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Discussion Starter · #1 ·
Okay, so the deal is that I've decided to just run the front large and middle chainrings. I'm going to ditch the granny. I"m not going into why I've decided to do this because I do not want to get into a discussion about that asped. My question is whether or not I should be running a narrower BB to move my remaining two rings in closer and possibly alleviate some of the cross chain problems in the big ring-big cog combo? or should I leave everything the same and just take the granny ring off. It seems like I shouldn't need to change the BB.

Secondly, I'm assuming for derailleur set up I should simply put the limit screw in far enough that it will not drop off the bottom of the middle ring. Is that correct?

I'm interested to hear if anybody out there has done this.

Thanks!

Gabe
 

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depends

With just two chainrings, you'll probably find yourself using both of them more, splitting time between two, rather than among three.
To be able to use all the available gears, including the larger cogs with the big ring, yeah, it wouldn't at all hurt to be able to shift the two rings inboard so the gap between the two rings lines up with the 5th cog of a 9sp. cogset or the gap between the middle 2 cogs of an 8 sp. setup, that'd be textbook good chainline.
Even more useful I should think would be swapping out the smaller stock chainring for an even smaller one. 44/29, 42/29, 42/28 are popular double crank gears with compact cranks. Assuming you got compact and your smaller ring would be a 32t, even with a 32t in back you only have 1:1, which might be limiting on the steeps, and definitely at altitude. I didn't look, maybe you live in Illinois or something where that's not even an issue.
I was running a 42/29 with a 12-34 in back for awhile until the frame broke. Plenty low for all local stuff, not even low enough for serious high altitude backcountry. I used a shorter spindle and could use the 42 ring with the second largest, 30t cog just fine. Nice thing about just 2 rings is you do more shifting on the rear, which is always easier than front shifts and you never have that mental dilemma "go to granny or stick it out in the middle?" trying to anticipate the trail ahead. That's what I like best about a double, relief is just a rear click away, no granny or not granny mental debate.
 

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ya I know, you don't want to get into that "asped" (aspect perhaps?)...but I just can't let this pass: WTF would you want to do that for? Are you looking to just bike on pavement? or do you live in an area so flat that the little ring is of no use? If so then I feel bad for you. I'm about to do the opposite and ditch the big ring. Just bent another tooth today. I haven't used the big ring in at least a month. Gonna slap a bash guard on in its place.
 

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both approaches are valid

don't feel sorry for me, I live and ride in Durango. On one bike, with a 20/32/42, 12-32t drivetrain, I NEVER use the granny ring at all on any of the local -- as in, ride to the trailhead -- rides. Never. 1:1 is the lowest I ever use. I leave the granny on for the occasional high/backcountry visit, though I usually take a different bike set up for those conditions.
Conversely, on another bike that I ride locally as well as in the high country, gearing is 24/34, 11-34, no big ring at all, and my high gear is 81" which is plenty high enough to get to and from the trailheads.
Both setups work well on the local stuff. So I'd have to ask, using your vulgar terminology, "WTF" are you questioning another's chosen gearing for without knowing more about their riding conditions?
 

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ballbuster
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heck, why not?

gabe0807 said:
Okay, so the deal is that I've decided to just run the front large and middle chainrings. I'm going to ditch the granny. I"m not going into why I've decided to do this because I do not want to get into a discussion about that asped. My question is whether or not I should be running a narrower BB to move my remaining two rings in closer and possibly alleviate some of the cross chain problems in the big ring-big cog combo? or should I leave everything the same and just take the granny ring off. It seems like I shouldn't need to change the BB.

Secondly, I'm assuming for derailleur set up I should simply put the limit screw in far enough that it will not drop off the bottom of the middle ring. Is that correct?

I'm interested to hear if anybody out there has done this.

Thanks!

Gabe
Roland Green runs a double up front, I think.

If you can find some older XTR Octalink1 cranks (with the removable spider) you can use that with an Ultegra BB for a double. That should bring in the cranks enough to center them on the cassette. Road bikes have a slightly narrower rear end (130mm vs mountain's 135mm), so the chainline probably won't be 'Dead On Balls' centered, but it will be closer. The added benifit here is that your pedals will be closer together, so you can get a tad bit more leverage on them.

