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'Rule of Four' in practice

1K views 11 replies 7 participants last post by  Fungazi 
#1 ·
At the mo, I have a 46Tx18T gear combo. It's a magic gear. Now, given that dark art called the 'Rule Of Four', if I want to use a 16T cog in the rear, what's that mean to the chainring size I need to go to? 48T chainring? 44T chainring?

I never fully grasped the concept of the rule, so I figure here's my chance to dive right in.
 
#2 ·
48t

or
46/18
48/16
50/14
44/20
42/22

SpinWheelz said:
At the mo, I have a 46Tx18T gear combo. It's a magic gear. Now, given that dark art called the 'Rule Of Four', if I want to use a 16T cog in the rear, what's that mean to the chainring size I need to go to? 48T chainring? 44T chainring?

I never fully grasped the concept of the rule, so I figure here's my chance to dive right in.
 
#5 ·
Hey yeah!!

Des, what the 'ell are you doing running that bigga gear anyways??? The heck with your knees, you're gonna have one a them cardiac episodes or sumthin'.

I'm an idiot for running 38/18 up here in hilly Preskit!

SpinWheelz said:
Cheers, fellas. If you'll excuse me, I'm off to set things on motion to completely wreck my knees now...
 
#6 ·
What a nosey bugger!

This gearing isn't for off-road use, mind you. At the mo, I've got a very nice setup with the 46x18 gear combo on my road-going MTB. I can handle most of the flats and mild ascends with ease with that gearing. But I know I can go faster and I know that I can try and build up a bit more strength if I tweak the gearing a bit, hence the tougher gearing. Granted, that's a helluva tweak, but what the hell, let's see where this goes.
 
#7 ·
The "rule of 4" isn't 100% foolproof when running "magic gears" on bikes with vertical dropouts. For example, 36/20, 34/18, and 32/16 all run really well on my bike, which has vert drops. That is just as the rule of 4 would predict. But the "rule of 4" also says I could do 36/16 and 32/20, without any chainlength change. Well, there is soem change, I know because 36/16 is really snug (doable with a "stretched" chain), and 32/20 is very loose and would require a tensioner for any real riding.
If you really want to get into just which ones will REALLY work (and why) you need to check out fixed inovatons / fixmeup. http://www.peak.org/~fixin/personal/fmu/php/fmu1_1_big.php
Fixemeup indicates that 44x16 will work exactly as 46x18 would (and is a good bit higher ratio) but that 48x16 requires a signifigantly shorter chainstay- IE, it would be very loose, and likely require a tensioner. There also a lot of gears lower than 46x18 that would work for yah, but not to many ones that are higher, at least not with practical chainring sizes.
 
#10 ·
Doesn't work this way

If you are running 36/20, how could you run 36/16 without a change in chainlength? You have an extra couple of links hanging around. Remember that the total amount of teeth has to be equal. Using the rule of 4 you actually get 36/20, 34/22, and 32/24 not 36/20, 34/18, and 32/16.

"]The "rule of 4" isn't 100% foolproof when running "magic gears" on bikes with vertical dropouts. For example, 36/20, 34/18, and 32/16 all run really well on my bike, which has vert drops. That is just as the rule of 4 would predict. But the "rule of 4" also says I could do 36/16 and 32/20, without any chainlength change. "
 
#11 ·
Your answer is:

SpinWheelz said:
At the mo, I have a 46Tx18T gear combo. It's a magic gear. Now, given that dark art called the 'Rule Of Four', if I want to use a 16T cog in the rear, what's that mean to the chainring size I need to go to? 48T chainring? 44T chainring?

I never fully grasped the concept of the rule, so I figure here's my chance to dive right in.
If 46x18 is magic then 44x16 is magic. You'll take out a link but that's not important.

(rules work unless they confuse people, so I use Excel)
 

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