Hello engineering folk

I’ve seen a wave of “boost adaptors” allowing you to run 142mm wheels in a boost frame.

These generic ones are not like the original wolftooth boostinators that actually modified the hub with a new end cap.

They have some spacers to just fill the gap and a shim to push the rotor out.

I’m not worried about a rotor… but surely just popping small spacers on the end of the hub held in place with the axle is a bad idea? Not just MEGA fiddly in the event of a puncture… but also adding huge sheer forces to the 148 maxle as the hub nonlonher sits directly in the recess in the drop out?

Thoughts?

Fiddly, yes, it would be a right pain in the arse.

Shear forces? Over thinking it. The maxle takes the forces the same way it does on a proper boost set up, you’re only spacing it out to make it fit in the frame, so that it can all be tightened.

There’s probably an element of ‘it depends’ involved.

The end caps dont take much load into the dropout on most hub designs, the load all goes through the bearings and onto the axle itself, the end caps just prevent it sliding up and down the axle. Even if it’s a design where the end cap sits inside the bearing then it’s just spreading the load onto the axle.

The obvious exception would be qr hubs which sometimes don’t have an axle at all and just rely on the end caps pressed into the bearings.

Having said that some hubs. Notably the original pro2 had a habit of snapping though axles which moving it 5mm out might exacerbate. But if it’s fine normally it’s probably still going to be fine.

Not fiddly at all either, I just roughed up the original end cap surface and glued the spacers on. Haven’t lost one yet in over two years.

You got dropout – spacer – end cap – bearing inner race – yet another spacer [more bits for rear hubs] – bearing inner race – end cap – spacer – dropout. Forming a thing under compression made of n + 2 things instead of n things, held in compression by the tension in the axle. I think the axle is there mainly to provide the tension to make the thing behave like a single thing, not several things.

So I think bearing loads are transmitted to the dropout via the end caps or spacers and the friction between them and the inner face of the dropout caused by the force created by the axle. But possibly it is via the axle acting as a beam and transmitting load via the threads/ bore in the frame, or a combination of the two.

Any engineery folk care to comment?

I ran them, didn’t have a problem, to be honest with stupid Rockshox torque cap drop outs the axle is supporting the hub rather than the drop outs anyway so can’t see any real issues

I run them on a DT Swiss wheelset which was expensive and seemed silly to sell just because they don’t now fit the bike I have.

Used a metal epoxy to glue them on at they haven’t moved since. Works fine.

If you think that a couple of mm of spacers is going to make your bike unsafe to ride I presume you never rode a bike with qr wheels…
No one ever died from that, but there were a few accidents and a couple of court cases.
On the grand scale of things though I’d go for spacers over QR’s any day.