A non circular chainring is completely specified by giving four characteristics: the number of teeth, the ovality, BCD and number of crank arms.
Number of teeth (e.g. 52 teeth or e.g. 38 teeth)
Install preferably oval chainrings with the same number of teeth as the circular ones mounted on your racing bike. The average gear ratio is identical, but the efficiency with PowerOval® is very much better.
Is the ratio of the largest to the smallest diameter of the PowerOval®.
This is e.g. 1.25. Usually written as 25 %.
Strive for the highest possible ovality to reach maximal efficiency gains.
But, the oblique course of the derailleur chain (misalignment) with 10- and 11 speed drivetrains, combined with the use of pronounced non-circular chainrings mounted on particular bike configurations and when cycling in rough conditions, may cause a chain drop off the outer ring when shifting towards the smaller cogs. Indeed in these circumstances the limits of the technical possibilites are reached.
For this reason the ovality of the PowerOval® has been limited to 25%
and the shape of the PowerOval® is optimized for “zero-chain -drop-off”
Indicates the Bolt-Circle-Diameter of the crank.
This is the diameter of the circle going through the centers of the bolts of the crank.
BCD is e.g. 130 mm or 110 mm (Compact).
It is also possible to derive the BCD by measuring the distance (say X) between two consecutive mounting holes for the bolts. See the figures.
Shimano ( -compatible) cranks:
X = 76.4 mm (5-arm crank) then BCD = 130 mm.
X = 64.7 mm (5-arm crank) then BCD = 110 mm
X = 90 mm / 63 mm (rectangular) (4-arm crank) then BCD = 110 mm
Campagnolo (only made to order)
X = 79.4 mm (5-arm crank), then BCD = 135 mm
X = 64.7 mm (5-arm crank), then BCD = 110 mm
The choice of the BCD also determines the ovality and the (minimum) number of teeth on the inner ring. Conversely the ovality and the number of teeth determine which BCD is possible.
As an example, a 42 teeth chainring with 20% ovality is possible with BCD 110 and with BCD 130. However with an ovality of 30% the 42 teeth oval can only be manufactured with BCD 110.
Mounting on the 5-arm crank BCD 130 or BCD 110
*5-arm crank without screw through the crank arm.
Position the chainring so that the “notches” in the inner edge of the outer ring are at the top. Rotate the crank arm approximately perpendicular to the direction of the seat tube. Take care that a screw bolt is positioned
in e.g. R or TT or …. as indicated on the outer ring.
The inner ring has to be joined to the outer ring so that the “notches” in the inner edge of the inner ring correspond to the “notches” in the inner edge of the outer chainring. Notice that the engraving of the inner ring must be oriented towards the frame.
*5-arm crank with screw through the crank arm.
Position the chainring so that the “notches” in the inner edge of the outer ring are at the bottom. Rotate the crank arm approximately perpendicular to the direction of the seat tube. Take care that a screw bolt is positioned in e.g. R or TT or …. as indicated on the outer ring.
See above. Analogous.
Mounting on the 4-arm crank BCD 110
- Disassemble the crank spindle from the bottom-bracket.
- Slide the outer ring over the crank spider and ensure that the oval chainring is positioned in that way that the “chain lay-on tab” is below the crank arm and that the 4 ends of the crankspider are located in the milled cutouts (small diameter enlargement of the inner edge). See photo 1.
- Rotate the PowerOval® chainring, counter clock-wise, until the correct mounting position (R or TT or…, your choice) is reached. See photo 2.
- Now position the inner ring (the engraving of the inner ring must be oriented towards the frame) and screw both chainrings with the supplied bolts.
- The mounting of the PowerOval® on the crank spider is correct when having the major diameter of the PowerCurve vertical, the crank arm is approximately perpendicular to the direction of the seat tube.
The bike frame and the crank spindle should allow the passage of the (inner-)chainring along the rear wheel fork.
Normally the outer chainrings provide no problem.
Possible solutions in case the chainring drags against the lower rear wheel fork:
-install a washer between spindle and crank so that the spider-crank shifts slightly outwards.
-install a larger crank spindle
*Adjustments to the height of the front derailleur.
The height of the front derailleur fork is to be adjusted to less than 2 mm above the passage of the largest chainring diameter.
In case of a shifting fork with clamping strap the adjustment can be done without any problem. In case of a fixed (brazed-on) front derailleur and a large outer oval chainring (e.g. 54 T) the mounting of a derailleur adaptor may be required. This adaptor can be ordered together with the PowerOval® chainrings. See “Accessories“.
*Front derailleur adjuster types
-rod shaped adaptor
-wedge shaped adaptor
The wedge shaped adaptor raises the front derailleur, tilts it and moves it backwards in a manner that the lower edge of the shift fork sinks approximately 9 mm.
*Adjustments to do when the chain drags in the front derailleur shifting fork.
Chain drags on the lower edge.
When using a small inner chainring with large ovality (e.g., 38 teeth and 25% ovality), it may happen that the chain, at the lowest gear (smallest diameter vertical), drags on the lower edge of the shift fork.
This can be solved
*by installing the appropriate adaptor type (see above).
*In case the adaptor doesn’t solve the problem a front derailleur with longer shifting fork must be installed.
The chain drags against the inner edge of the shift fork
In case the chain drags against the inner edge of the shift fork: widen the derailleur cage by the mounting of some thickness washers or deform the shift fork slightly.
The noise level of the PowerOval® rings seems to be different from ordinary round chainrings. This is caused by the special shape and ovality. A well lubricated chain is recommended.
For justification see “Science“.
Seat tube angle is about 73°
Crank set up in “R” (Road) position.
*Time Trial Bike
Seat tube angle is about 80°
Crank set up “TT” (Time Trial) position.
*In case of a highly forward seating position a “correction” can be done by sliding to the next mounting position: “R” to “TT” and “TT” is shifted clockwise to the next mounting hole.
Rotate the crank so that the largest diameters of the rings are vertical. If the crank arm is approximately perpendicular to the direction of the seat tube, the chainrings and the crank are mounted correctly (rule of thumb).
The PowerOval® non-circular chainring is yielding a crank power gain by a favourable kinetic efficiency effect. This efficiency effect occurs from around 50 rpm and becomes significantly larger from 80 rpm onwards. It continuous to grow “exponentially” with higher speeds.