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#51 2012-06-14 12:59am

IPD
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Re: Compare Ratios of Gearing

or you could go with 160mm cranks so you can pedal faster.

personally, i'll swear by 9/30 with 170mm cranks.  i've never felt like i couldn't accelerate fast enough with this setup.


It's like the man says, there's no problems...only solutions.

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#52 2012-06-14 5:06am

astroidbeltwalker
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Re: Compare Ratios of Gearing

your ratio is 56, have your tried a ratio of 46 to compair the acceleration. im @ 49 its fast very fast but tops out quickly
im stuck with the gt 36 if it fits or 43, (riding now) or 44 chainring's only changes to make are in the freewheel(s) an
cranks. feel like 175 is short now. when its up to speed dont feel stable on pedals legs are not fully extended, i have to
tweek my knees at feet to compinsate, im going to give them a try though if i could find some 160's never seen any for sale
also 165's but the difference from 175 to 180mm is only .3 175mm = 6.889 inches vs 180mm @7.086 inches
looked for the 177.5mm in one piece they are hard to find.


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#53 2012-06-14 5:59am

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Re: Compare Ratios of Gearing

No, the difference between 175 and 180 is 2x that.  Those are each arm.  The total difference is 170x2 vs. 180x2.

There are 155mm crank arms available:

http://flatlandfuel.com/stmartinevosplinedcranks.aspx

The physics behind smaller crank arms is that you should be able to go faster--with the same gear-ratio setup--by using a shorter crank arm.  This is because the rotational forces (pedals) are applied closer to the rotational axis (crank) which allows for higher RPM.  It's the same way with a car engine; longer stroke usually = lower RPMS (although with more torque).

By decreasing the crank arm length, you're essentially doing the same thing; decreasing torque to improve top RPM.  Since you have a lower gear ratio to begin with, torque isn't as much of a factor for acceleration as it would be with a higher gear ratio--thus you can probably afford to sacrifice some torque to hit higher RPM.

Just my $.02  I know that most people pick crank length based on rider size...but there's more to it than that.  Also, I'm not sure what you mean by "feels unstable".  If you are referring to sitting on the seat & at the bottom of travel your leg is fully extended...then you need to raise the seat.  If you don't want to do that, then I'd suggest longer crank arms & a higher gear ratio.

I used to ride everywhere on a 13/44 back in the day on a 1-piece crank (dunno length).  I'd consider that an option as well--since you already have a chainwheel about that size.  Then, if you decided you don't like it, you can downsize your chainwheel from there.  I'd never run larger than 13t on the rear if I could help it--you can make virtually any gear ratio you want with 13t or smaller, and the bonus is weight savings over the same gear ratio with 15t or 16t rear cog.


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#54 2012-07-15 9:06am

southernflatland
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Re: Compare Ratios of Gearing

I've found 43/18 perfect for flatland on my Supergoose. Perfect torque, perfect cruising speed. Damn prime numbers, can't reduce them exactly sad


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#55 2012-09-20 1:22pm

badmofo
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Re: Compare Ratios of Gearing

wired wrote:

This is kind of cool Hobbes.  At first I couldn't understand how you could get any value out of it for BMX.  After I started playing with it I compared a 20" gear ratio to a 24" and [think I] figured out these should feel the same when pedaling:

44/16 @ 20" ~ = 37/16 @ 24"


Wheel Diameter: 20.00 inches
Gears: 44   16
---------------------------------------------------------------------------
SP  CRxFW    GI      GIdf      DI      diff      DF        PRPM      RT
---------------------------------------------------------------------------
1>  44x16   55.00    0.00%   172.79    0.00   14' 4.79"   366.69   1:2.75

  Wheel Diameter: 24.00 inches
  Gears: 37   16
---------------------------------------------------------------------------
  SP  CRxFW    GI      GIdf      DI      diff      DF        PRPM      RT
---------------------------------------------------------------------------
1>  37x16   55.50    0.00%   174.36    0.00   14' 6.36"   363.39   1:2.31

Nifty (but made my head hurt)! lol

That's some "Good Will Hunting" crap right there!


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#56 2012-10-12 12:53pm

sharpbbb
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From: United Kingdom
Registered: 2012-10-06
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Re: Compare Ratios of Gearing

i was wondering what 44t and an 18t rear sprocket will be like with 2.125 mx tyres?

