# Technical Discussion: Vehicle Modification & Design Considerations

Discussion in 'Center Of Gravity' started by BorregoK5, Apr 21, 2002.

1. ### BorregoK51/2 ton status

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I've been absorbing myself in theory lately over different technical considerations as applied to suspension. While I'm looking to make some exotic changes to my own vehicle and gather knowledge pertaining to that change, I'm finding quite a bit of information that is pertanent to calibrating stock suspention, slightly modified (like lift kits) and right on up into complete custom designs. There is quite a bit of information available on calibrating and/or designing suspension targeted toward street, baja or rock climbing but very little on creating a well rounded vehicle which has adequite capabilites in all of the above at the expense of being exceptionally good at any one of them (hmm, sounds like a production sport utility, eh). If everyone is to get something out of this, I think a quick glossary of terms is in order and then we can play around with how they combine to excel in different areas. I want to clarify from the get go that I'm a complete novice and have never formally applied these elements on a full size vehicle so those of you who have, pipe up and add in the real world experience. So, from what I can gather:

Spring -
-A spring is an elastic device that resists movement in its direction of work. The force it exerts is proportional to the movement of one of its ends. Or to put this into a mathematical equation: Force = movement * spring constant. A high value for the spring constant makes for a stiff spring, and a low value makes for a soft spring. For progressive springs the spring constant will increase as the spring goes deeper into its travel, and for regressive springs it will decrease with travel. So math wise, springs aren't very complicated, but handling wise, they are. This is because the springs have to absorb the torques that are generated The problem is that they work in two dimensions: left to right and front to rear. Springs inhibit weight transfer, both front-to-rear and left-to-right: for the same cornering, acceleration or braking force a stiffer spring will compress less, resulting in less chassis movement and thus also less weight transfer, and a soft spring will compress a lot, resulting in a lot of weight transfer. Springs store and release energy.

Unsprung / Sprung Weight-
-Sprung weight is the weight supported by the springs. For example, the vehicle's body, frame, motor, and transmission would be sprung weight. Unsprung weight is the weight that is not carried by the springs, such as the tires, wheels and brake assemblies.

Dampening (shocks)-
- Damping is needed to absorb the energy associated with suspension travel and results in the generation of heat. In terms of energy, damping absorbs most of the energy the truck receives as it moves, unlike springs, who store the energy, and release it again. Dampers absorb all the excess energy, and allow the tires to stay in contact with the ground as much as possible. This also indicates that the damping should always be matched to the spring ratio: never run a very stiff spring with very soft damping or a very soft spring with very stiff damping. Damping that's a bit on the heavy side will make the truck more stable; it will slow down both the vehicle's pitch and roll motions, making it feel less twitchy. Note that damping only alters the speed at which the rolling and pitching motions occur, it does not alter their extent. So if you want your vehicle to roll less, adjust the springs, but not the dampers.

Roll Center/ Instant Center -
- A roll center is an imaginary point in space, look at it as the virtual hinge your car hinges around when its chassis rolls in a corner. It's as if the suspension components force the chassis to pivot around this point in space. The roll center is also the only point in space where a force could be applied to the chassis that wouldn't make it roll. The roll center will move when the suspension is compressed or lifted, that's why it's actually an instantaneous roll center. The roll centers/instant centers transfer the non-rolling forces to the road surface. The height of the roll centers and the instant center placement determines how much of the weight transfer at either end (front and rear)goes through the springs/shocks (rolling) and how much goes through the geometry(non-rolling). with low roll centers the springs and shocks transfer the majority of the weight. With high roll centers the location of the instant centers and any panhard bar take care of teh majority of the weight transfer.

Center of Gravity -
- Center Of Gravity is the spot within a vehicle where there is equal weight all around it.

Roll Moment -
- The vertical distance between the Center of Gravity and the Roll Center

Anti Squat-
- Anti-squat describes the angle of the rear hinge-pins relative to the horizontal plane. Its purpose is to make the truck squat less when accelerating. (Squatting is when the rear of the truck drops down when the truck accelerates)
More anti-squat will give more 'driving traction': there will be more pressure on the rear tires as you accelerate, especially the first few meters. At the same time, it will give more on-power steering, because the truck isn't squatting much. The disadvantage is that the truck has an increased tendency to become unstable entering corners, especially in the rear. Reducing the anti-squat angle has the opposite effect: a lot less on power steering, and more rear traction when the truck isn't accelerating as much anymore. The truck will also be a lot more stable entering corners. It also affects the truck's ability to handle bumps: more anti-squat will cause the truck to bounce more when accelerating through bumps, but it will increase the truck's ability to absorb the bumps when coasting. Reducing the anti-squat does the opposite: it improves the truck's ability to soak up the bumps under power, but reduces it while coasting.

