rear axle link bars on lifted truck

muskie · Post by **muskie** » Thu Nov 15, 2007 7:41 pm

on the rear 4 link bars do i lengthen the top bars or shorten the bottom bars to correct the angle of the rear axle??? truck lifted 3''

lengthen the tops seems the obvious way to me but what do you guys do??

have taken out panhard rods to modify aswell, i was surprised to see how much it pushed the axle over with 3'' lift, from what i can tell its only about 12mm i need to add to the panhard rods, i expected more.

doddzee · Post by **doddzee** » Thu Nov 15, 2007 7:44 pm

Other way around, you would either shorted the top or lengthen the bottom to correct the pinion angle.

I would get the bottom lengthened 16mm as it will return the rear axle to where it was factory.

muskie · Post by **muskie** » Thu Nov 15, 2007 8:02 pm

arrr yea i see now, thus levelling the spring seat with the top spring seat, back to parralel.

SupraLux · Post by **SupraLux** » Fri Nov 16, 2007 7:46 am

Dodzee, can you explain that? I think he's right - the top links are shorter than the bottoms so when the truck is lifted it causes the pinion to point more at the ground, so to my way of thinking the tops need lengthening to point the pinion back up and get the d-shaft angles right again.

Steve

doddzee · Post by **doddzee** » Fri Nov 16, 2007 8:12 am

SupraLux wrote::?: Dodzee, can you explain that? I think he's right - the top links are shorter than the bottoms so when the truck is lifted it causes the pinion to point more at the ground, so to my way of thinking the tops need lengthening to point the pinion back up and get the d-shaft angles right again.

Steve

When you lift it, it is "pulling" the bottom arms rotating the pinion up. We had this problem with my dads truck and the shocks were touching the springs.

doddzee · Post by **doddzee** » Fri Nov 16, 2007 8:20 am

Photo doesnt quite show it (stolen from DB) from factory you could see that the trailing arms on the diff sit slightly higher than the chassis mounts.

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GQTROL · Post by **GQTROL** » Fri Nov 16, 2007 9:21 am

The driveshaft vibration you get when doing suspension lifts is the result of the UJ's having to work outside normal parameters. The idea is to have the t-case output flange as close to parallel as possible to the rear diff input flange...apparently within 3 degrees of parallel is all good.

Suspension lifts rotates the rear diff and causes the input flange to point upwards more. The solution is rotate the rear diff backwards at the bottom (lengthen lower arms) or forwards at the top (shorter top arms).

A 4" suspension lift in a GQ will effectively shorten the wheelbase by 50mm, so I would prefer to lengthen the lower arms to restore driveshaft angles.

NJV6 · Post by **NJV6** » Fri Nov 16, 2007 1:19 pm

I think there are two seperate issues here.

One being the U joint angles and the other being the rotation of the housing leading to mounts (springs & shocks) being in the wrong place...

I understand what your say gq trol about getting parallel angles and lengthening the arms for restoring axle placement but lengthening the lower arms would cause an increase in u joint angles leading to the vibrations... :?:

Are you not wanting to point the rear pinion upwards to restore the angles? Then run into location of axle issues. Catch 22 i guess.

I really have no idea but to me that would make sense.....

doddzee · Post by **doddzee** » Fri Nov 16, 2007 1:37 pm

NJV6 wrote:Are you not wanting to point the rear pinion upwards to restore the angles? Then run into location of axle issues. Catch 22 i guess.

I really have no idea but to me that would make sense.....

No when you lift it the pinion rotates up therefore you lengthen the bottom arms rotating the pinion back down restoring the factory angles or close to it. The Unis should be running parallel to eachother to prevent vibs.

NJV6 · Post by **NJV6** » Fri Nov 16, 2007 2:29 pm

I understand the lifting changing angles.

But the distance between the trans case output and pinion is greater on a lifted vehicle..... So if they were to remain parallell then the angle the uj's are operation in would be greater.

Is this not why you lower the gearbox mounts on a shorty GQ when you lift it?

or have I completely lost it now... :shock:

Its actually quite interesting.

