Gerald81
Well-Known Member
Just to share a write-up on corner weighting for any interested trackies:
I believe that while most of us have heard of corner weighting, not many of us actually understand its benefits and what it involves. To make matters worse, most workshops that perform corner weighting services either fail to properly educate customers, or worse, share misleading information. To top it off, the costs of corner weighting are relatively high, ranging from $350 to almost $500. That, coupled with the fact that the benefits of corner weighting can only be fully appreciated on track, is what I believe turns most casual enthusiasts away. However, with more and more drivers participating in track days, and the upcoming Changi Motorsports Hub presenting the prospect of a local track, more enthusiasts may wish to consider corner weighting their cars. After all, to get the most out of engine modifications, proper ECU tuning is required. Likewise, corner weighting is the final step to getting the most out of your suspension setup. Anyway, enough rambling from me, and on to the subject topic.
What is corner weighting?
In order for a car to make full use of its tyres’ available grip on a road course, the full weight of the car should be evenly distributed across all four tyres. Unfortunately this is impossible for most road cars in general, where the choice of drive-train layout and driver sitting position offset significantly affects the weight distribution. For example, placement of the engine and transmission at the front is one of the most common drive-train layouts used, which tends to result in a high weight bias up front. Another example would be in right-hand drive cars, where the steering column and linkages (and the driver himself) tend to weigh heavily on the front right corner.
The goal of corner weighting is to achieve equality between the sum of the weight resting on the rear left and front right tyres and the sum of the weight resting on the rear right and front left tyres. This can be represented simply as RL + FR = RR + FL. In automotive speak, RL + FR is called the cross weight, and it should ideally be 50% of total vehicle weight.
To illustrate the impact of having uneven corner weights, we might consider a typical Subaru Impreza, with a front engine, right-hand drive layout with the exhaust muffler fixated at the rear left corner. In this case, the car is likely to have cross weight percentage higher than 50%, which means RL + FR > RR + FL. In a right hand turn, the weight of the car shifts towards the left. At the front end, part of the heavier load on the FR tyre shifts onto the FL tyre, equalizing the left and right tyre loads, thereby maximising grip. At the rear end, weight shifts from the lightly loaded RR tyre towards the already over-burdened RL tyre, overloading it and reducing available grip. As a result of better grip at the front and lower grip at the rear, oversteer is promoted. In a left hand turn, the weight of the car shifts towards the right. At the front end, weight shifts from the lightly loaded FL tyre towards the heavier FR tyre, overloading it, and decreasing grip. At the rear end, the heavier load on the RL tyre is evenly distributed to the RR tyre as weight shifts towards the right, thereby maximising grip. As a result, understeer occurs.
When the car reacts differently in left and right turns as illustrated above, it is less predictable and more difficult to drive with confidence at the limit. The objective of corner weighting for a road course is to tune the suspension such that the car reacts predictably and maximises the grip of its tyres in both left and right turns. For oval racing tracks (e.g. NASCAR) where the car needs only to turn well in a single direction, corner weighting can be used to tune the car’s handling characteristics to promote understeer or oversteer, but that is not relevant for most of us.
How is corner weighting done?
To corner weight a car, you will need to be able to adjust the suspension ride height (e.g. height adjustable coilovers), and a means of weighing the car at each corner (e.g. corner weight scales). For best results, you will want to do it on relatively flat ground. From what I’ve observed, most garage floors or driveways have a very slight slope angle built in. This is still workable, but at a minimum, all four scales should be more or less on the same flat plane, so you’ll need to measure the wheelbase and track width of the car, and find four corresponding spots that are relatively flat. It’s also preferable to drive the car onto the scales using ramps as opposed to jacking it up and dropping it down to avoid suspension bind, which may affect the readings.
Once the scales have been positioned, the car is put on the scales, the weight measured, and then the necessary adjustments can be made. However, before weighing, the car should be in track-ready condition. This means that all fluids should be topped to the appropriate levels used for track days (personally, I would use three-quarter tank of fuel, minimal windshield washer fluids and three-quarter tank of intercooler spray, to simulate a mid-session lap). Also, tyre pressures must be set to what you normally run on track (i.e. when hot), which for me was 39 psi front and 36 psi rear. Also remember to remove the spare tyre, jack, rear seats and soft toys (if you have the habit of doing so during track days), and setup any in-car video recording equipment that you typically use. And of course, the driver should be in the car during the weighing, with helmet (and full-body fire-proof racing suit and underwear if applicable). If your car has adjustable anti-roll bar end-links, these should be disconnected before weighing to eliminate any possible suspension preload. As you can see, I positioned the control unit so as to be able to check the readings from inside the car. Having weighed the car, you will now need to jack the car up, remove the wheels, and make adjustments to the coilover heights.
