Vehicle & Technical > Suzuki

Calmini Death Wobble!!!!!!

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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!

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