Just another cabin fever diversion.
The scenario is simple. You're emerging from a mild cruise toodle in a metro area, to the open highway. Or, you're wanting to pass around a slow truck. And, stay in 5th gear. What kind of acceleration can be expected when throttling up from about 45mph to 70-80mph, all while staying in top gear?
You and the bike present a total mass that has to be accelerated. The engine produces torque that's multiplied by the primary, trans, and final drive. An overall ratio that applies torque to the rear wheel, which produces a forward thrust to accelerate you and your bike.
The premise here is oversimplified, but somewhat reliable. Amazingly enough, a well designed 4-stroke motorcycle engine, in good condition and tune, will produce a torque value in ft-lbs that almost equals its displacement in cubic inches. Our 40 Cubic Inch XS650 engines should produce roughly 40 ft-lbs of torque, in the low to mid-range rpm zones, about 45-75mph in 5th gear. Outside that zone, the effects of restriction, reversion, and scavenge supercharging, provided by the cam and breathing systems, will affect the engine's volumetric efficiency (VE), and will nudge the torque value up/down a bit. Similar for other engines.
As an example, this early XS1 performance chart shows a torque curve that's somewhat depressed until it reaches higher rpms, attaining its 40 ft-lb torque, attributable to the more radical early 256 cam.
This dyno chart, provided by member Crashcourse, shows the flatter torque curve (tan line) of the later 447 engines, just below 40 ft-lbs.
Dyno charts of other bike engines also show a close relationship of displacement to torque. Some of the newer, hi-tech engines show nominal torque values a bit above their displacement size, attributable to the advancements in engine tech.
If you ever study a dyno chart, and see torque values that violate this general relationship, be suspect.
So, wanting to keep this thing simple, I chose to do comparisons based on engine displacements, expecting their torque values to comply with this general concept. The alternative would have been to acquire realistic torque charts, and convert them to a lookup table. The various, wild, and inconsistent published charts out there make that impossible, from my end anyway.
So, gyrating around with the physics and math, conversions, accommodations for rolling and wind resistances, wheel sizes, the whole complicated formula boiled down to a conveniently simpler version that closely predicts the expected acceleration within the typical tall-gear cruise region, of about 45-80 mph.
A = Acceleration in MPH per second
CC = Total engine displacement in Cubic Centimeters
TGR = Top Gear Ratio, the overall drive ratio from engine to rear wheel
BikeWt = The actual curb, or wet weight of the bike, in running order
RiderWt = Total weight of the rider, plus all the things worn and in pockets
BikeWt + RiderWt = The overall total weight of this thing that's rolling down the road
A = CC * TGR / (BikeWt + RiderWt)
The scenario is simple. You're emerging from a mild cruise toodle in a metro area, to the open highway. Or, you're wanting to pass around a slow truck. And, stay in 5th gear. What kind of acceleration can be expected when throttling up from about 45mph to 70-80mph, all while staying in top gear?
You and the bike present a total mass that has to be accelerated. The engine produces torque that's multiplied by the primary, trans, and final drive. An overall ratio that applies torque to the rear wheel, which produces a forward thrust to accelerate you and your bike.
The premise here is oversimplified, but somewhat reliable. Amazingly enough, a well designed 4-stroke motorcycle engine, in good condition and tune, will produce a torque value in ft-lbs that almost equals its displacement in cubic inches. Our 40 Cubic Inch XS650 engines should produce roughly 40 ft-lbs of torque, in the low to mid-range rpm zones, about 45-75mph in 5th gear. Outside that zone, the effects of restriction, reversion, and scavenge supercharging, provided by the cam and breathing systems, will affect the engine's volumetric efficiency (VE), and will nudge the torque value up/down a bit. Similar for other engines.
As an example, this early XS1 performance chart shows a torque curve that's somewhat depressed until it reaches higher rpms, attaining its 40 ft-lb torque, attributable to the more radical early 256 cam.
This dyno chart, provided by member Crashcourse, shows the flatter torque curve (tan line) of the later 447 engines, just below 40 ft-lbs.
Dyno charts of other bike engines also show a close relationship of displacement to torque. Some of the newer, hi-tech engines show nominal torque values a bit above their displacement size, attributable to the advancements in engine tech.
If you ever study a dyno chart, and see torque values that violate this general relationship, be suspect.
So, wanting to keep this thing simple, I chose to do comparisons based on engine displacements, expecting their torque values to comply with this general concept. The alternative would have been to acquire realistic torque charts, and convert them to a lookup table. The various, wild, and inconsistent published charts out there make that impossible, from my end anyway.
So, gyrating around with the physics and math, conversions, accommodations for rolling and wind resistances, wheel sizes, the whole complicated formula boiled down to a conveniently simpler version that closely predicts the expected acceleration within the typical tall-gear cruise region, of about 45-80 mph.
A = Acceleration in MPH per second
CC = Total engine displacement in Cubic Centimeters
TGR = Top Gear Ratio, the overall drive ratio from engine to rear wheel
BikeWt = The actual curb, or wet weight of the bike, in running order
RiderWt = Total weight of the rider, plus all the things worn and in pockets
BikeWt + RiderWt = The overall total weight of this thing that's rolling down the road
A = CC * TGR / (BikeWt + RiderWt)