# Using Magic gear calculator



## LaneDetroitCity (Nov 10, 2015)

I have found this calculator from other threads and im curious how to interpret it.

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

Does tire diameter and chain stretch have a huge impact on anything? It doesnt let you put 29er. And a chain doesnt stretch before i toss it.

Also how do i interpret the results. Are the ratios to the right of the line possible with a tensioner and the ones to the left not possible due to chainstay length? Or the opposite?

Trying to find a magic gear on a procaliber 29er. 38-32 chainring and as close to 2/1 as i can without a tensioner. And something for climbing like 32/20 ish.

Procaliber is 435mm chainstays









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## solo-x (Feb 16, 2010)

To the right is viable via use of a tensioner. Tire diameter doesn't matter. Chain stretch has a significant impact.

I thought you said you were going to use a 29+ ebay carbon frame? What about the Procaliber is making that a viable frame for you and why aren't you just going with a Pivot or Giant XTC and calling it a day?


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## LaneDetroitCity (Nov 10, 2015)

solo-x said:


> To the right is viable via use of a tensioner. Tire diameter doesn't matter. Chain stretch has a significant impact.
> 
> I thought you said you were going to use a 29+ ebay carbon frame? What about the Procaliber is making that a viable frame for you and why aren't you just going with a Pivot or Giant XTC and calling it a day?


Thank you. Its two different builds. My regular 29 ss broke. One9 RDO. So replacing that with the procal due to cost and opportunity. The 29+ is the ebay frame using a ebb. That will a different tool. The procal will be my main xc ss bike. Was hoping a magic gear around 2/1 was possible but i guess ill have to get a tensioner but would rather not.

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## eri (Sep 4, 2012)

I derived the equations myself, made a spreadsheet. It seems to work in practice. To me that above table doesn't look like it leaves enough slack.

I tried a few values and 34x16 (a 2 1/8 ratio) should work for your 43.5cm chainstays.

If you need exactly 2 then 36x18 should be good. Here's what my spreadsheet says for 36x18:

front radius	2.864788976
rear radius	1.432394488
theta	2.974119674
length	47.87184206
desired slack	2.2
span length	17.06597734
span with slack	17.10139125
slack needed	0.035413907

length with slack	47.90725596


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## Velobike (Jun 23, 2007)

What is important to remember is that a singlespeed can run reliably with a certain amount of chain slack.

The provisos are:

Straight chainline (kind of obvious)
Singlespeed chainring and rear cog (These have full size teeth to engage the chain, not cut down as you find for derailleurs)
Singlespeed chain, or at least no more than 7 speed. (These have less lateral flex, so less likely to jump off)

If you doubt this, crank up Google and look at pics of bicycles in 3rd world countries. You'll see folk happily pedalling along with incredible loads and a chain dangling down almost to the dust.

I don't bother with the calculator. It's singlespeed, so you're always going to be in the wrong gear anyway.

What I do is have a couple of chainrings a tooth apart, and about 3 rear cogs, and simply try the combos out. 

I started doing this because specified chainstay lengths are rarely millimetre accurate, and that's what you need for the calculator to work. You could measure it yourself, if you have sufficiently accurate tools, but it's easy to be a mm out.

Also I found that wear in the chainring quickly means slack in the chain, but for the last 5 or 6 years I have been using steel chainrings (try Surly) and solved the problem.


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## LaneDetroitCity (Nov 10, 2015)

eri said:


> I derived the equations myself, made a spreadsheet. It seems to work in practice. To me that above table doesn't look like it leaves enough slack.
> 
> I tried a few values and 34x16 (a 2 1/8 ratio) should work for your 43.5cm chainstays.
> 
> ...


Thank you so much. 34/16 36/18 would all be perfection for what i need. I use an oval chainring also so that can take up a tiny amouunt of slack. Now im real excited for possibilities with the procaliber.

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## LaneDetroitCity (Nov 10, 2015)

Velobike said:


> What is important to remember is that a singlespeed can run reliably with a certain amount of chain slack.
> 
> The provisos are:
> 
> ...


Thank you. I have all those things in a ss drivetrain. I have ridden ss only pretty much. I use 8spd chain, king or surly cogs, i9 ss specific hub, absolute black ovals.

I always leave a tad bit of slack where i see lots of people with the chain super tight. Ive had the chain slapping the chainstay once and it was fine.

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## LaneDetroitCity (Nov 10, 2015)

If 36/19 fits perfect after the chain is "stretched" what else should fit perfect? 

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