Claude asked;
>where is the fulcrum point
at the rear when the vehicle ...
Claude, there isn't a "fulcrum" per se-
I understand clearly what you're asking- but despite appearances, there
isn't a point where two axles 'pivot' around one. On a standard GMC, both
rear tires on the same side should carry identical loads- in this example,
the point that the rear 'swivels' around is mid way between the two axles.
In actual practice its much more complex than that-
Tires operate at all times when not driving straight ahead at 'slip angles'.
Slip angles are essentially the difference between the angle the WHEEL is
pointed, and the actual course followed by the tire treads, due to
distortion and slippage among other things. At very low speeds, slip angles
are very small, as speeds rise, or angle of turn increases, or loading
increases, or inflation diminishes, the slip angles grow wider and wider.
Turning any corner at any speed creates a slip angle on all six tires. Once
past perhaps 5 MPH or so, nearly any turn causes all six to slip OUTBOARD in
the turn. If the slip angle of the rear tires in a corner was slughtly less
than the front wheels, the vehicle will understeer slightly, if the reverse
is true it oversteers (tail out)- The forces acting on the rear tires if
they are the same size, pressure and load are pretty much identical, as they
are essentially reacting en masse. Essentially, one tire forces the other to
slip- so no tire gets a greater force as they are reacting against one
another pretty much.
Because the GMC is at heart a profound understeering vehicle, it is
reasonable to say the front tires likely are the most 'overburdened' by a
wide margin.
I suspect you could attribute any unusual wear or inclination to
flats/blowouts not attributable to overloading, or tire defects in any rear
wheel position to either where road debris is normally found (gutter), and
the fact that the rear tires hit said debris after the fronts have kicked it
up in the air. Some failures may be attributable to curbing etc also I
imagine.
Anyhow, an interesting question, but one that needs a less than satisfying
answer I'm afraid.
Brent Covey
Vancouver BC
>where is the fulcrum point
at the rear when the vehicle ...
Claude, there isn't a "fulcrum" per se-
I understand clearly what you're asking- but despite appearances, there
isn't a point where two axles 'pivot' around one. On a standard GMC, both
rear tires on the same side should carry identical loads- in this example,
the point that the rear 'swivels' around is mid way between the two axles.
In actual practice its much more complex than that-
Tires operate at all times when not driving straight ahead at 'slip angles'.
Slip angles are essentially the difference between the angle the WHEEL is
pointed, and the actual course followed by the tire treads, due to
distortion and slippage among other things. At very low speeds, slip angles
are very small, as speeds rise, or angle of turn increases, or loading
increases, or inflation diminishes, the slip angles grow wider and wider.
Turning any corner at any speed creates a slip angle on all six tires. Once
past perhaps 5 MPH or so, nearly any turn causes all six to slip OUTBOARD in
the turn. If the slip angle of the rear tires in a corner was slughtly less
than the front wheels, the vehicle will understeer slightly, if the reverse
is true it oversteers (tail out)- The forces acting on the rear tires if
they are the same size, pressure and load are pretty much identical, as they
are essentially reacting en masse. Essentially, one tire forces the other to
slip- so no tire gets a greater force as they are reacting against one
another pretty much.
Because the GMC is at heart a profound understeering vehicle, it is
reasonable to say the front tires likely are the most 'overburdened' by a
wide margin.
I suspect you could attribute any unusual wear or inclination to
flats/blowouts not attributable to overloading, or tire defects in any rear
wheel position to either where road debris is normally found (gutter), and
the fact that the rear tires hit said debris after the fronts have kicked it
up in the air. Some failures may be attributable to curbing etc also I
imagine.
Anyhow, an interesting question, but one that needs a less than satisfying
answer I'm afraid.
Brent Covey
Vancouver BC
