A fatbike using standard mtb parts

[Emyr]

Fatbike: a mountainbike with very wide tyres, typically 3.5"-5".

I've worked out it's possible to fit 3.8" tyres between normal mtb cranks and have a reasonably normal chainline without using hideously long chainstays, whilst using a symmetrical 135mm dropout spacing.

I'll need the stays to be no wider than 10mm.

Choices:
Use the front triangle from a cheap frame for the prototype or use 100% fresh metal.
Chromoly or SS (304 or 316, design in an "un-pivot" to avoid mixing at a weld).
Cut and weld rectangular sections or bend oval tubing

My other bike's a Burf special, BTR #7 Bantam but I thought itd be safer to test this idea cheaply first..

 

[DrNik]

Are the wheels and tyres easy to come by??

 

[chunkolini]

My mate Charlie The Bike Monger is a leading monger of these components http://www.charliethebikemonger.com/ I cant see the point of it myself but it seems t be a thriving scene.
Then again thirty years ago people laughed at stupid 26" wheeled 'mountainbikes' with their riduculous fat tyres.
I recon making your own frame like this would be a massive brainache but if it worked would be satisfying.
Me? I would make a radical new concept in cycling, something revolutionary that could change the world.

Emyr, do we know each other?

 

[Emyr]

Yes, Mr Prince of Darkness, I've done 4 Clunkers. One of Jarvis's bothers in law.

I've got some Weinmann DHL80 rims (559x80) on their way from Germany and Surly's 3.8" tyres are available from a few different shops including Charlie's joyful emporium.

 

[Emyr]

Weinmann DHL80 rims (559x80) on their way from Germany

Arrived!

Rough measurement suggests 76-559 bead, 36 holes offset 8mm from centre.

Matt black, no stickers.

Surly quote dimensions for their tyres mounted on different rim widths:
http://surlybikes.com//uploads/downloads/SURLY_Tire_Geometries.pdf

Salsa's blogpost about fatbikes mentions some EN-standard 6mm clearance between moving and fixed parts, and I'd like to make it 8mm if possible to allow for flex.

 

[Shox Dr]

Have you ridden a pushrod with really phat tyres, it's like wading through cold treacle, unless you run high pressures.

Dan Hanebrink has been building them for years. Rich Williams Mr Boulder Bikes built a full sus, bike with 20" rims running wide tyres in the late 90s. Well over 9" of travel, and only weighed 50lbs

 

[Emyr]

Do you mean a bodge using heavy stiff motorcycle tyres or proper MTB-oriented balloon tyres?

With a 3.8" tyre I'm not intending to have any frame or fork suspension. Pressures will probably be below 20psi.

 

[Emyr]

Here's an export from SolveSpace:

Using Surly's figures for Knard 3.8 on Darryl 82mm (External width) rims, I get 97mm@337mm for the knobbles and 95mm@320mm for the bulge.

Clearances set as contstraints:
6mm knobble - stay
7mm stay - crank
10mm stay - 32T chainring (67mm radius)

Clearances derived from constraints:
8mm sidewall - stay
13mm stay - spider

To achieve these clearances with a 440mm rear end, the 10mm wide chainstays will need to be bent through 53 degrees with a 50mm middle-of-pipe radius.

 

[Shox Dr]

Do you mean a bodge using heavy stiff motorcycle tyres or proper MTB-oriented balloon tyres?

With a 3.8" tyre I'm not intending to have any frame or fork suspension. Pressures will probably be below 20psi.

Regardless of the original use for the tyres. Wider tyres increase rolling resistance. Decreasing pressure will increase this resistance. Not to mention the increase in weight.

 

[Emyr]

Fatbikes are for when you're more constrained by traction than physiology, and when the terrain is soft or greasy enough that wider tyres mean rolling on top of a soft surface rather than ploughing a furrow through it.

Fatbikea are ideally for routes involving snow, sand, soft mud, one or more pubs and a grin so wide you could swallow a doughnut.

 

[Emyr]

Regardless of the original use for the tyres. Wider tyres increase rolling resistance. Decreasing pressure will increase this resistance. Not to mention the increase in weight.

This logic is perfect for roads, velodromes and hardpack.

 

[Shox Dr]

I spent 18yrs in the MTB suspension industry. I think I know what I'm talking about















(My username is a bit of a give a given away)

 

[chunkolini]

Emyr
I admit to being as perplexed by the fat tyre thang as I was by the single speed thang.
I rode a single speed bike for a day out with a certain bike monger and was left as perplexed as I was before.
Cant see the point in either of them. But if people enjoy it that is the main thing.

 

[Burf86]

Emyr, make a yoke with some thin plate (10mm'ish), then weld tubes to that.

Yoke:
http://w2.wideopenmag.co.uk/images/p1230373-300x225.jpg

 

[Burf86]

I spent 18yrs in the MTB suspension industry. I think I know what I'm talking about


(My username is a bit of a give a given away)

I would love to know more about these 18 years, please tell us more!!


It's true what Emyr says though, or at the very least, that's the way they market them...

 

[Rocketman]

How I see it, it's hard to nail exactly when a fatbike will perform better than a standard bike. It really depends how deep the snow, mud or sand is and add to that the consistency.

Look at it this way generally rally cars go for a very thin tires for the snow with deep tread - purely on the fact they can cut through the snow and get grip below it. But say the snow was very deep on the other hand if you have a thin tyre that can cut through the snow but all that does is get deeper and deeper due to the snow depth it's not working.

 

[Shox Dr]

I would love to know more about these 18 years, please tell us more!!

Well I started importing circa 91' ..... Until 2006 when I packed it in.

Is that enough?

 

[Burf86]

I was genuinly interested, sorry if made it sound otherwise.

Most of that was before my mtb'ing time. First race being '99, so I don't know much about Noleen, CVI or Boulder Bikes. Only thing I knew of Risse Racing was the rear swingarm kit they made, and that because Curtis built a frame with one. My old man bought a K2 frame once, was in the back of some wholesale bike place, had an electronic shock on it!

 

[chunkolini]

Shox Dr. Wow, those names are like a history lesson to some of us.
I have distant memories of the price tags.