SoSmellyAir

I would like to convert my 2017 Trek FX 2 into a 1x bike, and hope to learn which crank to buy without having to change the bottom bracket.

From my research, the FX 2 (previously sold as the FX 7.2) seems to have a JIS square tapered bottom bracket, on which there is a Shimano Tourney 3x (48-38-28) crankset.

Shimano Tourney FC-TY501 Crankset - 175mm, 6/7/8-Speed, 48/38/28t, Riveted, Square Taper JIS Spindle Interface, Black - Modern Bike

Except mine does not even have Shimano branded crank arms; it only has a pair of generic silver colored crank arms. Would this Sugino crank arm set fit?

Sugino RD2 Crank Arm Sets - Modern Bike

Even if they do, would the resulting chain line work with a Shimano Acera 8 speed rear derailleur in the back?

And are there any other (less expensive) 1X crankset that would work? Thank you.

dedhed

Put the chain on the middle chainring, remove the FD, done - 1X.

dedhed

If you look at your Sugino crank link it is for single speed which 1X isn't. Specifies 103mm track and 110 road without telling you what those chainline distances are.
Any given sq. taper crank has a required BB length based on it's design. If you want to use an existing BB for another crank you need to remove the existing crank and measure the spindle length or find that information from a different source. Then you have to choose a crank that will give you the desired chainline with that spindle length. Or you choose a crank and buy the BB that it requires to give you your desired chainline.
Realistically on a 1X (8/9/10/11) chainline isn't all that critical within a few mm given that it changes with every shift in the rear. You need to clear dropouts, chainstays etc. One thing to consider when figuring out what you want for a chainline is where on the cassette you will do most of your riding.

SoSmellyAir

True, although I am willing to spend a little money for a cleaner look. The FX 2 is the first bike in my adult life, and so has some slight sentimental value.

Also, no matter how I adjust the front derailleur, I cannot avoid rubbing when on the cog at either end of the cassette even in the middle chain ring.

SoSmellyAir

I thought the product description for the Sugino crank I linked says that it can be used as a double? In any case, I understand that the chain ring can be mounted on either in front of or behind the 5 star arms, so that would allow about 3 mm of adjustment either way. Hopefully that is enough given what you said about the chain line not being all that critical. This is my pedal around the community with my son (and pre-pandemic, going to nearby coffee shop) bike, so I mostly use the 2nd to the 6th gears (counting from the wheel).

oldbobcat

With the chain ring mounted to the inside, you might still need a shorter BB spindle. But try first. Eyeball the chainline with chain on a center cog of the cassette. It should be parallel to the top tube, give or take a degree or two. I don't know how other guys do it, but I've always found a bad chainline annoying and failure-prone.

SoSmellyAir

Would these work? This crank set says 118 mm / 113 mm spindle length.

Universal Cycles -- Dimension Cyclocross Crankarms [RSC6-RLH 170 BLK, RSC6-RLH 175 BLK]

However, the Shimano Technical Service Instructions state that the spindle length is 122.5 mm

SI-1LX0A-003-ENG.pdf (shimano.com)

dedhed

The linked crank is not a Shimano crank so any spec shimano gives for a FC-M131/171 doesn't apply. We don't know what crank you currently have on the bike to see what the specs are for the existing crank.
You either find out what crank you currently have or pull it and measure spindle length.

SoSmellyAir

I linked that because that seems to be the bottom bracket and chain ring set for the Trek FX 2.

That I have not found out because there are no markings on the crank arms.

I was hoping to avoid this, at least for now, but it sounds like there may be no other way to make sure.

dedhed

Look on the back of the arms. If it's a Shimano it'll say FC-MXXX whatever the model is. Anything else it's a crap shoot if it's marked beyond arm length (170/172.5/175 etc)
Maybe a picture can help someone else identify it as well.

70sSanO

For now, you should try the middle chainring to see if the gearing works. You can always remove the other rings and use shorter chainring bolts; or add a bash/chain guard in place of the outer ring, or you can put tab fillers where the outer chainring attaches.

The point is to see if a 1x8 works or do you need a1x9/10 to get the range you want. Once you know what works best, get a 1x chainring and do it right.

John

SoSmellyAir

Yes, I have ridden this bike using only the middle chain ring since this pandemic started; as mentioned above this bike is only for riding around the neighborhood with my son or going to a nearby coffeeshop.

