Frictional losses: standard versus compact chainwheels

Dorian Gray

I was always under the impression that frictional losses in the geartrain increased as the number of teeth went down, due to the chain having to bend more acutely around a smaller radius (so more frictional losses in the bushings). In other words, a 53 x 17 would be more efficient than a 50 x 16. Is this the case?

If so, why are so many people going for compact chainrings? I use a 53/39 crankset and when I'm humming along on the flat the chain tends to somewhere around the middle of the cassette, so ideal for avoiding an inefficient chain angle. I suspect this is the case for most half-fit cyclists. Given the above, it seems that using a compact double in the same situation would only decrease geartrain efficiency.

Is the compact revolution a canny ploy by the manufacturers to sell us new stuff or is it actually the better choice for most cyclists?

Monty Dog

For a well-lubed transmission, you're only talking losses of no more than 1-2% which proably amount to a couple of watts at the most - when 80% of your energy is expended overcoming air resistance, then it's not going to matter too much in the greater scheme of things.

Dorian Gray

I know overcoming aerodynamic drag is where the vast majority of one's power goes, but is it nevertheless the case that wheels with fewer teeth cause more friction? I realise it would be a tiny and probably negligible difference.

My point, I suppose, is that compact chainrings don't seem to make much sense given that at cruising speeds a 53T chainring seems to provide the ideal chain angle.

Climbing the Alps may be a different matter, but we don't exactly have mountains like that in most of the UK, yet people everywhere are flocking to compacts. Why?

LangerDan

I think that if it was in any way significant, your transmission would have seized up due to the far tighter radii experienced as the chain wound its way around the cassette and through the jockey wheels of the derailleur. If there are increased losses with a compact, they may be counteracted by the fact that the smaller chainring may allow them keep the chain in the middle of the cassette with a straighter (lower friction losses)chainline.

Dorian Gray

LangerDan wrote:

I think that if it was in any way significant, your transmission would have seized up due to the far tighter radii experienced as the chain wound its way around the cassette and through the jockey wheels of the derailleur.

Well the losses around the chainring (53T versus 50T) are presumably microscopic, but using the smaller chainring forces the use of a smaller sprocket too, e.g. 53 x 17 is equivalent to 50 x 16. It's the 16T versus 17T that intrigues me. The difference should be even greater on the small chainrings because compacts have a bigger difference between the large and small rings.

The losses around the pulleys would be vastly lower because the chain there is under very low tension.

LangerDan wrote:

If there are increased losses with a compact, they may be counteracted by the fact that the smaller chainring may allow them keep the chain in the middle of the cassette with a straighter (lower friction losses)chainline.

But isn't a slightly smaller cassette (overall) the norm with compacts, therefore you'd still be around the same area of the cassette? In any case, I find the chainline to be pretty much perfect at cruising speeds with a standard setup.

Just food for thought. The more general question is, why are compacts so popular all of a sudden? I'm in my mid-twenties so I'm not some old fuddy duddy railing against these modern contraptions, waxing nostalgic about the good old days of grinding up hills in 42 x 21. It just intrigues me that all of a sudden everyone feels the need to convert to a compact double. Have the hills in our little country been recently forced up by plate tectonics? :P

penugent

I'm in the process of whipping off my 53 and replacing it with a 49. My reason is simple, I have a chronic knee problem and so get almost nil use out of the 53:11 or 55:12 ratios as grinding along leaves me with an ache - I'm hoping that I will get some use out of 49:11/12 without increasing the load beyond the capabilities of my knee. I am not technical at all, but am just experimenting to see what happens - maybe others are the same.

chrisw12

A couple of points in no particular order.
1)Remember also that a smaller chain ring will be lighter and perhaps more aero, again small differences.
2) You are right about drivetrain efficiency of a bigger ring iirc there's a guy on the tt forum (a very good tt'er by the way) Nik Bowlder who has something like a 60+tooth cr he's so convinced of the advantages.
3) I run a 55x11 on my tt bike, not because I'm super fast but because I'm more likely this way to have the chain on the middle cogs. It does mean that when training I have to change down to the small ring perhaps more than needed, but the way new front quality derailer work these days, this isn't a problem.
4) Did compacts come about because (indirectly) of the Lance effect. People started to realise that the best way to get up mountains was to use the same cadence as they would on the flats instead of going to a lower cadence. Therefore much smaller gears were needed which you could only get by going less teeth on the chainring

