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My recent woes as regards my trailer bearings have caused me to look into this situation more closely. I learned several surprising things that I hope to share with you all. I also welcome any and all input, particularly where necessary to correct my math or any conceptual misunderstandings. The only thing I would request is that you back up any and all assertions, opinions and/or prejudices with Cold Hard Facts, NOT Anecdotal Evidence.
I don't want to hear about your cousin's, ex-wife's, brother-in-law, who inherited bearings from his Great-Granpa; who forged them himself before he left the old country and your family has been using them in boat trailers ever since. Numbers and references would be greatly appreciated. I have had to resort to data directly from a few manufacturers to compile this and if some of your equipment has different dimensions or specifications than what I have relayed here, your results may be skewed, compared to mine.
These are largely my own opinions, so I'd greatly prefer that none of this gets quoted as "Gospel"; but instead serves as a starting-off point for discussion to (hopefully!) increase our collective understanding of what keeps our wheels turning. If we can fill in enough blanks and correct enough misunderstandings, we might eventually be able to convert this into a useful reference article for the Readers Contributions section of the TSBB.
BEARINGS:
OK, the Very First Thing I learned is that all manufacturers LIE!! 
Whew!! Glad THAT'S out of my system!! 
Now, let's look at what they do. By and large, our trailers all use tapered roller wheel bearings. This is a useful design for trailers traveling down the road, but is difficult to quantify.
LOAD :
Types of load:
There are two different forces acting upon a tapered roller bearing:
1) An axial load and
2) A radial load.
This is because those suckers are sitting at an angle inside your wheel; hence the need for tapered wheel bearings. To confuse the issue further, these loads sometimes swap places a little as your rig moves (turns, for example) and the manufacturers rate their bearings for both types of loads. As a Very General Rule, the inner bearing carries the brunt of axial loads and the outer bearing carries a lot of the radial loads.
How much of which?
Well, tell me how far your inner bearing is from your leaf spring attachment point and how far apart your bearings are inside your hub and those forces can be calculated.
Usually, radial force is roughly around a third of axial force.
But not always.
If that weren't enough to give you a headache, the manufacturers also rate them at two different times, with two different weight ratings: dynamic and static load points. Obviously, trucking down the road places different stresses on a wheel bearing, than sitting in your driveway does.
Another consideration for the radial loads is the, . . . uhh . . . . . radial loading of the bearing.
We tend to think of this as the: 1/4 Turn Past Finger-Tight Rule.
Timken Bearings have on their web site, some very useful articles on how to set bearings.
Roughly summarized, it seems that a little too loose is MUCH better, than a little too tight.
The link is at:
http://www.timken.com/products/bearings/techtips/tip5.asp
So, Charles, cut to the chase and tell us: HOW LONG DO THEY LAST?
Well, . . . . . uh . . . . . . . They (the manufacturers) don't Really Know.
The best They can do is to test bearings to the failure point, until They get to some point where 90% of the bearings make it to some arbitrarily chosen number. So, even when They SAY: "We're guaranteeing our fine bearings for X amount of revolutions, for Y amount of hours" . . . . . Their collective fingers are crossed behind Their backs, since They KNOW only 90% of those bearings are going to reach those Magic Numbers.
Most Comforting.
Unless YOUR trailer bearing is one of the 10 in every 100 . . . . . . . . .
Some of these ratings are Truly Laughable.
One manufacturer rates bearings as guaranteed for 1 million revolutions, IF those million revolutions are at 33.3 RPM for 500 hours at full rated load. When you get these guys up against a wall and twist their arms, all of a sudden-like they know how to combine axial and radial loads as well as dynamic and static loads for an over-all load rating life. Another manufacturer rates THEIR bearings at NINETY Million revolutions,  but then drops the load and hour requirements commensurately. 
In Other Words: Figures Don't Lie, But Liars'll Figure.
Their Final Insult is to rate all this stuff in Kilogram/Newtons, instead of Pounds/Foot, making for a lot of tedious conversions.
There are two main things that make them hedge their bets:
Speed and Weight.
Which do you think is worse?
Sorry!  You're gonna have to read the WHOLE article!!
