[Gwagensteve]

I was just reading this on Pirate:

Quote:

Your roll axis should be sloping up from back to front. This means that in the worst case, your inner front tire will lift before the back inner tire. This will induce understeer at the limit of traction, and keep you from flipping over and rolling.

Suzuki forgot how to design suspension at some point in the 1980's, and, as far as I can figure, the roll axis on a Vitara runs the other way - It slopes down from back to front. That's why they don't handle properly.

The salvation is, however, that if you lift a vitara, the front roll axis rises, but the rear doesn't. This results in a great win for handling, as even though the car now body rolls more, the roll axis is closer to ideal. The car feels more balanced front to rear and feels less likely to roll oversteer.

Anyways, it was just something I read I thought was relevant.

If anyone is interested, here's the Pirate thread

http://www.pirate4x4.com/forum/general- ... ast-2.html

Cheers,

Steve.

[monley]

I never thought I'd see the day where g-wagen says you need lift

[watermouse]

That makes sense.
It would be really interesting to see a graphical interpretation of this for various suspension setups. In my mind I can't work out what it would look like but I get the feeling that roll axis is a somewhat constantly moving thing throughout body roll and would depend on spring rates as well as link geometry.

[Gwagensteve]

Not really. Roll axis for the rear of a vitara is completely static- it's the ball joint on the top of the diff. For the front, I agree, it's lift dependant.

Steve.

[watermouse]

I should have clarified as you were talking about Vitaras, I was speaking more generally.

What about the roll center on a Jimny rear (2 trailing arms + panhard) or GV rear (Upper and lower links + panhard)?



Going back to the front on IFS. Would it be better design if the lower arm was pivoting on the chassis on the opposite side of the car and as high as possible? (so they cross over).

[Gwagensteve]

Not really IMHO. The key is lowering rear roll centre, not raising the front.

In general, roll centre for cars with a panhard rod is 1/2 way along the panhard. Rear roll centre for a jimny or GV is lower than the rear of a vitara, however they rise with suspension lift, a vitaras doesn't.

Steve.

[atari4x4]

stock vit's are hell squirrelly.

my swb drove brilliantly, in the hills/fast dirt & offroad it was a pretty well balanced car all-round, the lwb is a fair way off it. not really sure what it needs but it needs something, have even considered putting the front sway bar on.

[Gwagensteve]

atari4x4 wrote:

stock vit's are hell squirrelly

God yes.

Since the mid 80's:

WT Sierra= fail- too high roll stiffness
Vitara= fail- roll axis
Coil Sierra= fail- roll axis
jimny= fail- roll axis
NGV= fail-bushes prevent any actual flex

Honestly- nothing since the NT Sierra has been any good.

[watermouse]

So as I understand it those with vitaras should engineer tricky low ball joints on the diff, under the drive shaft and those with panhards should lower their panhard brackets as much as they dare then choose a spring to match the driving style.

I just googled IFS roll center and found some good diagrams came up that clear some things.
But, as you say that lifting the front is good as the Roll Center comes up and the CoG to RC moment shortens, there are limits to what you can really do. I suppose longer arms and even more lift would help. Lowering the rear RC seems to be the easier thing to do for the average car modder to get the rear low/front high RC axis that is desirable.

Quote:

Honestly- nothing since the NT Sierra has been any good

A knee-jerk reaction to the poor perception Samurai's had in the States with regard to rollover, perhaps. Maybe seen to have made advances in suspension but not really.

[Santos]

You know there has been quite a few times in my last ten years of owning my 92 sierra where i had the world slow down and the clear detach thoughts of
"shit this time its definitely going to flip/roll" went through my mind.
Through luck or design it has not happened. Speeds, angles on road, off etc.

I wont claim to understand how it all works but its why its important to me to have stock tyres, lift and a swaybar. Calling me a pussy will not change me.
I was genuinely cheesed that i couldn't get stock height dakkar springs and lowest ption was 40mm above stock.

[fordem]

Gwagensteve wrote:

Not really IMHO. The key is lowering rear roll centre, not raising the front.

In general, roll centre for cars with a panhard rod is 1/2 way along the panhard. Rear roll centre for a jimny or GV is lower than the rear of a vitara, however they rise with suspension lift, a vitaras doesn't.

Steve.

I find this interesting ...

When lifting a rear 5 link design, not everyone keeps the panhard rod parallel to the ground, some folks just make it longer so as to re-center the axle and leave the rod at an angle (in my opinion, far from ideal), and all the relocation brackets I've seen for the GV raise the axle end of the rod, thereby raising the roll center.

It may be possible to lower the axle end of the rod, which would leave the RC in the original lower location (assuming no change in tire size).

