Keep in mind that these forces are shown as a percentage of the force on the piston. But the force on the piston due to combustion is not constant through the power stroke. Thus in reality the side-wall load and crank normal force are further modified by the changing combustion force. This graph details only the "geometric" effects of rod ratio on wall load and crank load. Another way of thinking of it is that these graphs assume that the combustion pressure is constant through the power stroke, sort of like MEP (mean effective pressure).

Regarding the side-wall load, we see that it is largest when the crank is half way through the power stroke (crank at 90° past TDC). We also see that as the rod becomes infinitely long that the wall loading goes to zero, as we would expect. This is true regardless of how the combustion force changes. A longer rod (stroke being equal) will always reduce side-wall loading, which is one reason why long rods are desirable. Longer rods also reduce peak piston acceleration at TDC and cause the piston to "dwell" longer at TDC which can work with combustion pressure to produce more work on the crank.

Now consider the normal force on the crank. For an infinitely long rod this force peaks at 90° past TDC on the power stroke. Also, for an infinitely long rod the maximum normal force on the crank is equal to the force on the piston (which we have assumed to be constant here). As the rod is made shorter the maximum normal force on the crank occurs sooner in the stroke, and it also becomes larger than the average force on the piston due to mechanical advantage. Sounds great right? But remember that the price is increased side-wall loading. Furthermore, the peak normal force on the crank is not the only important parameter in determining engine torque. We also want to maximize the area under the force vs. rotation curve.

Even though the exact variation of the combustion force through the power stroke may not be known, a few facts can be stated. Most performance camshafts will have the exhaust valve opening well before BDC. The combustion gases are "exhausted" while there is still some pressure in the cylinder so that this pressure can help start the gases flowing along the exhaust tract. This is referred to as "blow-down". Thus, on a typical performance engine, the last 70° or so of the power stroke does not directly contribute to torque production (which may surprise those not aware of it). For this reason alone it makes sense to bias the peak normal force on the crank more towards TDC, and this happens with a shorter rod.

Also, with ignition occurring well before TDC, the maximum combustion pressure is going to occur not long after TDC (never before hopefully ). Since the most torque will be produced when the crank normal force and the combustion force curves have the same shape, and since the combustion force curve is biased to the TDC side, then it again makes sense to bias the crank normal force curve towards TDC. This is a second reason that a shorter rod might make sense. Grumpy Jenkins (V8 drag race engine builder) was a fan of shorter rods for this reason.