why level your truck?

[underdrive]

Toymaster, actually him bringing up the weight distribution hitches does make sense in this case, tho probably not in the way he meant it Consider an improperly loaded trailer of the bumper-pull variety, unfortunately rearranging the load for better distribution is not always an option and sometimes you have no option but running the thing nose heavy. In those cases if you're all fancy you can air up the rear bellows and keep the truck's rear bumper height where it was before the trailer, but that still does absolutely nothing for the keeping the front axle properly loaded - the tongue weight will still cantilever the entire truck using the rear axle as a fulcrum, and thus the front will still lighten up and rise a bit. Which in some situations can push the steering into funky caster angles and thus lead to some less than desirable steering behavior - of course that kinda depends on what the initial settings were, but for the benefit of the argument let's just assume that is indeed the case and you absolutely do NOT the nose to rise. If you have front air bellows and for whatever reason you are regularly using them to support the truck's nose weight, you can now air them down a bit and allow the nose to drop back down to its unloaded height - hooray, your truck is now level again, all is good, right? Yeah, not so much - the reason the nose dropped is not cause you got your axle weight back, it's cause you reduced your spring rate, thus allowing the lower axle weight to compress the suspension more. Meaning even if your steering geometry is back right where you want it, the steer wheels are still lightly loaded and the tail can still wag the dog like nobody's business. This is where the load-distribution hitch comes into play - with it you need to add less air to keep the rear of the truck up, you need to release less air to keep the front of the truck down, and overall you have more weight up front to keep your steer wheels well planted.

So it appears that having the truck level has very little to do with its towing capabilities. Heck we can keep one of our trucks mostly level even when the front axle is about to lift off the ground (tall A-frame crane setup off the back bumper with a strong winch up front to reel the cable in), that still doesn't mean it will steer worth a darn. Extreme example of course, but it does illustrate the point I'm trying to make.

 

[underdrive]

The best way of keeping weight distribution even when braking, accelerating and cornering is to keep both the tongue of the trailer and the hitch as parallel with one another as possible. If they're not parallel the trailer may exert excessive upward or downward force on the hitch causing excessive weight shift on and off the steer axle and cause loss of braking and or control. Common folk may refer to this as the trailer pushing the tow vehicle.

It's far easier to establish correct towing geometry on a level truck than one with excessive rake or sag. Load leveling hitches do exactly as their name implies. They level the trailer tongue and hitch so they remain parallel.

No, the best way of keeping the weight distribution even when braking, accelerating and cornering is to balance your trailer properly - and for this you need to actually know how your trailer behaves with the load COG in different locations along its length. Once you have the thing loaded right, the tongue will be where it wants to be - this is the height you're trying to match with your truck's drawbar, the height of the hitch itself can be anywhere depending on what truck it is and how tall the suspension is and how big the tires are and such. For example it is very common for non-deckover trailers (prime example would be most landscaping trailers) to have very low frame rails, and when the tow vehicle is a mildly lifted 4x4 the truck hitch usually ends up quite a bit higher than the trailer's tongue - at this point it matters not if the hitch is level or the truck has a massive redneck rake to it, what matters is the height of the tongue with respect to the truck's axle. And you really can't change that, it is what it is based on your particular equipment. A trailer with the tongue above the axle height will tend to lift the truck during decel thus planting the front axle firmer into the ground, whereas a trailer with its tongue below the axle height will tend to load the rear axle more and thus unload the front axle. This is assuming a properly loaded trailer with a relatively low COG - if the COG is high enough (say a bumper-pull car-hauler with a mudding truck on it) then every time you get on the brakes the trailer tongue will dive and that will unload your steer wheels, regardless of how high your trailer tongue is and how level the hitch may or may not be.

You are however correct on that WEIGHT DISTRIBUTION hitches work best with mostly level trucks - for the sole reason that a truck that is very high in the rear will end up putting the spring arms of the hitch darn close to the trailer tongue frame, which can be annoying to hook up properly. But it's not going to stop them from doing their job, as long as they are tensioned properly they will take weight off the rear axle and shift it to the front one, regardless of whether the truck is sniffing the pavement in the front or digging its bumper in it in the rear.

