Laycock de Normanville

[lowact]

Good point, not just the gearstick wiring, also the gearstick switch imo is a bit lightweight. It’s all per Jaguar design, good design would have the fuse the weakest part of the circuit …

With the XJ6-S1 there is a second, in-line “Overdrive Solenoid/Reverse Light” fuse, that is rated at only 8 (eight) amps. I’m imagining that this would be the “good design” element, protecting the lightweight gearstick wiring and switch (without a relay)? Presumably, when there are no issues, the closing solenoid current draw, if more than 8 amps, would not be for long enough to heat up and burn out the fuse? At the same time it would provide the most immediate protection against what is seemingly the #2 error condition to be avoided, persistent high current? I’d like this, corroded solenoid contacts causing permanently high current is something I’d want to know about.

With the XJ6-S2 the in-line fuse has seemingly disappeared, maybe because 8A fuses kept blowing? “Seemingly” because there are ambiguities in the wiring diag and description, this was the height of the Leyland malaise. So I can’t trust that this is still “good design”. Might also be acknowledgement that the XJ6-S1 in-line fuse wasn’t good design either …

Then there is the LdA design, much more gung-ho, heavy wire from battery-side, no switch except for the relay, no fuse because, if there was one and it blew when you were pulling max revs in O/D 2nd, the instant overspeed could be catastrophic …

I’ve bought a relay, 30 Amp with built-in fuse, cost a tad more than $3… My thinking was to use a 10A fuse with it. The gear lever wiring/switch would be protected by the relay, the smaller current fuse would protect against any persistent high solenoid current. Relay winding would be supplied from the reverse-light circuit, solenoid would be supplied from either side of fuse #4 (hazard lights, brown wire battery-side supply). Ideally the relay would be located near the gearshift.

Having a dedicated and easily accessible O/D fuse would be a useful emergency isolation/testing provision imo. If this is only possible with an in-line (10A) fuse aka XJ6-S1 I might (?) try instead to do this. Also would be the least modification, entirely consistent with Jaguar O/D design and would avoid any relay reliability issues. But then, if the 10A fuse keeps blowing I would have to redo, run the extra wires and install the relay, just more fun to be had …

[MarekH]

Hopefully you'll wire it up in a way not to put 16amp pulses through the ignition switch now. Saving five pounds for a relay would have have meant spending more on a new ignition switch some time down the line.

kind regards
Marek

[lowact]

18 months later, I'm finally getting around to it.
I was going to add a gearbox isolation switch, to limit OD to only when in top gear.
Instead I'm thinking to enable OD in all forward gears (with isolator to prevent reverse gear destruction).
This would give my car, with 3.54 diff, gearing that is just a smidgen taller than a 2.88 diff, with ability to drop down instantly, at the flick a gear knob switch. Please, can anyone suggest why this might be a bad idea?

[Dave123m]

It is highly likely the torque in the lower gears will destroy the Overdrive fairly quickly.

It’s very rare for any car to have overdrive on 1st & 2nd but some have it on 3rd & 4th.

I have overdrive on 4th only & don’t miss it on the other gears.

Good luck with the installation

[christopher storey]

Stick to 4th gear only . The torque of any Jaguar ( except the 2.4) will destroy the OD if multiplied up by 1st and 2nd gear

[MarekH]

I'm not sure that analysis holds well in every scenario.

Doing whatever you are doing in 1st or 2nd gear presents a substantially lighter load to the engine than doing that same thing in fourth gear. If you have trouble visualising that, try pulling away or accelerating hard in top gear on a bicycle - and then try the same exercise several gears down from that.

kind regards
Marek

[Series1 Stu]

If you have trouble visualising that, try pulling away or accelerating hard in top gear on a bicycle - and then try the same exercise several gears down from that.

That's because you've got more torque available in the lower gears. The scientific name for a gear reduction system is a torque multiplier because that's precisely what it does.

The principle of moments applied to rotational equipment.

Regards

[MarkRado]

I had OD on 3rd and 4th on my Healey BJ8, the difference between 3rd with/without OD was minimal and of no practical use. The Jag gearbox has very close ratios, so it will be the same, except maybe if you have a 2.88 axle.

[MarekH]

I thought the torque curve for the v12 was almost completely flat judging from the posted dyno graphs people buy. Does this mean you have to buy a different graph from the dyno man if you do it in 1st gear not 4th?

