View Full Version : the debate about weight in the trunk

02-08-2006, 03:26 AM
If this is in the wrong thread then just move it, wasnt sure where it belongs.

I noticed in the jegs catalog they have a bladder you fill with water, it goes in the trunk and is a few hundred pounds. They say its for winter traction.This will be new to you california guys but northeasters put sand bags in their trunks for winter grip.

I know this helps put weight on the rear wheels and helps you get low speed traction in the snow, but doesnt it have a negative efect as the speed of the vehicle goes up?

Going around a corner doesnt the tail of the car have more weight pushing it towards the outside of the corner?

Under braking, isnt it just more weight that wants to stay in motion, even making the rear more likely to try and pass the front?

I am hoping for a scientific answer, not the " I do it and Ive never had a problem" response.

02-08-2006, 03:47 AM
On a similar note, this should help address the "light in the rear" comment as to why camaros and mustangs end up backwards into a tree so often. Is it weight distribution or a combination horsepower/posi/highspeed/drivererror.

A couple of times I have seen accidents where a camaro or mustang left the road at high rate of speed at three in the morning (speed?alcohol?) and someone in the peanut gallery throws out the "light in the rear" comment.

My feeling is that in both the cases as the speed goes up lighter is better.A trunk full of crap will help you get rolling but as the speed increases the advantage becomes a disadvantage.
I have never seen a driver crawl from a wrecked race driver and say "I spun out, not enough weight in the trunk" I have never seen weight added to help a car handle (required ballast can be strategically placed, thats a different story)

I put both these questions here in the hopes of a scientific answer.

David Pozzi
02-08-2006, 10:21 PM
There was a similar discussion in the Suspension section a while back. An awful lot depends on what car and where it's being run. You allways want the lightest car you can run, but rear traction out of corners is worth a lot, and I'd suffer with a little extra rear weight to get good corner exit traction.

When autocrossing, I tried removing the rear seat of my Camaro, also the front passenger seat, I tried removing the spare tire and ran a half-tank of gas. I had one of my fastest autocross runs with everything in, plus a passenger! One thing lacking was I couldn't easily re-balance the car and handling was off due to too much roll stiffness in the rear. (the car oversteered). It really wasn't a fair test because of that.

On the subject of light rear ends, think of this: A Camaro has say, 57% front weight. When the rear of the car is sliding around a corner, only 43% of the total mass of the car is sliding. If you had a car with 50/50 balance, 50% of the total weight would be sliding, it would seem the car would be tougher to control with a higher percentage of it's mass sliding. This assumes the rear only is sliding, but if the front were sliding a little, at least the front wheels can be turned to control it, controlling the rear slip angle requires the front end to change direction before the rear can "see" a change. Actually there is some weight jacking effects from turning the front wheels that can help the rear, especially if you use high caser angles.

Norm Peterson
02-12-2006, 08:10 AM
On a similar note, this should help address the "light in the rear" comment as to why camaros and mustangs end up backwards into a tree so often. Is it weight distribution or a combination horsepower/posi/highspeed/drivererror.Such comments typically come from those whose understanding of tire grip is limited to drag strip (or stop light) starts. More rear weight limits wheelspin, less rear weight aggravates it. Easy enough even for those who slept through high school physics class or who never took it in the first place to comprehend. But with little or no understanding of such concepts as lateral load transfer [in a turn] and friction circle (friction ellipse, actually), adding weight over the drive wheels is assumed to solve everything related to drive wheel grip.

Actually, the amount of grip available from the rear wheels relative to the amount of weight carried is better than the same comparison for the front wheels, at least in a front engine/rear drive vehicle. And that's not counting camber effects, which generally hurt the front grip more as well.

Adding a significant amount of weight toward the rear will increase the vehicle's polar moment of inertia, and I think this effect increases faster than does the increase in rear weight percentage. What that means is that once the vehicle with the larger PMOI gets "loose" it will be harder to bring it back in under control due to its greater tendency to keep spinning. Newton's First Law of Motion (Every object in a state of uniform motion tends to remain in that state of motion unless an external force is applied to it.) applies to bodies undergoing rotation as well as those simply translating in a straight line.


