Technical question on bigger chainring

Quoted post said:

Originally Posted by novetan [IMG]/img/forum/go_quote.gif[/IMG]

Currently I’m using Shimano Compact chainring 50/34T. I intend to change the bigger one to 53 or 54T while retaining the small one. I was told by the bike shop the difference between the 2 chainring shld not be more than 16T (50-34). If change to 53/54, the difference will be 19/20T. Will there be any abrupt problem for the chain to roll over?

And do you think i can reuse my present chain?

First, whether-or-not you can continue to use your present chain depends on how long the current chain on your bike is ... you may need additional links ... which really means that you will need a new chain.

BUT, without knowing the cassette you are currently use OR why you want to change to a larger chainring (why not simply buy a "standard" Road crank?) ...

Presuming you are using something like a 12-27 Cassette, then 'I' recommend that you simply buy-or-cannibalize an 11t cog & lockring AND THEN restack the Cassette to achieve a higher top end gear ratio ...

• BTW. Cannibalizing an LX cassette for the 11t cog + lockring will probably be a less expensive than buying the 11t cog + lockring a la carte from your LBS.
  
• 52t & 53t 110BCD chainrings are not common, and the cost is probably going to be surprisingly high until a 105 variant of Shimano's new 4-arm cranks becomes available -- of course, then you will have the added cost of the new cranksets, too!
  

If you insist on using a 53t or 54t chainring with a 34t inner chainring, then you may want to use an appropriate-to-the-shifter-model front derailleur designed for a Triple crankset ...

FYI. If you are currently using SRAM shifters, then you are SOL with regard to your front derailleur options.

I'm toying with the idea of a bigger chainring as I'm running out of gear on certain downslope. It may not justify havent a bigger wheel, I'm just toying with the idea. But I must admit I havent spin out or max out the gear yet which is something I still hv to work on it.

My rear cassette is 12/27T. So your recommendation is to replace the 12 with 11 and throw out the 27?

50/12 would let you reach 35+ MPH before spinning out. Certainly doable on a descent, but outside group rides/race settings, does it really matter?
You'll be spending some effort to rectify a fairly occasional, and quickly passing phenomena.
Me, I'd be happy to coast those precious second above 35 MPH.
Why not save it until wear calls for a chain/cassette replacement anyhow?

Appreciate all above comments. I hear you out.

Quoted post said:

Originally Posted by novetan [IMG]/img/forum/go_quote.gif[/IMG]

My rear cassette is 12/27T. So your recommendation is to replace the 12 with 11 and throw out the 27?

No ...

First of all, the 27t cog is probably attached to a spider and so it is part of a mini-cluster which can not be disassembled ...

If you were to replace the 12t, then you would re-stack the Cassette as 11-13-etc or 11-12-13-etc instead of 12-13-etc. where YOU have to decide which of the loose, intermediate cogs is superfluous to you & then exclude it from the stack.

BTW. I agree with dabac -- and, I'll add that 'I' think that coasting at slower speeds is a good thing, too!

All good advice above, but the simple answer is to replace your current 12/27 cassette with a commonly available 11/27 or 11/28 cassette rather than trying to swap out specific cogs and introduce a big jump between the 11 and 13 tooth cog or otherwise swap out one of the smaller cogs where gearing jumps are more pronounced. Unless you have unused cassettes of the appropriate sizes you'll likely spend more time, effort and cash trying to find just the cogs you want vs just buying a complete cassette with the gearing you're after.
-Dave

Quoted post said:

Originally Posted by daveryanwyoming [IMG]/img/forum/go_quote.gif[/IMG]

... the simple answer is to replace your current 12/27 cassette with a commonly available 11/27 or 11/28 cassette rather than trying to swap out specific cogs and introduce a big jump between the 11 and 13 tooth cog or otherwise swap out one of the smaller cogs where gearing jumps are more pronounced. Unless you have unused cassettes of the appropriate sizes you'll likely spend more time, effort and cash trying to find just the cogs you want vs just buying a complete cassette with the gearing you're after.

YOU are correct ...

