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It's all about gain ratio
Shorter cranks have a higher gain, so the same gear ratio feels harder (because it needs a higher force at a lower foot speed to get the same wheel speed)
When you are artificially limited to a low gear ratio, you can change crank length to get the gain ratio you would regard as optimal for the exercise in question, and with a bit of time to adapt you end up doing the speed you would with the same gain achieved with 'normal' cranks and a higher gear ratio.
gbj_tester
Many thanks to the five of you for answering.
I also found the Unicycle option, but floundered at how to get a chain ring attached.
The shortened cranks look promising, as do the shorteners,
but,
it may be rendered a bit moot.
He is just 14, 66kgs and 171cms tall,
and with a bit of saddle adjustment can ride my bikes, (54, 92 kgs, 6'1"),
so apart from this specific application has no need for for adjustable cranks.
I will have a look in a couple of BMX shops,
but it may just be he has to HTFU,
(in a softened, loving father kind of way).
I'm guessing the shorter cranks allow faster leg speed?
So training on a fixed wheel, 48:16 with 170mm cranks,
will make the 48:16/125mm cranks seem harder or easier?