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Are smaller front chainrings easier to turn, when gear ratios are the same? If so why? and How much easier?
Need some clever science/physics person to explain this to me. Or at least someone with a power meter.
Our cycle group is doing a silly hilly bike ride up Crowcombe Combe and Draycott steep.
Everybody is panicking and swapping cassettes and chainrings over before Sunday.
I have a 30 tooth chainring with a 27 toothed cassette on 105 Chainset (Triple)
My mate has a SRAM 1x Chainset
He has a 40 toothed chainring with a 36 toothed cassette.
So our gearing is 30/27 or 40/36 - both at 1.11 ratio.
Both ratios are therefore exactly the same.
I know from by gear calculator app, that in theory, if we were going up a hill at 90RPM, we would both go along at 7.9 Miles per hour.
In my opinion, it would "feel" exactly the same to ride up a hill. (assuming both bikes weighed the same, our bodyweights were the same, crankarms the same etc)
Another friend says that the Triple, with the 30 tooth Chainring would actually still feel easier to turn the cranks round, because of Torque.
But, due to my lack of physics understanding, I dont understand why this would be the case. and if it was true, How much easier would it be?
So my question is which is easier to turn? Is one easier than the other? Would one method use less watts than the other?
If there some kind of calculation that could be worked out?
And if the triple is easier to turn, and uses less watts, why isnt everybody riding triples? (Hey I know I should drink Triples and not ride triples, but it came with the bike and at the time I hadnt read the velominati! :-p )
I guess this question could be applied to any single bike, where the ratios overlap, e.g. is the smaller chainring version of a gear ratio always actually easier to turn the pedals, and uses less watts, than the big chainring version of that ratio?
I found this article, but never really had a definite answer.
http://www.velominati.com/technique/sur-la-plaque-physics-of-the-big-rin...
I'm hoping some of you guys with experiences of power meters will have tried "overlapping" gear ratios and have noticed a measureable effect.
Comments please!
If you're new please join in and if you have questions pop them below and the forum regulars will answer as best we can.
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22 comments
It's been established that larger chainrings/cogs combos have higher mechanical efficiency due to lower chain friction (see the huge jockey wheels that some pros use).
The diffrences are probably marginal.
Friction Facts did a test on this a wee bit ago, quite interesting and along the lines of what you might suspect regarding radius of curvature and chain line effecting friction.. was slightly surprised at the magnitude actually.
http://www.bikeradar.com/road/gear/article/friction-facts-free-speed-fro...
Same ratio means sane pedals turns per metre and same force multipliers. But the chain will move further on larger rings. Does your mate have an old rusty chain that results in significant extra friction? Fit a new chain or lube it. Or maybe larger rings and cassette weigh more. Maybe if you carry his spare tube for him it will balance up.
What this debate needs, is someone with a power meter to go and overlap some identical gear ratios and test it.
Assuming other factors are the same (e.g. chain line), the larger ring should be slightly easier. Because the chain follows a longer circumference around, and hence the links have to turn less, which should mean slightly lower chain friction losses.
Actually, I disagree because if you think of the crank arm as a lever with the chainring being the load partway up the arm, then physics shows that the further the distance between the place where the force is applied and the load the greater the torque. So the smaller chainring will require less force as its load is closer to the fulcrum, making the crank arm a longer lever, think of how you use a crowbar, that can lift a much higher load than say a screwdriver even though you are applying the same force or why using a wheelbarrow you can lift much higher loads than just by carrying it by yourself (the load is close to the fulcrum with a long lever, the handles), that's my theory anyway!
lever.jpg
The crank length and the wheel circumfirence remain constant though. These are your inputs/outputs. What the gearbox does in between isn't governed by its physical size, but by its gear ratio, which is ultimately the equivalent of your lever length.
Basically your nonsenese needs a re-think!
As an aside, and on your average wheelbarrow the fulcrum isn't right at the front of the barrow so if you load the front of the barrow properly it doesn't take much lifting.
I cycled up Draycott Steep a couple of weeks ago. Just past the hardest part, I passed a woman parked up. Out the window she said "well done" and offered me an orange!
