Bigger chainring

[DeivisAteivis]

Hello, I currently have a SCOTT Scale 970 MTB with SRAM SX Eagle DUB 55mm CL / 32T chainring and SRAM SX-PG1210 / 11-50 T 12 Speed cassette. Since i started using it more for road cycling, I would like to upgrade it to 42-48 t. My frame takes a maximum of 38 t. How can I modify my bike in order to fit the chainring with good chainline? Maybe spacers in the front and back if that's possible? Also 38 t just seems too slow for me and I have ridden higher chainrings before.

 

[Sharky]

My frame takes a maximum of 38 t.

What is the restriction with going over 38t?

 

[DeivisAteivis]

What is the restriction with going over 38t?

There's not enough space to fit a chainring that has over 38 t. Which means that it would be touching the frame.

 

[cyberknight]

I would be interested in what your current speed on the flat is ?
according to a speed calculator on the flat 38x12 at 90 rpm gives you around 25 mph

 

[chriswoody]

What is the restriction with going over 38t?

It will be frame clearance I suspect. This is the rear frame on my Sonder, with a 36 tooth chainring. There's no way I'm going any larger than 38t, there just isn't the room.

The OP's bike won't be as extreme as this, because I have boost rear hub spacing, the Scott has a narrower non boost rear end, so the chainstays are not as wide, however the chainstays will still restrict the maximum size of chainring available.

The Scott does run a 1x12 speed drivetrain and a 55mm chainline. The chainline is optimised to bring the chainring in line with the middle of the cassette, thus reducing wear and increasing shifting performance. Mucking about with this chainline in order to increase the size of the chainring will bring about a load of unintended consequences, especially with these wide range 12 speed cassettes.

There's an interesting deep dive into the murky world of SRAM chainlines here if anyone's interested:

https://theradavist.com/55mm_chainline/

If your going to be primarily going to be running on the road, I'd be looking at narrow tires, possibly gravel bike tires, as well as increasing the chainring to the maximum 38 tooth. Ultimately though, maybe you really need to be looking at a bike more suited to what you intend to do with it, maybe even a gravel bike would be better than a mountain bike running a 1x drivetrain?

 

[lostinthought]

Can you fit a 10-up cassette? 10/38 gives you much the same ratio as 11/42, for a a 100-ish inch top gear.

 

[Jenkins]

Can you fit a 10-up cassette? 10/38 gives you much the same ratio as 11/42, for a a 100-ish inch top gear.

Not without a new freehub. A road 12 speed cassette with a lowest 11 tooth cog would fit though.

 

[Jameshow]

You could change the BB as well to a wider one say 5mm so that for every mm you push the chainring out you get 3mm clearance? (Going by the pic above) But that means the chain lines poorer for the low gears which you say you hardly use.

 

[chriswoody]

You could change the BB as well to a wider one say 5mm so that for every mm you push the chainring out you get 3mm clearance? (Going by the pic above) But that means the chain lines poorer for the low gears which you say you hardly use.

Apologies, the pic above was from my bike sadly, which is very different to the Scott that the OP owns. I just popped it in there to illustrate the frame clearance issues, I hope it doesn't cause any confusion.

Looking at the specs for their bike on Scotts website, they appear to have a press fit bottom bracket a SRAM DUB PF 92 MTB Wide. They will have a 3mm spacer on the left hand side, so in theory they maybe could move this to the drive side and push the chain line out by 3mm, but that would be it. There would be no possibility of a wider BB or extra spacers because the frame and the spindle length of the crankset wouldn't allow it.

The other option is to change the chainring to one with a greater offset, if one existed. This Diagram from the Wolf tooth webpage should illustrate what I mean.

Whether any of this would work though is another matter.

 

[CXRAndy]

Use a twin chainset and remove the inner ring, or there maybe enough clearance just accept and dual crank
like this https://www.sigmasports.com/item/Sh...PS6t5zsd9pf-hPJslQBNNFYEG6bpz5DRoCRKIQAvD_BwE

 

[wafter]

I think @chriswoody has pretty much nailed it already..

The large grey trunked mammal in the room is that pushing the crankset outboard to give clearance on a larger ring will result in a less-than-optimal chainline at the rear. To the best of my knowledge this can't be changed at the back end as it's dictated by axle spacing and hub format - there's simply no more space to push the cassette further outboard.

While going to the max. of 38T would obviously give a useful increase in gearing, ultimately the setup sounds extremely sub-optimal for road use and I'd be looking at alternatives. Given how specialised MTBs have become a new bike is probably the easiest route to more appropriate gearing.

 

[CXRAndy]

Yes the chain line won't be optional, but it's only a few Watts of loss. The upside is you will be travelling much faster