Offset Fatbike wheel analysis…

23 11 2011

A little to the right please!

Update: upon further investigation I believe the calcs on the blog post I link to are in error or at the very least they don’t apply to the offset Large Marge rims I have. The offset of my LM rims is ~13mm [measured hastily so it could be +/-1mm] this should be entered in the wheel calculator as +/-13.0…the Phat Divide Blog uses +/-6.0 which would be correct for a 6mm offset rim. I have redone some calcs using what I think is the correct offset LM value and published them in them in this post. If you are motivated look at this and confirm I’m correct.

If you ride a Surly Pugsley or an offset 135mm 9:zero:7 fat bike your rear wheel uses a standard 135mm wide MTB hub, but it’s moved 17.5mm to the right to clear the big fat tires you ride on. This is an unusual situation for wheel builders and worth considering as you plot your next wheel set. There is a great post over at the Phat Divide Blog discussing this exact topic that is worth a read.

Large Marge Offset Rim with Alfine 8spd IGH

Since we are on that topic Joe over at the Pedaling in Place Blog has an interesting post about experimenting with Pugsley wheels/tires trying out a number of “What If?” scenarios you might have been pondering, but like me been too lazy to actually try out. 

Offset LM with an XT hub...

Above and below are offset and symmetrical Large Marge rims with XT 135mm hubs – which demonstrates why you’d want to use an offset LM on a Pugs with an IGH or a normal MTB hub.

Symmetrical LM with XT hub...

Sorry for not checking these calcs out before linking. Everything looked good until I punched in an offset LM with Alfine and the results didn’t match my wheel so I went out and measured the offset on my LM rims.

170mm Mukluk hub with symmetrical LM...

Above you’ll see the results for a 170mm fatbike hub with a symmetrical Large Marge rim. I also tried an offset LM rim with this hub with the offset going left and right – neither option was as good as using a symmetrical rim which makes sense

Symmetrical LM rim...

Just an observation if you look at the symmetrical LM rim shown above the spoke holes are actually offset to both sides of the rim. This needs to be entered into Freespoke as +3mm/+3mm [I’m guessing as I don’t have a symmetrical LM to measure]. I used +0/+0 which gives the same % spoke tension results, but the spoke angles will be different so if you have symmetrical LM rims measure them and use the actual value.

Overall Results:

  • 135mm IGH in Pugs with offset rim provides most even spoke tension
  • 135 MTB hub in Pugs has approximately the same spoke tension as a 170mm MTB hub with symmetrical LM in a non-offset fatbike
  • flange spacing is also a factor in wheel strength [170mm MTB hub has widest spacing followed by 135mm IGH]
  • offset spoke holes may not result in as strong a wheel a symmetrical rim, but this doesn’t seem to be an issue in practice
  • measure any spoke offset of your rims to confirm these results apply
  • if my calcs are correct than an Alfine offset LM Pugsley wheel is stronger [better spoke tension balance] than a normal Alfine MTB wheel with a symmetrical rim.



8 responses

23 11 2011
Rob E. Loomis

Wow, there’s a lot of info in those two posts. It’s curious to think that an offset fat tire is stronger than a standard tire. It’s also interesting because, if I’m wrapping my head around it properly, it seems like that would mean that a gear hub, which creates a stronger rear wheel normally by eliminating (or greatly reducing) the offset and evening out the spoke tension, would actually have the opposite affect in an offset, fat tire wheel because it would just increase the degree to which the rim needs to be shifted towards the non-drive side. This means I can’t use the “stronger wheel” argument to justify my choices and will have to fall back on “I like hub gears and they’re cool and shut up” argument. But I’m fine with that.

23 11 2011

@Rob – I added an offset LM rim + Alfine 8spd calc to the post. IGHs in a fatbike make total sense given the conditions they are operated in – especially since weight is not much of an issue with a 30lbs+ bike.

