Dave's Fall Wheel Blatherings Pt 2

My success or failure when I write these things is measured by "how many people who didn't know about or understand this topic will have a workable understanding of what's going on after reading this."  My goal is not to write a textbook that's on the way to a PhD in Engineering (and not even having a BS in Engineering - I have a BA in English - I'm not qualified), but to help people get a solid lay understanding of the topic.  The goal is to help people be more informed consumers and users of bike stuff.  Hopefully I'm successful in that.  This one's a bit long, so go get a cup of joe before you start.

Scratch the surface of any discussion on wheels and you will soon bump into the concept of "the bracing angle."  The bracing angle is simply the angle at which the spoke leaves the rim to travel to the hub.  Functionally, the higher it is, the better. 

There are a lot of reasons why deep section wheels (please, not "deep dish," that's wrong on several levels) feel so stiff.  A big one is that they are very stable circles.  Stand a box section alloy rim up, and push down hard on the top of it.  With not that much force, you will be able to push the top and bottom closer together, and push the sides farther apart.  A lot of people try to use this when they build wheels with alloy rims, using the spoke tension to manipulate the roundness of the rim.  It's a bad technique - if your rim is round, you are just trying to place the hub in the center of that circle, and then introduce enough tension to keep it there.  Now try the same with a deep section carbon rim.  You can push and push until the cows come home but that thing ain't going anywhere.  This kind of stiffness feels rewarding at first (among my many hypotheses is that short test rides have a great chance of leading you to a decision you'll regret, for this very type of reason), but is essentially undesired.  You feel it right away and you think "ooh, good, STIFF!, I LIKE it!" but then after a few hours you think "when can I get off this ass hatchet?"  Speed and comfort are sometimes at odds, but the speed here is thanks to the aerodynamics of the deep section rim, not the radial stiffness of that rim.  Rim makers try to make the sidewalls as thin as they can get away with both for weight and to reduce this stiffness, but at best you are minimizing and not eliminating. 

Another reason deep rims feel so stiff is the short unsupported span between the spokes.  The distance between 2 spokes on one of our RFSC85s with 24 spokes is 61.5mm.  The distance between 2 spokes on an FSW with 24 spokes is 79mm.  The distance between spokes on an FSW with 32 spokes is 59.25mm - nearly the same as on an RFSC85 with 1/4 fewer spokes.  This means each spoke is doing less work, as the circle the spokes support (the inside diameter of the rim) has many more spokes per mm of circumference supporting it.  The other diameter of the rim is supported by carbon, which is pretty balls strong (as we learned in the paragraph above, it's so strong that you're trying to actually design out some of that strength).  This is a beneficial trait in that it makes the wheel tougher, but again it can make for a pretty, shall we say, "disciplined" ride feel. 

Imagine two cardboard tubes like the ones inside a roll of paper towels.  Cut one in half, stand both up, and place the same amount of weight on each one.  Then add more weight.  The longer one will fold well before the shorter one.  Reducing the length of a column increases its stiffness.  The spokes in a wheel act as columns.  The spokes in an 85 average about 224mm in length, and the spokes in an FSW average about 290mm in length.  Big difference.  This one trends more toward the good than the bad.  I think ideally you'd have more equal column stiffnesses across all wheels (subject to the stress relieving properties I discussed in Fall Wheel Ramblings Part 1, but you don't want to just add weight and speed-robbing surface area willy-nilly.  On our newest FSW build (20/24 lacing, on an off-center rear rim), we chose to put Sapim Race spokes on the rear drive side.  It's about a 16 gram penalty, with negligible aerodynamic impact (it's pretty rare that the drive side rear spokes are in anything other than a very disturbed wind stream), but it adds measurable and palpable stiffness to the wheel.  Because spoke stiffness impacts both vaunted sides of the "laterally stiff and vertically compliant" grail, this isn't something that would benefit an 85 in the same way. 

The other thing that comes into play here is bracing angle.  Bracing angle is pretty much "the more the better."  Imagine a spoke that left the rim at 90* - you wouldn't have to pull very hard on that spoke to make the rim move toward you, but more importantly, you'd have to pull pretty darn hard against the spoke to make the rim move.  The rim is well supported.  Now imagine a spoke that leaves the rim at 0* (essentially the spoke is an exact radius of the rim).  As you pushed or pulled sideways on the rim, the rim would easily move with almost no effort.  So you see that for "lateral stiffness," the 90* spoke is good.  Front wheels get pretty good bracing angle without much work - the flanges can be set pretty wide (flange to flange distance in almost all front hubs is greater than in their rear counterparts), and the flanges are equally spaced (except in disc brakes - that's a whole other plate of nachos that we'll talk about separately).  Rear wheels are a challenge, but in short the very much reduced spoke bed circumference of an 85 from an FSW improves the bracing angle.  The short leg of the right triangle (lateral center of hub out to flange) stays the same no matter what kind of rim you put on, but the deeper rim makes the long leg of the right triangle way shorter.  That decreases the angle between the short leg and the hypoteneuse (the spoke is the hypoteneuse, get it?) sharper, and makes the angle between the long leg and the hypoteneuse (aka "the bracing angle") greater.  This is all good.  There is no bad with this.  This is one way in which deeper wheels just plain have an advantage.