I have front end wobble

[BigRAWesty]

Ok another subject I don't write about much due to the very odd ideas so many people have and seem to be a expert with but have no real idea of the maths or the dynamics or the science and engineering require to even gets a small perspective of. I am no expert but for a few book smarts because I wanted to know how the limited educated monkey on the wheel alignment machine could possible have any farken clue when he try's to tell me why my Nissan get the wobbles, and usually its just dribble being sprued from his no idea brain. In the US this subject is a 4 year degree course yet the kid on the alignment machine or the suspension shop owner or tyre shop owner is a bloody expert in the industry. There is one guy called Adams who has written many texts books and suspension software, I have a few of his books plus a few others use in the degree schooling of suspension dynamics and correct steering. Its odd really because the very first page of Adams suspension dynamics text book had me totally feeling like a complete noob yet our suspension shop dude is a bloody expert mmmmm somehow I have missed something here..

Anyway rant over, time for a little tiny incite to the world of steering dynamics. but more a tiny bit of understanding of our live or beam axle Nissan. Our Nissan engineer dude who is pretty smart produced a very basic but very robust front diff for us with a few compromises. In his wisdom he didn't account for us Aussies lifting our rigs even so he still did a good job. What he did for his std height patrol was to dial in a effective geometry for the std height condition with some nice angle geometry to be quite forgiving. One of the things he did was to give the tie rod a nice Ackerman angle so the wheels give a good compromise to stability when turning up to 10 degrees. This was to compensate for the crappy caster change we get with our short radius arms. He also set some odd angles on the panhard rod in relationship to the drag link to help stability for the wheel travel used which he limited so he didn't get to much change on the rear positioned tie rod.

Now you should have a little incite to what we have and a little bit on the why because as you would expect I could write a book just on those few items mentioned here. Anyway back to wobbles which is the OP's question. Our Nissan smart dude even though he designed a smart system he didn't quite understand the effect on how critical caster and king pin inclination would become or how tight the front end would have to be using a rear tie rod with such a high Ackerman angle on the arms. What happens is if you loose caster below 2 degrees the kingpin angle on a slight turn or steering angle moves the king pin inclination contact point of the tyre on the road to far back to keep the wheels in a stable condition hence if any of your steering components are slightly loose then stability is compromised dramatically hence the start of the wobbles. Even a slight lift of the rig puts a lot of this carefully thought out geometry at risk due to the short radius arms and the huge change in caster that happens when the suspension cycles. At normal std height the engineer has all the bits within safe parameters.

But little changes we don't consider like wheel offset has a dramatic effect on our Nissan it moves the contact patch or the rotation arc of king pin incline to a different balance point on the tyre if the geometry is just a little out this sort of thing can effect the inbuilt stability of the set geometry hence more prone to the wobbles if other bits are a bit loose etc. There are so many things I can suggest or a combination of things that cause this effect. Even camber with too little caster with a loose panhard will set the balance out. Sure most shops go to great length to suggest rims and tyres and balancing on and on but these things are just stop gaps to the real combination of issues causing the issue.

Just to prove a point or a suggestion of a different perspective on another OEM who designed this type of suspension, example Range Rover early models. They have the tie rod on the back with a closed Ackerman angle they used a different sweep angle for their radius arms but have the same caster kingpin angle and camber but they do not have anything like the tenderness of the Nissan to get the speed wobbles. Or even the early model F100 same basic system tie rod on the front, but a different design to the geometry, without the tenderness.

I have had a go at this phenomena a few times when the owner has given up, I get the usual story the geometry is perfect the suspension shop said so. Setting my trusty iphone on the wheel with an alignment app and you guessed it, it has less than 2 deg of caster, the book of god says 2.5 to 3 degrees there is a reason for this otherwise the Nissan dude would not have set that spec. For example a RR is 1.5 to 3.5 degrees mmmm funny that the RR has a lot more tolerance I wonder why eh.. On one particular rig he had changed his springs and the body had 2.5 degrees of angle down to the front then he had 3 deg caster correction bushers and drop boxes and still the front end had +1.2 degrees of caster. Hence he had massive wobbles and no matter what he or the suspension shop did, it was never going to go away. It just so happens not all Nissan diff geometry set in the factory are all equal the control arm plates had been welded on with too little caster built in. We lowered the back and had to bend the radius arms to get anything like -2.5 caster and a proper camber which is fix to caster of course. Funny thing is after the adjustment all was back just like STD.

Another thing I have had to adjust to fix this issue or more to the point extend the tenderness of the front end. Our Nissan dude in his design for STD height and parameters he set the panhard rod at a angle to the drag link in plan view. This is all good if both links are level in front view but lift the rig just a bit and this geometry gets a little to far out side of our set safe parameters so out tenderness to the wobbles is increased 10 fold on slight bumps. Had they been lever and parallel say like a Range Rover or F100 then this tenderness would not be as bad. I have had to lower this on the chassis end on a 3 inch lift so the rig could have different off set rims the owner wanted to use and yes problem solved.

I could go on and on here of course but really to fix this issue there is a bit more to it than meets the eye. I am not saying just because you changed you panhard bushers or tightened your wheel bearings or kingpin bearings the problem went away end of problem. What I am saying it shouldn't matter if there is a bit of movement in the bush or bearings because there is something else or a combination of other bits that has made your rig prone to death wobbles. And for god sake don't let a suspension shop fit those damn eccentric kingpin bearings that is not a fix its a bodgy if ever there was one. And that is for another time to explain.

