Categories: Technique

Sur la Plaque: Mechanics of the Big Ring

I’ve been riding for long enough to know that what “feels” good and and what “is” good in terms of technique are two independent sets with a small intersection; it’s very important to put a lot of thought and research into what you’re doing to make sure it offers a benefit. Research takes “work” and “time”, so I’m not very fond of that approach. Instead, I like to do a lot of “thinking”, leveraging both my inadequate expertise in mechanics and my unusually high degree of confidence in my ability to reason in order to jump to conclusions that benefit my initial assumptions.

For example, I believe there is an advantage to riding sur la plaque, or in the big ring, as opposed to riding in the same size gear on the small ring. I generally find that when I’m strong enough to stay on top of my gear, climbing in the big ring feels less cumbersome than when I climb in the small ring at the same speed. The downside is that it is like playing a game of chicken with your legs; it works very well if you are able to keep the gear turning over smoothly, but should you fall behind the gear, and your speed evaporates as you fall into a spiral of downshifting and decreasing speeds (not to mention morale).

All this can be explained away by having good legs or not (un jour sans), but I think there is a mechanical advantage as well.

First, there is the duration of the effort. As they say, it never gets easier, you just go faster, but I firmly believe faster is easier, provided you are strong and fit enough to support the effort. The faster you climb, the less changes in gradient and road surface impact your speed. Not to mention that while all athletes perform the same amount of work when they cross over the same climb regardless of the duration of their effort, athletes doing so in less time suffer for a shorter period of time than do those who go slower. Marco Pantani claimed that despite knowing the suffering that was just around the corner before his attacks, he was motivated to go as fast as possible in order to make the suffering end sooner.

Second, there seems to be a mechanical advantage of riding in the big ring. I’m a little bit hazy on the physics here, but it seems to me that the crank arm is in effect a second-class lever and, while maintaining the same length crank arm (lever) and fulcrum (bottom bracket), by moving into the big ring, you are moving load farther out on the lever, providing a mechanical advantage over the small ring.

WikiPedia defines leverage as:

load arm x load force = effort arm x effort force

In our case, since the speed is constant, that means that the load force (to turn the pedals) is also constant. And, since the load arm (crank) is a fixed length and the effort arm length is increased when moving the chain to the large chainring, the effort force is reduced in order to maintain a balanced equation, meaning that it doesn’t just feel good to ride sur la plaque, it actually is good.

All that said, this theory completely ignores the energy loss of bending the chain as you start to move the chain from straight at the center of the cassette towards the edge of your cassette, in particular when riding in the big ring and crossing to bigger cogs. Q-Factor has an impact on how much your chain is bending as you ride in bigger and bigger cogs, but I think there’s a measurable loss if you are crossing your chain completely (big ring to biggest cog); and I suspect is is entirely possible that the big ring’s mechanical advantages are outweighed by losses in chain friction.

frank

The founder of Velominati and curator of The Rules, Frank was born in the Dutch colonies of Minnesota. His boundless physical talents are carefully canceled out by his equally boundless enthusiasm for drinking. Coffee, beer, wine, if it’s in a container, he will enjoy it, a lot of it. He currently lives in Seattle. He loves riding in the rain and scheduling visits with the Man with the Hammer just to be reminded of the privilege it is to feel completely depleted. He holds down a technology job the description of which no-one really understands and his interests outside of Cycling and drinking are Cycling and drinking. As devoted aesthete, the only thing more important to him than riding a bike well is looking good doing it. Frank is co-author along with the other Keepers of the Cog of the popular book, The Rules, The Way of the Cycling Disciple and also writes a monthly column for the magazine, Cyclist. He is also currently working on the first follow-up to The Rules, tentatively entitled The Hardmen. Email him directly at rouleur@velominati.com.

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View Comments

Nathan Edwards says:

Jun 6 2010 at 2:46 pm

Ok Physics done... If the ratio of chainring to cassette is the same (basically if you are pulling the same length gear - how do you work out that inches thing for gears?) it doesn't actually matter how big either of them are, what actually matters is the length of the cranks compared to the radius of the wheel. Longer cranks mean less force required, but as Jarvis pointed out you have to push through further and less responsive. So it probably makes no difference at all.
frank says:

Jun 6 2010 at 3:19 pm

@Nathan Edwards
Are you taking into account the placement of the fulcrum? I mean, in your equations, are you also taking into account where on the crank the load is being placed? To make an extreme example, if you put the chain directly on the bottom bracket, you'll have very little leverage to turn the gear. Conversely, if you put the chain on the pedal, you'll have loads of leverage. The length of the crank is only a part of the equation.

Also, we're all ignoring the distribution of the load across the chain ring.