John.T

There are several reasons why I have converted to a compact chainset. I first fitted one when riding the Raid Pyrenean in 2003 and was very glad of the 34/27 bottom gear on the Tourmalet and other climbs. I went back to my 53/39 set up but realised that I was only using the small ring except for races and was just wearing out the 39 ring and the 15 and 16 cogs. With 50/34 and 12/27 10sp I have a wide range of usable gears (I almost never used 53/12) and am using all the cogs and rings so they will last longer. I have never noticed any increase in friction and am sure that it will not be worth bothering about. Don't forget that if there is more friction per pin with a smaller cog there are more pins per cog with a big one. I cancels it out somwhat.
I also am not a fuddy duddy wingeing about anything. I am a 64 year old rider. If you don't fancy compact then don't use it. Simple.
The main reason for using large chain rings for TTs is that a combination of larger rings and larger cogs gives closer gear ratios as the percentage difference in teeth is less.

Dorian Gray

John Tiffany wrote:

I also am not a fuddy duddy wingeing about anything. I am a 64 year old rider. If you don't fancy compact then don't use it. Simple.

Certainly, and I have no problem with people going for compacts; I'm just wondering why. And by "fuddy duddy" I just meant people who prefer the traditional methods and equipment, for no particular reason other than it was around when they were young (a valid reason in its own right, I hasten to add). Clearly you're not one of them and age has little to do with it in any case (I value tradition quite a bit myself, for instance, which is maybe one reason I personally prefer a standard double).

John Tiffany wrote:

The main reason for using large chain rings for TTs is that a combination of larger rings and larger cogs gives closer gear ratios as the percentage difference in teeth is less.

That makes sense. If you're fast enough to need the 12T sprocket in a time trial, it's quite a jump to 13T and especially 11T. Makes sense to move up to the bigger sprockets by using a larger chainwheel, where the difference to adjacent gears will be smaller.

By the way, George Hincapie mentioned after yesterday's TT in the Tour de France that he'd been using his 55 x 11 gear for most of the course. Crazy.

Airmiles

Overall I think it's to do with the trend away from bunch racing and towards longer/hillier events. My personal reasons for going compact were

1) coming from an MTB background I prefer spinning to grinding up hills, and particularly on longer rides/audaxes/sportives where your legs seem to stay fresher, longer if you avoid grinding.

I did the Maratona 106k on 34/27, (in 5:51) and I think anyone much less fit would've been better off with a triple - I passed literally thousands of people grinding along on 39/25 - and had plenty of people on triples twiddle past.

2) I don't race, so rarely ride in a bunch, so am going at slower speeds and/or higher watts for a given speed - which again argues for lower overall gearing. On a compact at, say, 95rpm at 19mph I'm probably hitting the same leg force per rev as someone in a bunch on 53/39 doing 25mph......

Finally, if pros pushing out 6 watts/kilo or more ride on 53/39, then surely it follows that anyone knocking out less than 34/39 * 6 i.e. 5.25 w/kg ought to be on a compact????? Or do you want people to think your other coach is a Ferrari?

Dorian Gray

Airmiles wrote:

Finally, if pros pushing out 6 watts/kilo or more ride on 53/39, then surely it follows that anyone knocking out less than 34/39 * 6 i.e. 5.25 w/kg ought to be on a compact????? Or do you want people to think your other coach is a Ferrari?

Well, if we're going to get that technical about it, we should remember that the relationship between power and speed is far from linear. In fact, doubling one's speed requires 8x more power (4x more due to the exponential increase in drag, 2x more due to covering twice the distance per unit of time). Therefore the power relationship between 34 and 39 would be (34^3)/(39^3) = 0.66, so if a pro athlete has 6 watts/kg then a compact double would suit someone with less than 4 watts/kg. Don't need Mr Ferrari's assistance to beat that!

But this may be overanalysing it. :P

Thanks for your thoughts though, very interesting.