New 29er Wheel Build: Time for CX-Rays?

[ColonelJLloyd]

I'm going to build up a wheelset for a hardtail 29er. Mostly XC type riding; no jumps or big drops.

32h front and rear SRAM 900 hubs to Pacenti TL28 rims. I usually build with Sapim Race spokes, but was thinking this might be a good build to try CX-Rays. They're not really necessary here, but I thought for $85 more it would be neat to have a slightly stronger, slight lighter (~100g) wheelset. I do have the Alchemy bladed holder somewhere. Thoughts?

[Mark McM]

I'm going to build up a wheelset for a hardtail 29er. Mostly XC type riding; no jumps or big drops.

32h front and rear SRAM 900 hubs to Pacenti TL28 rims. I usually build with Sapim Race spokes, but was thinking this might be a good build to try CX-Rays. They're not really necessary here, but I thought for $85 more it would be neat to have a slightly stronger, slight lighter (~100g) wheelset. I do have the Alchemy bladed holder somewhere. Thoughts?

What do you mean by "stronger"? Sapim's claim of CX-Ray spokes being "stronger" are intentionally misleading. In fact, the numbers they provide for these spokes show that in a practical sense they are actually weaker.

The number Sapim uses to claim that these spokes are "stronger" is the material ultimate stress (stress at rupture). But the force of rupture is the ultimate stress times the cross-sectional area. Since these spokes have a smaller cross sectional area, the force at rupture is actually lower.

In most day-to-day riding, the force at rupture doesn't really matter, since most breakages are due to fatigue, not a single overload event. Fatigue takes many, many load cycles. Although their ultimate strength may be lower, butted spokes (like CX-Rays) can actually be more durable in fatigue loading, because the extra "stretchiness" of thin butted section reduces peak spoke loading, reducing the magnitude of load cycles. In this way, butted spokes can be more durable, if not actually stronger.

That being said, do these spokes make sense for a 29'er MTB? Probably not, for several reasons:

1 - CX-Ray spokes are ovalized for aerodynamics. Aerodynamics makes little difference at MTB speeds, so you end up paying a large up-charge for a feature a feature with no benefit.

2 - The extra "stretchiness" of super-butted spokes not only makes them more difficult to build and maintain, but they also make the wheel flexier. 29'er wheels can already be a bit flexier than their 26" and 27.5" counterparts, so you may not want to make your wheels flexier still.

3 - The ultimate rupture strength of spokes only comes into play when they are hit directly, as when a stick gets into the spokes. On a road bike, this is an uncommon occurance, but can be quite common MTBs. The lower rupture strength of super-butted spokes like CX-Rays means they are more likely to break when something goes into the spokes.

[ColonelJLloyd]

Maybe not "stronger". I want a strong, stiff wheel and I want lighter where I can get it.

1. I'm not concerned about aerodynamics. Saving 50g per wheel is attractive, though.

2. You're saying CX-Rays are going to "stretch" more than a Sapim Race spoke in such a manner as to make a difference in the real world?

3. Stay away from CX-Rays because a stick might break them?

Thanks!

[Mark McM]

1. I'm not concerned about aerodynamics. Saving 50g per wheel is attractive, though.

Then you want Laser spokes, not CX-Rays. Laser spokes have the same cross-section area, the same weight, the same strength, and the same "stretchiness" as CX-Rays, but have a round shaped instead of ovalized. This makes them not only cheaper, but a little easier to build/maintain, because they are bit less twisty when turning the nipples. These are equivalent to DT Revolution or Wheelsmith XL spokes.


2. You're saying CX-Rays are going to "stretch" more than a Sapim Race spoke in such a manner as to make a difference in the real world?

Yes, all else being equal, a wheel built with any super-butted spoke will have more lateral flex than wheel built with thicker spokes. All steels has the same elastic modulus (flex per square inch of cross-section), so thick spokes will always be stiffer than thin spokes.

Whether the extra flex is meaningful will depend on the rim and number of spokes. if you use a light right with a moderate number or fewer spokes, the difference will likely be noticeable.


