What's the best way to balance big tires? I've heard all kinds of different things, but I was wondering if you had any practical experience with balancing tires above 36 inches in diameter.
Like you, we have seen and heard about a wide variety of big tire balancing methods over the years. And we've tried most of them. Conventional stick-on weights just aren't very effective with tires above 35 inches in diameter because the distance between the location of the weights and the tread face largely negates the effectiveness of the weights. This means turning to a number of alternative methods of balancing, some of which can be effective and some of which seem to be largely mythical in their effectiveness.
Truth be told, tire balance is getting to be less of an issue thanks to modern technology and tire manufacturing methods. For example, the author has been running 37-inch to 40-inch tires without any weights on them for over a decade and really hasn't noticed much more than a slight hop with a tire or two. Granted, most of these haven't been on daily drivers, and therefore tire balance is less critical than it otherwise would be, but modern tire manufacturing methods have yielded remarkably smooth tires in large sizes. It has been our experience that if you stick to well-known tire brands and wheels that start life round, tire balance really won't much of an issue.
With that said, a few different balancing methods are effective. The most common involves tossing a balancing media in the tire when mounting it on the wheel. There are a variety of products that work via this method that are geared mostly to over-the-road trucks and 18-wheelers, but we've also heard of people using everything from steel shot to Airsoft pellets. Regardless of the media used, they all work off the same principle: The centrifugal force of the rotating tire causes the media to move to the opposite end of the tire from the 'low' spot, counteracting the portion that is out of balance. That's a bit of an oversimplification, but it works for the discussion at hand. While effective, there are a couple of issues with this method. First, it's something of a guessing game figuring out just how much media to put inside the tire to correct its balance issues. Too little and there won't be enough material to balance the tire, while too much can create other types of balancing issues. The other and more common complaint is that the balancing media settles when the tire stops rotating, so you're going to have a bit of a "hop" until the tire gets rolling fast enough to distribute the media and balance the tire. This can be annoying, especially in stop-and-go driving conditions.
The main issue with both commercial and homegrown balancing media is that the results are often spotty, and they can make a real mess of things should you blow a tire or need to replace worn tires down the road. The media needs to be inert to rubber and its related chemicals, nonabrasive, doesn't clump when it's exposed to water, and can't degrade over time. Because of all of these things, results using this type of balancing method seem to be spotty and largely subject to interpretation.
A long time ago one of our editors tried a set of Centramatic wheel balancers () and reported good success with them. These work on the same principle of centrifugal force described above, but instead of putting balancing media inside a tire it is placed inside a disc sandwiched between the wheel and the hub. The disc had an outer ring filled with some sort of weighted media that distributes itself as the tire rotates. This is much easier to install than other forms of balancing media and much less messy, but we don't have any firsthand experience with them for anything other than over-the-road big truck applications for quite a few years.
What does all of this mean at the end of the day? It means there's no perfect solution, or even a solution that works best regardless of tire type or even the region the tires are being used in. There's also a bit of buyer-beware here: A big tire can weigh well over 100 pounds not counting the wheel it's mounted on. With that kind of rotating mass, even small imperfections in the tire are going to be difficult to balance with conventional weights, and large imperfections are going to be nearly impossible to balance perfectly regardless of the method tried. If tire balance is a major concern for you, then it might be best to rethink those no-name 40s you've had your eyes on and stick to some brand name 37s. Either that or prepare to live with some tire shake.