Prop Shaft / Overdrive for Derby

David Cocking · Posted: Sat Sep 10, 2016 4:04 pm Post subject: Prop Shaft / Overdrive for Derby

Does anyone know (or can measure) the length of a standard Derby 3.1/2L prop shaft assembly, from the front of the forward flange to the rear of the rear flange? Alternatively, does anyone have the equivalent measurement for the BDC overdrive kit? Before I go ahead and order the bits I just want to check that the standard kit will fit, rather than discover too late that the relative positions of my drive train components have been modified during the car's life.

Many thanks,
David

Christopher Carnley · Posted: Sat Sep 10, 2016 5:00 pm Post subject:

I will measure one tomorrow. Is it the Hooper saloon B79CW?

If you have the steel rear floor pan, you may have to alter the tunnel to fit over the overdrive.
There are a pair of adaptors in the kit to fit the Hardy Spicer UJ,s to the R-R ones.

Christopher Carnley · Posted: Sun Sep 11, 2016 9:11 am Post subject:

David,

About 56" from flange to flange but the gearbox flange is sandwiched inside the vibration damper. 57" from cross member to pinion drive flange.

David Cocking · Posted: Sun Sep 11, 2016 11:33 am Post subject:

Many thanks for this Christopher. Yes, it is the B79CW, but now ex-Hooper as that body rotted out and was replaced with a Harry Rose body in the '80's. I posted a separate question in the Specials SIG forum to see if anyone knows of any interference issues between the overdrive and the Harry Rose rear footwells.

While I am on, although the diff is relatively quiet, given the degree of backlash I have in it I suspect I may need to get it sorted before too long. For improving higher speed touring, any thoughts on the relative merits of the higher ratio crown wheel vs an overdrive? My current plan may end up with me doing both.

Very much obliged,
David

Christopher Carnley · Posted: Sun Sep 11, 2016 2:48 pm Post subject:

David,

The "backlash" is rarely the problem, by that I mean is that you may be experiencing the total accumulated wear between the planet assembly, the half-shaft splines, the driving dogs and the hub splines.

I am in the process of rebuilding a 4 1/4 litre axle,viz new hubs, drivers, half-shafts, 3.64:1 gear set, and pinion bearings. Parts? a mere 6.5K!

It is a mistake to fit a new gear set in a worn axle, and the knocky knocky wheel clonks give it away.
The ratio drop with the 3.64 set is about 10%, and the overdrive about 22%.
You have to be very careful in using the overdrive, selection must only be when driving in 4th, as if the circuit is on, a sudden drop by effectively 2 full ratios is enough to damage the works, and makes a right old bang.

An overdrive along with the 3.64 set means that you are effectively starting in 3rd gear, and bucking broncos comes to mind.

David Cocking · Posted: Sun Sep 11, 2016 4:50 pm Post subject:

Thanks again Christopher for your hard-learned pearls of wisdom. Yes, by "backlash" I was referring to the overall slack/wear in the system, i.e.. the amount I can turn the prop before the wheels show any interest in moving. A switch to the higher ratio crown wheel will only happen if the need for a complete axle overhaul becomes unavoidable.

Just measured what I have and the 57 inch checks out fine so it looks like I am in good shape there. Your warning re only engaging OD when already in 4th gear is duly noted and I will ensure an idiot-proof interlock is in place!

Thanks again,
David