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#57 2012-10-24 11:33am

astroidbeltwalker
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From: mount kisco
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Re: Compare Ratios of Gearing

42  / 16 right now have yet to ride the bike. thinking about 43/16. on a GT 43/17 was quick, slight top end. but quick. on a bike close to 13 lbs lighter, 42  / 16 might be to short


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#58 2012-10-24 11:34am

astroidbeltwalker
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Re: Compare Ratios of Gearing

20x1.50's


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#59 2013-01-21 2:43pm

Brian74
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From: Erial, New Jersey
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Re: Compare Ratios of Gearing

Is 25/16 with 165mm crank arms good for flat?


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#60 2013-01-26 11:12pm

Jeno007
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Re: Compare Ratios of Gearing

Running, on my GT Interceptor, same setup Ive always run on my bikes since I got strong enough to play with it has been 46-14 to keep it close to 10 strokes between the gate and the first jump, and 44/46-16 if I couldnt get 10.


Anything is negotiable.

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#61 2013-01-27 7:12am

Jeno007
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Re: Compare Ratios of Gearing

Oh, 180mm cranks. lol


Anything is negotiable.

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#62 2013-02-13 3:00pm

Hollywood 1
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Re: Compare Ratios of Gearing

I use 39 by 17 on my 24" (Cruiser).  I really like it.  I am racing with it.  Fast out of the gate and does pretty good even after the 1st turn.  cool


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#63 2013-04-06 5:08pm

neon
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From: Panama City, FL
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Re: Compare Ratios of Gearing

I use a 48 - 14 with 175 mm cranks 20x1.75 tire

Seems ok for me


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#64 2013-04-15 5:42pm

astroidbeltwalker
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From: mount kisco
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Re: Compare Ratios of Gearing

new bike first ride today 20x1.50 43x15 going to take some getting used to


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#65 2013-04-22 11:28am

cushman
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From: des moines, IA
Registered: 2012-02-12
Posts: 323

Re: Compare Ratios of Gearing

26 inch cruiser 180 cranks 39 front, what for rear??????

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#66 2013-04-22 1:13pm

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Re: Compare Ratios of Gearing

13 or 14.


It's like the man says, there's no problems...only solutions.

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#67 2013-05-12 11:10pm

Jeno007
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Re: Compare Ratios of Gearing

IPD wrote:

No, the difference between 175 and 180 is 2x that.  Those are each arm.  The total difference is 170x2 vs. 180x2.

There are 155mm crank arms available:

http://flatlandfuel.com/stmartinevosplinedcranks.aspx

The physics behind smaller crank arms is that you should be able to go faster--with the same gear-ratio setup--by using a shorter crank arm.  This is because the rotational forces (pedals) are applied closer to the rotational axis (crank) which allows for higher RPM.  It's the same way with a car engine; longer stroke usually = lower RPMS (although with more torque).

By decreasing the crank arm length, you're essentially doing the same thing; decreasing torque to improve top RPM.  Since you have a lower gear ratio to begin with, torque isn't as much of a factor for acceleration as it would be with a higher gear ratio--thus you can probably afford to sacrifice some torque to hit higher RPM.

Just my $.02  I know that most people pick crank length based on rider size...but there's more to it than that.  Also, I'm not sure what you mean by "feels unstable".  If you are referring to sitting on the seat & at the bottom of travel your leg is fully extended...then you need to raise the seat.  If you don't want to do that, then I'd suggest longer crank arms & a higher gear ratio.

I used to ride everywhere on a 13/44 back in the day on a 1-piece crank (dunno length).  I'd consider that an option as well--since you already have a chainwheel about that size.  Then, if you decided you don't like it, you can downsize your chainwheel from there.  I'd never run larger than 13t on the rear if I could help it--you can make virtually any gear ratio you want with 13t or smaller, and the bonus is weight savings over the same gear ratio with 15t or 16t rear cog.

This as far as I am taking your understanding of it is incorrect. The benefit of going to a shorter crank arm versus a longer crank arm (within comfort) is an increased holeshot acceleration, as it is reducing the torque necessary to turn the cranks. But the down side to this is you reduce your ability to continue to accelerate and gain more mph as you go faster, as you hit the top of your gearing. The crank is as much a part of your gearing as your sprocket/chainwheel and freewheel/cog, it all adds up. If you find yourself running out of steam between jumps or rhythm sections but you like the acceleration you get out of the gate, you should adjust your chainwheel/freewheel combo, go higher in the front or lower in the rear or both. If after doing so you find yourself close but not quite there, then try moving up some on your crank length and you will find more top end, but the trick is to find the length that allows you to keep your snap speed.


Anything is negotiable.

