Calmini Death Wobble!!!!!!

[DaveHenderson]

hi all,
today after what seems a long, long time i finally finished fitting the 3" lift kit from Calmini to my Sammy.

when on the on road first test drive it all felt fine until reaching 35mph when all of a sudden the car starts to violently wobble / rock side to side coursing near loss of control of the szuk.  :?

when pressing the brake and slowing down the wobble does not stop until about 20mph, can anyone help!   :?:  someone said it might be the tracking but i don’t know. :?

I think I’m going to go down the garage tomorrow but  don’t want to pay for tracking if that isn’t what it is.

any suggestions ,

thanks   :lol:  :lol:  


[/b]

[generation-x]

hi dave have you fitted the calmini drop pitman arm too??

simon

[DaveHenderson]

yes i got it on in the end, it all looks bolted ok.
will get some pics tomorrow.

[Evilgoat]

Sounds like some kind of resonance or dampers not working right. I take it the kit includes the dampers? Do the Ol dbounce test, car should come back and settle, no more than two bounced and then very small.

[DaveHenderson]

yes i tryed the bounce test, all seams very hard / stiff but it does move a little and stops bouncing almost instatly

[Damon]

My guess would be tracking.

Can't you get a free tracking check where you are?  Mind you, if the tracking is that far out you should be able to see it.

Get the drivers side front wheel perfectly straight and then look at the N/S wheel and see if it's pointing in a different direction...........

[rollazuki]

Front axle(inc diff) angle wrong, causing steering to over self centre, and giving the well known    'SUPERMARKET TROLLEY WHEEL'    effect.

On a link system with adjusters, pretty easy to dial out, on a leaf set up, Im guessing wedges between spring and axle will help to rotate axle back to normal.
 Are springs fitted correct way round? are correct shackles used, did springs come with kit?
Give me the full run down of what you've fitted and Ill try to give you an idea, in the meantime, dont run it on the road!!!!!!!!!!!!!

[DaveHenderson]

I’ve fitted Calmini +3" springs, Calmini bigger shocks all round, anti-roll bar extensions, new poly bushings and a big pitman arm, o and I’m using stock shackles.

It all came new as a kit from la supertrux.

I had to file down the pitman arm a bit to fit the steering damper bracket, and i painted the axles.

when it happened it did feel like a out of control shopping trolley :!:  :D

There are some pics of the springs/ pitman arm on my photo section.

i looked at the tracking, and it does look a bit out but not much.

Thanks   :D  :D

[generation-x]

dave for starters bin that anti-roll bar it may just be the problem as well as it will be over flexed
and as rolla says the axle might be rotated slightly too

simon

[jaeger]

i had the same thing its called bump steer, it not so much the speed your doing its a tiny bump in the road that starts it off once you are moving over a certain threshhold..

my castor angle was all cocked up, so i fitted slightly longer shackles to the front which rotated the kingpins slightly and cured it, adding wedges/shims at the back/front  of the leaf spring plate should do the same thing...

[V8MoneyPit]

I have to go with a caster angle suggestion.

Bump steer is simply steering movement when the suspension is articulated. Poor bump steer characteristics can cause the car to veer as it passes over bumps, but there is no reason for it to set off steering oscillations. When you fitted longer shackles, you would have been reducing castor angle.

However, it's hard to see how fitting 3" lift springs would change the castor significantly. The area that the axle sits on should be exactly the same angle. For sure, fitting long shackles totally screws up castor.

Is it just the Calmini 3" springs or the shackle reversal kit? The shackle reversal kit can affect castor.

Fitting lifted springs will not have any effect on toe in or toe out. The geometry is set by the link which stays the same relative to the axle. The steering rod from the box would be at a different angle which is why you fit a drop arm. But this would just mean your straight ahead position is not the centre point of the steering box.

Been there, done that!! I've got 3" springs and no drop arm. No problems at all. I just end up with slightly different turning circle left and right!

[rollazuki]

First point, Im 90% sure you are suffering from too much trail, which causes the 'shopping trolley' effect.
Ive had it, been there, got the t-shirt etc, and actually witnessed my front wheels leaving the ground, the effect was so severe.
Two things spring to mind:

1) If you have just fitted taller springs, they are tilting the diff nose downwards. To reduce trail you need to tilt the diff nose upwards, so hopefully by using an adjusting wedge between axle and spring pack you can help that out. Are the spring packs fitted the right way round? they may be Aysimmetrical, ie different length front and rear of the axle.
At this point a longer shackle would not help.

