alright guys i have a problem with my datto thats been hassling me since i got it started and is starting to get the better of me...

im stumped at the moment as to what to do to fix it so im turning to the datto family for some ideas... this is the last of the teething problems and its stopping me from getting my ute to the engineer so i can get it legal and enjoy it

for those who dont know its a standard 12a rotary engine with a rx7 5 speed and standard ute diff..

i have a slight driveline vibration which only occurs above 60-70kph and only when not under load..ie if you put the clutch in to change gear.. when accelerating theres no vibration... when coasting in gear theres no vibration.. only for a second when u let go of the accel pedal... if you put it in nuetral and let it roll vibration is constant..

the chap who made my custom tailshaft has looked at it and says there is a tiny bit of angle on the uni at gearbox end..

since then i have lifted the gearbox higher.. and dropped the engine slightly... this meant modifying my trans tunnel furthur.. making another set of engine mounts and gearbox xmember... and modifying my exhaust so it doesnt knock on the underside of the car...

the vibration isnt as bad but is still present...

tailshaft guy says the box has to go higher still...

now to move my box up... apart from modifying all of the above AGAIN... i will end up with a fairly big angle on the uni at the diff end....

so at the moment im stumped...

i can improve the angle of the gearbox side uni by making the engine lower... but doing this will involve recessing the original datsun engine crossmember... and making all new mounts etc.. but i havent heard of this being necessary in a 1200 with rotary conversion....

where have i gone wrong??

do i need to lift the gearbox higher and then have the angle on the diff changed to improve the uni angle's again??

im stumped... tailshaft guy isnt much help.. apart from saying box up box up...

and idea's or miracle answers to my problem before i start pulling the whole thing apart AGAIN...

thanks in advance guys and sorry for the longest post ever

ps. i bought an rx7 for the running gear and there was no vibration present when i drove that...

cheers

i say find a new tail shaft guy
but then again your rear end has been lowered as well hasnt it ange, maybe the problem is at the diff end with the angle not at the gear box. if you have to move the gbox up to reduce the vibration would moving the diff down (raising the rear end slightly) do the same?
Maybe experiment with some lowering blocks but use the to raise the rear end to see if it alieviates the problem. at least you wont have to fabricate any thing .... YET.

ive been down that path already mik...

ive raised the rear end which improved the problem ever so slightly...

bad news is this means i need even MORE angle than it looks like i need so that i can lower it again once the conversion is complete without making the problem return..

Try this may help diagnose the problem
Get a radiator hose clamp , big one, and put it near the gearbox end of your tailshaft, take car for a drive. Is the problem worse or better, rotate the clamp so the screw part is 180 degrees around, try it out again.
The move the clamp to the diff end of tailshaft and repeat procedure.
The screw acts as a weight and will help you tell if your tailshaft is out of balance.

the tailshaft has just been shortened and balanced...

althought i may take it to a 2nd party to check for balance since the original tailshaft guy wasnt interested in testing it for balance when i took the ute to show him...

another thing i thought of is that the output shaft on the gearbox has a fair bit of play in it...

althought it didnt vibrate in the rx7 it came out of im thinking maybe as i have a slight angle its causing the play in the shaft to slop around a bit...

thoughts??

No conversion here but after putting in the ute diff and lowering it I too have the same simptoms as you. A slight vibration when backing off. I think it must have to do with my tailshaft not been as direct anymore due to the change in angles though. But hey. Doesnt really bother me. Its a 32 year old car so I dont expect new car qualities in those departments.

The angle of the tailshaft doesnt matter, but you want the angle of the gearbox output shaft and the diff pinion to be the same. Angleing the diff pinion down is just like angleing the gearbox up.
They say that ideally you want the diff pinion angle to be 4 degrees lower than the gearbox shaft's angle. This means that underload when the springs windup a little - both angles will be the same. ( realisticly every car is different and springs loadup differently )
Different angles cause the uni's to rotate at different speeds throughout a complete revelution and this causes the vibration.

4WD shops sell angle shims for between the diff and the leaf spring.

phunk beat me with the 4wd shim bit .....
the shorter tailshaft will accentuate the angle between the 2 more than a longer one. could possibly pay to check the angles and shim the diff so the yoke points down a little more rather than trying to keep lifting the gearbox.

good to hear you got it all together so hows she runnin aside from the little wobble ??? give us some details

there is no vibration down low even under heavy load... it seems to be sensitive to speed rather than load...

do you think angling my diff down slightly might rid me of the vibration?


boof apart from the vibration she's running good... revs quickly and hard with wheelspin and axle tramp being its main enemy...

rotors are a lot like 2 stroke's...

0-4500rpm nothing... 5000+rpm powerband..

it goes real well... considering its a standard motor with about 100hp at the wheels give or take some...

sounds good might have to bring mine down for a bit of sticky beak could be an interesting match up

with the diff angle ... loosen off the shakles and stick in a washer or two at the back - tighten up and take for a drive (no wheel spinnin ) see if it makes a change and try a different thickness ... then if you can get it good see if you can get some proper shims to the right thickness