air pressure in the tire. The load varies with acceleration from cornering, braking and
getting on the gas. The will constantly change the area, and the shape, of the contact patch during riding. Otherwise
you couldn't do a stoppie or whellie. (Friction my man, friction.)
Toyracer,
The pressures inside the tire can change with heat and to some extent with the reduction or increase, in most cases
this is a small change and typically not worth counting.
Different size rims determine the basic shape of the contact patch and this determines the tire and motorcycles
response to rider input(acceleration forces). Regardless of the rim size and the tire width, the static contact area for
all tires with the same bike and rider, in a static condition (at a stop light) and same air pressure will be the same.
When a dragster is starting the terrific acceleartion forces generate additional load on the tires, mostly from friction
between thr tire and the pavement, not centripetal acceleration, the tires are barely spinning at the line compared to
the 200 plus at the end. The acceleration load increases the tire contact area.
Your comments about grooving the tire are well taken, however, on dirt you are also changing the frictional
coefficient of the motorcycle tire. I expect that the grooving, along with slightly higher contact pressures (please
note that I said higher pressures, not area) allow you to "bite" the dirt better. You generate a confining pressure in
the dirt increasing the dirts shear strength. Although I know nothing about Pro Dirt, I make a living with soil (dirt). I
expect that you had to use different compounds, grooving, etc, more due to the different soil types and
environmental conditions at each track rather than the bike or riders. Just as in Road Racing you will change
compounds and tires based on the air temperature and track surface.
Rocketman,
You hit on a hidden truth here. The contact patch is theortical, in reality, the actual contact pressure with a grooved
tire is HIGHER than the internal air pressure because the physical contact area is smaller and the tire carcass (that
part above the grooves)is the actual portion where theoritical contact area is located. This is required for static
equilibrium of the tires. If the contact pressure was equal on a grooved surface the bike would have to accelerate
down (towards the center of the earth as my physic professor used to say).
My understanding of roadrace tires is that the compound (chemical composistion of the rubber) and the heat
response of the polymer chains are what really effects the tires. The principal thing with the grooving is that you
have increased the shear stress in the rubber, making the tire exert more pressure for pavement contact surface
area, and are breaking more polymer chains. Remember we are talking about friction here, which means heat. A
smaller area (grooved) has to dissipate hear faster than a larger contact area (slick).
The article that Aaron pointed to is accurate that most riders won't generate the heat necessary to get the most from a
DOT Racing Tire. The compound used in these tires also doesn't respond well to multiple heating cycles. If you
can't get that tire on a track you aren't getting your money's worth from it.
Wow, I like a good technical argument. Bring it on.
Arvel
[This message has been edited by jimjib (edited 20 July 2000).]
