guy153

According to this doc:

https://www.servicearchive.sram.com/...019_road_0.pdf

Page 49, the chainline is 43.7mm. That sounds like a road chainline to me, implying 130mm rear spacing to put the middle of the cassette in the middle. But when I measure that scale drawing, it's a 73mm BB shell. This seems a little weird to me: usually 73mm shells go with 135mm OLDs, and 68 with 130.

Most commercial bikes available with this stuff have rear spacings like 147 and disk brakes. No idea how that's supposed to work-- surely the chainline would be way off?

Should I make the frame with 73/130? or 73/135 or 68/130? It's going to have caliper brakes and I think I can use a regular road or MTB 10 speed hub for the SRAM 11 speed cassette.

dsaul

What crankset? All SRAM road cranksets( except for the special Wide Axle Rival S522) are made to fit a 68mm bottom bracket with no spacers. They will not fit in a 73mm bottom bracket. Don't worry about the chainline, I've built several SRAM 1x frames with Rival1 and Apex1 with 12x 142 rear spacing and had no issues. The difference in cassette spacing between 130 and 135/142 is 2.5mm, which is less than the thickness of a chainring.

guy153

Thanks for the info! The crankset is a SRAM Apex 1 road and now I check their website it says 68mm. But weirdly it looked like 73 in that drawing. Perhaps I measured it wrong.

So definitely 68 BB and as you say it doesn't make much difference, but presumably 130 is the optimal rear spacing?

unterhausen

I would think the optimal is 142TA /135QR since they mostly expect for it to go on a disc bike.

dsaul

According to my calculations from the freehub body specs in that same SRAM document, the "ideal" chainlines are as follows:

130 OLD 10 speed freehub - 44.7mm
130 OLD 11 speed freehub - 43.8mm
135 OLD 10 speed freehub - 47.2mm
135 OLD 11 speed freehub - 46.3mm

The maximum variation between the "ideal" chainlines is 3.4mm, which is less than the spacing between cogs in a 10 speed cassette. In terms of shifting performance, especially with a single chainring, that is an insignificant difference. Let's not forget that MTB frames use the same 135/142 OLD hubs and the cranksets have a chainline of 50+mm, which is still outside the "ideal " chainline in the opposite direction. Chainline is not even something that I consider when designing a frame.

Andrew R Stewart 

I'll second that chain line is a far lower aspect of design on my frames. But then i don't push the chain stay shortness limits and don't build for 1x as well as expect the very few i build for to know about their gear system's limitations. Andy

guy153

Thanks everyone! I'm going to go with 68/130 as this looks like it's what it's meant to be and is also standard.

Agree it's not that critical but there are enough opportunities for mistakes on a frame build that it's best to be as careful as possible!

unterhausen

I spent 6 years interpreting drawings and the lack of that dimension offends me a little. If you are going to refer to a centerline, the width has to be on the drawing. Also not particularly happy about referring a dimension to a centerline. I'm pretty sure Shimano has it on their specs.

guy153

I also think the drawing might have been wrong. I opened it with the Gimp and used the measure tool to establish the scale and from that it looked like the BB shell was 73mm. Maybe they just copied and pasted a drawing from somewhere not expecting anyone to be daft enough to assume it was actually a proper scale drawing.

unterhausen

Yeah, it's not a scale drawing. But it needs to convey the required information. Maybe if they ever get any OEM customers they supply a properly dimensioned scale drawing

helixbikes

If it helps, I've built a frame for a SRAM Force 1 build using BSA68 & 130mm dropout spacing. No chainline issues, though plenty of tire clearance issues due to my sketchy decision to force 2" tires without bent chainstays/seatstays. You'll be fine!

guy153

SRAM 1x11

Thanks! Built the bike up today. 68mm shell, 130mm spacing, chainline is perfect.

Was worried for a bit I'd somehow done something horribly wrong when the chain kept falling off while I was setting up the shifting but then realized I just hadn't popped the quick-link properly

The frame is 100% purpose-built for 1x with a shifter boss / cable stop thing only on one side and a single brazed-on cable guide under the BB shell.

The deal with the road cranksets is you actually can't use a 73mm shell even if you wanted to as the axle is too short.

The tyres are 25s. You could maybe squeeze 28s. I just think it's nicer to use a road fork and road stays. But I've made the chainstays a little bit longer than "pure" roadbike and there is just enough room to mount a fender.

The 1x stuff is pretty nice but that huge rear cassette is rather heavy. When you pick the bike up the extra weight at the back is quite noticeable. This is Apex which is the entry level I think.

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