Lessons from my Prototype Stainless Fat Bike

I woke up this morning way to early and way too excited to finish my prototype stainless fat bike. However, being a Saturday morning I couldn’t go right to the shop and start making awesomeness (noise). Emily’s not down with that on her non-morning-chore day. If I can’t work on it at least I can talk about it.

I’ve learned a lot from this bike, some of it was expected and some unexpected. When I build frames for myself I have what I call A.D.D.S (Always Doing Different…Stuff). What I mean is that I use my frames as a testing ground for new ideas so that when you ask for them, I know I can execute them. In the case of this frame the big new idea was a new material, KVA MS2 Stainless Steel. I also played around with geometry as usual, and a segmented seat stay idea.

Stainless Feel
When you work with a material you start to get a feel for it: how it cuts, bends, forms, brazes, and welds. All the reading can’t compare to filing a miter, bending a chain stay, or drilling a hole. Stainless certainly was tougher in the drilling and cutting category because of it’s increased hardness and tendency to work harden. It was also more likely to buckle when being bent because of it’s reduced ductility compared to 4130. I was aware of the material properties before I started the process but working with them turned the numbers into feeling as well. How much can a tube be bent, how hard is that hole to drill, etc.

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S-Bend Chain Stays

Improved welding techniques
Stainless steel is significantly more challenging to weld than the 4130 that I’m more experienced with. The things I learned in this area were the most beneficial to all future frames.

With stainless steel it is essential to back-purge all your welds to preserve their strength and corrosion resistance. This frame was my first chance to test out the back-purging ability of my Anvil Frame Jig. It worked well and made the process of purging the front triangle very efficient. This was one of the challenges I was pretty comfortable with and it went smoothly.

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Checking the back-purging

Stainless steel is more sensitive to heat than 4130. Too much heat can reduce the strength of the weld. It also has a greater tendency to warp when overheated, affecting frame alignment. It’s a great application for the pulse option on your welder, but unfortunately I don’t have that feature. This pushed me to refine my technique: pulsing manually, keeping a shorter arc length, using smaller diameter filler rod, and spending extra time on even tighter miters. At the end I felt like working with stainless had revealed areas for process improvement that 4130 was masking. I came away a better welder, and will apply these techniques to my steel frames to make them even better.

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The welds aren’t anything to write home about, but for a first frame (with no practice joints ’cause I like the rush of discovery) I’m happy.

Segmented Seat Stays
This element actually became part of the frame out of necessity. When attempting to shape the seat stays, I realized there was no way they were going to make the bend I wanted without buckling. Initially I was pretty disappointed, but after a little thought I decided this was a good solution and could look really cool if done well. Also, it went right along with my ADDS, opening the door for a new stylistic element. I am also curious to see if the ride will be different, I have read that a wishbone setup can offer a more supple rear end. While this isn’t exactly the same as that, it might still feel different.

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Stays all tacked up…I can’t even describe how I managed to position them.

The Ride
It’s not done yet, but I think it’s late enough that I can get to work! More to come soon.

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