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Bottom bracket length for correct chainline?

Old 02-13-17, 10:59 AM
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Diewahreart
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Bottom bracket length for correct chainline?

I've been lurking on these forums for a while, but this is my first post! I am so appreciative of the fund of knowledge to which you all contribute here. Before buying my first adulthood bike in 2015, and every other bike related purchase and decision I've made, I've researched it here; you all have helped me immeasurably.

After a couple thousand miles on my 2015 Trek FX 7.4 WSD, I am getting to the point of considering how to upgrade the Shimano Acera triple crankset it came with. So far I am favoring a Stronglight Impact Triple crankset, which is apparently a rebranded Sugino XD (BTW is this considered a touring crankset? Mtb? XXcycle.com seems to think it's a road set); and I'd like to stick with a 9-spd mtb cassette. I want to replace the big ring on the Stronglight with a chainguard (with my current riding style and urban riding environment, I simply don't need a big ring), hopefully thereby circumventing any potential compatibility issues with my mtb front derailleur, as long as I adjust the inner/outer limit screws properly. (Please advise if my thinking on this is totally incorrect!)

What causes me the most uncertainty is the length of the bottom bracket to achieve the correct chainline with this crankset. According to Shimano specs, the UN26 square taper BB on my FX has a 122m spindle length, giving me a 50mm chainline. Sheldon Brown's site says that a 115mm BB with the Stronglight triple yields a chainline of 45mm. Treefort Bikes recommends a 113mm BB length for this crankset on mountain frames, for a 47.5mm chainline; and from reading various other threads and vendor websites, I have gathered that other recommended BB lengths include 110mm and 115mm. Also, some folk report that 118mm is wider than anyone needs.

Hence my confusion. If the desired chainline on my bike is 150mm, and a 115mm length BB with the Stronglight gives me a 45mm chainline, as Sheldon says, wouldn't I want a BB length of at least 125mm? I am assuming that any BB I get (probably a Shimano UN55) will be symmetrical, and thus increase distance equally on both sides, which may be incorrect? It seems to me that in order to work with a mtb cassette, the front chainline should be as least as great as it is now (with current mtb crankset and cassette set-up). Not only that, but removing the big ring in front, as I plan to do, effectively moves the front chainline inward (i.e., results in reduction) about 2.5mm, according to Sheldon. I do want to be able use the granny ring a little more than I do now, and potentially use it with the first (biggest) 5 cogs at least, since I will have only 2 rings. Does this mean I should be looking for a 127mm BB at least and/or use bracket spacers?

115mm (to get 45mm chainline) + 10mm (to get 50mm chainline) + 5mm (to compensate for removing outer ring) = 130mm.

On the other hand, if TreeFort's info, not Sheldon's, is right for my bike, then it seems the BB I have now would just about work.

113mm (to get 47.5mm chainline)+ 5mm (to get 50mm chainline) + 5mm (to compensate for removing outer ring) = 123mm

Which info is correct? And what am I missing?

Please forgive my ignorance if I am asking all the wrong questions and making the wrong assumptions/decisions, as I am still feeling pretty new to all of this (just changing tubes and tires by myself last year was a major spiritual and physical victory for me). I am trying to research this as well as possible ahead of time, rather than just buying different length BBs and experimenting (as was frequently suggested), because I will be attempting the installation myself at my local bike co-op; and even with their help, this is a job I'd rather not do several times. I did find several threads re: this crankset, but in none of those cases did they seem to be installing them on a bike just like mine. PLEASE feel free to advise me in any aspect of this upgrade I am contemplating. Many thanks for your patience and your help!
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Old 02-13-17, 11:43 AM
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Have you done the math to see if a cassette change can get you what you want?
Most of us have little use for an 11T and/or 32T cogs.
Maybe something like a 12 or 13 or 14-25T cassette would make your big ring "usable"?
It would certainly be less expensive to try.

What's the lowest & highest gears you actually use/need?
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Old 02-13-17, 11:54 AM
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Originally Posted by Diewahreart View Post
After a couple thousand miles on my 2015 Trek FX 7.4 WSD, I am getting to the point of considering how to upgrade the Shimano Acera triple crankset it came with.
Any particular reason you're looking to change out your crankset? Gearing change? Weight? Aesthetics?

