sdmc530

So just starting my sons first road bike build. My second build so I am gonna need some help from the BF people who are smarter than I.

Anyways first issue I am confused on is I am doing a 1x road bike. I have these cranks:

SR Suntour XCT Jr. Crankset - 152mm, 9-Speed, 44/32/22t, 104/64 BCD, Square Tape
SR Suntour XCT Jr. Crankset - 152mm, 9-Speed, 44/32/22t, 104/64 BCD, Square Tape 4717385006447 | eBay

I got these because 1. they have the proper crank size for my kid, adn I can remove the the chainrings and make it a 1X 9 speed.

So my question is BB size? Frame is a standard threaded 68mm bb but they make the square taper in 68x113 and 118? What do I get? Shimano has both out there but not sure what is proper....
Is the 118 for a triple set up or like that or am I just assuming off.

they have other sized but these seem to be the most common two sizes....any help would be AWESOME!

Here is the frame (Specialized Allez A1 Max Frameset 50cm Carbon Fork) for referance.

Specialized Allez A1 Max Frameset 50cm Carbon Fork | eBay

Iride01 

That's one of the things that drove me away from square tapers. Too much uncertainty about where the chain line is going to wind up.

And some square taper cranks will sit differently on different brands of square tapers even though both are JIS or any same of the other types.

Until someone comes along with a better answer, I'd just buy one and see if I guessed correctly. Or you can do like I'd really do and switch to anything but square tapers. 2 piece cranks of any sort have all the guess work figured out. Of course they don't allow for bikes that have unique or out of whack rears, but they sure don't have as many possible ways to be wrong.

delbiker1 

This link shows the correct bb is 113 mm. https://thebikehub.com/products/sr-s...terface-ck9841. I have a similar set up that is 113, but my crankset is an FSA, so not the same thing.

Russ Roth

It actually specifies "50mm chain line with a 113mm spindle" and is a setup for a mtb. You will have to take this into account with the 130mm rear end of this bike vs 135 of the mtb this is meant for. Presumably you'll be keeping the middle chainring position but you'll want to measure for the chainline and may need to add spacers between the crank and chainring to move it over for a better centerline. Use a 1x narrow/wide ring, they do actually work. Also keep in mind that Jr are gearing restricted if they're going to actually race and the bike has to meet a 26' rollout, which is a 41/11 with 23c tires, I'll be building my kid's bike with a 40/11 for a close enough.

sdmc530

This was going to be my guess if I have to. BB frustration is real.

sdmc530

Yes, for a road bike, should have been clear on that. Running 1X9 Sora and will get a narrow wide once I get a grasp on what sprocket he likes best. But yeah, didn't think of the 130 vs 135 either. but I would rather be a bit narrow than wide if I can only get "close". Spacers would be fine too if I have to.


No racing just a bike to match dads so no rules in play

Russ Roth

It may be the middle ring will line up but otherwise there are thin spacers that will allow you to move that middle ring more inboard.
Gearing wise, if not racing, you still want to give some consideration to what a kid can handle. From my experience, an 11-13 year old will do fine with a 40t-42t and will rarely need an 11t at that age. Training with a cluster of them, they had no problems maintaining 16mph for 20mi. I've even seen a 17yo with Jr gearing lead a pack of fast cat 5/4 for 3 laps of a crit and finish in the pack at the end of the 20mi.

sdmc530

We are super flat here so I know he will be fine on gearing. His current bike is a 7 speed not sure of granny gear but he doesn't use it much really. I am going to throw on a 11/34 on there and see how he handles it before I adjust. Will be some trial and error with gearing. Big ring might start at 38 and see how it goes. Just hard to tell what he will like.

I think I will try the 68/113 BB. With that crank I think I can get it close. IDK though....hate to buy multiple BB. Shops here don't carry much anymore inventory.

dedhed

Catalog suggests 113 mm for 50mm chainline

https://www.srsuntour.com/fileadmin/...atalog_Web.pdf

sdmc530

thank for that pdf! I will get the 113. Man the brains on here are super helpful.