Cycling UK Forum

my brother asked me to knock up a 48 tooth elliptical chainring. I'll get this made (laser cut) in 2mm stainless

the major & minor radii are 107mm and 87mm. The ring of holes near the middle are 110mm PCD so the thing will bolt straight onto most 5hole crank spiders.
Weight will be 200 grams.
lotsa holes so you can play with angles and swapping right/left or rotating 180degrees both give a stack more options.
They will probably cost about £15 including postage, maybe less so if anyone wants to try one, leave a reply on here by tuesday lunchtime - that's when I'll be round at the laser place.
It's as easy for me to get 5 cut as one...... ;^)
You will have to chamfer the tooth sides (file or grind) and clean off any flash. There are none of the "easyglide" ramps and bumps to aid front deraillieur changing.

Very, very interested to know how you are calculating the centres of the tooth cut outs. Since the demise of EggRings, I have been musing with the idea of CNCing elliptical chainrings, but laser-cutting seems to be the obvious solution to low / custom volume production. The bit I was having trouble with was calculating the positions of the tooth cut-out centres for a given number of teeth and eccentricity.

If you can design and produce smaller ones from 2mm stainless to my spec for anything like this cost, I'm very interested and I'm probably not the only one.

Hi DR,
many many ways of skinning that cat - I'm using an old solidworks seat for the basic design & in this case the chain roller "circles" are always dimensioned to be 12.7mm centres.
You can make the sum of the distance to 2 focii constant to force them onto an elipse
you can draw an elipse ("for construction") & constrain the centres to be on that line
you can make the distance from the chainring centre vary sinusoidally round the ring
Whatever you do you need to make whatever defines the elipse do be a "driven dimension" to allow the ring's roller centres to join up on the 12.7mm spacing.
I just worked out 90degrees & mirrored the result twice.
The lead-in part of the teeth can be centred on the next roller notch - on circular rings I allow an extra degree of lead-in, but on this that was too hard ;^)

If you want something custom PM me, I can probably help you out; the laser cutters are just a local firm at the end of the ind. estate where I work, nothing special, there will be several local to you too!!!

How about cassette sprockets?
You could probably sell those.

Elliptical cassette sprockets would probably induce great confusion ;D

I've finally bullied Excel into calculating the figures for me using its solver. Glad that I was right that it's a constraint / optimisation problem, I was tearing my hair out trying to find a solution otherwise!

I might give FreeCAD a try to see if it can do something as nice as yours...

With Chris Bell ceasing production of EggRings, I realised I had to figure out how to make my own.
After riding quite radical EggRings for the past 23 years, I discovered that returning to circular chainrings made the arthritis in my left knee flare up terribly, even on a ride of less than ten miles.
Laser-cutting chainrings works very well, and is relatively cheap, as long as you don't cost your own (valuable) time into the equation.
Getting the ellipse correct is relatively straightforward, provided you are conversant with computer-aided design (CAD) software and have time for a lot of trial and error.
I notice that Wiggo's TDF oval chainring had no retardation. This may have been intentional, depending to his saddle position. However, the average rider will benefit from a retardation of about 15 degrees. A recumbent rider will have to work out their required retardation by relating the angle of their hip position to the vertical.
If you are having a single chainring laser-cut from 2mm stainless steel, you should have no real problems mounting it.
If you're planning multiple rings, watch out for issues regarding the distance between each ring.
There may also be problems with the chain catching on the (now prominent) chainring bolts; usually recessed with a counterbore of 1mm.
If anyone is interested, I could post the process I use in TurboCAD.

I had another look at that Wiggo Chainset and notice that it is retarded by about 15 degrees.

Also interested to see the finished product of course. What grade of stainless are you using?

GeoffApps wrote:With Chris Bell ceasing production of EggRings, I realised I had to figure out how to make my own.

Did you end up laser-cutting them Geoff? Interested to know what grade of stainless or alloy you used and how they fare durability wise.

Now I have the calculations working in Excel (for teeth a multiple of four at least - other values shouldn't be too hard) I think I can probably script FreeCad to produce a blueprint for cutting for a given number of teeth, eccentricity, offset angle, BCD etc. But first I need to design a decent tooth profile to be replicated around the ring.

