Right now I’m working on building up my first tri specific bike. I’ve raced my triple equipped road bike with clip-on’s for the last three years, but am now committed to a double for my new tri bike. My road bike’s current gearing is 52, 42, 30 on the front with a 12-23 cassette on the back. The low gear of 30x23 is great and I want to maintain this on my new tri bike which will have a 53, 39 on the front. How big of a rear cog will I need with a 39 front to replicate a 30x23? Any help would be appreciated. Thanks.
You’d need a 30t rear cog to duplicate the low gear on your triple with standard cranks (53/39), or a 26t cog if you use a compact drivetrain (50/34), assuming you keep the same wheelsize. If you really need that gear, I would recommend going with a compact crankset.
How did you figure that out? Is there a formula for gearing conversion? (30 is to 23 as 39 is to ?).
I probably don’t need that gear (30x23), but it’s really nice - I have only ridden a couple of hills that I can’t spin up with it.
Becasue I’m already dedicated to a new Dura Ace 53/39 crank I’ll probably just go with a 12-27 and reconcile myself to standing a bit more.
Check out a couple sites
Both these places have good gear charts. Check out the century special cassette on Sheldon Brown’s page. It is a 13/30. I just put a FSA Energy on my bike to get a better climbing option and I like it a lot.
caleb,
Gearing conversions are just ratios.
For your case, a 30x23 (ie 30/23=1.304 ratio) is the same as a 39tooth/1.304 = 30 tooth
Or, if you use a compact 34 tooth chain ring then you need a 34 tooth/1.304 = 26 tooth.
Hope this helps.
Right now I’m working on building up my first tri specific bike. I’ve raced my triple equipped road bike with clip-on’s for the last three years, but am now committed to a double for my new tri bike. My road bike’s current gearing is 52, 42, 30 on the front with a 12-23 cassette on the back. The low gear of 30x23 is great and I want to maintain this on my new tri bike which will have a 53, 39 on the front. How big of a rear cog will I need with a 39 front to replicate a 30x23? Any help would be appreciated. Thanks.
**jasonk **
May 9, 2004, 11:58 AM
Post #2 of 4 (29 views)
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**Re: Triple to Double Gearing **In reply to] **Quote | Reply ** You’d need a 30t rear cog to duplicate the low gear on your triple with standard cranks (53/39), or a 26t cog if you use a compact drivetrain (50/34), assuming you keep the same wheelsize. If you really need that gear, I would recommend going with a compact crankset.
**caleb **
May 9, 2004, 12:08 PM
Post #3 of 4 (26 views)
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**Re: Triple to Double Gearing **In reply to] **Quote | Reply ** How did you figure that out? Is there a formula for gearing conversion? (30 is to 23 as 39 is to ?).
I probably don’t need that gear (30x23), but it’s really nice - I have only ridden a couple of hills that I can’t spin up with it.
Becasue I’m already dedicated to a new Dura Ace 53/39 crank I’ll probably just go with a 12-27 and reconcile myself to standing a bit more.
How did you figure that out? Is there a formula for gearing conversion? (30 is to 23 as 39 is to ?).
I probably don’t need that gear (30x23), but it’s really nice - I have only ridden a couple of hills that I can’t spin up with it.
Becasue I’m already dedicated to a new Dura Ace 53/39 crank I’ll probably just go with a 12-27 and reconcile myself to standing a bit more.
caleb,
A great book that will help you out immensely is:
BICYCLE GEARING A Practical Guide by Dick Marr.
Published by The Mountaineers.
This book should enable you to answer any gearing question that you might ever encounter. After reading it, you will be giving us advice on gearing questions.
Good luck.
By my math a 39x27 has a ratio of about 1.44 (39/27=1.44). A 30x23 has a ratio of 1.304 (30/23=1.304). What is the real world difference of .136 worth of gear ratio on a 700c bike spinning at 90rpm?
caleb,
Difference is about 10%.
Or 27.7 metres per minute, for 700c @ 90 rpm, all other things being equal.
I would stay with 39/27.
Gees Mark, if you are going to be so specific don’t forget to ask if he runs a 20 or 23 tire. How about the air pressure? ---------- Just kidding :~)
See you at the Marine Corps Tri?
Aloha,
Larry
Larry,
Yeh, its the engineer and “number cruncher” coming out in me. I should just get on the bike and ride rather than analyse!
See you at Kaneohe.
Hairy Legs,
If there is an engineer/number cruncher in there, why isn’t he talking in gear inches? The generally accepted terminology for expressing gear ratios on bicycles.
What is an “inch”?..oh yeh, that unit of length from the antiquated system of measurement that the rest of the world threw away years ago…except Liberia (in western Africa), Myanmar (Burma), and the US of A. Go figure!
Besides, I am a structural engineer who crunches numbers and just happens to ride a bike.
aloha,
Mark