Hi Guys,

I am building a Sierra based kit car and my differential is mounted in the centre and my driveshafts are equal in length. Every Kitcar I have seen has unequal driveshaft lengths. and the differential mounted off centre, I can’t think from an engineering point of view it would make a difference and this is purely down to a engine/gearbox installation location .. as I don’t have a builders manual and my expertise is in civil engineering not mechanical engineering, can one of you knowledgeable gents advise if I’m right.

Thanks in advance

The Sierra diff is slightly off-set to one side of the vehicle meaning the driveshafts are (or should be) unequal length. Although the diff may look as if it's mounyed centrally in the vehicle the actual outputs of it are not equally spaced around the centreline. I can't recall which side is the longer and which the shorter driveshaft (it's some years since I built and sold my Sierra based MK Indy).

I'm sure others on here will know which side is which.

Hi,

If the mounting points of the diff are equally distant of the car's centre line (diff is centred), then you should have unequal length drive shafts. It is the case on the Haynes Roadster, which uses OEM Sierra drive shafts (unequal), and I guess number of Sierra based kit cars.

The reason is that a differential is not a symmetrical mechanism, as there is the large crown wheel on one side. This crown wheel takes space on one side of the diff, hence a shorter drive shaft on this side if the diff is perfectly centred.

If you have equal length drive shafts, the mounting points of the differential should not be equally distant of the car's centre line (diff should be off-centre).

In my humble opinion, it is one or the other, and it sounds a bit weird to me that drive shaft could have the same length with a centred Sierra differential.

If your differential is mounted on the centre line, your left hand driveshaft will be shorter than the right.
Equal length driveshafts are found on cars where the differential is offset to the right, the engine (the only unit I've seen in the paddock so placed is a bike unit) then offset to suit. Looked good on a bike unit anyway, unsure how a car unit would suit.
Other ways of getting the driveshafts of equal length include an angular drivetrain installation, nice example of that in the chain drive world are the Speads cars.

It matters not, the only issues are the need to carry two shafts in the van to the circuit if it's an unequal installation. The only other (slight) issue is that the joints on the shorter shaft get a harder time in articulation.

Related to the above posts regarding driveshaft length...

The Sierra Lobro driveshafts are advertised as a certain length (e.g. 466mm, 480mm, 508mm, etc) but obviously the driveshaft spline must be able to move in and out of the lobro a bit (in order to accommodate geometry changes associated with suspension movement).

So, is the advertised length the maximum length, the minimum length, or somewhere in between.

Following on from that, does anyone know how much stretch (or squash) there is in a Sierra Lobro driveshaft?

Thanks,
Stuart