02-12-2006, 08:31 AM
And the braking at speed would be hampered by weight in the trunk also, correct?

02-12-2006, 08:51 AM
Anyone who has ever driven a tanker can attest that the last thing you want in the the trunk is a bunch of fluid. Dealing with RC lateral migration is one thing, trying to drive with actual fluid weight transfer while manuevering is downright dangerous. I guess for getting to work in the snow it's one thing but it should not even be a consideration for performance driving.

02-12-2006, 08:56 AM
I race slot cars as a cheaper form of racing. Slot cars have solid axle like a kart but they also have a fixed point for is slip angle pivot. Adding weight just in front of the rear tires aids traction, forward bite and lateral bite but once the lateral force of the back end trying to follow the front around the cuve exceads the bite of the rubber on the road the back ends up in a savear slide or a complete spin. But It has more to do with road conditions and inexperiance. The problem is that the transition from grip to slide is ofter a violent change do to road surface conditions and someone who is not anticipating that event will often times get caught out and be unable to recover. Also it can in some cases help braking but it is rare and it is just putting a bandaid and a more important problem.

Norm Peterson
02-12-2006, 09:07 AM
And the braking at speed would be hampered by weight in the trunk also, correct?That would depend on the current brake system capacity, the existing front:rear brake balance, and the rest of your passenger and cargo load, at least for the first stop. The tendency would be to shift the brake balance requirement rearward. In a car, and in the absence of ABS, I'd expect most OE systems to be slightly more likely to lock at least one front wheel first with that sort of ballast. Not sure what would happen in a pickup truck installation, as pickups may have a bit more rearward brake balance to begin with.


02-12-2006, 09:30 AM
Dennis The bag is supposed to freeze and be solid, I was just asking about a fixed weight of any kind.

Lets assume there is no load shift , brake fade , no abs , would the additional weight added make the car more difficult to stop at speed?, because of the extra forward momentum and the instability of the weight in the rear.

My thought was that added weight to the rear has more cons than pros.

02-12-2006, 09:41 AM
Lets put it this way: If you took a sedan that had neutral handling and added two hundred pounds to the trunk area, where would the handling improve and where would it worsen? Include braking and acceleration.

I guess there is a different ansewr for cars with a 50/50 weight bias and a different answer for "others"

02-12-2006, 10:52 AM
If the car handled neutral before, why in the world would upset the balance by adding a bunch weight?

In a hypothetical, simply adding weight wouldn't change braking a measurable amount assuming you are only adding a few hundred pounds at the CoG level.

If the water is frozen, and you run 5 hours at Buttonwillow where track temps are commonly 140*...what keeps it in a solid state? I thought so, which means by the end of the day you would have been fighting a constantly changing behavior that didn't need to there to begin with.

The added weight actually has no pros if it was neutral before, it would simply make the neutral car now have a slight push as the front would lose roll stiffness and you would have to re-set the chassis to the place you had it before adding the weight.

The comments about Camaros and Mustangs that are ass-light are erroneous. Most of the pony cars are somewhere in the 55/45 zone which isn't bad and the ones that are backwards in the tree are from no skill drivers who were hard on the brakes going into the corner which unloaded the rear tires making the inexperienced driver ASSume it was due to the cars light rear weight. Simply applying some power and shifting the weight transfer to the rear would have avoided said incident.

02-12-2006, 11:16 AM
Dennis I was using the water bag in summitt as an example, i dont even use sand bags in the winter. A set of tall skinny knobby tires works great for me.I wasnt talking about actually adding weight on a track day, I was justing trying to use a real world scenario to help debunk the l.a.c (light ass camaro) and the sand bag in the trunk theory.

You guys have explained to me what I suspected,and debunked the L.I.C. and L.I.M. myths.


