> >
>

# Math quiz

Mountain Biking Mountain biking is one of the fastest growing sports in the world. Check out this forum to discuss the latest tips, tricks, gear and equipment in the world of mountain biking.

# Math quiz

05-25-15, 07:24 PM
#1
sgtdirt
Senior Member

Join Date: Sep 2013
Location: Mentor, Ohio
Posts: 247

Bikes: 1988 Univega rover sport, 2014 Felt TK3, 2012 Jamis Nove Pro CX

Mentioned: 0 Post(s)
Quoted: 7 Post(s)
Likes: 0
Liked 0 Times in 0 Posts
Math quiz

is there a difference in gearing ratio between a 29er and 26" wheel. a friend has a SS 29 with a 32 chainring and a 22 freewheel. my 26" is geared with 36 chainring and 17 freewheel. he says they are about the same ratio (taking in wheel size difference).

can this be true?
05-25-15, 10:00 PM
#2
desconhecido
Senior Member

Join Date: Mar 2010
Posts: 1,461
Mentioned: 11 Post(s)
Quoted: 252 Post(s)
Likes: 0
Liked 0 Times in 0 Posts
Originally Posted by sgtdirt
is there a difference in gearing ratio between a 29er and 26" wheel. a friend has a SS 29 with a 32 chainring and a 22 freewheel. my 26" is geared with 36 chainring and 17 freewheel. he says they are about the same ratio (taking in wheel size difference).

can this be true?
No.

For the 29 (700C: 56-622), one crank rotation results in 1.45 wheel rotations (32/22). Each wheel rotation results in about 230 cm travel so each crank rotation gives 1.45 * 230 = 335 cm.

For the 26 (50-559), one crank rotation results in 2.12 wheel rotations(36/17). Each wheel rotation results in about 205 cm travel so each crank rotation gives 2.12 * 205 = 434 cm.

434/335 = about 1.3 so they are not almost the same ratio.
05-27-15, 09:47 AM
#3
ColinL

Join Date: Jul 2011
Location: Wichita
Posts: 4,903

Bikes: Santa Cruz Blur TR, Cannondale Quick CX dropbar conversion & others

Mentioned: 0 Post(s)
Quoted: 0 Post(s)
Likes: 0
Liked 0 Times in 0 Posts
Originally Posted by desconhecido
No.

For the 29 (700C: 56-622), one crank rotation results in 1.45 wheel rotations (32/22). Each wheel rotation results in about 230 cm travel so each crank rotation gives 1.45 * 230 = 335 cm.

For the 26 (50-559), one crank rotation results in 2.12 wheel rotations(36/17). Each wheel rotation results in about 205 cm travel so each crank rotation gives 2.12 * 205 = 434 cm.

434/335 = about 1.3 so they are not almost the same ratio.
I agree with this, except it's weird that you did the math based on a 29x2.25 and a 26x2.0. I didn't see the tire width mentioned in the OP?
05-27-15, 10:16 AM
#4
desconhecido
Senior Member

Join Date: Mar 2010
Posts: 1,461
Mentioned: 11 Post(s)
Quoted: 252 Post(s)
Likes: 0
Liked 0 Times in 0 Posts
Originally Posted by ColinL
I agree with this, except it's weird that you did the math based on a 29x2.25 and a 26x2.0. I didn't see the tire width mentioned in the OP?
You have to assume a tire circumference (or diameter or radius) to do the calculation. I have no experience with big 700C tires so I just looked up tire circumferences for these tires and it appears that 230 cm is a reasonable number to use in such a calculation. Likewise with the 559 mm tires.

Are these not reasonable tire sizes to use to answer the question? The ratio resulting comes out very close to 29/26 -- could have just used nominal diameters and gotten the same result. Or, rim diameters (622/559).
05-27-15, 05:15 PM
#5
sgtdirt
Senior Member

Join Date: Sep 2013
Location: Mentor, Ohio
Posts: 247

Bikes: 1988 Univega rover sport, 2014 Felt TK3, 2012 Jamis Nove Pro CX

Mentioned: 0 Post(s)
Quoted: 7 Post(s)
Likes: 0
Liked 0 Times in 0 Posts
those tires sizes are correct. nice SWAG!

although, i was incorrect on my friends gearing... he's running 32X20 mine is still the same (34X17).
we are both getting ready to switch to a 650 bike. haven't decided on gearing yet.
05-29-15, 03:05 PM
#6
ColinL

Join Date: Jul 2011
Location: Wichita
Posts: 4,903

Bikes: Santa Cruz Blur TR, Cannondale Quick CX dropbar conversion & others

Mentioned: 0 Post(s)
Quoted: 0 Post(s)
Likes: 0
Liked 0 Times in 0 Posts
Originally Posted by sgtdirt
those tires sizes are correct. nice SWAG!

although, i was incorrect on my friends gearing... he's running 32X20 mine is still the same (34X17).
we are both getting ready to switch to a 650 bike. haven't decided on gearing yet.
that was lucky.

32x17 on the 29x2.25 would be close to 36x17 on the 26x2.0 assuming average volume & diameter tires.

wait .. you wrote 36T on your bike twice, including the first post, and now you said 34x17. if you have 34x17, his closest equivalent gear would then be 32x18.
05-29-15, 03:19 PM
#7
sgtdirt
Senior Member

Join Date: Sep 2013
Location: Mentor, Ohio
Posts: 247

Bikes: 1988 Univega rover sport, 2014 Felt TK3, 2012 Jamis Nove Pro CX

Mentioned: 0 Post(s)
Quoted: 7 Post(s)
Likes: 0
Liked 0 Times in 0 Posts
oops...again. yes, i am riding 36X17. so he was correct.
05-29-15, 03:35 PM
#8
ColinL

Join Date: Jul 2011
Location: Wichita
Posts: 4,903

Bikes: Santa Cruz Blur TR, Cannondale Quick CX dropbar conversion & others

Mentioned: 0 Post(s)
Quoted: 0 Post(s)
Likes: 0
Liked 0 Times in 0 Posts
Originally Posted by sgtdirt
oops...again. yes, i am riding 36X17. so he was correct.
not really, 32x22 is way different than 32x18.

32x22 does sound wrong, though. That would give a top speed of something like 12-14 mph depending on how fast you can spin. (100 - 120 rpm) 32x18 sounds more realistic on a singlespeed 29er.
05-29-15, 05:42 PM
#9
FrozenK
Senior Member

Join Date: Jan 2009
Posts: 1,036
Mentioned: 2 Post(s)
Quoted: 174 Post(s)
Likes: 0
Liked 0 Times in 0 Posts
Originally Posted by sgtdirt
oops...again. yes, i am riding 36X17. so he was correct.
No. The easiest formula to compare gearing is probably gear inches. Gear inch= Wheel diameter (inches) x Chain ring Tooth# / cog tooth # Using nominal wheel diameter (26 and 29 inch in this case) is usually close enough.

For the gear ratios you have given:
32x20 on a 29r equals 46.4 gear-inch
36x17 on a 26r equals 55 gear-inch
Even if you are running 34x17, you are still not close. He is running a low gear (hill friendly) and you, if you are rrally running 36x17, a pretty tall gear.

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