eliptical chainring - want to try one?

[GeoffApps]

That's quit impressive bobc.
However, to get the maximum benefit from that ring, you'd best turn it three holes clockwise (looking at the photo).
The crank in the photo would ideally be at about two o'clock with the ring in that position.
In the photo the crank is at about four o'clock.
Try it out, although you mention that it was not yet set-up correctly, so you may have already rotated it.

[bobc]

yeah - it was assembled without the benefit of a protractor. I was aiming for 110degrees but it looks like I got nearer 120 ;^) I'm sure my brother will do a proper job before putting miles on it.

[Brucey]

I think Geoff is saying crank at about 60 degrees when long axis is vertical would be right, rather than 110 or 120 degrees.

I'm not convinced oval chainrings are really of any benefit in any orientation myself, but if I had to, I'd fit them at about 110-120 or a touch more; IIRC peak torque comes at about this point or a touch later (see Whitt and Wilson).

I note that (IIRC) this is also roughly how Wiggo's wonky chainrings are set, too.

edit;

cheers

[GeoffApps]

I know very much from my own cycling life experience; what goes for the mainstream does not necessarily work for the individual. I find riding a 'normal' bicycle a horrible experience; clearly I am the odd one out, or am I?
Who bothers to ask non-cyclists? Non-cyclists are the majority...

My view is that peak power is reached at between 90 and 110 degrees with a gradual build-up period from around 40 to 90 degrees; with 15 degrees retardation, the major axis is brought into play at 75 degrees, towards the end of the power build-up period. As I reach and pass through the peak power phase, my leg/foot is not meeting resistance, but a gradually reducing gear ratio, facilitating full harnessing of my energy input.
This is my experience; I use slow cadence a lot, it helps my balance and control in tricky off-road situations that cannot be rushed at. The description above could describe winching myself up a steep slope over slippery tree roots, nearly at stalling speed.
Tacking along a main road in a time trial is, obviously, a completely different cycling experience. I can well imagine that with a higher cadence, where you're not fighting for every last Nth of traction, steerage and balance, you're better placed to meet rotational resistance during peak power; you have momentum on your side, and speed is the principle aim. My previous comment may therefore not be appropriate, but it would be worth the experiment.

The opportunities now presented by the combined technologies of CAD and laser-cutting give us wonderful opportunities to experiment, simply not possible until now; also, I believe the variety of riding techniques and style, as indicated above, could become much more recognised than it has been, because of the enlightenment brought about by that broader experimentation.

[Brucey]

this;



is how real pedal strokes are repeatedly found to look when studied. This happens to be LA's pedal stroke... but Whitt and Wilson report similar from non-cyclists IIRC. As you can see there is little evidence of great force being generated at (say) 60 degrees.

The force plot is however slightly misleading; it looks as if there is force being deliberately generated by the rider at a time and in a direction that is not productive, i.e. at about 5-o'clock, downwards. In part at least this is not deliberate, it is simple Newtonian mechanics at work; the (rather heavy) leg is moving downwards at speed and must decelerate by the bottom of the pedal stroke. It cannot do this unless there is a force applied somewhere and although this would be supplied by the legs themselves if mimicing the pedalling action in free air (i.e. without pedals) this would be counterproductive on a bike, so there is a simple inertial force through the pedal instead. It looks as if it ought to be inefficient, but it isn't necessarily as bad as you might think.

Geoff's argument sounds (in part) a bit like the logic behind Biopace to me. IIRC that system was designed primarily to help non-cyclists cycle better; rightly or wrongly, many well-practised cyclists didn't like it.

One of the problems with this kind of thing is doing accurate measurements. At lower speeds (when climbing for example) the relative speed fluctuations of the bike within each pedal stroke are likely to be very much greater than when the bike is going faster. The force plots that are usually generated in laboratories are perhaps more representative of high speed riding. If anyone knows of any that are better representative of lower speed riding and climbing, do say so.

cheers

[GeoffApps]

I'm not aware of any corelation between my point of view, and BioPace.
It's possible I may lie awake tonight worrying about it.
With regard to those highly scientific pedal schematics; I do not doubt their voracity for one moment, my personal reservation is that they are highly unlikely to relate to any cycling that I do.
With these tests that scientists do, the usual thing is for a relatively fit athletic young(ish) person to be placed in a racing crouch on a static bicycle on which they pedal at regular and high cadence, in a competitive manner.
This is so far from my own personal experience of cycling that they may as well be simulaing cycling on the moon.
That being said, what they do show indicates there is considerable room for experimentation around the optimum orientation for faster, high cadence riding.

