Recently, I grossly misjudged (by 50%) the weight of some building supplies I put in the trunk of my '98 Ford Contour. What clued me in was the handling…it wanted to “ground loop” (swap ends) like a Piper Cub in a crosswind! I (very, very briefly) relaxed my grip on the wheel and confirmed it was clearly divergent in stability.
Later, I decided to peruse the 'net and my owner’s manual. The net gave me the “typical” weight (but not the F/R split), but the owner’s manual says nothing about cg location in any dimension other than to comply with GVWR and GAWR.
Does this mean that ALL factory-spec cars are safe at all loadings that don’t exceed these parameters, or is this a “caveat emptor” thing? Also, how much does spring compression change the weight loading over and above the mathematical caculations of weightXmoment?
GVWR = Gross Vehicle Weight Rating: This is the design maximum weight the vehicle should weigh. Exceed this value and even though cars are over designed, you are in uncharted territory and no one has tested to find out what will happen if you do.
GAWR = Gross Axle Weight Rating: This is the design maximum weight the AXLE can carry. So there will be 2 values - front and rear.
Add the 2 GAWR’s and they will be more than the GVWR - meaning the axles can carry more than the whole vehicle.
Don’t think in terms of “safe”. (Nothing is 100% “safe”.) Think in terms of the inverse - “risk”. By staying within the design parameters (GAWR’s and the GVWR), your risk is low (this area has been explored and those things that were found that could be improved, were.)
But even if you don’t exceed the GAWR’s, you still can have situations where you’ve increased the risk. Try to center the load you are carrying. The worst possible situation is hanging a load at the end of the vehicle (like hitching up a trailer). That not only loads the rear, but unloads the front. This will change the oversteer / understeer ballance and could lead to some strange handling (like you experienced.)
The Contour is a bad pickup truck and the weight you do have in it has to be evenly distributed. You can’t have it all in the trunk. You aren’t the worst offender. I saw a Dodge Colt with two pickup loads of wood on the back. It collapsed before leaving the parking lot. He had good balance. There were seven bags of cement in the car.
Aside from mathematical and scientific jargon and calculations, there are two rules for loading ANY vehicle or trailer.
One is check your owners manual or door post for max axle gross weights allowed and,
two: always load 60%-40%. In other words, load 60% of the load weight front of center of axle(s) and 40% rear of same.
This loading manner eliminates ‘fish-tailing’ which will put you in a “HOLY CRAP!” situation in a hurry.
Keep in mind that loading the 60% toward the front of a trailer MAY overload your vehicles rear axle.
When in doubt, ALWAYS take the unit across a weigh scale. There are very good reasons for this as any police officer will attest to.