29-er Gravel Bike with Boost Spacing - What Drivetrain?!

Interestingly it’s not actually the step that makes BSA30 a flawed design. It’s the thickness of material at the threads. To provide enough spindle clearance, BB manufacturers have to reduce the thickness of the BB at the threads more than they do for any other threaded BB design. This makes for a weaker BB more prone to flex and failure.

This is EXACTLY why Sram introduced Dub. They wanted to keep the axle OD as large as possible while allowing for sufficient thickness at the threads for BSA BB’s. It’s especially necessary for the low cost high volume bb’s they produce.

High quality BSA30 BB’s work great most of the time but as you pointed out, they can fail under heavy loads. Mostly because there just isn’t enough aluminum there.

The solution for this is a Stainless or Titanium BSA30 BB. The higher material strength carries the load better and should be rock solid. Below are the Stainless and Ti options I’m aware of

I really hate to admit it but sram did a really good thing with dub and I hope every 30mm crank manufacturer moves to 28.99 spindles. Did you notice how many large companies have moved back to BSA since dub was introduced? Thanks sram for keeping BSA alive!


This is some great info and makes a lot of sense. Really appreciate the explanation mate!


Ok, I think I have finally figured things out.

The Cutthroat has boost spacing. Ideal chainline - 51-53mm.

The chainline on the Easton chainrings is 45mm, which is a standard road chainline. However, the cutthroat also uses an MTB Bottom Bracket and an MTB crankset. The Raceface Aeffect R has a chainline of 51mm with a 137mm spindle.

What I was confused about for a very long time, is how the GRX front derailleur is working. For all intents and purposes, it’s a road derailleur, albeit sitting 2.5mm outwards.

The answer seems to be the Cutthroat’s “road boost” FD mount, which moves the FD further out to the boost chainline.

If my assumption is correct, then this little piece of metal is genius. It could, if the frame had the space for it, allow someone to run a full-on road groupset, on a boost spaced MTB.


About the Q factor, on a road bike, with a 105 crank, I used to have terrible back pain. When my bike fitter took a look at me riding, he discovered that I had a pretty serious hip impingement and the 170mm cranks that came with my bike were simply too long for my 74cm inseam. The narrow q factor of a road bike was causing even more problems.

I now ride a bike with a 160mm crankset from Croder and have pedal extenders. My q factor/stance width is effectively 190mm!!! And I have never been so comfortable on a bike before.

So, yeah, a wider Q factor isn’t just a non issue for me, it’s actually desireable.


Another great solution is this Jtek Eccentric Front Mech Shim. You simply use a 34.9mm clamp on a 28.6mm seattube. They don’t state the offset it allows but I’m sure it’ll get you there.

Todd from Black Cat made his own on the 2019 NAHBS bike below!