Bicycle Mechanics - Lacing Rear Disc Wheels
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07-18-07, 02:48 PM
I have built a few disc wheels and I tend to follow the adage that "trailing spokes on the inside of the flange" but I was wondering if any thought that the disc sides pokes should be the opposite.
Sheldon Brown's site (http://sheldonbrown.com/wheelbuild.html) says:
"Derailer rear wheels should be laced with the trailing spokes running up along the inside of the flange."
"Note: This is not an important issue! There is a sizable minority of good wheelbuilders who prefer to go the other way around, and good wheels can be built either way."
Which leads me to believe it doesn't matter. But I distinctly remembering seeing an insert that comes with Shimano hubs which explicitly tells you which side the spokes should go on, and I believe it was different depending on whether it was disc or not.
It seems like the disc side would get the opposite forces as the drive side, but would it? I would think the hub shells are stiff enough to transfer the forces equally to each flange.
Any thoughts? Should I not worry about it as per Sheldon "Flange Side Doesn't Matter" Brown?
07-18-07, 03:06 PM
My experience agrees with "Flange Side Doesn't Matter Brown".
That said, whenever I lace up a wheel onto a new hub I always do it with the trailing spokes on both sides coming from the inside of the flange. I've always done it that way so it's a LOT faster for me to do it that way. I laughed at how much trouble I had the last time that I tried to relace an old hub that had previously been built the other way around.
07-18-07, 06:04 PM
The leading spokes on the disc side will be the ones receiving tension under braking. Thus, they will be more highly stressed than the trailing spokes on the disc side, whereas trailing spokes on the drive side are more highly stressed than the leading spokes.
My philosophy about building rear wheels is "make spoke tension caused by external forces as low as possible to achieve the desired results". By "external forces", I mean forces which are not caused by other spokes or spoke nipples, forces which cause spoke tension to be greater than the tension inherent in a non-stressed wheel.
Keep in mind that, as heads-in spokes (those running along the outside of the flange) pull on the rim at a shallower angle to the hub rotation axis than their heads-out counterparts, they will be lower in tension to generate the same sideways (parallel to hub rotation axis) force on the rim. Similarly, they will be higher in tension to generate the same force on the rim perpendicular with the hub rotation axis.
The line about which disc braking force ultimately reaches the rim is perpendicular with the axis of rotation of the hub. Ergo, the closer to parallel with that line a leading spoke can pull, the better, as the spoke will be able to maintain lower tension while pulling with the same perpendicular force. So, lace the disc-side leading spokes heads-out. This will allow greater braking power before reaching the elastic limit of the spokes, which will result in a stronger wheel than if the disc-side leading spokes were laced heads-in.
Furthermore, if disc-side leading spokes were laced heads-in, they would apply a greater moment about the hub rotation axis under braking than they would if laced heads-out. This would translate into a greater sideways force on the rim, and therefore, a rim that would move sideways more under a given braking load than it should, not to mention the extra tension this would place on the drive-side spokes to offset the extra sideways pull.
The same logic applies to drive-side spokes, except in that case, trailing spokes should be laced heads-out, as they are the ones directly placed under extra tension by external forces (that is, the pull of the chain on the sprockets).
In summary: lace leading spokes heads-out on disc side, and lace leading spokes heads-in on drive side. Unless you're lacing wheels for Trebek...
Julian "Flange Side Matters" Renaud
07-18-07, 08:40 PM
Thanks man. That makes sense to me, and is what my gut feeling was.
Understanding what you say, do you think that perhaps the benefit is marginal and it is easier and faster to lace it the common way? I suppose your definition of 'marginal' would depend if the wheel was for your mom or if it was for Wade Simmons.
That's interesting. I've never actually considered this.
How many wheelbuilders do change up the spokes for a disc hub, and how many just go with more "standard" routing?
07-19-07, 07:33 PM
For a rider like Wade Simmons, the benefits of lacing the wheel properly (drive side leading spokes heads-in, disc side leading spokes heads-out) would definitely show up in greater ultimate strength and longevity of the wheel. Guys like that push their gear to the absolute limits on a daily basis. He might even notice that it is a bit stiffer under braking loads than wheels laced more traditionally.
For mom, ultimate strength is probably not the greatest concern, but the ability to stay true for long periods of time is. A wheel laced as I've described will stay true better than one laced the traditional, "leading spokes always heads-in" way. Less tension demanded from the spokes by external forces will equate to less stretch, less stress on hub flanges, less bending force on the axle, and a better wheel in general.
It is easier and faster to lace wheels the common way if that's what you're used to. Myself, I build each wheel with a specific purpose in mind, which depends on the weight of the rider, type of brakes (rim or disc), type of hubs (i.e. no radial spoking on hubs that don't have reinforced flanges), and intended usage. As such, I very rarely build two identical wheels, so there would be no speed benefit for me to lace disc wheels the way rim brake wheels are laced.
Personally, I don't care how long it takes me to build wheels. All that matters to me (and customers) is performance and durability. It's also a matter of pride - I won't put my name on something unless it's the best I can make it.
Banzai, good question about how common proper disc wheel lacing is vs. traditional lacing. I checked every disc-brake equipped bike at my shop (Alternatives) today, and found very close to a 50/50 split between disc wheels laced as traditional rim-brake wheels (which SHOULD have leading spokes laced heads-in) and disc wheels laced properly. The machine-built Trek wheels were mostly traditionally-laced, with a few laced properly, more likely due to fluke than anything else. The Rocky Mountain bikes, all with hand-built wheels by WheelTech (Rocky's in-house wheel builders), were more interesting. The builders at WheelTech sign the rims on each completed wheel. I found that spoke routing preference was not only different between builders, but even the same builders built the same wheels differently each time! For example, I was encouraged when I found that the builder who's initials are "MG" built the front disc wheel of a Rocky Flow 2 properly - leading spokes heads-out - but then the rear wheel of a Flow 1 built by the same guy had the leading spokes heads-in on the disc side! I also found that the drive sides sometimes had leading spokes heads-in, other times heads-out.
I can only conclude that the majority of builders don't care about spoke routing at all, and probably don't even know of the advantages of routing them a certain way. In my opinion, they should.
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