3. Stay away from CX-Rays because a stick might break them?

Well, for the combined reasons of easier breakage, more flex, and more difficult to maintain (retrue), I would personally avoid them for a 29'er wheel.

[ColonelJLloyd]

I thought the flat cross section was a benefit when building because you can use a tool to hold the spoke and prevent twisting.

This was noticeable when I built a 32/32 SS set with D-Lights.

[oldpotatoe]

I'm going to build up a wheelset for a hardtail 29er. Mostly XC type riding; no jumps or big drops.

32h front and rear SRAM 900 hubs to Pacenti TL28 rims. I usually build with Sapim Race spokes, but was thinking this might be a good build to try CX-Rays. They're not really necessary here, but I thought for $85 more it would be neat to have a slightly stronger, slight lighter (~100g) wheelset. I do have the Alchemy bladed holder somewhere. Thoughts?

Won't be 'slightly stronger' compared to Race spokes. Thinner spoke and not as strong as a Race spoke. There's nothing magic about CxRays. What Mark said. Thin spokes are thinner spokes. If ya gotta try to save weight, use lighter tires. Using all lasers or all CxRays won't make the slightly lighter wheel perform any better and probably worse, particularly on a MTB/disc wheel.

Not sure where ya get spokes either. About 3 times the $...$3.00 or so per spoke for CxRays. VS about a buck per for Race.

[ColonelJLloyd]

Won't be 'slightly stronger' compared to Race spokes. Thinner spoke and not as strong as a Race spoke. There's nothing magic about CxRays. What Mark said. Thin spokes are thinner spokes. If ya gotta try to save weight, use lighter tires. Using all lasers or all CxRays won't make the slightly lighter wheel perform any better and probably worse, particularly on a MTB/disc wheel.

Not sure where ya get spokes either. About 3 times the $...$3.00 or so per spoke for CxRays. VS about a buck per for Race.

Light tires and light wheels are not an either/or proposition. I do plan to use light tires. I'm pretty confident any spoke I choose for these wheels is going to be strong and stiff enough for what they will see. I totally realize that we're talking a handful of grams that really won't add up to much. I'm interested because I'm building wheels for myself and I can afford to experiment a bit with this wheelset.

So, as far as "lighter" spokes go, what would you suggest and where? I've never built wheels that didn't use the same spokes all the way around front and rear. Thanks.

At Dan's Comp Lasers are $.85 and CX-Rays are $1.75 and include nipples. Never ordered from there, however. I pay $.46 for Sapim Race from my "spoke guy".

[oldpotatoe]

Light tires and light wheels are not an either/or proposition. I do plan to use light tires. I'm pretty confident any spoke I choose for these wheels is going to be strong and stiff enough for what they will see. I totally realize that we're talking a handful of grams that really won't add up to much. I'm interested because I'm building wheels for myself and I can afford to experiment a bit with this wheelset.

So, as far as "lighter" spokes go, what would you suggest and where? I've never built wheels that didn't use the same spokes all the way around front and rear. Thanks.

At Dan's Comp Lasers are $.85 and CX-Rays are $1.75 and include nipples. Never ordered from there, however. I pay $.46 for Sapim Race from my "spoke guy".

For disc wheels, thinner spokes on one side don't help the wheel at all. For non disc, thin spokes non drive side rear but for disc....'maybe' Lasers front but it won't be as stiff as using Race all around.

I ordered from Dans Comp twice and they screwed up my order both times. But I sell Race for $1.15, Laser for $1.25 and CxRay for $3 per....all with SecureLock nipples...which I highly recommend for any wheel. These are $.25 per bought alone.

[ColonelJLloyd]

Good to know, thanks. Never used Dan's Comp, but my spoke guy hasn't flubbed any of my dozens of orders so I reckon I'll stick with what's been working and use Race all around.

[oldpotatoe]

Good to know, thanks. Never used Dan's Comp, but my spoke guy hasn't flubbed any of my dozens of orders so I reckon I'll stick with what's been working and use Race all around.

Good choice, IMHO