Explaining the efficiency

All you ever wanted to know about Working Efficiency

Wherever power is transmitted, friction is produced within the mechanical system. In general this friction is lost as heat and is referred 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 independent 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 works completely different. Whilst pedaling the friction loss is produced between the tooth flanks of the gears and all bearings 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 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 5W power loss. When the input power is increased to 300W, the power loss will rise 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 considerably. This example shows clearly how important a good working 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 increasing 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 7-speed gear hubs are reaching only 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 lose approx. 4% (8W) of output power, when riding in gear #4, which is comparable to the first gear of a 7-speed gear hub. With the same input power but using a 7-speed gear hub, our test rider will lose 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 up to 10W against 20W with the common 7-speed gear hub.

Conclusion: Power independent losses, e.g. like the friction of the seals, will have the same effect as a lightly rubbing brake block. I will bring a free spinning wheel to a halt but is barely notable whilst riding, which is easy to recognise on a bicycle repair stand.

In contrary, power dependant losses mean a particular percentage of the transferred power is lost, which is not so easy to detect on a bicycle repair stand, and a proper measurement can only really be attempted with the bicycle being ridden provided with the appropriate measuring aids. The Rohloff SPEEDHUB 500/14 has an extra fact to take into consideration: When the pedals are rotating backwards, the gearbox is activated. This means the sprocket is also rotating 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, now away with the theory and back to the drawing board.

There are many Rohloff SPEEDHUB 500/14 equipped bikes performing in special MTB marathon races and the results show the riders are either equally fast or even faster as the derailleur gear system riders. This shows quite plainly there is hardly any difference between the two gear systems as far as working efficiency is concerned. The faster times are results of the fact that SPEEDHUB 500/14 riders have less stress with 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, as long as you ride in the correct gear (the right pedaling frequency and force upon the pedals for the optimum riding situation) everything goes forward and you stay in a good mood. Approx. 25% of your breakfast's calories get burnt up in action through the muscles. If the gear selected is not correct, the rider has to increase the force applied upon the pedals (which might feel, as if the rider has lead in the legs, so the burning of those calories is no longer at the optimal level) or increase the speed of the rotating cranks, which will cause the pulse to rise and the blood circulation rises up into the red area.

The working efficiency of the human body will decrease to under 10%. Lots of sweat but very little action will be produced, and the mood of the rider will sink along with it. Nearly important as a good working efficiency of all mechanical parts is the use of the correct gear for the riding resistance, so quick gear changes and a large choice of different gears are possible. Only with these factors; overall working efficiency, breakfast, human body, bicycle and speed the opportunity of success and fun is achievable.

That's why the SPEEDHUB 500/14 has been constructed.


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