I have a 2016 Infiniti Q50. If I step on the brake (so that the wheels cannot turn) and shift from park to drive, the rear of the car squats and/or the front of the car rises, just a smidge. If I keep my foot on the brake (again, preventing the wheels from turning) and step on the gas, the car’s stance changes even further. Hard to tell for sure, but I think most of the change is about the rear-end squatting down.
Since the car is not actually accelerating during this exercise, this is clearly not due to dynamic weight transfer. Something about the suspension configuration is causing it to displace when the driveline gets loaded by the engine, even while the car remains stationary.
Can someone 'splain what sort of suspension is on this car, and why it behaves like this?
Torque. The engine is applying torque to the axle. Since you have the brakes engaged, the force from the engine torque is being applied to the brake calipers (through the pads.) the engine is (somewhat) rigidly mounted to the body, the calipers are rigidly mounted to the steering knuckles and suspension arms/struts, so that force is being used to compress the springs on the side opposite the direction of applied torque.
This squatting rear/rising front effect the OP notes is intentional. It’s an effect of what is often called “anti-dive” suspension geometry.
Without anti-dive suspension geometry, the front end of your car would dip precipitously under braking, which causes “weight transfer”[sup]1[/sup] to the front wheels. It’s more accurate to say that braking increases the vertical (normal) force on the front tires and decreases the normal force on the rear tires.
So braking tends to compress the front suspension and extend the rear suspension, causing an abrupt change in attiude. In other words, the front end dives under braking. While drivers perceive this to be unpleasant, it also chews through suspension travel, reducing the car’s ability to handle bumps (and stay on the road) under braking.
Anti-dive suspension geometry exists to ameliorate brake dive. To do so, the brake calipers are positioned in such a way that the reaction forces from the brake calipers tends to compress the rear suspension and to extend the front suspension. (You’ll notice that front brake calipers are often mounted about 180 degrees (relative to the wheel bearings) from where the rear brake calipers are mounted).
This greatly attenuates (but doesn’t quite eliminate) brake dive. As the OP noticed, a side effect is that when you try to start moving with the brakes engaged, car tends to squat. This isn’t from engine torque directly. Rather, the engine torque is providing something for the brakes to react against.
Under “normal” braking conditions (decelerating with one’s foot off the gas pedal) the brakes do the same thing without any engine torque at all. But the squat is imperceptible because it’s all going into counteracting brake dive.
On balance, most cars dive a little under braking. They’d dive a lot more without anti-dive suspension geometry.
[sup]1[/sup] This is a misnomer; the effect loads the front wheels and unloads the rear wheels, but it’s not due to a change in actual weight/mass distribution. But that’s what car enthusiasts call it, so I’ll use the same term.
It depends on the specific suspension geometry, but yes, most cars would exhibit the opposite behavior when shifted into reverse.
No, it’s not hard on the brakes. It’s how they work all the time. May I ask why you suspected this is hard on the brakes?
I’m not mocking you at all, just to be clear. I’m just so deeply enmeshed in the mechanical world that it can be hard for me to step back and think about these things in the way a smart-but-not-especially-mechanically-inclined person would. That’s why I’m curious about your thought process.
There’s a classic bit of automotive received wisdom that says one should downshift when approaching a stop “to save the brakes.” That was sort of true a generation ago, when drum brakes were common and they routinely overheated when descending mountains.
But modern disc brakes are designed to dump heat quickly, and “saving your brakes” with downshifting comes at the cost of increased engine wear, particularly at the valvetrain and piston rings. I don’t know about you, but I’d rather deal with brake pad replacement than a top-end engine rebuild.
no, because the car isn’t moving. Brake linings wear out when they experience sliding (kinetic) friction; i.e. the pads/shoes are pressed against a rotating rotor or drum which gradually wears material off of the pads/shoes and rotor/drum. If you’re just sitting there brake torquing it, nothing is moving and there’s no wear.
This is a surprisingly-common myth. In point of fact, downshifting causes no engine issues at all. Yes, technically, it causes extra wear–but only on parts and places that normally get little or no wear.
It just seems stupid to put the engine in gear and press the accelerator at the same time pressing on the brake pedal. Seem obviously counter-productive, and NOT what the car was designed for.
Without even getting into the environmental and global warming effects of such a move.
This is what drag racers do, basically, with automatic transmissions. The trans has a stall speed, maybe 4,000 RPMs. So lock up the brakes, rev the engine to 3,900 RPMs, light turns green, let off the brakes, and go fast.
