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Answer to cement truck puzzler is bogus

Re. the cement truck puzzler of 6-16 wherein the boys said you could tell the truck was empty because the drum wasn’t turning - not true, unless the driver was careless. I drive a mixer truck, and you always have the drum turning when the truck is in motion. The reason for this - aside from the obvious one of having leftover concrete and/or washout water in the drum is this:

The rear end of the drum has a steel track aroung its circumference. Under this track are two rollers which support the rear of the drum. There is nothing holding the rear of the drum down but gravity.

Under some conditions, especially when empty, it is possible to bounce the drum on the rollers, thus creating a flat spot on the track or rollers, which you don’t want. Rollers are expensive and a pain to replace; the drum even more so.

Therefore, empty or loaded, clean or dirty, the drum ALWAYS turns when the truck is in motion.

Wow, then I have observed many careless cement truck drivers in Las Vegas (which is under construction everywhere and affords innumerable opportunities to observe them in their natural habitat). Seriously, around here I see cement trucks on the streets with drums not turning all the time. I always assume they are on the way to pick up a load.

Of course, Las Vegas is not known for attracting the responsible personality type.


I frequently observe cement trucks on the road with stationary drums. Maybe the drivers are being irresponsible, but the fact remains that I see this several times a week.

I’ve nevr personally noticed, but your explanation makes perfect sense. Especially considering the weight of a cement drum…even empty!

I suspect that there are probably a lot of cement truck drivers that are unaware of this. I doubt that its a question on any exam.

why would the drum turning have any affect on the weight of the drum holding it onto the rollers? And I find it hard to believe that the manufacurers of cement trucks rely on weight holding the drum in place. are you saying that the turning drum keeps the rear of the drum from bouncing off the rollers???

Yes, rocking motion is far more wearing than rotating motion on ball bearings. That’s why the first roller or ball bearing to go on a motorcycle are the ones in the steering head. That’s also why U-joint needle bearings wear out so frequently. Oil well pump jacks wear out the walking beam pivot bearings before the other bearings wear out.
Even if some people don’t do it, keeping the drum turning is a good idea when traveling down the road.

Back to the puzzler: perhaps an empty truck may be observed with a turning drum, but you should never see a loaded truck with a stationary drum.

I’m saying that the turning keeps the drum from bouncing repeatedly in the same spot on the rollers/track, thus creating a flat spot. Obviously the weight doesn’t change.

And I find it hard to believe that the manufacurers of cement trucks rely on weight holding the drum in place

Consider a railroad car - they rely on gravity to keep the running gear on the wheels.

Scepticus is correct. Whether running an empty drum on a cement truck is advisable or not, if one was to see a cement truck with a stationary drum, that should indicate that the truck is not carrying a load of cement. On that basis, the Puzzler answer is not bogus.

Yup, even on mountainbikes (the pedal kinds) detents can develop in the “headset” bearings, those thrust bearings on which ride the steering fork. It’s common from the pounding taken on the trails. Mine was due a new set due to a detent having developed just about the time I exited the cylcling world.

Folks should realize that when two circular surfaces are in contact, only a very tiny portion of the actual surfaces is in contact. With a drum weighing thousands of pounds, one or both of those surfaces could easily be deformed with pounding.

Not on any exam, perhaps, but the subject is nicely covered in “The Truck Mixer Driver’s Manual”, c. 1999 by The National Ready Mixed Concrete Association.

If a driver was careless enough to not keep an empty drum turning, he’s do the same with a full one. You should never see a mixer truck on the road with a stationary drum. Period.

If a driver was careless enough to not keep an empty drum turning, he’s do the same with a full one.

That is quite an assumption. I have been more successful with logic than assumptions when it comes to puzzlers.

Excellent point.

We had some guys here doing Stucco, (plastering walls) and they managed to set two gas powered mixers by letting them stationaly while having lunch. Even managed to shear the shaft on one on “attempting” to restart it… idiots. So what exempts truck drivers from a similar fate.

We did once have a driver call in and offer us a load of free concrete if we would get it out of the tub, on which the motor had failed. Not needing concrete that particular day, he was forced to call the Fire Dept to hose out his tub. He removed a plate on the side of the tub and they did the rest.