Is the reson to keep them as close to parallel is to avoid vib's from the UJ's going through the 2 different speed cycles in one rotation...?

SupraLux · Post by **SupraLux** » Fri Nov 16, 2007 4:07 pm

All I'm missing is how the pinion goes up.

If the top links are shorter than the bottom ones, and they are near enough to parallel then pulling the axle away from the chassis (ie, lift) will cause the top of the axle to roll forwards due to the bottom ones having a bigger arc because they are longer

That needs to be rear in one breath, but might take a few more breaths to understand what I'm saying...

I'm probably missing something obvious about the setup of a GQ, I need pics...

Steve

turoa · Post by **turoa** » Fri Nov 16, 2007 4:16 pm

umm steve, GQ upper links are really short, and thats the same with lots of 4wds.

So when you lift it the pinion gets pushed up. By lengthening the lowers it gets the pinion flange nearer to parallel with the output flange on the t-case.

SupraLux · Post by **SupraLux** » Fri Nov 16, 2007 4:25 pm

are the uppers behind the diff instead of in front like the lowers? :idea:

...or are they on a really stupid angle down from the top of the diff when the truck is at rest?

GQTROL · Post by **GQTROL** » Fri Nov 16, 2007 8:52 pm

doddzee wrote:
NJV6 wrote:Are you not wanting to point the rear pinion upwards to restore the angles? Then run into location of axle issues. Catch 22 i guess.

No when you lift it the pinion rotates up therefore you lengthen the bottom arms rotating the pinion back down restoring the factory angles or close to it. The Unis should be running parallel to eachother to prevent vibs.

What Doddzee said is right on the money.

NJV6 wrote:But the distance between the trans case output and pinion is greater on a lifted vehicle..... So if they were to remain parallell then the angle the uj's are operation in would be greater.

The distance, longitudinally, between the t-case and diff pinion is actually less in a lifted GQ...i.e. the wheelbase is shorter.

NJV6 wrote:Is the reson to keep them as close to parallel is to avoid vib's from the UJ's going through the 2 different speed cycles in one rotation...?

Yes, exactly right.

SupraLux wrote:If the top links are shorter than the bottom ones, and they are near enough to parallel then pulling the axle away from the chassis (ie, lift) will cause the top of the axle to roll forwards due to the bottom ones having a bigger arc because they are longer

Unfortunately no, or atleast not in a GQ. Top of the axle will roll backwards with a bigger lift. Bottom arms are on a steeper angle than the top arms.

muskie · Post by **muskie** » Sat Nov 17, 2007 2:04 pm

so then i now see how it works, 16mm extension in the lower arms should do it. dont really want to make them adjustable as its a bit obvious when it comes to WOF time.

DaveM · Post by **DaveM** » Sat Nov 17, 2007 2:19 pm

muskie wrote:so then i now see how it works, 16mm extension in the lower arms should do it. dont really want to make them adjustable as its a bit obvious when it comes to WOF time.

The way things are going, you will need a cert for the lift soon anyway :roll:

muskie · Post by **muskie** » Sat Nov 17, 2007 9:31 pm

DaveM wrote:
muskie wrote:so then i now see how it works, 16mm extension in the lower arms should do it. dont really want to make them adjustable as its a bit obvious when it comes to WOF time.

The way things are going, you will need a cert for the lift soon anyway

yeah your about right there :lol:

nivaman · Post by **nivaman** » Sat Nov 17, 2007 10:32 pm

muskie wrote:
DaveM wrote:
muskie wrote:so then i now see how it works, 16mm extension in the lower arms should do it. dont really want to make them adjustable as its a bit obvious when it comes to WOF time.