And back to the OP's question: leveling your truck does not usually directly alter its towing behavior one way or the other, having the trailer properly balanced is way more important. But if you're shooting for the level looks when empty, you may wanna do something about the rear suspension so truck can remain level all the time - from the engineering standpoint this would be purely for steering geometry purposes, and from aesthetics standpoint that's just how you like it in the first place, right?

 

[Lykos]

Yes, weight distribution not load leveling.


Too many beers.



Great info in this thread!

 

[Lykos]

I'm pretty sure we are saying the same thing for the most part. I'm always assuming a properly loaded trailer btw. I like mine a bit on the nose heavy side whether bumper pull behind the Suburban or in an 18 wheeler.

A saggy rear end on the tow rig always seems to create an understeer issue. But that could be me using the wrong hitch as well.





I wonder if those who see significant differences in their towing with leveling kits are simply changing their tongue angles with the lift rather than using the proper hitch?

 

[RoyJ]

A saggy rear end on the tow rig always seems to create an understeer issue. But that could be me using the wrong hitch as well.

Assuming the weight is properly distributed (I think by now we've all cleared up the difference between real axle weights and optical illusions of sag and rake), this is due to the suspension geometry issues that I mentioned previously.

On an independent suspended axle, a sagging or lowered ride height reduces the lateral roll center, and vice versa, a lifted axle increases roll center.

When a vehicle sags (again, assuming actual axle weights are proper), the increase in front roll center causes less roll, or increase in roll stiffness. This creates less traction. The decrease in roll center in the rear reduces roll stiffness, and increases traction. Couple the two together we have understeer, in THEORY.

I say in theory because in real life, it's very unlikely you'll encounter a severely sagging tow vehicle with perfect weight distribution... Also unlikely it's an IFS/IRS vehicle, unless you're using a Land Rover, or Expedition (only two heavy IRS two vehicles I can think of). So chances are, the understeer is from actual poor axle loading / lack of front end weight.

In the sportscar field, tuners do use rake to dial in oversteer. And vice versa, as previously mentioned, pre-runners use "sag" to dial in stability or understeer.

 

[snowblind]

I'll try to offer some input as an engineer, hopefully it'll make sense to everyone.

Careful how you define the boundaries of your analysis: to compare a marble in the bed with a rake, would be like the truck sitting on a downhill slope, where the entire earth (mountain slope) is accelerating.

This is not the same as the truck itself having a rake / slope, acceleration on FLAT ground. In that case, yes, the Sin(angle of slope) * 32.2ft/s^2 - rolling friction = acceleration of truck. Likewise, drop a marble on a non-moving truck, with a certain rake, and the marble will accelerate at sin(angle of rake) * 32.2, minus friction.

OK. Let's see if I can keep up here.

I understand how a truck on flat ground is not applying more weight to the front wheels. The trucks wheels are level and the rear wheels would need to move physically higher than the front wheels to make that happen. For the marble... The "ground" is actually the truck bed. What puzzles me, is how a non-roling load would be affected. A flat box in the bed of a raked truck is feeling the same force as the marble right? Obviously friction is a lot higher but is the force the same?

For the truck ITSELF, with a rake, the acceleration and braking forces are only altered very minutely. Most of it is an optical illusion - more rake gives the illusion of less squat, and more brake dive.

Hhhhmmmmm... "minutely" eh? It's been my experience that "minute" movements can become "significant" in heavy vehicles as speeds rise.

The only real physical change, is the slight shift in center of gravity. When you accelerate or brake, the forces act on the center of mass (gravity), about the point of contact (tire patch). This force moment is what creates dive and squat forces. <SNIP>
Contrary to optical illusions, a leveling kit, because it raises the CG, would create MORE brake dive (in inches), assuming suspension is kept equal. It may *feel* less because the truck is more level. The only exception is if, by lifting the front, you've altered the longitudinal roll center MORE than the raise in CG. Example, you lift the front 2", CG goes up by say 1.7", and your roll center (due to short radius arms) raises by 4". In this case you may end up with less brake dive. But it's important to understand why - due to CG vs roll center relationship, NOT just because the truck is level.

