Lew.

Right - I would say I have quite a good knowledge of physics - arguably a big factor into why cycling is my sport of choice. But this is something I can't comprehend.
But I recently read an article on Velodromeshop.net that claims:
If I'm correct, 47x14 and 50x15 are incredibly close in gear ratio (if not the same?).
So - if you pedal either of these gears at 100RPM, you will be traveling at the same speed (I haven't worked the actual speed out), but acceleration differs between the gearing (atleast according to the article).
I only have one question: How?

(The article can be found at: https://www.velodromeshop.net/index.p...e_id=gearevent)

JerrySTL

3.357 versus 3.333 certainly isn't all that much difference.

Lew.

One thing I maybe should have clarified - the article seems to suggest that the chainring size is a determining factor, even if the cog makes the gear practically the same.
As the poster (thanks for the reply) shows, the gears aren't really going to be noticeable on the bike.

carleton

Yes, there exists a phenomena (real or imagined) where riders experience different acceleration and deceleration characteristics when using differing chainring/cog combinations that have very similar gear ratios.

Another ratio to consider is the "Sprinter" 45/13 (35.46) and "Enduro" 52/15 (35.33). The former seems easier to accelerate/decelerate which is why it is preferred by sprinters. The latter feels slower to accelerate, but once up to the desired speed, feels easier to maintain which is why it is preferred by time trial racers who seek maintain a constant velocity.

I have experienced this myself. I have no explanation as to why it feels that way.

To further this, there are also gear small chainring/cog ratios that, even though are larger, *feel* smaller than corresponding big chainring/cog combinations.

A LOT of people dismiss this. But, enough people mention it to indicate that there is something going on.

I think it comes down to an individual's sensitivity. Some people can feel when their saddle is 1cm too low or that their cleat has moved slightly. Some cannot.

Quinn8it

This theory pops up from time to time, and i believe has been pretty well de-bunked over at FGF...

People argue that the smaller ring starts faster or the sharper angles create more drag resistance. ..

The only decicing factor in gear ratio is the relationship between the rotation of the crank (at the pedal) and the rotation of the wheels (at the tire).. what happens in between is irrelevant

At the end of the day "gear inches=gear inches"

I have done some training on a bike with a 130bcd crank and used smaller ring/cog combos to hit GI that I was using on the track bike- and in my experience there is no difference..

Brian Ratliff

Two effects:

1) a small-small gear combo will have higher chain tension than a big-big combo of equivalent gearing. This means a small-small combo develops chain tension more quickly when the chain is being accelerated, which might pull the slack out of the drivetrain faster. This effect would be amplified by stiffer chains.

2) the big-big combo uses more chain (larger mass) at a larger radius (faster chain-speed), which should have the effect of lending the drivetrain more inertia. This effect would be amplified by heavier chains.

Both of these effects lend the small-small to feel "quick" and the large-large of equivalent gearing to feel "smooth".

I think people who use lighter, more flexible chains (mostly enduros) don't feel this effect much. Sprinters, who tend to use very heavy, stiff chains, might feel this effect more. Both of these effects are present and real; the question then becomes whether it can be felt or if it is all in peoples' heads.

Quinn8it

interesting points Brian... but im not sure they play out in real world scenarios.

first- for the sake of this debate, we should remove the variable of different weight chains.

second- in terms of chain slack in the system- at best that is a tiny tiny difference. The other aspect of the "Chain Slack" argument is that if the rider made his comparison between a big/big combo and a small/small without changing chain lengths (very possible with the right dropouts) this would actually work in the opposite direction- as the Small/Small combo would have more chain links not engaged by the teeth of the ring or cog.

as for the the effects of the longer chain (if you use a different chain length) on inertia- typically the difference in chain length would be about 2-links. I cant really see that having much effect on inertia..

Brian Ratliff

Don't forget the chain speed lending inertia to the drivetrain in the big-big combo. There are probably differences in driveline friction as well. In any case, this is all rule of thumb stuff unless someone wants to get the computers out and do a real analysis. Too much time; too little gain.

