48 tooth gear on a triple chainring?

[etsisk]

Read a comment on another thread to the effect that a 48 tooth gear on an MTB triple isn't advisable (at least that's what I think he meant). What's the consensus?

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[Pat Lamb]

Low gear matters more than high gear.  Pick your smallest chainring, and get a crank that comes with that.  The high end will be what it will be.

[etsisk]

I've got a 24, 42, 52 in front of a 11-36 cassette, with a few week long tours in it and a lot of local riding. So far I like it.

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[John Nelson]

Was 48 considered too high or too low in that other thread?

[etsisk]

Didn't say! Couldn't tell!

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[Pat Lamb]

I've got a 24, 42, 52 in front of a 11-36 cassette, with a few week long tours in it and a lot of local riding. So far I like it.

In every thread, there's a tendency to devolve into "analysis paralysis," where you spend more time worrying about some minute feature of your gear than it deserves.  I think you've hit that point. 

You've got a 24x36 low gear, which is almost low enough to pull a stump out of a field.  You've done a couple week long tours and lots of local riding, and you're not complaining about mis-shifts, excessive jumps, or any of the other things that goofy gears feature.

It's time to go ride your bike!

[etsisk]

I appreciate that! I've had the bike built for a year or two and have done 2 or more short tours each year with it. I'm actually happy about it and confident about it. But I know there's a lot I don't know, so I figured I would explore that assessment and see what other folks thought about it.

Ideally I would have a 24/38/48 crankset - but who knows, maybe I'll be strong enough by next year to get some use out of that big old 52 tooth chainring!

I appreciate the feedback, y'all.

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[DaveB]

Pat has it right that over thinking equipment choices can be a needless distraction but there is one real reason a 48T chainring probably won't work on your crank.  Your current 52/42 chainrings are 10 teeth apart.  Changing to a 48T will give you only a 4 tooth separation and your front derailleur cage's inner plate will hit the 42T chainring if the derailleur is positioned properly over the 48T chainring.   Front derailleurs have a "minimum tooth difference" specification and I'm nearly certain yours requires more than 4 teeth. 

In the past, there were front derailleurs that had shallow inner plates and would work with "half-step" crank gearing but they are no longer common.
 

[etsisk]

Thanks for that info! I've tried, and failed, to clear up that I'm not interested in changing to a 48, just curious as to the assessment that it was a "bad idea" and wondering if others agreed and why. Sorry for the miscommunication. The fault is entirely mine. But I hadn't heard/didn't know about the minimum tooth difference spec, so I've learned something else important!

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[RussSeaton]

If you go back in history to the first years of mountain bikes, you will find many triple cranksets on mountain bikes with 48 teeth.  Back then 13 teeth was the smallest cog on the cassette or freewheel.  A high gear of 48x13 was considered a good gear for bombing down mountain trails.  Its a trail, not a smooth paved road so in theory you should go slower off road.  But things changed.  Cassettes began to be offered with 12 and then 11 tooth cogs.  So a chainring of 44 or 42 with a 11 or 12 tooth cog was the same high gear.  And then the 29" craze began and people became obsessed with going down mountains at 99 mph.  So even the 44 chainring isn't really big enough now.  Now some cassettes come with 10 tooth cogs.

Derailleurs have capacities.  Front derailleur can only have so much difference between big and small chainring.  Otherwise the chain will drag on the back of the front derailleur if the inner chainring is too small.  Bad shifting and riding.  The rear derailleur is sized for wrap.  Difference in big and small cogs and difference in big and small chainrings.  Add them together to get total wrap.  Rear derailleur should cover this wrap.  You can exceed all these capacities by a little bit on most bikes and all is well.

If you make a mountain bike using a Bike Friday.  Then you will probably want bigger chainrings on your crank.  Because Bike Friday uses 20 or 16 inch wheels.  So you need bigger chainrings and smaller cassette cogs to get the same gearing as a 700C wheeled bike.  Gear inches is Chainring / Cassette Cog * Diameter of wheel.  You can change any of these three parts so you more or less end up with about 100 gear inches on the high end and about 20 gear inches on the low end.  That range will be good for about every up and down on earth.

[BrianW]

Shimano's "Mega9" drivetrain supports a 48 tooth large chainring. So an MTB FD that's marked with that will work on a 48t ring and can be readily found on eBay in both XT and XTR trims. XTR cranksets from that era often came with 48-36-26 rings. Shimano's modern trekking series cranks come with that setup too, I believe.