Cycling and bicycle discussion forums. 
   Click here to join our community Log in to access your Control Panel  

Go Back   > >

Bicycle Mechanics Broken bottom bracket? Tacoed wheel? If you're having problems with your bicycle, or just need help fixing a flat, drop in here for the latest on bicycle mechanics & bicycle maintenance.

User Tag List

Thread Tools Search this Thread
Old 07-11-12, 06:40 PM   #1
Senior Member
Thread Starter
Join Date: Jun 2012
Location: harrisburg, pennsylvania
Bikes: 1976 Schwinn Super Le Tour, tricked out with modern components. Shimano Alfine 11 internal gear hub. Dynamo hub. Titanium racks and bottle cages. Mercier Kilo Wide Tire dropbar singlespeed
Posts: 307
Mentioned: 2 Post(s)
Tagged: 0 Thread(s)
Quoted: 29 Post(s)
difference between 39 and 38 tooth chainring?

If I switch my 39-tooth inner chainring to a 38-tooth will there be a measurable lowering of the gearing? Say I'm in my biggest-tooth/lowest-gear cassette gear and were able to switch between a 39 and 38 tooth chainring, would I feel a difference? I'm exploring chainring options without having to change my bottom bracket parts.
motorapido is offline   Reply With Quote
Old 07-11-12, 06:51 PM   #2
Senior Member
AEO's Avatar
Join Date: Jun 2007
Location: A Coffin Called Earth. or Toronto, ON
Bikes: Bianchi, Miyata, Dahon, Rossin
Posts: 12,258
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
negligible difference.

This is all about gear ratios.

39:27 = 1.444
38:27 = 1.407

which ends up at around 1/4mph difference in the low range.
Food for thought: if you aren't dead by 2050, you and your entire family will be within a few years from starvation. Now that is a cruel gift to leave for your offspring. ;)

Last edited by AEO; 07-11-12 at 06:56 PM.
AEO is offline   Reply With Quote
Old 07-11-12, 07:07 PM   #3
Join Date: Apr 2007
Location: Syracuse, NY
Bikes: 2008 Novara Randonee - love it. Would have more bikes if I had time to ride them all. Previous bikes: 1968 Motobecane Mirage, 1972 Moto Grand Jubilee (my fav), Jackson Rake 16, 1983 C'dale ST500.
Posts: 6,785
Mentioned: 5 Post(s)
Tagged: 0 Thread(s)
Quoted: 211 Post(s)
For any change in front or rear it is simply the difference in teeth divided by the orginal tooth count, expressed as a percent. So 1/39 * 100 = 2.56% difference in gear ratio, speed or distance covered in a crank revolution.
cny-bikeman is offline   Reply With Quote
Old 07-11-12, 07:48 PM   #4
Senior Member
Join Date: Apr 2009
Location: New Rochelle, NY
Bikes: too many bikes from 1967 10s (5x2)Frejus to a Sumitomo Ti/Chorus aluminum 10s (10x2), plus one non-susp mtn bike I use as my commuter
Posts: 35,049
Mentioned: 100 Post(s)
Tagged: 1 Thread(s)
Quoted: 3441 Post(s)
While there's almost no real difference in the gears either will yield (2.5% or so) it can make a difference. Not in the low gear because 2.5% won't make or break anything, but it can affect the spread of the middle gears. Typical steps in a 9 or 10s are between 6 & 10% or so. If you get a gear chart, compare the ratios yielded by both inners, and compare them to the outer's ratios.
One may yield a gear progression about 1/2 step between those with the outer, while the other may be very close to duplicating them. So with one you get a nice batch of very closely spaced mid ratios (those that work well with either chainring) while with the other you don't.

Not a big deal, but if you've ever been on a shallow grade, or working a headwind and wished you had a gear between the 15 an 16, you'll be happy you could find one using the other chainring.
Chain-L site

An ounce of diagnosis is worth a pound of cure.

“Never argue with an idiot. He will only bring you down to his level and beat you with experience.”, George Carlin

“One accurate measurement is worth a thousand expert opinions” - Adm Grace Murray Hopper - USN

WARNING, I'm from New York. Thin skinned people should maintain safe distance.
FBinNY is offline   Reply With Quote
Old 07-11-12, 09:46 PM   #5
fietsbob's Avatar
Join Date: Jun 2010
Location: NW,Oregon Coast
Bikes: 8
Posts: 27,868
Mentioned: 58 Post(s)
Tagged: 0 Thread(s)
Quoted: 2819 Post(s)
a 135mm bolt circle, 39 is all you have, 130 will let you drop that 1 tooth smaller..
fietsbob is online now   Reply With Quote
Old 07-11-12, 10:20 PM   #6
Senior Member
DannoXYZ's Avatar
Join Date: Jul 2005
Location: Saratoga, CA
Posts: 11,633
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 38 Post(s)
difference is: 39-38 = 1 tooth !!!
DannoXYZ is offline   Reply With Quote

Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

All times are GMT -6. The time now is 09:00 AM.

  • Ask a Question
    get answers from real people!
Click to start entering your question.