Front suspension geometry...lets get in depth...

[MaXJohnson]

... go ahead and keep spinning my words.

no spin required. Just responing to your comments.

Suck it up, man. It's only the internet...

 

[Beezil]

Suck it up, man. It's only the internet...

I already sucked.

 

[Bob Sheaves]

Max,

I have been lurking on this board for a bit and have found your postings to be quite accurate. As a dynamics engineer, your description of the geometry and how it works is spot on. Congratulations!

Too bad others do not realize the importance of a balanced approach to suspension design. When an engineer makes a mistake, people die- a fact of life that too many dismiss because of ignorance or supposition.

Best regards,

Bob Sheaves

ADDENDUM:

FYI- the stock MJ & XJ have a bit over 18% A/D on the OE base tires...

 

[Beezil]

okay max and bob....

for the last time, I never told willis to throw away his playbook and wing it.

quit making it look like thats what I was trying to do.

I would be interested in seeing your designs for something like wilis wants to do that addresses all the difficulties of the xj platform. Since you seem threatened by my realistic and practical "heads up" approach, and don't seem to understand the value of what I'm trying to do on this thread, show me how it can be done. lets see a couple versions.

I think research is extremely important. Learning new things is very cool. There is plenty to be learned on this thread, and the infomation posted is fantastic.

To my knowledge, neither of you have ever made an attempt to incorporate the theoretical "PERFECT" link suspension set-up on a cherokee platform. I've never seen your rig bob, but max, isn't your rig a variety of various bolt-ons and possibly some non-exotic diy modifications? For someone that has such an excellent grasp on theoretical suspension geometry, why aren't you trying to incorporate any of this on your own rig?

 

[Bob Sheaves]

Ummmm....actually, I did on my 1990 XJ when I was at JTE-that car was used as a chase car for several "test sessions" including one out to Moab in the summer of 1991.

Have you ever driven a Dodge Ram of 1994 to 2000 vintage with the 4 link over constrained front suspension?

Best regards,

Bob Sheaves

PS- I never mentioned any specific names in my comments about people. I was making an observation about MOST people here and their collectively cavelier attitude about modifying suspensions-particularly the XJ/MJ. I am not saying it cannot be done-just that there are tradeoffs that should be accounted for that most people here demonstrate a distinct lack of awareness and distain for.

 

[Willis]

My long radius arms unloaded HORRIBLY. sure, a centered limiting strap worked WONDERS.

So does your parabola system unload at all? Did you have the need or desire for a limiting strap at all?

Also, is there a need to angle the lowers (as viewed from the top) with the parabolic link? Can they be completely straight? In a standard rear 4 link, the distance the mounting points are apart at the chassis is commonly around 50-70% what they are at the axle. This is to help combat rear steer. What would be the benefit of angling the lowers using a parabolic upper?

Steve

 

[Beezil]

bob, tell me why a dodge ram is at all relevant?

willis, no, it doesn't seem to....

btw, have you made any decisions yet?

do you have an idea of tire size, or which axles you are going to use, or maybe lift hieght?

how comfortable are you in loosing some sheet metal?

would you consider taking metal out of the floor?

what kind of boingers are you going to use?

 

[Willis]

Too bad others do not realize the importance of a balanced approach to suspension design. When an engineer makes a mistake, people die- a fact of life that too many dismiss because of ignorance or supposition.

It's true, many do go and just build a setup with no regard to geometry and characteristics. I, and others, who are trying to incorporate a link setup to the XJ do see that you may start with a balanced approach, but given the XJ chassis as your canvas, balance can not always be attained. Just because that balance is not attained, does not mean a mistake has been made, it just means that compromises have been made. You just have to choose your compromises and calculate them to achieve a result close to what you set out to build.

Of course we are building trail rigs, and not bridges or carnival rides here, so please leave out the whole death issue. Can some moron build a very unsafe suspension? Of course, but the majority of us here have enough common sense to build enough safety into our systems. Just because the lower links end up 3* off of perfect does not mean anyone is going to die.

