Utilization of R8000 Ultegra 50/34 chainset on gravel frame with rear spacing of 142mm

Question

I have an Ultegra 11 speed mech kit, R8000, with 50/34 chain-set mated to a 68mm BSA Kogel BB on a Strong TI frame. I'd like to use this kit on a new build; an all roads, or "gravel" if you will, bike, having 142mm rear spacing. Plan is to specify threaded T47 BB, mated to a shell of 68mm. Will this configuration work and will cross-chaining with large ring up front and next to largest in rear (11-32) work out satisfactorily? Many thanks. David

Answer 1

The question originally referenced the Road Boost standard. That standard uses a 110x12mm front hub, I.e. 110mm diameter with a 12mm axle, and a 148x12mm rear. (NB: 100 and 142mm are the standard spacings for road thru axle hubs.) On MTBs, the Boost standard enabled stronger wheels and wider tires. A version of this standard for drop bar bikes exists, called Road Boost. It might be helpful for heavier bikes, like e-bikes, and gravel bikes designed for really rough, MTB-like terrain.

The OP is building an “all road” bike. This term is understood to mean road-like gravel bikes that can tackle a mix of paved and relatively smooth dirt roads. They aren't designed for singletrack. In my experience, they can navigate some singletrack, but they definitely aren't optimal. Typical front gearing would be 50/34 or 48/32, maybe a bit smaller. Non-Boost wheels are plenty strong enough for this terrain. Additionally, these bikes can accommodate 40mm or slightly wider tires, which is also more than enough for the terrain. The advantages of Road Boost would be unnecessary for this type of bike.

If the OP were transferring components to a Road Boost bike, then I believe but can’t absolutely confirm that the cranks won’t be compatible, and the BB and front derailleur may also not be. Shimano’s road groups use a 43.5mm chainline. I believe that MTB Boost chainline should be as wide as 49-50mm, and given the hub spacing, Road Boost is likely to need a very similar chainline. Shimano's GRX and SRAM's AXS Wide double cranks (47.5mm chainline for the latter, close to that for the former). The 1x chainlines might be more comparable, but the OP did specify a 2x crank. If the chainline isn't compatible, then the bottom bracket and front derailleur would also need to be changed. The GRX FD might work.

The OP later corrected the question to refer to a bike with standard (for disc brakes) 142mm rear spacing. With this setup, all the components save the rim brakes and bottom bracket would be compatible. (NB: if the FD doesn't have a braze on mount, check the clamp diameter for the FD.) As to cross chaining, the OP would basically be moving to a bike with the same chainline. If the chainstay length remains constant (which may not be the case), then the amount of cross chaining will be identical between bikes. Generally, it's better not to ride in the largest 3 cogs and the big ring. However, nothing will explode if you do this.