Saturday, March 7, 2009

Spoked Wheels-Part 1 Intro

I've built spoked wheels since 1989. A lot of engineering mechanics and design concepts are nicely illustrated with a bicycle wheel. The concepts are not necessarily complicated but can still be hard to grasp. Even with engineering degrees (although I understood the physics behind the wheel) I did not really start to "know" spoked wheel until i sat down and starting doing calculations and thinking. For example things like 2-cross versus 3 cross, hi- versus low-flange, etc. Jobst Brandt's book and internet postings of course are great.

And here I am 20 years later, and i decided i want to start actually putting numbers, and showing the engineering behind the concepts. Although I am able to make generalized statements, it would be nice to be able to be specific, and dispel mis-information. Thus the reason for doing some calculations and sharing it in a blog. This will be a learning experience for me as well.

There are a lot of very expensive boutique wheels being sold, some are very good, some not. With some care and by incorporating a few techniques, a home or custom builder should be able to build a wheel as good as any (meaning as light, possibly more durable, and way less expensive).

This Part 1 blog entry will be the "generalized statements" and notes and current opinions (subject to change!). In subsequent blog will start to show calculations.

Wheel building an Art?
The wheel, when done, may look like, or be art, but the actual wheel-building process? No. It is aluminum and steel, tensioned. It's a bolted joint(s). If things are kept elastic (as they should be) we can do calculations and fully analyze a wheel.

Equal Spoke tension in wheels.
This of course is the holy grail (as it should be). Most wheel makers will even tout how even their tension is. (be wary if they state that the wheels may be a little out of true due to equal tension being their first priority. Wheel trueness should always be the first priority!)

But then a highly dished rear wheel gets built where the spoke tension on the NDS (non drive side) is about half of the DS(drive side). This is certainly not equal tension and the result can be a very poor wheel. Obviously not an issue with single speed wheel or really even 5,6, 7 speed wheels because the "dish" is not as great. [also Mtb (135mm spacing), and tandem(145mm+) wheels]. But with 130mm spacing and 8,9,10 speed road wheel, the issue is pushed just a little too far and problems arise.

Yes many wheels with this drastic un-equal tension have been built, and have functioned fine. But it is not ideal. Either the NDS spokes have too little tension and thread lock is needed (or constant "touchups"), or the DS spokes are so tight that either the nipple or rim fails (usually a "fatigue" failure a couple years down the road).

One solution is in the form of 2 spokes on drive side for every one spoke on non drive side (aka, G3, Triplet, etc, or 2-to-1) . This works well because the tension ratio (which is simply a ratio of the flange width distances) is 0.57 (for DA 7700 9s). There are other spoking patterns as well that I will present.

There are other techniques such as Offset rims, which is a good idea. Or a very-very poor idea such as moving the flanges closer together.

Paired Spoking.
It works. With such low spoke count, the rims have to be very strong (ie heavy and also means welded) and very straight. I have taken a number of Rolf Rims, which are super quality, drilled a few extra holes and built a nearly normal wheel with them. One of the reasons for the design of paired spoking was with nomally spaced low spoke count wheels , a "sine wave" could be put into the rim, from one spoke pulling one way and the next spoke pulling the other way.
So by pairing the spokes the problem is "solved". The paradox though is that paired spoke design requires a stronger stiffer rim. Which also solves the"sine wave" issue if used with normal spoke pattern/spacing. So you get there either way. However for an extremely low-spoke-count wheel, Paired is probably better. (future Blog Entry with some calcs..)

Low Spoke Count / Rim and Nipple Strength
I personally really like deep profile low spoke count wheels. (I consider low spoke count to be ~24 spokes or less). A lot Easier to build and more aerodynamic. The main issue is that a lot more spoke tension is required.

This then means extra care is needed at the nipple to rim interface to spread out that force. Rolf, and well most wheel builders, unfortunately do not do this (at least as far as I know) and nipples will break or the rims will fatigue and crack out sooner or later (might be 1 year, my brother's neuvation wheels.. or maybe 5 years depending on use ?)
What is the tradeoff between low spoke count wheels and rim profile?

Lacing PatternRadial lace is not a problem if your hub flange can handle it. If in doubt, or if a low spoke count wheel that requires high tension, don't do it.   Also the radial force can cause the but bearing fit to be lost.  

Obviously to transfer torque from the hub to the rim, the DS (drive side) flange needs some sort of 2, 3, or 4 cross lacing. with low spoke count wheel, I aim to have the spokes be nearly tangential to the flange. With higher spoke count ie 32 or 36 your don't need to be as concerned. Almost always have done a 3 cross. A 4-cross only to accomdate a given spoke length.

Note if have large diameter rear hub body, torque can be transferred to the NDS (non drive side) and you can do the 3-cross on that side and radial, or etc on DS. Some wheel wheels are seen with this. Any advantage? No. Keep in mind that most of the force in a spoke is preload from building the wheel. When torque is applied (to a wheel with lacing pattern, as just discussed) , the amount of additional force adding and subracting from the spokes is nearly insignificant, no matter how super strong the rider might be. (later blog entry to put numbers to this...)

An exception, obviously is a full radial lace to rear wheel, which is basically a "toggle-over-center" which can develop huge mechanical advantage (actually at point of toggle, it's infinite!) and possibly huge forces dependent on elasticity of members involved. (the concept is the same as Vise-Grips, Quick Release, or even why electric tension wires are purposely "sagged".)

2014 comments: I still think my comments are good.  I really like 2 to 1 spoking for a 8/9/10/11 rear wheel: equal spoke tension is just plain smart.  Unfortunately I have noticed a lot of hub manufacturers are decreasing the flange distance on their rear hub.  Bad Idea !