The Stiffness Saga, Pt. 1: Rims

If you've never read it, Edward Tufte's book The Visual Display of Quantitative Information is definitely worth a look.  With all of the info that we're shaking loose from our wheel stiffness testing and measuring, it's definitely a challenge to parse it out into pieces you can actually chew.  This project has actually made me need to go back and digest what it is I've thought I've learned several times.  

Since the wheels we make are really nothing if not assemblies of components, it's worth it to talk about the components themselves.  You have rims, hubs, and spokes.  Each affects the system, but they all work together.  You do a bunch of horse trading back and forth to try and tease out the biggest spike of the features you want to maximize, while paying the lowest cost in negative features like weight and drag.  

Once you have the right fixtures, it's easy to measure rim stiffness.  You support the wheel at 3 o'clock and 9 o'clock, and load it at 6 and 12.  Measure the deflection under load et voila, there you go.  A few pieces of aluminum extrusion, a weight, and a dial gauge were all we needed for this one.  

We measured every type of rim we have on hand, except the Iron Cross rims because some dummy already built them into wheels and can't wait to use them.  Mike channeled his inner Edward Tufte and this came out:


All of these are 700c/29" rims (which is why the Stan's Crest isn't there - some dummy ordered the wrong size of those).  Note that we have plotted them against weight, so the bias line you see is stiffness to weight ratio.  On the line can be said to have average stiffness to weight, above the line can be said to have good stiffness to weight, and below the line can be said to have less good stiffness to weight.  

Rail 34s do quite well measuring this way.  

Now, of course, stiffness to weight isn't the only important thing in a rim.  The Rail 52 is fundamentally a pretty similar structure to the Rail 34, but pays a weight cost for its depth and shape.  That depth and shape come with the benefit of making it among the fastest wheels anywhere near its depth (and any clincher near its weight).  Conversely, the Stan's 340 comes with a very light weight.  Light riders who aren't slapping out watts might not need all that stiffness, and can benefit from lighter weight.  And a Stan's 340 with a good hub and an appropriate spoke count can be a quite stiff wheel - a 28 spoke Stan's 340 was a standout in whole wheel testing.  In the case of the Arch EX, we're building those with 32 spokes, not the 20 spokes that a Pacenti SL23 front might have.  And it needs to durability as well as stiffness - the two are neither mutually in- or exclusive.  The Kinlin XC279 which has been the basis for our FSW23 wheel has a nice mix of all of the attributes.  

We'll talk about hubs and spokes individually before we get into the whole assembly, and note that this is lateral stiffness.  Radial or circular stiffness is another thing that we'll talk about.

All of these measurements take massive mounts of time - building wheels, configuring the test apparatus, etc etc etc, so this will come over some time period.  But we're happy to be out of the land of guessing and into the realm of knowing.  

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Having done lots and lots of stiffness tests on rims / wheels / same wheel built with different spoke counts / different spoke gauges, etc. I can tell you the next fun thing you will discover is that the Alpha 340 (or any other soft rim) built into a stiff enough wheel will be the one that provides the less deformation on the opposite side from the load which in real world translates into less brakes pads rubbing with a soft rim than with a stiff one in many cases (contrary to popular belief).So many times you will hear people saying "my wheels are too soft they rub on my pads" when in fact it's the rim that it too stiff relative to spokes count / spokes gauge / hub geometry so you have a very stiff rim that provide lot of 180° deformation (a rear brake on a standard road bike is most often around 150-160° but 180° measurement is good enough information) since it doesn't bend (push down on one side, it moves up on the opposite side) with a triangulation that's not strong enough.It only differs with brakes under chainstays like on most TT/Aero bikes meaning maybe wheels should be developped a bit differently depending on where rear brake is positionned…As a general rule what I'm saying is a light/flexible rim can be built into a super light wheel without much consequence (except for the loss in "precision" in the ride) as long as it's strong enough, stay true and hold tension of course whereas a very stiff rim "should" be built into a very stiff wheel because you need very little deformation under load in order to have little deformation at the brake pads.Hum… sorry I got carried away (that's the wheels passion talking), good first information on rims, looking forward to read what you find out when testing wheels then :-) !!!


Please describe what the labels each axis of the charts mean. Without them these are a nebulous group of data that is useless. Nobody has any idea that the horizontal is grams unless you label it! The vertical axis needs to be described even if the values can't be quantified, they need to be described by relative terms, I E the larger the number the (lower on the chart) means more deflection.Please explain what you are doing otherwise the graph is quite meaningless.


Mike,You're dealing with two components of stiffness here – the much derided "vertically compliant, literally stiff" schism.Lateral wheel flex is, as far as any evidence I've ever seen shows, a straight negative. It can cause understeer, it can cause rub, it stresses the components in the wheel, and yes at some decimal point level it wastes energy. There is no comfort gain to be made from lateral flex. Vertical compliance, or radial stiffness, or hoop strength, can get overdone. Quite simply, you need to ensure the amount of radial integrity to ensure that spokes are cycle loading the minimum amount, and any more is a waste. Whether or not anyone can actually discern a difference in vertical compliance of one wheel versus another (given exact same tires/tubes/ps, with all other variables controlled) is a hotly debated topic. I personally maintain that I can feel some difference (a recent switch from months on Rail 34s with T11 hubs and CX Rays to Pacenti SL23s on T11 hubs and CX Rays gave me a good data point – or just ride a disc wheel at some point!), but also know for absolute sure that a relatively minor tire psi change outweighs that difference. The stiff rim/brake rub thing is complicated. Assuming a perfectly stiff axle, when torque is applied to a very stiff rim with too few spokes, the rim's plane will move off the perpendicular to the hub's axis. With a flexible hub axle, the stiffer rim will flex the axle. In contrast, a flexible rim will "soak up" that stress by flexing at the ground, which is where that torque gets grounded. I wouldn't call one better than the other, but to the rider the flexible rim will likely be less evident. This is all getting covered in subsequent entries, but again, these tests take a ton of time so it will take us time to get it all done and published. Our convention is to do our readers the service of sharing things open book, in progress, and sometimes the conversation gets ahead of itself, as is the case here. On the marketing drivel point, yes you could easily say that some rims we've measured are "TWICE AS STIFF!" as others, but it would be meaningless to do so unless you were looking for a flashy copy line.

Dave Kirkpatrick

MikeM,Like I said… if you don't mind brake pads rubbing then yes you can use a stiff rim with fewer/thin spokes. it won't ride badly but you will get brake pads rubbing.If you do mind brake pads rubbing then a softer rim can actually be a good option of you need to build a stiff wheel out of a stiff rim.But if someone find a different solution I'm very happy to hear it… as I haven't found it and would like to ;-))) .


Does it necessarily follow that it might actually be a good idea to use fewer spokes on the stiffer rims?It almost seems like carbon rims may do "more of the work", so using more spokes might actually mean you've used too many. You don't want to get too rigid.When I read marketing claims on frames, they frequently say something like, "Now 34% stiffer at the seat cluster!", and find myself saying, "But it was already way too stiff there. We have to ride these things, you know."Wheels should flex an appropriate amount; that's part of the work they do. Stiffer may not be better at times.


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