Rob Siegel has been writing the column The Hack Mechanic for Roundel Magazine for 35 years, and is the author of eight books available on Amazon. He currently owns thirteen cars. Yes, his wife knows about all of them.

In the repair world, there are “hard failures” and “soft failures.” Hard failures are things that instantly drag a car to a stop when they break. Broken ball joint? The worst type of hard failure—the front wheel folds under the fender like a broken ankle and you lose control of the car. Bad fuel pump? Usually a hard failure—it either pumps or it doesn’t. (Okay, there are some exceptions. The one my E39 wigged out on a low tank in hot weather, then recovered. I replaced it anyway.)

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Matt McCormack, applications engineer for SKF gives us a little insight into what design engineers are frequently asking him. Here, he focuses on the basics of determining the loads on a bearing system.

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After the recording session ended, I began driving back toward the highway. As I approached a small service station, I debated stopping to have them throw the car up on the lift and check it, but again, since the rumble remained low and constant, I headed home. Still, to err on the literal side of safety, I stayed in the right lane, turned the flashers on, and drove at 50 mph.

On newer BMWs, the front wheel bearings are very different than on the 1970s-era cars. About ten years ago, I owned a ’99 E39 528iT wagon that needed struts, control arms, and other front-end work. The E39’s wheel bearings are part of an assembly that includes the front hub—the wheel bearing assembly bolts to the steering knuckle, and the wheel bolts directly to the bearing assembly.

Radial loadformula

I was driving my 2003 E39 530i up to do some recording in Chelmsford, about 30 miles north of me, when I began to hear and feel a rumbling coming from the left front of the car. It ramped up over several minutes, but then plateaued. If it instead had gotten VERY loud VERY quickly, with obvious metal-smacking-against-metal instead of the more gentle rumble of a worn bearing, that would be the hallmark of loose lug nuts, something you need to pay attention to IMMEDIATELY because you may have only five or ten seconds before the wheel falls off.

My ever-rational left-brain shot back indignantly. “You can’t be serious,” it sneered. “You actually had a wheel fall off that 1600 back in 1984. And you’ve had two close calls, both more recently than you’d like to admit. You know what loose lug nuts sound like, and this wasn’t it. And you learned your lesson. You conditioned yourself to always tighten lug nuts with a torque wrench. It’s not loose lug nuts. Really. It’s not. It can’t be. You are not that careless.”

Bearing loadcapacity chart

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Axialload bearing

Although bearing rumble is usually quite clear, part of testing for a bad wheel bearing is to jack the car up, set it on stands, grab the wheel at the 6:00 and 12:00 positions, and push-pull it. If you’re hands are at 3:00 and 9:00 instead, play in a front wheel can come from anything in the steering mechanism, but 6-and-12 play pretty much has to be coming from the wheel bearing. In an old-school car with adjustable wheel bearings, if there’s no rumble, you can try adjusting the bearing by pulling the cotter pin and moving the castellated nut by one notch, but play plus rumble equals bad.

What does radial and axial load capacity mean? ... The radial force is the force acting perpendicular to the shaft axis. Radial ball bearings optimally absorb ...

Often, our customers want to know “how do I determine the load a bearing will encounter?” Commonly, customers think of what the weight of a component is on the bearing (weight of the shaft for example) and they ignore many other influencing loads that are on the bearing. So when we talk about loads on the bearing, we’re talking about the reaction load at the given position.

I then jacked up the left front wheel and rocked it at 6 and 12. Yup, play. Then I spun it. Yup, rumbly. No doubt—a bad left wheel bearing. I set the car back down and began to walk back to the computer to order the bearings, vividly recalling how much of a pain in the butt they were to replace in the 528iT wagon ten years ago. I wondered if maybe still having those eight spare bearing bolts had made the whole thing a matter of fate.

Bearing loadcalculation example

Rob’s newest book, The Best of The Hack Mechanic, is available here on Amazon, as are his seven other books. Signed copies can be ordered directly from Rob here.

