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Newbie Doubt: Why do we need the large Chain ring in the rear ?

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Newbie Doubt: Why do we need the large Chain ring in the rear ?

Old 09-18-16, 08:47 AM
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Jibin
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Newbie Doubt: Why do we need the large Chain ring in the rear ?

I can understand why a large chain ring in the front and a corresponding small chain ring in the back generates speed. I also understand how a small chain ring in the front helps climb hills. But why do we need a corresponding large chain ring in the rear ? Wouldn't a medium or even small chain ring reduce the effort equally ?
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Old 09-18-16, 08:53 AM
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bulldog1935
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you can do the math here:
Sheldon Brown's Bicycle Gear Calculator

and even more here
https://home.earthlink.net/~mike.sher...ITLE=None&HL=1


Last edited by bulldog1935; 09-18-16 at 06:04 PM.
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Old 09-18-16, 08:54 AM
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Both front and rear contribute, because it's based on the ratio of the two.
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Old 09-18-16, 08:55 AM
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Huh? My understanding is that the larger the ring in the back the easier the peddling on whichever sized ring you choose up front. In the front, the opposite is true. With the same size ring in the back (regardless of size), the smaller the ring in front the easier it will be to peddle. I don't know why. So, it is all relative.

The larger the ring in the back the easier the peddling will be on whichever front ring you choose, but the smaller chain up front with largest in the back would be easier than the largest in back and largest in front. Since we generally limit the number of rings up front, the result is that the larger the ring in back the easier our peddling will be.

As an aside, I have seen a lot of new higher end mountain bikes with very small front ring with a large number of back rings from which to choose.
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Old 09-18-16, 09:03 AM
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its simple math, divide the front chainring teeth by the rear sprocket teeth, this will tell you how many revolutions of the rear wheel for each revolution of the pedals.

both the front chainring size and rear sprocket contribute to a lower gearing.
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Old 09-18-16, 09:03 AM
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Aye , It's MATH !! A divided by B or B x A.( A chain ring t, B wheel cog t )

That Ratio X the wheel diameter .. and the resultant Circumference
= how far you Go Down the road.
Every wheel rotation.
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Old 09-18-16, 09:06 AM
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Mostly to avoid cross-chaining, and to have smaller increments between the gear combinations. Small-big (front-back) is the easiest for going up steep hills, big-small is the hardest for the top speeds. Cyclists like to have a large range between those two, so having more gears front and back achieves that. But wanting small gaps between gears needs a lot of gears on back, as close together in size as is practical.

The big-big and small-small combination, and nearby gears on back, are "cross-chained" and generally avoided by shifting the front gear.
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Old 09-18-16, 09:39 AM
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Originally Posted by Jibin View Post
I can understand why a large chain ring in the front and a corresponding small chain ring in the back generates speed. I also understand how a small chain ring in the front helps climb hills. But why do we need a corresponding large chain ring in the rear ? Wouldn't a medium or even small chain ring reduce the effort equally ?
You must not have any big hills where you ride.
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Old 09-18-16, 09:50 AM
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Originally Posted by Berg417448 View Post
You must not have any big hills where you ride.
Or is a very strong climber.
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Old 09-18-16, 10:24 AM
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Perhaps a better question would be, "How do I best use the multitude of gear rations at my disposal to improve my performance on and enjoyment of my rides?"
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Old 09-18-16, 11:26 AM
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Technically you're not wrong, but it's just a trick to minimize the space needed to attain certain ratios. If all you had was a small ring up front, you'd need a massively deep cassette in the back to attain the same ratio.
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Old 09-18-16, 12:22 PM
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Jibin
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Originally Posted by cyclist2000 View Post
its simple math, divide the front chainring teeth by the rear sprocket teeth, this will tell you how many revolutions of the rear wheel for each revolution of the pedals.

both the front chainring size and rear sprocket contribute to a lower gearing.
You nailed it. This clarified some of my miss conceptions.


Thank you all for contributions.
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Old 09-19-16, 08:16 AM
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WizardOfBoz
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Look at it this way. The circumference of a 700c road wheel is about 2130-2150mm, say 2140. That's 85.25 inches.

However the gears on your bike work, it's the ratio of how many times the wheel spins for each crank revolution that determines how "hard" it is to push the pedals, and for any given speed what your pedaling cadence is.

