I discovered something really interesting about Pugsley wheels today, using the excellent FreeSpoke spoke length calculator. FreeSpoke is a very versatile tool that not only calculates spoke lengths for any imaginable rim and hub, but also tells you what the relative spoke tension will be on each side of the wheel. You can play around with it to design a stronger wheel.
Let’s take a look at the standard Pugsley offset wheel using the Large Marge offset rim. The LM is in FreeSpoke’s database of rims, so you don’t need to do anything except select it. FreeSpoke will fill in the details in the calculator window. The LM is offset 12mm, which FreeSpoke handles as two offsets, -6mm on the left, and +6mm on the right. (It uses two measurements instead of one to handle rims in which the spoke holes are not all drilled at the same offset — see what I mean when I say it’s versatile?) I picked a totally vanilla hub, the Shimano M525, which has nothing particularly special about it. To handle the Pugsley offset, enter 17.5 in the “hub offset” field. The results (for 36H 3x) are:
This is cool in several ways. First of all, the diagram generated by the tool shows you the wheel you’ve designed, so you can spot obvious errors you might have made. The red line is the centerline of the rim. On a normal bike, the right (drive-side) flange would be closer to the centerline than the left, but this is a Pugsley, remember, so it’s the other way around. Note, however, that the spoke holes are not on the centerline, they are offset too. The result is that the rim is moved back to the left side of the bike almost as much as the hub has been shifted to the right. But what’s really interesting (to me) is the “tension distribution”: the drive-side spokes are under only 66% as much tension as the non-drive-side spokes. To put it another way, the non-drive-side spokes are under 50% more tension than the drive-side spokes. The Pugsley frame offset clearly stresses the non-drive-side of the wheel.
How does this compare to a “normal” wheel? Take a look at a symmetric LM built for a symmetric frame (this is essentially what every bike is running in the rear, although with a narrower rim, which doesn’t affect these calculations). All I have to do is zero out the spoke offsets and the hub offsets and the result is:
This looks just like any other rear wheel; the drive-side hub flange is closer to the centerline of the rim than the non-drive-side, and as a result, the non-drive-side spoke tension is on 59% as high as the drive-side tension.
Interesting observation #1 is that, even though the offset Pugs wheel looks ridiculous, it is actually stronger than a “normal” rear wheel, in the sense that the spoke tensions are more even. Of course we can argue about whether it’s better to put the higher tension on the drive side or the non-drive side.
But what if, I thought, what if you were to use an offset-drilled LM on a symmetric frame? You would, of course, have to spin the LM around so that it offsets the other way.
Interesting observation #2: A symmetric fat-tire wheel is stronger if you build it with an offset LM rim. In fact it is nearly the holy grail of wheels!
This result blew me away. I expected the offset to help, but I didn’t expect the spoke lengths and spoke tensions to be virtually identical! The offset of the LM almost perfectly compensates for the lopsided construction of the standard freehub.
Interesting observation #3: Let’s say you were touring on a standard Pugsley with an Extrawheel trailer (see previous post). The trailer can handle a 3.7″ tire but it can’t handle the offset construction of the standard Pugs wheel, so you build a symmetric wheel with an offset LM as shown above. If one of the bike’s wheels suffered a catastrophic failure, it would still be possible to re-purpose the trailer wheel by unlacing the spokes, reversing the rim, and re-lacing. (The spoke lengths are close enough to not worry about.) Sure, you lose a few hours sitting by the side of the trail, but at least you aren’t stranded.
One last thought experiment. I want to build up a 29er wheel to try out in the back of my Pugs so that I can evaluate some 29er rubber, as I explained in the previous post. Unfortunately, a non-offset rim in a Pugsley rear triangle is going to be extremely weak. I got the following result by zeroing out the spoke bed offsets but leaving the hub offset at 17.5mm:
Ouch! The non-drive-side spokes are under two-and-a-half times as much tension as the drive-side spokes!
Interesting observation #4: Symmetrically-drilled rims are a really bad choice for an offset Pugsley frame!
One way to ameliorate the above problem is to use a Hi-Lo flanged hub with the high flange on the non-drive-side. Several economical SRAM hubs are available in this configuration (the 406, 506, and X-7), as well as some high-end hubs that cost 10x as much. I will leave it as an exercise for the reader to show that using an SRAM Hi-Lo hub increases the drive-side spoke tension from 39% to 45%. This still bites but it’s the best design I’ve found. AFAIK there are no offset-drilled 29er rims on the market anymore (but there used to be). Does anybody know differently?
Well, I hope you found this discussion interesting and that you get a chance to try out FreeSpoke. FreeSpoke now joins my collection of external links on the right side of the page. Good job, FreeSpoke!