Also, here's chart on the friction table for gear ratios. Here's an interesting block I just found in a blog post that linked to this article (which also claimed my assertion in the article is bullocks):

Big is Better, By Mick Hamer

The difference between winning the cycling gold medal at the Olympic Games in Sydney in 2000 or settling for the silver could depend on something as simple as the size of the bicycle's sprocket wheels. To save weight, world-beating designs of bicycle all try to minimise the size of the two sprocket wheels that carry the bicycle chain. However, Stuart Burgess, a mechanical engineer at the University of Bristol, has proved that this design philosophy is wrong. Tests by Burgess have shown that larger sprocket wheels are more efficient than smaller ones, because larger wheels reduce friction in the chain drive, which is more important than the marginal increase in weight. "Designers have concentrated on minimising losses from what they can see," says Burgess. "They can't see the chain losses." When Chris Boardman broke the world distance record for cycling in Manchester in 1996, he completed 56·38 kilometres in an hour-breaking the record by more than 1 kilometre. In a paper to be given to the Engineering of Sport conference in Sheffield in July, Burgess says that if Boardman had doubled the size of his sprocket wheels, he would have added 100 metres to his record. Although the increase in efficiency is small-equivalent to a saving of 6 seconds over 25 miles-a few seconds can mean the difference between first and second place, says Burgess. Modern aluminium sprocket wheels are so light, he says, that doubling their size is relatively unimportant. "It is extremely marginal, no more than fractions of a second over 25 miles," says Burgess. Far more important is the impact that larger sprockets have on friction in the chain drive. Competition cyclists typically have a chain wheel with 52 teeth and a sprockets with 13 teeth on the rear wheel. Burgess tested two different sizes of sprockets to demonstrate his point. One pair of sprockets had 26 teeth on the chain wheel and 13 on the rear wheel. He also tested double-size sprockets, with 52 teeth on the chain wheel and 26 on the rear wheel. At 50 kilometres per hour, the force on the chain with the smaller sprockets is about 45 kilograms. The force on the double-size sprockets is half that. "The cyclists is still putting in the same torque, so if the radius doubles the force would be halved," says Burgess. The reduction of the force on the chain is accompanied by a similar fall in the frictional force in the chain. Each link of the chain is coupled with a pin that fits inside a roller, and the chief source of sliding friction is the pin sliding in the roller, says Burgess. In his tests, Burgess showed that doubling the sprocket size increased the efficiency of the chain drive from 98·8 per cent to 99·4 per cent. Even so, doubling the size of the sprocket wheels may not be practical for all competition cyclists, says Burgess, because there has to be clearance between the chain wheel and the ground. But competition bicycles could easily be made with sprockets that are up to double the normal size, and racing cyclists could order to larger sprockets tomorrow simply by going to a specialist supplier
brett says:

Jun 6 2010 at 3:26 pm

@Jarvis "compact's generally come in 175."

Wrong. The crank lengths are determined by bike size. I've just done a scan of all the bikes on the shop floor, and for e.g Tarmacs (standard) and Roubaixs (compact) all have the same crank lengths for each comparitive size. Small (52cm) have 170s, Mediums and Larges (54, 56cm) have 172.5 and XLs (60cm) have 175.
frank says:

Jun 6 2010 at 3:28 pm

@brett
Have you ever found one in a 177.5? My failure to find them has kept me out of the potential client pool for a set myself.

It's a shame, too, because cranks are so easy to switch these days, it would be nice to engage in some pedal doping every now and again.
brett says:

Jun 6 2010 at 3:42 pm

Dura Ace compact comes in 177.5 and 180. SRAM Red comes in 177.5, and also offers a 52/36 compact.

Campy doesn't offer a 177.5.
frank says:

Jun 6 2010 at 3:51 pm

@brett
That's what I thought.

One more thing on the compacts before I retire from this absolutely awesome conversation:

The argument that a compact has less rotating mass is ridiculous; given the size of a crankset, unless you're comfortable spinning at 4356 rpm, there can't be a measurable difference. It makes a difference in wheels which, at 700c are much bigger than a crank, and rotate several orders of magnitude faster than you pedal. But on a crank set? I'm not buyin' it.
david says:

Jun 6 2010 at 3:53 pm

frank :One other thing to keep in mind, however, is that being overgeared is also bad. Despite the physics, you have to match your gearing to your ability to turn it. I have gradually been shifting to lower gears, and my speeds increase on the steepest slopes. For more on that, just watch Basso and Evans on the Zoncolan.

Of course I'm not, and I doubt Jarvis is, advocating 50 rpm big gear climbing. Like I said, the science is clear. Higher rpms on climbing is better. But surely spinning up hill at 90 rpm in a 39x23 is better than spinning up hill in a 39x25.
david says:

Jun 6 2010 at 9:19 pm

@Marko Sorry. Didn't see you were quoting someone there. "The tradeoff is easier climbing as the expense of a small loss of top speed. This is the lightest configuration for either set up." Which, any way, I still assert is crazy, either for a road racer or a tri-guy. There's some strange stuff in that article I"m not familiar with. Because fat is the most available source of energy, you should pedal at 90 rpm? Huh?
frank says:

Jun 6 2010 at 10:03 pm

@david
I'm with you, David. That article makes some big jumps that I'm not following. I'm not a physio, but I always thought many of the things the article attempts to standardize were based on physiology and vary pretty significantly from person to person. Oh, well. I guess next year every pro will be riding a compact at 90rpm, all day, every day.
david says:

Jun 6 2010 at 10:11 pm

frank :@david
Oh, well. I guess next year every pro will be riding a compact at 90rpm, all day, every day.

Then Brett will be able to move all his compacts off the floor, and get a new bike for himself.