As you say, you have to do it for the love of it...!

DoctorRad wrote:

GeoffApps wrote:With Chris Bell ceasing production of EggRings, I realised I had to figure out how to make my own.

Did you end up laser-cutting them Geoff? Interested to know what grade of stainless or alloy you used and how they fare durability wise.

Now I have the calculations working in Excel (for teeth a multiple of four at least - other values shouldn't be too hard) I think I can probably script FreeCad to produce a blueprint for cutting for a given number of teeth, eccentricity, offset angle, BCD etc. But first I need to design a decent tooth profile to be replicated around the ring.

As you say, you have to do it for the love of it...!

Yes, laser-cut from 2mm stainless steel.
Seem to work fine, but I've yet to get a magnifying glass out and give them a close inspection.
Multiples of four mean you have a complete number of teeth in a quadrant.
Multiples of two mean you have a number of teeth plus one half-tooth in a quadrant.
Odd numbers mean it would probably be wisest to work a half-ring (with a half-tooth at the end) to avoid any drift, which there shouldn't be, but just in case.

Tooth profile is from the man who should know, Chris Bell: two 8mm dia circles centred 12.7mm apart; draw a construction line parallel to the centreline of these circles 3mm outside; from the midpoint draw a 1.5mm line both ways; draw a tangent from each circle to the ends of the 1.5mm lines; trim perpendicular to the circle centres to the tangent and join polyline of the tooth profile, step and repeat.

I'll see if I can find time to make a drawing and post it later.
What we're talking about here is actually an elliptical polygon with sides of 12.7mm.
In my researches, I checked Wikipedia for the formula to ascertain the circumference of an ellipse.
Lots of mathematical formula type stuff that meant nothing to me, and at the end it said that this will only provide (or deliver in modern parlance, GRRR!) an approximation.
So, I'm quite astonished that you are confident that you can script all this stuff; I'm not interested in producing a variety of sizes, but my nephew is building a CNC milling machine with this eventual purpose in mind, and such tools would be very useful for him.

Tooth profile would be useful if possible, thanks, a close-up magnified screenshot from a CAD system would be great if possible.

You're right, there is no method for calculating the positions of the polygon vertices exactly, it must be done iteratively using a numerical optimisation model.

I now have an Excel spreadsheet up and running for 24, 25, 26, 29 and 37 tooth chainrings. Any other sizes should be easy. If the number of teeth is divisible by four, you need only calculate for a quarter-ring. Any other number of teeth has one axis of symmetry, as you say with a half-tooth if the total number is odd. The spreadsheet requires Excel 2010 as that version has a vastly improved 'Solver' (numerical optimiser) over previous versions.

The optimisation aims to make the distances ('D') between each adjacent pair of vertices of the polygon equal given two fixed vertices. I use this as a constraint in the optimiser, but as it ultimately needs a single figure to work manipulate, I set things up so that the average 'Delta D' is minimised, where Delta D is the absolute difference between subsequent D values. The results are then scaled to a 12.7mm chain pitch and the resulting X/Y co-ordinates spat out in a list to far greater tolerance than could ever be cut or machined.

The scripting engine in FreeCAD should then be able to place and orientate the correct number of teeth into a CAD model and add bolt holes at the correct orientation, but I need to learn to use FreeCAD before I can implement that.

Good luck learning CAD, it can be a minefield.

I've been at it for about a year and just beginning to feel some confidence.

Hi
I am the nephew.
A CNC milling machine is being made capable of making oval chainrings any size required, I intend to use aircraft grade aluminium 3 and 4 mm thick depending on the application.
The tooth shape I will be using is the same as was used on eggrings including derailleur gates if required.
Kendrick

hi
sprockets were mentioned at the start of this thread, its correct eliptical sprockets are not wanted but having round sprockets made of hard to get sizes might get some interest, custom spacers will probably be required as well.

Kendrick

hi
As asked for a picture of a tooth bulit around the golden length of 12.7mm.



kendrick