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Discussion Starter · #1 · (Edited)
General fat bike geometry is still not where I like it so I'm building my own frame. Chainstay and wheelbase are too long and BB too low for me on all bikes. I'm building a custom frame to test out my desired geometry. I've got the jig made and decided to repurpose old bike frames for the tubing this go around, just cause I have a lot of time an much less money.

The bike will fit the widest 5" tires, have 16.5" chainstay's, 120 mm BB shell, a 14" BB height, and 42.5" wheelbase. I've checked all clearances and everything works/clears so far. Haven't decided on HT angle yet, probably 70 deg. I'll post some pics soon. This should be a fun bike.

I'm just using my cheap wire feed welder to tack things together and then will be using my buddy's TIG welder for the full welds so I can do 90% of the build in my garage.

Jig1.jpg

The old Rockhopper chainstay's were too thick where they meet the BB so I reshaped them super thin to give adequate tire clearance. I'll be gusseting them top and bottom for extra stiffness/strength. With the current crank spacing, a 22 T chainring barely clears the CS. Chain line should be fine with the wider 120mm BB, but I can space out a little if need be.

jig2.jpg
 

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Discussion Starter · #2 ·
I used two standard BB to make a 120 mm BB. It's surprisingly straight and I used an press fit insert between the two shells to line them up and then bolted in the axle before welding the shells together. Need to fill in the holes though. This frame is just a free prototype to learn to TIG weld on. The BB seam were my first attempts at TIG welding. Not pretty, but good penetration and the welds are strong. I've done a lot of MIG. My next frame will be all new tubing and parts.
 

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Discussion Starter · #4 ·
My thoughts for optimizing chainline angle are to run a 1x5 using half as many sprockets as a 1x10 and with 10/42 as the smallest/biggest cogs. The jumps will be twice as big between gears, but the cassette will be half the width and still have the same overall ratio as a 1x10. Fun part will be figuring out what shifter to use and get a 5 speed with the correct index spacing. I thought about combining the sram 1:1 shifter with a shimano 2:1 derailure but I'm not sure if the indexing will line up perfect. COuld also just go back to old school shifters. Any other ideas?
 

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Discussion Starter · #6 ·
The Fox sus fork is only being used as a JIG for the HT. I'll be making a rigid Chromoly fat fork with a 20.5" A/C height, so I can run a SUS fork later if I want.
 

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aka bOb
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In for sure, some dude on another thread said we should be happy with what is coming out and to stop trying to make things work. I say :prft: to that and let the innovation continue. Well done sir!!
 

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Discussion Starter · #8 ·
In for sure, some dude on another thread said we should be happy with what is coming out and to stop trying to make things work. I say :prft: to that and let the innovation continue. Well done sir!!
Agreed! The stuff that's out is all aimed at what the market will buy which isn't necessarily the best tool for the type of riding I mostly do. If people who think the geometry I want is ridiculous came riding with me, they would see exactly why and what this geo is for. There will always be those who are content with whats handed to them and those who tinker until they find what they want. To each their own as long as we're all having fun!
 

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giddy up!
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My thoughts for optimizing chainline angle are to run a 1x5 using half as many sprockets as a 1x10 and with 10/42 as the smallest/biggest cogs. The jumps will be twice as big between gears, but the cassette will be half the width and still have the same overall ratio as a 1x10. Fun part will be figuring out what shifter to use and get a 5 speed with the correct index spacing. I thought about combining the sram 1:1 shifter with a shimano 2:1 derailure but I'm not sure if the indexing will line up perfect. COuld also just go back to old school shifters. Any other ideas?
You should be using the same shifters/derailleurs that you would if you left the cassette with 10 cogs......as long as you don't change the actual spacing between cogs.

The problem that you will wind up with is getting the pulley to clear the larger cogs since they would then be quite a bit closer to the hanger.
 

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I'm looking forward to seeing this.
 

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Hooligan
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You go man! I love seeing people fab up bikes and parts to suit their needs on the cheap. this year i built a fork so i could fit a fat tire.

IMG_3954[1].jpg

i have wanted to build a fattie frame for a while, i'm looking forward to seeing how you do the bent seat tube.
Rock on! :thumbsup:
 

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Discussion Starter · #14 ·
Update

So the jig worked awesome and everything is tacked up almost ready to weld, just a couple gussets to box in the bottom stay still. Also waiting for my buddy to refill his welding gas tank, might be a few days or more. The frame lined up great and everything is straight. The chainstay's are exactly 16.5" with enough clearance for a 30.5" O.D. 5" wide tire. Seat post was super easy, just cut, bent, and welded. A tube bender would make a nicer looking post though. The only thing left to make sure that fits is the chainline once the bike is built, but I'm 100% confident the chainline will be fine. Here are some pics:

Fat1.jpg

Fat2.jpg

Fat3.jpg

Fat4.jpg

Fat5.jpg
 

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Discussion Starter · #15 ·
The actual measured geo is:
HT-69 deg, Effective TT- 24", WB- 42.5", CS- 16.5", BB static- 14".

I made the DT to be able to clear the RS Bluto, although I doubt this frame will see one until they are outdated and cheap being this is just my first practice frame. Once I put a little riding time on this one, I'll be building another frame fairly soon with all new tubing and parts. A full sus is also soon on my list to make.....
 

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Discussion Starter · #16 ·
Oh, in case anyone is wondering how I made the flat gussets, I just cut some chromo tubing in half and flattened it out with a hammer then cut it to shape. Might have been better to torch it before hammering to maintain greater structural strength, but this is just a practice frame anyway.
 

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is buachail foighneach me
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Awesome!! Well done. My only concern would be that notching and bending the seat tube like that will result in a really slack effective seat tube angle.
 

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Discussion Starter · #20 ·
And I take it the crank arms will clear the chain stays? Looking cool BTW!
Thanks guys, yep everything clears, cranks, chainring, and all. I'm pleased with how it's turning out so far. Can't wait to get it done and ride it.
 
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