Adding weight to a vehicle


#1

Well, it’s snowy season again, and those of us driving pickups are considering adding weight to aid in traction/handling.

The conventional wisdom is to add weight directly over the drive axle. However, my experience in physics class, doing “weight and balance” calculations for flying, and time on a seesaw tell me that’s bull.

Basically, changing the CG of a vehicle is like a seesaw: you can balance a big weight with a small one, provided the small weight is sufficiently far from the pivot point.

To relate this to a vehicle, say you’ve got a 4000# vehicle with a 60/40 weight distribution that you’d like to get to 50/50. You could put 800# right on the axle, which leaves the front axle unchanged, and and ups the rear to a matching 2400. Alternatively, you could put 267# one wheelbase BEHIND the rear axle (granted you’d need a big bed!) which would transfer 267# rearward, to balance at 2133# on each axle…saving you 533# of dead weight. (Heck, construct an infinietly strong, infinitely light boom and place one pound a half mile back…)

So, what’s the downside? The only thing I can think of is polar moment of inertia, which would make it minimally harder to enter a spin (and minimally harder to recover if you did manage it). I’m not sold that PMI is relevant outside the race track; heck, if we gave two hoots about it, we’d all be driving mid-engined rides. (It should be noted that in aviation, where spins are a BFD, ballast is still added as far forward or aft as practical.)

Heck, I think the sloshing effect of half a tank of fuel is about a realistic safety issuse as PMI variances!

So, what do you think: ballast over the axle, or as far back as it’ll comfotably fit?


#2

The potential downside of putting weight behind (not on top of) the rear axle in pickup is that it will reduce the downforce on the front wheels, possibly affecting front end traction.

That’s why you usually read instructions saying to put the weight directly over the rear axle, because it neither overloads nor underloads the front end.


#3

In addition to what jesmed stated, the OP should think about, “the pendulum effect”.
Placing excess weight all the way in the rear has the potential to make any loss of traction in the rear into…an exciting event…with the rear end swaying out of control.


#4

+1 to both jesmed and VDC’s comments.

Also, be sure you secure the weight load very well, such that it doesn’t become a projectile in the event of an accident. People have been killed from things coming through the rear window.


#5

Here’s another ice driving situation that nobody thinks of or realizes untill it’s too late.
And there’s not much you can do about it except put it in neutral.

– brake bias –

The wrong brake bias can explain the un-drivability of many vehicles in icy situations and yet it’s often never diagnosed as such.

Even with weight in the bed of my 79 Chevy pickup 2wd…
The fronts can be locked up yet the rear is still turning, pushing, and rolling.
In the past I have been completely stopped…front wheels not moving …yet the rear is still turning, causing the truck to walk sideways on the ice.

Putting it in neutral is the only answer.
Thinking ahead, you put it in neutral to properly stop in the first place with out the threat of the rears pushing uncontrolled.


#6

Put the weight directly over the rear axle or closer to the center of gravity. Weight in the extreme rear increases the “polar moment of inertia”, and as mentioned, this pendulum affect is bad for spinning out of control. The Corvair had its engine hanging way out and spun out easily.


#7

^

Doc–Just imagine if the Tucker 48 had gone into series production, with its massive H-6 engine (372 ft lbs of torque!) hanging out behind the rear wheels.

A skilled driver would have probably been able to deal with its handling, once he got used to its very pronounced oversteer, but imagine if a little old lady had to suddenly deal with the rear end of that big sedan swinging out on her. That car would have been a bear to deal with in low-traction situations.


#8

In the years just before WW II, Mercedes, Alfa Romeo, and Auto Union were the top 3 contenders on European racing circuits. While the Mercedes & Alfa racers were conventional front engine/RWD cars, the Auto Union racers were rear engine/RWD, and the rear placement of their TRULY massive (V-12, IIRC) engines gave them awful weight distribution and oversteer that was–literally–deadly.

After losing a lot of their drivers to one-car accidents resulting from that oversteer, Auto Union decided to hire only motorcycle race drivers, and to retrain them to drive the unconventional Auto Union race cars.

It seems that even very experienced race car drivers who had always driven conventional cars couldn’t be retrained sufficiently to stay alive in the AU cars, but if the factory people began from square one with motorcycle drivers, those men were able to deal with the incredible oversteer of the AU cars and not be killed by them.


#9

Alternatively, you could just get 4WD pickup and not worry about this sort of thing :slight_smile:


#10

A couple of things. Real good points about the weight behind the rear axle increasing the yaw rate to extreme by going too much outside the axis and lightening the front.

The idea of adding weight correctly is two fold. First, to gain traction in the rear wheels which even 4 wd needs and secondly to balance handling, which a 4 wd pick needs as well. Getting weight 50 / 50 is better for handling but I have always strived for near 45/55 to the rear with weight just forward of the rear axle. The handling is still good and the tendency to loose the rear end is lessened. You can’t drift through a corner as fast, but that is never the intent of a truck.

