The 'E' Type Forum

OD does reduce torque, here is sample calc. for 3.54 FD in OD in 1st gear:
If engine output torque = 350 Nm
1st gear ratio = 2.94. So gbox output (OD input) = 350 x 2.94 = 1030 Nm.
OD ratio = 0.78. So OD output = 1030 x 0.78 = 800 Nm. I.e. OD has reduced torque from 1030 Nm to 800 Nm.
Same calculation working back from the wheels:
Car mass = 1700 kg, 50% on rear (driving) wheels. Tyre/ahphalt static friction coefficient = 1. Tyre radius = 0.34 m. Therefore max wheel torque before wheelslip = 1700 x 9.81 x 50% x 1 x 0.34 = 8340 Nm.
FD = 3.54. We’re working backwards so FD input torque = 8340 / 3.54 = 800 Nm
OD ratio is 0.78. So OD input torque = 800 / 0.78 = 1030 Nm
I.e. as before. The OD has reduced the 1030 Nm input to 800 Nm output. Moving on:
Gbox 1st gear is 2.94. So gbox input is 1030 / 2.94 = 350 Nm. This is then the wheel-slip limited engine torque in 1st gear with OD applied and 3.54 diff. N/A to any other configuration.

Colin : I think the difference of opinion here is that you are looking at the effect of OD in reducing the torque at the end point i.e. the wheels, and you are correct about that in mathematical terms. However, what is really under consideration from the point of durability is the torque received by each individual stage of the transmission, and the torque which the input shaft of the OD receives is the engine torque multiplied by whatever gear ratio is selected, and in the case of 1st gear on the V12 Jaguar that multiplying factor is of the order of 2.933 . The maximum torque is given as 433NM or 319lb/ft and thus the torque which would have to be dealt with by the OD input would be 319 X 2.933 = 935 lb/ft. The highest figure that I can trace for which a Laycock OD was designed was 265 X 1 = 265 lb/ft on a Jaguar 3.8 XK150S, so that what you are proposing is an excess load factor of 935/265 = 3.53 to 1 . This is why we are concerned at the lowest, and certain at the highest, that you will break your OD if you adopt your proposed scheme

Christopher - you have to look at the entire powertrain from crank to contact patch. There is no work done without resistance. A formula one engine and a moped engine are some senses functionally equivalent if their back wheels are off the ground. If there were no torque needed to overcome internal resistances of their respective mechanisms, neither would be doing any work. Mass is an integral part of the load calculation both from an inertia perspective and traction.

That’s why Laycock in Sheffield told me I was wasting my money asking them to upgrade an OD to competition spec once I told them I was going to put it in Series 3 E. They said it was unnecessary as the E was a lighter car than an XJ with/without passengers and luggage. i had to insist they take my money and upgrade, with the excuse “..just in case I used it in something different.” I felt a bit of a fool for arguing the point, which is probably because I was...

It’s also why an overdrive from a heavy XJ suffered no ill effects being used on all four gears behind the fastest XK engine I’ve ever driven to date, presumably because it was in the lightest ‘Jaguar’ kit car I’ve ever driven. Anecdotal evidence, admittedly, but still...

Seems Laycock de Normanville became the overdrive division of GKN which was bought out by Gear Vendors (GV). GV still produce these units and market them as under/overdrives, to emphasize the opportunities/benefits of using OD in all gears, called “gear-splitting”.
GV website: https://www.gearvendors.com/index.html

One of the many gear-splitting example from GV website:


One of the magazine articles reproduced on GV website, in this one they say that early LdN Od's weren't that strong …
https://www.gearvendors.com/rodandcustom-sep05.html

So I've asked the question:
Hello, i'm about to install a Laycock deNormanville Compact type A OD to my V12 XKE, bolts straight onto the Jaguar 4 speed box, replaces the extension housing. I want to use it for gear splitting, I'm told the V12 torque would break it if used in lower gears (ratios 2.94, 1.9, 1.39 & 1). Pls can you suggest if there is anything I should be concerned about and whether there could be anything done to fix this?

I'm not going on for ever about this, Colin, but I think you are trying to compare apples with oranges. I quote from the Gear vendors article : " The unit Gear Vendors provided to the restoration and hot rod market is the same unit used in muscle cars, drag racing, and off-road racing with up to 1,200 hp, ". This is fundamentally different from a Compact A type overdrive. However, if you want to do it, it's your car and your money......