Your Ass is Mass


I know you have written many times, especially in some of the write-ups on the One Lap Valiant, about how important cutting out weight is. Can you expand on this a bit more? I would think that adding more power could give the same performance improvement, no?

Sure, weight-to-HP ratio is important. In the muscle era, a number around 8 was as good as it got for production cars. Looking, however, simply at that number, while it is surely a guide as to whether you are speaking about a performance car or a grocery getter, leaves a slew of important factors out of the picture.

Say, for example, you swap your 318 for a worked 528 stroker, so you double the power and torque numbers. Most likely you have added mass over the front wheels, so traction goes away, so now you need much larger rear tires.

The speeds that can rapidly be attained go way up, so you need larger brakes. You need to beef the rear suspension to handle the torque, and the trans and rear axle has to be swapped for larger units.

All this adds mass, negating some of the HP gains.

Let’s look at it from the opposite per- spective. The 318 car made, say, 200 HP, and weighs in at 3,000 pounds, for a weight / HP factor of 15. You swap to body panels to fiberglass, and lighten the engine with aluminum components, add lightweight seats, light wheels, small battery, etc. Say you have reduced to weight to 2600 pounds. Now the number is 13, which would be “peppy.” You soon discover that the car handles way better, and the brakes, which before were only so-so, now seem excellent. Since the engine doesn’t have to work as hard (pulling around 13.5% less mass), a smaller radiator is now just fine, and maybe you can use smaller brakes, surely an A727 to A904 swap, etc. Now the car might be down to 2400. Maybe the body roll is now mini- mized, so the swaybar(s) can go. Now you are thinking about a carbon fiber propshaft, fiberglass rear leafs, etc.

You can see where this is going: Light- weighting brings many benefits: Better fuel economy, everything lasts longer, etc. The engineers call this cumulative effect “mass decompounding.” Dick Landy was said to have a rule that his guys had to bring him an ash tray containing one pound per day of mass trimmed, shaved, or ground off the drag cars of the day. True? I simply don’t know.

The other facet, often overlooked, is reducing the polar moment of inertia. The entails keeping the mass as close as possible to the center of gravity—in other words, towards the center of the car, low. This really helps the car respond to transients—think: Twisty road, auto-X, etc.

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