maagg

Hi
I have a couple of questions after reading and following so many parts in this forum, I got and old mtb I want to convert to 1x10 and I think I am close....

I have this rockhopper comp year 2000
this came with a 3 x 8 drivetrain
For my understanding if I grab a 10 speed shimano cassette Shimano CS-HG500 11-42T it will fit in my current 8 speed hub
The thing get me confuse is about the chainline and how to pick a new BB and chainring and crankset
I was looking for 34T Wolf Tooth Components Drop-Stop narrow wide chainring and I wonder if I pick any BB for my 68mm shell is ok, I mean, I wonder if this thing about the chainline is most trial and error or what kind of extra measure should I have to know what is the correct BB + crankset

If any of the bright minds of this forum can clear my confuse newbie mechanic minds I would deeply appreciate

tFUnK

Are you sure your BB shell isn't 73mm?

Kapusta

Can’t you just pull the rings off your current crank and put whatever narrow/wide ring you want in place of the middle ring?

You can also put a Shimano 11 speed cassette (or any others that use the HG hub) on your 8 speed hub.

maagg

Yes, I am sure, why? you think it doesnt match with the year's bike?

maagg

Saddly not, it have one of these triple ring all together
I was reading about spacers for crankset, how they can help to get inwards in case is necessary

I dont know, still reading around to see what to buy

tbenjaminsen

Hi!

I did the same kind of conversion on my son’s Trek. I swapped the square taper BB with a regular 68/73mm modern BB that I mounted with the included spacer ring on the drive side. Replaced the crank with a cheap, secondhand Shimano XT crankset. Bought a no-name 30T narrow-wide chainring (Chinese), all plug-and-play, chainline was spot on with the new 10s cassette.

Good luck with your conversion!

-Tom

FrozenBiker 

Yeah, the current popular Shimano hollowtech II bb's do come with a few spacers if needed.

SoSmellyAir

I very recently converted by 3x8 hybrid to a 1x8 drivetrain, as detailed here:

Converting Trek FX2 to 1x - Bike Forums

You are essentially doing the same plus going from 8 speed to 10 speed in the rear, which should both fit the same freehub. So the first steps are to determine the existing (1) bottom bracket, (2) crankset, and (3) chain line of the bike. From your description about the 3 chain rings being riveted together, my guess re: #2 is that you have a Shimano Tourney triple crankset.

maagg

I am trying to do the conversion in this bike



I did found yesterday the are not riveted so I can unscrew the triple chainring, it have 76mm BCD but man, so hard to find 34-36 t 76 mm chainring for 10 speed in UK right now
Does that mean I am better just changing the BB also so I can grab a 104 BCD??? I would like to avoid to change the BB tbh

maagg

Thanks Tom, I did have a look into this, If I need to change the BB and crankset I think I will go this route, around 60£ for a secondhand Shimano XT crankset, I think that is alright

maagg

Why I cant find 76mm bcd 34 -36 t 10 speed chainrings to buy? is this rare???
Does this mean do I need to change BB + crankset and then grab an easy chainring?

Any help is appreciate

DorkDisk

You’re looking for the wrong BCD. 34-36t bolts to the outer holes, and it looks like you have a compact Strongarm crank. These should have a BCD of around 96 (off the tip of my head, you want to confirm this)

maagg

I was looking on the raceface website about how to measure this
But know I am confuse again

I was measuring from the middle to the hole to the next hole, the one I can unscrew




what do you mean is the wrong BCD?

Ronsonic 

BCD stands for "Bolt Circle Diameter." Not hole to hole, but across the thing. From one side to the other:

maagg

Thanks so much
After a while I figured that I misunderstood the way to measure, now I understand that I have a 104 BCD
I managed to order a new single chainring
Today I managed to remove the BB because it feels a bit hard when spins
I found out that is a BB UN40 68 1.37x24
I wonder if I can replace this with BB UN 300, it says is 68x110mm and 1.37x24 like my BB says but between LR says LL123 (mine says M110)
worth to mention that I will keep using the original crankset

SoSmellyAir

Difficult to follow what you are asking? Which bottom bracket (existing or replacement) says LL123? Which bottom bracket (existing or replacement) does "mine" refer to?

From your photos you seem to have a JIS triple crankset, which you will reinstall onto a new bottom bracket. A JIS bottom bracket has two key measurements: (a) shell width, and (b) spindle length. From what you have asked I can only tell that (a) = 68 mm. The spindle length of the replacement bottom bracket depends on where you plan to mount your 1X chain ring on your triple crankset: (i) middle ring or (ii) outer ring? If (i), buy a bottom bracket with the same spindle length as your original bracket to maintain the existing chain line, assuming that the middle ring had previously lined up with the center of your cassette. If (ii), buy a bottom bracket with a shorter spindle length. How much shorter? New spindle length = old spindle length - 2 x horizontal distance between the outer ring and middle ring of your crankset, measure from the tip of the teeth.

If you look at my other thread about the 1X conversion of my Trek FX 2, (ii) is the path I followed, because I think mounting a 1X chain ring on the outer ring of a triple crankset looks better. A shorter spindle length also decreases the Q-factor (i.e., horizontal distance between pedals), but I am OK with that because I am more used to the narrower Q-factor of my road bike anyway.