I agree, these effects, if they exist at all, are very small. But under certain conditions, people might be able to feel a difference, even if there is no quantifiable difference in speed or acceleration. Like a saddle height being off a couple mm. Shouldn't matter, but sometimes some people can feel it. I would call this less of a time/speed advantage issue than it is a preference/feel issue. Some people like the feel of one saddle over another, one handlebar shape over another, one pedal system over another, one tire over another, etc. Big-big vs. small-small fits in this category, IMHO.

For me, choosing big-big vs. small-small gets me into a mindset. I tend to use big-big on mass start and small-small for sprinting. This puts me in the mindset of steady speed vs. heavy acceleration. I also tend to change handlebars.

One other difference is, when you are working in the 49-50-51/15 (88.2-90.0-91.8 inches) range, a tooth on the front is less of a change than when you are working in the 46-47-48/14 (88.7-90.6-92.6) range. The small-small "sprint" gears almost start looking like the "big boy" version of the large-large enduro gears, and if you are a sprinter, this might put you in a certain mindset as well .

Lew.

Thanks guys... Seems like I was thinking inside the box a little too much (my thought was, also, that what happens inbetween the pedals and tyre was irrelevant other than gear inches). A lot of ideas I hadn't thought of, that make some sense.

Carleton - on the bike fit. I'm very, very anal about my fit, I'd notice a cm change easily.

Quinn8it

ok.. so it sounds like some people have opinions based on feel or how it makes them feel... but the OP quoted pretty exact claims about the differences in big/big and small/small...

this is where the arguments on this lose me:
to simplify this ill pick 2 gears that line up- (45x15) and (48x16) are both 81.0"

i assume that everyone agrees that both gear combinations are 81", cadences would be exactly the same at any given speed, and based on a standard gear chart that would mean at 30.12mph both would be creating an RPM of 125 at the crank... the quote says in the big/big combo it will be "easier to maintain". How? Why? the biggest creator of resistance on the drivetrain is the wheel- wheel speed and cadence are connected, wheel speed is the same. where is the difference coming from?

same goes for the claim "cadence will be obtained quicker" (with the small/small combination)- at every cadence the wheel speed is the same across both set-ups.. where is the de-coupling between the crank and wheel? or is it a reduction in the necessary power to accelerate the wheel?

according to the original quote- it is easier to spin a small/small gear up to a speed- but then it suddenly gets harder to hold????

none of this makes any sense to me

Brian Ratliff

Surely you can see how a big-big gear combination will have the chain (even if it is the same length chain) moving faster: with the 48 tooth ring, one revolution of the crankset will move the chain 48 links, while in the other combo, one revolution will have the chain move only 45 links. The greater speed of the chain in the big-big combo will result in more energy stored in the drivetrain in the form of rotational inertia. Kind of the same effect as two wheels which weigh the same but one with the weight concentrated at the hub and the other concentrated at the rim. The one with the weight concentrated at the rim will spin up slower, but hold its speed better.

With the small-small being faster to take up tension... Say you are going from applying back pressure to the chain to full acceleration. This is a situation that comes up quite often in sprinting. It might take, say, a quarter inch of pedal movement to take up the slack in the chain with the big-big combo. This quarter inch is essentially unopposed by chain tension. Now, switching to the small-small combo, the pedal will have to move something more than a quarter inch to take up the same amount of slack because the smaller chainring makes a shorter lever arm. Because this is unopposed motion, the pedal will accelerate all through this slack-taking-up phase. In the big-big case, the pedal has a quarter inch to travel while accelerating before hitting chain tension. In the small-small case, the pedal has more than a quarter inch to travel before taking up the slack, which mean the pedal will be moving faster when the chain hits tension and the force will hit the drivetrain faster. This will make the drivetrain feel like it is "snappier" than in the big-big combo. The rider doesn't notice the slightly larger play in the pedals with the small-small combo because this movement is unopposed; but she does notice the force slamming onto the drivetrain more sharply once the slack is taken out of the chain.

