Does this lead make my car look fat?
When looking at the performance of a vehicle, weight is one of the most important factors in the equation. Heavier vehicles require more energy to accelerate and are harder to stop. They are also more difficult to control through the turns. All in all, what makes a vehicle heavier usually has a ton of drawbacks in both performance and efficiency. You want your racing car to be as light as possible.
Every now and then, however, automakers have found reasons to intentionally put vehicles on high weights. We’re going to look at some key examples and discuss why this weird design decision sometimes is exactly what the engineers ordered.
Anyone who’s taken a course on vibration at university will know that adding mass tends to change things for the better when it comes to vibration. More mass reduces the natural frequency of a system. If the natural frequency of a chassis or subsystem in a vehicle falls below that of stimuli such as the road, the drive train vibrations or the engine, the vibrations that can be felt throughout the vehicle can be greatly reduced.
A well-known example is the Porsche 912E, which replaced a rattling Volkswagen four-cylinder with a smooth-running six-cylinder. In developing the vehicle for this engine, Porsche engineers decided to add a whopping 12 pound weight under the transmission cross member. The weight is listed as a “vibration damper” and will likely change the natural frequency of the rear subframe to be less than that of the vibrations generated by the engine. Adding twelve pounds of curb weight to a cheap, underperforming “sports car” has little noticeable adverse effects, but it can improve the feeling of refinement with less vibration from the engine to the cab.
Porsche isn’t the only one who pulled such a trick. Toyota used a bolt-on weight on certain Hilux models to reduce vibration in the drivetrain, which was mounted on the transfer case to dampen excessive vibration. Weighing about six pounds, it was just enough to make a meaningful difference. However, the additional weight would hardly be noticed in a vehicle that weighs more than 3480 lbs.
Volkswagen installed a similar item, around 25 pounds, in the rear of Golf convertible models of the mid-1990s. The “rear added mass” was apparently included to reduce vibration.
This technique is used to this day, albeit often in less obvious ways. Engineers work hard to reduce noise, vibration and harshness on new models known in the industry as “NVH”. Pinning a large chunk of metal a la the 912E is a quick way to solve a vibration problem in an existing model. However, when you design a car from scratch, you can be neater at your job. Frames and cross members can be designed with thicker cross sections, or additional material can be added to things like motor mounts or mounts to change the image more subtle. However, when a vehicle is about to go into production and there is no time to change the shapes for other parts, sometimes a bit of screwed-on weight is added to dampen those nasty little vibrations.
However, automakers sometimes go further. Dodge has installed active mass dampers in its RAM trucks that shake weights out of phase with the vibrations generated by the engine. The shaking caused by these weights destructively disturbs the shaking of the engine and, if properly coordinated, can greatly reduce the vibrations in the entire chassis of the vehicle.
Mass dampers were also used in motorsport, with the Renault Formula 1 team pioneering the technology in 2005 before it was banned by the sports association. In this case the damper consisted of a 19 pound disc that could move in a vertical plane, held in place by springs and dampened by oil. The disk would help absorb the car’s energy hitting bumps, much like tuned mass dampers would help reduce vibration in skyscrapers and was considered a significant competitive advantage for the team until it was excluded from competition.
Of course, we must remember that one of the most common instances of adding weight to a vehicle is when balancing the wheels. Tires and wheels do not always have a perfectly homogeneous construction and can have heavy spots. Spin them up to speed and you will experience all kinds of terrible bumps and vibrations all over the car. However, with the help of a balancing machine, some weights can be glued on to balance all of this.
However, sometimes large weights are added to a vehicle to improve balance and grip. A great example is the 1957 Willys FC-150 truck, which carries a whopping 265 pound weight over the rear wheels. In this case, by design, the truck has the engine and the passenger cabin well forward on the chassis, which means that all of the weight is directed very much forward. This has the rather negative effect of causing the vehicle to want to raise the rear wheels when braking hard, and generally makes the vehicle unbalanced and difficult to control. The solution was obvious: put a large weight on the stern to keep the wheels on the ground.
This is of course a very simple solution. In the 1950s, little things like fuel consumption and efficiency just weren’t in the foreground. So it was easy to put a weight on it and deal with it. Nowadays trucks are designed with a little more finesse and are usually very heavy all around so that such obvious measures are not necessary. Regardless, it is still important that the weight distribution of a vehicle is appropriate, whether laden or unladen, in order to ensure good drivability.
In fact, a common solution for pickup trucks to increase traction in winter is simply to add a dummy load to the truck bed. A few hundred pounds of sand or cobblestones can help reduce fishing or rear wheel spin in the snow. This comes at the expense of fuel economy, but if it keeps the truck out of a snowdrift, you save a ton of towing fees to make up for it.
Similar measures are often used on tractors to help them pull implements and keep traction on muddy terrain. These can be attached to frames on the front or back of the tractor, or even directly to the wheels themselves to increase the force that pushes the tires into the ground for more traction. These can weigh thousands of pounds on the high end.
It’s not all bad
As it turns out, gaining weight isn’t always bad. If a little extra weight helps reduce annoying vibrations or solves some tricky handling problems, it’s usually worth the trade off. If you compete in motorsport and need the ultimate performance, it makes sense to forego that last bit of weight. For daily use on the street and elsewhere, however, a little extra often helps to solve some horticultural problems!