Yesterday's post about Zipp's new 303 S wheels was a pretty popular topic. The amazing thing is that you can't even find news about them on cycling media sites this morning. The news cycle moves that fast. Campganolo released some very expensive very deep wheels, Specialized released their new Diverge, and Mavic was placed into receivership. We'll no doubt take a look at that in a bit more depth, but first we have to cop to some blame for that; our lukewarm review and subsequent cancellation of their Open Pro rims was clearly the killer there. And then the normal Zwift racing news (which Mike and I just can not get on board with - more on that later, too) and a sporty new tire or two and 303 gets moved to page 2.
So, as the world moves inexorably to disc brakes, offset rims become an easier thing to justify from a production stand point if you want to. Flip an offset front rim around and like magic it's an offset rear rim now. You don't need to make a separate "normal" rim for the front and an offset rim for the rear. Which gets us to the question of "if we can, does that mean we should?" And that answer depends.
To start with, offset rims do one thing: they equalize tension between the spokes on each side of the wheel. This makes it less likely that the lower tension side spokes will go slack, fatigue, and eventually break. That's all. I posit that this is more important in mass produced wheels, as we take great pains to ensure that all spokes are as equally loaded as possible. The extra minutes we spend "cleaning up" the build, transferring tension from tighter spokes to their less tensioned neighbors, really seems to bear fruit, and it's something that wheel building machines are just poor at.
They do not create stiffer wheels. To do that, you would have to increase the overall bracing angle (angle of the spoke between the hub flange and the rim). Using a 590 ERD rim (similar to HED Belgium+ or Boyd Altamont Lite) and a WI CLD front hub, with 28 spokes, the total bracing angle is 11.2 degrees - 4.5 on the disc side and 6.7 on the drive side. Offsetting the rim 3mm, we still get an 11.2 degree total bracing angle, only now it's 5.1 degrees on the disc side and 6.1 degrees on the drive side. Tension ratio goes from 67% with the non-offset rim to 84% on the offset rim.
If we go to a deeper rim with an ERD of 550 (similar to an All Road 38 or RCG) the tension ratio stays about the same (it rounds down to 83% for the offset rim and stays 67% on the non-offset rim), but the total bracing angle moves up to 12.2 degrees. This splits as 4.8/7.3 on the non-offset rim and 5.5/6.5 on the offset rim. A whole degree of bracing angle, or just shy of a 10% increase in bracing angle, makes a bit of a difference. This is part of how ERD influences our spoke count calculus that we talked about the other day.
As we discussed when we talked about rim compression (same link as above - the spoke count post), shallower rims which have higher ERDs are likely to compress more and lose more spoke tension as a result. This makes whatever spoke tension you have on the "off" side more precious. That's simply why offset rims are a bit more attractive on shallower rims. With deep rims, you very often run into funky spoke hole angle things, and it gets tough for the nipples to "seat" properly into the rim bed. The rim has to be nearly perfectly molded to avoid that. And since we keep more of the spoke tension we start with in deeper rims, we're willing to make fewer sacrifices to preserve it.
White Industries chose to go offset with the G25A, RaceFace uses an offset for the ARC25, and Astral has an offset on the Wanderlust. These are all appropriate applications of offset. The Wanderlust has the smallest ERD of all of these, and that's 596. HED chose not to use an offset on the Eroica, which has a 592 ERD. The offset is a fine tool when it's used in the right application, but it's by no means a silver bullet. You can build good wheels with offset and without.
After 8 straight weeks of posting every weekday, we are likely going to a bit more of a normal posting schedule next week. This has been fun and super challenging, and we appreciate all the readership and comments. We're now at a point where quality will likely suffer in the service of quantity, and no one wants that. Also we've used literally every photo we have (most of them 7 or 8 times), so we need to go take some more.
Have a nice weekend. RI is easing things a bit, which thanks to a crappy weather forecast probably means nothing. Stay safe, stay sane.