Pinion angles- opinions please

db · Post by db » Wed Feb 27, 13 9:24 pm

In trying to fix a nasty vibe at 80-90mph I've been checking angles with my groovy new gadget.

Using the chart, I've corrected the diff (as close as I can) by adjusting the 4-link bars.
If my sketch makes sense, does it look right-ish?

Dave-R · Post by **Dave-R** » Wed Feb 27, 13 9:31 pm

If i understand what your drawings mean then no. But very close.

Basically you need the angles in both UJs to be exactly the same when driving. So just 1 degree "down" from the trans end (with the car at rest) is best.

You can also shim the trans mount if needed.

Dave-R · Post by **Dave-R** » Wed Feb 27, 13 9:38 pm

I go by this little drawing.

Pete · Post by **Pete** » Thu Feb 28, 13 7:07 am

I use Dave's drawing.
We had to shim the Transmission tailshaft to get things in line.....

Dave-R · Post by **Dave-R** » Thu Feb 28, 13 8:36 am

What is not in that little drawing is that you need at least one degree. For some reason I don't understand, the UJs like a little angle in them rather than being directly in line with the centerline of the crank.

Pete · Post by **Pete** » Thu Feb 28, 13 10:08 am

Again, I agree with Dave on this.

In your case it could even be a balance issue, not UJ angles.

Wheels, Tyres, Prop (I am sure you havethis balanced but you can still get harmonics...)

Dave999 · Post by **Dave999** » Thu Feb 28, 13 10:27 am

this is just a guess

when the suspension compresses or the engine moves on the mounts the CVs have to bend.

if there is no pinion to prop to tail shaft angle the cvs have a choice which way to go
that choice is assisted in the correct or wrong direction by the direction of change AND the shove forward or backward felt at the slip yoke

if you bias them one way or the other they can only go the way they are biased.

firstly this probably good from the point of view when the slip yoke has gone as far as it can forward, you won't shove the prop shaft up towards the floor out to either side or down towards the ground becasue it can never have a bend in the same direction at both ends. all it can do is get lower or higher at the back which if angled correctly means shorter as well

I also think its necessary to do this becuse any CV joint when rotating causes the prop shaft to speed up and slow down as each quadrant of the cv passes the inner radius of the bend. i.e it tries to go straighter than the bend then is happy to bend further as each lobe of the joint passes the inner radius.

With a big bend the finger from the prop in the bend as alinged with nearly the centre of the slip yoke shaft and the other finger from the prop is sitting outside the radius or rotation of the slip yoke faster slower faster slower etc like inner wheel and outer wheel in a tight corner with a spool in ya diff neither goes the correct speed for the manuver

Now this is the bit i don't understand

with it set up correctly this equal and opposing angle either avoids one end trying to go faster than the other at any point in time

OR

it sets it up in absolute oposition to that, which i guess would smooth out the pulsing seen at the pinion

not sure which.

if you take any prop shaft style jointed bar and bend the joint to an extreme and spin the shaft you can feel the notchyness at the articulated free end...

try it out with lego

the only other thing i can came up with is that that 1 degree is reduced to 1/2 or 0 degrees once you start moving forward and maybe there is no angle during speedy progression on a dead flat surface

but that seems likely to be a scenario that plays out for less than 1% of your motoring time. roads are lumpy and we all like to gun it from the lights

as stated its a guess and could all be rubbish but some of it fits right in my world and until i think of something else

Dave

db · Post by db » Thu Feb 28, 13 11:06 am

I believe that's something to do with the self lubing of the roller bearings- they need some movement or they'll stick.

I like my chart as it's the only one I've seen that gives specifics for a 4-link. That doesn't make it right of course

I'm happy to listen to the Voices Of Experience instead.

If I shim the trans mount to level the trans at 0 and drop the pinion nose to 0 that should give a bit of an angle in the UJ's.

Ok I'll try to give that a bash today, I imagine shimming the trans will be an arse of a job tho...

Dave-R · Post by **Dave-R** » Thu Feb 28, 13 11:39 am

I had to shim the trans on my Challenger after fitting the overdrive because of the longer tailshaft and shorter prop. Not sure if you have the same kind of mount but it just needed a bit of ally plate in there.

db · Post by db » Thu Feb 28, 13 12:01 pm

Yes mines the same Dave.
It's just compared to bunging a few turns on the 4-link bars (5 mins with a couple of spanners) To half a day getting the car up on stands, jacking up the tailshaft, cutting a plate dropping it all back on the deck to find it's not enough/ too much... Sorry feeble whining I'll stop now

db · Post by db » Thu Feb 28, 13 10:58 pm

Cracked it! Told you it'd be a bloody nightmare though

First I raised the trans about 1/2 inch on the jack. This brought the prop uncomfortably close to the prop loop and only changed the angle 0.5 degree.
Next I took the weight off the motor mounts with the engine crane and dropped to a 2nd set of holes in the chassis plates which I'd miraculously drilled 'just in case'. (just in case of what I didn't know when I did it!)

This combined with an 8mm spacer plate under the trans mount did the trick.
I now have the trans and the diff at 0.3 degrees sloping up towards the rear, and the prop angled down by 2.9.

pete walton · Post by **pete walton** » Fri Apr 05, 13 8:55 pm

They say the pinion angle is differant for each type of suspension set up i.e SS prings is one , ladder bars is another ,fourlink etc..... i read that somewhere the other week when i was looking up what mine should be...

db · Post by db » Fri Apr 05, 13 9:43 pm

That's pretty much what the chart at the top says Pete.
Though as you suggest every set up will have it's own right 'n' wrong

Dave-R · Post by **Dave-R** » Fri Apr 05, 13 10:28 pm

The differences in static set-up are down to how the suspension reacts under acceleration loads. But the final position under load should be the same angle in the UJ at each end of the prop. That way they rock from side to side the same amount and therefore at the same speed.