[NATURAL ANKLING]

If anyone is interested, I could post the process I use in TurboCAD.

Hi,
Yes please I am interested how you do that.
Thanks,

[Brucey]

I'm not aware of any corelation between my point of view, and BioPace.

with biopace IIRC the 'low gear' part of the pedal stroke was also the part where the crank might have most mechanical advantage, and/or most people generate the highest forces. However I don't think the orientation of the oval was exactly the same as that you propose.

It's possible I may lie awake tonight worrying about it.
With regard to those highly scientific pedal schematics; I do not doubt their voracity for one moment, my personal reservation is that they are highly unlikely to relate to any cycling that I do.

That is kind of what I was driving at with my final comment in my previous post; I don't think those force plots necessarily well represent climbing/slow cadence/slow riding in sticky conditions etc. I would love to see some data that did represent this.

I have been pondering the whole topic and I suspect that it is only leg/pedal inertia that carries the bike through the dead spot. At high speeds this isn;t a problem, but at low speeds (road speed and/or cadence) it might be. Some of the forces generated vary as V squared, so half the cadence might mean 1/4 the kinetic energy, for example.

If you are going to pedal at a slower cadence, maybe it makes sense to allow the maximum leg speed just before the dead spot, so as to best carry through it.

I shall try not to lose sleep over this, but who knows...I might....

cheers

[bobc]

Those osymetric rings look interesting - there's no obvious reason to use a pure ellipsis. They seem to have gone for max radius over 70degrees or so of the crank rotation, with an essentially flat bit to reduce torque over the dead spot. Obviously they can't make it flat, there's no way a chain would stay on, so they put a "maximum radius" section in instead.
Even easier than an elipse to make if anybody fancies spending a tenner or so? (you've already got a computer & the likes of freecad will produce the design....)
I'll add the zipped dxf to this mail, pictures are above, it's 48 tooth with major radius 20mm bigger than minor radius and fits on a 5 hole 110mm PCD spider in 100 distinct angular positions.

[james-o]

I've used oval rings on my SS mtb for a year or so. The major axis is vertical when the drive crank is at 4.30pm. This seems to be the place where leg speed /momentum is higher so although leverage may not be at the maximum, power output should be. It may be better if it was variable by a few degrees each way to see if I can feel any benefit, but since I'm out of the saddle as much as seated when climbing I'm not to fussy about the exact angle. I've swapped between round and oval rings a few times and prefer the oval ring - it seems to let me ride a 2 teeth higher ring for similar perceived effort, as others using them report.

OP, if there was a possibility of buying a 34T ring with a defined ovalisation for a 104mm BCD MTB chainset I'd like to buy one, I'm doing a ride this summer that will probably wear out my Al oval ring so a stainless version would be perfect.


The Goldtec ring I use and a new Doval to try out - a dual oval / osymetric style with a 3-position adjuster via a seperate piece.


A basic comparison of shape where '+' is a greater radius.

[GeoffApps]

... so although leverage may not be at the maximum, power output should be...since I'm out of the saddle as much as seated when climbing

Two very good points there:
Power output verses leverage; these arrive at different zones of the rotation.
Riding out of the saddle moves the hip forward, which effectively moves the zone of peak power lower down.

[NATURAL ANKLING]

Hi,
I have now constructed an accurate model With "Autodesk Inventor" Cad Software, which is an vertual model on computer.
If anyone requires any model file output or a drawing file whatever, just holler and I will attempt to help.
You need to tell me all the details that you want -
Tooth number and variation of elipse eg, Max effective tooth number if it was a circular sprocket. The total number of teeth will drive the Min effective tooth number.
Max and min effective tooth number eg, 46 T x 38 T, this will drive the actual number of teeth on sprocket.
Spider size, and any adjustment for position of the sprocket to crank arms, with additional mounting holes.
I will construct the model in full so Material to spec and thicknes etc, etc,
The sprocket will be an elipse but I would also be possible to construct other curve shapes
I could also construct a sprocket / chainwheel which could be a linear over part and eliptical on other parts of the sprocket.
I just need the details in full. Free of course........................

[kendrick57]

hi
natural post an example of your files to make the chainring, compress them all into a zip file,these can be posted on the forum.

kendrick

[NATURAL ANKLING]

hi
natural post an example of your files to make the chainring, compress them all into a zip file,these can be posted on the forum.

kendrick

Hi,
SP EL is the Inventor file, only opens with Autodesk Inventor or similar CAD
SP EL 48 is a dxf file.
These are sample files only.

[kendrick57]

Hi
Will have a look at that later when at home should be interesting to look at.
Kendrick