So if we just look at the fan shaft, you have your fan on the left-hand side. This is the fan color. And then you have the shaft, and then you have your drive pulley on the right and then you have your two pillow block bearings supporting. So when we’re talking about bearing loading, we’re loading on the bearing, the loads from the application are the blue arrows.

Perhaps the down comforter of soft failures is a bad wheel bearing. Loss of lubrication will cause them to wear, resulting in a rumble that sometimes can be more felt than heard (they may also squeak or squeal), but the failure process is gradual. They’ll get louder as wear and play embrace each other in an ever-widening downward spiral, but you’d have to be an idiot not to hear it and address it, making outright failure rare. Wheel bearings are pretty robust in BMWs. After owning over 70 cars, I’ve only ever seen one wheel bearing that completely self-destructed—a ’67 2000CS I bought came with one that had destroyed the stub axle. I had to replace the entire front strut assembly.

I can only speculate what happened. Due to the space constraints in my garage, when I swap winter/summer wheels, I’ll usually pull one end of the car in, jack it up, swap both wheels, snug the lug nuts with a ratchet, let the car down, torque the nuts, air the tires, then pull the car out, flip it around, and do the other side. I must’ve gotten interrupted. Maybe I received a text I felt that I needed to respond to. Maybe my wife was leaving and I walked outside to kiss her goodbye. Maybe I had back pain and took a break. I don’t know.

Then I checked the lug nuts on the right front wheel. While none were loose to the point of being free-spinning, all were substantially below the 88 foot-pound torque spec. The rears were fine.

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Radialand axialloadonbearing

Bearing loadcalculation

Still, to be careful, I pulled into the right lane, slowed down, and listened. The somewhat rapid onset was uncharacteristic of a wheel bearing, but the other symptoms fit. Since I had a recording appointment with a fiddle player, I preferred to keep going, unless of course I couldn’t for safety reasons. The rumbling remained at a constant low level, so I assumed it was a wheel bearing doing the soft failure thing, and continued on.

On 1970s-era BMWs, there are inner and outer front wheel bearings with each sitting in its own race that’s pressed into the hub. Since the front discs on these cars sit on the inside surface of the hub, and since the bearings lift out of their races and present their greasy private parts to you when you pull the hub off the spindle, the advice is that, if there’s no documentation on how old the bearings are, to replace both sets of bearings on both sides when the front discs are replaced.

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So here we have a shaft and we have a ball bearing, and the radial load is the load that’s perpendicular to the shift. So it doesn’t matter if the shaft is going towards vertical. The radial load will always be perpendicular to the actions of the shaft. The axial load and the thrust load will be parallel to the axis. And if you ever combine those situations, then you can have both radial and axial.

I snugged them down, grabbed the wheel at 6 and 12 and verified that the play was gone, spun it and verified that the noise was gone, let the car down off the jack, and tightened them up with a torque wrench.

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Bearingaxialloadcalculation

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In contrast, soft failures are things that either give you ample warning before they fail completely, or their failure doesn’t immediately drag the car into the breakdown lane. A textbook soft failure is when the alternator or voltage regulator dies. Even though the battery is no longer being charged, the car can continue to run for some amount of time off the battery alone. On a vintage car like a 2002, you can likely drive for hours during daylight before the battery no longer has enough juice to light the spark plugs. On a late-model car loaded with electric motors and control modules, you might have more like fifteen minutes to an hour—enough to get to a rest area, possibly even a repair shop.

When I arrived home, I pulled the nose of the car right into the garage, then hopped on the laptop to get a set of front wheel bearing on order. I looked on FCP Euro, and was more than a little surprised to find that a front wheel bearing kit with German FAG wheel bearing assemblies now cost $312. Of course, I still had that set of bolts, so I could save myself the $4.29 per bolt right there. I mean thirty-four bucks is thirty-four bucks, right? One FAG bearing was $139. I began entertaining replacing just the left bearing, then price-shopping other manufacturers, when I decided that I should be certain that the wheel bearing was really the problem.

