I ended up going the bearing cups route but went with 6002 bearings and made my cups a 34mm press fit. 1-1/2x.095 tube for the bearing shell and ream with a 34mm head tube reamer
I have a few rides on that bike now and no issues so far.
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I completely agree with using the press in aluminum cups over a BB, just a better solution all around. The 34mm headtube reamer is also a cool idea.
With this general configuration, I would add a spacer between the two inner races. Otherwise when you tighten the axle you’re introducing some axial load into the bearings. Too tight and the bearings will stick or wear prematurely, not enough and and axle will work itself loose over time.
Another option, which I stole from Neko’s Frameworks, which is stolen from Transition is to use the hardware below. This is for an upper link but the arrangement could be used for a single pivot as well. The pins (item 21) both thread onto an 8mm stud (item 20). In reality I think you could use an 8mm set screw of whatever length you need to get the correct width axle assembly. OD on the pins is 15mm so you could still use the 6902 or 6002 bearings. For the frame I would use .75" x .095" tubing cut to length which leaves around .030" on the ID to clean up with a 15mm reamer (about $50 on McMaster) after welding.
One of the downsides to this is you still need a way to capture the outer bearing race on the swing arm. I see a lot of frames using a split collar type design that uses a bolt to clamp the bearing but it’s probably not trivial to fabricate.
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That is a great suggestion! Then you only need one part number (21) for both sides, and even multiple designs. The stud gives you more flexibility to hack a pivot together.
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You guys make some good points. If the pivot shell diameter becomes an issue then it will be worth looking at other options. Existing suspension pivot arrangements and purchasable hardware could certainly be utilized.
Personal philosophy
I have found that many big brand pivot arrangements are slightly under built. This has been partly addressed with the suggestions to use ordinary size bearings instead of really thin low profile bearings. The localized pressure in off the rack suspension hardware gets kinda high for aluminum. This is largely combated by the use of 7075 over 6061 but aluminum male threaded fasteners are not my favorites.
Why a headset pivot
I find making precision fits is a big hurdle for most of us. Those can be addressed by purchasing parts and using internal fits that have an existing reamer. The headset cups are already aluminum press in cups with shoulders, and we already have a reamer to make the fit. If you clamp the axle then the 44mm hole is the only press fit you have to make. I have seen others use a split Paragon steer tube clamp. This way you have all standard off the shelf parts that are not specific to a bike model that could change.
Summary
I think using a zero stack 44mm headset follows the original intent of the post because it is easy in that it all the parts are off the rack and It requires no precision machining.
Devil’s advocate
The pivot shell is really big. There’s no way around it… You have to machine, make, or purchase a big shell tube. This will likely result in a lot of material removal.
Disclaimer
Anyway the above is subjective and largely influenced by personal preference. In the end the main thing is for people to come up with something they’re happy with. I think these discussions are helpful. Sorry my reply is so long…
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I use 15mm 304 stainless for the pivot pin. Its heavy, but only in comparison to aluminium. The part probably weighs in at 50 grams or so.
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That’s a whole lot of work vs keeping it simple. Most of what you are getting custom made…is just a headset top cap.
Most full suspension bikes on the market are undersized on bearings and axles to save weight or cut costs.
With the 44mm headset route one doesn’t need any special items or tools other than what they would already need to build a frame (44mm headset reamer and 46mm holesaw).
Axle is an off cut from a fork (if long enough) or stop by a local shop and ask them to hack the steer off one of the many warranty creaking crowns they have kicking around.
I’ve done it multiple ways. First and second full suspension frames had the 44mm headtube. Frame three I used hardware from a Devinci bike but this was dictated by needing an open shock tunnel and they had components that fit the bill. Fourth frame I used axle and bearings from a Kona in a similar layout to the Salsa above and I made delrin sleeves to press into a custom tube in the frame. With all that I am going back to the 44mm headtube, it’s easier and a nice clean solution.
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I’d be careful with the assumption that bigger bearings and axles are better, especially with what you are proposing.
Headsets use angular contact bearings so they can take an axial force when the top cap is used to preload the assembly. Because of this, their radial load capacity is going to be less than a comparable deep groove ball bearing found in suspension pivots. Headset bearings also have smaller ball bearings than most of the suspension bearings which further decreases radial load capacity.
As far as the pivot axle, steering tubes need to resist some bending load so it makes sense to have them be a larger diameter tube (higher moment of inertia) rather than a smaller diameter solid shaft. However, suspension pivots are virtually all shear force which is a function of cross-sectional area. A 15mm solid axle has more (slightly) cross section than a steering tube in a significantly smaller package. That’s even before considering shear strength differences between a 7075 axle and whatever alloy the steering tube is made from.
Headset bearings will probably hold up fine and their accessibility may make it a viable option, but it’s going to be bulkier and weaker than more traditional pivot bearing arrangements.
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