Ride Height -
- Proper ride height is very important, too low and the vehicle will bottom out a lot, too high and the risk of traction rolling will be unnecessarily big. Equal ride height front and rear is a good starting point. Raising or lowering ride height on one end of the truck changes the steering characteristics of the truck, the lowest end will have a slightly bigger percentage of the trucks static weight. But, more importantly, the roll center will also be lowered, making that particular end of the truck roll deeper when the truck corners, making it sit even lower and thus having more grip.

Suspension Travel-
- The amount of negative suspension travel (downtravel) a truck has can have a huge effect on its handling; it influences both the mount of roll and the amount of pitch the chassis will experience. An end with a lot of downtravel will be able to rise a lot, so chassis pitch will be more pronounced, which in turn will provide more weight transfer. For example: if the front end has a lot of downtravel, it will rise a lot during hard acceleration, transferring a lot of weight onto the rear axle. So the truck will have very little on-power steering, but a lot of rear traction. A lot of downtravel at both ends, combined with soft springs, can lead to excessive weight transfer: on-power understeer, and off-power oversteer. The cure is simple: either reduce downtravel, or use stiffer springs.

Sway Bars-
- Anti-roll bars are like 'sideways springs', they only work laterally. Here's how they work: if one side of the suspension is compressed, one end of the bar is lifted. The other end will also go up, pulling the other side of the suspension up also, basically giving more resistance to chassis roll. How far and how strongly the other side will be pulled up depends on the stiffness and the thickness of the bar used: a thin bar will flex a lot, so it won't pull the other side up very far, letting the chassis roll deeply into its suspension travel. Note that the bar only works when one side of the suspension is extended further than the other.

Weight Transfer -
- Newton's third law, force = mass * acceleration, implies that whenever the vehicle accelerates in any direction, additional forces occur. For example, when your car lands after having taken a jump, its downward velocity decreases rapidly. Basically, it stops falling down quite suddenly. The extra force associated with this equals the mass of the car times its acceleration.

Transverse Weight Transfer-
- Transverse weight transfer occurs because the driveshaft torque reacts between the frame mounted engine and the axle. The engine, which is a part of the sprung weight, produces the torque through the transmission. The rear axle has to resist that torque at the rear tire patch. Transverse weight adds cross weight under acceleration and removes cross weight durring deceleration. The manatude of teh transverse weight shift is a function of the instantanious engine torque and the roll stiffness.

Lateral Weight Transfer -
- Is the Weight Transfer from one side of the vehicle to the other.

Longitudinal Weight Transfer-
- Is the Weight Transfer from one end of the vehicle to the other.

This is a good start, any feedback or corrections before we start throwing all this together?

2. ### DaveCowley1/2 ton status

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/forums/images/icons/smile.gif See You In Moab

3. ### 4X4HIGH1 ton statusPremium MemberGMOTM Winner

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I think you are spending too much time thinking about this stuff. I finally looked at your profile to see that you are an engineer, go figure. LOL. Just build the truck like everyone else and drive the damn thing. LOL

4. ### Triaged1/2 ton status

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Springs inhibit weight transfer, both front-to-rear and left-to-right: for the same cornering, acceleration or braking force a stiffer spring will compress less, resulting in less chassis movement and thus also less weight transfer, and a soft spring will compress a lot, resulting in a lot of weight transfer.

<hr></blockquote>

This isn't quite right...
Spring rate does not have anything to do w/ front-to-rear weight transfer (well not much). The only diffenence they make is if the front dives more on breaking the CG will move forward (what maybe 1" max?) and that will cause a percentage of the weight from the rear of the truck to be transfered to the front. It is such a small amount that it is usually neglected. Weight transfer (front-to-rear) is considered to be a function of CG location and acceleration (like breaking).

Lateral weight transfer is affected more by the springs than side to side because both front springs will be the same rate and the rear will also match side to side (at least they should...LTO not encluded). If one end of the truck has a lower roll rate (dependant on the spring rate but not the same...it also depends on how far apart the springs are) less weight will transfer to on that end. That is if the front has a softer roll rate than the rear the rear will transfer more weight to the outside tire (and resulting in less rear traction) than the front does.

Shocks:
It should also be noted that the energy they absorb will be turned into heat.

Anti Squat:
I aguree with the def...but not with what the effects are...but don't have the time to explain (see above about weight transfer).

