We have a lot of wheels to get out today, so I'm going to publish this without some charts and graphs that I'll add a little later. The words will convery the point adequately enough for the time being, but charts and graphs always make a nice impression on management - those guy are short attention span theater, what can I say?
Anyhow, spoke strength is generally expressed Newtons/mm2. To get the functional strength of a spoke, you multiply that number and the spoke's cross sectional area. A CX Ray has a strength of 1600 N/mm2, and an approximate cross sectional area of 1.98mm2, for a total strength of 3168. I say approximate because I give the cross sectional area as if a CX Ray was a rectangle, but it's actually an oval so my stated cross sectional area is a bit high. The total strength and weight per spoke of an array of common spokes is as follows:
CX Sprint: 3564/5.2g*
Race: 3306/5.7g
CX Ray: 3168/4.25g
D-Light: 2928/4.8g
Laser: 2650/4.26g
*Sapim hasn't published the strength yet, so this is a very conservative but very educated guess
Given these numbers, you can see that if you build a wheel using exclusively CX Sprint or Race spokes, you'd need fewer of them to have the same overall spoke strength. The dynamics of a wheel, however, allow us to engineer a little more elegantly than that.
The most loaded spokes by far are the rear drive side. They are loaded to the highest tension (around 20% higher than a front spoke, which is next), and are subject to the most dynamic forces. Without adequate total spoke strength (spoke per strength times number of spokes), a rear's drive side spokes will slightly elongate over time, and won't have adequate strength to cope with the dynamic forces that they see. The front spokes are fairly highly tensioned, but a front wheel is symmetrical and doesn't see near the dynamic loads that a rear wheel sees. A front wheel built with 20 Lasers, depending on the rim and hubs used, will do pretty extraordinary duty without batting an eye. The rear non-drive spokes? They're kind of along for the ride. They get loaded to a maximum of 50% of the drive side tensions, they see less torque than the drive side spokes, and the rear hub's left flange placement means that they don't have very hard work to do to keep things in line.
Given that, you want a ton of total strength on the drive side rear, and you need somewhat less on a road front (disc fronts act similarly to rim brake rears), and you don't need very much on the non-drive of the rear (or non-disc side of a disc front). Against that, you are going to balance weight. The lowest weight at which you can get adequate total strength is the best.
You also have a couple more things to consider. One is points of control. When you build a wheel, the more spokes you have, the more opportunities you have to correct minute rim issues. If the rim has a hard spot, more spokes allow you to taper that correction by making small corrections to several spokes rather than larger corrections to fewer spokes. This makes a better build, and the points of control also work for you in counteracting stresses that the road puts into the wheel. Many hands make light work.
Another thing is simply redundancy. My dad was an aeronautical engineer for his entire career, and did some pretty neat stuff. A trip to Udvar-Hazy with him makes for a very interesting day (and a lot of "I don't know if that's actually been declassified yet"s). Anyway, one of his cliches is that you strive for infallibility but you always use redundancy. If a spoke breaks on a 20 hole wheel, or a 24h rear, your ride is almost certainly toast - this is not a blanket statement but it's generally accurate. The more spokes you have, the less true that becomes. With 32 spokes, if one breaks you rip out the debris and ride on, maybe you have to open your brakes a little. On a race wheel optimized for lightest weight and absolute best aerodynamics, sure fine. On a wheel that you rely on all the time for all of your riding, or on which you are going to go out into remote places beyond "honey can you come pick me up?" Not so much.
Okay, so that was long, and miles to go before I sleep and all of that, but with that done you have a very good picture of a lot of what we're thinking about in terms of spokes when we recommend a particular setup.
Have a great Memorial Day weekend, and please keep in mind the staggering volume of day drinking that America does this weekend - please be careful on the roads.
5 comments
AJ – Sorry, I missed your comment. More Newtons would be stronger in tension, yes. But spokes don’t break because of tension. They break because of fatigue. So more Newtons is stronger in tension, but it doesn’t always build stronger wheels.
Sorry I missed my physics class. The more the number of newton, the stronger that stuff? Lets say, I need strongest spoke for my Dirt Jump bike that will never broke, and at this point I didnt think about flex or anything else, just the number of newton. Should I choose Sapim Race or Sapim D-Light?
Very interesting! Just curious, how does spoke cross sectional area and Young's modulus (material stiffness) affect ride qualities?
Jim – Excellent question. Baldly honest answer is we have no way of objectifying or quantifying it so it's tough to answer. Increasing either will increase the wheel's ability to resist flexing, making the wheel more "laterally stiff." If spokes were able to be built with a material with a higher Young's Modulus than current spokes are (spokes we use are 18/8 stainless) and retain acceptable properties of price, reliability, not make it impossible to actually build wheels, all that good stuff, then using them on the drive side of the rear would be good. I'm not sure it's needed on the front, as fronts are pretty insanely strong and if you start going much lower than 18 or 20 spokes in the front, it gets hard to build the wheel very straight and round. You need a certain number of control points to do the build correctly. For the rear non-drive, you don't need anymore than what currently available spokes bring.On cross sectional area, for one it adds weight, to the point that a wheel built entirely with Sapim Race spokes or DT Competitions is going to be notably heavier than an equivalent wheel built with all Sapim Lasers or DT Revolutions, or done like we do it with Lasers on front and NDS but slightly thicker/heavier/stronger D-Lights on the drive side. You also have aerodynamics to consider with cross section. Aerodynamic speed at any given angle may not be changed too wildly one way or the other, but crosswind handling would suffer. The degree to which that would happen I can not say but a lot of people (a lot of people) have complained of crosswind handling of wheels with shallow rims but with wider bladed spokes. Haven't tested it, but I' very willing to believe those big flat spokes cause handling issues.As far as "would the wheel feel different to the rider?" I believe that the higher Young's modulus rear drive side spokes could solve some problems that are symptomatic (brake pad rubbing being the big one). I don't think "wind up" or "spin up" would be affected because I'm skeptical that anyone could actually blindly correlate a feel to any measurable property having to do with that. We've covered that crosswind handling would suffer, plus weight consequences. That's likely to cover it completely, I think.
For readers not familiar with the term, Young's modulus is a material property that describes how a material reacts under force. Basicaly, it's how stretchy it is. Rubber has a very low modulus. Steel is high, aluminum and titanium are in between.There is little variation in modulus among steels. CrMo spokes would get you a few percent more stiffness, but they would have to be plated for corrosion.So for stiffer wheels, the options are really changing the number or cross section of spokes, or the hub geometry. I guess I was really asking about the tradeoff between spoke selection and ride qualities.