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Mark Ortiz Automotive is a chassis consulting service primarily serving oval track and road racers. This newsletter is a free service intended to benefit racers and enthusiasts by offering useful insights into chassis engineering and answers to questions. Readers may mail questions to: 155 Wankel Dr., Kannapolis, NC 28083-8200; submit questions by phone at 704-933-8876; or submit questions by e-mail to: firstname.lastname@example.org. Readers are invited to subscribe to this newsletter by e-mail. Just e-mail me and request to be added to the list.
EFFECT OF DAMPING ON LOAD TRANSFER
I’ve found conflicting information re weight transfer in pitch with regards to braking. I have a C5 [Corvette] Z06 and when I add too much rear rebound, without any other changes, the car seems to go into “ice” mode and the braking forces are near null until the car gets a chance to reach steady state and braking forces return.
I thought it was due to the rear shocks not allowing the front tires to load properly due to ‘slow’ weight transfer outlined in the JRZ quote below, but I’ve also read the opposite which may be correct.
From JRZ’s web site:
“Rebound damping can also effect weight transfer, cornering, and feel of the motorcycle. The lighter the amount of rebound damping front or rear will greatly affect your weight transfer of the motorcycle. If you lessen the rebound damping in the front forks of the motorcycle it will transfer weight quicker to the rear of the motorcycle as the brakes are released or under acceleration. The same goes for the rear shock, if you lessen the rebound damping in the rear it will quicken the weight transfer to the front of the motorcycle especially as you apply the front brakes, and on turn in. “
I understand drag racers use soft front rebound to increase grip which would be similar to my situation but on the opposite end of the car. Just not sure of the correct definition/mechanism.
A simple “JRZ is wrong” (or right) answer would be sufficient.
Many people make the mistake of confusing weight transfer (dynamic wheel load transfer) with sprung mass or suspension displacement, or make the mistake of supposing that weight transfer can be inferred from sprung mass or suspension displacement. Actually, weight transfer due to x and y axis (longitudinal and lateral) accelerations is not mainly the result of sprung structure movement with respect to the wheels. It occurs even in vehicles with no suspension. We cannot even say that a suspension change that results in more displacement change at a given corner, end, or side of the vehicle implies more load change there.
However, we also cannot quite say that the amount or speed of suspension movement has no effect at all on weight transfer. The biggest effects come from differences in resistance to roll displacement at the two ends of the car (for cornering) and differences in resistance to pitch displacement at the two sides of the car (for braking and forward acceleration). Such differences will affect dynamic diagonal percentage. There are also smaller effects even when front/rear or left/right differences are absent.
Because these effects are small, I don’t see any way they could result in wheel lockup on initial brake application. If an ABS system is reacting to slow rear suspension extension by going into an “ice mode”, it is responding to something other than actual wheel lockup or any actual delay in wheel load transfer. I will have more to say about the specifics of this in the case of the Corvette in a bit.
First, though, let’s consider what happens upon abrupt application of the brakes, assuming the car is running straight and assuming the suspension is entirely symmetrical.
For racing or autocross, this situation typically occurs at the end of a straightaway. It upsets the car least if we apply the brakes with a gentle “squeeze” rather than a “slam”. However, we don’t want to waste any time. We want to bring the car up to full retardation as quickly as the car will tolerate. The more we reduce jerk (change of acceleration), the sooner we have to get off the throttle and start brake application. The more abruptly we can apply the brakes, the longer we can delay braking.
When the brakes are applied swiftly and the car is brought as quickly as possible to straight-line braking at the limit of adhesion, assuming that the car has less than 100% anti-dive and anti-lift, the sprung structure pitches forward, with the rear suspension extending and the front suspension compressing. With some delay, it assumes a steady state with a forward pitch displacement, and holds that until the driver starts releasing the brakes. During the delay period, the sprung structure accelerates forward in pitch, possibly briefly attains a fixed forward pitch velocity, then accelerates rearward (decelerates forward) in pitch, to a pitch velocity of zero. At this point, the car is in a steady state of straight-line limit braking.
At any point in this process, longitudinal load transfer depends almost entirely on only three things: the amount of rearward acceleration, the height of the c.g. (center of gravity or center of mass), and the length of the wheelbase. Suspension displacements matter to the extent that they influence c.g.
height. Anything that reduces rear suspension extension or increases front suspension compression lowers the c.g. and reduces load transfer.
In steady-state braking, with pitch velocity at zero, this will depend entirely on the anti-dive, anti-lift, and springs. However, when there is some forward pitch velocity, the dampers also have an influence.
We can slow the pitch motion by increasing front compression (bump) damping, or by increasing rear extension (rebound) damping. However, while these have similar effects on pitch velocity, they have opposite effects on the magnitude of load transfer while the effects are present. This is because they have opposite effects on c.g. height. Slowing front compression temporarily increases c.g. height, and therefore temporarily increases load transfer. Slowing rear extension temporarily reduces c.g. height, and therefore temporarily reduces load transfer.
