Chain harmonics - what does this mean?

[fastpedaller]

moving off slightly........ What is the view on moving chainrings around to even out wear caused by the pedalling power fluctuation?
I know the chainrings are marked so they align for easy changing (Spa triple), I tend to use mainly the middle ring anyway, so a 'slow change' may not be a problem. If I move all 3 chainrings around by 1 or 2 positions ie 72 deg or 144 degrees etc etc, will this even out wear, but give a poor change? I've disregarded the effect of the pin on the outer ring between it and the crank, as It's never had to catch the chain (yet)!

[Samuel D]

Overall, (given a free choice) most folk are probably better off with a 46-50T big ring and a 13 or 14 to something cluster for touring purposes. Not only is it more efficient gear-for-gear (than smaller sprockets and chainrings), but if you are fussy about your gear gaps, 2T intervals in the cassette give you better spaced gears in the cruising range when you are using slightly larger sprockets.

It is a source of some annoyance that it is almost impossible to buy a suitable cassette off the shelf, BTW.

I agree with every word of this. The effect of these 11T-up cassettes is that I, with my 13–26T 8-speed cassette, have about as many useful gears as the cyclists I ride with who use 11-speed cassettes. I recognise a 13–26T cassette won’t suit everyone either, but the standard 11-speed cassette starting at 11T incurs all the downsides of 11-speed with no good upside! In combination with typical chainring sizes, it’s a waste of cassette width.

[Tigerbiten]

If you want a noisy drive train, I think mine can be set to be about as noisy as it gets.
I can warn pedestrians from behind that I'm approaching just from the noise .....

A Rohloff in 7th gear can make a slight noise due to it's internal 2.45 step down epicyclic gear spinning the fastest.
Now match it up with a Schlumpf HSD in overdrive mode (2.5 step up epicyclic gear) with a 53t chainring.
The whole lot resonates and sound like an electric motor.
It's a lot less noisy if it's on the 38t chainring or if I change gear down and quiet from 8th gear upward in either chainring while in overdrive mode.
The only thing I can think of to explain the noise is it's the sound of the Rohloff in 7th being extremely amplified somewhere in the drive chain while in overdrive mode and using the big chainring as the sound board.

[elPedro666]

I've often wondered about the efficiency of larger chainrings, in the early nineties I was shrinking my Shimano mtb setup a-la Suntour Microdrive to save weight and get the closest ratios I could with the widest overall range. I think the result was 20/33/47 with a pieced together 11-23t cassette. Large tables of ratios are how I used to pass the time in English lessons...

However, on my road bikes I invariably hang on to the big ring for as long as possible* as it feels smoother and more efficient; to this end I tend to err on the side of smaller outer rings (currently ranging from 46 to 50t). Hard to believe that I've never actually checked if this is a real thing or just psychological... If anyone can point me to actual information such as Brucey alluded to that would be appreciated - about time I got some facts I think!


*effort-wise; although I leave enough chain length etc that all gear combinations are possible, I will avoid cross-chaining in all but the most dire situations (again it just feels wrong). Generally I set front mechs to start to rub on the large ring and the third largest sprocket so that I have to trim the mech and know I'm running out of downshifts before I have to drop to the inner ring.

I'm a trendy consumer. Just look at my wobbly using hovercraft full of eels.

[Brucey]

regarding reading matter there are a couple of articles that are worth reading

http://www.ihpva.org/HParchive/PDF/hp51-2001.pdf

http://www.ihpva.org/HParchive/PDF/hp52-2001.pdf

However there are always possible errors in the way the tests are carried out and there are also discrepancies in the way the data is presented and analysed.

[From memory, I would not have come up with identical conclusions from the data that Kyle & Berto obtained, and I certainly would not have presented it the way that they did. For example presenting the derailleur gear efficiency data in a chart that plots efficiency against 'gear number' (where gear number is an artificial construct of some kind) is almost completely pointless and tells you nothing about what is really going on. If you look for trends with chainline and sprocket size there are real effects there that they did not allude to in their discussion and conclusions. Loosely, when you use the big ring the efficiency varies only little through the gear range; the effects of chainline and sprocket size almost cancel one another. By contrast on the small ring the small-small combinations suffer from a double-whammy of both poor chainline and reduced efficiency because of sprocket sizes effects. IIRC there is also a marked drop in efficiency in all gears that use the middle chainring of the triple chainset that they tested; I suspect that this is because the teeth are weird shapes (to help shifting) but they don't really mention that. The chain they use was (I think) PTFE treated, but they don't mention that when they discuss how efficient the transmission is when different lubrication strategies are employed. Plenty of folk have read that report and believe what they 'conclude' i.e. that bone-dry chains are still very efficient. The coating on the chain would reduce the cross-chaining losses whilst it was still there but it would not have lasted indefinitely. Since the same chain presumably would not have run on all the other transmission types (which normally use 1/8" sprockets in most cases with IGHs) it throws the comparison between derailleur and IGH into question too.]