A slight bias ( not as much as VDC interestingly described) to the rear i feel is safest and gives you better traction overall for acceleration, climbing and taking corners with the power on up hills. Living on steep hills even with 4wd Is made easier. So 4 wd trucks need added weight in the rear nearly as much as 2 wd. It’s a fallacy that they don’t. Even in the summer, when unloaded, I still kept a 70 lb bag of tube sand or two in he back, then removed them when carrying loads.
So, unofficially, because I see it was in jest, I will disagree with @fodaddy. ;=) Think of the added front diffential alone adding too much weight to an unloaded 4wd pick up when the 2 wd version is poor in front weight bias to begin with.


#11

^
Yup!
Pickup trucks–whether RWD or 4WD–tend to have very light rear ends when unladen.
So, adding some weight above the rear axle is a good idea in the winter with any pickup truck.


#12

Even in good weather, unladen 4 wd pick ups can handle very poorly with rwd and so much front bias. Traction and stability control help, but there is nothing like a balanced chassis to begin with. It may hurt mileage a little, but it’s always helpful both in ride and handling to have a little extra weight added back there.


#13

I agree with the physics involved here. When I was 16 I drove a 2WD 1974 F-100 with a 4bbl 390 through unplowed roads during the blizzard of 1996 (blizzard being somewhat relative as I’m in central VA), and while I didn’t get stuck or anything, it wasn’t the ideal vehicle to drive in such conditions. These days, I have an old 1997 F-150 4WD that sees maybe 500 miles a year of use. When it snows I don’t even bother putting anything in the back, the 4WD and BFG A/T tires are enough to get me through anything I’ve encountered since owning the trying. Adding some weight to the back probably wouldn’t be bad thing, but it’s unnecessary in my experiences.


#14

@fodaddy
Around here, every rwd vehicle is enhanced by adding weight and wearing snow tires. I tried an experiment some years ago. I drove my Suzuki Sidekick to pick up tube sand in 70 lb bags for my pick up truck. At the VIP store, I loaded over 400 lbs in the back. It squatted some, but I drove back in 2wd with AT tires without spinning a tire. When I drove out I needed 4 wd engaged. I drove in 2 wd back out to the end of our mile and a half road just to check the differnce again. 2wd with all that weight, went as well, easier in some areas, as 4 wd without. Now, my Zuki wouldn’t like that diet every day, but the extra weight was surprisingly effective . Any one who experiments with added weight, sees immediate advantages…in rwd, 4 wd.
I even throw a bag or two in the back of my wife’s RAV Awd, and it helps when it’s really bad.


#15

You add the weight above the rear axle or slightly forward.

Weight that’s added to the rear of the axle can and does cause a whipping of the rear end. I’ve experienced it a couple of times. Yes…adding weight to the rear of the axle will add more overall weight to the axle…but it can cause whipping. So just add MORE weight right over the axle instead.


#16

I found mother nature the perfect solution. With an open bed because if the wind patterns my bed would fill up with snow, Enough weight to provide decent traction. In town driving the snow would eventually be gone by the time I did not need the weight.


#17

I always add weight in the form of tube sand, between the axle and seat or cab at in the 4Runner, same for the RAV and trucks but stacked vertically so it stretches to the axle area. That said, the 4 Runner is so good in snow anyway with the cab weight, I stopped doing that… Throw a tarp over it and just throw loads on top of everything. Always had a cap or cover on trucks so the snow idea never appealed to me. Letting it fill with snow just gives you a two passenger car that gets 18 miles per gallon.

If you tow any thing substantial with a heavy tung weight, it’s worth adding more weight up by the cab, as far forward as possible to help handling if it’s not a load distribution hitch.


#18

I’ve not bothered trying to add weight in my car in the 3 years I’ve had it; mostly because it’s an FWD biased part-time AWD setup. Though, I can get the rear to “kick out” a bit if I apply enough throttle as I’m going around a corner. I only do that when I feel understeer kicking in so I’m not heading straight when I want to turn; a bit of opposite lock and I can get her righted.

The snow in the truck bed idea is nice, if you get enough snow to fill the bed up good enough. I think the worst we got in my area was maybe 6~8 inches one day. I know the drift in my old driveway going to the garage was up to my knees, but that part of the driveway was essentially a wind tunnel(my house, detached garage and neighbor’s house were really close together). A few occasions the unplowed parts of town had me scraping the top of the snow with the bottom of my Civic. Since I’ve bought my CX-7 I’ve not had to even shovel my (current)driveway because it has much more ground clearance.


#19

I had good luck with four wheels and tires, three pallets and two bags of sand in back of my 1974 Ford 150 with 8 ft. bed. It was about as evenly distributed as you could get with four wheels… I never got stuck.


#20

Hear something scary ? At work several years ago after the first snow storm a lady who was a fellow teacher made this comment in the teacher’s room. I heard you guys talking about adding weight to the back of your trucks so I tried it in my car. It didn’t help at all. It was terrible. After a little questioning, we forund out she had all season tires and…fwd.

@bscar2 yes, adding weight to the rear of any Awd, even with fwd bias, helps. In the new ones, as much as 50 to 90 percent of the torque can be shifted to the rear. At that point, balance weight distribution really helps. Like I said. Wife’s car is that way in her RAV. It helps a lot. We just do it selectively when our road is iced up or deep snow and leave the bags at the end of the garage normally.