mechanical advantage

[Blodwyn Pig]

if there IS a real gain, why don't the pros run a 70T chainring, with correspondingly large rear cassette?

[frankly frankie]

Shimano tried to introduce a smaller chain pitch of 1cm (down from 1/2") to make the whole drivechain smaller and lighter.  They seem to have failed.

It would have broken one of the last Great Universal Standards (bike chain pitch, camera tripod thread, plastic drink bottle top).

[Samuel D]

if there IS a real gain, why don't the pros run a 70T chainring, with correspondingly large rear cassette?

It’s a good question, precise chainring size aside. The pros are finally starting to use larger chainrings, e.g. 54T is common on flatter stages. However, more would probably be gained by going larger still.

I think there is still a counter-productive weight obsession in the pro ranks as there is elsewhere.

And the pros can only use what the manufacturers (often sponsors) can supply. Shimano does not make very large chainrings, presumably because there wouldn’t be much of a market for them. Thus the pros can’t use them.

Another consideration is aerodynamic. At the very high speeds of pro racing, the drag of the large chainring (and chain, and high front derailleur, etc.) may offset the small gain in efficiency from the large sprockets.

One other thing to consider is that the drivetrain tests may not tell the whole story in that they are usually done without a derailleur in the run. Larger chainrings increase chain speed and therefore losses in the derailleur pulleys. However, this is a small effect and can itself be offset by larger pulleys with better bearings, as many professionals use anyway.

[Brucey]

AIUI the friction facts tests are carried out with a tensioning mech.

 As I mentioned the differences in the results would be even greater (between chainrings) without this.

cheers

[Samuel D]

I’ve just checked and you’re right: the test was done with a derailleur in place. The model was not specified so we don’t know the pulley size and bearing type although we can guess (Dura-Ace).

[ElyDave]

My current on-road set up is

52/39/30
12-28 (12-13-14-15-17-19-21-23-25-28)

So the way I'm reading this, for reasons of straightline efficiency AND larger sprocket efficiency, I'm better off when in audax cruising mode in the middle ring and the 15-21 range, which is where I habitually sit anyway giving me a nice easy 25-28kmh at 90rpm.

When climbing, I'm seemingly better off moving to the 30T up front when I'm getting to around 23-25T on the rear, which might mean a 30x21 to start with, slightly lower gearing, but more efficient?

On the flat/downhill/following wind, I'd be better off moving up to the 52T on the front and 15-14-13 etc on the rear rather than progress much beyond 39 x 14

[Brucey]

the way I see it....

well sort of; as I mentioned earlier, cross chaining to the left, even a fair amount (on a bigger chainring/sprocket with lower chordal losses) is usually more efficient that cross-chaining the right at all. The results graph I posted very clearly shows this. This means that 52/19 is almost certainly slightly more efficient than 42/17

Better than either is of course running a straight chain run.  One of my favoured triple setups looks like a half-step chainset (ie both middle and outer chainrings are quite large) but isn't. The result is that in the cruising gears (which are used most) there is a good choice of ratios that both have a straight chain run and don't suffer excessive chordal losses.

A good test is to see which gear ratios in any given setup are most efficient and then to ask yourself if

a) you could get your most used gear ratios in a more efficient fashion and

b) whether the extant most efficient gear ratios are indeed ones that you would use much.

I think that with your extant setup the answers are 'yes' and 'no' respectively, unless you spend most of your time riding at either over ~23mph or under ~14mph. With your setup I would immediately ditch some of the smaller sprockets thus reducing the cross-chaining losses in every sprocket (that you use much whilst cruising) whilst on the big ring.

cheers

[ElyDave]

Thanks,

I tend to spend more time in the middle ring than anywhere else, typically in the 17/19/21 at 25-30kph, trying to keep a reasonable cadence of 90-100.  As per the other thread, I'm going to evaluate this weekend with a view to changing the 12-28 for a 13-30 SRAM cassette, I think the medium cage 105 rear mech should be OK with that.

The SRAM 12-32 gives  12,13,14,15,17,19,22,25,28,32 - which would have more cross chaining to the left in the middle ring, gives me a lower bottom gear, but does nothing for the relatively unused top end.

