Roll steer. Anyone encountered it and have a fix?

[Mud Camper]

I've built a camper, based on an LN65 chassis and suspension. It has some unusual driving characteristics which I'm trying to eliminate.

From what I can find on the net, i suspect it's suffering from roll over steer, caused by the body roll extending the track of the vehicle on the weighted side, as the axle moves through the spring compression arc. Essentially rear wheel steer.

I'm a little unclear on whether I may also have bump steer - and figured guys here will know shed loads about that part!

Here is the symptom: road camber changes require significant steering wheel correction to compensate for. The vehicle steers itself quickly towards the lower camber section of the road. Also, during cornering, at mid-corner the vehicle turns itself more sharply, essentially feeling as though you have turned in too much, requiring a correction. All in all a bit odd to drive!

Suspension mods are minimal. More or less std spring heights, but extra leaves front and rear. Rear also has airbag assist, running at 45lb and a sway bar from a light truck. The rear may be up an inch or two from stock due to the airbags.

Shocks are adjustable ranchos I the back, set to rock hard, and heavy duty munroes or similar in the front.

The whole rig weighs 2500kg wet. 1100 over the front, 1400 over the rear. Corner weighting is pretty close to even.

Oh - the cab has a 2 inch lift, which is what I don't know whether will create a bump steer issue.

So there's the basic set up and problem - photo attached I hope!

So here are my two questions. Easy one first:

Could the 2 inch body lift affect the steering geometry, and if so is a high steer kit the solution?

Also, has anyone tried control mechanisms, such as panard rods, rear links etc, to assist in altering the rear axle geometry to reduce roll oversteer?

[meatc]

You did the bushes in the front diff to chassis rod aye?

The fat tyres won't be helping either, they tend to follow the camber and depressions in the road.

[Mud Camper]

No, I didn't end up replacing those bushes. I couldn't get any dodgy movement out of the old ones, even using a crowbar to lever on end of the control arm, so I figured they're good and best left alone.

Fair point on the big tyres. They're so good on the sand though.....

[Trundle]

Give the steering toe in to help it track straight ,
Adjustable shocks in the front and going to a load bag or simular to allow stiffeneing of the springs ,
even putting airbags on top of the leafs just hooked up to air lines that you can fill up with compressed air ,
Also much havier sway bars i.e using a tortion bar (nzv8 ) swaybar setup with a disconnect for road use , all of these things will assist you issue but is hard to pin point without having driven the vehicle ,

[Smurf]

Are your rear airbags linked?
If so as one side is compressed the other gets filled more which may exaggerate your body roll?

[Mud Camper]

No, they're not linked - for the exact reason you mention!

[Smurf]

??

[Mud Camper]

Innovative, but perhaps not ideal...

[churchill]

Sounds like you might not have enough castor on the front end. I went for a drive in the neighbours Fiat with modified suspension and it was doing what you described. It would dive into corners and he was having to fight to keep it from turning tighter. He give the front end more castor and the problem was fixed.

Measure the king pin inclination, Hilux's solid axles are set to 3°. I'm not sure if yours is solid front. It's worth checking.

[suzolla]

Hi,
Regarding your question on body lift, this should not affect steering as steering box is attached to chassis.
Is it more noticable turning left of right.
When stationary is the link rod between the steering box and the C arm on the knuckle more or less horizontal or on an angle.
Cheers
Tim

[Mud Camper]

Sounds like you might not have enough castor on the front end. I went for a drive in the neighbours Fiat with modified suspension and it was doing what you described. It would dive into corners and he was having to fight to keep it from turning tighter. He give the front end more castor and the problem was fixed.

Measure the king pin inclination, Hilux's solid axles are set to 3°. I'm not sure if yours is solid front. It's worth checking.

Cool, thanks for that. Mine is solid axle. Is the castor able to be altered on this set up?

[Mud Camper]

Hi,
Regarding your question on body lift, this should not affect steering as steering box is attached to chassis.
Is it more noticable turning left of right.
When stationary is the link rod between the steering box and the C arm on the knuckle more or less horizontal or on an angle.
Cheers
Tim

Thanks Tim. I think it's more obvious to the right, but it's hard to say for sure.

The C arm is on an angle, downward from the steering box idler arm to the knuckle. It's not a huge angle, maybe 7 degrees at a guess. Does that sound correct?

[tweake]

have you got a rear sway bar? reducing amount of sway will help a bit. disconnects for when going offroad.

[De-Ranged]

Unless you have a large suspension lift your roll steer will not be the problem
Roll steer comes from short links at extreme angles under lots of articulation.... on standard suspension the effective extention of wheel base would "turn" the front axle a fraction of a degree..... not what you are discribing
My guess is you have toe-out on the front axle or possibly a bent rear axle with toe-in on the passanger side..... as the camber or corner throws weight onto the outside wheels it steers it in that dirrection

I would replace the suspension bushes with neolothene, not something I like to use on a 4wd but with your high CoG you don't want bush compression
With that done take it in and get a wheel alignment from a truck outfit not a car shop (most car alignment places will give you what is on there data sheet without knowing how it really effects your car lol) a truck shop on the other hand has staff that will know and can deal with beam axles

Cheers Reece

[Mud Camper]

Thanks Reece, that sounds like good advice re the alignment. It's been done twice, but both by car guys, so your suggestion to go to a truck specialist given the beam axle front and high cog makes good sense.

Trundle mentioned giving more toe in to improve stability earlier, so perhaps it's as simple as that, and the car alignment guys aren't up to play with this sort of rig.

And it would be a cheap fix too, so that would be even better!

A bent rear axle is also a real possibility. The s.o.b I bought the ute from had installed a 4.3 diff in the rear, but left a 4.8 in the front. Test drives in 4x4 by potential buyers could have done some damage there for sure.

Of course there's still the trainer wheels suggestion from Smurf...

[churchill]

Cool, thanks for that. Mine is solid axle. Is the castor able to be altered on this set up?

Yes it can but only with wedges. What was the castor readout on the alignment sheet?