Rear banging noise sound like propshaft out of balance

[boyle39]

my 4wd bongo has an intermittant noise from the rear which sounds like a banging noise on the floor which gets faster as speed increases once over 50mph,, it may not do it for weeks then comes on. i have had transfer box looked at, new front propshaft, oil change in diff, transfer box etc. now run up with wheels off the ground sounds like rear diff, but suspect thats because of axle not true and rear prop is out of true,, which leads to transfer box again,,, wondering if this is going out of line and making props run un true. all mounts look ok,,, any one know what the dynamic damper does,,, getting frustrated now...

[Ian]

What are the sizes of your tyres?

[rita]

Hi boyle39,

Have you checked the prop shaft centre bearing ? and the universal joints/pinion bearing for lift,dont think these vehicles have a Dynamic Damper fitted.

Cheers..

PS the prop shafts often have balance strips on them,make sure they have not fallen off..look for the marks..

[Northern Bongolow]

bongo.s have them on the front edge of the rear diff, both 2 wheel and 4 w drive.
http://en.wikipedia.org/wiki/Dynamic_Vibration_Absorber.
http://en.wikipedia.org/wiki/Tuned_mass_damper

[rita]

bongo.s have them on the front edge of the rear diff, both 2 wheel and 4 w drive.
http://en.wikipedia.org/wiki/Dynamic_Vibration_Absorber.
http://en.wikipedia.org/wiki/Tuned_mass_damper

Hi,
Is that a Damper or a Carrier,if it is a Damper how does it function.?

Thanks.

[Northern Bongolow]

as i understand the subject, all rotating shafts in machinary have a vibration point at a certain speed, like out of balanced wheels at around 50 mph, they shake. the problem with the prop is that it has a wide range of rotation speed so its going to hit the vibration/resonance speed at some point between just moving and the rotation speed at say 80mph, this is far more than the revs noted on the rev counter as the rev counter is representing engine speed, but as the prop is a true representation of rotation at the wheels depending on the diff gearing the revs here are much higher.
the damper is made to, and tuned to, take out this vibration by putting in a lump of rubber that is flexibly mounted on a hanger, this stops the build up of ossilation on the shaft.

[rita]

as i understand the subject, all rotating shafts in machinary have a vibration point at a certain speed, like out of balanced wheels at around 50 mph, they shake. the problem with the prop is that it has a wide range of rotation speed so its going to hit the vibration/resonance speed at some point between just moving and the rotation speed at say 80mph, this is far more than the revs noted on the rev counter as the rev counter is representing engine speed, but as the prop is a true representation of rotation at the wheels depending on the diff gearing the revs here are much higher.
the damper is made to, and tuned to, take out this vibration by putting in a lump of rubber that is flexibly mounted on a hanger, this stops the build up of ossilation on the shaft.

That's brilliant I am glad someone understands these things,it all sounds googly gooch to me,well done

Thanks.

[mikeonb4c]

That explanation sounds odd to me. I can understand that shafts, just like guitar strings, could have a tendency to vibrate around nodes. But it would seem to me that prevention would have a lot to do with accurate balancing, and ensuring that shaft bearing positions don't coincide with node positions. The idea that Rolls-Royce jet engine shafts rotating at 50,000 rpm might rely on crude damping, just feels weird. This is the kind of techie subject where someone like The Great Pretender might be able to provide enlightenment.

[boyle39]

Several garages have had a look at the problem, one said transfer box,, it was removed tested an ok, one said front propshaft, replaced with second hand part still there, all bearings and joints have been looked at, common link it is the transfer box when it is touched, the noise stops for a while or is instant, the Dynamic damper is on the bottom of the transfer box, see the parts diagram on the fury site. Now mazda would only put one there if they thought the transfer box would move, it is only suspended by one point and if it vibrated it may throw a perfectly balanced propshaft out of true...

the noise can be gone for weeks and a journey from newcastle to oxford and back only resulted in it being there twice once there once back for no more than a few minutes,


the tyres are smaller on the front than the back,,,

[Northern Bongolow]

the 2 wd and 4wd both have the same 2 dampers fitted, one at each end of the rear prop, there is nothing different added to the 4wd so that is saying to me that its the rear prop that needs the damping not the transfer box.
the transfer box splines can wear badly giving movement, is there any lift in the front prop as it enters the tranfer box. grab the front prop and twist.does it click.

[Northern Bongolow]

mike.
just chatted to a bloke that makes the blades for rolls, he said they make the blades all different sizes, if they made them all the same they have severe balancing problems when the engine is assembled . work that one out , but i think i get what he's saying .

[Northern Bongolow]

the tyres are smaller on the front than the back,,,

this is correct.
195 70 15 front.
215 65 15 rear.
this gives 6mm dia-- 18mm circumference larger at the rear. however if your transfer box is worn this may be your problem, as the transfer box may not let the normal rear bias operate correctly, especially if you have part worn tyres on.
any weird wear patterns on the front tyres?
try 4 wheels /tyres all the same size and wear level.

[teenmal]

the 2 wd and 4wd both have the same 2 dampers fitted, one at each end of the rear prop, there is nothing different added to the 4wd so that is saying to me that its the rear prop that needs the damping not the transfer box.the transfer box splines can wear badly giving movement, is there any lift in the front prop as it enters the tranfer box. grab the front prop and twist.does it click.

Hi,I Think you will find one of these doofers fitted to the transfer box casing.I would imagine on these old vehicles they could be classed as Sky Hooks.

Cheers.

[teenmal]

Several garages have had a look at the problem, one said transfer box,, it was removed tested an ok, one said front propshaft, replaced with second hand part still there, all bearings and joints have been looked at, common link it is the transfer box when it is touched, the noise stops for a while or is instant, the Dynamic damper is on the bottom of the transfer box, see the parts diagram on the fury site. Now mazda would only put one there if they thought the transfer box would move, it is only suspended by one point and if it vibrated it may throw a perfectly balanced propshaft out of true...

the noise can be gone for weeks and a journey from newcastle to oxford and back only resulted in it being there twice once there once back for no more than a few minutes,


the tyres are smaller on the front than the back,,,

Hi,
just being nosey,how do you touch the transfer box when you are doing 50MPH.?