Before going further, it is useful to note that weight cannot be shifted between the front and rear, or left and right. To shift weight in this manner requires actual physical movement of weights in the car, such as relocation of the battery or the use of ballasts (read: a hefty ICE setup), etc. Any workshop that tells you otherwise is completely ignorant. On the other hand, corner weighting re-distributes diagonal weights. The theory behind this is that lowering the coilover spring perch at any one corner (this will typically result in lowering the ride height, but a distinction in moving the spring perch versus altering ride height should be made, and you’ll see why later) will decrease the load on that particular corner and its diagonally opposite corner, while at the same time increasing the load on the other two other corners, and vice versa. Overall front-to-rear and left-to-right weight bias does not change from corner weighting, I repeat. So if you’re driving a Subaru Impreza, and you want a 50-50 front-to-rear weight ratio, you will need a really really hefty ICE system in your boot, otherwise you’re stuck with 60-40, sorry.
The typical analogy used to explain the phenomenon that is corner weighting is that of a four-legged table. If one of the legs was trimmed such that it is slightly shorter than the other three legs, the table will be prone to wobbling. The manner in which the table wobbles is the key to understanding corner weighting. When the table wobbles, it is the shorter leg and its diagonally opposite leg that tend to fluctuate between lifting off and landing back on the ground. The other two legs tend to remain firmly planted while the table wobbles. This demonstrates that the load resting on the shorter leg and its diagonally opposite leg has decreased. This is a logical argument given that if any leg is off the ground, it is not carrying any load. This also means that the load on the other two legs has now increased, since the weight of the table top resting on the four legs is a constant. The conclusion from this example is that, the longer table legs carry a heavier share of the load, while the shorter table legs carry a lighter share of the load. When applying this concept to a car’s suspension, it implies that raising the spring perch height at any one corner will increase the load on that particular corner and its diagonally opposite corner, while decreasing the load on the two other corners, and vice versa.
Once this concept is understood and applied, corner weighting is a relatively straightforward process. Don’t believe everything the workshops say when they tell you that it’s a complicated process and you cannot hope to achieve equal ride heights. The fact is, you can achieve your target ride heights during corner weighting as well, it just takes a little more effort. Just to quote an example, if the RL + FR corners are overweight, you will need to lower either the RL or FR, or raise the RR or FL, or all of the above, in order to shift some of the excess weight to the RR + FL corners. However, you find that the ride height at the FR is already slightly lower than the FL, especially with the driver in the car, and you in fact wish to raise it a little to make it even with the FL. Other than the FR corner, you don’t wish to change the overall ride height by too much. One possible solution to this conundrum is to lower the RL corner, and combine that with raising the FL and the RR corners. This combination of adjustments should reduce the load on the RL + FR corners and increase the load on the RR + FL corners, which is what you want. But what about the ride heights? Firstly, raising both the RR and FL corners will also raise the ride heights on the RL and FR corners (unless your chassis is as soft as a wet noodle). Secondly, lowering the RL corner will actually have the side-effect of raising the ride height on the FR corner (think of a see-saw with the RR and FL corners acting as the fulcrum). The net result is that you actually end up with a lighter FR corner, while actually raising its ride height! Of course, in reality there will be some chassis flex in play but the impact is minimal. The possibilities are virtually endless, and any workshop that claims that corner weighting will always give you uneven ride heights are probably just too lazy to take the time to make the correct adjustments at the correct corners (unless your chassis is bent out of specification). Indeed, a perfect 50% cross weight can be achieved by making a large adjustment to just one corner, which might be a fast and easy approach, but would probably screw up your ride heights.
Once the adjustments have been made, mount the wheels and lower the car down. I strongly suggest tightening the lug nuts by hand first, followed by a torque wrench at approximately 100 Nm of force. This ensures that all 5 wheel studs are evenly stretched, preventing the brake rotor from warping. Remember to shake the car vigorously to release any suspension binding before weighing it again. Or you could go for a leisurely drive around the neighbourhood, if you prefer. Once done, put the car on the scales and check the weights. Make further adjustments as necessary and repeat until you get as close to 50% cross weight as possible. It should not take long once you get the hang of how corner weights change in proportion to a change in coilover length. The proportion of change depends on the stiffness of your springs. For any given adjustment, the stiffer the springs, the greater the change in weight. Once you’re happy with your corner weights, adjust the anti-roll bar end-link lengths (if you have adjustable end-links) so that you can re-attach the anti-roll bar without any twisting and tensioning. Finally, remember to release the excess tyre pressure or they might result in over-inflation once your tyres heat up from normal driving. Unless the corner weighting process resulted in a significant change in ride heights (which as explained above need not be the case), chances are you will not need to get your wheel alignment checked. The trick here is to set your desired ride heights first, get the car aligned, then corner weight it. With a proper, methodological approach and patience, your ride heights should not vary significantly after corner weighting.