SPEED:
But let's start with speed.
The first problem, is that most of us haven't a CLUE as to how many RPM's our wheels are spinning. Below is a chart that has some dimensions for popular trailer wheel sizes. Sorry to say, there are some holes and flaws in it. Bias tires have different dimensions than the equivalent radial tire and some tires have different dimensions as the load rating changes. If anyone can get me definitive numbers on trailer tire dimensions for the various types, I'll be happy to amend the table. The first thing we did was to multiply pi times the outside mounted diameter to arrive at a rough circumference expressed in inches. How rough? Well, for example, changing the tire pressure by 15 psi can affect the circumference measurements up to 5% in some cases, so your tire as measured by you might be different from these numbers, as calculated by me.
Deal with it.
Tire Size |
Load Range |
MTD DIA. |
Max Load in pounds |
Max PSI |
Tire Circum. in inches |
4.80-8 |
LRB |
16.25 |
590 |
60 |
51.07 |
4.80-8 |
LRC |
16.25 |
745 |
|
51.07 |
4.80-12 |
LRB |
20.5 |
780 |
|
64.43 |
4.80-12 |
LRC |
20.5 |
990 |
|
64.43 |
5.30-12 |
LRB |
21.8 |
780 |
|
68.51 |
5.30-12 |
LRC |
22.1 |
1045 |
80 |
69.46 |
5.70-8 |
LRD |
18.9 |
1075 |
100 |
59.40 |
ST175/80D13 |
LRB |
24.7 |
1100 |
35 |
77.63 |
ST175/80D13 |
LRC |
25 |
1360 |
50 |
78.57 |
ST205/75D14 |
LRB |
26.9 |
1480 |
50 |
84.54 |
ST205/75D14 |
LRC |
26.9 |
1760 |
50 |
84.54 |
ST215/75D14 |
LRC |
27.3 |
1870 |
50 |
85.80 |
ST205/75D15 |
LRC |
27.1 |
1820 |
50 |
85.17 |
ST225/75D15 |
LRC |
28.1 |
2150 |
50 |
88.31 |
ST225/75D15 |
LRD |
28.6 |
2540 |
65 |
89.89 |
Table 1: Tire Size, Loads and Circumference in Inches
The Good News is that we now have some circumference numbers to work with. If we take some number of miles per hour, then multiply by 1056 and divide that number by the circumference of the tire in inches, we get pretty close to the RPM that wheel is spinning at a given speed on the highway. Which leads us to our NEXT table!
Tire Size |
Tire Circum |
RPM @ 45mph |
RPM @ 50mph |
RPM @ 55mph |
RPM @ 60mph |
RPM @ 65mph |
RPM @ 70mph |
RPM @ 75mph |
RPM @ 80mph |
4.80-8 |
51.07 |
930.5 |
1033.8 |
1137.2 |
1240.6 |
1344.0 |
1447.4 |
1550.8 |
1654.2 |
4.80-8 |
51.07 |
930.5 |
1033.8 |
1137.2 |
1240.6 |
1344.0 |
1447.4 |
1550.8 |
1654.2 |
4.80-12 |
64.43 |
737.6 |
819.5 |
901.5 |
983.4 |
1065.4 |
1147.3 |
1229.3 |
1311.2 |
4.80-12 |
64.43 |
737.6 |
819.5 |
901.5 |
983.4 |
1065.4 |
1147.3 |
1229.3 |
1311.2 |
5.30-12 |
68.51 |
693.6 |
770.6 |
847.7 |
924.8 |
1001.8 |
1078.9 |
1156.0 |
1233.0 |
5.30-12 |
69.46 |
684.2 |
760.2 |
836.2 |
912.2 |
988.2 |
1064.3 |
1140.3 |
1216.3 |
5.70-8 |
59.40 |
800.0 |
888.9 |
977.8 |
1066.7 |
1155.6 |
1244.4 |
1333.3 |
1422.2 |
ST175/80D13 |
77.63 |
612.1 |
680.2 |
748.2 |
816.2 |
884.2 |
952.2 |
1020.2 |
1088.3 |
ST175/80D13 |
78.57 |
604.8 |
672.0 |
739.2 |
806.4 |
873.6 |
940.8 |
1008.0 |
1075.2 |
ST205/75D14 |
84.54 |
562.1 |
624.5 |
687.0 |
749.4 |
811.9 |