Sounds right. In the GMT800 Chevy's the Lower Control Arm angles start to resist brake dive (and bump compression) as you crank the torsion bars. But yes, this is a result of suspension design and other vehicles may perform differently.

As I mentioned previously, the rake vs weight transfer is mostly an optical illusion. A high rake APPEARS to put more weight on the front wheels, and vice versa. A fellow engineer did the calculations (backed up with measurements) on a sportscar forum, and the actual difference was around 1 to 2 lbs! In other words, very little STATIC weight transfer.

I have more experience with sports cars and I totally believe that rake has little effect there. However, when talking about trucks don't you think the weight of the truck, the truck's increased ride height and the trucks softer spring rates and much greater suspension travel would all serve to magnify those forces compared to a sports car?

So, in conclusion, a "leveling" kit does not affect the weight transfer much, either static or dynamic, except due to changes in roll center. Most of that is an optical illusion: squats more because truck sits level to begin with, and dives less because level to begin with...

Hope that helped somewhat, feel free to correct me or discuss more!

I think your observations are correct but there might be other conclusions. EG: Stock truck pitches forward at X degrees angle to the road under hard braking. Lifted truck pitches forward at (X-Y) degrees to the road. With the weight of a truck that Yº more of angle might create considerable force.

Beyond the theory I can only offer personal observations. Since leveling my GMT800 the handling and braking when empty has improved. The front suspension does not feel overloaded when braking in a turn and there is much more rear wheel traction in the snow. I should note that it took three or four tries to determine the best compromise of front lift versus ride quality.



Matt

 

[AA1PR]

what about accelerated wear on front end parts ?

 

[RoyJ]

OK. Let's see if I can keep up here.

I understand how a truck on flat ground is not applying more weight to the front wheels. The trucks wheels are level and the rear wheels would need to move physically higher than the front wheels to make that happen. For the marble... The "ground" is actually the truck bed. What puzzles me, is how a non-roling load would be affected. A flat box in the bed of a raked truck is feeling the same force as the marble right? Obviously friction is a lot higher but is the force the same?

Let me start off by stating: I am not saying the rake of a chassis doesn't affect things like brake dive and weight transfer - it does, and I have provided multiple reasons why. What I am trying to explain is the reason behind that, which is predominantly suspension geometry changes, and not because the truck is less "visual heavy" in the front.

Back to the marble - you nailed it, the marble is on a sloped ground, which is why I said it's a bad analogy (truck is on level ground). That's like parking our truck on a hill. In these two cases, yes, there's a very large change in axle weight. Put it this way:

• A truck with a 10 degree rake, on level ground, has a very low axle weight change, maybe 10 to 20 lbs or so heavier in the front.
• A truck parked on a 10 degree slope, with the brakes applied, has a very high axle weight change. Depending on the CG, we're talking 500 lbs heavier in the front, and 500 less on the rear! This is akin to a marble sitting in the truck of the first bullet.

It's tedious to explain in words, but if I were to draw it out, then it becomes clear - the 10 degree force vector is acting on two completely different moment arms.

On a parked truck with a rake (which is essentially a hill), a marble and box experiences that exact same force. Only different is the marble starts rolling. Once it hits the front of the bed (or if you glue it), it comes to rest, and then acts identical to the box sitting on the bed. Both are not moving, experiencing a static force of sin(angle of rake), which is countered by friction (box), or the front of the bed (marble at rest).

But again, let's not focus on this too much, because it has nothing to the forces the truck ITSELF experiences with a raked chassis.

Hhhhmmmmm... "minutely" eh? It's been my experience that "minute" movements can become "significant" in heavy vehicles as speeds rise.

