Horsepower isn't the only thing that will make your car faster. The weight of your car is of near equal importance. To elaborate, generally every 100 pounds lost will net you .10-second faster in the quarter-mile. There are hundreds of pounds to be spared from our cars to help us meet our performance goals.
Weight borrowed from some places on the car has a better outcome than other places, and if you're going for the best bang for your buck, you should know where to focus. Knowing the difference between rotating weight, sprung weight, and unsprung weight will help you to determine where you want to concentrate your efforts.
All the components between the crankshaft and the tires are part of the rotating-weight family. Any weight you can trim from those spinning parts helps them rev more freely, making them accelerate-or decelerate-faster. The diameter and mass of the part determines its effect on the engine. For example, lowering the weight of a 20-inch wheel will help way more than a 3-inch driveshaft. If you can picture twirling a short lead pipe compared to a long one, the short one will spin and change direction easier if input is applied.
Sprung vs. Unsprung Weight
Let's start with a definition: Sprung weight is any part of the car that is supported by the suspension, and only moves when the suspension is active. That means that unsprung weight is any part that is directly connected to the road without cushioning from the spring.
The reason it's important to differentiate between the two is because removing unsprung mass is a more effective move than shaving sprung mass. There is no rule of thumb, like "for any 1 pound lost for sprung weight, it's like 10 pounds unsprung weight," or any conversion like that. With the exception of rotating mass, less unsprung weight will not make you go any faster in the quarter-mile than if it were sprung weight, but it does have handling benefits. The lighter the wheel and other unsprung components, the easier it is for the tire to follow bumps in the road. On a vehicle with extremely high unsprung weight, the inertia of the wheel and associated assembly can't move fast enough to follow the road, resulting in a jarring, crashing ride. What's more, a heavy wheel/tire combo requires a heavier spring and shock package to control it, upping the ante with even more weight, bigger brakes (still more unsprung weight), in a situation that spirals out of control.
Lowering the unsprung weight and rotating weight yields multiple dividends, with better acceleration, better braking, better ride, better fuel economy, and better handling. Money spent to reduce unsprung weight may be great, but it's a much better payoff than with sprung weight.
Fiberglass Body Panels
They don't make cars like they used to. You see an older car tangle with a little Honda, and the import comes out looking like a crumpled-up garbage can and your old-school musclecar comes away with just a scrape. This is because the sheetmetal on the older cars is so much thicker than newer cars. Though it's a plus for an accident, it's also an easy way to drop a ton of weight.
Year One offers a wide range of fiberglass body parts to replace the stockers. Hoods are the most common to replace, and come with many different mounting and styling options. If you're a traditionalist, you can order a flat hood that uses the factory hinges and latch. On the other hand, you can also get a 9-inch cowl and use hood pins to retain it.
The hood isn't the only place you can replace panels. Fiberglass replacements are available for near any bolt-on panel, including fenders, decklids, bumpers, and doors.
Shown is Year One's fiberglass hood for '67-69 Camaros (PN S800), weighing in at 44 pounds versus the 56-pound stock steel hood. It comes with a sticker price of $449, which turns out to be a pretty good deal.
Price per pound lost: $15.80
Relocation Of Weight
It is not always possible to remove all the heavy items from the car. In these cases, you can move them to a more desirable location. If you must carry the weight in a car that you drag race, you might as well shift it as far back as possible to help weight transfer. The battery is about one percent of your car's total weight, so changing your front-to-rear static weight distribution that much from one modification is quite good.
Relocating the battery is a simple and inexpensive way to shift the weight. You can get kits that come with the box, mounting hardware, and cables to complete your installation. Check out Taylor Products' battery relocation kit, PN 48101. It's made from aluminum, and comes with everything you need to complete the project.
Battery relocation kit:
Taylor Products (PN 48101)
$149.95 from Summit Racing
A lightweight flywheel isn't for everyone, as it does change the way the car drives, but there are big gains to be had, strictly outside of its reduction in mass. In the September 2008 issue of PHR, we got into more detail about steel versus aluminum flywheels. The gist is a steel flywheel stores more energy and keeps things running smoother than an aluminum one because of the high rotational inertia. This is why a lot of companies make lightened steel flywheels to create a middle ground. The corollary argument is that a light flywheel also accelerates quicker in its rotation while providing an overall reduction in vehicle mass.
|Flywheel: ||Part Number: ||Summit Price: |
|Aluminum 15-lb ||560130 ||$466.99 |
|Steel 22-lb ||460122 ||$301.69 |
|Steel 30-lb ||460130 ||$323.39 |
|Steel 40-lb ||460140 ||$338.99 |
Price per pound lost: $5.00
Shown here is a driveshaft from Inland Empire, which we used on our project '68 Chevelle. It was almost 11 pounds under the stock weight of the old steel shaft. Though not as effective as an aluminum flywheel in lowering the rotational mass, the driveshaft is still an easy way to shed some pounds.
Unlike wheels, driveshafts don't have the diameter to have much effect on reducing rotational inertia-that is, reducing the motor's effort to spin up the shaft-so the weight savings is mostly to bring down the overall weight of the car. Looking at the price difference of having a factory replacement driveshaft made compared to a custom aluminum or chrome-moly version, it's really just a matter of how much lighter you want to go.
|Driveshaft: ||Weight: ||Price: |
|Stock, 50-inch ||17 lbs ||$245 |
|Aluminum 50-inch ||10 lbs ||$285 |
|Chrome-moly steel 50-inch ||12.5 lbs ||$275 |
Price per pound lost: $5.71
inland empire driveline