Why Do Some Cement Trucks Have Extra Wheels At The Top?

Cement trucks are some of the most distinctive vehicles you'll see on the road. The most obvious thing about them are their gigantic, perpetually rotating drums (here's the reason cement trucks are always spinning, and what happens if they stop). But in some cases, that's not the only oddity. Some cement trucks are equipped with an extra pair of wheels near the drum's rear spigot. It's understandable to be confused by a pair of wheels that are nowhere near the road — are they supposed to be spares, or do they somehow help with pouring cement? 

The answer is neither of those. Those wheels near the drum spigot are part of what's known as a tag axle, and if they're sitting at the top of the drum like that, it's because the drum isn't currently carrying any cement. If the drum did have cement, those wheels would be unfolded down onto the ground, rolling behind the rest of the truck. The extra wheels go a long way toward keeping this exceptionally heavy vehicle properly balanced on the ground, ensuring it doesn't break down under its own weight.

Cement is extremely heavy. Even with it in a semi-liquid state, the combined weight of cement and truck can easily exceed 60,000 pounds. That's a lot of weight to haul around, which is why you need a special driver's license to operate Class 8 trucks like cement trucks.

No matter how strong a truck is, all of that cement weight sits forcefully on its axles. Not only can that weight gradually wear them out, it can also damage any pavement the truck drives over, violating road and bridge weight limits and potentially endangering other motorists. This is where tag axles come in. By attaching an additional axle and wheels to the rear of the truck, you can take some of the pressure off its rear axles and improve its overall weight distribution. This enhances the truck's longevity and brings it and its load more in line with road weight limits.

Tag axles are typically designed to be raised or lowered as necessary. When a cement truck is carrying cement, they're lowered to the ground to serve their purpose, but when the cement has been dispensed, they can be raised back up to decrease the truck's profile and tighten its turning radius.