[Closed] Fat bike geo

I wouldn't touch a slack fatbike with a bargepole if I was going use it for what fatbikes are good for.

Steep HA's with big trail work better in ruts.

How come the LLS doesn’t seam to apply to FB?

I wouldn't imagine given the little amount of fatbike sales and thus development that anyone would bother.

I'd love a lls fatbike, my smokestone Henderson is a great trail bike and Graham has also designed a LLS ti pinion frame, which has been highly rated by everyone who's ridden it in the area.

As to why there aren't any, it's now a niche market within a niche market. Vast majority of fatbike's seem to be n+1, a lls fatbike may be a n+1 too far for many.

I imagine that steering that big wheel/tyre combo with a vey slack head angle could be problematic.

Depends on tyre choice I'd have thought. Some FB tyres are much worse than others.
Bare in mind FB tyres start a 3.8 so only 1" bigger than a + tyre, the jump isn't all that drastic if it's a decent tyre.

Aye, some self-steer worse than others but even the lightest are heavy compared to standard sized tyres.

FWIW, the 1 degree change between my old 9ZERO7 and my "new" Cube Nutrail makes it a much better trail bike.

I'm not talking 63deg slack - I'm just saying I'm not seeing any 67deg bikes (Scott/Trek/Kona). Why is that?

My hardtail feels a lot better on flat twisty trails at 65deg than it did at 67deg stock.

Angleset? But wouldn't make the bike longer or lower...

The best thing would be to ride one or two to see how they feel with the standard HTA.

the old voodoo wazoo had fairly modern geo - something like a <67 head angle from memory?

Getting to throw my leg over one is pretty tricky now days as the are getting very niche. I don't know anyone who rides one - nor have I seen one on my local trails.

Been watching a few youtube vids and it looks so fun.

So the bigger and heaver the wheel - the steeper the geo should be?

FWIW, the 1 degree change between my old 9ZERO7 and my “new” Cube Nutrail makes it a much better trail bike.

So is the Nutrail steeper or slacker than the 9ZERO7 ?

Old - 70 deg

New - 69 deg

Wazoo had 69.5 HTA apparently, I ran my 18" one with a 110mm stem for ~2 years for road duties until the driveside crank sheared through the pedal threads back in June.

https://www.voodoocycles.com/wazoo

Colin, influence of tyre pressure on head angle?
Pumped up = slackerer?
Aired down = steeperer?

Aye Rick, there must be some change, though I guess it's pretty minimal and would also be affected by front:rear pressure differences. The Nutrail also sports Blutos so there's the usual "was it measured with sag" question.

but even the lightest are heavy compared to standard sized tyres

I've got Jumbo Jim's on mine and they weigh less than some other 29" tyres I have in the garage. just over 1kg each.

I’ve got Jumbo Jim’s on mine and they weigh less than some other 29″ tyres I have in the garage. just over 1kg each.

So the 2nd lightest fat tyre is only 20% more than a tough-ish 29er (specialized ground control 2.3 Grid).

My minion 4.8's upset CERN once upto speed!

All that weight does a lot for their stability at speed. You can really feel it when you have to brake/turn into a corner.

You have to look at the whole front-end geometry i.e. head tube angle, fork offset, tyres size and trail to get the full picture on how it affects handling. I'd be wary of simply copying regular MTB geometry because the of the effect of the tyre size and contact patch size e.g. the gyroscopic effect of a fat bike tyre adds significantly to the stability of the bike at speed so countering the need to slacken the HTA. Go too slack and you get wheel flop, which is the last thing you want when riding on soft surfaces like snow and sand as it takes more effort to keep the bike tracking straight.

The sheer size of the tyres also affects the effective geometry.

With rims available from 55mm to 120mm, the same tyre can have a completely different profile which is not usually considered in bicycle applications.

Why is that important? When you turn the steering on a bike you are effectively lifting the front wheel off the ground*, and the curvature of the tyre accentuates that. The Head Angle has a direct effect on the amount of flop.

Have a play with this http://yojimg.net/bike/web_tools/trailcalc.php
.
*Of course that doesn't actually happen normally, because gravity pins the wheel down, but if you flick your bars really quickly you will notice it. It's one of the reasons a rut is so deadly, it turns your wheel to the side and momentarily it is not in contact with the ground so you then get smacked on to full lock, followed by an OTB. With a steep HA and little flop, this behaviour is considerably mitigated. Experiments I have done suggest a HA of 75º and a heap of trail for those sort of nasties (talking rigid only).