Hopefully, this post may serve to shed more light on corner weighting and its theory and application.
I believe that while most of us have heard of corner weighting, not many of us actually understand its benefits and what it involves. To make matters worse, most workshops that perform corner weighting services either fail to properly educate customers, or worse, share misleading information. To top it off, the costs of corner weighting are relatively high, ranging from $350 to almost $500. That, coupled with the fact that the benefits of corner weighting can only be fully appreciated on track, is what I believe turns most casual enthusiasts away. However, with more and more drivers participating in track days, and the upcoming Changi Motorsports Hub presenting the prospect of a local track, more enthusiasts may wish to consider corner weighting their cars. After all, to get the most out of engine modifications, proper ECU tuning is required. Likewise, corner weighting is the final step to getting the most out of your suspension setup. Anyway, enough rambling from me, and on to the subject topic.
What is corner weighting?
In order for a car to make full use of its tyres’ available grip on a road course, the full weight of the car should be evenly distributed across all four tyres. Unfortunately this is impossible for most road cars in general, where the choice of drive-train layout and driver sitting position offset significantly affects the weight distribution. For example, placement of the engine and transmission at the front is one of the most common drive-train layouts used, which tends to result in a high weight bias up front. Another example would be in right-hand drive cars, where the steering column and linkages (and the driver himself) tend to weigh heavily on the front right corner.
The goal of corner weighting is to achieve equality between the sum of the weight resting on the rear left and front right tyres and the sum of the weight resting on the rear right and front left tyres. This can be represented simply as RL + FR = RR + FL. In automotive speak, RL + FR is called the cross weight, and it should ideally be 50% of total vehicle weight.
To illustrate the impact of having uneven corner weights, we might consider a typical Subaru Impreza, with a front engine, right-hand drive layout with the exhaust muffler fixated at the rear left corner. In this case, the car is likely to have cross weight percentage higher than 50%, which means RL + FR > RR + FL. In a right hand turn, the weight of the car shifts towards the left. At the front end, part of the heavier load on the FR tyre shifts onto the FL tyre, equalizing the left and right tyre loads, thereby maximising grip. At the rear end, weight shifts from the lightly loaded RR tyre towards the already over-burdened RL tyre, overloading it and reducing available grip. As a result of better grip at the front and lower grip at the rear, oversteer is promoted. In a left hand turn, the weight of the car shifts towards the right. At the front end, weight shifts from the lightly loaded FL tyre towards the heavier FR tyre, overloading it, and decreasing grip. At the rear end, the heavier load on the RL tyre is evenly distributed to the RR tyre as weight shifts towards the right, thereby maximising grip. As a result, understeer occurs.
When the car reacts differently in left and right turns as illustrated above, it is less predictable and more difficult to drive with confidence at the limit. The objective of corner weighting for a road course is to tune the suspension such that the car reacts predictably and maximises the grip of its tyres in both left and right turns. For oval racing tracks (e.g. NASCAR) where the car needs only to turn well in a single direction, corner weighting can be used to tune the car’s handling characteristics to promote understeer or oversteer, but that is not relevant for most of us.
How is corner weighting done?
To corner weight a car, you will need to be able to adjust the suspension ride height (e.g. height adjustable coilovers), and a means of weighing the car at each corner (e.g. corner weight scales). For best results, you will want to do it on relatively flat ground. From what I’ve observed, most garage floors or driveways have a very slight slope angle built in. This is still workable, but at a minimum, all four scales should be more or less on the same flat plane, so you’ll need to measure the wheelbase and track width of the car, and find four corresponding spots that are relatively flat. It’s also preferable to drive the car onto the scales using ramps as opposed to jacking it up and dropping it down to avoid suspension bind, which may affect the readings.
Once the scales have been positioned, the car is put on the scales, the weight measured, and then the necessary adjustments can be made. However, before weighing, the car should be in track-ready condition. This means that all fluids should be topped to the appropriate levels used for track days (personally, I would use three-quarter tank of fuel, minimal windshield washer fluids and three-quarter tank of intercooler spray, to simulate a mid-session lap). Also, tyre pressures must be set to what you normally run on track (i.e. when hot), which for me was 39 psi front and 36 psi rear. Also remember to remove the spare tyre, jack, rear seats and soft toys (if you have the habit of doing so during track days), and setup any in-car video recording equipment that you typically use. And of course, the driver should be in the car during the weighing, with helmet (and full-body fire-proof racing suit and underwear if applicable). If your car has adjustable anti-roll bar end-links, these should be disconnected before weighing to eliminate any possible suspension preload. As you can see, I positioned the control unit so as to be able to check the readings from inside the car. Having weighed the car, you will now need to jack the car up, remove the wheels, and make adjustments to the coilover heights.
