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Maybe someone (perheps even myself) have already covered this: My own experiences from under-motorized vehicles is that high weight can actually be beneficial in some situations. Mass in motion can act as a "linear flywheel" and help overcome smaller inclinations that would otherwise require downshift and loss of speed. An under-motorized vehicle is very sensitive to gearing ratios and spacing so the need of downshifting to go uphill may cause an "inverted avalanche effect" if the gear spacing is too high. My three-wheeled 1959 Heinkel Kabine is a good example of that. -Just a very little tail wind can help me keep över 70km/h on 4:th gear in the same uphill slope that otherwise will force med to shift down to 3:rd gear where I get good torque at less than 50 km/h.
When I ripped out two pistons from my 1981 Fiat 127 I got a lot of experiences from heavy vs light load in the same road conditions and can definitely say that heavy weight IN AVERAGE makes a lot less impact on travel time than windy weather.
With a high weight-to-power ratio it will offcourse take a longer time to reach cruising speed, but the weight will then help to keep that speed.
Low weight will definitely be more beneficial to a vehicle mainly operated in city traffic. As I see it there are only two factors that will make high weight a loosing concept: Braking and tyre friction. -A heavy vehicle will add more friction to the tyres, perhaps demanding wider tyres to carry the load, increasing friction even more. I suspect however a wider tyre with high pressure may be better than a thin tyre running almost flat. I also guess a typical MC-tyre with it´s rounded shape is a lot more sensitive to weight and tyre pressure than the flat shape of a car tyre. Perhaps a narrow car tyre may be a good solution even for a two-wheeler if driven mainly on highway? I think some cornering qualities can be sacrificed to fuel efficiency.
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