2) Have you fitted bigger wheels and tyres? Believe it or not, a larger diameter tyre also increases trail, and if the car was borderline before, a bigger tyre can tip it over the edge and cause instability.

At this point, Id not even worry too much about steering dampers etc, because they wont control such a major problem, You'll just end up shearing mounts off(guess how I know)

If you have original/smaller tyres, fit em and see if it helps, bue above all, ring LA supertrux or similar and get some degree adjusting shims and refit the axles with the diff nose pointed upwards, it, the thicker end of the shim towards the rear of the car if its a regular spring under axles arrangement.
Rolla

[V8MoneyPit]

With respect Rolla, why would the taller springs tilt the axle? The axle is fitted mid point along the spring. Surely the point at which the axle sits would be at the same angle? I suppose it depends on the design of the spring.

Mine (3" lift springs from KAP) certainly give no problem with 31" tyres fitted.

If you tilt the axle with wedges, you will not only be changing the trail, but reducing the castor angle.

Not questioning your experience, just can't get my head around it!

[rollazuki]

Not all springs are made equal. Some have the location pins for the axle slightly ahead of centre, some behind, (hence the Q, are they fitted the correct way around!) Just a long shot that the Calmini springs may move the axle slightly, and hence transfer the mounting point onto either the up slope of the spring or the downslope.(Ive never used these springs)

Ive fitted lifted leafs to my old zook, (approx 3 inch) and had no real problems myself.

If the rotation of the axle has changed(causing more trail), or the tyre size has increased(again causing more trail) then you can help this out by reducing the KPI(kingpin inclination angle)(changing the castor) This is done by rotating the axle casing(using wedges) and will effectively reduce the trail for a few mm for every degree you rotate it.

The whole trail thing can be proved at the next shopping trip. look at the trolley castor. Run a vertical line from the castor pivot point to the floor, then another from the 'axle' to the floor. (its about 1/2 the diameter of the tiny wheel , yes?)
Thats a lot of trail, which is great at slow speed, but get running back to the car, and you'll see the wheels flap about like crazy. Thats cos the huge amount of trail is correcting/over correcting/correcting/overcorrecting/correcting   etc  all the time.

This is what this truck is doing.


Taking the trolley again,If the castors pivot point was moved backwards over the wheel(almost over the axle, but not quite) then when you pushed it, the self centring effect would be reduced, the trolley would be more wayward and harder to push in a straight line, but the wheels wouldnt flap when you got running.

Hope this helps a bit. Rolla

One day Ill knock up some sketches to try to explain this, its pretty simple really, but its amazing, some changes to running gear really screw up your truck.

[V8MoneyPit]

I have training in suspension design (ex-Lotus design engineer) so I understand trail/castor/etc, but what I couldn't understand is why the axle would change angle. Thinking 'out loud', if a spring is designed to lift the vehicle and the manufacturers decided they wanted the propshaft to remain the same length, the axle would have to be positioned slightly further back along the springs. This would have the effect of tipping the diff nose upwards though, reducing trail and castor angle, not increasing it. I can't imagine why a manufacturer would want to move the axle forwards. I suppose the actual curve of a leaf spring can vary considerably though.

As I understand it, the trail is to provide the self centering effect and the castor angle stabilises it. Too much castor and the steering becomes heavy, too little and the wheels can set into oscillations, as here. But innitiated by excessive trail as you say. The castor is acting a little like a damper.

In the case of a shopping trolley, the caster is probably zero! Extremely unstable! The amount of trail probably affects the frequency of oscillation and at what point it is induced. iI you were to have 12" of trail, it would still happen because of the zero castor, but it would be at a different speed and a different frequency (lower).

Ultimately, it is a combination of the two that sets the geometry correctly. One directly affects the other.

Oh, and when I get taken away for fiddling with a Sainsbury's shopping trolley, I'll blame you  :lol:  :lol: But officer, I was only measuring the steering geometry!

This isn't highjacking this thread is it? Apologies if it is. But it is still on topic, so hopefully of interest. I'm certainly finding the discussion interesting... but then I should get out more!

[Boggert]

Steve,
Do you think it could be a problem with the leaf springs?