Given the safety factor we build into our vehicles like cages, the safety factor is pretty high. We are not engineers here, but the proper design and execution of a link type suspension is what this thread is about. The proper design may differ from the execution given the platform, and that's what Beezil is saying. He never said throw out all your designs and put your links wherever you want.

 

[Bob Sheaves]

The Dodge pickup suspension fixes a lot of the inherent limitations of the XJ/MJ design. How do I know....I was responsible for all the 4x4 BR suspension design and parameters at JTE in the old PreProgram group. There is a distinct difference between the two in perfromance of the suspension. As it relates to the anti-dive discussion, the XJ has 18% and the Dodge has 36%. The Dodge has far less non-symetrical axle steer due to linkage compound ratios when co,mpared to the XJ/MJ.

Best regards...

Bob

 

[Beezil]

"balance" is a compromise between what you want and can to do, and what you CAN'T do.

that's great bob.

but that doesn't help willis now does it.

 

[Willis]

willis, no, it doesn't seem to....

btw, have you made any decisions yet?

do you have an idea of tire size, or which axles you are going to use, or maybe lift hieght?

how comfortable are you in loosing some sheet metal?

would you consider taking metal out of the floor?

what kind of boingers are you going to use?

Ya, 64" HP D44 front (already have), 35" MTRs (already have), hoping to keep lift as low as possible, I have 7" planned in my sketches. The parabola is perfectly parallel with the ground but 8" above the lower link. I need to check oil pan clearance though.

Looks like I may not have to remove metal from the floor for the front, but it depends on where it would be. This will not be daily driver, and I already have my MJ cab to do a chop, plus dovetail the rear is planned. Front fenders are already trimmed to rear TJ flares.

And, what are boinger? Ya lost me there.

 

[Beezil]

And, what are boinger? Ya lost me there.

springs!

 

[Willis]

Gotcha! Coils front, unsure on brand. Maybe 5.5 REs with spacers? Using leaves rear. I've got some 4.5" RE leaves I plan on taking apart and using some stock waggy front springs in the pack. The waggys have 7 thin leaves per pack and flex well. I'll combine the leaves until I get the flex and lift I desire. That's first, custom leaves come second if I can not make what I have work first.

 

[Bob Sheaves]

That kind of comment Beezil, is what I would expect of someone that caused of the demise of the CJ's . Too bad...

Let me spell it out for you in simple terms....

A. By directly comparing the BR to the XJ, in stock form, you can see how the compromises made improved the design to where it had greater travel potential, less driveline change (leading to improved joint durability), greater driver control under extreme travel positions, less suspension induced uncontrolable dynamics, and less lateral axle movement relative to vehicle centerline. All of these things are what is striven for in the off road community.

B. "Balance" has nothing to do with what "you" (as an individual) can or cannot do-anything can be done, given time, money, and understanding- but rather, the balanced compromise between street legal drivability and off road capability.

Willis....dying is something that you had best be concerned with, if not for yourself, than be VERY concerned with the responsibility towards others if you take your vehicle on a public road or highway. If you EVER take a modified vehicle on public roads-YOU are responsible for the results of those changes in an accident. A good lawyer will take you to the cleaners for the rest of your life if you make a mistake. Good intentions don't mean a damn thing in a court.

That being said, I wholeheartedly approve of your thought in asking the questions you have. I would only hope you listen to the advice given by Max. You have to make the decisions on how to do the modifications, but thinking before cutting and welding is far more productive than simply dismissing as irrelavent, comments that you disagree with.

Best regards,

Bob

 

[Bob Sheaves]

Additional thought Willis...

You may be interested in looking for some old pictures of Mike Leslie's MJ and XJ off road race cars from the late 1980's and early 1990's. The 7's (MJ's) has 19 inches of travel from jounce to rebound (don't ever say "droop" or "bump" to a real engineer...LOL!) in the rear and, as I remember, 22" in the front...all with the equivelent of a 6" ride heigth increase relative to axle centerlines (means discounting tire related increases).

Check out the boards at racedezert.com-there are some of the old guys there that may have some.