Again though, these are all very minute differences. But that said, people can feel a lot of very subtle things. It makes no rational sense that moving a saddle three millimeters up or down will make any difference on the pedal stroke; a leg is, what, on the order of 900 or 1000 millimeters long. That's a change of just 0.3%. But people feel it and act like it affects them, even though a researcher would probably have a real tough time finding any quantifiable performance differences.

slindell

There have been some studies (I think they were referenced on BikeTechReview) that showed lower chain losses with higher chain tension. So my theory is that the small/small combination gives a stiffer chain ring and lower chain losses at the high tensions at high power/sprinting. The large/large combination seems to run smoother on the cog so feels better at the steady state.
That said with a degree in physics I use the gear that requires the fewest changes from whatever is already on the bike and the chain.

Quinn8it

yes.. moving faster.
so at about 26mph/100rpm the 45t ring would spin 4500 links per minute and the 48t ring would spin 4800 links per minute... i know I'm not that smart- so feel free to correct me if I'm wrong.. isnt that a speed increase of about .07%??

how much "more energy" is stored in a chain that is going .07% faster? whatever it is i'm sure it adds up to tons of "rotational inertia"!

Brian Ratliff

The difference in chain speed is about 7% (4800/4500 = 1.0667). Kinetic energy is proportional by the square of velocity, which means there is about 14% more energy stored in the large-large drivetrain over the small-small. With a heavy sprint chain like the Izumi V, it's something that might be felt.

Baby Puke

I love this argument, and you guys are giving it a more interesting treatment than I've seen before. I've heard enough old-school guys and gals talk about this that I'm curious. I'm about to do some racing with the 13t for the first time, I'll let you all know if I "feel" anything different. And Quinn, 14t? 13t? What do you use?

Quinn8it

ok so- clearly i was out-mathed!!!

im still not buying it, and i don't think you do either ("its something that might be felt"!!??)

This side of the debate- is a big/big combo easier to hold at speed than a small/small, is actually fairly easy to test. Ill have to lengthen a chain but then i can play around with 48x16 versus 51x17 both at 81"- if i get any data thats worth looking at ill share it.. i could at least have some power files from roller sessions on both gears by the weekend..

BabyPuke-
im strictly a 14t guy these days... aside from warming up on a 16t

carleton

Just because we can't relate to or explain something doesn't mean it doesn't exist.

Many track Racers from multiple generations mention this phenomenon. They ask questions about it and others declare it a myth.

Remember, bicycle parts are measured in millimeters. Heck, cranks come in 2.5mm increments! That's HALF of a millimeter! Guess what. Some people can feel the difference between 167.5mm and 170mm cranks...and some cannot. Just because one person cannot feel the difference doesn't mean that another wont. The human body is remarkably sensitive in so many areas.

Something is going on. As soon as some grad student proves it unequivocally many will say, "Duh. We've known that for decades."

Velocirapture

Nice debate :-)
I'm with Brian on the inertia idea - not so much on the chain tension idea, because unless the angles of the links have a big impact, i'm not too clear how a shorter chain holds more tension, if you set it up the same. But i'm open to correction

Anyway, Quinn8it, you say you are still not buying the idea of the difference in inertia being detectible..

Another way of thinking about it is in terms of rotating mass, instead of speed of the chain. (The chain on the bigger ring goes faster, simply becuase it has further to travel). There is more weight - the chain - sitting further away from the pivot point -the BB.
Anyone who has gone from one wheelset onto a lighter wheelset will testify that the lighter wheelset 'rolls easier' it will attain the same speeds with the same pedal cadence etc, but it is easier to accelerate.
And 'an ounce off the wheel is worth a pound off the frame' is not a saying people tend to disagree with. A bigger chainring has more chain/ more mass wrapped around it than a smaller one, so its heavier. and like the heavier wheels, harder to move from stopped to going.

Not only have you got more rotating mass to deal with, which is already harder to accelerate - the same inertia that will keep it going when up to speed will resist the acceleration to speed - your lever (crank arm relative to chain and chainring) also changes.

