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Thread: Suspension selection and design for technical terrain

  1. #1
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    Lightbulb Suspension selection and design for technical terrain

    I want to start a series of three threads on suspension modifications.

    1. Technical Terrain
    2. Load Management
    3. High Speed Terrain

    There are compromises required to move in any one direction; However, now technologies and designs can allow for minimal compromise.

    So this first thread will cover technical terrain, which many of us love. That is the 3.0+ trails with large rocks, crossed-axle terrain, etc.

    It will be a fun discussion.
    Scott Brady
    Overland Journal
    D1 | LJ78 | LR4 | MKIII | J8 | G-Wagen |

  2. #2
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    Default Lift height: Dispelling a few myths

    As a general rule, you should only add lift as you add travel and width. I.e., with each inch of lift that you add, you need to incorporate an additional inch of wheel travel or track width. That is the only way to ensure proper performance and stability in technical terrain.

    A big lift (like drop brackets or body lift) with no additional articulation or width is not appropriate for 3.0+ trails and technical terrain where crossed axle washouts are encountered, cambered ledges, angled traverses, etc.

    That is the whole key behind minimal lift and maximum tire diameter. The ability to gain clearance under the axles, but maintain the lowest COG possible.

    1-2.5" lift and 33-35" tires: IFS

    The key here is to take advantage of the high speed performance of the suspension design. That is why I kept my IFS. My truck has 33.3" tall tires, 1.5" of lift, and a very low COG. I can drift this 5,000lb+ truck around corners and still run 3-3.5 trails. The tall, narrow tires tuck into the wheelwells and the dual lockers provide the slow speed control and traction. 5.29 gears allow mechanical advantage.


    3-5" lift and 35-37" tires: SAS

    SAS gives you all of the off-road advantages. You can go wider on the axle and gain 30-50% more articulation. You will end up with a vehicle that is more stable and capable than stock. Front articulation numbers in excess of 10-12" is not uncommon. With the additional width, you can lift 3-5" and be very stable, with excellent traction and articulation.

    In detail:

    1-2.5" of lift desired, and up to 35" tires-
    Use longer travel coil overs, disconnect the swaybar while on the trail (I removed mine all together). I also gained 2" of travel in the rear suspension with Deaver springs and longer shocks. So, with my 1.5" lift and 33.2" tall tires (255/85) I have better suspension performance than stock. In addition, I reduced my wheel off-set from stock by .5".

    To break it down:

    Front:
    1.5" lift puts me at -1.5"
    1" more wheel travel from longer coils in front puts me at -.5"
    1" greater width from wheel off-set puts me at +.5" for the front (better than stock even though I am lifted)

    Rear:
    2" lift puts me at -2"
    2" additional wheel travel from springs and longer shocks puts me at net 0"
    1" greater width from wheel off-set puts me at +1"

    So overall, my articulation, travel and performance is better than stock even though I am lifted.

    3"+ Lift Required:
    4+ trails require tires 35" and larger and good ground clearance. So how do you lift your truck that high and it still perform?

    You cannot do it with a drop bracket kit. You will end up with a tall truck, but no engineering changes to compensate for the height. The most you can gain back is an inch of additional travel in the front and maybe a few inches in the rear. And the lift brackets do nothing to improve the strength of the front diff. 33" is the max recommended for stock CV's

    So to lift big, you must gain wheel travel (articulation). This can only be done with a SAS. Many that have SAS'ed their trucks have reported 4-6" of additional travel after the modification. Some have achieved even more.


    Exceptions to this rule? Yes. If the truck is being run in deep, but not technical mud trails, or on the sand, then a drop bracket kit is fine, but still not preferred.
    Scott Brady
    Overland Journal
    D1 | LJ78 | LR4 | MKIII | J8 | G-Wagen |

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    Default Fitting Large Tires: No lift required ;-)

    Another common myth is that you must lift a vehicles suspension to fit large tires. Well, believe it or not, that is not true (there are some exceptions).

    What allows larger tires to fit and not rub, etc.?

    1. Lowered bump stops
    2. Fenderwell trimming
    3. Tire width
    4. Changing steering stops
    5. Change in fenderwell size or height

    Follow my logic for a moment with this Tacoma example: When you install the new, shiny coil-overs and dial in the preload to give you 2" of lift to fit taller tires, what has changed to allow fitment of a 33" tire?

    Nothing! Yep, all that has changed is the amount of ride height. The suspension still moves in the same arc as when stock. The bumpstops haven't been changed. You have just increased compression travel and decreased extension by the same amount.

    So when off-road, and you stuff the tire into a rock, the suspension will compress all the way to the bumpstops. If you have taller tires, they will rub on the pinch weld, flares, etc. So it is the trimming and hammering of the pinch weld that allowed the tires to fit, not the lift.