[Brucey]

BTW I also think it very likely that whenever there are (avoidable) chordal losses, (through using a setup with small chainrings and sprockets to give a certain gear ratio), there are likely to be other losses that are not accounted for in most transmission efficiency tests.

Specifically I am referring to the fact that when there is any chordal effect, either the output or the input has to vary in speed as each roller passes over each tooth on the sprocket. My bet is that, on a real bicycle going at a brisk pace, it is not the output that varies in speed, but the input.

If so, one's pedals, cranks, feet and legs are fluctuating in speed which is unlikely to be a process that conserves energy (in much the same way as riding on a bumpy road doesn't either). The smaller the sprocket the greater the variations in speed that are seen.

For example if riding 36/12, (assuming each power stroke occupies a quarter turn of the cranks) there will be 36/4 = 9 fluctuations in speed during the power stroke.

Each fluctuation's size (in percent) is given approximately by

100x (1- Cos (360/2n))  where n is the tooth count on the sprocket.

In the case of a 12T sprocket, it is about 3.4%.

If the same gear ratio is achieved by using 45/15 instead there are ~10 fluctuations of ~2.2% instead, and if the gear is 55/19 (which is not the exact same ratio, but close) there are ~13 fluctuations each of ~1.35%.

Because the change in kinetic energy of the fluctuating element is dependent on changes in (V1^2 - V2^2) this effect is unlikely to be insubstantial. For example if V2 is 3% less than V1, the change in kinetic energy is of the order of 6%.

 So if using a 12T sprocket there is a loss in the power stroke of

~ 9 x 6.7%

by contrast if using a 19T sprocket the losses are closer to

~13 x 2.7%

The latter quantity is only a little over half the former. 

So far as I understand it, until now, no-one has attempted to allow for this (side-)effect of chordal losses in bicycle transmissions. It might be at least (if not more) significant than the pure mechanical losses.

 If so, the chances of feeling a change (because of chordal effects) that only measures about 0.5% on a test rig are likely to be higher, as indeed is the variation in real speed.

cheers

[Brucey]

Thanks,

I tend to spend more time in the middle ring than anywhere else, typically in the 17/19/21 at 25-30kph, trying to keep a reasonable cadence of 90-100.  As per the other thread, I'm going to evaluate this weekend with a view to changing the 12-28 for a 13-30 SRAM cassette, I think the medium cage 105 rear mech should be OK with that.

The SRAM 12-32 gives  12,13,14,15,17,19,22,25,28,32 - which would have more cross chaining to the left in the middle ring, gives me a lower bottom gear, but does nothing for the relatively unused top end.

Indeed. My suggestion is that you would (literally) be better off with an 8s setup or a shortened 10s cassette, running (say) 13,15,17,19,22,25,28,32, because then you could use 52/19,22 for cruising (without incurring cross-chaining losses) which would be more efficient chordally than 42/17,19 etc.

[edit; BTW moving the chainline to the left will have a similar effect and is a good dodge too; shame that modern chainsets  don't allow this to be adjusted so easily.]

It is all a matter of priorities; for any kind of real racing losing the 12 and 14 sprockets might be a disaster, but for anything that isn't racing, it almost certainly won't be.

cheers

[grams]

that is a simplification. Fortunately there is some test data that casts some light on this;

What's the input power on this? Discussing watt losses without knowing watts input (or at least speed) is completely meaningless!

[Samuel D]

250 watts. Buy the report! It’s well done.

Power losses as a percentage of input will be greater at the lower power outputs typical on audax rides.

[mzjo]

the way I see it....

well sort of; as I mentioned earlier, cross chaining to the left, even a fair amount (on a bigger chainring/sprocket with lower chordal losses) is usually more efficient that cross-chaining the right at all. The results graph I posted very clearly shows this. This means that 52/19 is almost certainly slightly more efficient than 42/17

Better than either is of course running a straight chain run.  One of my favoured triple setups looks like a half-step chainset (ie both middle and outer chainrings are quite large) but isn't. The result is that in the cruising gears (which are used most) there is a good choice of ratios that both have a straight chain run and don't suffer excessive chordal losses.