23 11 2011
Rob E. Loomis

Yes, there are other benefits, but I take some comfort in the fact that a hub gear is more centered and therefore stronger than a gear cluster. Looking at your calculations (thanks for that) compared to the ones on the linked post, it looks like not only is a hub gear wheel not as strong when compared to gear cluster on the Pugsley, but it also is not as strong as a gear cluster hub on a standard wheel. I have no idea what is “strong enough,” but being on the heavy side myself, I try to keep wheel strength in mind when making wheel choices. The numbers on the hub gear are not much worse than a standard bike wheel, but they do seem to make the hub gear the weakest option for an offset fat tire wheel. I guess the front wheel would have similar issues, but then the front wheel takes less weight, so it can certainly handle that situation if the rear wheel can.
But now I’m wondering about a narrower IGH, like 132 or 120, that had spacers on the drive side to push it back in to where the cog would be closer to the inside gear on a cluster. although as far as I know that would limit the hub gear choices to 3 or 5 speed Sturmey or Sram. Not the range I would want on an all-terrain bike.

23 11 2011

@Rob – the Phat Divide used the wrong rim offset values. I’ve updated my post…the IGH is a better option. The values Phat Divide didn’t match my own wheels so I went and measured my wheels.

23 11 2011
Rob E. Loomis

That is interesting and encouraging, but confusing to me. It seems like a standard rear wheel is somewhat offset with the rim shifted towards the right relative to the center of the flanges. When trying to compensate for an offset fat tire frame, the rim needs to be shifted to the left. It makes sense to me that some of that need for right shifting would cancel out the need for left shifting, making the gear cluster hub the stronger option.

I guess the factor here is the rim offset, which seems to more than compensate for the necessary left offset, meaning that once again the more symmetrical hub becomes the strongest option.

Now my head hurts. So far my wheel building has been very simple because I’ve only built front and IGH wheels, so they are mostly centered. Should I take the fat tire leap, it seems like I will be jumping in at the deep end of non-standard wheel builds. Good times.

23 11 2011
Greg Weber (@onespeedgreg)

You have done some nice homework here. Thanks..

23 11 2011
Micheal Blue

Vik, as I couldn’t find your email address on your website, I’m asking here. Next year I’d like to bike-tour Vancouver Island (well, a part of it). Either flying to Victoria and then cycling to Pacific Rim National Park or flying to Tofino and then enjoying Pacific Rim Hwy. To Victoria I could perhaps send my Dahon in a suitcase OK, as Air Canada flies there with big jets. To Tofino my luggage would be very limited, as the airline that flies from Vancouver to Tofino uses small planes. So I’ve been thinking about getting either a Tikit or a Brampton. They can (at least the Brompton) fold small enough. Are the roads there in good enough shape not to bite my tongue off riding 16″ wheels? What month would be the best for visiting?
I was thinking either June or Sept (August is out for me). June would be nicer due to the longer days and warmer temps, but I wouldn’t want to be eaten by black flies or mosq; but perhaps you don’t have any there? Thanks for your answers.

23 11 2011

@Michael – if you travel the main highway it’s paved all the way. If you want to get off the beaten path it will be dirt/gravel roads.

You can mail your Dahon ahead to a business in Tofino and ride it back then box it up and take it home on the plane. You can also bus to Tofino from Victoria and they’d take your Dahon. I wouldn’t buy a new bike for the trip unless you want a new bike regardless.

The weather will be better in June, but the traffic along the main paved road and only road into Tofino will be insane. The weather is less certain in September – although it can be great, but the hordes of tourists that visit Tofino in the summer will be gone.

There can be some bugs in June if you are camping in the forest, but none that would be a problem while riding. It’s not Yukon bad for bugs. I go camping most weekends in the summer and take no precautions and I hate bugs.

If it was me I’d go in September to have less traffic and miss the tourists. I simply don’t go to Tofino before September because it’s a zoo!

Your Dahon will handle any paved roads on the Island just fine.



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