Explained: Weight transfer vs body roll (part 2)

This is the second part in a 2-part series…

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Last time, we dissected the components of weight transfer, understood why we want to reduce it as much as possible, and saw that our only means through which to do so are vehicle weight, CG height, and width. We also established that reducing body roll makes no appreciable different to the amount of weight transfer.

However, body roll brings about various problems of its own which we need to understand and try to solve for. The trouble we run into is that every solution for body roll creates some other problem(s), and we are left needing to decide where to make compromises.

Let us be crystal clear: Even the most sophisticated race car in the world is full of suspension compromises. With all the variables we need to account for (tire flexion, bump compliance, weight transfer, body roll, etc), physics dictates that there is no solution that addresses all of them completely. The only way to eliminate every issue is to have a car whose CG height is zero, driven on tires that somehow make grip while being perfectly solid, on a flawless glass smooth race track.

As such, because we live and drive in the real world where these perfect conditions do not exist, let’s work through the various problems that body roll creates.

1. Camber loss

Why do we need negative camber? We actually only need it while cornering, primarily to maintain a good contact patch when the tire flexes sideways under cornering loads, and secondarily to enjoy the benefits of camber thrust. Ideally, we would want our tires perfectly vertical when we are driving in a straight line, and for the outside tires to get negative camber in a corner.

This by itself is a fairly easy engineering problem. The solution is to have an upper control arm that is shorter than the lower arm so that under compression the top of the tire is pulled inwards, creating negative camber. As a matter of fact, that is exactly how a double wishbone suspension works. Unfortunately, we can only go so far with this approach because if we engineer the suspension to gain lots of negative camber under compression, then during braking we would end up with both front tires riding their inner edges and have absolutely horrible braking performance.

The problem becomes even more complex, because a good suspension has to meet not just those needs, but various others as well. It must allow for 4 wheel independence, have appropriate camber during braking/acceleration, have appropriate camber during cornering, have minimal/no change in track width during suspension movement, have minimal movement of the roll center during suspension movement, have minimal/no toe change during suspension movement, have minimal/no compliance in the suspension links, and be light weight.

Turns out, we do not have a way to meet all of those needs simultaneously, which is why we try to limit body roll. The more roll we have, the more our the desired suspension angles change, and we lose performance.

2. Timing of weight transfer

In part 1 we talked about the various components of weight transfer. Unsprung weight transfer and geometric weight transfer (i.e. the ones responsible for the tipping/jacking force) occur instantly. Elastic weight transfer, on the other hand, doesn’t completely arrive until the suspension has completed its movement (i.e. until the body reaches its final roll angle).

This means that the amount of grip our tires have changes while the weight transfer is occurring, and it is to our benefit to speed up the weight transfer so that we are better able to read the amount of grip we have! Furthermore, slower weight transfer means that the car will have a slower reaction time, resulting in poor transient behavior. While that is a detriment in any form of racing, it is particularly problematic in transitional elements (slaloms, offsets, etc) which are very common on autocross courses.

3. Rotational torque of body roll

When a body rolls, the motion generates rotational torque which must be overcome every time we want to change direction. The amount the body rolls is affected by the stiffness of the springs/bars, and the speed of the roll is affected by the stiffness of the shocks. The more the body rolls and the faster the body rolls, the more rotational torque it generates and the more force it takes to overcome that torque.

This further worsens the reaction time of the vehicle when it comes to changing direction, and is a big reason why (in SCCA autocross) Street Touring vehicles transition so much better that Street class vehicles. By lowering the car, using stiffer springs/bars, significantly stiffer shocks, etc, Street Touring vehicles reduce (and slow down) roll, thereby greatly reduce roll inertia, and as a result change direction very quickly.

4. Aerodynamic performance

The effect of body roll on aerodynamic efficiency only really matters on cars that are heavily reliant on aerodynamics to generate grip. The gist of the problem is that in order to enjoy maximum aerodynamic benefits, we need as little air as possible traveling under the car. When the body rolls, there’s more of a gap under the car as the inside lifts, which reduces the ability of the splitter, side skirts, and other devices to keep air from getting under the vehicle, which wreaks havoc with the aerodynamic performance.