By way of analogy, if a plank is lying on the floor, with a weight in the middle of it, it will take a certain amount of energy to lift up one end of that plank. If you put the weight closer to you, and lift up the end again (by the same distance), it will be more difficult, becuase your lever is shorter (even although your plank is not shorter).

Same thing with the bigger chainring, imo.

And it must be noticible, because people have noticed it. ha ha ha!

Brian Ratliff

The small-small has higher chain tension for the same reason the chain moves slower. The power transmission is the same, so to make up for the lower velocity, the tension must be higher. Another way of thinking about it is noticing that the pedal has more leverage on a smaller chainring, like your analogy with the plank.

Baby Puke

I have also been a scoffer. Seems like a gear ratio is a gear ratio, and conversely, the Japanese pros ride big chainrings (and very big gears), but I think it's for the look of the big ring more than anything else. That said, it's Walthour who's convinced me to give this a try. First time I saw this in practice was on Anna Meares' bike, which was set up with 45x13. I was 14t only too, Quinn, until Bobby took a look and called my set-up "pursuiter gearing", haha!

Quinn8it

good analogy- except not only does it not illustrate your point- but i think it backs up mine.

as you have written it- it is easier to move the plank if the weight is near the pivot point- what you are leaving out is that since the plank is moved the same distance with the longer lever the weight moves a shorter distance.. this beautifully explains differences in total Gear Inches and or changes in crank length and how those affect the system- but does not explain how a gear could "feel easier" while accomplishing the same work... yes the work you did was easier, but you did less of it..

in this example:
i think we all agree that our lever is an 81" drive train. whether accomplished by 48x16 or 51x17. I think we all agree that at any given crank RPM wheel speed will be the same on either system.

the "weight" is the force acting against the drive train. in this case that is the wheel. the wheel creates almost all of the resisting force against the cranks..

so if at same RPM's either drive train is spinning the wheel the same speed and creating the same opposing force on the drive train... what changes to make it "more difficult"? or less difficult?

the "weights" are in the same place on the "Plank" and the Plank is moving the same distance and so are the weights...

Quinn8it

i don't think this is really happening..

take a look at your weighted drive-train. apply force to the pedals and hold the bike in place. check out your chain. the only links that are actually holding real tension in the system are the first 7 or so, you can wiggle the rest of the links that are engaged around the chain ring..... that's about 15% of the ring on a 49t.. compared to an extreme small ring, say a 40t- that might drop 1 link of engagement? with this small area of engagement the circular shape of the ring doesn't really come into play.. i cant see that affecting leverage

Brian Ratliff

Oh, this part is really happening; this is just the physics of levers. Torque is force multiplied by radius. The torque at the crank is the same regardless of gearing and is set by the force you apply to the pedals multiplied by your crankarm length. As you reduce the radius of the chainring, with the torque remaining constant, the force on the chain has to go up to counteract the decreasing radius of the chainring. A chain wrapped around a smaller chainring has more tension for the same torque as one wrapped around a larger chainring.

slindell

Look at the other end of the chain.
To drive the wheel with the same force a the tire the force on the chain is proportional to the ratio between the radius of the tire and the radius of the cog the chain is acting on. Going from a 14 to a 13 cog reduces the cog radius 7% which will require an increase in the chain tension of 7% to net the same force on the tire (smaller lever arm).

Brian Ratliff

You are right, there is no reason one gear will feel easier than another if they are the same gear combination. Steady speed in either a big-big or small-small of the same gear ratio will take the exact same power and cadence.

Any change in feel would come only when the gear is either accelerated or decelerated. The difference being, if you are accelerating (or decelerating) the drivetrain, you are doing work against the inertia of the wheels, cranks, and chain. A small-small combo will have less inertia than the big-big which means you are doing slightly more work to accelerate the big-big combo at the same rate. Once the bike is at speed, conversely, the big-big combo, having more inertia, will slow less quickly once power ceases being applied.