From my limited experience of such things it would appear that 170mm and 135mm are the most popular widths for fat bike hubbage. I also notice 190mm and 150mm used on certain models, am I right to assume that the purpose of wider hubs is simply to alow the use of 100mm rims and 4+" tyres? If my assumptions are correct, are the 100mm rims / 4+" tyres a positive or negative thing once you remove the bike from sand and snow and stick it on a mountainside in Wales?

Input, experiences appreciated, ta.

135mm offset will let you run a 4" tyre on 82mm rims with just about enough chain/tyre clearance.
170mm was the next step - to let you run a non-offset wheel. 5" tyres on 100mm rims are possible, but many had to faff around with chainline and removing cogs from the cassette
190mm gives all the clearance you need for 5" tyres on "hundies" but requires an even wider crankset.

IMHO, 4" tyres on 82mm rims are all that's necessary for fat grip rather than fat floatation. In fact, a narrower rim would likely be at least as good (55-65mm). There is a point where too fat a tyre just causes you to lose grip and spin out or wipe out. Think mud. A narrow tyre will cut through it, a fat tyre tends to float/skim over it. You can correct this a bit by the use of a proper knobbly tyre like a Nate, but the effect is still there.

Summary: for other than sand/snow I reckon 4" tyres are more than adequate.

Can't help on the question about riding anything else but snow, but it quite often can get tight with 170 mm rear spacing and 4.8" tires. If you want to get the max out of fatbiking with not too big investment it's probably best to go for a 65-85 mm rim and then 4.0 tires for anything but snow and sand and change to 4.8 for the soft stuff.
The difference in width of a 4.0" tyre on a 65 mm rim compared to the same tyre on a 100 mm rim is only a few mm. According to Surly 4 mm... Now use the same rim, but a 4.0" and a 4.8" tyre and the difference will be between 11 and 14 mm. From here:
http://surlybikes.com//uploads/download ... etries.pdf
Compare Larry to Big Fat Larry or Nate to Lou/Bud on var. rims.

Alpinum wrote:The difference in width of a 4.0" tyre on a 65 mm rim compared to the same tyre on a 100 mm rim is only a few mm. According to Surly 4 mm...

That's "Knob Shoulder Width" Casing width impacts the chainline and the difference is quite a lot more - 14mm or so. I recall being the first in East Lothian to turn up with Rolling Daryls and everyone remarked on how much wider the tyres were when compared to the Marges they were used to.

There's also 12x177, 12x197 for thru axles. More 'standards'.

ScotRoutes wrote:That's "Knob Shoulder Width" Casing width impacts the chainline and the difference is quite a lot more - 14mm or so. I recall being the first in East Lothian to turn up with Rolling Daryls and everyone remarked on how much wider the tyres were when compared to the Marges they were used to.

Oh... I must have been in a hurry - you're absolutely right.
So the difference between 100 and 65 mm is indeed mostly 14 mm.
The difference in tyre casing width is 8 mm for Larry vs. BFL and 11 mm for Nate vs. Bud.
But yet, we're talking about the effect of 1.4" on rims vs. the effect of 0.8" on tires.
I let my statement stand, 65-85 mm rim, and adjust with different tyre sizes and pressure.

p.s.: my 170 mm frame only takes a Lou on a 83 mm Robsson with pressures up to 0.8 bar. From 1 bar upwards it rubs when the frame flexes.

150 front allows for the use of the new Bluto squishy fork; If you are wanting to go that way in future, it saves having to change hub/wheel at that time.

Watch the Salsa bucksaw video: they chose 4 inch over 5 due to the autosteer on firmer ground. I can't confirm or deny as no first hand experience in riding 5" on firm stuff, but QBP (Salsa, surly, et al) have been the biggest player in Fatbikes for a long time, and aren't exactly the most conservative group out in bike land, so would seem they have pretty good reason to go that route.

Here in Aus, we have a lot of sand and a lot of folk still run 65-80mm-ish rim with 4-4.5 wides; enough float tbh, especially as rides often include more than 'just' sand; rocky outcrops, coastal paths, salt pans, etc.

Also seems to be a greater choice of tyres in the 'narrower' sizes (though that is changing now more folks are producing fatties).