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Hopefully by now we agree that body roll brings about some rather undesirable effects. So far, we have talked about roll and weight transfer with the assumption that it happens uniformly at both ends of the vehicle. Of course, in reality, that’s rarely true. A general rule to keep in mind is that whichever end of the car has more roll resistance will have more weight transfer, and will thus have less grip. All else being equal, if there is more roll resistance at the front of the car, it will understeer. If there is more roll resistance at the rear, it will oversteer.

Given the methods we have for reducing body roll, it turns out that we can affect where the weight transfer goes, and how quickly it gets there. As we will see below, that can be a really powerful handling tuning tool! Let’s look at 4 primary means through which to affect the amount of roll, velocity of roll, where the weight transfer goes, and how quickly it gets there.

Springs

Obviously, using stiffer springs reduces roll. Applying what we have covered so far, we see that by using stiffer springs the body reaches its final roll angle more quickly (less rotational distance to cover), and thus the elastic weight transfer takes less time, reducing the time for total weight transfer. That’s a win.

The downside is that we lose bump compliance, which can severely hurt performance. A tire can only make grip if it is in contact with the ground. Unless we are in a very light weight vehicle, it is unlikely that we will be able to reduce roll to optimal levels, or get to an ideal handling balance using stiffer springs alone without severely affecting the vehicle’s bump compliance.

Anti-roll (sway) bars

Which brings us to anti-roll/sway bars. A sway bar is a torsional spring (a fancy way of saying, “a spring that works by twisting”) which connects the suspension on one side of the car to the other. When the car goes over a bump and both sides compress, the bar simply rotates in its mounts and doesn’t add anything to the mix. However, when going around a corner, the outside suspension (which is compressing), now exerts a compression force on the inside spring (which is trying to extend) because the sway bar connects both sides, and this force resists body roll. So we have managed to reduce body roll without affecting bump compliance!

But of course, a problem arises. The stiffer the bar, the more weight is transferred across that axle, and the less grip that pair of tires makes.

This is a critical point to understand! When we use a stiffer sway bar, we increase the proportion of roll resistance on that end of the car, which (we established as a general rule) increases the weight transfer on that axle. We do not change the total amount of weight transfer across the entire vehicle, but we take some additional weight off that inside tire and distribute it to the other 3. This means we reduce grip for the pair of tires when we use a larger sway bar.

So, while there are good reasons for using larger sway bars, we must recognize that it always comes with a loss of grip on that axle.

Shocks (more accurately, dampers)

While springs and bars affect how much the body/suspension move, shocks affect the rate at which they move. The physics of how shocks affect suspension motion and weight transfer is very complicated, so the following statement may not be intuitive: a stiffer shock slows down the motion of the suspension, but speeds up weight transfer. If you are interested in reading more about this, I recommend “Tune To Win” by Carroll Smith. For our purposes, we can summarize the effects as follows.

Stiffer shocks give quicker response because they speed up weight transfer. The more compression force the shock has, the faster it will receive weight. The more rebound force a shock has, the faster it will give up weight. Needless to say, this is an invaluable tool for tuning transient handling balance. Shocks have no effect on steady state balance, as they only contribute anything meaningful when the suspension is moving.

Taken to the extreme, a shock that is too stiff can overpower the spring to the extent that it barely moves at all. The result is that we not only lose bump compliance, but the tire itself becomes the only “spring” on the car, and being undamped, it chatters/skips across the surface.

Roll center adjustments

For most of us, discussing this is purely academic because we do not have a competition-legal way to adjust our roll centers. But for those who can, it can be a useful tuning tool. Recall from part 1 that the closer the roll center is to the CG height, the less roll (and more tipping/jacking force) we will get. This has the dual effect of speeding up total weight transfer (because there is less elastic weight transfer), and increasing the weight transfer across that axle. Therefore, by adjusting the roll center heights on one end of the car vs. the other, we can tune the understeer/oversteer balance of the car.

Adjusting roll centers is very much like adjusting toe though. Just as a little bit of toe-out on the front tires can improve turn in, a slightly raised front roll center improves turn in response. But there’s only so far we can take it before there are other unintended/bad effects.

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Hopefully this series has given you an easily digestible understanding of how weight transfer and body roll work, and what some of the compromises are that we must make when setting up our vehicles. There is no such thing as an “ideal” suspension setup, and everyone will have a slightly different idea of what compromises they are comfortable making. A more precise driver may trade some stability for more ultimate grip. A driver more prone to taking risks may favor a more forgiving setup that sacrifices a bit of front grip. And so on.

Knowledge is power, or rather in this case, it is balance.