Not speed, but acceleration (all directions, forward, reverse, lateral, and even vertical).

When I say minute, it's relative. A 5 degree rake is minute compared to say, installing springs with 3 times the spring rate...

Also, our truck generates relatively low acceleration (in terms of g-force), which makes things minute. An F1 car doing a 5g threshold brake, may see more weight transfer variance with different chassis rake angles, but our truck at .65g won't see nearly as much!

Sounds right. In the GMT800 Chevy's the Lower Control Arm angles start to resist brake dive (and bump compression) as you crank the torsion bars. But yes, this is a result of suspension design and other vehicles may perform differently.

Ah, I didn't realize you were referring to a GMT800 (should've looked at your avatar!). I was thinking a solid front axle with radius arms the whole time.

On the GM IFS, the anti-dive geometry is built into the control arm mounting mounts itself. If you look at your front suspension from the side, you'll see the upper/lower control arm tilts at an upward angle. Since this angle, relative to the frame, is unchanged, we won't see any anti-dive improvements here.

But what does happen, is an increase in lateral roll center, since the control arms are at a sharper angle. This doesn't directly affect dive, which depends on longitudinal roll center, however, the sharper control arm angle increases the affect spring rate of the torsion bars, as the moment arm changes.

If we take it to an imaginary extreme, and have the control arms dangle straight down somehow, then the spring rate would be infinite, and have zero brake dive.

I have more experience with sports cars and I totally believe that rake has little effect there. However, when talking about trucks don't you think the weight of the truck, the truck's increased ride height and the trucks softer spring rates and much greater suspension travel would all serve to magnify those forces compared to a sports car?

It sure does, under DYNAMIC situations. That's why I was very clear on distinguishing the static and dynamic weight transfers of a raked chassis.

What I was saying, is that the static weight transfer of a chassis rake is very very small, and equally small for both a lightweight sportscar and heavy truck, as suspension travel / CG / mass does not matter in a static situation.

I think your observations are correct but there might be other conclusions. EG: Stock truck pitches forward at X degrees angle to the road under hard braking. Lifted truck pitches forward at (X-Y) degrees to the road. With the weight of a truck that Yº more of angle might create considerable force.

Very good point you brought up, and this is where we go back to the marble / truck parked on a hill situation.

The truck itself experiences very little force difference due to a rake, this is what I've been trying to explain. However, you as the passenger, can experience a much larger difference in force (between a raked and level truck). You are the marble sitting in the bed.

But this change in force is a vector change, not magnitude change. I'll explain: in an extreme case, say a truck with 30 degree rake, you're already fighting gravity. Your seat belt digs into your very hard even sitting there, because you're almost sliding off your seat.

If you were to brake at 1g in this truck, the seatbelt is downright painfull, because it has to hold 1 g, plus the force generated by a slope truck. However, the TRUCK chassis doesn't know that! It still "feels" only the 1g. The rake has very little affect on IT.

On the other hand, if you have a -30 degree rake (squat), your seatbelt is hardly doing anything, as you're leaned back in your seat. If you did a 1g stop, you'll barely feel the seat belt, as it only has to hold back around .25g.

This is why a leveling kit gives us, the cargo of the truck, a perceived improvement in brake dive. But it's important to realize it's only perceived, and is not felt by the truck chassis/tires...

Beyond the theory I can only offer personal observations. Since leveling my GMT800 the handling and braking when empty has improved. The front suspension does not feel overloaded when braking in a turn and there is much more rear wheel traction in the snow. I should note that it took three or four tries to determine the best compromise of front lift versus ride quality.

Matt

Absolutely.

Like I said , I'm only trying to explain the theory of static weight transfer on the axles with a given rake angle (which is small). In real life dynamic situations, there are many more factors in play, some which may be perceived from the driver's seat.

Perhaps you're hitting the bump stops less, or in the increase in roll center reduced your front end body roll. There're many reasons why a level kit can make a truck feel better (at the cost of payload and aerodynamics).