Originally Posted by
kamtsa
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As for your suggestion, will the pain stick to the chrome plating? Any special pain to use? I tried for example to draw a hight marker with permanent marker but it did not stick. Will grit wrapped with thin tape work better?
The custom seatpost will be done by a friend so the cost will be reasonable. Any suggestion how to spec it?
I've found nail polish and a number of rattle can spray paints to stick to chrome reasonably well. The key is to have a 100% dry, oil free surface. Use acetone (not nail polish remover), or naphtha, to totally strip any oil or wax film then you can mask at just below yout target height so it wont show. Toy might get by with some grit sprinkled on 2" cello packing tape then wrapped around. The grit will cut through and engage both the tube and the post and frame and do it's job.
If you want a better job you can prep the post a bit by gripping medium sand paper around it and twisting a bit while squeezing hard. Unlike most sanding work, the object isn't to get a nice finish, but to leave scratches which will be anchor points for the paint.
Normally I use coarse lapping compound on slipping posts because it's easier, but Knowing your application calls for raising and lowering often, II suggested a painted band that would stay put and not make a mess, by smearing all over the post.
As for making a post, I'd suggest 6061 T^ as about ideal, but you have to know the extended weight to calculate the bending moment. One way to approach it is to take a reliable post like a thomson, measuring the usable length and wall thickness, then increasing the thickness in proportion to the added length (proportional to the bending moment, assuming Thomson got it right), then adding 15% for the jell of it as an added safety factor. Be aware however that the friction properties of anodized aluminum aren't that much better than steel (or chrome) so unless your situation close to marginal, you still may not solve the problem.