A 54 front and 11 rear give you (54/11 * 85.25") 418.5 inches per crank revolution. Efficient cadences are in the range of 90 per minute (some higher, some lower: if you have a favorite that's different, do the math). This works out to (90 * 418.5" /12 /5280 * 60) 35.7 mph. This is a pretty "high" (hard to pedal) gear. By contrast, a 39 front and 28 rear gives you 118.8" per crank revolution. If you can maintain a 90 cadence, that's 10.1 mph. That's a "low" (easier to pedal, but slower) gear.

Hop into the Wayback machine, to a day when "corncob" freewheels had 11-21 (or even 11-19!) cogs. To get the same low gear with a 21 tooth rear cog, you'd need a 29 tooth front. To your point: you COULD just have a smaller gear in front to get to the same low gear. In fact, three-ring fronts do have small cogs like this.

Because most folks want to have the ability to go fast, you need a 52-54 front. To get the low gear without a huge rear cog, you'd want the small front chainring with very few teeth. But for a two-ring front road bike, there's two reasons you wouldn't want to have a big difference between large and small front chainrings.

First, shifting. There's a limit to how much of a difference in size between adjacent chain rings a front derailleur can be expected to handle. On my bike, The difference between chain ring sizes that Shimano offers is only a difference of 14-16 teeth. For example, they offer a 53/39, but do not offer a 53/36. At least stock. In addition to shifting reliability and performance, one has to think of the overlap between all of the combinations of front/rear gear ratios. For technical reasons, one generally does not run with the largest front chainring and the largest rear cog, which limits the range of the big front gear. And some overlap between the ratios for the big front ring and for the small front ring are desirable. A super small front ring can limit that overlap.

You CAN get a lot of ratios by running just a rear derailleur with a single front chainring. But to get the same range of ratios you get with a front derailleur, your largest rear cog needs to be huge, which is the opposite direction from what you hint you want. Also, the same shifting efficiency/reliability point applies for large differences between rear cogs. And a huge largest rear cog requires a very long idler arm to handle shifting between the largest and smallest rear cogs. A large front large to small chainring difference makes for an even larger idler.

Only using a front derailleur is another idea (fixed rear gear, but with a front derailleur). You could do this, but it would be weird (you still need a rear idler for the chain). This is why we have single-front-ring bikes, but not a lot (have never seen any) of single rear cogs with front derailleurs.

So, to your point, you COULD get a gear ratio as low as you like by making the front ring small, but there are many practical reasons why the most efficient bikes have a limited range in front and a wide range in the rear.

Last edited by WizardOfBoz; 09-19-16 at 08:21 AM.
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Old 09-19-16, 08:25 AM
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Because I like riding my bike up hills, not pushing it.
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Old 09-19-16, 08:58 AM
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I can think of some long ascents I like, too. It's a very pleasurable feeling when your gear and your watts perfectly match your climb.
I'm a very strong rider and also live in big hills.
I still like having gears below 30", because it's a 400' climb with a couple of spots at 14% grade to get home.

There were a couple of guys on a group ride yesterday who said my Raleigh was a really nice cruiser. I smiled, explained I bought it new in college, then smoked them up the final 3-mile climb.



This gearset has half-step big rings which split the steps on the wide-7 rear, and a 26T granny. The full range is 22" to 104" in 5" steps.
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Old 09-19-16, 09:10 AM
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Originally Posted by bulldog1935 View Post
I can think of some long ascents I like, too. It's a very pleasurable feeling when your gear and your watts perfectly match your climb.
I'm a very strong rider and also live in big hills.
I still like having gears below 30", because it's a 400' climb with a couple of spots at 14% grade to get home.

There were a couple of guys on a group ride yesterday who said my Raleigh was a really nice cruiser. I smiled, explained I bought it new in college, then smoked them up the final 3-mile climb.



This gearset has half-step big rings which split the steps on the wide-7 rear, and a 26T granny. The full range is 22" to 104" in 5" steps.
Please tell me more about your pedals, really like how they look.
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Old 09-19-16, 09:20 AM
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Those are VP-001, and I ended up selling them, because they were bad about strikes with the low-BB. I replaced them with narrower Blackspire El Gordo, which work well with my ice-skate feet (and match nicely in anodized blue).

If you like the VP-001, Rivendell has maybe the best price at $68.

Another thing I didn't like about them, is they have sharp metal that sticks out beyond the platform spikes farther than needed.
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