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I am using a road bike as commuter its and old bike and have weird front gear selection 52 and 42.

So I took it to work this morning and thought that for a change I will ride it on big gears. While I was getting more tired and lowering the gear I had a thought is there actually a difference between going on low front and back gear and going high front and back gear, but both having the same gear ratio.

Criggie
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kifli
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    First time I've seen normal described as weird. 52/42 was typical for road bike front rings for many years, until the introduction of compact gears with 50/34. Sometime competitive cyclists would use different front rings such as 53's or 54's, even 56, for specific events, and sometimes they'd vary the size if the small ring too. – andy256 Nov 03 '16 at 10:22
  • Well if you hadn't encountered bikes 10+ years old, then yes indeed 52/42 would seem weird. 52/39 has been the 'standard' size for almost as long as I can remember. – Andy P Nov 03 '16 at 10:25
  • every road bike I see now days it is 50/30 something. I started caring about gears recently. – kifli Nov 03 '16 at 10:43
  • @kifli that's a "compact" crank set, which is about 99% of the big gearing, and more smaller gears for going up steeper hills. Personally my road bike still runs a triple, so my lowest low is 26/28, essentially an underdrive. – Criggie Nov 03 '16 at 11:15
  • I've tweaked the question - the front ones are chainrings (or "chain rings") and the ones in the cassette are called sprockets, or sometimes cogs. If you don't like that, use "revert" to back out my changes. – Criggie Nov 03 '16 at 11:18
  • For most purposes a gear ratio is a gear ratio. Often, especially with 3-freewheel setups, there are several gear combos that are identical or very close. This is useful, since it allows you to "fine tune" your gear ratio with just the rear derailer, vs having to jiggle both shifters to get the specific gear you want. The only caveat is that it's best to avoid "cross chaining" where you go extreme big-big or small-small, but with narrow chains this is less of an issue than it once was. – Daniel R Hicks Nov 03 '16 at 11:54
  • One other caveat is that the extreme "cross" settings either stretch the chain all the way out or place it in it's most slack position. With a modern drive train in good condition this is usually not a problem, but if the chain is a hair short or long, or the rear derailer a hair haywire, it can cause drive train lockup or a jumped chain. – Daniel R Hicks Nov 03 '16 at 12:00

2 Answers2

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Depends - you are cross chaining when in the big/big and small/small combinations. See this question:

How bad is cross chaining?

Also, theres a question of ratios, but this is more theoretical.

Assuming your rear cassette ranges from a 12 to a 28 tooth, and the front chainring is 52 and 42, then Sheldon says these are your Gain Ratios.

 Gear chart using Gain Ratios
 For 700 X 28 / 28-622 tire with 12-13-14-16-18-21-24-28 8-speed cassette

      52        42    23.8 %    
 12   8.7       7.0   8.3 %
 13   8.0       6.5   7.7 %
 14   7.4       6.0   14.3 %
 16   6.5       5.3   12.5 %
 18   5.8       4.7   16.7 %
 21   5.0       4.0   14.3 %
 24   4.3       3.5   16.7 %
 28   3.7       3.0

so expressed another way, here are your overlaps, where 1 is the biggest cog and 8 is the smallest. (42/13 is almost the same as 52/16)

 52       1   2     3   4 5   6 7 8
 42   1 2   3   4 5   6   7 8

So if your comfortable speed for a road is in the middle of the range for 42 tooth chainring, its at the bottom of the range for the 52 tooth.

To say that another way, the 42 tooth chainring has more ratios closer together in one are, at the cost of having no ratios elsewhere in the range.

So on our hypothetical 8 speed there are two gears almost identical, 42/13 and 52/16. Why choose one over the other?

  • Cross Chaining - the smaller chainring will be "happier" with the cassette at 1-5 and the larger chainring will go better with 3-8. There is overlap.
  • What's coming up - if you know theres a climb, a descent, or an obstacle/stop then you might choose to be in 42 for the climb, or in 52 for the descent. For a stop, you'll probably change gear, 42/13 would be a bit hard to pull away from a dead stop.

If everything else was the same, I'd choose the bigger pair over the smaller. More teeth in contact with the chain means slower wear.

Play about with Sheldon's calculator at http://www.sheldonbrown.com/gears/

Criggie
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Yes, there is a small difference. It is more efficient to ride in larger gears as the chain wraps around them at lesser angles resulting in less friction.

http://www.cyclingpowerlab.com/DrivetrainEfficiency.aspx

Andy P
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    Note that the link is suggesting something like a 0.5% efficiency difference between big cogs and small cogs. You're going to get much more than that by making sure your tyres are properly inflated, using a more aerodynamic position, wearing more aerodynamic clothing, etc. So, while this answer is accurate, I think it would be improved by quantifying what "a small difference" means and how it compares to other differences one might easily make. "Small" on its own is hard to quantify. – David Richerby Nov 03 '16 at 10:12
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    I believe the hierarchy of savings go along the lines of Position -> Clothing -> Wheels -> Frame -> Drive-train, but I had neither the inclination to find data to support all that, nor do I think they are relevant to the actual question. – Andy P Nov 03 '16 at 10:30
  • it say so but the next statement is : “Cross Chaining” really hurts efficiency. So it will maybe counter rest that gain. – kifli Nov 03 '16 at 10:41
  • @AndyP where does that ordering come from? I assume it means on the flat too ? – Criggie Nov 03 '16 at 11:05
  • @Criggie Ordering comes right out of my head from years reading blogs and magazine articles. To the best of my knowledge it is correct, but happy to be corrected if anyone chooses to do so. Flat will clearly make the biggest difference, but with the savings we are looking at in drive-train efficiency, i'd be surprised if it didn't hold for all but the steepest uphills too – Andy P Nov 03 '16 at 11:23
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    @kifli both examples in the question (big/big, small/small) will both be cross chained, so that doesn't factor into the comparison between them. Obviously the 'perfect' gear would be using the largest possible gear/cog combination that maintained a good chainline – Andy P Nov 03 '16 at 11:25