Tailshaft – 1 or 2 piece?

[e30guy]

Not to get offtopic

That is mega shit if the front uni is a slip joint.

15 mins to drop the diff thanks to some new holes in the boot floor, no big deal.

Pretty certain E30 have a slip in the middle of the TS, for this exact reason?

Correct, all that goes out the window when you fit an SR and 300ZX gearbox, check the racers thread. Clicky

Pssi, Any idea how long a single piece would end up? Mine is close to 1200mm and Hardie Spicer were a lil concerned with only being able to fit a 3" tube, even for a piss weak SR20. The only reason we went single was because it didn't mean attempting to graft some CV arrangement onto the diff flange.

2 piece effective halves the span of the tube and increases it's torque capacity a great deal.

[Paydirt]

How did the original shaft length go with lining up?

[psi999]

How long would the single shaft be and what rpm will it see?

How did the original shaft length go with lining up?

Pssi, Any idea how long a single piece would end up? Mine is close to 1200mm and Hardie Spicer were a lil concerned with only being able to fit a 3" tube, even for a piss weak SR20. The only reason we went single was because it didn't mean attempting to graft some CV arrangement onto the diff flange

Pete - no dice, wrong spline and too short (if anything i thought the stock shaft would have been too long!)

Momus & E30 - here are the measurements:

Stock S15 SR20 shaft: 510mm (centre flange to gearbox output shaft)

And some rough measurements of what is required:

a) Front driveshaft - would need to be ~ 660mm (centre flange to gearbox output shaft)
b) Complete new single piece ~ 1300mm (diff flange to gearbox output shaft)

As for RPM, im not sure. I'll be turning the engine to 7500rpm and being a 6-speed sky's the limit for 'road' speed - will have ratio for 300km/h+ (not that i have the balls however)

[tanman]

I had spicer make up a complete new two piece for an S14 200SX.
Think it was 3" will have to measure to confirm.
If you need a new centre bearing a navara one from their catalogue is the same.
They used Heavy Duty Commodore Uni joints and a Ford transit bit in the centre.
Was not cheap ans sometimes I wonder if I should have got someone to make me a one piece instead.

[Momus]

Ok If you have a max of 7500 engine rpm and an overdrive top gear of say 20% the shaft will see about 9000 rpm. This is getting up there; even if you don't reckon you will top end it, it may still see high rpm if the gearing is shortened.

Assuming good workmanship, the critical speed of the shaft is the limitation, not the torque. Basically, the critical speed is when the shaft starts to whirl; to run off centre, and that speed is closely related to shaft length and diameter. If you double the diameter of a shaft you increase its stiffness approximately 16 fold so a huge diameter increase is not needed. You are possibly going from two short shafts to one relatively long one so the diameter will have to be increased. I've lifted the chart and info below from Mark Williams, and it gives you a general guide.

You can see straight away that 3" steel tube, regardless of wall thickness is not enough to be safe with a 1.300 metre (51") shaft. You probably want 3.5". 3.5" 065" cr-mo 4130 would be my choice but will not be available easily. The 3.5" mild steel .065 or .083" will probably be available though at this diameter the yokes will probably have to be altered or replaced to suit the big tube. Altering them is not hard if the driveshaft people are worth the name, and it could involve welding a concentric ring to your existing tube ends to get the diameter.

If you go this way ask for new tube to be used or at least make sure that any used tube is run in the same direction as it was in the original shaft.

-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Critical speed is the speed at which a spinning shaft will become unstable. This is one of the single largest factors in driveshaft selection. When the whirling frequency and the natural frequency coincide, any vibrations will be multiplied. So much that the shaft may self destruct. Another way to think of this is that if a shaft naturally vibrates at 130 times a second, and one point on the shaft passes through 0 degrees 130 times a second (7800 RPM) then the shaft has hit a critical speed. There are several ways to raise the critical speed of a driveshaft. You can make it lighter, stiffer, or increase diameter without increasing weight. This is the reason carbon fiber makes a good driveshaft, it is stiff and light and can be made to any diameter or wall thickness. Aluminum, while it has a very good critical speed is not quite as strong as steel. Steel, with good strength characteristics will have a lower critical speed.

Critical Speed

--------------------------------------------------------------------------------

[Glenno]

I'd personally go for a 2 piece its a lot nicer for alignment ect

Get a second opinion from UDSQ at Geebung they know their stuff.

[chartreusehj]

Momus,

Im of the opinion that the critical speeds arent as "critical" unless you plan to sit on that harmonic for a while, if the rear gearing lets the engine rev through it quickly enough the effects arent as catastrophic as say, a looong slow dyno pull.

am i an idiot?

[Momus]

I think vibrations and frequencies that are a bit nasty but occur briefly as the engine is accelerating and passes through them are less of a worry than potential tailshaft whirl that can start very quickly at the speed you least want it, or can control it, and become a disaster. There are quite often a few of these more minor vibrations on modified and amatuer race cars, that are blythely ignored.

I've never whirled and failed a car tailshaft but I have seen a similarly long slender part (a thick wall chrome moly stabiliser bar) that was being run at a very modest 1800 rpm in a lathe let go and smash the top slide half off the saddle.

I wouldn't take the chance with a tailshaft, particularly one that was running a slip yoke on a live axle; which is not what's happening here.

[bpm]

i have had three of the fucking things let go on my dyno, always around 7000rpm and it scares the absolute shit out of you. and then makes a mess of the exhaust, dyno, floorpan. always been 1 piece shafts. last one broke into three pieces and launched the "middle" bit across the workshop

[chartreusehj]

thanks!

[MRMOPARMAN]

what momus said is exactly why the old live axle falcons were speed limited to 180kmh. they used to get whip up and fail (aluminium tube tailshafts).

if you needed extra uni angle to allow for extra wheel travel, youd go single piece. in your case, id go 2 piece.

[Glenno]

what momus said is exactly why the old live axle falcons were speed limited to 180kmh. they used to get whip up and fail (aluminium tube tailshafts).

if you needed extra uni angle to allow for extra wheel travel, youd go single piece. in your case, id go 2 piece.

Hotgemini and myself have stripped the gearbox and what was left of the engine out of an AU ute that was burnt out, quite clearly the single piece tailshaft on it had let go and taken out the fuel tank in the process.

[bpm]

Hotgemini and myself have stripped the gearbox and what was left of the engine out of an AU ute that was burnt out, quite clearly the single piece tailshaft on it had let go and taken out the fuel tank in the process.

have seen tailshafts explode when cars are burnt.

[bpm]

Hotgemini and myself have stripped the gearbox and what was left of the engine out of an AU ute that was burnt out, quite clearly the single piece tailshaft on it had let go and taken out the fuel tank in the process.

have seen tailshafts explode when cars are burnt.

[psi999]

Thanks for your help guys (especially Momums - great info!)

At this stage im leaning towards a single 3.5" 6061 Aluminium drive shaft made with spicer 1350 uni's

Consulting Momus's attachment (driveshaft length = 51 inches) = critical driveshaft RPM will work out at 7718.5 RPM

(Same length in 3" mild steel = 6603.5 RPM, which is obviously unaceptable)

I have also been offered a 4" 6061 shaft for not a lot of extra dollars which works out at 8826.5 RPM - have to get under the car to see what sort of clearance i have though.