The mechanics of levers

[Lonerider]

Having got to grips with gearing using the bike gear calculator recommended by the good folk on here I find I still have a gap in my knowledge.

It relates to the chainring, and levers, the cranks. My Mercury has a 43t chainring, 17t sprocket and 165mm cranks. My gravel bike has a 40t chainring, 42 biggest sprocket and 170mm cranks. Based on the gear calculator the Mercury has two lower gears than the gravel bike. However, climbing hills on the Mercury is a lot harder than the gravel bike. Fully appreciate the difference in weight between the bikes, Mercury being heavier, but I'm thinking the problem I'm experiencing relates more to the cadence I can achieve on the respective bikes. I can't spin the pedals fast enough!

The Mercury has a bigger chainring with shorter cranks. Gravel bike, smaller chainring and longer cranks. I can pedal faster on the gravel bike. My, often flawed, logic suggests that using a longer lever on a smaller chainring results in easier pedalling and higher cadence. Does anyone understand the mechanics of levers?

I was going to change the Mercury gearing by putting a bigger sprocket on the hub but I have an inkling that reducing the chainring size would give a better result.

Ray

[Danneaux]

Generally, one can spin shorter cranks faster (higher cadence) and this favors people with a "spinning" pedaling style that is light, smooth, and pedals in round circles. A "shorter" crank would be something sub-170mm or so, say 165mm. I have a French folding bike dating from 1970 that is equipped with very short 145mm crankarms. Coupled with the S-A 3-sp IGH and 46mm/20in wheels, I can spin fast but sure can't produce the same torque I can with longer cranks.

170mm seems to be regarded as a mid-point between the shorter and longer cranks. This is a common length for road-bike cranks and until the MTB, was regarded as a common size on many if not most adult bikes.

Longer cranks favor "mashers" -- people who pedal at lower cadences in higher gears and with more torque applied to the pedals. A "longer" crank would be longer than 170mm, say 172.5, 175 and even up to 180 or more for very large size frames with appropriately high bottom brackets to prevent pedals striking the pavement, especially in turns. MTB bikes usually have higher bottom brackets for more ground clearance and are more often equipped with 175mm cranks. Given the varied terrain encountered off-road, a mashing pedal style is a good match.

Me? I'm of average size for my location and age group and find myself locked into 170mm cranks as most comfortable for my height and pedaling style with a hummingbird-like pedaling cadence of 110-120rpm for hours at a time with a fast, light, round cadence. I favor quadrants in the pedaling circle when I want to rest while riding or to emphasize development of certain muscle groups. if I try to ride longer cranks it makes my knees hurt, I can't pedal as fast and I feel like I am making too-big circles with my feet and legs.

If your cranks are compatible, it would be a fun exercise to swap arms and rings between the bikes so you could experience any difference firsthand.

Don't forget, your gearing makes a difference apart from crankarm length. You'd have to compare a given gear like-for-like to get some idea of how crank length affects your pedaling style and comfort.

Best,

Dan.

[martinf]

My own experience suggests that the actual size of chainring/sprocket doesn't make much difference to the ease at which the cranks can be turned, what matters is the ratio.

For a given ratio, tests show that a large chainring/ large sprocket is more efficient and more durable than a small  chainring/ small sprocket. So, unless there is another constraint, such as Chainglider compatibility in my case, I generally go for the largest practical chainring/sprocket combination. I experienced this in practice when I had a small-wheel Moulton with derailleur gears, the 11T sprocket ncessary to get a decent top gear with the largest chainring I could find at the time felt rough and wore quickly.

As far as crank lengths go, despite my 1m80 height I use 150 or 155 mm cranks, which I spin faster than the standard 170 mm cranks. By reducing the gear ratios, I get about the same speed as before I changed from 170 mm (now a few decades ago), with the advantage of no longer having the knee problems I had with standard cranks.

In addition to a higher average cadence, I found that I could vary cadence more than with long cranks, increasing if necessary if I need to go faster for a short period, or reducing on very steep hills if I have run out of lower gears. But crank length is personal, I knew a small lady who used 175 mm cranks without problems.

One issue with comparing a Mercury with a gravel bike is that the first has a hub gear while the second is a derailleur bike. If the hub gear is reatively new, it will probably be less efficient than the derailleur, as hub gears have a "breaking-in" period after which they are a bit more efficient than when new. The Rohloff is also most efficient in gear 11, and generally in the top 7, and a bit less efficient in the lower 7 gears, while a derailleur gear is generally least efficient in the highest gears with the smallest sprockets. There are charts on the Internet that show this effect.

[Moronic]

It miht be worth elaborating, Ray, on your report that climbing on the Mercury is a lot harder.

The shorter crank arms would decrease the torque you could produce for a given effort but we're talking only a couple of per cent and the Merc's shorter gearing would much more than make up for that.

Similarly, the weight differential will be small as a proportion of total weight, including rider.

So on its face, your report doesn't make much sense unless your Merc has some brake drag. But you might mean something that you haven't  captured explicitly.

[mickeg]

When it comes to bicycles, generally the word lever is either for a shift lever or brake lever.

Crank arms or crankarms are the parts that your pedals are attached to.  Some abbreviate crankarms to cranks.

[PH]

Re Chainring/sprocket sizes - The improvement in efficiency when using larger is measurable and quantifiable, not just the mechanical advantage, there's also the reduction in bending forces on the chain, this later being the reason they wear better.  Against that, there's the weight penalty.
Will you notice? I sometimes think I can, but in a blind test I wouldn't put money on it, not my own anyway.  You could maybe try it out for yourself, unless your derailleur bike is 1X the chances are it has duplicate gears, use a gear calculator to find a close match and see if it makes you fly 

Re crank lengths - I have no idea, I suspect a lot of it is personal preference with added familiarity.  I bought a bike from someone who kept the 165mm cranks, they were 6'+ and he said it vastly improved his riding.  I've always chosen 175's, but when I had a bike that came with 170's I never noticed the difference.  Maybe try it, it isn't an expensive experiment unless you go OTT.

[Lonerider]

Thank you for all the useful comments.

Martinf - your insights are very useful. Particularly the large chainring and large sprocket comment. Additionally, the comparison of efficiency between the Rohloff and a derailleur. I spend most of my time riding in 9th or 10th. Going slightly larger on the sprocket will put me firmly in 11th!

Mickeg - a newbie to the forum I may be. A newbie to cycling i am not. I am fully aware of the nomenclature used in cycling! My post clearly talks about cranks and crank length. Do you really need to point out that 'cranks are the parts your pedals are attached to?' The title of my post gives you a clue. The mechanics of levers! Any debate about the influence of crank length normally involves framing the discussion in terms of basic physics. When viewed in these terms a crank is considered a lever, and hence, any increase in the length of the cranks has the potential to provide the rider with extra leverage. I bow to your status on the forum but please refrain from being condescending!

Again, thank to those who took the time to comment in a constructive manner. Adieu and farewell!

[mickeg]

... The title of my post gives you a clue. The mechanics of levers! ...

After a few minutes of trying to figure out how brakes or shifters pertained to your gearing, I moved on.  It was later looking at Dans comments that I realized what was going on.