5. ### BorregoK51/2 ton status

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You make some really good points. One of the reasons I wanted to get this discussion going was to validate the different reasons for having the different suspension designs, get people familiar with what they have and the reasons the factory chose those options. I think the majority of people here drive their truck to the area they off road so highway manners are very important. I've been searching for that fine line of minimal road sacrafices with minimal off road sacrafices where adjustability and disconnects can help improve the advantages. I like the idea of exotic setups to combat axle wrap, alter stock antisquat characteristics and change travel/dampening based upon terrain.

6. ### BorregoK51/2 ton status

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I think I understand what your saying. Rather than springs inhibit weight transfer, it should read springs provide weight transfer. If the springs were removed and solid mounts were to take their place, there would be no axle travel and aside from the tire sidewalls, the body movement would stay flat and thus eliminate stored energy previously associated with the spring.

Good note on the shocks as heat can change a shocks characteristics.

I'd like your input on the anti-squat when you have time.

7. ### laketex3/4 ton status

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Argh, now I know why I changed majors from engineering. There's no way I can even read all of that without going nuts. Btw, my dream suspension on the rig would be 1/4 elliptical rear and modified leafs front. But I've been spending way too much time designing and way too little time driving, thus my toys are beginning to own me. Hate it.

8. ### BorregoK51/2 ton status

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I know what you mean, just getting familiar with all these terms is one thing but then turning around and applying them while trying to determine their effects on each other gets complicated. I like the buggy spring idea too.

I'm finding it realtively easy to determine an approximate instant center on link suspension and how running parallel links which are parallel to the frame have different anti-squat characteristics as parallel links which are angled to the frame. When you apply the same concepts to a 1/2 eliptical setup (stock leaf spring), changing mounting points such as a shackle flip should also change instant center but having a link which can bend makes it more difficult to calculate it. Kind of like a buggy spring would be ...

http://www.echobit.com/images/buggyleaf.gif

9. ### m j1/2 ton status

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roll centre and instant centre are not the same thing.
off the top I cannot think of a suspension where they are the same point. was thinking Unimog torque tube but they have a panhard that I believe creates a roll axis

10. ### m j1/2 ton status

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I like the 1/4 elip more n more
i like that you can add leaves under the pack to prevent droop and hopefully transfer some more wieght to the tractive tire.
that cant be done with coils or bags.
I still like the bags for their constant ride hieght regardless of vehicle loading. also some of the antics you can perform with incab controls like, lift any tire so no-jack tire changes are possible in some vehicles, or dumping the air to allow easier egress
other then that they act like coils.
new they are \$\$\$ used I have bought them for \$50cdn

11. ### BorregoK51/2 ton status

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Your right, roll center and instant center are different. I clumped them into the same description because they had similar traits. I tried to differntiate between the two in the description but perhaps I should reword it or sepperate them and inlcude a graphic.

I'm still looking into the air bags as an option. Its a neat solution.

What do you think about the shackle flip and potential squat characteristic change, it seams that axle wrap could change the instant center as well.. any thoughts?

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I'm with MJ on this one. I like the 1/4 eliptics, but I want to combine them with air bags for adjustable ride height and higher rate springs (for when the bags are deflated, like fire roads or off camber). I'm dreaming again, I'll never get around to doing it, but I think that's the way I would go if I got the wild urge, lots of time, and "found" a bunch of \$\$\$.

Anyway, still following this thread with a great deal of interest. I've seen allot of this stuff referenced but never quite understood allot of it. No reason to spend much time on it since I don't plan to pursue it. Also, some of the guys on POR have referenced a book that cover’s allot of these suspension calculations and considerations. Might do a search there.

13. ### BorregoK51/2 ton status

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BadDog - I've been through about 6 off road forums so far and found lots of information on this topic... but nothing here, so I'm starting it.

Traiged, I understand what you mean now to a further extent. Movement of the spring mass does not cause weight transfer but does change the center of gravity height.

"The only thing that causes weight transfer is lateral or longitudinal force acting on a center of gravity (CG) that is above the ground. All cars, no matter the suspension geometry or wheel rate (indeed, even with no suspension at all), have a CG above the ground and therefore must transfer weight when acceleration force is applied. The CG forms a lever between itself and the ground through which the acceleration forces acts, and the wheelbase forms the lever through which the force acts on the wheels. That means that the only way to reduce weight transfer is to lower the CG (shorten its lever arm), or increase wheelbase or track (to decrease the mechanical advantage the force has on the wheels), or to lighten the car overall (which reduces the force at its origin)."

Where springs inhibit weight transfer is in inhibiting the movement of that center of gravity height. So I should change the wording to springs inhibit the change in the center of gravity height and further define center of gravity. Sound better?

