vacuum belts
Hoover round belts have gotten less reliable over the years unfortunately just like many other things manufactured these days.
For a better belt for your Hoover Decade 80 upright, I would get the Hoover commercial style belt, part number: 044783AG. It uses a slightly softer rubber than the household ones which will last much longer. Also as Hoover Convertible/ Decade uprights are very easy to access the belt on, you can detach the belt on your Hoover upright between uses.
Most anything vacuum that has a revolving brush uses a belt of some sort. The belt provides power from the motor to the brush roll. The exception to this rule were some Kenmore uprights from the early 2000s where the motor was actually in the brush roll. That design did not stay on the market for too many years though and it was a much more pricey upright compared to other Kenmore's at the time.
There are 3 typical belt types:
1. Stretch Belt-- these require high tension to maintain good contact with motor pulley and brush roll. Generally these belts are rubber. There are vacuums with round stretch belts such as your Hoover Decade 80, older Eurekas, newer Sanitaire direct air units and some others. Most vacuums with a stretch belt have a flat belt. The Eureka power team canisters you were looking at would have this style belt. Flat belts generally last a good bit longer than round belts.
The best flat stretch belt came out from Kirby and was textured. It is otherwise known as a gnarled belt. Royal also has gnarled belts for some of their models over the years. Kirby gnarled belts last a very long time and have worked successfully on many power nozzle canister vacs too as a replacement from the OEM of that particular unit.
2. Cogged belts-- Cogged belts are like a geared system. There is a gear patterned on the motor pulley, agitator and belt that all match up. This system is much easier to install belts on than a stretch belt since the connection is tight but not near as tight as a stretch belt configuration. Life is generally very good with these belts. Many vacuums with a cogged/ geared belt had a belt/ motor protection system built in that shuts off the brush if an obstruction is detected. The most common machine that used a cogged belt over the years were Kenmore/ Panasonic canisters of the 1990s with the 14" power nozzles. The power head style was also known as the Cen-Tec CT14DX. That power head also came on later Royal PowerTank canisters, and various central vacuum attachment kits including Hayden and many Nutones. Many Electrolux vacuums over the years are also cogged belt systems.
3. Serpentine belt-- Serpentine belts are the newest technology belt. They used a ribbed system. The ribbed belt has a continuous traction pattern spanning the entire belt. This pattern coincides with a matching traction pattern on the motor pulley and brush roll. Serpentine belts have become very popular on Tacony vacuums and others. Many vacuums from Kenmore and Panasonic from the early 2000s on had serpentine belts. Serpentine belt power nozzles from Kenmore and Panasonic were much quieter running compared to their cogged belt predecessors while offering continued great power too. Like many cogged belts systems, generally a serpentine belt unit does have a motor/ belt protection system which shuts the brush roll motor off if an obstruction is detected.
Sebo/ Windsor is one brand that does come to mind which doesn't use a belt system. Sebo Windsors have a brush roll that easily removes from the vacuum with out needing to touch a belt at all. They are direct drive units and typically are very well built, quiet running vacs. Some collectors, including myself, however don't feel that Sebo/ Windsor vacs deep clean as well as many other vacuums despite their high build quality. These vacuums tend to typically get used in commercial environments for low profile carpet that doesn't require much agitation.
Best of luck to you with your new Eurekas!! I hope that you will continue to expand your collection and come across a variety of power nozzle canisters and uprights to try out.
https://www.amazon.com/Hoover-044783AG-Belts-Industrial-Uprights/dp/B0082VCF3O