874.3 |
936.8 |
999.3 |
ST205/75D14 |
84.54 |
562.1 |
624.5 |
687.0 |
749.4 |
811.9 |
874.3 |
936.8 |
999.3 |
ST215/75D14 |
85.80 |
553.8 |
615.4 |
676.9 |
738.5 |
800.0 |
861.5 |
923.1 |
984.6 |
ST205/75D15 |
85.17 |
557.9 |
619.9 |
681.9 |
743.9 |
805.9 |
867.9 |
929.9 |
991.9 |
ST225/75D15 |
88.31 |
538.1 |
597.9 |
657.7 |
717.4 |
777.2 |
837.0 |
896.8 |
956.6 |
ST225/75D15 |
89.89 |
528.7 |
587.4 |
646.2 |
704.9 |
763.6 |
822.4 |
881.1 |
939.9 |
Table 2: Tire Size, and RPMs at varying highway speeds
These are the approximate RPMs at differing speeds for the more popular trailer tires. They range from 528 RPM to a whopping 1654 RPM for those poor abused 4.80-8's which should probably be restricted to lawnmowers. 
Unpleasant Fact #1: Many tire manufacturers rate trailer tires only up to 65mph.
Ya wanna go faster, they want you to swap to a safer (read more expensive) technology.
This table was moderately eye-popping to me.
I think every TSBBer, needs to print this out, laminate it and glue it to the dash of their tow vehicle, just above the speedometer. OK, Remember those guys telling us about 33.3RPM for 500 hours continuously at full-rated load?
Well they assign a Speed Factor to their bearings and the guys that go 33.3RPM (That's about 2.7mph for most of us! ) are assigned a factor of 1. The REST of us, get this table:
(Only ran those values pertinent to the speed range, from Table 2, above.)
RPM |
500 |
600 |
700 |
800 |
900 |
1000 |
1100 |
1200 |
1300 |
1400 |
1500 |
1600 |
Speed Factor |
0.444 |
0.420 |
0.401 |
0.385 |
0.372 |
0.361 |
0.350 |
0.341 |
0.333 |
0.326 |
0.319 |
0.313 |
Table 3: RPM versus manufacturer Speed Factor
Some Real World Examples:
ME! Tooling along at 75mph up to BEER 2005 on my ST175/80D13's (we'll get into why this was nearly suicidal, a little later) means that my trailer bearings are turning at around 1020 RPM and the manufacturer says that de-rates my life expectancy to .350 or thereabouts, or roughly 1/3rd of what more conservative guys would get. SLOW DOWN, CHARLES!! is what Everybody says. OK, slow down to 55mph or 680 RPM and my expected bearing lifespan goes from .350 to .400 of nominal. Not a very big change. Probably roughly equivalent to one more trip to the Keys.
Maybe back, too.
Unless I'm one of the Un-Lucky Few in that 10-out-of-every-100 group . . . . . . . . . .
Gene: 53mph all the way up to Atlanta in his 15 inch trailer tires increased his Life Factor number all the way to .420!! Hmm . . . . . . Not a whole lot better than my .350 ratings and takes him half again as long, to get there. But these figures at roughly .3 to .4 times the rating, means that we're getting slammed pretty hard and that those lofty bearing life specifications are fudged pretty severely. Off the top of your head it looks like you're only getting a third of what they promise you.
Unpleasant Fact #2: Wheel Bearings need to have a hardness of Rockwell 90 to be useful. Many don't.
The other side of this coin however, is weight.
