Understanding suspension and steering

[RMP&O]

I thought this may be useful info to some people.


Let's start with caster. What is caster? To keep it simple all you need to remember is caster is the angle the the steering arms are at on the knuckle. If you take an angle finder and place it on top of the knuckle you will get a reading. On Patrols I find this to be 2 degrees (2*) with decent but old suspension. Now 0* would be totally level. Keep that in mind. Forward of the 0* is called negative caster. That is bad. Back from 0* is called positive caster and is what you want. When you lift a truck or the suspension is very old and sagged it changes caster. This applies to both leaf spring and coil spring trucks. Lifting the truck rolls the axle forward reducing caster. Lowering it rolls the axle back increasing caster.

I have looked in the manuals and they only talk about caster in terms of how it is designed. The manuals only mention how to figure caster using a very old school (but very sound) method that is complicated. These days an angle finder or digital level does the trick easily. Point is though, there is no reference to caster in the books as far as what it should be for a stock Patrol. Again, my best estimates are 2*, that is 2* positive as anything in the negative range is a big nono.

For more reading on caster go here
http://en.wikipedia.org/wiki/Caster_angle

Now what happens to a truck when the caster is off? Lets say it is 2* negative or -2*. Going down the road the truck is going to be squirrelly, wander a lot and not corner well. You will notice this most at speeds above 40mph. If caster is way out, let's say -5* the truck will be almost un-drive-able and dangerous. So caster is very important in how the truck handles and drives.

Caster is really only going to be an issue on an old Patrol if you lift the truck. On a leaf sprung truck generally shims are used between the leaf spring and the axle housing to bring the angle back into specs. I personally really dislike shims. What I do is called a "cut & turn". I think a C&T is a bit advanced though for 90% of Patrollers and only needed if lifting over 3" or doing a spring over axle conversion. So I think I will skip describing it and what it does for now. One thing to remember though is the bigger the tire you run the more caster you want. For example, if stock caster on a Patrol is 2* with a 31" stock tire I would want my caster at 4-5* with a 35" tire. Increasing caster with a large tire makes it drive like the tires are small.

One last thing about caster. I am not sure it applies to a Patrol but it does apply for any more modern vehicle, let's say from 1980 onward. Caster is different side to side from the factory. It can be different by as much as 1-2* but generally .5* or there abouts is normal. Now why would caster be 2* on one side and 2.5* on the other from the factory!? Simple, it accounts for the crown in the road. If caster is the same on both sides and the road has a good crown on it the crown will push the truck to the shoulder. If the caster is off a bit like .5* it makes the truck push into the crown (not noticeable) which in turn makes the truck track straight down the road.

Camber does not apply on a solid axle truck like the Patrol. Camber is the angle left to right if looking at the front of the truck that the tires sit on. This is built into a Patrol and can not be changed. Lifting the truck or anything you do to it will not change camber.

Ackerman angle is another thing that is important. Ackerman angle is how far each tire turns in a corner. If you move the steering wheel full lock to right or left and look at the tires or take a reading using an angle finder you will see one tire is turned inward more than the other. This is ackerman angle. This is important in corners as one tire needs to turn more inward for the truck to track correctly.

Ackerman angle is relatively simple to understand but is also complex by design. For more reading on ackerman angle go here
http://en.wikipedia.org/wiki/Ackermann_ ... g_geometry

Toe in is another steering angle that is good to understand. All vehicles generally have a small bit of toe in. When you go get an alignment of the front end this is one of the things they adjust. A slight toe in of the tires is ideal because it allows the truck to track straight and true, gives even tire wear and keeps the front tires from fighting each other if the surface is not perfectly smooth. Usually after I work on a front end I set the toe in using a tape measure in the shop. Give it a good guess, adjust, check and so forth. Once done I take it in for an alignment since they have the tech machine and equipment. Nine times out of ten I am very close on my shop adjustment with a tape measure. It is very easy to do, don't be intimidated by it! Always get it professionally aligned though, if nothing else you get a print out of the specs on your truck showing toe in, caster, camber and so forth. More on toe (in) here, http://en.wikipedia.org/wiki/Toe_(automotive)

