Suntour XCT-T308-PB Crankset Replacement

NewBikerBiker

Hi,

I am new to cycling and I recently bought a very old 'Drag Ontario" to learn how to ride, and I wish to upgrade that bike and replace old parts with new ones, I really hope I could find some assistance from more experienced bikers here.

The first part I wish to replace is the crankset, the bike originally comes with a Suntour XCT-T308-PB 48/38/28. I am wondering if a Suntour CW-XCT-T312 42/32/22 would be a good replacement, since I noticed the original one have bigger size numbers 48/38/28 thatn the one I intend to replace it with 42/32/22, will that be much of a problem?

In that case, what do you think about a Shimano Altus FC-M31 48-38-28 instead? it has the same size numbers so I think it could be compatible with the bike?

I have tried to search online those size numbers but all info that I could find about crankset sizes is the lenght... I will really appreciate your help.

Thank you.

cooldad

If the chainrings are worn, you should consider replacing the cassette and chain as well.

The Shimnano would work. If you get one with smaller rings you'll need to move the front mech down a bit.

Whatever you use, you may have to replace the bottom bracket as well to get the right chainline. You'll need to see what axle length your current BB has, and what the replacement needs to check.

The link to Parktools in my sig has plenty of useful how to info.

NewBikerBiker

Thanks mate for the helpful info and the useful Parktools link.

What I understand from your answer is that I can use a smaller rings crankset, and adjust the front derailleur position afterwards. Are there any advantages using smaller rings crankset (I am considering getting the Suntour one in hope I will not be forced to change the bottom bracket)

Cheers.

cooldad

It will just lower the gearing, make it easier to pedal, but top speed will also be lower.

The smaller (42/32/22) set is more mountain bike ratios, the larger rings would be better on road.

Depends what you want to ride, really.

NewBikerBiker

Cheers for the great help, man. I'll go with the bigger rings Shimano crankset then. Do you know which bottom bracket size is compatible with that one? Maybe cranksets with the same rings size have the same bottom bracket's size?

Thanks again.

cooldad

Nope but it will be in the tech docs that come with the crankset. They are available on the Shimano website as well.

NewBikerBiker

According to this, it would be a UN26 bracket:https://www.mec.ca/en/product/5037-329/Altus-FC-M311-Crankset-%2828-38-48%29-7-8-Speed

Thanks a lot mate for the great help.
Cheers.

The Rookie

You can use any 122/123mm BB with JIS square taper, however the Shimano UN26 (or a for a little more the better UN55) is a good option.

NewBikerBiker

The Rookie wrote:

You can use any 122/123mm BB with JIS square taper, however the Shimano UN26 (or a for a little more the better UN55) is a good option.

Would it make much difference if UN55 is used instead of of UN26?
Thanks.

The Rookie

None at all, other than the UN55 being slightly better made with better bearings.

02gf74

The lower end un BB have plastic LH cup whereas higher end have alloy, I prefer the latter as they dont crack and less chance of the BB tool mullering the BB Cup.

The Rookie

But it is heavier, I used a 55 with a 23 cup on my commuter to save weight (sad I know).

NewBikerBiker

Hi again,

The bike came with a SR Suntour BB8-XCT-CBT 68/122 mm Bottom Braket, is shimano BB-UN26 68x122.5 a good fit? I couldnt find 68x122 nor 68x123 sizes (Much apologies for my ignorance :oops: ) .

Thanks a lot.

The Rookie

It will be fine, that measurement is just the axle width, a 0.5mm width difference will move the cranks 0.25mm (even assuming the parts were well enough made for that not to be =/- 2mm before you even started!)