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#68 2013-05-12 11:16pm

Jeno007
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Re: Compare Ratios of Gearing

It all really comes down to your track layout, your strength, and your stroke length. My local tracks always had a majority of technical sections, and getting out of the gate first wasn't as much of a priority for me as having the most speed at the end of the first straight and turn as I could.


Anything is negotiable.

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#69 2013-05-13 12:55am

Oaktown Massive
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Re: Compare Ratios of Gearing

Jeno007 wrote:

The benefit of going to a shorter crank arm versus a longer crank arm (within comfort) is an increased holeshot acceleration, as it is reducing the torque necessary to turn the cranks. But the down side to this is you reduce your ability to continue to accelerate and gain more mph as you go faster, as you hit the top of your gearing. The crank is as much a part of your gearing as your sprocket/chainwheel and freewheel/cog, it all adds up. If you find yourself running out of steam between jumps or rhythm sections but you like the acceleration you get out of the gate, you should adjust your chainwheel/freewheel combo, go higher in the front or lower in the rear or both. If after doing so you find yourself close but not quite there, then try moving up some on your crank length and you will find more top end, but the trick is to find the length that allows you to keep your snap speed.

Yeah, I just went thru all this, it was a process to work out my gearing.  My top tube length also affected my setup.  My 18.5" TT would wind out way too fast with 44/16, 180.  I moved to 46/16 and my speed was good but my rhythm sucked.  I dropped the cranks down to 175 and my speed off the gate dropped as well.  At 44/16, 175, my 18.5 tt is fast off the line and has ok top end speed.  But my timing feels real good.
   My 20" TT had the same problem at 44/16, 180.  I tried dropping my cranks to 175 but I wasn't getting the pop I wanted.  The 175's felt smaller on the 20", so I raised my cranks to 180 and my gearing to 46/16.  This setup works well for me with the 20"tt - maybe not as quick a first stroke as I might want, but all kinds of speed after that.  And it gives me an extra gear at my top end, which is great big_smile

Just my .02 - everybody has to set up their own ride to their tastes.


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#70 2013-05-13 2:24pm

IPD
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Re: Compare Ratios of Gearing

Jeno, shorter crank arms DECREASE the amount of torque you can generate.  It's simple physics, with a shorter lever, you can generate less force.  The opposite end of this is that your motion of travel is greater with a longer crank; which means that you won't be able to go as fast at top-speed.  As I mentioned before, it's a similar effect to the stroke length of a piston in an engine.


It's like the man says, there's no problems...only solutions.

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#71 2013-07-05 5:03pm

astroidbeltwalker
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From: mount kisco
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Re: Compare Ratios of Gearing

thinking from 43t -15 to 44t - 16 i'd made a wise decision, could not have been more wrong, my bike feels like its broken. i've never rode 44 - 16 & can't wait to go back.


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#72 2013-07-05 5:13pm

wildbillo68
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From: Palm Bay, FL.
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Re: Compare Ratios of Gearing

What length cranks you runnin? Yesterday I rode my 1984 Diamondback Super streak with OG length cranks and 44/16 ratio, and i just a while ago finished a 1983 redline PL-20 that I took out for it's first run and that one has 43 t ring and a 16 t rear sproket.. I don't know if it's the extra inch in the crank length or the gearing of the teeth or what but- the RL was kickin way easier...& FAST. I think it was easier on the calves too hahaha.. I was wondering about the tooth thing.. I am curious what people have to say... I have to read the other posts...

Last edited by wildbillo68 (2013-07-05 5:24pm)


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#73 2013-08-15 5:04pm

astroidbeltwalker
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From: mount kisco
Registered: 2011-10-07
Posts: 5461

Re: Compare Ratios of Gearing

New ratio is 44-15 1.50 rear / 1.75 front 180mm cranks liked iT first time I rode it 43-15 / 180mm feels slow in comparrison but still nice one day I will try the 180mm 44-16


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#74 2013-08-15 11:07pm

IPD
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Re: Compare Ratios of Gearing

way bigger difference going from 44/15 to 44/16 than going from 44/15 to 43/15.


It's like the man says, there's no problems...only solutions.

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#75 2013-08-23 9:38am

badmofo
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Re: Compare Ratios of Gearing

The first post needs to be edited.  4 out of 7 of the links are now DEAD  sad


Grammar is important. Capital letters are the difference between helping your Uncle Jack off a horse, and helping your uncle jack off a horse.

Ezekiel 25:17 ~ "The path of the righteous man is beset on all sides by the inequities of the selfish and the tyranny of evil men....."

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