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I am glad that you are aware of the difficulties with truck/SUV chassis and suspension. It gladdens me that at least one person won't become a statistic.
I know I'm probably preaching to the converted, but there's a few things I want to get out to anyone who drives a truck/SUV.
First of all, there's the issue of weight. This is a double-edged sword; Initial starting traction is significantly increased because of increased static friction on the wheels. This means that in inclement weather, trucks will generally get off the line a little quicker. This traction bonus is lost almost immediately, since the tires are now relying on dynamic friction. The weight of the vehicle plays less into this particular equation, depending on the substrate.
Weight will also make a truck feel more solid while at speed. This is because it takes more energy to change the direction of a truck, since momentum dictates that the truck "wants" to stay in a straight line. This is the transient response that I was talking about earlier. Due to the increased mass, the truck's directional VECTOR will tend towards the direction of travel. Which means that when you tweak the steering wheel, it will resist turning. Many people equate this with high-speed stability. To some extent this is true, but bear in mind that this means the tires have to work extra hard to change the travel vector.
For those with no prior physics, a vector is simply a combined statement that includes a direction (say, southbound on I-95) and velocity (60mph).
The problem is when you have inclement weather and reduced traction. The weight balance of most trucks is less than ideal. I feel that the reason so many people end up in the ditch has to do with the fact that they rely too much on the supposed stability. When something goes wrong (brake too hard for a deer) you throw a whole bunch of new forces and variables. On a straight highway, even when iced, it's very rare to find even traction across the highway. "Slippery" can mean almost zero friction on the polished "ruts" and a frosty coating on the centerline.
If you brake with one foot on the polish, and one on the frost, you have a considerable traction differential. On a truck with a LOT of forward momentum, this differential will cause the truck to ROTATE, since it can't overcome the inertia to make the truck change directions. Remember, that force has magnitude and direction. So does a travel vector. In this particular scenario, you still have the same vector; You're still going southbound on I-95 at 60mph... Only now, you're facing sideways.
On my little dodge colt this makes the car lurch towards the side with more traction. The car is light, so the force needed to make it change direction is several orders of magnitude smaller. There is some rotation of course. for argument's sake, we'll say that my colt has rotated 20º, and the truck is now at 45º rotation and in a full 4-wheel slide, at 60mph.
In either car, this is a scary proposition. The vast majority of people at this point will try and correct the skid.
with a FWD car, this is a simple matter of pointing the wheels in the direction you want to go, and giving it some gas, keeping your inputs at about 1/3 of what you think you would need.
RWD is a little trickier, since you have to steer out of the skid, you can't just power out of it. Steering into the skid, keeping gas static, being careful not to drop or jerk the throttle. The idea is to get one set of wheels beck in line with the direction of travel, then let the rest of the car settle into it.
I'm going to venture a guess and say that in an emergency situation, 95-99% of the drivers on the road will steer instictively, but either hit the brakes or let go of the gas. Magazines rave about throttle-response in vehicles, but nobody mentions the role that accurate throttle will play in skid control. Especially in RWD cars.
Now the weight poses a second problem. Since the car is heavier, it will have more rotational (around the vertical axis, in a skid) mass also. This means that once it starts spinning, it will want to KEEP spinning. It's very tricky to put enough counter-rotational energy into a skid to stop it. But then you have to worry about adding too much correction, and having the tail whip back hard enough to completely reverse the skid.
Given the skidmarks that I get to see while passing these guys perched on their hoods calling the towtruck, this is what seems to happen every time. They will start skidding in one direction, over-correct, and end up in the ditch opposite to the original skid. All the cases I saw showed the telltale striped skidmarks of ABS brakes pulsing the tires, or else a flat polished skidmark from start to finish.
Now, it gets even more complicated in 4x4 terminology. The major problem with 4x4 at any kind of speed is that the front and back driveshafts tend to be locked together (unless you're lucky enough to have a viscous coupled AWD setup... Not that common in trucks). This means that your capacity for skid correction is reduced, because both front and rear wheels are turning at the same rate.
Especially in the case of extremely poor traction, this means that all four wheels have to "catch" at the same time to stop the slide. If the car is at all yawed over, all four wheels are relying on the drag-coefficient of the rubber vs. the ice. Anyone who's driven on ice will know that with the wheels turning in the direction of travel, you have a LOT more traction than if you lock up.
With one set of wheels turning freely, they will "grab" anytime they are moving at the same rate and direction of travel. The only thing you can do is keep the wheels pointed in the direction you want to go and give it some gas. The hope is that the thrust from the engine will eventually point the truck in the right direction. Given that most of these situations occurs because something bad is happening up ahead, this rarely happens.
The 4x4 spinouts I saw had similar characteristics, except that the skid they started tended to continue until they hit the ditch. Best I saw was a full 540º. Part of the problem I think is that people assume that 4-Hi on most trucks is a "whenever I feel like it" mode. You can't do it on concrete because you'll wreck your differential. But they do it on Ice because it helps them off the line faster. Most 4x4 manuals warn about taking it over 70kph (40mph?) citing drivetrain damage. Nevermind the fact that you lost whatever cornering stability you had since you will be essentially gripping with one tire only; the outside rear tire. The rest are trying to turn at a different rate.
*pant*
*pant*
*pant*
I think I'm about done my rant. Anyone else at work, angry, and bored?