Although these effects are real, for a low vehicle with a fairly long wheelbase and fairly stiff suspension the effects are of very small magnitude. For a realistic range of settings, we are probably talking about at the most a quarter of an inch difference in c.g. height around peak pitch velocity.
In the case of drag racing sedans set up for maximum front end lift, we are looking at much greater suspension movement. To obtain this, the front springs are made extremely soft. The front dampers are valved to extend very freely and compress very reluctantly. There is then perhaps an inch or a bit more increase in c.g. height at launch, compared to a general-purpose setup.
The first few milliseconds of the launch are the most important part of the run. The car is accelerating the full length of the strip, and any gain in acceleration right at the start translates to greater speed over the entire run. Therefore, we want the front end up as quickly as possible.
We should note that with advances in drag tires and pavement, we don’t necessarily set up drag sedans for maximum front end lift anymore. With really good tires, even a nose-heavy sedan may be limited by wheelstand rather than wheelspin. In that case, we don’t want to maximize front end rise. The ideal is to launch the car with all the weight on the drive wheels, and none at all on the front wheels or the wheelie bar casters, with the rear tires at optimal percent slip. But if such a car is on any sort of street tires, we probably will want maximum front end lift.
In addition to the effect from c.g. height, there are small secondary effects from pitch inertia. When the sprung mass is not moving in pitch, it doesn’t want to start moving in pitch. Once it is moving in pitch, it doesn’t want to stop moving in pitch. At the start of brake application, pitch inertia reduces forward load transfer. As steady-state braking is reached, pitch inertia increases forward load transfer. Any increase in damping, front or rear, reduces pitch velocity, pitch acceleration, and pitch jerk. That correspondingly reduces effects from pitch inertia.
But again, when pitch displacement is small, any effects due to pitch inertia are small, even with fairly light damping.
So, to answer the basic question: does adding low-speed rebound damping at the rear temporarily increase forward load transfer, or temporarily reduce it? It temporarily reduces it. However, the effect is so small that it would not cause wheel lockup and trigger the ABS due to that.
There are cars that have so much anti-lift that the rear suspension doesn’t extend at all in braking, regardless of damping. In some rear-engine cars with trailing arms, the rear suspension even compresses slightly in braking. Such cars do not exhibit any abnormal braking behavior, with or without ABS.
Well, then, why does this Corvette do what the questioner describes? My first inclination was to suspect that maybe the brake pads have poor initial “bite”, or perhaps there’s a pinched line somewhere. But the questioner says the effect goes away when the rear rebound is softened, and comes back again when the rebound is stiffened again.
I have not gone so far as to comb the world for a Corvette expert who can explain this to me (perhaps one will respond to this newsletter), but I have corresponded further with the questioner, and also gone on-line and found a wiring diagram for the car’s ABS system.
The car is a 2002 model. The shocks are not JRZ’s; the questioner was just citing that company’s literature. The shocks on the car are actually Penske single-adjustables. These are currently set up to have the adjustability on the rebound. They can also have the adjustability on the compression instead. The adjustment is a bleed with a check valve. The check valve can be reversed. I would think it might also be omitted entirely, making the bleed work in both directions.
The car has ABS, traction control, and stability control. It does not have the computer-controlled “real-time damping” (RTD) found in later versions. There are four wheel speed sensors, a steering position sensor, a brake fluid pressure sensor, a lateral acceleration sensor, and a yaw rate sensor. There are no suspension displacement sensors, and no pitch accelerometer is shown. However, there is something called the body control module (BCM) that apparently handles the stability control. The schematic doesn’t show what’s in the BCM, but it communicates with the ABS controller through a serial bus, and the stability control works through brake and throttle intervention.
The traction control and stability control can be turned off, but not the ABS, although maybe it can be deactivated by removing its fuse.
I think what must be happening is that when the rear rebound is stiffened, something in the BCM is not sensing the expected pitch that would normally go with the brake fluid pressure rise, and concludes that the car must be on a low-mu surface. This prompts the ABS to exercise “prior restraint” and not apply enough brake to cause lockup on ice.
The reason for having such an “ice mode” rather than just letting the system respond to wheel lockup is that when wheel lockup occurs on ice, a layer of water forms at the contact patch, and once this happens, there is so little friction that the wheel may be reluctant to start turning again even if the
brake force is modulated. If the car has begun to slide laterally when the wheel is locked, that alone may keep the contact patch melted. Therefore, it can be desirable to not let the initial melting at the contact patch occur. This requires preemptive intervention.
The questioner has been using the stiff rear rebound setting to improve transient behavior for autocross (US style, on paved parking lots or airfields). He has asked if using stiffer bump damping at the rear instead might work. My diagnosis of the cause of the problem is somewhat tentative at this point, but if I’m right about that, then more compression damping at the rear probably would be a good approach. Note that this would involve different shim stacks in the shock, not just flipping or removing the check valve.