I ride a small Airdrop Edit and a medium Canyon Dude, the biggest thing I notice once the hits start coming is the difference between a Pike and a Bluto. At slower speeds the fatty is very capable on steep tech due to huge grip and that wide lazy contact patch, very confidence inspiring, can't say I miss the slacker head angle of the Edit.

TBH, I don't know why different rules apply to my fatbike, but they do. I was a pretty early convert to the Church of LLS and even stuff like proper XC bikes just doesn't work for me now. But my fatbike is pretty short and steep and it just works. I suppose the weight of hte wheels and the contact patch gives it stability, or somethin?

I also assumed I'd get a bluto, but that didn't happen either. I suppose, basically, fatbikes are just different enough for the same rules not to apply. I wouldn't mind trying a #enduro fatbike, but I've done enduro races and ridden EWS stages and dh trails on my short/steep/rigid one and it just worked.

And that's another point too - they seam old school short. The 17" Cube Nutrail has the same reach as the current crop of small bikes. TBH its quite scary buying a bike with very much old school geo numbers.

I also assumed I’d get a bluto, but that didn’t happen either.

Conversely, when I bought the Nutrail I bought rigid forks for it too as I assumed I'd be using them most of the time. Having experimented with both the Blutos have basically been adopted permanently.

Nortwind
But my fatbike is pretty short and steep and it just works.

I think that's because a fat wheel tends to steamroll over the sort of obstacle that can stop an ordinary wheel and then OTB you.

I like long geometry, I used to buy bikes that were "too big" to get a longer toptube, so when long toptubes became fashionable I was happy.

I also think it's quite hard to get "low" with a 26*4" tire on the front. They naturally have a high front end.

My aluminum beargrease certainly handles a bit xc like but at the same time I couldn't imagine trying to go through tight corners with a slacker geometry. There's a real need for some "body English" to persuade those big tires to change direction.

The high front end and big contact patch means it's surprisingly good in slow steep tech until the brakes run out of power....

TLDR. Most bikes are designed for flat(ish) snow trails.

sweaman2
Free Member

I also think it’s quite hard to get “low” with a 26*4″ tire on the front. They naturally have a high front end.

Not really. 26x4 is smaller than a 29x2.3 wheel/tyre combo, and most fatbikes are rigid or short travel, so a low front is pretty much the norm. The BBs are often a bit higher mostly for pedal clearance though

Where are you based Op? Happen there might be someone who'll let you swing a leg.

I'm in Warwickshire (but get about) and have a Dune, which I believe had slightly more 'traily' geo than other bikes of its ilk.

My mate (not in UK) has a Pole Taiga - that is very LLS, he runs it with 120mm mastodons, 100mm clown shoes and 4.8" bud and lou. He loves it very much indeed. He came off a 2014 Salsa Mukluk (borrowed from me) which is deffo Short and steep. He regards the Pole as significantly more fun, but you do have 'push' more into the corners.

I built my ChiTi Full Fat/29+ frame on LLS principles early last year: https://singletrackmag.com/forum/topic/yikes-its-being-built-no-turning-back-now/

Mostly ride 29+ where the geo works like a charm, but been riding full fat (100mm clown shoes, bud and Lou) for last few weeks as I smashed my rear 29+ rim to bits. Now that weather is turning and trails are sloppier, it is an absolute weapon.

However, when there is a lot of grip, and its tight wiggly bits that require a lot of 'steering', it is a bit more hard work than is ideal to get it going in the right direction. As usual, the more I ride it, you start to use those tendencies to your advantage. Going back to 29+ is going to feel twitchy! 😆

Finally worn our my Lou on the back, and the sidewalls have gone on both Bud and Lou (they are about 5yrs old tbf), so replacing with Minion FBF/FBR. should be 'a bit grippy' 😂

Earl, if you're in central Scotland and not too wee you're more than welcome to have a go on mine.

As an aside, for most of the year a plus size tyre, ie 3", is a better choice than full fat, especially if you're not trying to ride bogs and mudholes.