# CHANGE ROUND TO OVAL

A letter from our reader V. Zasyadko prompted the idea to go back to the old idea of replacing the round sprocket of the bike Ellipso, which leads to the alignment torque generated by the efforts of the cyclist when pedaling. According to cyclists, the idea deserves attention, and the embodiment of her life — not too time consuming even at home. The recommendations V. Zasyadko calculation ellipse-tion of stars and of its manufacturing technology.

According to people who exploited the bikes with ellipsanime stars, when pedalling the chain sags unusual that supposedly can cause it to jump off. In fact, the chain is not slack, but simply changes its position in space, and four times for one revolution of the rod. As a result, when certain conditions (speed of rotation of pedals, road condition and so on) can start its resonant oscillations. To avoid this phenomenon it is necessary to set the lower horizontal tube of the frame of a simple bike roller chain guide.

For many years I have kept the article “the Ellipse for the two wheels” (“modelist-Konstruktor” No. 7 of 1982). And just recently I had implemented this idea, making for bike Ellipso,or rather oval, approaching to elliptical, star. Otezdil two seasons and was very pleased. Increased throttle response of the bike, it is better to be able to slides, less tired when riding long distances.
The star I attached to the already existing on the bike. To increase rigidity, has welded thereto a steel sheet of a smaller size. The bolts connect stars supplied remote sacontrol bushings and washers with otherkey.

Fig.1. The construction of the dividing ellipse:

R is the radius of the arcs of the transition, z is the radius of the main arcs; L — length of the ellipse, M width of the oval.

Fig. 2. The formation of the sprocket teeth.

Fig. 3. The relative position of the crankset and sprockets.

Fig. 4. Installation diagram of the stator circuit.

The number of teeth on the oval of stars was taken to be 46 instead of the proposed in article 48. In this case, the preserved center-to-center distance. When the drive chain changes although the amount of sagging, but nevertheless, does not fly.

I would like to clarify something. Given in that article, the size of a star is not to do not fit into the pitch ellipse 48 steps of 12.7 mm.

In this regard, here is the calculation for any number of teeth and further manufacturing technology.

The calculation of the pitch of the oval

Original data: L – length of the ellipse, M width of the ellipse, t is the chain pitch, z — number of teeth designed oval sprocket, z1 – 1,26 z — number of teeth in the full circle described by the radius R (standard stars).

Then

In this case:

L= 1,7915 R; M = 1,411 R; z = R/1,88.

All coefficients are derived assuming that L= 1,27 M.

For example, for t = 12.7 mm and z = 46 obtain the following values: L = 210,3 mm; M = 165,6 mm; R = 117,4 mm; d = 62,11 mm.

For t = 12.7 mm and z= 48, these values are respectively equal of 218.6; 172,1; 122 and 64,9 mm.

The technology of manufacturing sprocket

Draw on thick paper separating the oval.

Install on the caliper size of 12.7 mm.

Remove the caliper size drawing of the meter with a screw adjustment. In this case the tip of a needle to put a zero on the risk scale, and the tip second — first the risk of Vernier.

Check meter is placed whether on an oval is drawn dividing the calculated number of teeth. If necessary, adjust the line of the oval.
To meter the contour of the oval, making the drawing of the puncture needles.

Attach the drawing to a sheet of steel with a thickness of 1-2 mm and transfer the marks on the metal core, razor-sharp.

Remove the drawing and deepen the resulting holes with a Kern. Drill them holes with a diameter of 2 mm. This steel sheet will serve as a template in case if you want to make a few stars.

For sheet steel with a thickness of 2-3 mm which will be made a star, to move a tag with the core custom-tailored hole in the template.

To make notches and stake points on the line of the tops of the sprocket teeth (Fig. 2).

Drill holes holes pitch oval — first drill with a diameter of 2-3 mm, then 8 mm.

Trim the workpiece at the points of the line peaks.

To prepare the teeth with a file.

With the new (unstretched) chain to verify the correctness of teeth, bright features of the chain around the sprocket. If necessary, correct the teeth.

To attach the star, focusing it against the rods, as shown in figure 3.