The way things are going, you will need a cert for the lift soon anyway

yeah your about right there

Damn it they have to spoil our fun.

carver65 · Post by **carver65** » Sat Nov 17, 2007 10:42 pm

I faced the same problem when I bought a jap import swb. I haven't figered it yet (maybe someone can tell me?)but I think about 3" lift. There's roughly 6 1/2" between 31"tyre and bottom edge of flare. Anyway I reduced my vibes by making my top arms adjustable. I shortened the tops forcing the pinion down and increasing the UJ angles which I have to admit look pretty ugly.But this allowed me to get the flanges as close to parallel as pos thereby reducing the vibes significantly. As far as rear axle location goes the wheel seems to sit fairly centered in the arch.There is however still a small amount of vibe creeping in at around 95-100 km/hr,bugger. So I'm thinking about lowering the trans mounts so I can reduce angles in the drive line, but don't know how far I can go before upseting, T shift and engine angles ( any help?). Plenty of black paint and a bit of dirt and the wof man doesn't even know it's not standard unless he's a Nissan owner, but then he probably would'nt mind.

GQTROL · Post by **GQTROL** » Sun Nov 18, 2007 8:57 am

carver65 wrote:I faced the same problem when I bought a jap import swb. I haven't figered it yet (maybe someone can tell me?)but I think about 3" lift. There's roughly 6 1/2" between 31"tyre and bottom edge of flare. Anyway I reduced my vibes by making my top arms adjustable. I shortened the tops forcing the pinion down and increasing the UJ angles which I have to admit look pretty ugly.But this allowed me to get the flanges as close to parallel as pos thereby reducing the vibes significantly. As far as rear axle location goes the wheel seems to sit fairly centered in the arch.There is however still a small amount of vibe creeping in at around 95-100 km/hr,bugger. So I'm thinking about lowering the trans mounts so I can reduce angles in the drive line, but don't know how far I can go before upseting, T shift and engine angles ( any help?). Plenty of black paint and a bit of dirt and the wof man doesn't even know it's not standard unless he's a Nissan owner, but then he probably would'nt mind.

Most 2" susp lift require about 10mm of spacers to the gearbox cross-member. Since you've already got the adjustable top arms, throw a handfull of washers between the cross-member and chassis till the vibs go away. Then replace the washers with proper spacer plates of the required thickness. That way it absolutely no more than it needs to be.

carver65 · Post by **carver65** » Sun Nov 18, 2007 8:17 pm

Thanks for the advice.

muskie · Post by **muskie** » Mon Nov 19, 2007 5:08 pm

took bottom bars to be lengthened 16mm today, he is going to cut and chamfer the arms and make up push fit chamfered inserts then weld fully also drilling and and spot welding at the end of the inserts. box of pi$$, happy with that. also going to do panhard rods for me if i get the threaded rod and nuts. awesome, didnt even know there was an engineering firm renting one of the spare outbuildings at work :shock:

, aparently an engine reconditioners

glad i asked him what he did now :wink:

DaveM · Post by **DaveM** » Mon Nov 19, 2007 5:12 pm

muskie wrote:took bottom bars to be lengthened 16mm today, he is going to cut and chamfer the arms and make up push fit chamfered inserts then weld fully also drilling and and spot welding at the end of the inserts. box of pi$$, happy with that. also going to do panhard rods for me if i get the threaded rod and nuts. awesome, didnt even know there was an engineering firm renting one of the spare outbuildings at work , aparently an engine reconditioners glad i asked him what he did now

Good stuff. Just remember if you go adjustable panhards, the wof man will more than likely look for a cert plate for them.

muskie · Post by **muskie** » Mon Nov 19, 2007 5:36 pm

DaveM wrote:
muskie wrote:took bottom bars to be lengthened 16mm today, he is going to cut and chamfer the arms and make up push fit chamfered inserts then weld fully also drilling and and spot welding at the end of the inserts. box of pi$$, happy with that. also going to do panhard rods for me if i get the threaded rod and nuts. awesome, didnt even know there was an engineering firm renting one of the spare outbuildings at work , aparently an engine reconditioners glad i asked him what he did now

Good stuff. Just remember if you go adjustable panhards, the wof man will more than likely look for a cert plate for them.

yeah i think i will pretty much have to get it certed in the end, didnt really plan on doing this much but its a hobby and keeps me busy.