WEIGHT:
Load Ratio |
1,000,000 |
# Hours |
0.7 |
300,000 |
150 |
1.0 |
1,000,000 |
500 |
1.1 |
1,370,000 |
685 |
1.2 |
1,840,000 |
920 |
1.3 |
2,400,000 |
1,200 |
1.4 |
3,070,000 |
1,530 |
1.5 |
3,860,000 |
1,930 |
1.6 |
4,790,000 |
2,400 |
1.7 |
5,860,000 |
2,930 |
1.8 |
7,090,000 |
3,550 |
1.9 |
8,500,000 |
4,250 |
2.0 |
10,100,000 |
5,040 |
2.1 |
11,900,000 |
5,930 |
2.2 |
13,800,000 |
3,620 |
2.3 |
16,100,000 |
8,030 |
2.4 |
18,500,000 |
9,250 |
2.5 |
21,200,000 |
10,600 |
Table 4: Manufacturer Load Ratio, affecting bearing life.
Notice the Load Ratio of 1.0 (2nd row) is the million revolutions for 500 hours at full load, that they originally promised us.
Some More Real World Examples:
Travis Votaw is in the process of upgrading his rig after discovering that at 3220 pounds, that it's about a half a ton heavier than he thought!
After soliciting opinions from all of us, I recommended he upgrade to 14" tires and a 5000-pound axle using 1-3/8X1-1/4-inch bearings. He elected to go to 14" tires, but with a 3500-pound axle and 1-3/8X1-1/16-inch bearings. Let's see how that works out on the Load Life chart. 3500 / 3220 = (approx) 1.09. Round it up to 1.10 and we see the manufacturer thinks that Travis' bearings can turn 1,370,000 times for 685 hours, before they would expect 90% of them to wear out. Checking the OTHER factor, (Speed), we see that Load Range C 14" tires will turn 923 RPM at 75mph and 677 RPM at 55mph or again, roughly .4 and .3 times that number. If we divide his revolutions by 1/3rd we get about a half a million revolutions for about 228 hours.
(By The Way, any nitpickers and/or Math majors out there that want to take me to task for not calculating everything to 9 decimal places out, can do so ONLY if you're willing to stand on the trailer fender and count EVERY revolution yourself! )
Is Travis really in THAT bad a shape with his 3500 pound bearings?
Nope.
Another detail we've left out (until now) is the nature of those dynamic loads the manufacturers keep yammering about. Owing to a hefty tongue weight, Travis' axle weight while on his truck is 2680 pounds. Re-do the numbers, re-check the tables and we get: 800,00 revolutions for 400 hours of road time, a most reasonable rate of usage. In our previous example, we were calculating for worst case performance. In this example, we are calculating for best case perfomance.
But! Had Travis installed a 5000-pound axle, the numbers would have been very different. (Including the ones for his bank balance!  ) We would now have 5000 / 3200 = (approx) 1.6 Checking the table, we see that gives us 7,090,000 revolutions for 3500 hours. Factor in how much they take away for driving on our nation's highways and it still comes out to 2.36 million revolutions for 1,183 hours or effectively, the rest of Travis' lifetime trailering the boat!!!
(Just kidding, Travis! )
These figures would seem to indicate that while my solution was conservative (and more expensive!), Travis' final decision, was a pretty good balance between prudent and conservative, both fiscally and mechanically.
Until he hits that 400-hour mark somewhere out on the road! 
Another example (an extreme one, as it so happens) is Gene's trailer for First Born. Gene has tandem axles rated for 5000 pounds each and is pulling about 4400 pounds all together. Let's check the math on that. 10000/4400 = (approx) 2.3 Table 4 shows 16,100,000 revolutions for 8,030 hours. After factoring from Table 3, that gives us 5,366,666 revolutions for 2,677 hours!! This is why Gene has had his bearings so long, with so few problems. If he can keep water out of them, they very probably will out-live him!
A Final Example is Bob K. who has been dragging his Catalina 22 to various races, with no problems whatever. (At least on the trailering part! ) His (measured) trailer/boat weight is 2900 pounds put on two 2500-pound axles. Back to the numbers, we get 5000/2900 = a Load Factor of 1.7 for 5, 860,000 revoutions and 2,930 hours and factoring in our speed factor of roughly 1/3rd we get 1, 953,333 revolutions for 977 hours!
This must be the Manufacturer's Subtle Method of telling all of us:
Hey! Don't overload the bejesus out of our bearings and they JUST MIGHT LAST Ya!