Ratios are the same, it will come down to weight and power applied who gets up quickest/easiest. Weight being the important factor. A 100kg rider with 300-325watts climbing will get beaten by a 75KG rider producing 235-250Watts on a longish climb
They'd actually get there pretty close. With W/Kg within the same range, aero comes into play. Push comes to shove I'd put my money on the heavier guy on flatter climbs and the lighter guy on steeper climbs.
Gravity will eventually tire the bigger rider first on steeper climbs. I agree that less steep power wins most times. I have seen this when racing against lighter riders on Bkool, I can power away on slopes but then get pulled back on steeper hills
Imagine a two different see-saws*, on with a 10 kg weight half way along one end and the other with a 5kg weight at the end. Which would be easier to lift?
*weightless, frictionless, in a vacuum, perfect.
If I'm going for hills, I use a triple up front ( 50:39:30 )and a 9 speed 12-27 cassette. I've not found a climb that I can't get up with the 30 / 27. I find the triple gives me a lot of flexibility.
A shorter way to say that is that the gear choice with the straightest chain line and the least difference in teeth between the sprocket and the chainring will be the (undetectably) easiest to pedal.
I'd think for better gear engagement what would matter more is simply the largest total number of teeth. That's why hour records rides are done on 54/13-14 and not on 46-42/11. Which falls in line with your earlier post, if I read it correctly. And yep, this should be a second or two on a twenty minute climb sort of stuff, the difference between taking a racing line in one corner or filling your water bottle to the thread or to the brim.
It will be very very similar (not detectable to a human, and barely detectable to strain meters). The reason is:
If all other factors are the same, where the gearing is the same (from pedal to wheel rim) then the torque required to drive said wheel rim from the pedal only differs by the efficiency of the power transfer. The biggest of these efficiency characteristics will be lateral chain line. Beyond that there are 2 intersecting curves of the power transfer which are to do with the number of teeth on the chain ring, and the number of teeth on the sprocket. If you have less teeth on your sprocket then the angle between where the effort is applied by the chain tension to the sprocket is slightly larger at the inception of the load application, and this leads to a slightly lower efficiency. (the turning moment, as measured perpindicular to the chain, applying the torque is slightly shorter there hence slightly less efficient, also there are greater deformation chracteristics etc) At the other end, the more teeth on the chain ring you have leads to slightly greater engagement and over all friction losses. Where these 2 curves meet is the most efficient. These won't be noticable to a rider, and at 200w my guess is that they would make less than a 10th of 1 percent difference (though this is a standard random internet indexed guess value) If you have 34 chainring and 17 sprocket there is no noticable difference to using 56 chainring and 28 sprocket (chain line not withstanding)
As a previous poster said, Sheldon Brown is a great initial resource.
That's a reason for SRAM 1x system and changing to double from a triple chain set, because of the duplicate ratios... So yes it would be the same with minor differences from geometry and rider attributes, but these are not down to the chain set. Also crank arm length will contribute.
It will be exactly the same
Ok. Let's assume everything is completely identical apart from the teeth on the gears used. Is it easier to ride on identical ratio gears, if one of those ratios has a smaller front chainring than the other? Or is it the same?
You would be correct in that they would both feel the same, but you would have to use both bikes to determine this. Both bikes would also have to have the same geometry, because once you start adjusting saddle height etc (to suit rider and geometry) that then affects everything else, mind you the differences would most likely be negligible. Even with identical geometry to change crank length will change the gear gain. For example. A bike with 175mm cranks and same ratio as a bike with 170mm cranks the former will have a lower gear by about 3%. Also the dimensions of the rider come into play as you are part of the gearing as soon as you climb onto the bike! So two identical bikes with two riders of different stature will produce different power outputs.
rather than looking to the velomati, I would refer to sheldon brown in regards to the physics and mechanics of the bicycle for a greater insight of all things velo.
I would not trust Sheldon Brown as a source for reliable physics, last time I looked he was talking rubbish about the length of crank altering gear ratios.
Is that why giff77 seems to think a 175mm crank will give a 3% lower ratio than a 170mm on the same gearing? I'm surprised nobody has picked up in this.