Best regards,

Bob

 

[Willis]

Willis....dying is something that you had best be concerned with, if not for yourself, than be VERY concerned with the responsibility towards others if you take your vehicle on a public road or highway. If you EVER take a modified vehicle on public roads-YOU are responsible for the results of those changes in an accident. A good lawyer will take you to the cleaners for the rest of your life if you make a mistake. Good intentions don't mean a damn thing in a court.

That being said, I wholeheartedly approve of your thought in asking the questions you have. I would only hope you listen to the advice given by Max. You have to make the decisions on how to do the modifications, but thinking before cutting and welding is far more productive than simply dismissing as irrelavent, comments that you disagree with.

For the record, I don't want to die, and don't want to kill anyone. Of course that is a concern, but that chance everyone takes when making modifications to the XJ. The guy who lifts it 2" and removes the rear sway-bar can be held liable if some lawyer decides to pursue that avenue during prosecution of an accident where death was the result. Sure some XJs came with a 1" lift and no rear sway bar, but that extra inch has added anti-dive to the suspension dynamics. Some aggressive lawyer can argue that extra anti-dive did not allow enough weight transfer during braking and caused the accident. Does that mean the custom linked suspension I build will be more dangerous than his? No. That's what I mean my leave the whole death issue alone. Since this will be used on road, safety will be my #1 concern. I plan on building a suspension with better road characteristics than a 4" lifted XJ with 4 short arms.

Max has provided a great basis for building a suspension. If the 'perfect' suspension can not be adapted to the XJ, it does not mean it is unsafe. I am listening to Max, his knowledge on this subject is much appreciated and his advise is heeded. But from a real world standpoint, which Beezil has been, some of the ideal positioning may have to be changed. Again, with safety in mind. I have yet to dismiss anything as being irrelevant.

Steve

 

[Willis]

Additional thought Willis...

You may be interested in looking for some old pictures of Mike Leslie's MJ and XJ off road race cars from the late 1980's and early 1990's. The 7's (MJ's) has 19 inches of travel from jounce to rebound (don't ever say "droop" or "bump" to a real engineer...LOL!) in the rear and, as I remember, 22" in the front...all with the equivelent of a 6" ride heigth increase relative to axle centerlines (means discounting tire related increases).

Check out the boards at racedezert.com-there are some of the old guys there that may have some.

Best regards,

Bob

I'll check into that out of curiosity. His suspension will be miles away from what I plan to build. There is no way I need that much 'jounce'. This thing will never (intentionally) get airborne.

Thanks

 

[XJJPR]

The Dodge pickup suspension fixes a lot of the inherent limitations of the XJ/MJ design. How do I know....I was responsible for all the 4x4 BR suspension design and parameters at JTE in the old PreProgram group. There is a distinct difference between the two in perfromance of the suspension. As it relates to the anti-dive discussion, the XJ has 18% and the Dodge has 36%. The Dodge has far less non-symetrical axle steer due to linkage compound ratios when co,mpared to the XJ/MJ.

Best regards...

Bob

Bob,

Seeing your saying you're a Dodge engineer, working for Dodge either directly or indirectly thru a subcontractor, if I'm wrong please let me know. If I'm not where do you work?

Couple questions on the Dodge.

First off I truley hated my 95 dodge 4x4 2500hd quad cab long bed suspension/braking system.

What was it in the suspension that allowed for massive side pull during braking with oversized tires verses stock size tires. All the people who have explained it to me have never done a great job on this.

With all the beef in that front suspension why was such a crummy track bar used. It's gotta be one of the most under engineered things on any truck suspension in the last 10 + years.

I'm no where close to understanding all the suspension design you guys are talking about but have messed with the XJ/MJ suspension in it's relative stock/modified forms. Staying with basic stock type mounting. I do know what works on and off road, maybe not by design stats but by what it does.

BTW I always love when engineers boast about if things aren't engineered right people die. Do you realize how many people have died from engineers making mistakes? All engineered or DYI aren't perfect but most try to be as good as they can be, because noone wants to see anyone die from a mistake.

I will say I'm always amazed at how many DYIs do do such hugh suspension changes and what they get away with. But then some of the big suspension manufacturing companies blow me away with what they get away with too.

mark

 

[Bob Sheaves]

I'll add my comments inline, if you do not mind....

