Hey all
So the time has come for me to sort out the diff for my sedan!
As i am using EL Falcon axles i have to get both shortened anyways and cant use the short one in the long side blah blah

So what i was thinking, instead of doing it the normal way to use the untouched Short 31 axle.
Would i be better off just shortening the diff more evenly (eg around 80mm each side) And get the axles cut and splined to suit? Or will this move the pinion the opposite way too much?

Or should i just get it done the tried and proven way, 50mm off one side and 105mm off the other?

I was thinking there may be some advantage in doing it more even... but maybe not lol

cheers Wardy

hi wardy (our nic names are so close this is confusing) because you are using ford axles and have to shorten both of them i would do whatever it takes to get the pinion as close to the center of the car as possible ,this is to make the driveline as straight as practical , it will help with tunnel clearence as well . overall it will help later on when you start playing around with pinion angle to optimize power down out of corners .WARDS

stock r31 diffs dont have the pinion in the centre to start off with. and when you shorten them to suit a 1200 they end up closer to the centre but offset to the opposite side.

but as wards said, find out what you can actually do with the ford axles as you have to cut them down past then undersized part of the axle so you may only be able to do it one way
think the undersides part of the axle goes for about 100mm or so. so you might find that you need to shorten 2 long ford axles rather than a short and a long one

cheers for the reply guys!

I havnt actually measured how far off center the pinion is on the 31 diff as yet, ill have to get the tape measure out tomorrow and do a bit of math lol.. i hav ealso already got both the axles, unfortunately i got the short and long falcon axles, which may cause a problem. But we will see

HI guys
Just out of curiousity what is the width of a R31 rear end as opposed to a 1200 ute rear end ?
Also how much of a difference is there from side to side

Cheers
Bob

Bob,

The diff in my sedan measures in at around 1330mm drum to drum
Stock R31 disk to disk is 1485mm

Hi Wardy
And now if you cut the long side down so the short axle fits this side what width have you got.
I will be going down this path shortly but have not got a rear end to measure. So all info is greatly appreciated.
Thanks
Bob

bob, 50mm off the long side(to suit the short axle)
105mm off the short side to suit the long axle cut down 155mm

ok so ive done a bit of measuring and what not..

Seem the R31 pinion is offset to teh passenger side approx 40mm.

So ill be shortening the LHS tube by 98mm
And shortening the RHS by 57mm to acheive the standard 1200 width and make the pinion pretty much smack in the center!

As u said pig i cant use the short ford axle as it will need to be splined on the undersized part of the axle (approx 80mm from the end) which is a bummer but ah well.

So ill be using 2x Long Ford axles

1x cut down 165mm
1x cut down 175mm

just thgought i would chuck it up here for future reference for someone lol.

Theoretically there is 'nothing' wrong with the tailshaft being on an angle (they are already on an angle due to the trans output shaft being ever so slightly different height than the diff yoke in 99% of cars out there anyway) AS LONG AS the two yokes are on 'parallel' angles.

Due to the way universal joints work, believe it or not, when the gearbox output shaft and diff pinion are rotating at a constant speed, the tailshft itself will be speeding up a fraction, slowing down, speeding up, slowing down, during every single revolution. As long as the gearbox output shaft and diff pinion are parallel (but not necessarily aligned) this all evens out (you also have to have each uni joint flange welded on in perfect alignment at each end of the tailshaft, it'll perform precisely how it is meant to and be (assuming it's balanced and not buckled) vibration free. But if you (hypothetically) altered the diff pinion angle a lot, and or the engine setback angle a lot, so those angles weren't parallel to one another, then it can and will vibrate (and obviously a degree or two won't be enough to be noticeable usually, but 10 or more might be, the more the difference, the more significant the vibration)

Having said all that, having the diff pinion and the tailshaft on an angle laterally (rather than just the usual slight horizontal) risks putting the tailshaft closer to the tunnel, esp if the rear leaves start to wind up with a lot of force (and naturally much higher output engines are why this diff would be put there in the first place) and so forth - it's basically just adding another variable which isn't optimal in there.

Although, as mentioned, the uni joints 'will' function properly when the output shaft and diff pinion angles are parallel - even if the tailshaft it on an angle, well this 'speeding up and slowing down' stuff becomes more severe (i.e. the difference between it's lowest and highest speeds in any one rotation gets higher and higher) the larger the angle. This in and of itself can eventually get to a point where it will be a vibration from the abrupt changes in tailshaft rpm. So it will be more problematic eventually, and will cause faster uni joint wear. If you absolutely had to do it on large angles then the 'real' answer would have to be to custom make CV joints (which as the name suggest have 'constant velocity' and the rpm of the intermediate shaft (the tail shaft in other words) doesn't speed up and slow down, it runs at constant/steady rpms in sync with the output shaft and diff pinion. For this and other reasons CV joints can handle greater angles (it's not just the vibration etc, the amount of metal around the flanges of a uni joint take up space and make it difficult to allow a severe angle without them hitting each other and jamming).

If you can at all possibly do it without costing a million in new custom axles, I'd personally aim for having the tailshaft parallel and straight when viewed from overhead.