RWD rear end and pinion angle q's

[chris davey]

This is likely how it is now. Lower control arms are parallel with the ground at rest. Upper controls are inclined downwards at teh front. The Instant Centre is the imaginary point of intersection between these lines. Their location relative to the line from the contact patch of the rear tyre to the centre of gravity for the whole car determines the characteristics of the rear end. If the intersection is on that line, the car will be neutral- ie not squat and not seperate off the line. You see that a lot in well set up cars with reasonable rubber. As you can see the stock rear setup puts the IC below the line, which makes a torque which causes squat. As it squats, the IC moves further from the line and shorter, which makes it squat more and more aggressively. BAD.


This is neutral. By moving the front pickups up you might be able to get to this point- neutral. The IC is at or about the CG and this will drive straight down the track with no squat or seperation. There will appear to be some squat, but that will be because of the front lifting and not the rear compressing. Any compression will come from mass transfer and that is ok. You can run pretty hard springs and shocks with this type of setup because there is not much movement. Problem is that tuning is limited for the same reasons. But that is fine for a streeter.

If you are really lucky then with a raised rear ride height and some raised front mounts and maybe some lowered rear mounts you can get the IC above the line. Then the car will seperate off the line and plant the tyres hard. That will help LOTS with a small tyre/ big angry 2j

Firstly, I have no idea how you find the centre of gravity. Also in diag1 which one is just normally sitting? And which is under compression ie. launch? squat and launch is the same to me.

So there is the tech info.

Now my problem I think may be due to pinion angle. I have just fitted a hilux diff into my toyota corona. The mounts were already on it and it was using a 2 piece tailshaft. I was going to reuse the 2piece but took it to the tailshaft guy and he basically said for my needs (predominatly drag racing, street tyres) I should go with a 1 piece. So I did that as I had used a 1 piece with the old diff.

It seems to me that the pinion angle is further down on this new diff as a) the exhaust doesn't fit under the tailshaft anymore (there was small clearance before) and b) you can visually see a fair angle on the rear uni.

I measured the angle of the tailshaft to the output of the diff at 6.5deg. Is this too much?

As per muz's advice above I have also done some measuring of the top and bottom mounts. The measurements are as it would sit on the ground. The space between the top and bottom mounts is 190mm.

See MS paint




and here are some photos






So the issue is that I was just adjusting the drums and put the car in first and started to accelerate with it on the jack stands and it had a constant shudder like...donk...donk...donk...donk... not like donk,donk,donk,donk until the speed was up but it is speed dependent. Other info. which may be required is that the diff is welded and the axles had to be shortened about 9mm each as the fella who welded it welded the plate too close to the axle holes. The shudder is definitely from the rear and happens with the brakes on or off. There didn't look to be any movement at the flange of the tailshaft.

Please help

BTW all angles on MSpaint drawn with portactor on screen

[brett_celicacoupe]

I measured the angle of the tailshaft to the output of the diff at 6.5deg. Is this too much?

that angle isnt too much of a problem as long as the diff carrier and gearbox have parallel rotating axis.


is the car manual? the 'donking' could be caused by backlash in the diff centre

[chris davey]

car is auto. I was going to turn the wheels by hand and check the uni but it seemed to be ok. Another guy said that it is normally 3deg or less.

AFAIK the diff carrier and gearbox are basically the same angle. I will check it.

[30psi 4agte]

I looked in to this a little while back ..... There is plenty of info out there.
From memory you want -1.5 - 3 deg for a oil sprung car with control arms and -5 deg on a leaf spring car.

You run more neg pinion angle on a leaf spring setup as the diff will "walk" and change the angle under acceleration.

Good info here : http://www.carcraft.com/howto/91758/

As far as the control arm angle goes and setting up squat ........ sorry but its still fairly new to me too.

Im keen to find out more!

[thechuckster]

unbolt tailshaft
get strongest low-speed drill you have
get a v-belt pulley with small centre hole
attach long bolt to pulley so you can spin the pulley with the drill
get long belt to suit pulley
wrap around drill-powered-pulley and drum circumference
get helper to crank drill over (probably in low gear) to rotate drum and you feel/listen to diff for noise.

[Robbos_Toyotas]

my auto ma70 used to do it too, is mroe than likely diff backlash or worn bushes around the diff...

[brett_celicacoupe]

a little bit of backlash starts a chain reaction.

the wheels start spinning, then 'freespin' through the lash. as the wheels start spinning it loads and slows the engine. then the wheels clunk as they are slowed by the engine. then the motor gets the wheels spinning again, as it does so it loads the motor and the process starts again. the power pulsations also helps to cause this

try putting the handbrake on a little.

the same thing happens with motorbikes at low speeds

[chris davey]

Hmm...I am tending to think that the diff is the culprit. bushes are all new as well.

I will try and test the diff like Charles said. I am thinking that even if it is the diff I should probably still fix the pinion angle as it can't be good for the rear uni at that angle I don't think.

Also, if it is diff backlash it would be more noticable while the rear of the car is jacked than when it is on the ground driving right? My old car had a bit of backlash in it (manual) and I only noticed it a little while driving.

got no handbrake yet, still sorting shiznit out

[Mr DOHC]

use foot brake then

[chris davey]

did that, no difference, big clunks

[brett_celicacoupe]

when you launch the diff would nearly be inline with the tailshaft.

you might find if you adjust the diff so that there is less angle, the diff might hit the floor; depending on spring rates/dampening rates

[chris davey]

but isn't it too much angle at the moment? 6.5 deg instead of 3deg like was said.

[Robbos_Toyotas]

that pinion angle is fine mate, absolutely fine!! ....if you have a look at the uni and move it around with your hand, you'll notice you currently are using fuck all of its capability. a perfect example was the madass fj45 cruiser at dubbo, it had a 393 chev with a hugeass bottle of nos and was very quick - it had the stock offset rear diff and was just going off the auto tranny on it - the angle was farkin huge both sideways and height!

You get away with quite a bit of angle, if it gets too severe it will start to vibrate and be a prick to ride in, which you will obviously notice before something goes bang

[sv_i]

I saw this thread on performance forums and didnt answer there for fear of being shouted down, but from memory, there are cars out there from factory (I think the Shelby Cobra is one, but it has IRS?) that have a far steeper tailshaft angle.

honestly, it sounds like diff clunk/backlash. My hilux diff does it all the time and as brett said, so do motorbikes.

[chris davey]

Thanks for the input guys. WIll do what Robbo said tonight and give it a thourough check out.

Chris