Suspension set up

[SteelRat]

Having a bit of trouble with the suspension set up on one of my AX10s. It's on an XTrail chassis, and I've made some high cleareance lower links with some 6mm tube and threaded rod. All fine and dandy, and the wheelbase is just right for the Yota shell that I've got fitted to it.

The only problem is that when the dampers compress the wheels dont go straight up and down - in otherwords rather than moving at 90 degrees to the ground, the back wheels move towards the rear of the truck, while the front ones move to the front.

At first I thought I'd got the length of the top links wrong, because the axles aren't parallel with the ground, from the point of view of where the drive shafts connect to them, so I thought the axles were pivoting around that point. The thing is, no matter what I adjust I get the same problem.

Is it one single thing that causes this problem or a combination of things? I'm pretty sure I need a better understanding of suspension geometry to work out where I'm going wrong, and would be grateful for any light anyone can shed on this.

[MadForce]

Around here this is what we call axle steer. It is something common in lifted defender with increased articulation. Also found in leafed rigs but less severely.

If you picture your upper and lower links as geometrical shapes then you will see that a stock AX10 (pretty close to your xtrail from what I understand) has the uppers mounted like a triangle while the lowers are mounted like a rectangle or trapeze. To completely eliminate this effect both your uppers and lowers should look the same shape (triangle, rectangle etc) with same lengths.

This is a crude explanation of how it works geometrically. There also some other factors that play a role in this effect but mainly it is a geometry explanation.

[SteelRat]

Riiight that makes sense. And you know, I've always wondered why upgrading from a 3 link to 4 link system is seen as a "must do" mod. I guess that's it. With a 4 link set up you get the flexibility of making your uppers the same shape as your lowers.

I've actually got a 4 link plate for the rear, but I've joined the ends at the two provided holes at the front most part of it, which of course gives me no benefit at all because the uppers are still triangular in shape, which is the cause of my problem.

That's brilliant, thanks for your help! I'll have a play with that later and see what I can come up with

[MadForce]

Glad I could help a bit.
Note though that there are cases when dissimilar upper and lowers still have no axle steer. This where other factors come into play like suspension travel, link lengths, how parallel links are etc. etc..

[SteelRat]

I realise this is likely to be a bit of a "how long is a bit of string" question, but are there any kinda rules of thumb that I should bare in mind to minimise the chance of axle steer other than the links issue you've already identified? From what you're saying that sounds like the most likely cause of my issue, but it'd certainly give me a better idea of how things work, and minimise the chance of me getting it on other projects.