# "magic" gear ratios?



## mack_turtle (Jan 6, 2009)

i am getting a converted SS mtb with a 32/18 ratio and i am told this ratio allows for the perfect chain length without a chain tensioner of any sort. i think this ratio may be a bit spinny for me, so i want to try something a little harder like a 32/16. but i would like to avoid needing any sort of chain-tensioning device. i am not afraid of putting a half-link on it if necessary.

how can i determine (mathematically?) what harder gear ratios might work with my chainstay length? like i said, half-links are ok but i want to avoid chain tensioners.

edit- looking through that FAQ section, i might be able to figure this out on my own. i did a calulation and it says i have 46.2 gear inches. what does that mean?


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## broz (Feb 3, 2007)

about not needing a chain tensioner, that's hogwash. 32/16 on a fixie means your skid is always in one of 2 places on the tire (which isn't so great). but chain lenth is a function of chainstays and gear diameter.


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## mack_turtle (Jan 6, 2009)

this bike has a freewheel, broz.

i can measure the chainstay length later if that will help with the calculation.


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## ShadowsCast (Mar 23, 2008)

The difference between too loose, properly tensioned, and too tight is probably less than one cm... There isn't some magical gear ratio that always works with every bike. There are certain gear ratios that work with certain bikes, with certain chainrings, with certain chains, and certain cogs. And who knows, maybe your rear triangle is tweaked. Perhaps you've got enough power to flex the frame enough to drop that chain. Maybe that gear ratio only works for 100 miles until the chain stretches. If you want to figure it out, go start changing your rings and cogs around, the forum is little help. Sheldon's site does have a few suggestions for improving your chances, however there's no special equation that will determine your options.


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## forkboy (Apr 20, 2004)

*Special equation...*

Linkie...

http://www.eehouse.org/fixin/formfmu.php


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## mack_turtle (Jan 6, 2009)

thanks forkboy. i saw that link on the FAQ section, but the link doesnt work. who has the power to change the hyperlink in the FAQ section?


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## byknuts (Aug 9, 2008)

can only comment on the bikes i've built, but I found 36/17 or 38/15 worked perfectly on both an old steel brodie and a rocky mountain hammer race (both mid-nineties frames I believe)

back then almost every frame had what was termed "norba standard" geometry so most of them had identical (or damn close) chainstay lengths...

try it out.
a chain link being roughly analogous to 4 teeth I assume you could run a 32x17 or a 34x15 with identical results (but I haven't bothered to try it myself)

never believe anyone who just says "32x18 is a magic gear" unless you're 100% positive they're talking about YOUR bike frame...
as noted above, the chainstay length will change from year to year, model to model (more now than back in the day) and what was a magic gear for their frame won't necessarily work on yours, unless it's the same frame, and even then, sometimes there's enough of a difference from one year to the next to nullify their statement


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## bigfish (Nov 16, 2005)

First find the ratio you desire

Try your current chain with out a tensio to see how tight or loose it will be.
If its to tight try a half link or a more worn out chain or a new chain with a half link.
If its to loose try a new chain.

I have a *Big* box of used chains and half links that I use to get proper chain tension. if you dont have a big box of used chains around go to the LBS and ask them if you can sort through their steel scrap/old chain box.

Chains stretch fast at first but then slow, so find a chain that fits a little tight and it should wear in


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## mack_turtle (Jan 6, 2009)

i was hoping to find something like this: https://www.bikeguide.org/guides/drive/gearchart.php

this is for bmx bikes though. no one in their right mind would ride a bmx bike with a chainstay longer than 15."

according to Gunnar's site, the chainstay on my Rockhound should be 415 cm, which is about 16.33 inches. if a 32/18 fits "just right" on my bike, i need to figure out what higher ratio would also fit on a 16.33 inch chainstay. i am going to go do some maths, but if you have a better answer or a gear chart for mtb's, lemme know.










if i can figure out this based on these sprocket radii, i can figure it out mathmatically. this might be more complex than necessary, but i enjoy the challenge.


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## wv_bob (Sep 12, 2005)

mack_turtle said:


> thanks forkboy. i saw that link on the FAQ section, but the link doesnt work. who has the power to change the hyperlink in the FAQ section?


This will get you there http://eehouse.org/fixin/javafmu.php

It'll do what you want without all the math


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## mack_turtle (Jan 6, 2009)

i guess the question then is, can i run a 2:1 ratio on a bike with a 16.33" chainstays without too much slack in the chain that i would need a chain tensioner? and if so, what combo would work? the smallest freewheel i can put on is a 16t because it's a Paul Word hub.