The pink arrows are the reaction loads that are happening to the bearings. So we think of the three different loading components that make up what we need to consider for this application. The center arrow is the rotor assembly weight. The shaft is going to weigh a given amount, the fan collar on the left is going to weigh a given amount, and the pulley system is going to weigh a given amount. Just as an example, let’s say the weight is 150 pounds. So, this shaft weighs 150 pounds and it’s cradled essentially on these two bearings supporting both. When you drive, you drive the belt, the belt will tension to this pulley, or there’s some kind of stretching dimension port that’s putting a

I intentionally jacked up the car at the right front wheel—the one I didn’t think was making the noise—to get a baseline. I grabbed it at 6 and 12, rocked it, verified it was tight, and spun it. As expected, it was quiet.

Those are the three basic load components on the bearing. We have to consider the weight of the component, the force, and the drive forces. In this case, it was a belt, but it could be chain-driven or gear driven. And then you have the fly bills. If we know the distances, we can use stats to determine the load on the bearing. We can calculate whether they both are load bearing solutions.

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There are also loads that don’t necessarily think to consider. Like in the case of the fan, there might be imbalanced loads. So if the fan is not in balance, it wasn’t balanced properly when they first installed it, or if they’re aware over time it starts to become imbalanced, those imbalances will increase the load behind the bearing, and then any misalignment. If that pulley was misaligned or the belt was misaligned during the initial setup, that can cause misalignment to load down the bearing.

Bearing radial loadcalculation

Apr 25, 2023 — What are the signs of a bad wheel bearing? · A humming noise in the car getting louder with speed: This can happen when the bearing isn't ...

So, this shaft weighs 150 pounds and it’s cradled essentially on these two bearings supporting on both. When you drive, you drive the belt, the belt will tension to this pulley, or there’s some kind of stretching dimension port that’s putting a radial load on the pulley. The last loading is coming from the work that the collar is doing. This fan is pushing a large amount of air with great force to get it through the duct work to make it go throughout a building. So any force this fan collar is pushing, there’s an equal reaction force that’s happening on the bearing inside this yellow point, obviously.

So these are the loadings that we’re talking about. This is the stuff that we’re after when we’re sizing the bearing. It’s important not to just think of the weight, because many customers just think in terms of “what’s the weight?” There are other options that need to be considered.

Rear wheel bearings on old BMWs are incredibly robust. I’ve only ever had one go bad. It was on a Bavaria that my wife and I took on vacation to Martha’s Vineyard in 1986. We were foolishly driving in near-hurricane conditions and got caught in a tidal surge on the road that runs on the barrier beach between Edgartown and Oak Bluffs and drove through water that was deeper than we expected (as I said, foolish). On the drive home, the right rear wheel bearing began rumbling ominously. At the time, I didn’t own the necessary bearing puller, and had to take the car into the late great Beaconwood Motors who had the back page ad of Roundel for a generation. It was one of a handful of repairs the past 40 years I’ve paid someone else to do.

And so, just to be certain, to placate my left brain and make sure I didn’t go to all that work unless it was absolutely necessary, I took a 1/2-inch ratchet and a 17mm socket and checked the wheel’s lug nuts.

And I’m going to have some sort of voodoo ceremony with that bag of Locktite-coated wheel bearing bolts. I can’t tell whether they were bad juju that triggered the problem or a talisman that warded off disaster, but clearly they played some role outside the normal Western vectors of Newtonian mechanics and Cartesian cause and effect. They deserve reverence.

To make sure this doesn’t happen again, I’m instituting a new system of checking the lug nuts on all four wheels after I’ve done any work where a car is jacked up for any reason.

As an example, here is an industrial fan. This is a very large fan to move air. A hotel has many industrial fans in it, hospitals, schools, any large building where you need to move air for heating, cooling, or ventilation — you’ll find something like this.  The large cylinder portion is where the big fan blade is and at the top there is a box where the buffer will be attached to this fan to move the air through it. All of this is being driven by an electric motor. The electric motor is belt driven from the fan shaft, so you have a buffer to cut between the pulley and the belt. You can’t see it, but there’s a pulley system and a belt and then there’s two bearings supporting this fan shaft.