It’s how manufacturers and all the car reviewers do their 0-60 times (for automatics, at least) You’ll never get close to the published times without doing it. But yeah, max acceleration runs are never about being environmentally friendly.
Along with Edelweiss’ excellent explanation, which pretty much covers the likely scenario, I have one more;
There can also be slight movement or the perception of weight transfer on a vehicle with poor maintenance due to engine and transmission movement caused by torque converter engagement. Especially so with vehicles that do not have decent quality engine or transmission/torque strut mounts or those that are beginning to fail or have. Just a possibility, but with a vehicle so young your mounts should be good and this should not happen. Seems to be more noticeable on smaller vehicles rather than larger ones. If you want to see it in action, have someone sit in the the car and shift while you look under the hood. The engine will move (if bad mounts, much more than slightly, you can feel the entire vehicle move a little if really bad) this is from the torque converter being engaged, when it moves it can give the sensation that the car has a weight shift or ‘transfer’. Probably not the same thing but can’t rule it out either.
Also, “throttling” the transmission (hitting the accelerator with another foot on the brake) can actually be detrimental to the transmission as it creates excess heat. Too much heat breaks down ATF, when it brakes down it turns to varnish and sludge and a thin remainder stays around. This can cause transmission components (bands) to slip. It’s not wise to do it very often. Same reason why vehicles need their trans fluid changed out (not ‘FLUSHED’, just a drain, drop pan, clean and change filter, refill). Because heat will break the fluid down over time, overheat it more often and it will break down faster.
Isn’t it the torque converter that’s getting the heat? The transmission is locked–nothing is slipping, so while it’s undergoing significant internal forces, there should not be much heat. All of the slippage is in the torque converter, with the engine power dissipated via viscous friction in the fluid. Oil has pretty decent heat capacity, but there’s a lot of power here, and I don’t think you’d want to be in a launch control situation for more than a few seconds at a time in a traditional automatic.
The dipping of a vehicle’s front end when decelerating does not cause weight transfer; it’s the other way around. Weight transfer is also not a misnomer: under forward acceleration, some of the vehicle’s weight really is transferred from the front wheels to the rear wheels, and under rearward acceleration (i.e. braking), some of the vehicle’s weight really is transferred from the rear wheels to the front wheels. It’s the reason that sportbikes will happily put their front wheel in the air when you whack the throttle open, transferring 100% of their weight to the rear wheel, and the reason they will go ass-over-teakettle if they hit the front brake too hard. As many a bicycle rider can attest, a vehicle with no suspension whatsoever (i.e. zero dive) will still transfer weight to its front wheel(s) if it decelerates using its own tire traction.
Here’s a few pics of a Q50 (not mine), showing that the front and rear calipers are both mounted forward of the axle. The angular mounting position of the caliper can’t possibly affect suspension behavior. During braking, the caliper produces a force on the rotor that is exactly countered by a radial force from the wheel bearing. This produces a net torque on the rotor/wheel, but zero net force on the suspension during the stationary exercise I describe in my OP.
The anti-dive feature you mention is due to the tractive force exerted by the road on the tire tread. Many BMW motorcycles have a "Telelever’ front suspension design that does exactly this; the rearward force from the front tire tends to extend the fork sliders, cancelling the weight transfer’s tendency to compress the fork sliders. Weight transfer still happens as the bike decelerates - physics dictates that it must - but the Telelever front suspension remains near the middle of its range of travel for any bumps it may encounter.
I don’t doubt that cars may be designed with similar anti-dive features in their suspension, but it won’t have anything to do with the angular mounting positions of the brake calipers. A [for the rear wheels would exhibit the behaviors we are discussing: tractive forces at the tire contact patch during vehicle deceleration would tend to compress the suspension, preventing the rear end from rising during braking; and torque applied at the wheel axle by the driveline would cause it to squat during the exercise I describe in my OP. But it appears the Q50 does not have a trailing-link rear suspension: [url=https://cars.usnews.com/cars-trucks/infiniti/q50/2019/specs/q50-3.0t-luxe-awd-400441]it has a multi-link](trailing-arm suspension[/url) suspension. Pics of multi-link rear suspensions are so complicated I can’t sort out what the heck I’m looking at.