    Something to think about
    Scott Brady
    Overland Journal
    D1 | LJ78 | LR4 | MKIII | J8 | G-Wagen |

  4. #4
    Lots of stuff to think about, Scott.

    I have limited experience in 'rock crawling'. Have done the Rubicon trailer several times, but that was in a stock FJ40 with stock tires and so on. This is before any of us had heard of lockers, etc.

    I have a little experience with suspension modifications and tires for off-highway use. The really important issue s in my driving are:

    1. Suspension - Ability to smooth the ride on corduroy roads at speeds between 25 and 40 MPH. The MOE suspension on my FJ60 really improved this capabilty over the stock suspension. I had to live with the 2-1/2" lift, because they have no offering that offers stock height on this vehicle. I would not like more lift.

    My CJ2A is sprung over with Interco TSRs. It is fine on trails, scary on side hills, and never driven on the road.

    2. Tires - in mud and snow, resistance to sidewall cuts, ability to run slightly over-inflated for efficiency on the highway. I have only run a few tires that I like off highway. I do like the GY MTRs. They are the only tire that GY makes - and that I have tried - that comes close to being an 'off-road' tire that will perform both on and off-highway, really offer acceptable traction in both situations. I will try BFGs ATs and BS Revos soon, also. I have also found that the selection of suitable tires (load ranges & sizes) offered for 16" wheels to be much better than with the 15" wheels. In 15" the taller the tire, the fatter the tire. Fat flotation tires are inefficient on the highway and, in mud and snow at least, do not perform well.

    Unfortunately, tire manufacturers seem to think that all 15" 4X4 tires are for sandrunners and rock crawlers.

    M
    Mike S
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  5. #5
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    Here is an example. My Jeep Wrangler only has 2" of lift and 33" tall tires. The suspension in front has 2" more travel than stock and the rear has nearly 6" over stock with OME L shocks and a change in shock angle.

    With this small lift and great flex the Jeep is incredibly stable, even on side hills. It will ramp 1000.

    If I change the skidplate to a high clearance version, I will have no concern running 4.0+ rated trails.



    Video of Jeep flex
    Scott Brady
    Overland Journal
    D1 | LJ78 | LR4 | MKIII | J8 | G-Wagen |

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    Scott,
    What do you think about "drop out cones" for rear coil springs?

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    Brian,

    If properly designed, they can be a good solution. I always recommend incorporating a limiting strap to prevent hard bottoming at the limits of shock extension.

    The one downside is balance, as there is no opposing force on the extended axle which results in hard extension and over extension. A better solution IMHO is a retainer system like the one from Safari Gard
    Scott Brady
    Overland Journal
    D1 | LJ78 | LR4 | MKIII | J8 | G-Wagen |

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    That's pretty slick!

  9. #9
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    AAGGHHHH! A topic to which I have many thoughts and opinions (some may stink, too). but not much time to type out informative posts. But I will offer some points to consider when adding a lift of any sort to any type of vehicle...things that are usually overlooked. Much of this can be applied to suspension systems for any type of terrain (not just technical terrain). But as a general rule of "reality", the harder the trails you run, the more lift you are likely to have. As lift height goes up, all of the info below becomes more and more critical, because you are reaching farther outside of the design parameters of the OEM's.

    Steering geometry.

    With the typical inverted y setup (common strait axle steering where the drag link connects the pitman arm to the passenger knuckle, and the tie rod connects the drivers side knuckle to a point on the drag link directly in front of the passenger side track bar mount), it is imperative that you maintain the angular relationship between the track bar and the drag link. If you do not, you will have huge problems with bump steer. Your drag link and track bar must be parallel (to each other), and the pivot points must be the same distance apart. Common mistake is when lifting a jeep TJ, adding a "drop pitman arm"...this changes the relationship between the track bar and drag link, and creates problems. Drop pitman arms are included in many lift kits, so people install them and can't figure out why every time they hit a bump, their jeep turns a corner.....

    On IFS, the larger the lift (via drop brackets, longer shocks, whatever), the greater vertical distance you will have from the knuckle to either the center link, or the rack and pinion unit (whichever your vehicle has). When you load up the front drive train (climbing a large step, for example), your tires will want to point towards each other (toe-in). This is caused by the axis your knuckles rotate on (imaginary line between the ball joint centers) being inboard of the tire. As this happens, traditional (non-rack and pinion) steering setups will see upward force being placed on the center link. This will rapidly wear out idler arms and pitman arms, and in extreme cases, can cause steering box output shaft seal leaks. Something that ties in with suspension geometry (addressed below), but affects steering, is the more vertical distance between the ends of the tie rods (basically, the verticle distance between the knuckle and center link), is that the farther down in the arc the knuckle end of the tie rod is, the more horizontal movement you get per unit of verticle movement. This causes changes in the toe-in measurement as the suspension cycles. The worst case I have seen was so bad that anytime you hit a dip in the road with any speed, the tires would toe out so much that they would start to howl, and the truck would wander unpredictably.

