Centramatic Wheel Balancers

After the Overlander was refurbished for active camping, we took several week-end trips to gain familiarity with both the best way to camp in the Airstream, and how to safely get to & from the campground. I don’t believe I have ever had so many learning curves converge at one time.

In time though, we became comfortable with everything involved, and my attention shifted to some of the finer details associated with RVing. The first issue, since our next destination would require two days of travel, was to address vibration inside the Overlander while under tow. It was nothing bad like popped rivet heads littering the floor, but rather the annoyance of having to pick things up off the floor that should have been able to ride comfortably atop the center twin beds.

Out-of-balance tires are usually the culprit in situations like this, but these were new, balanced tires. The next potential culprit was the brake drums. I had every reason to suspect them because their design, as it included a raft of piece-parts, virtually screamed for the assembled drum to be balanced after assembly. But trailer hubs are not routinely balanced; Even on an Airstream.

Apparently, in days of old, many tire shops had the ability to balance the wheel/hub assembly on the vehicle. I’ve never seen the equipment involved, and the only people I could find who have seen it are older than me. The technique looks like it would have been effective, though.

Then I caught a thread on airforums which discussed Centramatic wheel balancers. After a bit of reading, this product appeared to be the ticket to alleviating my Overlander’s vibration issue. So I ordered a set of four. As seen in the image above, the potential problem solver fit perfectly on the brake hub. But, as seen in the right-hand image, the product did not fit flat on the wheel. To tighten the lug bolts would have damaged the balancer. Unfortunately, Centramatic had neither problem insight nor another stock number of their product for me to try.

While discussing my lack of joy with others on airforums, a clever man from MN suggested the fit problem might be because my wheels were made with an offset i.e. the wheel face which bolts to the hub might be closer to one side of the wheel than the other. Offset trailer wheels are apparently now a thing of the past as apparently all trailer wheels manufactured nowadays have zero offset. Since everyone with later model Airstreams had no problem mounting Centramatics, it appeared that I would need to replace my wheels if I too wanted to enjoy the benefits of wheel balancers.

Between the cost of new wheels, and the fact that part of the vibration might be attributable to the original axles, I opted to send the balancers back to Centramatic.

A couple of years later, my Airstream received new axles complete with new brakes & one-piece brake drums. The improvement, with a fully loaded trailer, was remarkable. Within reason, everything stayed where it had been put. While no specific testing was done, I think the bulk of the improvement was a result of better hub manufacturing techniques.

On a side note, I once helped a buddy recover an Airstream identical to mine except his was made in California whereas mine was made in Ohio. Part of the effort included swapping out wheels, and I noticed his Overlander’s wheels were difficult to get out of the wheel wells. My conclusion was that wheel change-out on my Overlander might be easier because of the wheels’ offset.

Now skip forward to the present. The Overlander’s bias ply tires are now seven years old, and I would feel better replacing them with radials before our next long distance trip in hot weather. In checking the Internet for the best tire brand & proper size, I noticed an Airstream dealer in California was selling Centramatic wheel balancers he claimed were specially made for Airstreams. Since my vibration issues virtually disappeared with the new axles & hubs, I was not necessarily interested in purchasing his product. But I was interested in why this second-party offering would work on my Airstream when the Manufacturer itself was unable to help me.

In subsequent emails with the dealer in California, I found him unusually evasive in identifying just what was so “Airstream special” about what he was selling. His defense was “just return them if they don’t fit” to which I would parry with “I’ve already returned a set, and will not order anything unless I’m 99.9% sure it will fit”. At one point, he said that as long as the in-side of the wheel had at least nine inches of space, his product would fit.

A week or two later, with new tires slated for delivery in a day or two, it was time to remove the Airstream’s wheels for a trip to the tire shop. My Airstream's wheels appear to have more than enough flat surface:

The time was also right to check for wheel offset:

Subtracting the two measurements, and rounding, I would say my Overlander’s wheels are offset one-half inch.

A mockup of the balancer Airstream dealer is selling was made based on his website photo:

I believe his product will work with my offset wheels:

And with my “new” Henschen axles:

Then it finally hit me. The Dealer’s product will fit my application because it is smaller than what Centramatic sells for 15-inch wheels: The reduced diameter, though, will affect the balancers’ ability to correct out-of-balance conditions. Perhaps this is why the Dealer was so evasive about answering specific questions. Now that the “fit” question had been answered, “function” was now in question. Is the product too small to be useful? With what gauge should the product be measured?

In response to “Should the tires/wheels be pre-balanced?”, the Centramatic web site’s frequently asked questions section stated, “In most cases, pre-balancing is not required.” So I decided a reasonable goal was to analytically determine if the Airstream dealer’s product would be capable of balancing, my Airstream’s wheel with the most balancing weight attached to it if the weight was not there.

To aid the effort, a still from a training video from Centramatic, a photo from the Dealer’s web site, and photos of the original Centramatics I bought & returned several years ago were analyzed. While I know my analysis would have more validity with either design prints or sacrificial wheel balancers, the former was not available, and the entire topic is not important enough to purchase & butcher perfectly good balancers just to reverse engineer performance. Please accept that errors may have occurred. This is nothing but a paper exercise.

The worst-case wheel had 4 ounces (113 gram) of lead on the front side, AND 3.75 ounces (106 g) of weight on the back side (at the same location) for a total of 219 grams balancing weight mounted eight inches away from the hub’s centerline.

The Dealer’s product, with its reduced size would need to position 350.4 grams of weight at the same arc to provide the same balancing result.

While I got a kick out of this entire self-imposed task, I know most people could care less about the technique & actual number crunching. I will spare you except for one image:

My analysis concluded the Dealer’s product can only accommodate 214.5g of imbalance. This is only 61.2% of the goal.

It was only fair at this point to analyze what Centramatic sells for 15 inch wheels. As the diameter of their product is 2-1/2 inches larger than the Dealer’s product, a more favorable result was expected.

Centramatic needed to provide 292 g of balancing weight on its six-inch radius to provide the same end goal. My analysis yielded only 271 grams. While better, it would only be 92.8% effective on the same wheel.

In conclusion, either balancer should be capable of correcting for latent imbalance in the cast-iron hub. But it appears to be prudent to balance the wheel/tire combination before depending on either product to make your Airstream’s ride smoother.

Oh, and the perfect balancer would have measured ~13 inches in diameter.

The new tires (Goodyear Marathon radials) were mounted today without wheel balancers. I may, one day, mount wheel balancers but there is certainly no rush.