02-12-2006, 11:50 AM
That water bag is used quite a bit in straight line pickup trucks. Thats about the only half reasonable way to use it.

Skinnies with knobs and studs work wonders in Canadian winters. Sandbags only come in handy when theres 300+ pounds in the back lol.


David Pozzi
02-12-2006, 10:15 PM
Imagine you had a well balanced car, both in weight balance and handling, and you can lighten it as much as you want, but only on the rear, also you can re-spring and change bars all you want to get the handling balanced.

How much rear weight would you remove before it hurt performance and where would the loss be?

Rear cornering traction would be better, less weight on a tire moves it into a better part of it's performance curve yielding more lateral grip per unit of download. The rear would gain lateral grip, the front would not, in fact, weight outside the wheelbase - behind the rear axle, acts like a lever arm, so vertical load on the front tires would increase, but probably only a fiew pounds. On a fairly well balanced car, this wouldn't hurt much, but a front heavy car on a tight course like an autocross, the front tires are going to have more load, and may be in a much worse part of their performance curve allready.

Polar moment would improve, but assuming a front engined car like a Camaro or Mustang, the front has most of the mass, and I think that is more of a concern than the rear. A lot depends on where the car is being driven.

Braking should improve with less total mass, but again if weight is removed behind the rear wheels, the front tires will see more load, but it will be offset by less weight transferring forward.

Acceleration out of turns is going to suffer at some point with rear weight removal. Perhaps not with the first 50 or 100 lbs, but at some point the rear will get light enough to cause excess wheelspin and times will suffer.

Jim Nilsen
02-18-2006, 11:47 AM
Something to ponder? Richard Petty was on tv one night talking about the cars in the late 60's and made the comment that they added 400lbs to the lower inside rocker of the Talledega car they had. It was much slower accelerating in the striaghts but when they hit the corner they would have a lot more traction and were able to keep the speed up throught the corners and they never let up passing the others in the corner. Since they had more exit speed the others never caught up. I know this goes against most logic and the weight was in a place that it made the most of the polar moment for the track it was on but it does make you think about whether or not weight is sometimes the big problem? Where the weight is make the biggest difference and weight can be translated into traction which is why aerodynamic downforce is so much desired at speed. A car that physically weighs less but is dynamically heavier is the key but if you compare apples to apples you have to look at Jeff Scwhartz's Cadillac and just accept that weight is traction. I once read that an Army tank could turn at + g forces so it all depends on what you expect to achieve.

I vote for the lightest car and the best balance you can get and get the stickiest tires you can buy and then drive the hell out of it. Brakes next and then all the horsepower you can afford and you will be keeping up with the best of them.

When the ice in the parking lot at work showed up while I was there , I can only say that I wish I would have had an extra 500lbs in the back since starting or stopping were a rather significant part of whether I was ever going to take a corner and at the looks of it I was never going to want to go at any speed around a corner. Getting home slowly was the only thing on my mind and with those closely balanced cars in the ditch it didn't seem to matter how much they weighed. So weight would have been an important factor to me at the moment. Afterall I am not going to race you in my pickup the way GM built it.

Jim Nilsen

02-20-2006, 03:29 PM
This question wasnt going to effect anything on my car, I was just trying to understand the way that additional weight makes a car handle.

The Petty thing might have been an unusual instance where weight in a particular spot led to a faster lap time.

02-20-2006, 05:35 PM
This is my take on this.

Anyone driving a Porsche 911 in the snow fully understands that having a heaviness factor towards the rear is good and bad. It is an extreme example but it illustrates it very well. The weight over the rear makes the vehicle travel very well in the snow BUT when the back end starts to come around AND goes beyond a certain point you instantly become a spectator. In the context of how weight distribution affects handling on a road course, my only experience was with my nephew's racing kart. It seems that different weight in different areas gave no clear answer. It seemed to be a constant game of chasing the setup due to never ending changing track and weather conditions. What worked one day did not the next.