Driving Force Comparison

Driving Force Considerations

My first problem was presenting the information in a standard format.  Because I am an old-school American I prefer to see anything related to an engine as horsepower and torque in pounds-feet.  I realize this feels like an abomination to some.  Anyone with a bathroom scale measuring in newtons can cast the first stone.  

As previously mentioned, driving force can't be determined solely based on the motor itself.  But assuming the rear wheel has a standard diameter, it's simply a matter of also considering gearing.  The gear ratio is always set up to provide a decrease in speed and an increase in torque.

Although driving force may be calculated from rear wheel torque and tire radius, that radius is not constant.  At high speeds, it grows due to centripetal force.  A trials tire is quite deformable due to the very soft sidewall and low inflation pressure (typically about 4 psi).

The circumference of a new Dunlop D803 is about 86.7", unweighted.  I use 82" as a reasonable working circumference. 

Some typical tire circumferences and their equivalent radii (rear axle heights) are shown below:

Getting this wrong can represent as much as a 6% discrepancy but it's more like 2% (worst case) when some deformation of the tire is assumed. 

This value affects not only the driving force calculation but also those for road speed versus engine rpm.  Using the same circumference for all bikes and all calculations mitigates this error source.

Also, note that no attempt was made to account for the loss in the gears.  A loss of 2% is reasonable for each mesh of gears.  Synchronous (aka toothed) timing belts are about the same, as are chain/sprockets in good condition.  There is a loss for the clutch swimming in gear oil as well.  Some sources report that a clutch never really locks up 100% either.  So, all the true driving forces will be somewhat smaller than reported.

Maximum Driving Force

For an ICE, maximum driving force occurs in first gear at the torque peak.  For an EV it is quite different due to the flatter torque curve and single-speed transmission.  Equating everything to road speed gives a basis for comparison.   For example:

EM 5.7 is essentially constant from 0 to 18 mph at ~300 lb-ft.

EM ePure race is essentially constant from 0 to 13 mph at ~425 lb-ft.

     Montesa 242 peaks at 500 lb-ft at 7 mph (1st gear).

Aprilia Climber peaks at 700 lb-ft at 8.5 mph (1st gear).

Montesa 315R peaks at 600 lb-ft at 10 mph (1st gear)

Montesa 4RT peaks at 550 lb-ft at 11.7 mph (1st gear)

GasGas TXT 300 has a long region of ~450 lb-ft from 7 to 14 mph (again, too good to be true?)