How do you know what size bearings?

[Ziradog]

So if you are looking at a trailer sailor that is not nearby, it is usually pretty easy to get wheel/tire info beforehand so you can purchase & carry new ones with you.  But what about bearings?  How do you know what to take?  Most sellers seem clueless.  The axle weight limit MIGHT be available.  Waiting until you get there seems like it is looking for trouble, especially if you & the boat are several hundred miles away.  Asking for a friend...

[Charles Brennan]

Ziradog, WOW!! What an Optimist, YOU are!! 

it is usually pretty easy to get wheel/tire info beforehand so you can purchase & carry new ones with you

Really? 
4 lugs, 5 lugs, or 6 lugs? 
4.5" lug spacing patterns or 4.75" lug spacing patterns?
What speed range? L? M? N? 
What load range? B? C? D? 
Get a new tire with the wrong combination of letters, and you can easily overload the tire AND exceed it's rated highway speed. 
And without getting the entire rig weighed on a scale, you are doing very little more than whistling in the dark.
If you KNOW what the weight is, THEN you can figure out what load range to get. 
If you KNOW what speeds you are going to travel at (slow rural roads, or high-speed interstates?) THEN you can decide what speed range you need to purchase.
After all, you have no way of knowing if the trailer size/numbers being given to you are in fact, the right ones for the rig.
A tire might be wildly overloaded with the seller having no idea that it is.
Weight slip is everything, My Man.

But what about bearings?  How do you know what to take?  Most sellers seem clueless.  The axle weight limit MIGHT be available.

Bearings are very similar. 
To be sure, they are far better than in Ye Olde Dayes, when different manufacturers had different ways to measure bearings.
Nowadays though, you can walk into a well-stocked trailer store and ask for trailer bearings for a 3500 lb axle and the counter guy will immediately know that you need L44643 inner/outer bearings and L44610 inner/outer races and 204507 seals and it won't even matter if the bearings are made by Dexter, Timken, DTK or whoever.

Should work the same for a 2000 lb axle, right? 
Uhh . . . . . . . . no. 

Some 2000 lb axles use a 1" spindle and some 2000 lb axles use a 1 1/16" spindle.
Everybody has an opinion, but a caliper dial always  reads in black and white.
Asking a potentially "clueless seller" to measure the axle with a tape measure, is probably beyond them.
Even if they did remove the hub dust cap, remove the cotter pin and nut and measure across the front of the axle face, how accurately do you think they are going to be able measure it? 
Nope. Trust your own Mark I eyeballs on a dial caliper, or nothing.

Your best chance (and it's a very small chance, indeed) would be if the seller can find a number on the edge of the bearing, when they pop the dust caps off, remove the cotter pin, the nut and the washer and wipe off all the grease.

Hope this helps,
Charles Brennan

[Ziradog]

Thanks.  So basically what you are saying is that if I know it's a 3500# load rating axle, I can get the correct bearing from that, otherwise measure the spindle diameter?  Really sounds too easy to be true.  The few times I've replaced bearings I have gone in with the info off my old ones. And I use either digital calipers, but to each his own.  I suppose a good micrometer would be even better.

On wheels & tires, even a clueless seller can (usually) count lug nuts, measure the spacing (close enough), and take a picture of the markings on the current tires.  Boat weights are usually published & trailer weights are usually easy to estimate within a couple hundred pounds (at least in the range I am work in) to compare with what the old tires were.  So not exactly whistling in the dark.

[Charles Brennan]

Ziradog, Extra points, for using a digital caliper!! 

Really sounds too easy to be true. 

Well, bear in mind, I left  out intermediate ranges like the 2500 lb axles used on personal watercraft, that come in either 1-1/16" or 1-1/8" spindles.  And there are not too many of the 3000 lb axles around any more, the ones that had a 1-1/8" bearing in the front and a 1-¼" bearing in the back. 

On wheels & tires, even a clueless seller can (usually) count lug nuts, measure the spacing (close enough), and take a picture of the markings on the current tires. 

Agreed.

My contention was that you cannot KNOW, what you REALLY need, without a NUMBER!! 
("If You Can Put A Number On It, It's Science; If You Can't Put A Number On It, It's VooDoo!!")   

I only bring it up, from bitter experience.
I bought an Arrow trailer in 1983, after giving the dealer all the weights and specs from the sailboat manufacturer.  I was assured the frame, undercarriage and wheels were all up to spec and should have no worries.

And after incidents like this one:
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Or incidents like this one:
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Ya know, I started getting just the least bit SUSPICIOUS!! 

Started investigating and eventually discovered the trailer frame was indeed the right one for my hull.
The entire undercarriage was not.
They put a 2000lb undercarriage on a 3500 lb trailer frame, probably to make more profit.
My rig, 1400 lbs from the manufacturer and (ostensibly) 300 pound trailer (according to the manufacturer) and maybe 100 lb of gear aboard, when weighed on a truck scale, came out to a whopping 2600 pounds!! 
Nothing like exceeding your undercarriage capacity, by 30%!!   
No WONDER, I was eating bearings like M&M's!! 

Ripped out the entire undercarriage and replaced it with a 3500 lb undercarriage.
The tires I kept losing? 
Yeah, Load range B and very close to max capacity.
The current ones are load range D.

I may very well be over-reacting, but I have good reason to do so.
(Hint: Check the pics above.)
If everything is off-the-shelf and done by competent and honest dealers and the seller didn't borrow some trailer wheels from his brother-in-law, in order to sell the boat or whatnot, you should be in good shape.

Just be skeptical, is all I'm saying.

Hope this clarifies,
Charles Brennan