Saturday, November 03, 2007

Equipment - G3 or not to G3

To misquote an classic writer, "To G3 or not to G3, that is the question."

G3 is actually the trade name for a spoke pattern used by Campy. It's an uneven spoke pattern - the spokes are arranged in groups of three, two on one side, one on the other.

On a rear wheel it makes a lot of sense. The drive side spokes of the rear wheel is under much more tension. This is because the rim is not centered on the hub flanges. With equal spoke counts on each side, the drive side spokes always have increased tension - this can lead to reduced spoke life. The wheel also goes out of true easier because of the bigger difference in spoke tension left to right.

Different companies have tried different ways of addressing the uneven spoke tension. One method is to bring the left (non drive side) flange closer to the center of the hub - this reduces the difference between the left and right flange-to-center distance. Another method is to offset the spoke holes in the rim - move them all as far as possible to the left side of the rim. By offsetting the rim holes, you effectively help recenter the rim on the spokes. A third method involves increasing the spoke count on the drive side, spreading the required spoke tension over a greater number of spokes.

The first method results in a wheel built with a very narrow flange-to-flange distance - this in turn causes the wheel to lose lateral rigidity. Although not a problem when going in a straight line, it reduces stability when turning or when making out of saddle efforts.

The offset rim holes works reasonably well but it requires manufacturers to tool up for front and rear rims, doubling inventory and tooling costs. In addition the rim is uneven in strength left to right.

The final method seems to have found most favor with wheel and rim manufacturers. It uses standard hub designs, standard rims, and involves drilling the left rear flange in a different pattern.

If you double the spoke count on just the drive side (and therefore halve the tension) you can increase reliability. Or if you're going aero, you halve the non-drive-side spoke count to bring the spoke tensions closer together. Either way the effect is the same - distribute the very high spoke tension on the drive side over a lot of spokes and use only as many spokes as needed on the non-drive and lower tensioned side.

Ten years ago I used the third technique to build 24 spoke wheels. The rims were, at the time, relatively radical low spoke count and deep section rims. However, hubs in such low spoke counts were not readily available (32 spoke was standard), and spreading the high spoke tension on the drive side over only 12 spokes meant there were incredible stresses on those spokes.

The third wheel building technique addresses both hub availability as well as the high drive-side spoke tensions. A wheel builder could buy a standard 32 spoke hub and use 16 spokes on the drive side and 8 on the non-drive side. This simply required skipping every other hole in the hub on the non-drive side. The built wheels were very reliable and had reasonable spoke tension on both sides.

Now you can buy wheels like that. One company using this build technique is Campagnolo. They call their particular design "G3". Fulcrum wheels also come like that, they call it "2:1". As Fulcrums are to Campy like Chrysler is to Dodge, they're made by the same people.

The G3 is the same thing but the spokes are grouped in three, not spread out evenly. On the rear you'll have a drive spoke (D), a non-drive (ND), and then a drive (D). Your spokes, if you went around the rim, would be like this:

D, ND, D,,,, D, ND, D,,,, D, ND, D

Note that every ND spoke is opposed by two D spokes. In Fulcrum-speak it's "2:1". The D's (drive side) would have less tension than a normal wheel with just one D for each ND spoke.

An important reminder - all this information about uneven spoke tensions only apply to the rear wheel. In comparison to the nightmare at the back end of the bike, the front wheel is wheelbuilding paradise. Unless you have a disc brake, there is no difference in tension between the two sides. The rim is centered on the hub flanges and, again, unless you have a disc brake, you do not have to deal with hub torque.

Therefore, in a front wheel, there is no need for an uneven spoke pattern.

Here lies the problem.

Campy, in their marketing wisdom, has marketed, and is selling, a G3 front wheel.

A G3 front wheel would look like this:
D, ND, D,,,,ND, D, ND,,,,D, ND, D

Since the spokes are grouped together, the trio with a D bias will tend to pull to D. The ND biased trio will pull to ND. The spokes will naturally pull the rim from left to right. Since that's not a goal for a front wheel, the spokes will have to be tensioned unevenly. So in a D, ND, D triplet, since the three spokes need to exert equal tension left to right, the two Ds need to equal the opposite and single ND.