Lifting a truck also changes pinion angle. As you lift the truck the axles rotate. That changes pinion angle. Without a double cardan or CV style driveline and pinion angle out of specs you will get driveline vibrations. Patrols use a standard driveline but you can easily fit a DC shaft, it just has to be custom built. Outputs on the t-case and the pinion on the axle with a standard driveline need to match. Meaning, at the same angle. Pinion angle can be changed by cutting off the leaf spring perches on the axle and rotating them but you can not change the output angle on the t-case. So unless you were to run a DC shaft the angles need to remain the same.

Here is a good video explaining drivelines and how they work in relation to angles.
https://www.youtube.com/watch?v=gmV4qwLfOMY


Now how about steering? First, the design of the Patrols steering is known as "cross over" steering. It is a tired and true design that goes way back in automotive history. Cross over steering simply means the tie rod and drag link are located in front of the axle. Technically it is often called "T" steering since true crossover means the drag link attaches to the knuckle arm not the tie rod. But if you just call it crossover you are right unless somebody wants to get anal on you! What the Patrol has is actually just like a GM style, known as a "y-link". All this means is the drag link connects to the tie rod rather than directly to a steering arm on the knuckle. Two different principles apply and are important depending on the type of suspension, ie leaf sprung or coil sprung. Since Patrols are leaf sprung let's just talk about that. What you need on a leaf sprung truck and how they are designed is with a flat or relatively flat drag link. That means left to right when you are looking at the front of the truck the drag link is level. As you lift a truck this obviously increases the angle. What that does is cause what is known as "bump steer". With a heavy angle on your drag link going down the road, you hit a bump, even can be a small one, and the truck will swerve side to side even though you are firmly holding the wheel straight. That is bump steer, it is no good.

While I don't think all this stuff will help many Patrollers because you need to lift your truck 3 or more inches before things get to far out of specs and problems are encountered. What this does do is give you a basic understanding on how it all works and why things are important. That is solid info very useful to any hobby mechanic. The more you understand a Patrol and how things all work, the better off you are! Even if you take your truck to a mechanic for work, being educated means they can't pull a fast one on ya!

Other problems can be encountered in an old truck that will give similar results to bad caster, bad drag link angle and so forth. But that is all going to be from worn out parts. Worn out and shot leaf spring bushings or badly worn pins will give wandering and bad handling results. That has nothing to do with caster though, simply that the parts are worn out causing poor handling.

Cheers

[moore_rb]

Great info and writeup...

Only one minor nit-pick:

Cross over steering simply means the tie rod and drag link are located in front of the axle.

Actually, cross-over steering (or "Y-steering) refers to a system where the drag link transfers the steering load to the steering knuckle on the side of the vehicle opposite from the steering box. Old school hot rods often have crossover steering systems that place the tie rod and draglink behind the axle, but it is still a crossover system since the draglink carries the force to the opposite knuckle...:

http://www.speedwaymotors.com/Tech/susp ... ering.html

68-90 Chevy 4x4 trucks were designed with "same side" steering (Or- "T-Steering" ), meaning that the steering box/pitman arm (on the driver's side) delivered force to the draglink, which carried it backward to the driver's side steering knuckle. if you were to look down at the system from above, the drag link and tie rod are at 90 degress to each other (forming a T )

There is a huge aftermarket industry in crossover steering conversion kits out there for old Chevy trucks, because, as you say (and as I totally agree) - Y style crossover steering is a far superior design.

So Nissan definitely got the steering right with the Patrol

I agree a million % with your points about caster and pinion angle - The 60 series Patrol is not designed for simple, bolt on lift kits, that is for sure. 2 inch lift springs are absolutely the max for driveability, and even they will probably require clearancing between the front driveshaft and crossmember.

Beyond that, any Patrol suspension/steering project is 100% re-design, re-engineering, and custom fabrication.

[RiverPatrol]

Moved this to Suspension & Steering category and made it a Sticky.

[RMP&O]

I just call it crossover, my buddies and myself know what we are talking about. But ya, technically it is not crossover and you are spot on Rob, as usual!