The reason I'm asking is that your chainrings are bolted to the crank arm, meaning you can swap them out without having to change the whole crankset. You may be able to accomplish what you want without buying a crankset or bottom bracket.

Originally Posted by Diewahreart View Post
So far I am favoring a Stronglight Impact Triple crankset, which is apparently a rebranded Sugino XD (BTW is this considered a touring crankset? Mtb? XXcycle.com seems to think it's a road set)...
There's nothing that really makes a crankset a "touring crankset" other than chainring selection -- basically making sure your small chainring gives you low enough gearing to get over whatever hills or mountains you might have to cross. Thus, "touring cranksets" are typically triples with a small (30-tooth or less) inner chainring.

Triple "mountain bike cranksets" can often be found with small inner chainrings ...and they can make fine touring cranksets.

Originally Posted by Diewahreart View Post
I want to replace the big ring on the Stronglight with a chainguard (with my current riding style and urban riding environment, I simply don't need a big ring), hopefully thereby circumventing any potential compatibility issues with my mtb front derailleur, as long as I adjust the inner/outer limit screws properly. (Please advise if my thinking on this is totally incorrect!)
Sure, you can use two-out-of-three rings on a triple, replacing the outer ring with a guard -- a "bash guard" in mountain bike parlance. You're right that you'd want to set the limit screw such that your derailleur can't shift up and onto the outer ring. That's a perfectly functional setup. The only oddity that you might notice is that you won't be able to shift into the "3" position on your front (left) shifter.

Originally Posted by Diewahreart View Post
What causes me the most uncertainty is the length of the bottom bracket to achieve the correct chainline with this crankset. According to Shimano specs, the UN26 square taper BB on my FX has a 122m spindle length, giving me a 50mm chainline.
47.5 to 50 mm, measured from the bike's centerline to the middle ring, is textbook for a Shimano mountain triple setup. (50 gives a little extra clearance for the derailleur to swing if your frame has a large-diameter seat tube.) 45 mm is the spec for road triples.

Originally Posted by Diewahreart View Post
Sheldon Brown's site says that a 115mm BB with the Stronglight triple yields a chainline of 45mm...

115mm (to get 45mm chainline) + 10mm (to get 50mm chainline) + 5mm (to compensate for removing outer ring) = 130mm.
Your math is right, but this is one of those instances I might be tempted to obtain just the crankset and test fit it on your current bottom bracket. If it works, great! If not, it's an easy measurement to figure out what bottom bracket spindle length you need.

Originally Posted by Diewahreart View Post
Which info is correct? And what am I missing?
As for what's correct... My guess, based on Stronglight's own web site, is that a 115 mm spindle will give you a 45 mm chainline. Stronglight's site specifies a 115 mm spindle but doesn't explicitly list chainline. Since it's a road crankset, though, and 45 mm is typical chainline for triples in a Shimano road bike drivetrain, I think it's a reasonable inference. The chart on Sheldon's page (which may have been sourced from an authoritative source) seems to agree.

As for what you're missing... What chainring sizes do you intend to run? The derailleur may have to be repositioned to accommodate. If the chainring sizes are significantly different than the inner two rings on the original, the sculpting on the derailleur cage that's intended to aid shifting might not line up quite right. (Probably not a huge deal and may not be an issue at all... just trying to dot all the i's and cross all the t's.)
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Old 02-13-17, 01:49 PM
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Originally Posted by SkyDog75 View Post
Sure, you can use two-out-of-three rings on a triple, replacing the outer ring with a guard -- a "bash guard" in mountain bike parlance. You're right that you'd want to set the limit screw such that your derailleur can't shift up and onto the outer ring. That's a perfectly functional setup. The only oddity that you might notice is that you won't be able to shift into the "3" position on your front (left) shifter

It would be better to set up the shifter so that the 2 and 3 positions correspond to the inner and outer chainrings, with the 1 position leaving the wire slack. The reason for this is that if you forget and try to shift against the stop you risk jamming the shifter ratchet, causing an inability to shift back down without undoing the wire to release the tension or even breaking the mechanism. Having the wire slack in the unused 1 position will do no harm.
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Old 02-13-17, 11:25 PM
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I love you guys.

