Been a long time since I rock and rolled, mmm-mmm

Hello poetry lovers. I haven’t blogged in a blue moon, but since there actually was one a couple of nights ago, it’s time!

So much is new… let’s start with the bike:

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It’s a 23″ Carver Gnarvester. I pirated the 29+ wheels that Mike C built for my Pugsley and re-laced them to some hope hubs. Running Sram’s cheapest 11 speed (X0?). Cable discs of course, but not the old standby! Using TRP Spykes which beat the old BB7s hands down. With carbon fork, the bike comes in at about 29 pounds.

Turns out my surly frame bag fits passably. I’ve chucked the anything cages, they were too fragile. But I am trying out the Blackburn cargo cage on my down tube. My tent fits under there! (Except poles, which are in the frame bag.) The old Viscasha holds my clothing. The top tube bag is a new addition. It was made by Greg Wheelwright in my home town of boulder. Check out his stuff, it’s top-tier.

More to come…

Psyched!!

Well, lots of activity around here lately. My ex is taking the kids on a 2-week trip right after July 4th, so that gives me two weeks to hit the Divide! I’m planning to drive from my home in Boulder, CO to Butte, MT. From there either I’ll ride north to Polebridge, circle back to Columbia Falls, and rent a car back to Butte, or else I’ll rent the car at the beginning and do the same route in the other direction. It would be somewhat easier to navigate the route N-S because that’s how the map narratives are set up. The advantage of going S-N is that if I find myself running short on time, it’s easy to cut the trip short. Would hate to have to rent a car twice, once at the beginning and once at the end. Still pondering that one.

Incidentally, I may have to change my blog name, because I might not be as “Phat” as originally planned :-). Just this very evening I received these bad-boy hoops from MikeC

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Yes, those are genuine Rabbit holes, laced offset for the Pugs, and folding Knards. MikeC is one of the only shops that has all these goodies in stock and ready to go!

I’m going to try these out for the next few weeks to see if I’d rather do the ride on these. I suspect the answer is going to be “yes”, but anything is possible. I’m anxious to see if these will roll as smoothly as the fat wheels. I haven’t weighed them yet, but I’m pretty certain I will save at least 2 pounds per wheel vs the LM/Larry donuts that I’m using right now!

I’ll report back after I’ve aired them up this weekend!

Free wheeling

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!

The Q Factor – or – another half-fat idea

I have to admit I’m having some misgivings about touring on the Pugs.  The problem is the Q factor of that 100mm bottom bracket shell.  It tweaks my knees just a bit.  It doesn’t really bother me at present, commuting 8 miles at a time, but I worry about riding 30-40 miles, day after day while on tour.  A small annoyance can become crippling in circumstances like that.

Another problem with touring on the Pugs is that the offset wheels are not something you are going to find at every small-town bike shop.

So I started thinking about how one could fix these problem on a fat bike.  Some fat bikes use a 170mm rear hub, which allows for a symmetric wheel while still providing proper chainline.  But 170mm hubs are a rarity.  You could build a symmetric frame with 135mm rear spacing, and simply not use the 2 or 3 lowest cogs on the rear (or space the cassette over and lose the 2 or 3 highest cogs instead). But if you think about it, the rear tire size mandates a minimum bottom bracket width: even if you built a custom frame, the BB would still have to be wide to prevent the crankarms from striking the chainstays.

This led me to thinking about going half-fat: fat on the front, fat-lite on the rear. Surly frames in general have a reputation for taking fatter-than-average rubber.  Many people report having run 3″ wide tires on a Karate Monkey for example.  It got me thinking that the perfect off-pavement touring bike might be a KM or the new Ogre 29er. With a straight (non-offset) Pugsley fork on the front, you could have symmetric wheels both front and rear, with room to run a Larry up front.  I’ve found that Larry on the front eliminates the need for a suspension on anything but really gnarly stuff.

Now the question is, is this enough cushion in the rear?  According to my calculations, the volume of a Larry is about 13 liters of air; the volume of a 29×3 is about 10 liters.  (Compare with other tire volumes on this chart.)  This is not too bad.  I guess the only way to answer this question with certainty is to try out a few different tires on the rear of the Pugs. Worse comes to worst, a Cane Creek Thudbuster could be used to smooth out the bumps.

This all begs the question, is this even a good idea? The biggest problem with using a setup like this for expedition touring is that the tires are not interchangeable between the wheels.  Worse yet, you can’t even carry a single spare that would work on both wheels – Larry fits on a 26″ rim, and the rear wheel is a 29er!  This would seem to spell doom for this approach.

But hold on a second.  When I tour, I plan to use an Extrawheel Voyager trailer to tote my gear.  The Extrawheel is rather unique among bike trailers in that it uses a full-sized wheel.  This allows you to use it as a spare wheel in an emergency. The trailer (w/o wheel) is extremely light – about 5 pounds.  The 3rd wheel is not all wasted weight, however, since it obviates the need to carry a spare tire, at least.  In the extreme, if you were to completely destroy one of the bike’s wheels, you could ditch the trailer (literally) and cannibalize the trailer’s wheel to get you to the nearest town where you could have a replacement wheel shipped.

So the strategy would be to use the same 29 x 3 semi-fat on the trailer as is on the rear of the bike.  Now the trailer wheel can swapped with either of the bike’s wheels (the Extrawheel is able to accomodate a 3.7″ tire). Note that the big disadvantage of using the Extrawheel with a Pugs is that Pugs wheels are damn heavy, and having to accelerate a third one is not something I’m looking forward to!  But with the half-fat setup, two of the three wheels are lighter than a Pugs wheel.

To summarize, the propsed setup is:

  • Karate Monkey or Ogre or similar 29er frame
  • Replace fork with a Pugsley non-offset fork
  • Larry on the front
  • 29 x 2.5-3.0 on the rear, whatever fits
  • Extrawheel trailer with the same 29er wheel
Is this a good idea? Opinions are appreciated!  In any case it’s a great excuse to buy and build another bike! 🙂