14. ### m j1/2 ton status

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in my webshots album 'chev power manual' there is a section on the hotchkis suspension.
it shows a point C as the instant centre but I am having trouble determining what locates C.
the length is 3/8 of spring length but the angle is a mystery to me so far. will have to read it a few more times

15. ### m j1/2 ton status

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I think this forum is more capable of a useful discussion then P4x4.
there are some extremely knowledgeable folk there but the XXXXXs drown out any intelligent ponderings.
all that is really needed is a list of rules of thumb and some generalities of what trends cause what effect at the limits.
something like "the rollcentre works well within X distance from the cg hieght, higher and Z is the result, lower and Y is the result"
for extreme anti squat I know that under braking you can have it nearly lift all the pressure from rear tires.
I would also like to see if the front straight axle math is a mirror of the rear.

16. ### Triaged1/2 ton status

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17. ### Triaged1/2 ton status

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More anti-squat will give more 'driving traction': there will be more pressure on the rear tires as you accelerate, especially the first few meters

<hr></blockquote>
This is the part I don't really agree w/. Longitudinal weight transfer is caused by acceleration, distance of the CG above the ground, and wheelbase. The only other way to get more weight on the rear tires is for the body to be accelerating upward! That could only be kept up for a short period of time and could never (in any normal truck) be that great in manitude. Anti-Squat will also bind up the suspension under acceleration (what prevents the squat) which will cause the tires to hop and loose traction when they hit a bump (and arn't bumps what we are looking for when we go offroading?).

At the same time, it will give more on-power steering, because the truck isn't squatting much

<hr></blockquote>
How would the truck steer better w/ anti-squat because it does nothing about weight transfer? The weight on the front wheels will not change as a result of anti-squat (of course the caster angle will change as the rear end squats and the driver will see more hood and less road but that is beside the point).

It also affects the truck's ability to handle bumps: more anti-squat will cause the truck to bounce more when accelerating through bumps

<hr></blockquote>
This is 100% right

but it will increase the truck's ability to absorb the bumps when coasting.

<hr></blockquote>
This is a very small change and most drivers would not even notice it...wheel hop they will.

18. ### 77K5Blazer1/2 ton status

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That buggy setup looks pretty "spiffy", LOL. I really like that idea. Would it require any locating links to keep proper geometry? I have never really paid attention to buggy designs, mostly flexy leaf and multi-link w/ coils. But, I think a retaining bracket could be devised where the buggy rests on the frame to fix it in place for DDing or when not needed. I don't know from personal experience how a buggy would react on the road, but I could almost guarentee that fixing it to the frame, effectively removing all of it's Pros, would remove all of the Cons of using it on the road. Does anyone have any more info on this type of setup? TIA!
Later...Allan

19. ### BorregoK51/2 ton status

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"More anti-squat will give more 'driving traction': there will be more pressure on the rear tires as you accelerate, especially the first few meters"

Is your disagreement on the nature of antisquat surface related? On a vehicle with lots of antisquat, lets say a drag car on smooth level asphalt, the vehicle will rise under accelleration moving the chassis upwards which essentially forces the tires down(directly proportional to torque and weight). This results in traction. A truck on level asphalt should exhibit the same characteristic and does. Where the bouncing comes from is the stored energy in the suspension getting triggered by an imbalance. On large trucks its usually a characteristic of soft suspension without an antisway bar on the rear. For a rock crawler, this is kind of mandatory and hence the different reaction. But this doesnt change the fact that it also affects the truck's ability to handle bumps: more anti-squat will cause the truck to bounce more when accelerating through bumps because of the torque applied to the wheels, which is trying to extend the suspension downward (remember, forcing the vehicle up) and adding resistance to its full range of motion. I think, and could be mistaken, that your jumping ahead to the application of antisquat as opposed to the concept of it which I simply want to define for later discussions.

As far as anti-squat binding up the suspension, I agree that excessive antisquat would do just that, just as excessive squat would do the same.
Great point of discussion though, the application of antisquat is totally different depending on what your using it for.

The on power steering is something more relational to the position of the front geometry as opposed to weight transfer. With more antisquat, the front tends to extend less into its full range of motion as teh vehicle rises. With a lot of squat, your more likely to have the front suspension extend further(as the back drops) and put the suspension geomety in a less desireable state. This is where the increase in stearing comes from, alignment. This is especially important on independent front ends.

I agree that most people will not notice the increased bump absorbsion durring squat but it follows the same properties of torque applied to the range of suspension motion.

I apreciate your involvement in this thread, it helps the discovery process and makes for some good info exchange. How does the above sound? Think were ready to start applying it to our rigs... its where the most debate starts.

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