Is there a smoother ride

I had them both . I think the dual axle rides a little smoother, but the single axle on a 21 footer is plenty adequate if done right. I had blow out and the single axle just laid down without any sway or issues whatsoever. In addition cheaper tire/wheel bearing, brakes maintenance and less on the tolls if you live or travel in heavy toll laden areas like northeast.You pay per axle. The nice thing about dual is could drive a ways if you get a flat on one of em before changing it out. The single has to be done immediately.

When I had my construction business I had two 16'-18' cargo trailers used as tool trailers for two different job sites.
I tried single axle trailers at first but after a year and 88-90k miles myself and other drivers got tired of the beating. Both were a pain to get loaded without lots of bucking with the trucks rear suspension ....plus contents of the trailer would all be scattered in the floor.

Swapped them for two tandem axle cargo trailers and no worries about loading or balance or tools scattered about the trailer or the best part was no more beating the trucks on those long 300-400 miles runs..

A smoother ride (tandem axle) costs way more $$$ though.

For example: A dealer near me has plenty of single axle TT's (19') at $8995. But his cheapest tandem axle TT starts at $14,995.

my experience is tandem is smoother than single axle. tandem will try to balance weight and movement between the two axles as you go over bumps etc. single axle can only balance between it's ONE axle and the TV axles.

now on small pop ups, utility trailers etc. single axle seems to pull fine.

My experience over the years towing everything from snowmobiles to race haulers is that more length and weight is smoother that short and lighter. A tandem will feel smoother but not because of the extra axle but the greater weight that required using a tandem to begin with.

Just my opinion.

Torsion axle(s) is the answer.

In the last 50 yrs I have towed numerous types I have only had one incident and it was a trixaxle it all revolves around weight and balance Multiple axles are more forgiving but added upkeep

I have a single axle trailer, because they do not make a dual for such a small trailer (which is 15 feet long, coupler to rear bumper). But I would prefer a dual. Not just for the smoother ride, but for safety. If you have a blowout on a dual, you will be able to keep control of the trailer. On a single axle, it is not so easy.

So bottom line, if you can find a dual axle in the length you want, I would get it. Yes, you will sacrifice some storage space (because the wheel wells stick up into the cabinetry). And yes, you will have to buy an extra set of tires. Well worth it, I think.

I belive so, but you can do even better with a better equalizer than OEM rigid one. There are more like Dexter EZ-flex. Also look for wet bolts and metal bushings.

Tandem axle trailers always seem to pull better to me.