Bill: re: gearing, I was afraid you all would laugh at me if I said how low gearing I want. Right now my set-up is 26-36-48 in front and 11-32 in back. The first time I went up the hill to my house in my lowest gear, I giggled because I couldn't imagine wanting a lower gear, or staying upright in a lower gear if I had one (without spinning a zillion rpm). But I have since changed my mind, after going up some unfamiliar and steeper hills with my full load of groceries; and now wouldn't mind one gear even lower than what I have now. So although like you I don't use the 11t cog much, unlike you I DO use the 32t already.

But most of my riding is between 40 and 75 gear inches (is that how you say it?). I can and do use the third ring now, especially just before stopping, so I can downshift a lot by just moving to the middle ring, rather than shifting a bunch on the rear cogs. But I don't think it's an absolute necessity for me, if the middle ring is big enough to give me gearing range; and I MUST have a chain guard, given how clumsy I am. I assume it's possible, depending on the bash guard, to install it WITH a big third ring on a new crankset; but I guess I thought if I don't need a third ring, why not lose it? After diddling a bunch with Sheldon's gear calculator, I was thinking of 24t-38t in front, and keeping 11-32 in back. This would give me gearing even lower than my current lowest, and the 38 front/11 back combination would be more than required by even my most desperate speeds! Actually the main issue is that if I go faster, my reaction time is not quick enough to respond evasively to the car drivers here who either don't see me or don't care. I wouldn't mind even going to 11-34, except that in combination with the 24-38 rings in front, it created a lot of redundancy, which bugs me a little bit (though I'm not sure it should). I suppose I could still go lower in the cassette, like a 12-36; but I haven't seen much besides 11-32 or 11-34 in my online shopping so far. What do you think?

Skydog: two main reasons for wanting a whole new crankset rather than replacing worn chainrings, the first being aesthetic. That Acera crankset is ugly and heavy. Re: the latter, I'm not a weight weenie in terms of riding my bike, wanting to go faster (obviously not, if I'm willing to lose the big chainring), etc., but because in order to put my bike on the rack at the front of the bus, I have to lift it to my shoulder height. This is already almost beyond my utmost capacity, straining every muscle I have; and in so doing I usually bash my head and hands, staggering and teetering under the weight, until I get my bike on there. (And for what it is, my bike is on the lighter side, isn't it?) So while weight is not a primary consideration in upgrading, if, while I'm at it, I can decrease weight even the tiniest bit (without a great deal of extra trouble or $), that can only be a plus. 24t/38t shouldn't be a problem for my mtb FD, should it?

I think I might eventually, as you suggest, just try out the Stronglight on the BB I have now and see if it works; and maybe also order a 127mm BB just in case? Do square taper BBs commonly come in lengths longer than that? The name Phil Wood somehow sticks in my mind...spendy, though.


dsbrant: so my shifter should "think" that the middle chainring is the outer ring, and the granny gear is the middle ring? Do I understand this correctly? So that if I forget and move the shifter to 1st position, what happens = nothing. <-- Stuff That Would Never Have Occurred To Me!
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Old 02-13-17, 11:36 PM
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"Some" Deore 9 speed RDER's can handle a 36T largest cog. Can you find the model#?
How about this cassette?
Shimano HG400 12-36 9-speed Cassette - Harris Cyclery bicycle shop - West Newton, Massachusetts
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Old 02-14-17, 08:19 AM
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"dsbrant: so my shifter should "think" that the middle chainring is the outer ring, and the granny gear is the middle ring? Do I understand this correctly? So that if I forget and move the shifter to 1st position, what happens = nothing."


Exactly. Just set your shifter to "2" instead of "1" and then set up your front derailleur normally. This procedure is a good one: Front Derailleur Adjustment | Park Tool Then, when you accidentally shift into "1" the wire will slacken but the FD will sit against the low stop.
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Old 02-14-17, 08:37 AM
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Would it not be better if you measured the chain line of the middle cog on the cassette and matched that number with the middle chain ring (or half way between the rings on a double). You cant do this with at two piece crank, but with a separate crank axle Id sat thats the way to go.
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Old 02-14-17, 10:40 AM
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I'm trying to figure out some of the responses, but I got a headache so I'll give it a go...

