The short version
- The hitch sits behind the rear axle, so tongue weight acts on a lever.
- 100 kg on the ball adds about 136 kg to the rear axle and takes 36 kg off the front.
- While towing, rear bearing life runs roughly 35 percent shorter at that tongue weight.
- But the cost scales with the fraction of miles towed, so occasional towing barely registers.
Towing and wheel bearing life have a clear relationship, and a more reassuring one than the raw figures suggest.
The load increase is real and larger than people expect. The lifetime cost is smaller than feared, because of how few miles most vehicles cover hitched up.
Where tongue weight actually goes
The instinct is that tongue weight simply sits on the rear axle. It does not, because the ball is not above the axle.
Weight on the ball pushes the back of the vehicle down and pivots the body about the rear axle, which lifts the front.
The rear axle therefore gains more than the tongue weight itself, and the front loses some.
| Tongue weight | Rear axle gains | Front axle loses |
|---|---|---|
| 50 kg | About 68 kg | About 18 kg |
| 75 kg | About 102 kg | About 27 kg |
| 100 kg | About 136 kg | About 36 kg |
| 150 kg | About 205 kg | About 55 kg |
So the rear bearings take roughly a third more than the figure on the ball. The lever is doing that.
What that costs while towing
Bearing life falls with the cube of load, which magnifies a modest load increase.
| Tongue weight | Rear corner load | Life while towing |
|---|---|---|
| 50 kg | 1.08 times | About 20 percent shorter |
| 75 kg | 1.11 times | About 28 percent shorter |
| 100 kg | 1.15 times | About 35 percent shorter |
| 150 kg | 1.23 times | About 46 percent shorter |
Why life is cubic in load rather than proportional is on the lifespan page.
Towing and wheel bearing life: the number that matters
Those figures apply only while the trailer is attached. Most vehicles spend a small share of their miles hitched up.
| Share of miles towed | Overall life |
|---|---|
| 5 percent | About 97 percent |
| 15 percent | About 93 percent |
| 30 percent | About 86 percent |
| 60 percent | About 76 percent |
Towing a caravan twice a year is a rounding error against bearing life. Towing daily for a living is not.
The right question is not whether you tow. It is what proportion of your mileage is towed.
What matters more than the weight
Three things affect bearings more than the tongue figure does, and all are within your control.
Tongue weight distribution. Too little makes the trailer unstable, too much overloads the rear. Around 10 to 15 percent of trailer weight is the usual target.
Exceeding the rear axle rating. The cubic law is brutal past the design load, and that is covered on the vehicle weight page.
Sustained heat. Long climbs and descents under load put real heat into hubs, and heat drives grease out.
It uses spring bars to transfer some of the tongue load forward onto the front axle rather than concentrating it at the rear.
That restores front axle load and reduces the rear increase, which helps steering and braking as well as bearings.
The trailer’s own bearings
Worth stating plainly, because it is the more likely failure by a wide margin.
Trailer bearings carry the rest of the trailer’s weight, they are usually adjustable tapered sets, and they are far more likely to be neglected than the tow vehicle’s.
They also sit unused for months between trips, which brings its own problems. That is on the sitting page.
Their service routine is on the trailer section.
What to actually do
- Stay inside the rear axle rating, not just the towing capacity. They are different numbers.
- Set tongue weight properly, around 10 to 15 percent of the loaded trailer.
- Consider a weight distributing hitch on anything substantial.
- Feel the hubs at stops on long trips, comparing corners rather than judging absolute heat.
- Service the trailer bearings, which need it far more than the car’s do.
How to read hub temperature properly is on the hot hub page.
Common questions
How do towing and wheel bearing life connect?
Yes, but less than expected overall. Rear bearing life runs about 35 percent shorter while towing at 100 kg tongue weight, and the total cost scales with the share of miles towed.
Why does the rear axle gain more than the tongue weight?
Because the ball sits behind the rear axle and acts as a lever. 100 kg on the ball adds about 136 kg to the rear and removes about 36 kg from the front.
How much does occasional towing cost me?
Very little. Towing 5 percent of your miles leaves roughly 97 percent of the bearing life of a vehicle that never tows.
What if I tow constantly?
Then it matters. Towing 60 percent of your miles at that tongue weight leaves around 76 percent of the untowed life.
What matters more than the weight itself?
Staying inside the rear axle rating, setting tongue weight to 10 to 15 percent, and managing heat on long climbs and descents.
Does a weight distributing hitch help?
Yes. It transfers some tongue load forward onto the front axle instead of concentrating it at the rear.
Towing and wheel bearing life: which bearings matter?
The trailer’s. They carry the trailer’s weight, are usually adjustable tapered sets, and are far more often neglected.