Check out this video (indexed to t=23s) of an Ocelot Force Protection Vehicle experiencing a mechanical failure during a braking test. Under heavy braking, the rear tires lock up and the axle begins to bounce; the amplitude of the bounce becomes so severe that the tires finally develop enough traction (when slamming hard against the pavement) to rip the whole axle assembly free of the vehicle.
An analogous phenomenon sometimes happens on front-wheel-drive cars under acceleration, provided the engine has enough torque to break the front tires loose and get them spinning. My own car (2003 Maxima) does this occasionally. I always ease up on the gas (or push the clutch in) when it starts to happen; I’ve wondered what would happen if I kept my foot to the floor, and now I guess I know.
Interestingly, my car doesn’t do this all the time. Snow or gravel, for example, will allow the tires to spin smoothly without incident.
Finally, my question: Given that this behavior does not occur under all traction conditions, what are the relevant factors that determine whether or not this sort of violent suspension chatter/hop is going to happen?
When the wheels start to skid, they “skip” slightly along the road surface. That vehicle presumably has a live axle on leaf springs or airbags, thus nothing to locate the axle in the vertical plane. So when the tires start skipping along the road surface, the high unsprung mass of the wheels + axle amplifies this skipping into what you see in that video. Inadequate damping by the shocks can also play a role.
Some newer pickup trucks have staggered shocks and/or shocks mounted as far outboard as possible to try to reduce wheel hop.
Would traction control cause something similar to this? My Mustang has it and if I try to do a burnout with it enabled it causes the rear end to judder pretty badly, I assume because the traction control is pumping the rear brakes in an effort to get traction (which is why I don’t normally do this…it sounds awful and plus I can do a clean, smooth burnout on demand just by turning the thing off).
That just looks to me like a crappy ABS and the suspension basically reacting to the violent on/off forces* - I don’t think it’s braking per se. I am anything but a suspension expert though…
As to the OP, are you absolutely sure it’s not your traction control kicking in? The only “interesting” thing I can think of under accelleration that only hits FWD cars is torque steer which is nothing to do with this.
*Appreciate that the wheels look as though they are locked but that can easily be for a similar reason to the way some helicopters when videoed by these types of cameras look as though their rotor blades are stationary
What’s that rythmic banging after the wheels go south? Is the engine fried as well?
Last month I had what sounded like wheel hop and felt like it on the steering wheel in my FWD automatic with traction control.
I had backed down a driveway into the street and had cut the wheel hard right. I put the car into drive after stopping and gave it gas only after it started crawling forward (I’m on a hyper-mileage kick). The right-front wheel went noisily nuts‚ bangbangbangbangbangbangbang.
Before that, the car sometimes would jerk forward after about two seconds in drive from neutral, after crawling forward at an engine idle without the steering turned either way.
It turned out to be a transmission pressure valve they replaced on warranty.
Wheel hop happens in nearly every FWD car if you accelerate hard enough.
When you hit the gas hard, the wheels(or wheel) grip and move forward in the wheel well, changing the alignment(toe) of the wheels. This causes a loss of grip, and the wheels spin. During the wheel spin, the wheel moves back to it’s original position. The car regains grip and the process starts over.
If you have a car with a complex racing suspension with extra trailing arms and hard suspension bushings, this doesn’t happen.
OK, so it’s primarily a problem of undamped elasticity in the suspension linkages. I understand mass-produced passenger cars typically have relatively soft rubber bushings in the suspension pivots so as to provide a comfortable ride, so I guess that’s not surprising that it can occur in most of those cars, but can be eliminated in race cars with hard bushings.
So reduced-traction situations (e.g. snow, gravel) don’t cause this problem because there’s not enough tractive force to sufficiently distort the suspension in the manner you describe (or at least not enough to develop that horrible hopping)?
Absolutely sure: my car is not equipped with traction control or electronic stability control. If I put my foot to the floor in low-traction conditions, I can spin the front wheels until the cows come home.
Leaf sprung axles can do this when the accel or decel forces twist the spring in to a slight “S” shape. The spring deforms, and then breaks traction and unloads. I’ve seen jeeps bust pinion shafts this way. Its usually worst when the spring is very flexible and mounted far from the centerline of the axle, (think lifted 4x4s.) Linkage suspensions tend to be more rigid and won’t wrap as much a leaves, well at least properly designed ones. The one in the video looks like a heavy truck on some stout leaves.
Here is a crummy videofrom inside a Maxima during wheel hop, it can be a violent shake. Apparently Maximas are known for this, there are a couple videos like that on youtube.
What happens in the OP’s video is probably more like what 60wag is describing.
It all depends on how hard Machine Elf is accelerating, and how worn his suspension bushings and struts are. The more the suspension geometry deforms, the worse the wheel hop.
Nope, my experience is pretty much what is depicted in ExcitedIdiot’s video: the whole car violently shudders as shown, and if I kept my foot on the accelerator I’m pretty sure I’d eventually break something in the driveline. Definitely not hitting the rev limiter, it happens way before the RPM’s get that high.
Relevant factors: My car has 107K miles on it, so suspension bushings are likely pretty worn. Tires are Goodyear Eagle F1 GS-D3’s, which are really grippy (but sometimes not quite enough to maintain static grip at WOT in first gear). As discussed upthread, this sounds like a reliable recipe for inducing hop.
By the end of the video the banging is very slow and steady, but not stopped. The frequency is too low for a crankshaft (I’m guessing it idles around 500-1000 RPM); I think the sound might be from the remnants of the driveshaft flopping around slowly at the back of the transmission (in first gear) as the engine goes to idle RPM.
And why do dirt roads get that accordion pattern that is so consistent? There are many different vehicles with different suspensions using the road but distance between the peaks remains the same.
It’s called “washboarding” and it has its own wikipedia page:
Experts argue over its exact causes, but the best explanation I’ve heard is that you start with just one bit that’s slightly uneven, and that causes your tires (and the car) to pop up a bit, and when they come back down they make a dent and push some of the dirt forward, making another bump, which causes subsequent cars to hop over that, making an endlessly repeating series of dents and bumps. The reason (supposedly) that the pattern is so consistent is that while suspensions differ quite a bit, most cars weigh about the same amount, and so at a particular speed they will rise up and fall roughly the same distance regardless of their suspension. This makes all of the bumps and dents end up in roughly the same place, so over time the washboard effect gets more and more pronounced.
Like I said though, folks tend to argue about this and I doubt you’ll find a definitive consensus anywhere about it.