On your chainline, if your bike is setup correctly now, in theory the chain will the perpendicular to the hub when you are in the middle chainring and on the 5th cog (middle of the cassette). If Stronglight recommends a 115mm it is based on trying to the get middle chainring to be lined up to the middle of the cassette. There may be quirks to each bike so it may not be a perfect alignment.

Because you are only going to use the inside and middle chainrings, in theory, the middle alignment is technically centered between the inside and middle chainrings. The reason why some people are suggesting a 118mm is to push the crank further out and try to get the chainline centered between the inside and middle ring. I don't know why Treefort is suggesting 113mm. If it is based being familiar with your setup, then it is sound advice. If it is because it is close enough, then I would probably go 115mm. Keep in mind, all the best math sometimes doesn't play out on practice.

As far as shifting, you don't do anything but set the limit screw to block out the outside (large) chainring. There is no tricking 1 becomes 2, or 2 becomes 1.. 1 is 1 and 2 is 2 and there is no 3. Apart from adjusting the front derailleur for the new setup, there is little you need to do.

John
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Old 02-14-17, 02:11 PM
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Originally Posted by 70sSanO View Post
As far as shifting, you don't do anything but set the limit screw to block out the outside (large) chainring. There is no tricking 1 becomes 2, or 2 becomes 1.. 1 is 1 and 2 is 2 and there is no 3. Apart from adjusting the front derailleur for the new setup, there is little you need to do.

John
Perhaps you should re-read my response and you would know why I suggested that method; blocking out the high shifter position with the limit stop leaves one open to jamming or damaging the shifter mechanism as a result of attempting to shift against the stop. I have had this jamming happen to me so it is a real issue. Shifting into the non-functional low position will do no harm, and will only result in a slack wire which is easily recovered by merely shifting up again.
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Old 02-14-17, 03:01 PM
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Lets talk about what "chainline" means:

1. Ideally, you'd like for the chain to be parallel to the bike's main frame. With multiple sprockets on both the front and back, that obviously isn't going to be the case all of the time. Not to worry because it doesn't have to be perfect.

2. The biggest single factor in determining what BB length you need to achieve that perfect chainline is the crankarm design. Older cranksets tended to need bottom bracket spindles that are quite wide by modern standards. Frankly, I think that 122 mm is pretty wide. Unless there is something else that's unusual in the design of your bike, I'd rely on the spindle length recommended by the crank seller.

3. The second factor in determining BB spindle length is (surprise) rear dropout spacing. Road bikes typically have 130 mm dropouts. Mountain bikes generally have 135 mm dropouts. That means that the cassette on a mountain bike is going to be spaced 2.5 mm farther outward. That means you need to move your chainrings outboard to achieve that perfect theoretical chainline. If your crankset seller recommends a range like 110/113mm for spindle length, choose the longer for 135 mm dropouts and the shorter for 130 mm dropouts.
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Old 02-14-17, 08:57 PM
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RG... Great catch on the drop-out width. The 7.4 has a 135mm rear hub (shimano FM30) so going narrower BB spindle will only make the chainline worse if the crank is for a 130mm.

As for the shifter jamming by the use of the limit screw. I have never heard of that and I don't know any mountain biker that has converted a 3x to a 2x ever do that. I can't imagine ever inadvertently shifting to a non-functioning position just to cause a shift to not happen when I really needed to shift to the middle gear. I also wouldn't want to ride off-road with a hanging cable. FWIW, I have two 3x to 2x mtb's and I set my wife's up as a 2x.

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Old 02-14-17, 09:53 PM
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The Stronglight Impact Triple is considered to be a 9/10 speed road crank, so the rear hub would be 130mm. Since the Trek FX 7.4 has a 135mm hub, (mtb), it would appear that a 118mm might be a better length. Have no clue why Treefort would suggest 113mm, except they think it is going on a road bike.

Do new road bikes still have a 68mm shell width or do they have 73mm?

John
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