Weight is something we all deal with in racing. We have to speed weight up. We have to slow weight down. We have to make minimum weight at the end of a race, and we have to try to eat fewer cheeseburgers and to lose some weight. How we each handle the weight we are dealt can make the difference between winning and losing. In this story, we will focus on strategies for weighing cars, leveling a garage floor for accurate static weight measurements, and corner weighing/cross weights.
The first step in weighing a race car to determine corner weights is by doing it on a level surface. This isn’t always available in a race track paddock, but taking the time to find a good level place to set up your scales will help you in being more accurate with your adjustments. A big space to work and good lighting always helps with any car project.
To corner-weight our car, we purchased the Proform 7,000-pound Slim Wireless Vehicle Weighing System. We chose the Proform scales because they were reasonably priced — around $800 for the entire set —were wireless, used over-the-counter batteries, and came with their own carrying case. The system turns on and off with ease and the wireless system finds each of the four scales perfectly every time we use the system. We take these scales in their handy wheeled case to each event and use them all the time to check overall weight (to ensure we don’t go below the minimum) and to weigh our gas jugs before and after each fill up (for fuel consumption calculations).
Once we had our Proform scales in place on the shop floor, based on the track width and wheelbase of our Honda Challenge car, it was time to see how level our shop actually was, or wasn’t. We did this with two equal-size sockets, a piece of aluminum square tubing and a level. Once we saw that our floor was in fact, not level at all, we set out to make some adjustments. It was much cheaper to use some thin floor tiles under some of our scales than it was to jackhammer out the shop floor and re-concrete the entire shop to a perfectly level float.
The thin Proform scales are angled at each end and are designed so you can drive directly onto them. We found that driving the car onto the scales often caused the scales to move a bit and made rolling the car off and on to unsettle and unbind the suspension difficult. We built some plywood roll-on/roll-off ramps and made them the same thickness of the Proform scale pads. This allowed us to easily roll the car off the scales, zero the scales, unbind the suspension, and then roll the car back onto the scales for accurate measurements.
When doing corner weighting or alignments it is important to have the car in the same mode it would be in when on track competing, which means it needs a driver in the driver seat, the correct amount of fuel in the car, splitters attached, cool suits filled, etc. We work hard to ensure the car is exactly as it would be if it were on track. If we don’t have a driver to sit in the seat then we fill the seat with ballast at the exact same weight as the driver (fully dressed, helmet, etc.).
Jacking a car up or making adjustments to the suspension can add bind to a suspension. There is quite a bit of friction in suspension bushings, especially rubber or urethane, and also friction between the tire interface with the ground. This can affect the wheel weights and thus your overall weight distribution readings on your scales. This is why rolling the car back and forth as well as bouncing the suspension at each corner is important: try to eliminate the bind to provide more accurate weight distribution measurements.
Once we had a level garage floor, the scales set up, the car set up with the correct parts/weight added, and the suspension unbound, it was time to get some data. We always roll our car off the scales, zero the scales, and then roll the car back onto the scales carefully ensuring our tires are perfectly on the center of each scale. Then it is time to look at the numbers. We use the wireless scale head from Proform to see where our car is at in terms of weight and cross weight percentages.
Once we had weight data, it was time to determine what we wanted to do with it. First thing is to start to document the weight at each wheel position. This can be done with simple pen and paper, white boards or by using a database. I’m sort of an Excel nerd, so I love using a database to track changes. Even though the Proform scale head calculates cross weight for us, I created my own spreadsheet to calculate all sorts of weight distributions.
Once we have data from the scales and we examine our weight distribution, we need to decide how we want to rectify the distribution of weight. We have options: add or remove ballast, move ballast, or adjust ride height to get cross weight closer to 50 percent. Depending on your racing class rules, you are limited to where and how much ballast you can install. But looking at the wheel weights and cross weights can help make strategic decisions in a car’s build. For example, our Honda Challenge cars are light on the right rear wheel. Moving our cool suit box closer to the right rear wheel position can assist us in getting more even weight distribution from left to right and cross weight.
In a perfect world, or in a physics problem, we would have 50/50 weight distribution laterally and longitudinally. That isn’t reality for most racers competing in sedans designed to get a family of four safely to Disneyland. In a front-wheel-drive car, we will generally always be front biased in weight. Our Honda Challenge car has 58.7 percent of the weight on the front tires. That isn’t always a bad thing, namely under acceleration. Where too much weight on one particular tire isn’t good is at corner entry under braking. Even though we can’t get the car to a 50/50 weight distribution, what we can change is the cross weight. Cross weight is the difference between the weight on the right front tire combined with the weight on the left rear tire versus the weight on the left front tire combined with the weight on the right rear tire. Essentially, it is like drawing an X through the roof of the car and comparing weights along the lines of the X. If cross weight is closer to 50 percent, that will assist with handling during left and right turns, which is exactly what road racers need to be fast at each portion of a winding road course.
If you have a coilover suspension system, then you have the ability to adjust your ride height. By making small adjustments to the ride height at different corners individually — by raising or lowering the spring perch — you can load a particular corner of the vehicle, essentially asking that corner to carry more of the vehicle’s weight. Through these adjustments, we can try to get a vehicle closer to a 50/50 cross-weight distribution.
Using the Excel spreadsheet I created helps give us a direction to go when adjusting the ride height to get us closer to the magical 50 percent cross weight destination. The spreadsheet calculates what the wheel weights need to be — the target corner weight — to get me to the goal. Once I see I need to add weight to a particular corner of the car, then that is the corner I will go to and raise the ride height. I have found success in raising two opposite corners at the same time, for instance the left front and the right rear. By adding ride height to those corners together, I add weight to those corners and thus they get more of the cross weight, helping my percentage get closer to 50 percent. I keep track of every adjustment using a small millimeter ruler against the shock. I measure the lengths of the thread as I raise or lower the spring perch.
Corner weighting a car is a time-consuming process. Getting the cross weight to exactly 50 percent may not even be possible depending on the design and layout of the car — or how many cheeseburgers the driver enjoys. You will spend hours adjusting ride height, lowering the car back down, rolling off and on the scales, punching the numbers into the spreadsheet, and fine-tuning adjustments. It isn’t a super fun Friday night by any means, but it can make the difference between first place or second place.
Rob Krider is a NASA National Champion and author of the novel “Cadet Blues.” To read more, or to contact him, go to www.robkrider.com.