Question: I've heard that 8 inch wheels and tires on a trailer are a problem but I'm not sure. I've heard the bearings run hot. What is your opinion on smaller tires and wheels compared to the larger ones?
Answer: The short answer--8" wheels are fine. On some trailers they are the only choice due to the design. To achieve the proper ride height based on design, sometimes an 8" wheel is the only choice. For example, the Mini Mate and two of the Time Out camper models use 8" wheels. I've towed all of them thousands of trouble-free miles. I would not make a buying decision based solely on wheel size.
The longer answer is that the problems people have with 8 inch wheels and tires has more to do with poor maintenance than with the size of the tires. It is true that an 8" wheel turns faster at the same highway speed than a 12" wheel, that's just mathematics (or geometry or algebra or physics or something, I dunno.) However, today's axle assemblies use bearings that are designed for much heavier loads than a motorcycle trailer or camper will EVER carry. Running at 80 mph on the interstate is no big deal. Properly inflated tires and lubricated bearings can handle that all day long, day after day.
The key is keeping the bearings packed with fresh grease and this is not a big deal either. If you are a casual tower and don't ride in a lot of rain, you may need to service the bearings just once every few years. This consists of pulling the dust cap and castle nut off the hub, pulling out and wiping off all grease from the bearings and hub, then repacking with fresh high speed high temperature grease. Better yet, install bearing buddies and you can always be assured your bearings are good to go.
12" wheels do have a couple of advantages but, as I said, they are not deal makers or breakers for me. A 12" wheel will not step as deeply into a rutted road surface, so it pulls a little smoother over rough surfaces. And there are a few more design/material options in 12" wheels, chrome and alloy, so you have some different looks. But that's about it.
Finally, it is NOT a good idea to swap out taller tires on a camper or trailer designed for 8 inch wheels. Doing this makes the trailer taller and changes the center of gravity. If the trailer wasn’t designed for the added height, the change can create sway at high speeds on the highway. You can also run into problems with tire clearance in the wheel wells.
Bottom line, I would not hesitate to pull a trailer with 8" wheels I wouldn't worry about maintaining a specific road speed to "save" the bearings and I would not substitute one tire size for another without first consulting the manufacturer.
Have a question about motorcycle trailering? Feel free to e-mail me, firstname.lastname@example.org and I'll try to answer your question, or point you in the right direction.