Offset Rear Triangle = Stronger Rear Wheel

By Mark Stonich

For 20+ years I've been singing the praises of building frames with an offset rear triangle. Now I see that ATP, the makers of Vision recumbents, is doing just that. I've used a 10mm offset, Vision uses 1/2" (12.7mm). The difference is that I started doing this when 7 speeds was the maximum. Now that 8/9 speeds is the norm, 1/2" or 12.7mm will give a dishless wheel (Nearly dishless in most cases, as flange spacing varies somewhat between different hubs.)

This means that if I use a 130mm rear axle, the inside of the right dropout is 70mm from the center of the frame, and the left is 60mm.

The most durable wheel is one where the spoke under the least tension is tight enough that it will still be stretched when it is the bottom spoke. This is so the bends don't fatigue. And why 2.0mm - 1.5mm - 2.0mm spokes are more durable than 2.0mm straight gauge. Spokes always break at the ends, and a light center ensures stretch without excessive tension.

A. If you have 60mm flange to flange, with offsets of 20mm and 40mm the right spoke tensions will have to be twice the left spoke tensions. (I did all the tricky vector math, only to discover that relative spoke tensions are simply inversely proportional to the flange offsets)

B. If you have 60mm flange to flange, with offsets of 30mm and 30mm, the right and left spoke tensions will be exactly the same. (See illustration below)

In example A, the right side spokes are either twice as tight as they need to be, overstressing them, and the rim. Or, more likely, the left side spokes aren't as tight as they should be, leading to a tight/loose/tight/loose cycle that eventually causes fatigue at the bends. In cases of excessive dish or a softly built wheel, this can even allow the nipples to back off, leading to wobbly wheels.

If you trash a dishless wheel on tour, you would have to remove the dish from any replacement wheel (much easier than adding dish). However, you are much less likely to have wheel problems in the first place. We've never had trouble with 36 14ga. (2.0mm) spokes on the rear of our recumbent tandem, which is easily the most heavily loaded wheel in the club.

The illustration is a digital photo (Scanners? We don' nid no stinkin' scanners!) of page 45 in Gerd Schraner's excellent "The Art of Wheelbuilding".

In case you disregard this priceless wisdom;
To get a really tough dished wheel use a high quality heat treated rim, deep and/or heavy enough to be nice and stiff. Use 2.0mm - 1.5mm - 2.0mm spokes on the left and 2.0mm - 1.8mm - 2.0mm on the right. With the unequal tensions on a modern rear wheel, the lighter center sections on the left will stretch almost the same amount as the heavier ones on the right.

Unless rotational inertia is your primary concern, save weight on spokes, not on rims.

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