[DaveHenderson]

interesting stuff, and your right i don’t understand how but the diffs are pointing slightly downwards. :lol:
Are these wedges hard to fit/adjust correctly?
 :D  :D
thanks

[rollazuki]

The contact point of the tyre will always be in the same place, in a vertical line exactly below the axle centre. this never changes, biger/smaller tyres, rotating the axle etc. the contact patch is exactly that, the point where the tyre touches the road.
draw a line thru the king pins (imagine a line thru the king pins, or the axis where the steering turns), and this projects down and forewards to a point on the road in front of the tyre. The distance between this and the tyres contact patch is the TRAIL.
Effectivley this is a triangle with the base being the measured trail, the height being the vertical line from road to axle centre, and the hypotenuse being the lind drawn thru the kingpins.

By fitting a larger tyre, you increase the height of the triangle, increasing the length of the base, and hence the trail.

by rotating the axle casing you change the angle of the hypotenuse, also increasing/decreasing the length of the base and hence the trail.

Thus by rotating the axle casing we can alter the amount of trail the steering sees on the road.

A tiny amount of trail will result in a damned vague steering feel, with little or no self centering, wandering around with no 'centre point' feel.

An excess of trail will give an over correcting effect, causing the steering to self centre like hell, and to flap around like a mad thing when in motion.

A happy medium has to be reached........

on a link suspension, you can adjust mounting points or link lengths, on a leaf, the easiest way is to place wedged shims between axle and spring pack.

Ill try to make some sketches(haha)

[rollazuki]

See pics from bottom to top, lower pic showing steering basics, next one up showing change in trail due to an increase in wheel/tyre combo, top one showing change in trail due to rotation of axle/diff assbly

Rolla(not an artist!)

[V8MoneyPit]

interesting stuff, and your right i don’t understand how but the diffs are pointing slightly downwards. :lol:
Are these wedges hard to fit/adjust correctly?
 :D  :D
thanks

I believe the wedges come in different 'angles'. The simply fit between the axle bracket and the spring. At worst, you might need replacement U bolts if yours snap undoing them.

Steve,
Do you think it could be a problem with the leaf springs?

Hard to think of a problem with the springs that could cause this effect. I have to agree with our resident artist and chief picture drawer  :lol: It's a geometry issue. It is a matter of playing with the angles until it works. Hopefully, changing the castor to achieve different trail won't give unpleasant side effects.

[rollazuki]

Hard to think of a problem with the springs that could cause this effect. I have to agree with our resident artist and chief picture drawer

If I didnt know better Id think you were taking the pee..... :lol:

[V8MoneyPit]

Me????  O:)  :lol:

[P.R.Howler]

So basically Dave needs to know the original trail angle dimensions from a standard vehicle to find out what degree shims/wedges he needs now.

[rollazuki]

best way is to get a degree finder(or a protractor with a bit of string and a bob weight attatched) and find the angle the diff casing sits on a standard vehicle on FLAT LEVEL ground.
Then get your truck on same ground, and see what angle your front diff casing sits at. 1 minus the other, and youll know how many degrees you need to adjust.

Of course, double check all nuts and bolts are tight, ubolt clamps are tight, shock mounts are tight , and all that stuff.

Good luck
Rolla

[Boggert]

Hi Dave,

have you got it sorted yet??

[DaveHenderson]

Nope not yet anyway
I’ve ordered some wedges that will come tomorrow but on reading rolla's last post i still don’t know the correct angels!! :roll:

the wedges i ordered are 6degrees witch i think will rotate the axel too far the opposite way, im assuming this will not be good  

thanks for all the help everyone :D  :D ,
hopefully it will be driving soon

[rollazuki]

give it a go, whats to lose?


remember tilt the diff nose upwards to reduce trail. This also gives the front prop an easier life. a spacer on the transfer case end helps extend the U joint past the chassis X member as well......

[rollazuki]

Any new yet? any success killing the wobbles?

 :D

[DaveHenderson]

Still waiting on those wedges.
 i seam to have the worst luck when it comes to mail order places!! Ordered  them from explorer UK, they say they may take another week.   :roll:

[baz2236]

Hi i have been browsing the archives so to speak and came acroos this one just wondered did you every get this sorted out. I have a similer issue that, sometimes my wheel wobbles i have spring over axle running standard springs and 16" tyres. it might just be the wheels are not balanced very well but they are quite diffucult to balance and they have a really big off set. maybe i will just try some standard tyres.