Brilliant mate. That makes complete sense.

[P4trol]

It sounds like you need your own thread so this doesn't get lost.

[sooty_10]

And I thought oldmav only knew intakes lol. Great info and also appreciate.

[OldMav]

And I thought oldmav only knew intakes lol. Great info and also appreciate.

Lol yeah sometimes you would get that idea, I do get passionate about manifolds among other things.

[Drewboyaus]

Thanks @OldMav. That is a great post. Has given me something to think about in the diagnosis of my own Q's wobble.
Speaking of which, I have been leaning towards a wheel or balancing issue because it only happens within a very specific speed variance before smoothing out and I was thinking it was related to a specific frequency of vibration at a given road speed........

[Robo]

So raising the vehicle upsets the steering geometry.
result, any lift above std height moves the diff back away from panard, drag link and damper, right-o understand that.

Humm thinking, thinking.
Installing lower arm spacers that push diff forward, move large tyres forward to clear guards.
And in return geometry to a more std arrangement.

CAN by doing this in turn help to reduce wobble?.
or at least reduce bump steer?.
can spacers serve 2 purposes?.

I'm told less angle on steering arms help to reduce bump steer so why not wobble to boot!.
Eg- drag link as parallel as possible to diff to reduce bump steer.
should this be a must do if lifting? even if not fitting large tyres.

[OldMav]

So raising the vehicle upsets the steering geometry.
result, any lift above std height moves the diff back away from panard, drag link and damper, right-o understand that.

Humm thinking, thinking.
Installing lower arm spacers that push diff forward, move large tyres forward to clear guards.
And in return geometry to a more std arrangement.

CAN by doing this in turn help to reduce wobble?.
or at least reduce bump steer?.
can spacers serve 2 purposes?.

I'm told less angle on steering arms help to reduce bump steer so why not wobble to boot!.
Eg- drag link as parallel as possible to diff to reduce bump steer.
should this be a must do if lifting? even if not fitting large tyres.

I will try and explain this simple like.
First question : Raising the vehicle changes the angle of the radius arm hence caster/camber is changed the change is not linear it gets exponentially worse the higher you go.
Yes the wheels move back as you raise the vehicle but the relationship to panhard and drag link only change slightly in top view but the relationship in front view is very different the arm become less parallel. This relationship effects bump steer.

Second : MMMmmm this bit is a bit misunderstood, Moving the diff forward will change the top view position relationship but this only has a very slight effect to bump steer when the vehicle articulates so no effect for on road feel. The front view doesn't change so the arm relationship is still the same no change to parallel-ness. so no change to bump steer. But to be very pedantic there would be the most slightest may be a minute in caster change due to the slightly longer arm.

Third : As above that's a big NO. The spacers have no real effect to geometry that is enough to be detected. Their purpose is the stop tyres rubbing when you lift the vehicle Due to radius arms pivot arc relationship.

Forth : Less angle is not quite right its a parallel relationship in front view the drag link and the panhard rods must be parallel to have no bump steer for this link relationship. When the wheels articulate you have bump steer regardless as the drag link and the panhard have to be of equal length to remove this condition. In top view having the links parallel only helps the articulation relationship. Our Nissan setup is not set even close to parallel and this relationship gets worse as the vehicle moves up and down due to the diffs action rolls the diff under. Larger diameter tyres changes nothing with link relationship but it does change components of geometry like COG, instant centre, anti squat/dive. Changing the distance relationship between sprung and unsprung weight changes every geometry component on a live axle of the Nissan design. the radius ARM system is not a good system when you raise the vehicle due to its fixed no ability to adjust these dimensions easily.

To note : death shakes is caused by a combination of non ideal geometry or more to the point for our Nissan patrol there is not enough safety margin built into the geometry to eliminate it when the vehicle is raised above the STD height. Non ideal wheel balance should only shake the steering wheel not cause Death shakes just like a normal road car. Bump steer can START death shakes if there are other geometry components settings are not ideal or components are loose or not adjusted correctly but bump steer by itself will not cause the death shakes. Oh just so you are clear wheel balance does not cause Death wobbles, non ideal geometry cause death wobbles wheel balance is only a trigger factor to START Death Wobbles loose components do not cause death wobbles it is only a trigger to START death wobbles. And again non ideal geometry is the cause of DEATH SHAKES loose and unbalanced components only mask the root cause, to FIX death wobbles you FIX geometry mainly CASTER/ CAMBER relationship.

Ok I will stick my neck out and state something profound mmmm if you do not have between 2.2 and 3 degrees caster vehicle loaded TO unloaded you have a extremely high probability to experience DEATH Wobbles when components become loose or unbalanced or bump steer movements. Also remember for our Nissan radius arm system if you have a vehicle empty with its ass sticking up in the air skyward then you have reduced your caster by that amount in degrees off level. for example I see many rigs with HD springs fitted in the rear with the vehicle 2 degrees off level. The so called expert suspension dude puts in the required caster plates the lift requires But neglects to adjust for the vehicle angle. If the vehicle is 2 degrees out of level then it has 2 degrees reduced caster. So you see this CRAP stuff is important, and a logical knowledge is required to fix this issue.