Today I wanted to optimize and optically adjust the flatmount bosses a bit. But now I’m asking myself whether I might not have milled too much away from the bosses? In general, the question arises as to which requirements must be met in order to attach flatmount bosses to chainstays?
Can’t upload any pictures… so maybe later
My tightly calibrated FEA-eyesight tells me that you’re going to be fine with this. By far the “softest” bit of that arrangement is the plate style dropout in torsion from the offset braking force. So if this dropout style in general is stiff enough, you’re not making it any worse with filing away the bosses a bit…
My eyeball FEA agrees as well. It definitely wont break or yield (bend). The only thing you would be compromising is stiffness under braking load. My eye FEA says the stiffness is trivial.
From this article, you can see the braking torque they tested was 120Nm
With a 160mm rotor, the bosses are experiencing 750N. Split between two bosses, it’s roughly 90lbs per boss.
I have a question about flat mount bosses too.
I only made one bike with it and I made my own bosses. I did it because I wanted to use a bunch of small 5/8 leftover tubing I have lying around. So I had some caps laser cut with the slots and I welded two caps on each tube. The issue is that I need to use a longer bolt. I am wondering if I can weld a cap on just one end of the 5/8 tube and use a smaller bolt? Would it be strong enough or the short bolt would cause a problem?
As far as I know, SRAM and Shimano say the minimum thickness is 10mm. I can’t seem to find it: a forum member reached out to SRAM directly, about using a 5mm thick boss. The only downside was a shorter bolt would not “stretch” as much, and therefore not have as much preload to keep it from unscrewing.
Theoretically, I don’t think the bolt tension and the stiffness would be an issue. However, I would be worried about introducing warping to the bosses. Flat mounts gotta be flat!
It looks like @englishcycles is doing some DIY bosses. I’ll keep on doing with the caps on both sides of the 5/8 tube and a long screw. Caps are 1/8’’ thick. Thanks!
I have a feeling that the spec in the SRAM and Shimano docs is supposed to be universal and it does not differentiate between materials. I can certainly see how a carbon fibre or aluminium boss that’s thinner than 10mm could pose a problem for yield strength or flex. Steel and Ti though… probably not as much.
@JMY Yes! I remebered seeing something about the 10mm but could not rembember where.