Before I installed a suspension fork onto my Diamondback cruiser,
all my oranges would bounce outa my front basket at 15mph.
Hitting potholes at 20mph was horrific.
JMO, I'd rather have my front suspension fork than my engine.
It's relatively easy to retrofit a mountain bike suspension fork onto your
cruiser frame.
Find one with disc brakes!
That's if you can find the right donor bike.
First, determine the length of your head tube (frame) and
diameter of the steerer tube on your cruiser bike.
Measure your head tube. That's the part of the frame that your fork lives in.
Then mark the position of your cruiser bike's handlebar in reference to your frame.
That way, you can reinstall your fork in its exact previous location.
Loosen the mounting bolts of the handlebar stem.
Have someone help you pull the frame up from your fork a few inches.
You don't have to completely remove the fork yet.
Stick a measuring caliper between the top of the fork and the bottom of the frame.
Measure the diameter of the steerer tube; It should be 25.4mm(1") or 28.6mm(1.125").
Measure the diameter of the steerer tube at its base.
It should be 26.4mm or 27mm on a 1" tube.
If your cruiser frame's tube is 25.4(1") diameter,
you'll have difficulty finding a mountain bike donor with the correct-sized fork.
You might have to buy a 1" suspension fork from a bike shop or ebay.
If your cruiser fork's steerer tube measures 28.6mm (1.125"), you're in luck.
Most mountain bike forks measure to be 1.125" diameter.
The first thing to do is measure from the bottom of the donor bike's
head tube(or the top of its fork) to the top of the steerer tube,
where the cap is.
Your donor bike's steerer tube extends well past its head tube.
You need to decide if you're going to keep your cruiser bike's original handlebar,
or if you're gonna use the mountain bike's handlebar.
JMO, it's much easier if you use everything from the mountain bike
to retrofit onto your cruiser bike.
If the gods smile upon you, the head tube from the donor bike will be the same
or longer than your cruiser bike.
If it's longer, add spacers between the bike stem and
the top of your cruiser bike's head tube.
If the new fork's tube is short, you'll have to
use your cruiser bike's stem and handlebar.
Take the old fork and the donor fork to the local bike shop.
Have them cut it the same length, PLUS an extra 1/4"(just in case).
Reinstall everything, line up the handlebar and the mark on the frame.
Install the wheel, brake lever, cable and adjust your brakes.
Salvage the fork, brake, wheel, bearings and spacers on the fork,
everything on the handlebar except the shifter.
If your cruiser bike has 28.6mm(1.125"),
everything should bolt into place.
If the handlebar sits too high, remove a few spacers
and trim the donor fork to fit.
If you do it this way, you don't have to thread the steerer tube.
Good luck.
If you visit your local bike shop, and you tell them what you're trying to do,
they might look at you funny, and recommend you buy
a comfort bike or a mountain bike.