Oil on the inside of the wheel? Here is what a leaking wheel seal means for the trip.

What the wheel seal is protecting

Every wheel on a truck or trailer turns on a pair of tapered roller bearings, and those bearings live in a bath of lubricant inside the hub. On many trailer and steer axles that lubricant is oil, held in the hub by a hub cap at the outer end and a wheel seal at the inner end, where the hub meets the axle spindle. Some wheel ends are packed with grease or semi-fluid grease instead, and on drive axles the wheel ends are usually lubricated by the axle's own gear oil.

The wheel seal has one job: keep the lubricant in and keep water, dirt and road salt out. When it stops doing that job, lubricant escapes through the back of the hub and is flung outward by the spinning wheel. That is why a leaking seal shows up as a wet, dirty streak on the inside of the wheel, across the tire sidewall or around the brake drum, often on one wheel only.

Reading the sight glass

Oil-filled hubs usually have a hub cap with a clear window, often called the sight glass. Wheel-end manufacturers such as STEMCO describe it as the way to check two things at a glance: that the oil is at the right fill level, and that the oil itself is in good condition. Most windows are marked with a fill line or a band that the oil should reach when the truck is parked on level ground. The CDL pre-trip inspection asks drivers to check that hub oil and grease seals are not leaking and, where there is a sight glass, that the oil level is adequate.

Read it from a step back, without touching the hub.

• Oil at the line and clean. The hub has lubricant. If there is still oil on the wheel, the leak may be slow, or it may be coming from somewhere else.
• Oil below the line. Oil is leaving the hub. Where it is going is the next question, and the inside of the wheel is the usual answer.
• Glass empty, or no oil visible at all. The bearings may be running without lubrication. Treat this as a stop-now fault.
• Milky, cloudy or foamy oil. Water has got in, usually past a failing seal or a damaged vent. Water-contaminated oil does not protect bearings properly.
• Dark, burnt-looking oil or a discolored window. The wheel end may already have run hot. That is a warning sign about the bearings, not just the seal.

Is it really the wheel seal?

Not every wet wheel is a failed seal, and it is worth ruling out the look-alikes before you assume the worst. The most common other sources are these.

• A new seal settling in. STEMCO notes that a newly installed seal can weep some of the grease pre-lube put in at the factory, and that this is not a leak. Seepage that extends well out onto the hub and stays wet is a different matter, and that seal should be replaced at the first opportunity.
• The hub cap, its gasket or its fill plug. Oil around the outer face of the hub points to the cap end rather than the seal at the back.
• A blocked vent. Wheel ends heat up in use and build pressure. If the hub cap vent cannot let that pressure out, STEMCO explains, the seal is likely to start leaking. Overfilling the hub can cause the same problem.
• Brake grease. Grease pushed out of the brake camshaft bushings or slack adjuster when they are lubricated can end up on the inside of the wheel. It is thick and grease-colored, not thin oil.
• Oil from somewhere else on the truck. On a drive axle, the axle shaft flange gasket at the center of the hub can leak gear oil that looks like a seal leak.

Why a leaking seal is not a small problem

The first risk is the bearings. As lubricant leaves the hub, the bearings run with less and less of it, and eventually with none. A starved bearing heats up, wears quickly and can seize or break apart. That is how a wheel comes off a trailer at highway speed, and in the worst cases it is how a wheel end catches fire, because the heat from a failing bearing, a hot drum and oil-soaked components is exactly the combination a fire needs.

The second risk is the brakes. Oil and grease that reach the brake linings soak into them. Contaminated linings do not grip the drum the way they should, so that wheel's brake can fade, grab unevenly or pull the combination to one side under hard braking. Linings that have soaked up oil are not cleaned and reused; they are replaced. A seal leak left long enough can turn a bearing job into a bearing, lining and drum job.

What you can safely check

Get stopped somewhere safe first, with hazard flashers on and warning triangles out; our post on where to place warning triangles covers the placement. Then check from the outside, without climbing under anything.

• Do not grab the hub. A hub or drum that has just come off the highway can be hot enough to burn, and a failing bearing makes it hotter. Stand back and look first. If you want to compare heat, hold the back of your hand near the surface rather than on it, and compare the suspect wheel with the others on the same axle.
• Read the sight glass. Note the level and the color of the oil, and whether the window is intact.
• Look at where the oil is. On the inside of the wheel and tire, on the brake drum or backing plate, around the hub cap, or dripping on the ground. Take a photo if it is safe to.
• Watch and smell for heat. Smoke, a hot oil or burning smell, or oil sizzling on the drum all mean the wheel end is running hot.
• Do not open a hot hub. Leave the fill plug and hub cap alone while the wheel end is hot. The oil inside can be hot and under pressure, and topping up a leaking hub does not fix the seal anyway.
• Stay out from under the unit. If seeing the back of the hub means going under the truck or trailer on a shoulder, do not.

When it is a stop-now fault

Some signs mean the wheel end should not turn any further than it takes to get out of traffic. Stop and call if the sight glass is empty or shows no oil, if oil is being thrown across the wheel and tire, if there is smoke or a burning smell from that wheel, if the hub is clearly hotter than the others on the axle, if you hear grinding or rumbling from that corner, or if the wheel looks as though it is wobbling.

If the oil level is still at the line, the hub is no warmer than its neighbors and there is only light seepage, the fault is less urgent, but it still needs a proper repair soon and it needs watching until then. Where exactly that line sits for your unit is a decision for someone who can see the wheel, and your carrier's maintenance or safety department may have its own rule.

Roadside repair or a tow to the shop

Here is the honest boundary. Replacing a wheel seal means taking the hub off the spindle, inspecting and usually replacing the bearings, cleaning or replacing contaminated brake parts and refilling the hub, and we do not do that work at the roadside. It is shop work. The same precedent is set out in our post on a dragging brake or a hot hub: a bearing that has run hot means a tow, not a roadside fix.

What we will do is tell you on the phone, from what you describe, whether the unit sounds safe to move to a shop under its own power or needs a tow. When it needs a tow, a trailer can be lifted at the affected axle and moved without that wheel carrying load, and the tractor does not have to wait with it. The trailer repair and mobile truck repair pages list what the service truck does handle at the roadside.

Details that help us make the call

Tell us whether it is a tractor or a trailer wheel, which axle and which side, and whether it is an oil-filled hub with a sight glass. Describe what the window shows, where the oil is, and whether the hub is hotter than the others, smoking or noisy. Say whether the unit is loaded, where you want it to go, and exactly where it is sitting. Those answers decide whether it is safe to move and which truck comes. For how inspections treat wheel-end and brake defects, see DOT out-of-service faults.

Related reading

Leaking Glad Hands or Trailer Air Lines: Causes and Fixes — Most leaks at the glad hands come from a worn, torn or missing rubber seal, a bent or mismatched coupler, or a hose that has chafed or kinked where it hangs between tractor and trailer.

Trailer Landing Gear Won't Crank or Has Collapsed — Landing gear that will not crank is usually seized from rust or lack of grease, stripped inside its gearbox, or being asked to lift a load in the wrong gear.

Trailer Lights Not Working? Troubleshoot the Seven-Way — Most trailer light failures start at the connection between tractor and trailer: a corroded or loose seven-way plug, a worn pigtail cord, or a bad ground.