The other possible issue may be that the big and middle rings could end up too close to your right chainstay. I ran into this on my SS and had to move my one chainring to the outside of the crank arms.

Do your own research, but this could work out well. I would even check in the weight weenie board to see if anybody over there has done it.

And as far as cross-chaining, I do it all the time. Just don't spend many long miles there. My favorite gear for tight techincal decending is big ring/3rd cog from the big cog.

As far as loosing the granny, go for it if you really don't use it. I ride with Impy all the time, and that chick hardly ever goes to the granny, even on stupid steep stuff. It's amazing. She just mashes everything. She's got legs like tree trunks (in a good way). Me, OTOH, rely on that 22x34 granny when things get steep. I'm kinda heavy and need all the mechanial advantage I can get to get up the stupid steep stuff.
 

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-arschloch-
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gabe0807 said:
Okay, so the deal is that I've decided to just run the front large and middle chainrings. I'm going to ditch the granny. I"m not going into why I've decided to do this because I do not want to get into a discussion about that asped. My question is whether or not I should be running a narrower BB to move my remaining two rings in closer and possibly alleviate some of the cross chain problems in the big ring-big cog combo? or should I leave everything the same and just take the granny ring off. It seems like I shouldn't need to change the BB.

Secondly, I'm assuming for derailleur set up I should simply put the limit screw in far enough that it will not drop off the bottom of the middle ring. Is that correct?

I'm interested to hear if anybody out there has done this.

Thanks!

Gabe
In going to a narrower BB the thing you are really gonna have to watch out for is chainring and crank-arm to chainstay clearance. An adjustable BB is a good solution for getting your chainline and crank spacing just right. What set up are you running now? That may help some of the product-knowledgeable folks give you more info.

As for the why-do-that-question, no worries. If you got the guns to handle it, go for it. 95% of the time I found the granny to be pretty useless even when I lived on the CO front range. Now I run either single speed or single front chainring on all my mtbs.
 

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I ditched the small chainring before converting to SS. Later I dumped the big chainring and replaced it with a rock guard (should be called a calf-guard LOL). The middle ring can give "excessive" speed for all local trails.
I wouldn't change my BB just to adjust the big-big chain line, I'd rather shift to middle-middle. A narrow BB is good for a road bike (spinning) but on a mtn bike you give up (slightly) the ability to shift your weight from side to side.
 

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Vecsus said:
ya I know, you don't want to get into that "asped" (aspect perhaps?)...but I just can't let this pass: WTF would you want to do that for? Are you looking to just bike on pavement? or do you live in an area so flat that the little ring is of no use? If so then I feel bad for you. I'm about to do the opposite and ditch the big ring. Just bent another tooth today. I haven't used the big ring in at least a month. Gonna slap a bash guard on in its place.
Pretty much anyone who races at the expert level or higher has no use for a little ring; even on hills. So, who cares why.
 

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Some racers use a double set up but many still use a triple. Tinker Juarez for example has always used a double. Typically racers/riders who go double switch to a 29/42 or 30/42 gearing at least, since a 32/44 or 34/46 standard size will be too heavy for many conditions. Its strongly recommended that you do use a shorter BB (if your frame allows) to center the rings a bit more and keep a straighter chainline. Thats very important given that you wil be using the big ring a lot more in a 29/42 setup.
 

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Discussion Starter · #10 ·
Thanks everyone for your input. I don't have the bike all the way together yet so I don't have exact specs of what I will be using; however, your suggestions confirm what I was already thinking. I very well might end up using a road BB and see how it works. I know my old C'dale Jekyll used one to keep it narrow. I'll experiment a bit with this.

I did not want to get into my reasons for running two front rings because I know that there are differences in opinion which on this board seems to mean flame wars. Some people don't see any benefit to taking either off. Some don't run a large gear. Some don't run a small gear. Some people run only one front and one rear. I'm just trying to discuss the mechanics of the situation not the rationale.
 
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