Does anyone know if a 21" front wheel and 18" rear wheel off a soft tail springer fit my 81 lowrider
You running the stock narrow glide front end on the 81 no the springer 21" is hubbed for the wide glides , the rear depends on how wide the 18" is . Most those 18 rear wheels had wide rims . If you mean a 16" that will work if it's off a 1999 or earlier bike with the 3/4" bearings but you will have to play with spacers to get things to fit, 2000 they went to 1" bearings.
I've got about 20lbs of odd axle & spacer combinations just for doing just this kinda jabberwocky and small lathe when it needs that special touch That kinda stuff is my thing