What I always wanted to know about working efficiency, but never trusted myself to ask!
Wherever power is transmitted, friction is produced within the mechanical system.In general this firction is lost as heat and is refered to as power loss.
When observing gear systems, two different kinds of power losses can occur. These are power dependent and power independent losses. Power independen losses are e.g. the friction within the seals. Whether the bike is being pushed, ridden down a hill or ridden with extreme force upon the pedals up a hill, the friction force within the seals remains the same. Here the friction losses vary according to the condition of the seals (smooth or rough surfaces) and depending on the speed are very small producing between 1W and 3W of power loss. This small power loss is enough that it can bring a free spinning wheel to a halt after just a few rotations, or make the cranks rotate whilst the bike is being pushed.
The power dependent losses work completely differently. Whilst pedaling the friction loss is produced between the tooth flanks of the gears and all bearing in use. Every different gear system produces a different percentage of power loss depending upon the mechanical construction. When the working efficiency of a gear system lies at e.g. 95%, that means that the 5% of the transmitted power is lost through friction. So, when a rider is pedaling with 100W of input power, the rear wheel will only transmit 95W of output power onto the road because of the 5W power loss. When the input power is increased to 300W, the power loss will be increased to 15W.
Should the rider increase his tempo to a fast, high blood pressure sprint with 1000W of input power, 50W of will be lost through friction reducing the output power conciderably. This example shows clearly how important a good workig efficiency is. The input power given by the rider defines the amount of power loss. In the examples this could be 5W, 15W or 50W. Through special constructive developments like an increased number of teeth per gear and the choice of using roller bearings for the planetary gears, running the gearbox within an oil bath and extremely rigid construction of the overall gearbox, the working efficiency of the Rohloff SPEEDHUB 500/14 is brought up to 96% (for gears #1 to #7) and 98% (for gears #8 to #14).
This working efficiency is comparable to a derailleur gear system. Common seven speed gear hubs reach a working efficiency of approx. 90% (gear #1) and 98% (direct drive gear). Again we observe our test rider, now pedaling up a hill with 200W of input power. When using the Rohloff SPEEDHUB 500/14 or a derailleur gear system, he will loose approx. 4% of output power (when riding in gear #4 as this is comparable to the first gear of a seven speed gear hub) which is 8W. With the same input power but using a seven speed gear hub, our test rider will loose approx. 20W of output power. Adding approx. 2W of power loss caused by the seals for the Rohloff SPEEDHUB 500/14 would bring the overall loss of power with the Rohloff SPEEDHUB 500/14 upto 10W against 20W with the common seven speed gear hub. Conclusion: Power independant losses like the friction of the seals, will have the same effect as a lightly rubbing brake block. This brings a free spinning wheel to a halt but are barely noticable whilst riding.
This is easy to recognise on a bicycle repair stand. In contrary, power dependant losses mean that a particular percentage of the transferred power is lost, this is not so easy to detect on a bicycle repair stand, the measurement can only really be attempted with the bicycle being riden with the appropriate measuring aids. The Rohloff SPEEDHUB 500/14 has an extra fact to take into concideration: When the pedals are rotated backwards, the gearbox is activated, This means that the sprocket is rotated backwards and therefore the planetary gear system is run in reverse.
The sun gears, that are always fixed in position on the axle (when the wheel is being pedaled forwards), then rotate with a ratchet function against the planetary gears when pedals are rotated backwards, this in turn causes more friction. When the rear wheel is freewheeling forwards, the sun gears rotate with the planetary gears around the axle creating no excess friction. Now it is easy to see why the working efficiency of a derailleur and a hub gear system can not so easily be compared, so, away with the theory and back to the drawing board.
There are many Rohloff SPEEDHUB 500/14 equipped bikes in operation in special MTB marathon races and the results show that the riders are either equally as fast or even faster as the derailleur gear system riders. This shows quite plainly that there is hardly any difference between the two gear systems as far as working efficiecy goes. The faster times are results of the fact that the SPEEDHUB 500/14 riders have less stress over the shifting because the system is quicker and more positive to find the correct gear. Next to the working efficiency of the gearbox, the working efficiency of the human body is just as important.
Every rider knows that when you ride in the correct gear (the right pedaling frequence and force upon the pedals for the optimum riding situation) then everything goes forwards and you stay in a good mood. Approx. 25% of the breakfasts calories get burnt up in action through the muscles. If the gear selected is not correct, then the rider must increase the force applied upon the pedals (as if the rider has lead in the legs so that the burning of those calories is no longer at the optimal level) or increase the speed at which the cranks are being rotated (which will cause the pulse to rise and the blood circulation to rise up into the red area).
The working efficiency of the human body sinks to under 10%. Lots of sweat but very little action will be produced and the mood of the rider will sink along side. Alongside the good working efficiency of the mechanical parts is the use of the correct gear for the riding resistance (which is also just as important), so quick gear changes and a large choice of different gears. Only with these factors; overall working efficiency, breakfast, human body, bicycle and speed is the posibility of success and fun achieved.
This is the reason that the SPEEDHUB 500/14 was constructed.