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WheresWaldo

Bicycle Digital Spoke Tensiometer based on Jobst Brandt design

by WheresWaldo Dec 2, 2018
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Looks good - got the specs or a part number on the spring?

Cheap Chinese spring, do you see a pattern developing here. 1 mm x 12 mm x 50 mm, really cheap in lots of 10 - 50 pieces. Not that I need 50 of them.

With a 12 mm diameter spring, you need to sand down the knurling on the end of the plunger cover to make the spring work smoothly. What I did was to chuck up the cover into a drill and then spin it against a sanding block to knock down just the top section. Only power tool I used for this.

Curious if the longer spring without the spring seat might not have enough force. Eyeballing the number of active coils at 12 and feeding the rest of the info into a spring calculator, comes out at about 3.5 lb/in which is pretty close to the original Lee spring 4 lb/in (and it'd probably be dead on the money if it was music wire and not stainless) - but without the spring seat you're giving up quite a bit of preload so the force at the spoke might be lighter. I'm thinking the spring seat in the original design served the purpose of guiding the spring and giving it preload to hit the desired 4lb side force.

Other than that, I'm all for swapping out the spring as long as it's commonly available, and that seems like it would be. I did buy a pack of larger diameter compression springs with about the same spring rate and was going to design for that as it fits very nicely over the top with plenty of room, but didn't try that once I realized the dual purpose of the original spring seat. Looks like the one I found is very close to the one you found.

You may be correct with regard to spring rate. If we increase the wire diameter slightly to say 1.2 mm in stainless steel, we could calculate the spring rate with 12 active coils all else remaining equal of approximately 6 lbs/in. that could be enough. A music wire spring would be about 8 lbs/in.

The spring pictured on this particular print is music wire. The stainless springs are on their way, the slow boat arriving before tariffs.

Yeah, and it's like you said - at this point without a calibration rig it's all theory. Having the rig that gives repeatable results will prove or disprove what works and what doesn't.