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## Doggity (Mar 27, 2007)

I plugged the data from my bike into the ee.house link above, and it swore that 34/20 was _the_ magic gear, for my bike. Well, 'tweren't...the chain hung. I coulda fixed it maybe with a half link, but I just put my Singleator back in, and all was well. Besides, I have a flip flop hub, and I can ride more than one ratio, this way. Tensioner doesn't give you as much range to change gearing as a frame with track ends does, but it's better than nada.


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## Letsinger (Aug 10, 2004)

You are tilting at windmills here.
Sometimes, someone might get a no-tensioner system to work for a little while, but there is no _way_ to do it. Ya just get lucky sometimes. 
Chains wear, cogs and rings wear. If you try to force it to work, you'll just spend your time fighting with your driveline and your chain will come off constantly.
Do yourself a favor and just buy the correct components. Sounds like an Eno Eccentric hub would be the solution you need. 
Really, it's worth it. SS is about simple, reliable, and fun - a bike that doesn't work right is none of those.
Tom


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## Fat Bob (Mar 5, 2004)

Jonathan,

Here is the link I was telling you about. I'll send you the .pdf on mojo if I can figure out how to send the attachments, it'll make getting your tension just right much better.


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## Upchuck (Jan 30, 2004)

I fully agree. Eno eccentric hubs have worked really well for me on several bikes.


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## fishcreek (Apr 10, 2007)

here's a graph from eehouse, i entered your chainstay length and by the look of it 32x18 will be too tight to be magic. 34x15 and 36x17 fell on the shaded area which is magic but in reality it will be a bit slack. but who knows, 16.33" may not be your EXACT chainline. you still have to do trial and error experiments.


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## Andy R (Nov 26, 2008)

The problem with all this "magic gear" stuff is that the difference between a correctly adjusted chain and one with a load of slack is a really small change in chainstay length/wheel position. I mean, even .010" makes a big difference to the amount of vertical deflection possible in a chain.

I agree with those who say forget all the faffing around and do one one these things -

Get an eccentric hub, an eccentric BB converter, get track ends fitted to your frame or put up with running a good tensioner. 
Unless you are *extremely* lucky you'll need to do stuff like filing flats on the rear wheel spindle or opening up the dropouts to make the "magic gear" work well. And then what happens if the ratio isn't quite ideal for you ?

I've done all these things and it just isn't worth the hassle, in my opinion.
Spend a bit of money, do it right and then just ride the thing....:thumbsup:


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## mack_turtle (Jan 6, 2009)

thanks folks. this has been an interesting thought experiment. i think that eehouse tool may be the most useful thing i have seen on the internet in a long time! considering the fact that chain stretch is a fudge factor, i'll have to try a few first hand to see what works in the real world, not just in theory. i'll ride it with the ratio it has now and mess with different combinations later if i feel they become necessary and try a few of the ideas posted here.

i know a guy who does custom machine work, so i will ask him about that Angry Asian eccentric axle for my hub.

btw, Fat Bob sold me the bike. thanks bob!


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## bikeny (Feb 26, 2004)

mack_turtle said:


> i am getting a converted SS mtb with a 32/18 ratio and i am told this ratio allows for the perfect chain length without a chain tensioner of any sort. i think this ratio may be a bit spinny for me, so i want to try something a little harder like a 32/16. but i would like to avoid needing any sort of chain-tensioning device. i am not afraid of putting a half-link on it if necessary.
> 
> how can i determine (mathematically?) what harder gear ratios might work with my chainstay length? like i said, half-links are ok but i want to avoid chain tensioners.
> 
> edit- looking through that FAQ section, i might be able to figure this out on my own. i did a calulation and it says i have 46.2 gear inches. what does that mean?


If 32/18 is truly a Magic gear ratio for this frame, than you could try 30/16, which will be a slightly higher ratio, 1.77 ratio versus 1.87. The '4 tooth rule' applies here. If you have a ratio that works, you can add or subtract 2 teeth from both the ring and cog to have the same exact tension. The trick is you have to add or subtract from both, not add to one and subtract from the other. Basically, this keeps the chain angle from ring to cog the same. So if 32/18 works, so will 34/20 & 36/22, onlt those will be lower ratios. As mentioned, 30/16 will work and be slightly harder, and 28/14 would also work. But that is not really a practical ratio.

35/19 and 33/17 may also work with a stretched chain according to Fixmeup. You really need to wait until you have the bike and find a ratio that fits right, and then apply the 'four tooth rule' to see if there are any ratios that will work for you. If you lucky and find a ratio that suits your riding & fits the bike, it will work great.

Good luck!
Mark


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## RSW42 (Aug 22, 2006)

32/18 has been doing well fo rme.

:thumbsup:


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## fishcreek (Apr 10, 2007)

RSW42 said:


> 32/18 has been doing well fo rme.
> 
> :thumbsup:


not all frames have the same magic ratio.


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## ernesto_from_Wisconsin (Jan 12, 2004)

the only thing that comes to mind is David Blaine's street magic.


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