Viva GT: Prop shaft
So this afternoon, I slipped under the car (no mean feat!) and un-did the propshaft and made a note of a few things:
The prop only has about 8-10mm clearance, so it might be bottoming out (compressing in fact between gearbox and axle?) as the axle moves through its range? Albeit the distance between axle and gearbox “should” get longer as the axle lifts up or compresses, as the trailing arms are fixed lengths, as such the rotation movement will lengthen the distance between the two. I listened to the advice from Dave Mac props they feel this space should be about 15mm but 8-10mm shouldn’t pose an serious issue (because of the above way the distance should increase not decrease under dip and lift of the rear axle).
The balance of the prop is also being checked at Dave Mac Props, so hopefully that will result in a confirmation the prop is fine. But there maybe an issue with two other factors around the propshaft:
One: the rotational speed of the propshaft, I believe I am very close if not exceeding the limitation of the prop, in terms of the need to drive it all the way to a max 7200 RPM in fourth gear (1:1). As such that’s potentially too fast for the length and dia./wall thickness of the prop as its stands. I should hear back later this week on the limitations of the prop, at the same time as the re-balance tests.
Two: Pinion angles, I can work out the angle of the gearbox, easy as all I need to measure the angle of the rocker covers. The rear axle is going to be harder, but I think both ends point ever so slightly downhill and this can (especially with high RPM) cause a phasing issue and drive a prop into an unfavourable harmonic situation (vibrations).
So how to fix it all in one easy step (more than one way to skin a cat obviously), is to fit a two piece prop (yes Nat your words are haunting me “why didn’t you get a two piece prop?”.
Assuming that the prop is deemed unsuitable it will be time to order up a two pie unit, and set about a crossmember to support the centre bearing. More work but much needed in order to drive safe.