Bob,

Seeing your saying you're a Dodge engineer, working for Dodge either directly or indirectly thru a subcontractor, if I'm wrong please let me know. If I'm not where do you work?

At the time, I was a direct employee of Chrysler Corporation in the PreProgram Engineering Group (R&D-we set the parameters for vehicles then handed off the designs to the various production groups). Currently I am consulting with DCX as an independent on "business process analysis".

Couple questions on the Dodge.

First off I truley hated my 95 dodge 4x4 2500hd quad cab long bed suspension/braking system.

Sorry to hear that-over 90 percent of those that bought the truck initially bought another.

What was it in the suspension that allowed for massive side pull during braking with oversized tires verses stock size tires. All the people who have explained it to me have never done a great job on this.

While I cannot comment on other people's hearsay, I am afraid you have not given me enough information to determine what was "wrong", if anything. Oversize tires change the response geometry due to a greater didtance from the center of the hub to the ground contact patch-greater leverage means increased loading from braking transferred to the pivot axis (the hub centerline). Changing the load in the vehicle will alos make a difference-as little as 3/4" change in the relative height of the frame mounts for the control arms and the frame end of the panhard rod will alter this reaction to braking.

Under your circumstances, what was the loading of the truck? (The design point is called BPL-Body Part Loaded- and is an SAE and industry standardized design process.) How many passengers? How much fuel? All of these things alter the geometry as they are changed. In some cases, as little as 2% weight shift can cause drastic changes in the manner a car responds to steering and braking input.

With all the beef in that front suspension why was such a crummy track bar used. It's gotta be one of the most under engineered things on any truck suspension in the last 10 + years.

What makes you say the problem lies with the panhard rod? That bar is far stiffer than anything the XJ has, proportionally. Perhards the fault lies elsewhere.....like the frame bracket loading???

I'm no where close to understanding all the suspension design you guys are talking about but have messed with the XJ/MJ suspension in it's relative stock/modified forms. Staying with basic stock type mounting. I do know what works on and off road, maybe not by design stats but by what it does.

BTW I always love when engineers boast about if things aren't engineered right people die. Do you realize how many people have died from engineers making mistakes?

Yes, as a matter of fact I do, since I deal with this stuff everyday, and have for over 28 years.

All engineered or DYI aren't perfect but most try to be as good as they can be, because noone wants to see anyone die from a mistake.

I will say I'm always amazed at how many DYIs do do such hugh suspension changes and what they get away with. But then some of the big suspension manufacturing companies blow me away with what they get away with too.

mark

You are exactly right-"THEY GET AWAY WITH IT"...for a while. Does anyone here remember Jackman Wheels? Enough said...

Best regards,

Bob

 

[XJJPR]

Bob,

Sorry to hear that-over 90 percent of those that bought the truck initially bought another.

That's hard to believe unless you meant "another brand" I bought a GMC sold that now too.

While I cannot comment on other people's hearsay, I am afraid you have not given me enough information to determine what was "wrong", if anything. Oversize tires change the response geometry due to a greater didtance from the center of the hub to the ground contact patch-greater leverage means increased loading from braking transferred to the pivot axis (the hub centerline). Changing the load in the vehicle will alos make a difference-as little as 3/4" change in the relative height of the frame mounts for the control arms and the frame end of the panhard rod will alter this reaction to braking.

My basic believe that the whole thing had problems as a suspension system. The braking problem was wide spread from all I could ever reaserch on it. DC ofcourse denied everything.

What makes you say the problem lies with the panhard rod? That bar is far stiffer than anything the XJ has, proportionally. Perhards the fault lies elsewhere.....like the frame bracket loading???

The joint fails way to soon for the socalled HD the truck was supposed to be.

Yes, as a matter of fact I do, since I deal with this stuff everyday, and have for over 28 years.

It's nice to see a real engineer here, because the wana bes would have argued that one until the cows came home

Does anyone here remember Jackman Wheels?

Yes I do!!!!!!! They weren't DYIers...

thanks for your info

mark