A4 scale = 70%
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A4 scale = 70% |
EGGring Ovals page 4
A more accurate question would be "What ovality?" since even round chainrings are oval - a circle is just an oval with 0% ovality, just like a square is a rectangle with equal sides. So, on that basis... maybe Ovals aren't quite so outrageous after all!
It may be hard to believe that a round chainring doesn't provide a smooth power transfer to your rear wheel, since 'round' would seem to be the ultimate in smooth shapes - but this misses the point. In engines which convert linear motion to rotary motion (which is what you are doing when you pedal), a flywheel is usually included to smooth out the uneveness and its removal should prove the point to any sceptic. The jerky power transfer on a cycle is most apparent when you pedal up a steep hill and lurch forwards with each stab on the pedals. The more Oval your chainring is, the less pronounced this becomes.
The graph shows how the potential to transfer power to your rear wheel varies during one complete crank revolution, for the same size round and Oval chainrings. The variation between the extremes is reduced by the Oval and the resultant curve is smoother.
On the graph, notice that the Oval's maximums are wider with less time wasted between them. This represents an opportunity to transfer more energy to your rear wheel if you still have more to give (ie. there's a greater area under the Oval's curve.) However the basic mechanical efficiency remains unchanged as your aerodynamics and rolling resistance are not affected in any way.
Nonetheless research done in some academic institutions has suggested an increase in physiological efficiency but, although it remains a distinct possibility, this hypothesis remains unproven.
However an Oval does allow you to work harder for short periods, especially when pedalling slowly. So, although you will be able to climb hills that defeated you before, it will still be you that has to do the work - you don't get something for nothing!
The manufacturers of Osymetric Oval chainrings claim a 5% to 15% increase in efficiency. We would be delighted if this were really so and would be fascinated to see evidence of their research!
This specifies how out-of-round an Oval is in percentage terms and indicates how much bigger the major-axis is than the minor-axis. Round chainrings have 0% ovality.
ovality = ( major-axis - 1 ) × 100%
minor-axis
The table of ovalities has been compiled after trials on- and off-road. Road racers can use round outer rings because they spin their pedals fast and pedal flat-out already. At the other extreme, offroad trials demand the utmost control at low speeds and benefit from the use of very oval chainrings.
Although you should generally choose the same number of teeth as you would for round chainrings, the smallest Oval that will fit your cranks will be a few teeth larger than the smallest round ring. This is because an Oval is narrower across its minor axis and the teeth inevitably pass closer to the fixing bolts at these points. The minimum number of teeth may be estimated as follows:
smallest Oval = smallest round where V = ovality (%)
( 1 - A×V ) A = 0.004 if 1,3 or 5 fixing bolts
or A = 0.0025 .. 4 or 6 .. ..
| © Chris Bell, 2004 |
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| HIGHPATH ENGINEERING Cornant, Cribyn, Lampeter, Ceredigion, SA48 7QW, Wales, UK phone / fax: +44 (0)1570 470035 (UK office hours only) email: 
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