2 x D = 1 x ND

In other words, the Ds need half the tension.

At the next triplet, the opposite is true:

2 x ND = 1 x D

A G3 laced front wheel requires double the spoke tension on the single spoke (of a triplet) versus the pair on the other side of the rim.

This is absolutely illogical.

Not only do you have to tension the spokes unevenly, since spoke tension on the paired side is half that of the single side, the paired spokes will tend to loosen quicker, especially when flexing the wheel side to side (like when the rider stands up on the pedals). To counter this the overall tension of the wheel has to be increased so that the "half-tension" of the paired spokes is high enough to resist this "automatic spoke loosening". This means the single spoke opposite the pair will be tensioned to an excruciating pressure, reducing the lifespan of said spoke.

Wait, it gets better.

The relatively long expanse of rim between the G3 spoke trios is unsupported and therefore needs to be stiffer - this is in addition to the requirement that the rim fight the tendency to wander left to right due to the left-right spoke-triplet bias. Ultimately this requires a much stiffer than normal rim - in other words, a heavier and not optimized rim. You'll still get unusual rim movement around the spokes since there will always be local distortion due to spoke tension. So the rim snakes back and forth anyway, it just does it really quickly.

Think about what would happen if the same wheel were built without the G3 pattern. If the wheel has evenly spaced spokes and they alternate left to right like normal, the tension for each spoke will be about the same. The rim only needs to be stiff enough to bridge the standard gap between each (evenly-spaced) spoke hole. The rim moves back and forth between spokes in a normal fashion and can be optimized for a given spoke count.

Looking at it from a different point of view, if the G3 spoke pattern is so good for front wheels, Campy's top line wheels would be G3. Study Campy's top line wheels - they are distinctly not laced in a G3 pattern.

It is because G3 front wheels are an inferior design.

Along those lines, if G3 was good for front wheels, that would mean 2:1 would be good for front wheels too. There are no 2:1 laced front Fulcrum wheels. Again, it is because a 2:1 spoke pattern in a front wheel is an inferior design. In fact, on their spec page, they describe the 2:1 pattern as a rear wheel specific design.

Incidentally I had Vento wheels (G3 front and rear), and the G3 up front was so flexible it was downright scary. The rear wasn't bad so I'd use the rear, but the front I virtually never rode. The Vento rim is not that stiff and therefore the rim, and the bike, moves around a lot, especially when standing up.

In contrast I also have the Eurus wheels - G3 in the rear, a standard radial spoke pattern in the front. They are extremely tough, dependable, and have performed above my expectations. They are rigid, fast, reasonably light, and very strong. I double-pinch-flatted both front and rear tires (approx 130 psi in 700x23 tires) when I hit a pothole while drafting a truck at about 45 mph. Although the tires were unusable due to massive snakebike cuts and I had to ride back on flat tires, the wheels were perfectly in true. Impressed me for sure.

So to answer the question, to G3 or not to G3, it's both yes and no.

G3 for rear wheels is great.

G3 for front wheels is stupid.

2 comments:

Anonymous said...

Another reason to not G3 is aerodynamics. According to a very informed and well-placed engineer who works on bike aerodynamics, grouped spoke patterns like the G3 are less aero than evenly spaced lacings. The main reason for these non-standard lacings are to be visually unique and recognizable from a distance and to have a marketing story to tell. With the aero disadvantage, you might as well just go to a higher spoke count since the aero importance of a few additional spokes is mostly negligible - especially on a rear wheel.

And you are right about the problems with the large unsupported rim sections in these grouped-spoke laced wheels. You either get undulations in lighter rims or an unnecessarily heavy stronger rim.

Commentator said...

So based on the early comment of "With the aero disadvantage, you might as well just go to a higher spoke count since the aero importance of a few additional spokes is mostly negligible - especially on a rear wheel". We can also say that the G3 aero disadvantage is negligible. :P