[Gwagensteve]

I agree - generally, it's easier to raise the axle end of the panhard than lower the chassis end. I think the reason this isn't more of a problem is that the front roll centre also rises with lift, so the cars roll axis likely stays pretty close to standard. Also, as COG rises with lift, roll centre rises to keep approximately the same relationship.

I run a very high roll centre in the rear of my car - it's probably quite close to the cars COG for the rear. Whilst this isn't the best solution for road handling, it's incredibly stable off road, without a sway bar and with very soft springs.

Santos - adding width will counter added height in relation to stability. The front sway bar doesn't, in my opinion, do anything to improve the handling of a sierra.

[britvit]

Gwagensteve wrote:

I was just reading this on Pirate:

Quote:

Your roll axis should be sloping up from back to front. This means that in the worst case, your inner front tire will lift before the back inner tire. This will induce understeer at the limit of traction, and keep you from flipping over and rolling.

This is exactly how my manual LWB Vitara behaves, it lifts the front wheel. It has the Calmini 3 inch lift, 30 inch tyres and still has the sway bar fitted. I think the car handles great for a lifted 4 wheel drive, although I always drive slowly and carefully.

On the other hand my auto LWB Vitara is horrible to drive. It feels very unstable, it has a small suspension lift 215 tyres and no swaybar.

[Gwagensteve]

And that's also my experience of the Calmini kit. Even though the rear roll centre is raised by way of A arm spacer, the front comes up far more as a consequence of the lift.

Steve.

[Chop]

britvit wrote:

although I always drive slowly and carefully..

[atari4x4]

Chop wrote:

britvit wrote:

although I always drive slowly and carefully..

[watermouse]

Gwagensteve wrote:

Vitara= fail- roll axis

This got me thinking of what the roll axis really looks like in my GV so I took some measurements, based on the image from this page, under the front of my car.
http://www.scielo.br/img/revistas/jbsmse/v33n4/a07fig05.jpg

and with a bit of Rhino I came up with this.


Which looks about right. I imagine this would be a similar case to the Vitara.
The blue lines are the struts and arms The black rectangles represent the tyres in cross section. The arm length is measured perpendicular to the arm pivot axis which is not parallel to the centreline. The roll centre comes to 246mm from the ground.
My car has a "40mm" lift, dobinson struts.
By comparison the centre of the panhard at the rear (RC as suggested by Steve) is 350mm from the ground which is not quite horizontal on account for the rear lift.
Overall the Roll Axis drops over 100mm from rear to front.

What got me wondering though is what happens to the roll centre when you roll and the front articulates. I had the impression it would not stay central as the front arms don't pivot from the centre of the car.
Some more Rhino...


Now I don't fully understand what is happening here but the suspension geometry is fixed, only the arms angles have changed and the struts have shortened/lengthened due to the changing loads of the roll. The roll centre moves towards the drooping arm and lower while the body rolls in the other direction.
It looks highly unstable to do this but I think that is where the spring rates and swaybar start to counteract. While the COG over RC moment increases the spring rates and swaybar rates are rising quicker for the restoring force. This for only 5 degrees of body roll. The worrying thing is what happens if the compressed strut sees a sharp bump. This might be enough to lose stability and control.

For me I want to set up my car to be stable on rough dirt roads while loaded and handle the odd sneaky bulldust hole. So high speed stability over ultimate articulation is important.

Thoughts?

[atari4x4]

any chance of a couple more with a bit more lift say 65 & 75mm & bumping the control arm length out 10mm, adding some offset (-15) & some 31's? eg ideal calmini set up

i don't ask for much.

thanks for taking the time to model the front end, it's awesome to see some real tech on the zook ifs front end.

[watermouse]

atari4x4 wrote:

any chance of a couple more with a bit more lift say 65 & 75mm & bumping the control arm length out 10mm, adding some offset (-15) & some 31's? eg ideal calmini set up

i don't ask for much.

Done.

Pretty much looks the same although you do have a higher neutral RC as expected. This is just over 4 degrees of roll.
You will need to measure the ball joint height on your rear diff to get the Roll Axis.






I have made an assumption on your tyre diameter and that the top of the struts on your SV620 are the same spacing as on an SQ.
Track increased with offset by 30mm, arms lengthened 10mm.
In no way do I guarantee that this is an accurate representation of the suspension geometry.

I should clarify that the blue strut line is the king pin axis as the strut actually sweeps a conical path to clear the inside top of the tyre. Also there is no allowance for the KP inclination which may change the geo a little bit but I think we are getting the gist of it. I am not sure if the green lines should intersect the KP axis at the ground or use the centre of the tyres contact patch but this will only change the rolled RC marginally.

SAS FTW!

[Grags]

my brain hurts

[atari4x4]

^ thanks for adding tech to this thread, tb


and thanks to watermouse for adding tech to this thread.