    With rack and pinion steering setups, this isn't as much of an issue, as the steering gear is located behind the front axle, so when you load up the front drivetrain, you actually pull on the links, which isn't as hard on the components (well, that and you don't have a centerlink, idler arm, pitman arm, etc). Just be aware that as your lift height increases, the length of your links from the rack and pinion unit to your knuckles also must increase. At some point you will need longer links (where that point is, I am not sure, and will vary from model to model).

    Similarly, suspension geometry must be considered. (damn, this is getting long and I gotta get up early....). As you lift the front of a IFS vehicle without a drop bracket kit, you are increasing the angle of the upper and lower control arms (aka: A-arms), relative to the ground. This will do 2 bad things. First, it will transfer more road noise and vibration up to the frame. Second, it will create excess camber change as the suspension cycles. The upper and lower arms are different lengths, which keeps the camber where some engineer has decided it should be (reference your post about inherent understeer up above). But by moving both of the outer ends (ball joints) lower in their arc of travel, you will get more horizontal travel per unit of verticle travel. This can cause all sorts of undesirable handling characteristics (bump steer, understeer, etc). Obviously, the more lift you get, the worse these get. I had 2" of lift obtained by "cranking" the torsion bars on my old chevy (along with 5" of lift obtained by drop brackets), and the camber curve was affected so bad that you could literally see the camber change with your eyes as I turned a corner. Along with this same line of thought, you also need to consider the increased angle that your half shafts operate at. As the angle increases, the weaker the CV joints are, and the more wear the boots will have.

    Now, please keep in mind that all of these things are affected to some degree. Wether or not any or all of these issues is affected so much as to be a realistic problem depends on lift height, driver preference, etc. Generally, a small lift will not affect things so much as to create many problems. But you should be aware that changing one thing affects pretty much everything else, and as lift height increases, so do the effects of these changes.

    Ok, the talk about strait axle suspension designs will have to wait until later....stay tuned for talk about 3 link vs 4 link, heim joints vs cartridge joints vs bushings, links vs leafs, leaf spring designs (there are 4 major types), what is anti-squat, roll axis and why do I care, what role do anti-sway bars play (yes, there is a very good reason to run anti-sway bars on even the toughest of trails), "long arms" vs "short arms" vs "mid-length arms", what are "bar-pins and why you should get rid of them, leaf spring rates and placement to control axle wrap, coils vs coil overs, and the list goes on......

    This is gonna be a good thread.

    **edited for clarity
    Last edited by goodtimes; 02-09-2006 at 03:56 PM.

  10. #10
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    goodtimes is offline Expedition Portal Moderator Expedition Poseur
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    I'll break this into short, hopefully digestable topics.

    Linked suspensions, and why they have been developed.

    As vehicle manufacturers strive for improved handling, smoother rides, and longer component life, they realized that they needed a suspension that would offer better control of axle movement during suspension cycling and cornering.

    Linked suspensions (typically known as "coil suspensions" due to the spring design), refer to the way the axle is located under the vehicle. Through a series of "links" (commonly called control arms), the axle can be located front to back, side to side, and radially with regards to the axis of rotation of the axle shafts. Using different materials, link mounting points and link lengths, one can very precisely control the movement of the axle to obtain precise and predictable handling characteristics.

    Without going into design details of suspensions (which is outside the scope of this website), here are some of the effects of installing lift kits on vehicles with linked suspensions.

    Link angle, relative to the ground. As the link angle increases (due to a lift kit being installed), there are a couple things that happen to the suspension (ignoring the steering geometry changes).

    1) Road noise and vibration are transferred to the frame at a greater angle than a non-lifted vehicle. Vibration travels strait up the link, and when it hits the frame, it will is transferred to the frame....the angle at which this happens will greatly affect how much you feel. Vibration transferred to the frame at a low angle causea the frame to vibrate end to end, vibration transferred perpendicular to the frame will will cause the frame to vibrate up and down (as opposed to end to end). Due to the relative rigidity of the frame (end to end vs up and down) means you will feel that vibration more and more as the link angle moves away from parallel to the frame and closer to perpendicular to the frame.