I see SoA mentioned a few places here in the forum. I don't see why a person would do that with the stock axles and also 5" or more of lift is a ton on a SWB truck making it tippy. Going SoA will get you more lift and is the way to go if you want more than 3" but why would you do that to the stock axles? Sure it will flex more but without the traction ie lockers, to me there seems to be no point. A locked truck will go much farther than an unlocked even if the unlocked truck has great articulation. Traction comes before flex as far as importance offroad and what is to be gained. Once traction is covered, then flex matters. On top of all this a SoA with stock axles has one major drama. That is the spring perch on the front axle at the diff side. Diff center is cast making welding difficult, perch needs to go right on the edge of the diff, same as it under it. On my Ute I am using the C200 front axle but it has the same issues. Think I am going to outboard the spring perch and hanger (frame side) to get the perch most of the way off the diff.

Any ways when it comes to SoA cutting and welding is required. Spring perches on the axle should be cut off, rotated and welded back on to correct pinion angle. After this the knuckle balls should be cut off, rotated and welded back on to set caster. That is called a "cut & turn". I have done a few of them now, both on my FJ60 Cruiser and my FJ80. The FJ60 is leaf sprung and the FJ80 is coil sprung. On the FJ60 (leaf sprung) it is a lot easier than on the FJ80 (coil sprung). The FJ60 had about 4" of lift, the FJ80 has 4-5". Both trucks I set caster to about 4* and run 35" tires. On the FJ80 after lift the caster was out by a whopping 9*!! Rather than buy bolt-in caster correction I opted to cut & turn the front end. With the caster out by 9* the truck was barely drive-able, very sketchy especially at 70mph or more. A total white knuckle drive 24/7. After the C&T and setting caster to 4* I can do 80mph down the HWY with two fingers on the wheel and it is super nice. Drives like a dream. On my 77 Ute I am going SoA so a C&T is part of the suspension build. It also has a high steer set up which keeps the drag link nice and level. The combination of the two will give me a truck that is lifted 5 or so inches on 35" rubber but drives super nice like it is all stock.

I do agree about lift and #2 cross member. However, the 64 soft top has around 4", even a touch more, and right now in the shop the front driveline has plenty of clearance. On droop though I am sure it will be an issue. Although I do recall drooping it out when new suspension was being installed and recall it had 1/4-1/2" of clearance, think I even have pics of this. Rule of thumb for me is 3" is max for lift on a SuA truck. The reason being, the more arch in the springs the less they will flex. Also because drag link angle gets ugly if going over 3". If more than 3" is desired, SoA is the only way to go. To keep it all fairly bolt in, 2-3" should be max on an old Patrol. Although even with just 2" the front limiting rods probably need to go or be extended. Sway bars are a bit more forgiving but should also be lengthened if lifting and retaining them.

Cheers

[limited slip]

So i read this thread and pretty much liked everything in it

very little to disagree on, and its a very good outline as to why sus mods have to be done correctly.
Of note, any change to the gearbox- esp an mq/mk 5 spd or marks 5spd- results in further reducing the frint drive shafts clearance angle over the gbox cross member. Scalloping out of the cross member or other heavy modification of it is very definitely required once both a longer g box and lift are used in conjunction with each other. With the 5 spd i would go as far as to say the cross member is more a worry than bad enough angles to require a double cardan joint being fitted.

i also particularly agree with the comment re swb g60s and tall lift. They will become unstable, tippy and dangerous with anything more than 2 inches of lift. Given no one in their right mind would be looking to lift and put big tires under an unmolested patrol, that means that a basic 2 inch lift, and 2 inch guard/fender trim will see 35s fit without any hassle and even 37s dependant on rim offset......not that im suggesting that stock c216 diffs and cvs would handle 37s in any case......
red six the swb paddock wagon ran 35s with next to no scrubbing with nothing more than a 2 inch guard trim- and that was on very sagged original sus...... well sort of, the back end has a bolt on add a coil sus kit/load leveler. Hermit will end up with this kit soon......

cheers
kev