In my last post, I discussed briefly and rather simplistically how the unsprung mass affects handling and ride comfort. The other major factor affecting performance is the rotational inertia of the wheel/tire/brake package. Rotational inertia (mass moment of inertia NOT area moment of inertia which also happens to be important in wheel design) is essentially the rotational mass of an object. It is defined by the distribution of the mass to the square of the distance from the center axis of rotation. What does this mean? It means that the more mass you have farther from the center of rotation, the more energy that is going to be required to speed it up and slow it down and the effect is squared!! The simple example I use during my tours to explain this is the ice skater moving their arms in or out while spinning. The closer they bring their arms in, the faster they spin. Their mass didn’t change, but their rotational inertia definitely changed and by a lot.

Now what does this mean in real terms in regards to wheels? Well basically the heavier and larger in diameter the wheel, the more rotational inertia it is going to have and the more energy required to spin that mass up under acceleration and to spin that mass down under braking. For us, this is the advantage of properly engineered forged wheels in that you can maximize acceleration and minimize your stopping distance. The converse is also true. If you were to put 24″ wheels on your Cayenne, it isn’t going to accelerate as quickly and it isn’t going to stop in the same distance as it did before. This isn’t only a performance issue, it can be a safety issue so it helps to understand this and be aware of what you’re putting on your baby that might be carrying your baby. Lower rotational inertia also means you’ll use less gas and at a time like this when we’re all trying to be more environmentally conscious this isn’t a bad thing. Efficiency can be a performance boost or an energy saver. It’s just how you apply it (with your right foot of course!!).

I’ll skip the details of its effects on gyroscopic forces as this is most important on motorcycles. When we start making wheels for motorcycles I’ll come back to this in more detail but basically, the more rotational inertia, the harder it is to steer the wheels. On a motorcycle this translates into the “flickability” of a bike and can be seen when going from heavy cast wheels to light magnesium race wheels. On cars this translates into slightly more steering effort.

Imagine now what happens when you use a carbon fiber rim barrel… Hmmm. You basically minimize the mass at the farthest edge of the wheel and have a tremendous effect on the rotational inertia. Now you know why HRE has teamed up with Dymag to make forged/carbon wheels.

So now that you understand the importance of lightweight wheels and you’ve decided to step up and get a forged wheel to ensure you get this. Does that ensure quality or that it’ll be lightweight? Well if it did, HRE wouldn’t have needed me and I think that’s a good lead-in to my next topic: Design.