Kyle and Berto may not have had (or may have had) a particular axe to grind but they were using borrowed facilities to do the tests, owned and operated by a company that did. They clearly didn't have enough time to do the tests they really wanted to, which is understandable, but that they didn't analyse their data more fully is IMHO a poor old do.

There is data presented on the Rohloff website that purports to demonstrate that their IGH is more efficient than a 3x9 derailleur system is. Well they are correct, if the derailleur system is operated by a cretin; said cretin uses the small-small combinations as they shift upwards through the gear range, then doubleshifts to the next highest ratio. This strategy completely avoids the use of the most efficient gear ratios available, which any averagely competent cyclist would not do. The Rohloff hub has many other virtues; they should surely not resort to deception to try and prove something that probably isn't really true...?

There is also data available from such websites as friction facts, but most of it is behind a pay firewall these days. The chap that does these tests appears to be competent and has no axe to grind in particular except perhaps that of making money from his data, which is a fairly recent development.

Note that for recumbent machines and others in the real world with complex transmissions, there are other factors which may skew the results; for example chains may become inefficient at high tensions (e.g. because of flex in the frame; this kind of thing is quite noticeable in tandems with crossover drives), long unsupported chain runs may develop lots of slack between pedal strokes and this may result in additional losses. Chain guides and pulleys may be lossy at high chain speeds in a way that does not happen with standard bicycle chain drives.

Many modern bicycle transmissions appear to be designed as if cross-chaining losses and losses arising from the use of small sprockets don't really exist; well they do and they are at least as significant as many of the other things that folk worry about. One of the 'marginal gains' made by the UK track cyclists at the Rio games was the use of larger chainrings and sprockets for any given gear ratio; not a new idea but one that netted them an estimated 0.5 to 1% increase in transmission efficiency for no real loss.

IIRC within the last few years a US magazine reviewed the options for a touring bike transmission and they concluded that -despite the added weight- a transmission using larger chainrings and sprockets was indeed the better choice overall. I agree with their conclusions, although I have not subjected their argument to minute scrutiny.

cheers

[brumster]

I seem to recall at the last Olympics the GB track Sprinters were running larger ( than traditionally normal ) chainrings - Marginal gains?

[Gattonero]

Can anyone help me understand Tinnishills comment "13t is a prime number, and so extends chain life by eliminating chain harmonics. " What does a prime number have to do with anything, and what are chain harmonics in the context of bicycles?

My opinion is that this is by large overshadowed by the way most of the bicycle chains do work: exposed to elements, so far from "ideal" working conditions.
Dust, grime, lubrication, play a very big part in the performance and life of a drivetrain.
Let alone those factors like chainline and -for single speeds- concentricity between the sprocket and the chainring, which affects the chain tension.

The latter, is something that is pretty obvious when you deal with many single speed bikes, hardly any has a good "round" drivetrain that won't give a noticeable tight-spot. This is far more important than "harmonics".

OTOH, is a good idea to not have chainring and sprocket to be a multiple of each, i.e. the often used "48x16t" gear ratio happens to be 3 to 1 (16x3=48), so for every rotation of the chainring the sprocket will do exactly 3 turns.
If "skidding" the rear wheel this is likely to wear the tyre in the same spot.

[busb]

I use snake oil on my transmission - I get no harmonics or even sub-harmonics!

[Gattonero]

I use snake oil on my transmission - I get no harmonics or even sub-harmonics!

I just ride a bicycle

[busb]

I use snake oil on my transmission - I get no harmonics or even sub-harmonics!

I just ride a bicycle

You don't to be doing that - it sends your legs into oscillation.

[elPedro666]

I use snake oil on my transmission - I get no harmonics or even sub-harmonics!

I just ride a bicycle

You don't to be doing that - it sends your legs into oscillation.

I feel like this conversation is just going round in circles now.

I'm a trendy consumer. Just look at my wobbly using hovercraft full of eels.

[Tigerbiten]

I just ride a bicycle

You don't to be doing that - it sends your legs into oscillation.

I feel like this conversation is just going round in circles now.

No it's not.
It's just having it's natural ups and downs.

[elPedro666]

You don't to be doing that - it sends your legs into oscillation.

I feel like this conversation is just going round in circles now.

No it's not.
It's just having it's natural ups and downs.

Well that's piston my suggestion!

I'm a trendy consumer. Just look at my wobbly using hovercraft full of eels.

[Stevek76]

Tangenting this thread (further?) off topic with some mild pedantry Strictly speaking the partial frequencies on piano strings are a little sharp of purely harmonic due to some physics. As a result piano tuning is also stretched a little wide in an attempt to make it sound as in tune as possible.

[Gattonero]

Tangenting this thread (further?) off topic with some mild pedantry Strictly speaking the partial frequencies on piano strings are a little sharp of purely harmonic due to some physics. As a result piano tuning is also stretched a little wide in an attempt to make it sound as in tune as possible.

Good thinking, but all goes down the drain on a transmission part that works in a variety of conditions, thus being subject to a lot of different forces and chances to wear or not working in optimal conditions.
No offense, but this is a bit OCD in my view: I'd like to see those bicycle chains in real use conditions, if a re kept in such pristine state that one needs to take a step further