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Discussion Starter · #1 ·
I think I got it all into the subject line. I am 100% happy with the 100 mm rim in the front, and very happy with 100mm rim in the rear, but have to rebuild or build a new rear wheel because of a junk hub. I bought an 80mm rim during the sale when they were $60 shipped, and I'm thinking of building an 80mm rear.

reasons for doing so:

I can only get 6 speeds with the 100mm rim and my frame (20.5mm offset 135, 17" stays). I really want 7 so I can use all adjacent sprockets from a loose cassette and still run a 32T or 34T low gear instead of the max 28 or 30 I can get now.

With 6 speeds, the chain occasionally rubs the tire (like if I am late to upshift on a rough downhill.)

I seem to get way more sidewall scuffs both sides from rocks and stumps and ice chunks in the rear than front, and a more rounded profile might keep the tire alive longer by scuffing the tread some of the time?

reasons not to:

flotation on ski trails. right now, I don't do any damage to the skate ski track in most conditions, but I am right on the edge of punching through a lot of the time. I've been running 3.5 psi, which seems to be the absolute bare minimum. I figure with 80mm rims I will need more pressure...?
 

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It should work fine. You will get less float, but in 95% of conditions (at least in AK) endos on any rim work equally well. The main reason to use 100mm rims is if you do long races/rides in remote areas and may need the extra float to avoid walking. If you mainly do local rides on packed trails there will be a few, but not many, days when 100mm will make a difference. IMO, less weight, less cost and more gears is a good trade for slightly less float.
 

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I'm deciding to buy fat - probably Surly. Now I know I do not need 100mm. But what about Large Marge 65mm and 80mm? In the middle of Europe we have usually lass snow and I'm planning to use this bike occasionally in bat weather also during rest of year. LM are double ply and 80 single? Does it make big difference?
 

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Discussion Starter · #4 ·
My 80 mm rim is double walled. I am not qualified to evaluate the difference, given I have only tried the 100mm so far, but my experiences differ from those on 65mm rims by enough to convince me the 100 mm rims are better for my use.
 

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Discussion Starter · #6 ·
Murphy1976 said:
"my use" means only snow, or also sand, rocks and average backcountry? 80mm rims allow only 7speed? Do 65mm allow 9speed?
I wrote another book (and so no editing, sorry). I hope something in there helps.

"My use" means snow all winter, not much sand, and heavily forested glacial till terrain like in this photo taken about 220 km south of where I live:


It also means a ~120 kg rider who wont much notice the difference in weight between a 100 mm wide rim and an 80 mm wide rim. If it were just summer riding, it would be absolutely silly to run the 100 mm wide rims. The ground profile of the tire, the tire sidewall protection, the rim scrape protection, and the gearing options/chain to tire clearance would all be better with an 80 mm rim. It might/probably would even be better to run the 65 mm rim... I don't know.

As another point, I missed riding together by about an hour, but I rode in the tracks of a much lighter rider (Chris, what are you? 60 kg?) on 65 mm rims with endos in varying snow conditions (snow shoe trails as well as snowmobile tracks, and frozen groomed ski trails). My observation is that the lighter rider had a MUCH easier time in the soft conditions, even with the much narrower rim, and presumably higher tire pressure. Since the bike's primary purpose is winter riding, and I believe the 100 mm rims offer... how should I say it... use-ably... more float than narrower rims in some conditions, and do so without a downside given I don't really care about getting where I am going a few minutes faster when conditions don't need the extra float.

drivetrain implications: It depends on the frame offset, rear hub spacing, chainstay length, cassette spacing, rim width, chain width (shimano is slightly narrower than the other 9 speed options), front ring position choice, your tolerance for the chain rubbing the tire (or collecting mud from it), how true you run your tires, and perhaps more I am forgetting. For example, ditching the granny gear small chainring and running only a middle ring probably clears 9 speeds on a pugs frame with 100 mm rims).

I run 135 mm hub spacing, offset 20.5mm with a granny and middle ring up front. 100 mm rims allow a theoretical 7 speeds on 9 speed spacing with a 6.6mm wide 9 speed chain and 17.2" chainstays with very true tires but I have found in practice that the chain will rub the tire in some conditions racking up noticeably more sidewall wear on the right side of the rear tire than the left over time, and the chain will also collect anything (like mud) that sticks to the sidewall all the time. 6 speeds gives comfortable clearance. There is a thread here where I posted a long explanation of what works and what doesn't based partly on personal experience, and mostly on measurements and engineering drawings, and then several people posted corrections. It looks to me like I can get 2 more gears by going to an 80 mm rim. I only want one more (for 7 speeds all taken from adjacent positions on a 9 speed cassette for perfect shifting and the gear range I want). Based on posts by others here, I assume the pugsley can get 9 speeds just with a little less clearance on 80 mm rims.

Random note: I run 2x6 on a 135mm hub with spacers behind the cassette. I should be running it on a single speed hub for better brace angle, but i thought 7 speeds was going to clear when I bought the parts. Spacing the largest sprocket to the right some works fine, but does not make the rear derailleur 100% happy, depending on how far over you push it. I built my frame with this in mind and spaced the derailleur hanger over 8 mm anticipating running 7 speeds. This puts the derailleur further into jeopardy from rocks and stuff, makes it shift better, allows the stock limit screws to be used, and theoretically allows a larger biggest sprocket to still clear the derailleur cage. If you don't find a way to space the derailleur over, you may be limited to a largest sprocket as small as 28t. I couldn't get a 36t to work with my spacing. 34 works great on 7 speeds, but just barely on 6. I ended up with a 30t for now, which is about like a 27t on a 26er.
 
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