Before going further, it is useful to note that weight cannot be shifted between the front and rear, or left and right. To shift weight in this manner requires actual physical movement of weights in the car, such as relocation of the battery or the use of ballasts (read: a hefty ICE setup), etc. Any workshop that tells you otherwise is completely ignorant. On the other hand, corner weighting re-distributes diagonal weights. The theory behind this is that lowering the coilover spring perch at any one corner (this will typically result in lowering the ride height, but a distinction in moving the spring perch versus altering ride height should be made, and you’ll see why later) will decrease the load on that particular corner and its diagonally opposite corner, while at the same time increasing the load on the other two other corners, and vice versa. Overall front-to-rear and left-to-right weight bias does not change from corner weighting, I repeat. So if you’re driving a Subaru Impreza, and you want a 50-50 front-to-rear weight ratio, you will need a really really hefty ICE system in your boot, otherwise you’re stuck with 60-40, sorry.
The typical analogy used to explain the phenomenon that is corner weighting is that of a four-legged table. If one of the legs was trimmed such that it is slightly shorter than the other three legs, the table will be prone to wobbling. The manner in which the table wobbles is the key to understanding corner weighting. When the table wobbles, it is the shorter leg and its diagonally opposite leg that tend to fluctuate between lifting off and landing back on the ground. The other two legs tend to remain firmly planted while the table wobbles. This demonstrates that the load resting on the shorter leg and its diagonally opposite leg has decreased. This is a logical argument given that if any leg is off the ground, it is not carrying any load. This also means that the load on the other two legs has now increased, since the weight of the table top resting on the four legs is a constant. The conclusion from this example is that, the longer table legs carry a heavier share of the load, while the shorter table legs carry a lighter share of the load. When applying this concept to a car’s suspension, it implies that raising the spring perch height at any one corner will increase the load on that particular corner and its diagonally opposite corner, while decreasing the load on the two other corners, and vice versa.
Once this concept is understood and applied, corner weighting is a relatively straightforward process. Don’t believe everything the workshops say when they tell you that it’s a complicated process and you cannot hope to achieve equal ride heights. The fact is, you can achieve your target ride heights during corner weighting as well, it just takes a little more effort. Just to quote an example, if the RL + FR corners are overweight, you will need to lower either the RL or FR, or raise the RR or FL, or all of the above, in order to shift some of the excess weight to the RR + FL corners. However, you find that the ride height at the FR is already slightly lower than the FL, especially with the driver in the car, and you in fact wish to raise it a little to make it even with the FL. Other than the FR corner, you don’t wish to change the overall ride height by too much. One possible solution to this conundrum is to lower the RL corner, and combine that with raising the FL and the RR corners. This combination of adjustments should reduce the load on the RL + FR corners and increase the load on the RR + FL corners, which is what you want. But what about the ride heights? Firstly, raising both the RR and FL corners will also raise the ride heights on the RL and FR corners (unless your chassis is as soft as a wet noodle). Secondly, lowering the RL corner will actually have the side-effect of raising the ride height on the FR corner (think of a see-saw with the RR and FL corners acting as the fulcrum). The net result is that you actually end up with a lighter FR corner, while actually raising its ride height! Of course, in reality there will be some chassis flex in play but the impact is minimal. The possibilities are virtually endless, and any workshop that claims that corner weighting will always give you uneven ride heights are probably just too lazy to take the time to make the correct adjustments at the correct corners (unless your chassis is bent out of specification). Indeed, a perfect 50% cross weight can be achieved by making a large adjustment to just one corner, which might be a fast and easy approach, but would probably screw up your ride heights.
Once the adjustments have been made, mount the wheels and lower the car down. I strongly suggest tightening the lug nuts by hand first, followed by a torque wrench at approximately 100 Nm of force. This ensures that all 5 wheel studs are evenly stretched, preventing the brake rotor from warping. Remember to shake the car vigorously to release any suspension binding before weighing it again. Or you could go for a leisurely drive around the neighbourhood, if you prefer. Once done, put the car on the scales and check the weights. Make further adjustments as necessary and repeat until you get as close to 50% cross weight as possible. It should not take long once you get the hang of how corner weights change in proportion to a change in coilover length. The proportion of change depends on the stiffness of your springs. For any given adjustment, the stiffer the springs, the greater the change in weight. Once you’re happy with your corner weights, adjust the anti-roll bar end-link lengths (if you have adjustable end-links) so that you can re-attach the anti-roll bar without any twisting and tensioning. Finally, remember to release the excess tyre pressure or they might result in over-inflation once your tyres heat up from normal driving. Unless the corner weighting process resulted in a significant change in ride heights (which as explained above need not be the case), chances are you will not need to get your wheel alignment checked. The trick here is to set your desired ride heights first, get the car aligned, then corner weight it. With a proper, methodological approach and patience, your ride heights should not vary significantly after corner weighting.
Hopefully, this post may serve to shed more light on corner weighting and its theory and application.