However, this is a Shimano Tourney 3x chain ring set in which the three chain rings are riveted together.

squirtdad

What is the goal? Is it make it look cleaner? you can just run it in the middle forever, but still have the super low and higher ranges available if you need for future.

70sSanO

Ahh... got it. I don’t blame you for wanting it to look like a 1x. Normally I’d recommend going on eBay and picking up a 90’s XT or LX, but the price on old stuff has become rudiculous.

John

SoSmellyAir

Mostly for the cleaner look, but also to eliminate the chain slightly rubbing against the inside of the front derailleur cage when I am on either the biggest or the smallest rear cog.

I have watched the Park Tools and other videos on YouTube on how to adjust the front derailleur for a triple chain ring, but no matter what I cannot avoid both.

SoSmellyAir

The non-drive crank arm says: SMA (upside down near the bottom bracket), 170, TOP, and F-4 at the pedal end. It does not appear to be a Shimano part.

SoSmellyAir

I have bought both a crank arm removal tool (Park Tool Crank Wrench and Puller | REI Co-op) and a bottom bracket tool (Park Tool Sealed Bottom Bracket Tool - BBT-22 | REI Co-op). I have also ordered the Dimension Cyclocross crankset (110 Crankset | Dimension Bike Products). My plan is to:

(1) Remove the stock crank arms;
(2) Install the new right crank arm onto the stock bottom bracket;
(3) Roughly assess the resultant chain line;
(4) Remove the new right crank arms;
(5) Remove the stock bottom bracket and confirm that it is indeed a Shimano BB-UN26 68x122.5 (SI-1LX0A-003-ENG.pdf (shimano.com)); and
(6) Order a new bottom bracket Shimano (BB-UN300 68 x 1??) depending on what I saw in Step #3.

Any pointers?

SoSmellyAir

After learning about the relationship between JIS BB spindle length and chain line, I finally realized what you meant.

SoSmellyAir

So I mounted a Sugino 38T middle chain ring onto the outer big chain ring position of the Dimension Cyclocross crank arm, and installed the crank arm onto the existing bottom bracket. This resulted in a chain line that was almost in line with the third smallest cog of the cassette.



The cog to cog (center to center) distance in my 8 speed Shimano cassette is about 5 mm (not shown). The chain ring plus carrier is about 7 mm thick (shown below). This means that if I had mounted the chain ring onto the middle ring position, its center would sit about 7 mm closer to the bottom bracket, giving me an almost perfect chain line between the fourth and fifth cogs of the cassette. But a single chain ring looks best mounted on the outer position, especially if it is a different color than the crank arm.



So I removed the stock bottom bracket and learned that it was a cheap VP model with a 121.5 mm spindle length. I ordered a Tange Seiki LN-3922 (made in Japan!) bottom bracket with a spindle length of 107 mm, which is 14.5 mm shorter, which should theoretically move the crank arm and chain ring 7.25 mm inward.

SoSmellyAir

What's up with repeating what I said in post #4 using white text on white background (which is only visible if the text is selected or quoted)?

Much as I like seeing my prose repeated -- imitation being the sincerest form of flattery and all -- I hope you can either contribute to or derive some use from the information in this thread.

SoSmellyAir

No worries; I was just hoping to chat with others who might be crazy enough to spend > $100 on upgrading what was a < $500 (pre-pandemic) hybrid bike ... to reduce its gear range!

SoSmellyAir

I am happy to report that the installation of the Tange Seiki bottom bracket had the intended effect hypothesized in post #19 above, moving the chain ring to line up more or less in between the fourth and fifth cogs of an eight speed cassette.

SoSmellyAir

After dropping the chain inward from the chain ring a couple of times while shifting onto the biggest cog of the cassette, and experiencing the difficulty of extricating the chain from the space between the 74 BCD mounting nubs of the right crank arm, I learned that chain drop is one of the disadvantages of going 1x. So I shortened the chain to its proper length and added a chain guide:

Amazon.com : CyclingDeal Mountain MTB Road Bike Bicycle Chain Guide - Chain Drop Catcher - Chain Clamp Guard - Single Chain Guide - Size 31.8mm & 34.9mm : Sports & Outdoors.

The chain guide does detract from the overall clean look but is worth avoiding the aforementioned difficulty.