    2) Axle movement. As the links move lower in their arc of travel, you get move horizontal movement per unit of vertical movement. This means that when you hit a bump, your axle can't move strait up, it will move in a arc, forward and up at the same time. So, as you hit that bump, not only is your vehicle moving forward in relationship to the road, but the axle is now also moving forward in relationship to the vehicle. This increases the vibration transferred through the suspension and into the frame. It's basically a double wammy.....you have increased vibration intensity moving up the link, and you are changing the angle at which these vibrations are transferred into the frame, resulting in a increased "feel" of the vibration.

    ***oops, gotta run, I'll continue with this later......

    Ok, it's later.

    3) This is a special case of #2. As I already pointed out, after a lift, the links are lower in their arc of travel. Now, if you have ever compared lengths of upper and lower links, you will notice that upper links are a little shorter than the lowers. This allows the castor angle to remain constant as the suspension cycles (more consistant handling). Basically, as the suspension cycles, the castor will remain close to the same angle, despite the change in the angle of the arms relative to the road. This works great as long as both sides of the axle are moving in the same direction. But what happens when you put one tire on top of a big rock, and one tire is still down on the trail? The side that is "up" will try to change the pinion angle one direction, and the side that is "down" will try to change the pinion angle the other direction....so in effect, you are trying to twist your axle housing. With a stock height vehicle, this twisting is minimal, because all of the arms are pretty close to being parallel with the ground (and frame)....this means that you get very little horizontal movement relative to the vertical movement. But when you lift the vehicle, this changes. Now you have a much larger difference in where the links are trying to position the axle (due to the links being much lower in their arc).

    4)Soft bushings usually allow adequate movement (to compensate for issue #3), but be careful. Soft bushings wear quickly, and do not allow as precise of control of the axle as compared to harder bushings. The downside of harder bushings is increased road noise being transferred up the links, and additional stress being placed on the links and link pockets (mounts). This last bit of stress mentioned is created by a misalignment between each end of the link. As your vehicle sits on the road, the bolts running through the bushings on each end of the links are parallel to each other. As one side of your axle lifts up , the bushing on the axle end of the link stays parallel to the axle, while the frame end stays parallel to the frame...but the axle and frame do not stay parallel to each other....basically trying to twist the link. So, again, here hard bushings are not so good. A well designed lift kit will make an allowance for this twisting by either making a 2 piece link that will twist around itself (like Tera), or by using a spherical joint on one end or the other (like Rubicon Express, Currie, Full Traction, Fabtech, etc). This is definately something you want in a lift kit. Interestingly enough, OEM's deal with this in a different way. They design their links to twist, which is why they are almost always stamped pieces, and never boxed.

    Now, all of the above applies to 4 link suspensions....and most of it applies to 3-link suspensions as well. The difference here is that on a 3-link, one of the upper links is not used. This eliminates the issue of the suspension trying to twist the axle housing in opposite directions. You need 2 attachment points to twist the housing in either direction....remove one of the 4 points, and it will only twist one direction at a time. The downside is that if the single upper arm fails, your stuck until it is repaired.

    I have not mentioned track bars (AKA panhard bars) either. These are usually not counted when counting "links". There are a couple things to watch out for though....as you lift your vehicle, the track bars will need to be lengthened to keep your axle centered under the vehicle. If you don't do this, your axle will be offset to one side, which can contribute to tires meeting sheet metal when you stuff a tire into the wheel well. Some people simply move the axle end of the track bar closer to the drivers side of the vehicle to compensate for the longer length between the mounts....the problem here is that you are now moving the pivot point away from the pivot point of your steering system, and can induce bump steer. It is better to have a proper length track bar. One last thing to remember, is that the bar will now be at a steeper angle at each of the joints, so be sure that the joint can operate in the new range.

    Ok, 2 more things.....I'll keep them short, because most of you have already skipped over this post anyway......

    A special case of the "3-link" is becoming very popular on the rear axle. A "triangluated 3-link" is just what it sounds like. 2 traditional lower links, but a triangle shaped upper link. This link has an attachment point on each side of the frame (driver and passenger), and one single attachment point in the center of the rear axle. This design allows the upper link to keep the axle centered underneath the vehicle, making the track bar useless...so you can get rid of it. It is a very functional design. But you will rarely see it on front axles because of the offset differential moves teh front driveshaft off to one side, and the transmission and engine oil pan get in the way.

    And finally...WHY would someone want to deal with all the problems of a linked suspension? predictable handling, no axle wrap, the ability to tailor the anti-squat number to the application, easier to tune for specific applications, and generally a better ride quality. The only real downsides are the complications when setting them up (which is one of the reasons quality lift kits are quite expensive), and when the link angles get too extreme, the suspension has a tendancy to do some quirky things...most of which must be experienced to be fully understood.
    Last edited by goodtimes; 02-09-2006 at 10:12 PM.

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