Hi guys,
my truck has developed a vibration and I was wondering if any one of you can come up with a solution for me.
vibration comes under load only
45mph to limiter
any gear
only with lift axle up
So far
front prop sent away had new bearing carrier and bearing, rebalanced
both parts of prop sent away, had new slider and both uj’s, rebalanced as a whole
diff taken out checked, all in tolerance, replaced
Robbo.
what truck?
Vibration from where?what kind have you had tyres changed recently had some fitted last year that were shocking I hit 45-50 and the bloody wing mirror glass almost fell out?!
have the tag axle lift bracket checked.
had 1 on a cf snap a few years back and it vibrated badly under weight over 40 mph.
robbo863:
Hi guys,
my truck has developed a vibration and I was wondering if any one of you can come up with a solution for me.vibration comes under load only
45mph to limiter
any gear
only with lift axle upSo far
front prop sent away had new bearing carrier and bearing, rebalanced
both parts of prop sent away, had new slider and both uj’s, rebalanced as a whole
diff taken out checked, all in tolerance, replaced
Robbo.
Let it develop! 
Turn the radio up.
what make/model/year/horsepower etc.■■? the man tga,s would vibrate when the exhaust valves were on their way out !!!
Have you made sure that all the prop joints are in “phase”?
Hardy spicer joints do not turn through 360 degrees without “wobble” if used on an inclined plane
as all props are.Thats why cars use Constant velocity joints or “double UJs” on steered drive shafts.
All the prop crucifixes must be in line or the front part of the prop will try to turn at a different rate
to the rear on different parts of its rotation cycle.
Even if they are only one spline out on the slider they will cause vibration and premature needle bearing failure.
And the reason the vibration reduces when you lower the 3rd axle is because it has changed the “resonance” length of the chassis.
The prop shaft will be rotating at about 30 revs per second at about 80 km/hr which will form a “harmonic” with the vehicles chassis rails and start to “ring”.As you lower the third axle you either in effect lenghten or shorten the chassis and it drops out of the range of this harmonic and the vibration is “damped”
I’d ask the workshop to check the propshaft and if they’ve done anything to the truck recently which might be put back together out of line which could cause the vibration.
BB
gearbox, clutch, wheel bearings
It sounds like the angle of the prop, as you lift the midlift the rear airbags inflate slightly, lifting the chassis and changing the relative angle of the prop shaft, so a UJ will be the most likely cause, the rear one is my guess as the front one would be felt through the gearstick (if it has one) 
You haven’t had new bushes fitted in the rear axle locating A frame or longitudinal (torque control) arms recently have you? It’s not unknown for the legs of the bushes to have been fitted the wrong side of the relevant brackets altering the angle of the propshaft and axle. It’s not necessarily easy to see that this is wrong without another vehicle alongside for comparison. This will create a speed related vibration.
totally agree mercman which is why ive replaced the prop and its balanced as one, how ever that has just been an expensive day out as its no better, could it be a phase of the tyres? nothing was changed it just developed.
going to swap all drive axle wheels and tyres to eliminate that theory.
newmercman:
It sounds like the angle of the prop, as you lift the midlift the rear airbags inflate slightly, lifting the chassis and changing the relative angle of the prop shaft, so a UJ will be the most likely cause, the rear one is my guess as the front one would be felt through the gearstick (if it has one)
Had this problem with the Renault Midlums we used to run. Vibration was so bad you couldn’t hold the steering wheel and nearly caused a few accidents before we found the Toyo’s we had fitted were the problem. Tried other makes and all caused the same problem, only Michelins didn’t cause it much to my gaffers dismay!!! 
shuttlespanker:
what truck?
as asked earlier make model of truck may help,as people with specific knowledge of a specific make can help track down the issue
Seems unlikely with the midlift thing but I would still give all the engine/exhaust/air box and intake mountings a good check with a pry bar, just to rule out that side of things, it’s sometimes surprising how much vibration some things can cause.
It could be a balance issue, with the wheels up and no load, it sets it up dead right for a vibration, a little bit more weight and you overcome it. If it’s tyres then the tyre itself can tell a story, alignment problems give a feathered wear across the treads, if it’s wheel bearings any irregular wear will be smooth.
You could also have a centering problem, either a tyre/s isn’t set on the rim properly or the hub spigots are worn and the wheel itself is out of whack, this makes it out of round, so the wheel rotates like an egg.
The way to check this is to jack it up and spin the wheel with a hammer pointing shaft to the air & touching the tread of the tyre, if the hammer rocks, there’s your problem.
organised to have another set of wheels at weekend to see if its in the wheels/tyres.
its a Scania twin steer.
Did you get any results with this yet?