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i've narrowed the shortlist down to middleburn ro2 and white industries a30. not entirely sure if the middleburns fit with a 73mm bb, but assuming they do with certain spacers.
middleburn ro2 (q factor stated online as 153mm, i've added 2.5mm to compensate for the 73mm bb)
Middleburn tout the RO2 as a 'road' crank so not sure why it would be compatible with a 73mm bb* and even if it is, the bb shell width won't change the q factor**.
*Shimano make their HT2 mtb cranks compatible with either 68 or 73mm shells, you add spacers between the bb cups and shell to make up the difference, HT2 road cranks are 68mm only. The spindle is shorter and you fit no spacers between bb cups and shell.
**bb shell width has no bearing on crank position. Imagine square taper bb and cranks, a 68mm bb with 122mm axle puts the cranks at the same position from centreline as would a 73, 85 or even 100mm bb with 122mm axle. The only way you could get a wider q factor with the 'burns would be to fit a longer spindle. They are commonly photogrpahed with the spindle fitted in the nds crank but the picture below from the middleburn site shows they are separable and the following page lists different axles that are available - https://www.mountainbikecomponents.co.uk/product/rs8-x-0003-73-mm-2/. That shows a 138mm axle as being for fitting RS8 Xtype cranks into a 68/73mm shell. I'll leave it to you to research whether the RS8 and RO8 cranks share a spindle interface.
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M_V
launchpadboi
i'm about to build a new gravel/bikepacking frame, which i'd still need a crankset for. i've narrowed the shortlist down to middleburn ro2 and white industries a30. not entirely sure if the middleburns fit with a 73mm bb, but assuming they do with certain spacers.
i'd like to keep the q factor as narrow as possible and made some highly theoretical calculations on the crank arm clearance using my road bike's current setup as a reference. ta'da:
...
reference road bike with 2010 sram force 110bcd cranks (68mm bb)
q factor: 145.2mm
outer stay width @ 19cm (from middle of axle to the outer edge of crank arm on 170mm cranks): 103mm
gap between crank arm edge and chainstay outer edge: 12mm
...
middleburn ro2 (q factor stated online as 153mm, i've added 2.5mm to compensate for the 73mm bb)
q factor 155.5mm (5.15mm wider per side vs. the reference sram force)
new frame outer stay width @ 19cm: 120mm
gap calculated against the reference: 12mm - 8.5mm + 5.15mm = 8.65mm
...
white industries a30 (stated q factor 163.5mm with a 73mm bb)
q factor 163.5mm (9.15mm wider per side vs. the reference sram force)
new frame outer stay width @ 19cm: 120mm
gap calculated against the reference: 12mm - 8.5mm + 9.15mm = 12.65mm
if the calculation is in any way valid, the conclusion would be both models clear the stays easily while the middleburns give a slightly narrower q factor. i've got a chris king bb, which i'd use either the hollowtech kit (middleburn) or the 30mm axle kit (wi) with. not sure if and how this variable changes the calculation.
perhaps someone more knowledgeable can confirm what did i miss and advise how this kind of situation is best approached :D thanks!