To folks who think this is hard on the brakes or the drivetrain:
A) the drivetrain is designed to tolerate loads due to wide-open throttle. That’s a lot of torque. In the example I described in my OP, I’m just giving the accelerator a gentle nudge, and only for a brief second. The torque converter is indeed the thing that takes up the slip in this situation. Heat is produced above and beyond that which is generated while idling at a stop with the transmission in drive, but again, only for a second, and only a trifling amount. Modern cars can tolerate ascents up long mountain grades with the torque converter passing high loads in slip mode, but generally speaking, the transmission doesn’t get dangerously hot unless you’re towing a trailer up a long, steep grade with the torque converter slipping the whole time. I have memories of family vacations in a big ol’ station wagon with a pop-up trailer in the mountains. My dad installed a transmission oil temp gauge so we could keep an eye on things. We could grind up a steep mountain pass for fifteen or twenty minutes before the transmission fluid got hot enough to cause concern.
B) the brakes are designed to generate/tolerate forces strong enough to cause the tires to slip on clean, dry pavement. In the example I described in my OP, there is no movement of the brake rotor through the caliper, so no heat is generated in the brake pads/rotors at all. And again, with a brief, gentle blip on the throttle, brake caliper forces are relatively tiny.
I knew a guy who had a highly-modified Camaro as a track-only drag vehicle. As you note, the trans had a stall speed of around 4K RPM. A few seconds before launch, he did two things:
#1: pressed-and-held a button that kept the transmission in drive, *but simultaneously engaged reverse, preventing any power transmission to the driveshaft. This same button also activated a rev limiter that kicked in at 3900 RPM.
#2: floored the accelerator.
Under these conditions, the engine stumbled and rumbled at 3900 RPM, modest torque was being transmitted through the torque converter, and the vehicle remained stopped because the transmission was in drive and reverse at the same time.
When the light turned green, he released the button. The transmission disengaged reverse and deactivated the rev limiter. And away he went with all haste.
Knowing now that my car has multi-link rear suspension, I guess I still don’t see an answer.
Trailing-arm suspension, I can understand: the carrier is fixed to the trailing arm, and there’s a single pivot axis at the front of that trailing arm, which really makes analysis simple. When the wheel hub is locked to the carrier by a clamped brake caliper, torque delivered via the driveshaft will try to rotate the hub/rotor, which tries to rotate the carrier, which rotates the trailing arm and causes the suspension to compress. Traction forces at the contact patch generated during braking/decel will likewise exhibit a torque about the trailing arm’s pivot point, tending to compress the suspension (and countering the tendency of the rear end to rise during braking decel).
But I can’t wrap my head around how driveline torque makes a multi-link suspension squat when the brake caliper has the rotor locked up, or how contact patch traction forces could cause multi-link suspension to squat. The kinematics of multi-link suspension are just a bit too complicated; there are pivot points all over the place at a bunch of different angles.
It’s all rubber.
The drag Camaro that you describe had a trans brake, and the rev-limiter is commonly referred to as a “two-step” rev-limiter, although strictly speaking it’s not necessary with a high stall torque converter (aka a “stall converter”) and he may not ever have one. All torque converters have a stall speed (which changes with engine torque) – stand on the brake and gas at the same time (colloquially known as “brake boosting”) and the engine RPM is going to hit a certain point and stay there, that’s the stall speed of the torque converter. Drag racers want it to stall higher so the engine launches in its power band, but you can do that mechanically based on the torque converter fin design rather than with an electronically controlled ignition. He may have had both, though, but two-steps are a lot more common with manual transmissions (and cars with turbos).
Anyway, here’s a great video of a Mustang launching from a trans-brake, exhibiting the exact squat you describe but quite exaggerated due to the power.
When you brake boost like that, power is sent to the rear differential, which is mounted to the unibody in a diff carrier that also likely provides all of the mounting points for the rear suspension. The power from the driveshaft is going to try to rotate the differential, which is going to try to rotate the diff carrier, which is going to try to twist the unibody. There’s some springiness to the steel, but most of that twisting force is going to go straight into the rubber bushings that hold the diff carrier to the unibody. That’s going to torque the suspension some, which is also held to the diff carrier with rubber bushings.
Whether or not the car squats or lifts based on this twisting motion and the energy stored in the rubber bushings depends on suspension design – control arm mountings, sway bars, etc can all be asymmetrical to achieve the desired effect. I’m not a suspension engineer so that’s about all I understand about that, but I know there’s a lot of products on the market for drag cars to adjust the squat level. In any case, what’s happening then is that the load is transferred from the rubber to the springs, and in the video you can see the springs actually bounce as the load hits them.
The bushings in question are small, diff bushings are maybe 3" diameter and control arm bushings smaller than that, but everything in the suspension is a lever, so a quarter inch of compression gets magnified.
And fun fact, this is less of an issue in cars with torque tubes, like the Corvette or the Porsche 928, because the transaxle can be hard mounted to the torque tube and therefore any rubber bushings don’t need to handle driveshaft twist.
