Why Should I Corner Weight My Car?
Within many motorsport categories, corner weighting a car before an event is crucial due to the large affect it can have on the lap times and capabilities of the car. It can significantly increase lap times and make the car feel much more predictable for the driver.
Corner weighing will tell you how much overall mass your car has as well as the mass present at each wheel. If you have uneven mass on your wheels then your tyres will all have different levels of grip which can lead to an unpredictable car on circuit and make it corner faster in one direction than the other. Front to rear weight distribution is also important because it allows traction to be increased on the driven wheels by increasing the front or rear mass depending on which wheels transfer the most power to the ground.
In many motorsport categories, a weight distribution of close to 50/50 is aimed for because this gives each tyre the same amount of grip, reducing oversteer and understeer tendencies as much as possible, allowing the car to carry more speed through the corner.
Another important reason to corner weight is to make sure that your car meets mass regulations to compete in the championship and also lets you see if you are overweight. If your car is overweight then you can work to reduce the mass of the car to regulation weight to give you the competitive edge in the championship.
Car Preparation Before Weighing
Before you begin putting the car on the scales, there are a few parts that can make the most out of the weighing process and some steps to take to ensure that the measurements will be accurate.
Some parts to install to your car if you want to have freedom of adjustment to give your car the best mass distribution and use it on track are:
- Adjustable Coilovers
It is important that you are able to finely adjust the ride height at each corner of the car. Therefore, you need to install adjustable ride height coilovers. They usually come in two forms, adjustable body or adjustable spring base. Adjustable body are where the damper screws out of a larger body to increase or decrease the length of the coilovers without changing the length of the spring. The image below shows this type of coilover.
The other type of coilover is where you adjust ride height be moving the spring base up and down. This does still work but it can alter the pre-load of the spring which can cause an unsettled feeling on circuit if the spring lengths are a lot different. If you have this style of coilover then adding a helper spring will help to try and keep the spring rate the same with height adjustments. This style of coilover is shown above.
- Adjustable Drop Links
Installing adjustable drop links is a very important process to achieve a well handling corner weighted car. This is because when the rides heights have been altered for optimum weight distribution, the height at each side is often different. This would normally put the anti-roll bar in tension when the car is sat statically, increasing the static spring rate of the car making the handling more unpredictable in a straight line. Fitting adjustable length drop links means that the strain can be removed from the anti-roll bar by compensating the difference in ride height with the drop link instead of through the roll bar.
With the correct parts installed on the car you can now carry out the following simple steps to ensure that you get the most out of the corner weighing session.
- Fill your fuel tank to the level that you stat the race with – This is free weight as it is present for scrutineering and will reduce out on track, allowing your car to get lighter and go faster.
- Set all tyre pressure to an even setting. Alternatively, set them to how they would be set for a race if they are different front to rear or side to side. Having incorrect tyre pressures can change the corner mass significantly due to an increase in tyre pressure raising that corner of the car.
- Disconnect one drop link at the front and one at the rear. This disables the roll bar and prevents it pulling the opposite side to be the same which affects the corner mass significantly.
- Put the driver in the seat of the car or their equivalent mass in the driver’s seat. This allows an accurate measurement of the corner mass when the driver is in the car which they are always if the car is on track.
- Finally, make sure the scales are placed on a perfectly even surface and that the surface is flat so that the car is not sat on a slight angle which can again affect the results of corner weighing.
How to Weigh and What to Aim For
With the scales sat on a level ground, jack the car up and place each of the 4 wheels on a scale, making sure to put the rear scales under the rear wheels and the front under the front wheels.
With the car on the scales make note of the current mass distribution. It will show individual mass at each wheel but it will more importantly show the percentage difference side to side, front to rear and something called the cross weight percentage. The cross weight percentage is shown on the screen below as “CR” and is the difference between the diagonal mass sums of the car (LR+RF and RR+LF). The percentage figures are the numbers that will be tuned and changed to make the car balanced. There will also be an overall mass figure so you know the total weight of the car.
To achieve a well handling track car, the side to side percentage needs to be as close to 50/50 as possible. Getting a front to rear percentage difference of 50/50 is also favourable. However, this can be changed to increase front or rear grip of the car. The very important percentage for a track car is the cross weight percentage. This has to be as close to 50% as possible so that the car corners the same on left and right hand bends.
The optimum percentage difference front to rear changes depending on whether your car is front, rear or four wheel drive. For a front wheel drive car, having slightly more mass on the front wheels is favourable for grip so a 55/45 front to rear percentage split would be favourable. For a rear wheel drive car, a 45/55 front to rear percentage split would provide more rear traction. However a lot of rear wheel drive racing cars are closer to having a 50/50 split. Finally, for a four wheel drive car, a 50/50 split is favourable to provide even traction at all four driven wheels.
To increase the mass on a particular wheel, the ride height must be made larger. Making the damper longer, raising the body further away from that wheel. People who have never corner weighted a car are often confused by this as it seem counter-intuitive to raise a corner of a car to make the mass on the wheel increase. However, increasing the ride height of a corner increases the moment arm (or leverage) of the body to the wheel, therefore amplifying the mass on the wheel.
On the other hand, if you want to decrease the mass present at one of the wheels, then simply lower the car on that corner and the mass on the wheel will decrease due to the same principle mentioned before.
It can take some time to get the perfect percentage difference side to side as when the mass is changed on one wheel, it affects the mass at all of the other wheels. Often the diagonally opposite wheel to the one being adjusted will be affected in the opposite direction. Therefore, if you increase the ride height on the rear left wheel, to increase the mass on the rear left wheel, then the mass on front right wheel will decrease. This knowledge can help speed up the corner balancing process.
If you are struggling to get a 50/50 percentage difference side to side or are finding it difficult to achieve your target front to rear distribution and are heading into a situation where your ride heights are widely different then stop. Sometimes, it is better to have a slightly uneven set up and a more level car than having 100mm height differences all around the car to achieve a target weight as the different ride heights will also make the car handle poorly.
Also, be aware that when you are moving your ride heights up and down you are also affecting the roll centre location which can increase or decrease the roll moment at the end being adjusted (front or rear). This can lead to creating under or oversteer if the roll centre is being moved. Therefore, try to stay away from very different ride heights front to rear unless the vehicle suspension geometry is being designed to work for the much increased or decreased ride height. Jacking the car too high can also raise the centre of gravity position which will cause the car to roll more excessively.
If you are adjusting ride heights by moving the spring base up and down (such as in the image above) then you will also be altering the pre-load of the coil spring. Similar to above, be careful not to set the spring heights too different as this could cause very noticeable differences in spring rates when out on circuit which will unsettle the car considerably.
If you are unable to achieve the correct mass balance by adjusting the ride heights within reason (within 75mm of damper length of each other) then it might be a better solution to add ballast mass in the vehicle.
Ballast masses are plates that you can bolt into the car at different locations which will make that point heavier and will change the mass distribution of the car. The only downside to this is that the overall mass of the car will increase, which will ultimately reduce the top speed and acceleration slightly.
With all the ride heights set and the target mass distributions achieved the coilovers can now be locked off and the adjustable drop links can be adjusted in length so that the anti-roll bar re-connects with the suspension with ease meaning that no strain is being put in the roll bar when the car is sat stationary.
With the corner weighting now complete, it is likely that the alignment will have been affected considerably. It is highly advisable that the alignment is corrected following a corner weighting session. The next article explains how to align a car track side.