Death Wobble problem 80-130AW

[80-130AW Norwegen]

Sorry for posting this in english but my german is real bad.

I got a 80-130AW 1986 w/short Wheelbase with a issue i cannot resolve, yet

In winter time when roads over her are bumpy due to road base freezing I sometimes experience severe Death Wobble
Truck hits bump in speeds of 60-80 km/t and front wheels start shaking so it is difficult to keep the truck on the road.
I can't find any wear nor slack in the suspension. Truck is only 42000km. Toe-in is checked OK. Shock absorbers OK.
I Wonder if the absence of a front track-bar is the reason for the wobble?
I recently noticed that the front leaf springs are kind of straight they don't form like a curve as the rear ones do.
They are not inverted but almost straight.
Truck has been used as a airport snow plower. It used to have a heavy snow plow mounted to the front.
I speculate in if this truck has been standing all summer (no snow) with the plow attached and leaf springs loaded.
Can this be the reason for the wobble? Worn out leaf springs will affect Caster angle?

Any one got suggestions on how to fix this ?

[Ulf H]

... are the wheels balaced ... sind die Räder ausgewuchtet ...

Gruss Ulf

[brummiwomo]

Old tyres ? --> stand plates

(Standplatten)

br
Carsten

[80-130AW Norwegen]

Both tires are brand new and balanced. The older ones got outside wear as if to much toe-in, so they were replaced.
Measured toe-in and it looks neutral.

[Ulf H]

... how are the rims fixed to the brake drums ...

... stimmt eventuell etwas mit der Zentrierung nicht ... würde auch erklären, warum das Gewabbel bi den neuen Reifen auftritt ...

Gruss Ulf

[felix]

Hey,

straight leaf springs on the front axle are not unusual - the front axle is proper loaded compared to the rear axle.

On a flat road the truck is running smoothly buy when it hits a bump it jumps sideways and tries to leave the road? So we are not talking about instable/wobbely driving on flat ground, we talk about the trucks response to a bump/disturbance?

Maybe they put different/wrong front suspension to carry the snow plow while ignoring it's impact on stability: normally the first joint of your steering rod should be in line with the eye of your front suspension. Because only when suspension and rod rotate on the same axle the suspension amplitude has no impact on your steering. As soon as the joint it's not in line with the suspension eye, "up" and "down" of your suspension will turn your steering left/right. On the left front wheel worse than on the right one.

Can you provide a picture of the first left eye of your front suspension and the steering rod?

Properly worn steering pins can also cause instability: normally your steering pin is slightly slanted so that steering left/right means lifting the whole front axle. If you let go your steering wheel it always tends to turn straight again - this is because the pins are slanted and the front axle weight turns the wheels straight. The more slanted the steering pin the harder it gets to turn the steering wheel but the more/faster the truck likes to drive straight and the more stable it drives on bad roads. Wear of the steering pin or it's bearing can infect the steering pin angle and therefore the vehicle response to an bump.

Put a jack under your axle, lift the wheels and observe the steering pin bearings whine you rapidly lower the wheel again: there might be axial (up/down) free play (play up to a millimetres is acceptable) but there should be no recognizable radial (left/right/front/back) play. Do the test with straight wheels because straight is the main position of your front wheels - therefore also the most worn position.

Aircooled regards,
Felix

[Ulf H]

... where are you located ... may be there is one who can visit you on his way north ...

Gruss Ulf

[Uwe]

Hi,

what's the actual pressure in the new tyres?

Cheers
Uwe

[80-130AW Norwegen]

Hi all. I experience that the wobble starts when truck hits bump, then it will not stop until I lower the speed quite much.
For example if wobble starts at 70 km/t I have to reduce speed to aprox 50 km/t, then the wobble goes away.
Its quite scary when this happen so I hit the brakes.

Rims are bolted to the drums using all orginal nuts and washers.
This is 8-bolt steel rims, 22,5 inch diameter. Tires are narrow:9 inch.

Tire pressure was 9 bars from the tire shop, truck is running with no load so I tried reduce to 4,5 bars; no difference.


I'm located 45 minutes south of Oslo, 90 minutes with this death wobble

[felix]

Hi,

any news from the wobbeling truck?

45min south of Oslo? I worked for a steel company in Horsens for a few weeks, might be close to your place.

Best regards,
Felix