I bought a cheapo adult tricycle and looking closely at each tire there is stamped on it a maximum weight capacity of 175lbs. Do I multiply 175lbs x 3 for the max load it can carry or does this mean only a person weighing less than 175 lbs can safely operate this trike? Thank you very much!
It is unlikely that the “weight” listed is the capacity of the tire to support weight, but more likely the maximum internal air pressure in PSI (pounds per square inch).
If it is weight capacity, then yes, each tire will support 75lbs, so a balanced load over 3 tires would support 225lbs. That assumes equal weight on each tire, though, which is difficult to achieve.
Since the weight listed on the side of the tire is actually 175lbs, that is much more realistic as a max weight capacity per tire. That would make the total supportable weight for a balanced load 525 lbs.
"Maximum tire load" reflects the maximum that any individual tire should carry. For instance, with a 175-per-tire limit on a trike you may encounter the situation where the singleton tire inherently carries about 50% of the load, due to the weight is distributed. This would mean that the entire load would be limited to 350 pounds, even though the paired tires each carried only 87.5 pounds.
As to what creates such a limit, the strength of the wheel (in particular the spokes and hub) is part of it, but also consider the deformation of the tire. If the limit is 175 pounds and the tire pressure is 35 psi, this means that the load will normally be spread over about 5 square inches. This is easily accommodated by a normal tire on smooth pavement, but when you hit a bump (or a curb) the tire deforms unevenly and a rim strike (or other nasty effect) is likely, because the load is concentrated on a smaller area.
There are a lot of factors that affect the maximum weight your tricycle can handle.
First, there are the tires. Tires are incredibly strong but there are limits especially if you hit a pothole or rock. Also, as Jeffrey Bell notes, it’s also possible to roll the tire off a wheel if you go around a curve too quickly.
Second, there is your rim and spokes. Your rims may be a limiting factor. Exceed the max weights for these and your rims break or spokes break.
Third, is your axle. On trikes, this is a huge limiting factor because the paired wheels usually mount with single-sided axles (aka wheelchair hubs). It’s very easy to bend these axles with a load that’s too heavy. Exceed this and bend an axle or more rarely break the hub or bearings.
Fourth, your speed is a huge factor. What you might be able to get away with at a snails pace won’t work at 30kmh or 20mph.
Fifth, your riding style will also be a factor. It’s much harder to bunny hop or even shift weight on a trike which is a problem.
Finally, an unsuspended bike will withstand much stronger forces at the axles than one with suspension.
In other words, you should go by what your trike manufacturer lists as the max weight of the trike rather than any single component.
Remember the rims, axles, wheels, frame, etc will all have their own maximum ratings. As such, the bike manufacturers specifications should be the final word on overall maximum loading.
For example the Walmart adult trike is a Schwinn, with an overall weight maximum of 350 pounds (159 kg) according to http://tricycleworld.org/schwinn-meridian-adult-26-inch/
That's excluding the 70 pounds (31.8kg!!) of bike, but including the rider, clothes, contents of pockets, and anything you put in the carrier.