I get the question all the time when people chat with me or call in:
It's a great question and it's why I have this chart on our homepage slider:
Sure, it's really brief and probably oversimplified because it doesn't breakdown by the hardness of the type of wood, but it's a starting point, but there is a reason for that. The hardness of wood has nothing to with it being considered "hardwood" or "softwood" based on the toughness of the wood.
Wait, what? Let me explain. Trees are defined as hardwood or softwood not based on the actual hardness of the wood, but by the seeds of the tree either having a shell or vessel (eg. fruit) when they fall or they just fall to the ground with seeds exposed to the elements. If it has a vessel, it's considered a hardwood. If not, softwood.
The actual toughness of a tree's wood has to been defined by the Janka Hardness Test since 1906. It was created by Gabriel Janka who was asked by the Department of Agriculture to find an “objectively and scientifically” sound way to measure the hardness of a piece of wood.
The test involves measuring the average amount of force required to “embed a .444-inch steel ball to half its diameter” with a ram in some type of wood.
Then an average is used so that the difference in hardness between heartwood and “live edge” wood keeps things comparable on average. The test also distinguishes “side hardness” and “end hardness." So whether the test was done on the cut surface (eg. the surface of a log or stump), which is called the“end hardness," or the surface of a plank which is the "side hardness," you can differentiate. When we split logs we want to consider the end hardness.
My favorite part of figuring this out is that most people find they were overestimating their need with regard to the amount of force they need because they think, "I'm splitting oak and it's a hardwood so I'm going to need a lot of force." In reality, white oak isn't really all that much harder than pine. It's only 400-500 lbs of force difference in the test. Do note that this is typically an average of side and end hardness as well.
If we go back to the chart in the image above, you can see why it's not necessary to worry too much about the type of wood or it's relative hardness. It's much more important to consider the size of your logs and how green or seasoned they may be.
I'll be the first to tell you that getting the most force possible is probably just a waste of your money, but there are situations where it may be advisable to consider one size up.
Let's say you just felled a few oak trees and the smallest diameter is 2 feet. You want to split them as soon as possible and let them season as split wood. In this case, you'll want to get at least a 30-ton splitter and likely need to push up to a 35 ton or high splitter.
Now if you let that wood season as felled logs or as segments you could probably get away with a 25 or 28-ton splitter. If we changed the type of wood, it doesn't make a huge difference in tonnage needed. Consider our example above being only 400-500 lbs difference, that's just 1/4 of a ton.
Of course, there may be other things to consider as part of your equation like how knotty is the wood or did it rain on them for the past week, but in general, the guidelines in the chart are most of what you need.
Long story short, focus on the diameter of the logs and how seasoned the wood is, if at all, in your choice of tonnage for your splitter. If you want to bump-up one size (eg. 20 to a 25-ton), that may be helpful if your logs are from a tree with wood that's a higher toughness on the Jenka scale.