Chain line and other measurements on 1990 3.0 MTB frame

[FPrue1304]

So I took some measurements on a 3.0 MTB frame (SM 400, size 20), built 6/1990 (great VIN decoder on this site!), what makes me believe these frames would actually work very well with a modern road/gravel group:

• Drop out space is 130 mm.
• BSA 68 mm bottom bracket.
• I measured the chainline with the installed Suntour 3x cranks and I read for the middle chain ring approx. 45 - 46 mm. Looking at the frame from below, I estimate that the chainstay leaves enough space for a 42 t chainwheel at that chainline. At roughly 50mm chainline (approx. outer chainring), I think up to 50 t should work. A chainline between 45-50 mm should work fine with 130 mm rear hubs.

Does this confirm with your measurements of chainline and space for chain wheels?

 

[black lightning 1987]

What chain rings are you thinking about using? I've never tried fitting a road double to a 3.0 frame but I'm sure it would be fine as far as chainline goes. Can't say about ring size but I could test fit a double crank to a 3.0 frame if I have one with the rings you are considering using.

 

[FPrue1304]

I consider a 1x set up and the appropriate chainwheel could be between 38 and 44, depending in what rear cassette I will find.

I have marked the current chainrings: mid is 36 and the largest is 48.



But I think the q-factor might be the bigger issue, because there is not too much space for narrower cranks, so I will measure that next:





I have the choice to go with road cranks that have a chain line of roughly 43-46 mm or with mtb cranks with a chain line of 49 mm. Road is more desirable to me.

 

[black lightning 1987]

I wasn't thinking about Q factor. Pull one crank arm and reinstall it 180 degrees from where it was, then measure the Q and the gaps to the chain stays. Should give you an idea about how much lower you can go on Q. I'm not sure about finding Q specs for road cranks, other than those printed on Campagnolo Ultra-Torque cranks. I can measure a number of other road cranks given enough time.

 

[FPrue1304]

I found drawings for the cranks I am considering (SRAM Force 1) and they have 120 mm clearance between, while the chainstays are 115 mm wide in the area of the cranks. So it should work but it will be tight. I have to try it but with the current shortages in Europe I cannot get the cranks before May.

 

[black lightning 1987]

Guessing that crank uses an external cup bottom bracket. In a perfect world, the gap to the chain stays will be equal side to side, but it might take a shim behind the cup on one side to equal it out. Worst case would be that you have to take some off the BB shell on the opposite side to add the shim, but hopefully not.

I measured the "inside Q" of a Campagnolo Centaur AL Ultra-Torque crank (Q=145.5) at 119 mm, so it seems that there isn't much variation in modern cranks.

 

[FPrue1304]

A Google search tells me that for biomechanical and aero reasons the Q factor should be as low as possible, ideally even less than 145 so I am not surprised that despite the advent of disc brakes and 12sp it has not increased.
In fact I am using the same crank system (SRAM DUB) on other bikes and by default there are 1 or 2 shims and a preload adjustment wheel so I hope I can finetune the crank.

And yes you are right: these are external screw-in bottom brackets, very similar to these BSA-BB30 adapters you find from Rotor and many other vendors.