Unpleasant Fact #3: When a sphere is jammed against a plane, it either flattens the sphere or dents the plane, depending on which is harder.
Manufacturers know that their bearings are going to flatten out slightly, under load. Like sitting in your driveway for example. They also know that the plasticity of the material (even in steel) will return to its original shape when removed from the load.
Up to a point.
Then it just squashes and stays that way.
What point IS that?
Well, unfortunately for me, that point is about 150% of Max Load.
At that point the manufacturers feel that all bets (and guarantees) are off, and they don't have ANY idea when the bearing will fail except for the possible guess: Real Soon.
Just the Right Bounce at Just the Right Speed at Just the Wrong Time and the bearing will deform and destroy itself and all the other rollers inside the wheel. This is why I have never had anything to analyze, after the bearing failures I have experienced in recent years.
A Last Real World Example:
So this guy is on his way to Pensacola from Hollywood, Florida and heTHINKS his rig is under a ton, when it's really closer to a ton and a half and so he exceeds the manufacturer's specifications to the tune of 150%! 2000/2950 gets us pretty close to .7 which is 300,000 revolutions for 150 hours. Multiply by our speed factor and we get 100,000 revolutions for 50 hours. OR, almost EXACTLY where they failed on me three years ago and EXACTLY where they failed me, this year! Imagine that! 
I believe this is why my bearings have been giving me problems in the past few years. All those beef-it-up projects have taken their toll in added weight and now I have to figure out how I intend to get a load off of those bearings. Ideally, I intend to get (somehow!) to a Load Factor of 1.5 or better.
LUBRICATION:
Grease vs. Oil.
Of increasing popularity, is the use of oil-filled hubs for trailer wheels.
At first glance, this seemed to me like: Niche Marketing to the Paranoid.
After all, the manufacturers clearly specify their bearings at up to 7500RPM with grease and up to 10,000RPM with oil. If our trailer bearings are only ranging between 500 and 1600RPM (even when SHANE'S driving! ) then there shouldn't be any Rational Reason for using oil instead of grease, should there?
Well . . . . . . .
Seems that the bearing manufacturers build in about 3mils of "slop" into the top and 3mils into the bottom of their bearings, to make room for the grease to coat the bearing rollers with a thin film of grease. The pressure on the bearing rollers wants to "squeeze" it's way off the rollers. At highway speeds when the hubs are up to temperature the viscosity of the grease is lessened and it is easier for grease to re-flow back into the bearing cage and re-lubricate the rollers. When it gets cool (like after sitting on the trailer all night) the grease is more viscous and does not want to re-lubricate the bearing quite so easily, so now the rollers are turning with more wear and tear (until the grease warms up), than if they were being lubricated more efficiently. Turns out (get it? "turns"? Sorry! Couldn't Resist! ), the bearing manufacturers know how to calculate exactly how much that on-again, off-again, lube cycles will decrease their bearing life.
Without actually admitting to anything, it seems that there is pretty good evidence that using oil-bath hubs instead of grease, will increase bearing life nearly 15% at most loads in the range that Trailer Sailors typically operate.
Another side-benefit of the oil-bath is surprisingly: Heat dissipation.
Grease against the bearing rollers gets pretty warm, much more so than the grease closer to the axle, just floating around inside the hub. The oil-bath hub does not have this same problem.
While I am not ready to run out and purchase a set of oil-filled hubs just yet, neither am I dismissing the notion out of hand, any more.
What To Do About It:
1) Determine your REAL Rig Weight.
Go to a certified scale, get some real numbers and some real tongue weights (they affect the life calculations significantly.)
2) Determine your True tire circumference in inches.
3) Determine your true RPMs. MPH * 1056 / Tire Circumference (in inches) should approximate your RPMs.
4) Determine from your driving habits your REAL Speed Factor.
5) Figure out from your total weight, what your bearing Life Factor is. (This is worst case.)
6) Recalculate minus tongue weight, what your bearing Life Factor is. (This is best case.)
7) Figure out how to get your Load Factor in excess of 1.5 (Best Case)
8) Decide what is the best way for you to lubricate your bearings, based on your circumstances.
Hope this helps, Charles Brennan
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