I know I'm probably preaching to the converted, but there's a few things I want to get out to anyone who drives a truck/SUV.
First of all, there's the issue of weight. This is a double-edged sword; Initial starting traction is significantly increased because of increased static friction on the wheels. This means that in inclement weather, trucks will generally get off the line a little quicker. This traction bonus is lost almost immediately, since the tires are now relying on dynamic friction. The weight of the vehicle plays less into this particular equation, depending on the substrate.
Weight will also make a truck feel more solid while at speed. This is because it takes more energy to change the direction of a truck, since momentum dictates that the truck "wants" to stay in a straight line. This is the transient response that I was talking about earlier. Due to the increased mass, the truck's directional VECTOR will tend towards the direction of travel. Which means that when you tweak the steering wheel, it will resist turning. Many people equate this with high-speed stability. To some extent this is true, but bear in mind that this means the tires have to work extra hard to change the travel vector.
For those with no prior physics, a vector is simply a combined statement that includes a direction (say, southbound on I-95) and velocity (60mph).
The problem is when you have inclement weather and reduced traction. The weight balance of most trucks is less than ideal. I feel that the reason so many people end up in the ditch has to do with the fact that they rely too much on the supposed stability. When something goes wrong (brake too hard for a deer) you throw a whole bunch of new forces and variables. On a straight highway, even when iced, it's very rare to find even traction across the highway. "Slippery" can mean almost zero friction on the polished "ruts" and a frosty coating on the centerline.
If you brake with one foot on the polish, and one on the frost, you have a considerable traction differential. On a truck with a LOT of forward momentum, this differential will cause the truck to ROTATE, since it can't overcome the inertia to make the truck change directions. Remember, that force has magnitude and direction. So does a travel vector. In this particular scenario, you still have the same vector; You're still going southbound on I-95 at 60mph... Only now, you're facing sideways.
On my little dodge colt this makes the car lurch towards the side with more traction. The car is light, so the force needed to make it change direction is several orders of magnitude smaller. There is some rotation of course. for argument's sake, we'll say that my colt has rotated 20º, and the truck is now at 45º rotation and in a full 4-wheel slide, at 60mph.
In either car, this is a scary proposition. The vast majority of people at this point will try and correct the skid.
with a FWD car, this is a simple matter of pointing the wheels in the direction you want to go, and giving it some gas, keeping your inputs at about 1/3 of what you think you would need.
RWD is a little trickier, since you have to steer out of the skid, you can't just power out of it. Steering into the skid, keeping gas static, being careful not to drop or jerk the throttle. The idea is to get one set of wheels beck in line with the direction of travel, then let the rest of the car settle into it.
I'm going to venture a guess and say that in an emergency situation, 95-99% of the drivers on the road will steer instictively, but either hit the brakes or let go of the gas. Magazines rave about throttle-response in vehicles, but nobody mentions the role that accurate throttle will play in skid control. Especially in RWD cars.
Now the weight poses a second problem. Since the car is heavier, it will have more rotational (around the vertical axis, in a skid) mass also. This means that once it starts spinning, it will want to KEEP spinning. It's very tricky to put enough counter-rotational energy into a skid to stop it. But then you have to worry about adding too much correction, and having the tail whip back hard enough to completely reverse the skid.
Given the skidmarks that I get to see while passing these guys perched on their hoods calling the towtruck, this is what seems to happen every time. They will start skidding in one direction, over-correct, and end up in the ditch opposite to the original skid. All the cases I saw showed the telltale striped skidmarks of ABS brakes pulsing the tires, or else a flat polished skidmark from start to finish.
Now, it gets even more complicated in 4x4 terminology. The major problem with 4x4 at any kind of speed is that the front and back driveshafts tend to be locked together (unless you're lucky enough to have a viscous coupled AWD setup... Not that common in trucks). This means that your capacity for skid correction is reduced, because both front and rear wheels are turning at the same rate.
Especially in the case of extremely poor traction, this means that all four wheels have to "catch" at the same time to stop the slide. If the car is at all yawed over, all four wheels are relying on the drag-coefficient of the rubber vs. the ice. Anyone who's driven on ice will know that with the wheels turning in the direction of travel, you have a LOT more traction than if you lock up.
With one set of wheels turning freely, they will "grab" anytime they are moving at the same rate and direction of travel. The only thing you can do is keep the wheels pointed in the direction you want to go and give it some gas. The hope is that the thrust from the engine will eventually point the truck in the right direction. Given that most of these situations occurs because something bad is happening up ahead, this rarely happens.
The 4x4 spinouts I saw had similar characteristics, except that the skid they started tended to continue until they hit the ditch. Best I saw was a full 540º. Part of the problem I think is that people assume that 4-Hi on most trucks is a "whenever I feel like it" mode. You can't do it on concrete because you'll wreck your differential. But they do it on Ice because it helps them off the line faster. Most 4x4 manuals warn about taking it over 70kph (40mph?) citing drivetrain damage. Nevermind the fact that you lost whatever cornering stability you had since you will be essentially gripping with one tire only; the outside rear tire. The rest are trying to turn at a different rate.
*pant*
*pant*
*pant*
I think I'm about done my rant. Anyone else at work, angry, and bored?
. my uncle had a rwd aerostar, and the camper he was pulling about ripped the hitch off, i dunno how, it was a simple pop up camper and was within the load limit. you just cant beat a frame mounted hitch.
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