A good test is to see which gear ratios in any given setup are most efficient and then to ask yourself if

a) you could get your most used gear ratios in a more efficient fashion and

b) whether the extant most efficient gear ratios are indeed ones that you would use much.

I think that with your extant setup the answers are 'yes' and 'no' respectively, unless you spend most of your time riding at either over ~23mph or under ~14mph. With your setup I would immediately ditch some of the smaller sprockets thus reducing the cross-chaining losses in every sprocket (that you use much whilst cruising) whilst on the big ring.

cheers

This reminds me very much of an article that I read in the CTC magazine ("Cycletouring"?) in a YHA sitting room nearly 50 years ago. At the time we rode 5sp freewheels (or 4sp for the impoverished amongst us) and aspired to two chainrings. The article advocated triple chainrings and only 3 sprockets on the freewheel (giving effectively 7 gears, if one eliminated big-big and small-small, with minimal cross-chaining - against 8 gears with considerably more cross-chaining for a typical 5x2 gearing set-up). Chainline was given a lot more importance in the late 60's and 70's it seems to me (and has probably contributed to my rejection of the current war of numbers in cassette design).
Brucey, how would you design a 3x3 system with 85" top and 25"-30" bottom (better 25")? The middle (in line) would have to be 60-65". It would be quite easy to build using single-speed conversion cassette spacers to correct the chainline (although the advantage of reduced dish would be lost with a cassette hub).

Edit: it doesn't work with a 65" middle gear, it needs to be closer to 50" to give a sufficient number of lower gears.

[Brucey]

re a 3x3 system, I think you can make a decent one that uses a half-step between the middle and big rings and a larger step between the middle and small chainrings. Something like this;

http://www.gear-calculator.com/?GR=DERS&KB=25,42,50&RZ=13,19,28&UF=2185&TF=90&SL=2.6&UN=MPH

but in this case it only makes sense if you use the 'cross-chained' gears...

cheers

[mzjo]

re a 3x3 system, I think you can make a decent one that uses a half-step between the middle and big rings and a larger step between the middle and small chainrings. Something like this;

http://www.gear-calculator.com/?GR=DERS&KB=25,42,50&RZ=13,19,28&UF=2185&TF=90&SL=2.6&UN=MPH

but in this case it only makes sense if you use the 'cross-chained' gears...

cheers

I don't know this calculator but I think that representation is wonderful; it really shows me the way to go. I personally wouldn't want to go that high with this sort of set up, 85" is plenty high enough for me (which means a ring between the 42t and the 25t). I will play with that, I have a bike that could be converted!

[Bianchi Boy]

Reading this in detail reminds me of a wise old cyclist from the first club I ever joined who said "the ratio you use the most should be on the large ring in the middle of the cassette"

Enough said

BB

[Brucey]

re a 3x3 system, I think you can make a decent one that uses a half-step between the middle and big rings and a larger step between the middle and small chainrings. Something like this;

http://www.gear-calculator.com/?GR=DERS&KB=25,42,50&RZ=13,19,28&UF=2185&TF=90&SL=2.6&UN=MPH

but in this case it only makes sense if you use the 'cross-chained' gears...

cheers

I don't know this calculator but I think that representation is wonderful; it really shows me the way to go....

I think it is a good gear calculator too. You can play with the settings easily enough but a few things to note;

- if you want to post (or save) a link to a scheme you have been working on, the link to copy is the one in the bottom right of the page, not the one at the top

- for some reason it will let you display gear inches but it won't let you link to a version that displays this as a default

- you can compare two setups if you like

- you can add/drag chainring/sprocket elements around using the cursor.

- the chain angle appears to be calculated using an assumed chainstay length that appears not to vary with wheel size

- the range of allowed chain angle limits is a little smaller than I'd like; if you want to vary it (or any other parameter) beyond the limits allowed by the graphical interface, you can manually edit the url if you take notice of the code within. For example you can include chainrings and sprockets with fractional values (which need is occasionally  real but complicated to explain why)

- the calculator is one of the few that supports the use of an IGH with multiple sprockets etc

cheers