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Weight-to-Power Ratio

Created 2008/08/01, modified 2011/02/25
Feedback welcome; contact, email daveclarkecb@yahoo.com


Weight-to-power ratio (WTPR) usually applies to vehicles. It is the weight of the vehicle, in kilograms, divided by the vehicle's power, in kW. (The inverse of WTPR, power-to-weight ratio, is conventionally used for engines rather than for vehicles.) The maximum acceleration available to any vehicle depends directly on its WTPR, halving the WTPR doubles the potential acceleration.

In general, the greater the weight-to-power ratio the more fuel-efficient, but the less 'responsive' will the vehicle be. Most engines work at their greatest efficiency when they are operating near their maximum power. If you must have a vehicle that will get from 0 to 100km/hr in a few seconds you need one with a low weight-to-power ratio.

 
The numbers on this page are approximate, if any reader believes that they are wrong I'd be pleased to hear about it.

The power developed by a bicyclist can vary greatly depending on the fitness of the rider, and of course a cyclist can produce a relatively high amount of power for a short time.

Also see payload ratio.

An electric-assisted bicycle motor might develop 200 Watts (0.2kW). A typical, smallish, motor vehicle of the early twenty-first century might weigh a little over a tonne, say 1050kg, and have an engine capable of developing 70 kilowatts; 350 times the power of the electric bicycle. The bicycle and passenger might weigh 100kg, the car and passenger 1120kg. So the WTPR of the bicycle, in kg/kW, is 500 (100/0.2), while that of the car is 16 (1120/70). The bicycle rider could add another 0.2kW, decreasing the WTPR to 250 (100/0.4).

Some figures in table form
'Vehicle'WeightPayloadTotal weightPower (kW)Ratio
Small car105070 11207016
Bicycle107080 0.2400
Elec. bicycle3070100 0.2500
Elec. bicycle and
rider pedling
3070 1000.4250
Oscar W59 00024 000 83 000126 917
The Oscar W is a Murray River paddle steamer built in 1908 at Echuca.

So, some people find a WTPR of 250 or even 500 acceptable, while others (most of us?) demand something nearer 16. Why? Are we really in such a rush that we must have high levels of acceleration available to us? Would it really matter if it took an extra 5 seconds to accelerate to 60km/hr?

I need hardly say that the greenhouse gasses produced by recharging the electric bicycle would be tiny compared to those produced by burning the fossil fuel that powers the car, with 500 times the power.

Why can we not buy cars with WTPRs closer to the electric bicycle? I believe the fact that they are not available indicates that we are not yet taking climate change/greenhouse seriously.

The responsiveness of a vehicle can be improved by either increasing its power or by decreasing its weight (either way, by decreasing its WTPR). Why, then, are we not seeing more ultra-light vehicles on the roads; they could both provide for peoples' desire for relatively high rates of acceleration and produce less pollution.

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