Unfortunately I’m not able to verify if 18 inch wheels will fit or not - I’ve never had a Strida 3 in hands.
However, below some thoughts:
According to the first pics of this post it should be possible.
I guess that the frames of Strida 3 and 5 are identical (except the bottom bracket) and it’s no problem to mount 18 inch wheels on a Strida 5.
You’re right, here is not much room leftover, but it does last.
Please note also that the width of suitable 18 inch tyres will be limited to 1,35 inch - in case of 16 inch wheels it is possible to use tyre widths up to ~ 2,00 inch.
I do have also concern, but they are regarding the hubs itself:
Did you notice the amount of spokes?
These hubs are built for 24 spokes, the Strida 18 inch hubs do have 36 spokes…
(I suppose these are 16 inch hubs, they’re simply not made for 18 inch rims.)
I do know that a few fans from Hong Kong did use 16 inch hubs to build lightweight 18 inch wheels, hmmm, that’s not a really safe decision…
Besides of the amount of spokes are the flanges of 18 inch hubs more wide than that of 16 inch versions.
(Already mentioned here:
[url]savings... - #4 by Blackstridaaustria])
And another argument might be the costs:
[url]Tiny wheel improvement - thoughts & facts]
(Please scroll up for ~ one screen height.)
Caution: Estimation without assembling costs!