For inexplicable reasons, I was thinking that the OP had a 9-speed Cassette.

novetan,
Double FD are designed to 16t max. difference for chain rings. A triple FD will not work due to the shifter throw limit. You would have to get a triple front shifter to help that issue. Not sure what you are trying to accomplish changing rings on your current setup. More info would help with suggestions. What is your current group? 9 or 10?
Cheers,
Rick in Tennessee

Quoted post said:

Originally Posted by cycledogg1 [IMG]/img/forum/go_quote.gif[/IMG]

novetan,
Double FD are designed to 16t max. difference for chain rings. A triple FD will not work due to the shifter throw limit. You would have to get a triple front shifter to help that issue. Not sure what you are trying to accomplish changing rings on your current setup. More info would help with suggestions. What is your current group? 9 or 10?
Cheers,
Rick in Tennessee

Guys,

Tks so much for all the help.

After what hv said, I will stay put my current compact chainring, 50/34. You guys hv suggested the better way is to change the rear cassette from 12T to 11T and I really do think that is a much cooler idea. Never thought of the gear in. reaps better ratio even by just 1 in. I'll look into that direction.

Quoted post said:

Originally Posted by novetan [IMG]/img/forum/go_quote.gif[/IMG]... Never thought of the gear in. reaps better ratio even by just 1 in. I'll look into that direction.

Yep, a single tooth on those small cogs makes a very big difference.

It was probably mentioned above, but a 50x11 is a bigger gear at ~123 gear inches (faster for the same cadence) than a 53x12 at ~119 gear inches. It's surprising how big a difference a single tooth can make on the smallest cog.

So stick with the 50/34 crankset, pair it up with an 11-27 or 11-28 rear cassette and you'll get a bigger top gear and the same or lower hill climbing gear for a ton of useable range.

-Dave

daveryanwyoming said:

Yep, a single tooth on those small cogs makes a very big difference. It was probably mentioned above, but a 50x11 is a bigger gear at ~123 gear inches  (faster for the same cadence) than a 53x12 at ~119 gear inches. It's surprising how big a difference a single tooth can make on the smallest cog. So stick with the 50/34 crankset, pair it up with an 11-27 or 11-28 rear cassette and you'll get a bigger top gear and the same or lower hill climbing gear for a ton of useable range. -Dave

+1. For future reference a rough rule of thumb says that that 1 tooth change in the number of teeth in the rear is approximately equal to a 3 tooth change in the front. So going from a 12 tooth to an 11 tooth cog while on a 50T chainring is about the same as going from a 50T to a 53T in front while staying on an 11T cog. Remember it's a rough rule of thumb. BTW, coasting very fast is why the gods invented mountain descents. It'd be criminal to do a climb only to putter down the descent.

Quoted post said:

Originally Posted by alienator [IMG]/img/forum/go_quote.gif[/IMG]

+1.
For future reference a rough rule of thumb says that that 1 tooth change in the number of teeth in the rear is approximately equal to a 3 tooth change in the front. So going from a 12 tooth to an 11 tooth cog while on a 50T chainring is about the same as going from a 50T to a 53T in front while staying on an 11T cog. Remember it's a rough rule of thumb.
BTW, coasting very fast is why the gods invented mountain descents. It'd be criminal to do a climb only to putter down the descent.

-1
Let toss out the rough rule of thumb.
For future reference, grade school arithmetic works well here. Can you do 12/11?
That little 11 in the back will give you a gear 9% bigger than a 12.
A huge, fun gear.

Paul, just to be a critic, which is always fun here, I'll say your illustration provides nothing useful because it only goes half-way. To determine whether a change of the front from 50 to 53 makes a bigger change than going from 12 to 11 on the cogs, we need to pick the bigger of the two ratios (53/50; 12/11). Just saying 12/11 produces a 9% change doesn't answer the question really. Looking at both ratios, it's obvious that 12/11 > 53/50, so changing the cassette will produce the bigger increase in gearing.

Actually, what I normally would do is just look at the two gear ratios, 50/11 (4.54) and 53/12, or 4.41. By taking the ratio of the two, 4.54/41, you can see that the 50/11 gear combination is 3% bigger (or "faster"😉.

But for some people, the concepts of arithmetic and ratios aren't all that easy. For them, I don't see why we need to toss out the old 3-to-1 rule of thumb. It's rough, but answers the question.