Yes, the gear ratios simply get duplicated (except between 1st and 2nd) so there is no point in engaging overdrive in anything other than top.

kind regards
Marek

[johnetype]

Yes, the gear ratios simply get duplicated (except between 1st and 2nd) so there is no point in engaging overdrive in anything other than top.

Except that with third overdrive being very similar to top, you can enjoy driving around twisty country lanes in third and use the overdrive to quickly switch between third and (effectively) top. No need to use the clutch and slowly physically change gear. It's fun and effective.

[lowact]

Most of the criticism of a 3.54 diff is the high revs at highway speeds. For me, equally tedious is when taking off in 1st gear, the very short distance travelled before wanting to change gear. OD unit fixes the highway issue but not the taking-off issue, unless it was possible to use OD in 1st also…

I note lot’s of advice that 1st gear OD would likely overload/damage the OD, including: https://www.uniquecarsandparts.com.au/h ... _overdrive

Thus a four-speed gearbox might be made part of a seven-speed transmission. It was never thought safe to apply the overdrive to first gear, as the ratio of first was so low as to multiply the torque of the engine three or more times, the resulting output torque from the gearbox being too much for the overdrive to stand. When allied to large engines, the overdrive might not be able to take the torque in second gear either, and in powerful cars it was thought more prudent to limit its operation to top gear only.

I calculate as follows:

1st gear, torque is limited by loss of tyre traction, what is the corresponding OD (input) torque?

Start with a known baseline, 2.88 diff with OD disengaged (not being used): 1st gear tyre slip would be caused by 330 Nm (250 lbf.ft) from the engine, input to the OD unit would be 980 Nm (730 lbf.ft). Drive through a disengaged OD is via the sprag clutch.

Compare to this, a 3.54 diff with OD engaged, tyre slip would need 350 Nm (260 lbf.ft) from the engine, input to the OD would be 1030 Nm (760 lbf.ft). Drive through the OD is via the planetary gears.

1030 is only 5% more than 980, this cannot be critical? Good design means the sprag clutch and planetary gears would have similar strengths? In which case, if Marek and Peter’s cars (2.88 with disengaged OD) can withstand 1st gear wheel slip with impunity, a 3.54 with engaged OD should be ok in 1st gear also?

[Series1 Stu]

If you ignore losses, whatever torque you have at the flywheel can simply be multiplied by the reduction ratio.

Regards

[Ole-xke1974]

Please, can anyone suggest why this might be a bad idea?

I wouldn't have the OD available in all gears. The torque from the engine is transmitted un-reduced (in 1st) and less reduced in 2nd and 3rd, which will be too much for the OD to handle. I'm mainly stating this based on the fact that all factory applications of the LdN OD was in forth and in some cased 3rd and 4th (as far as I know)
I have it fitted as per Jaguar, available only in 4th. With the torque of the V12, the car pulls without effort from under 1500 RPM with the OD engaged.
When the Sheffield company "Overdrive Repair Services" rebuilt my OD, they suggested to upgrade it to 4.2 specifications. (springs and oil pressure, I believe)
Just my $0.02 worth ........ Ole

[Series1 Stu]

The torque from the engine is transmitted un-reduced (in 1st) and less reduced in 2nd and 3rd, which will be too much for the OD to handle.

Not quite correct. The torque is multiplied by the lower gears according to their respective ratios. At a ratio of 1:1 the gearbox output torque is equal to the engine's torque at the flywheel (assuming no losses).

Hence, the lower the gear, the higher the torque seen by the overdrive unit.

Regards

[PeterCrespin]

I was going to add a gearbox isolation switch, to limit OD to only when in top gear.

Every LdN Jag box came with a second switch on the top cover, just forward of the identical reverse lamp switch. The switch is wired in line with the solenoid feed, thereby only permitting OD selection when in 4th gear and automatically preserving the OD from inadvertent reverse gear selection.

Instead I'm thinking to enable OD in all forward gears (with isolator to prevent reverse gear destruction).

I had a D-type replica with direct-fed OD on all gears, operated via a semi-covered dash switch.Since you can't accelerate with the clutch disengaged, the only sensible usage pattern was to drive in sport mode with the overdrive always off, or cruise mode with overdrive on all gears. This worked quite well, with rwo caveats:

A D-type replica weighs far less than a Series 3.

Without some kind of home-brewed electromechanical gear lever interlock, having the OD available on all forward gears normally means it is available in reverse also. Contrary to popular belief, reversing with the overdrive engaged does not inevitably or instantly destroy the mechanism, although it is definitely not recommended.

[Ole-xke1974]

The torque from the engine is transmitted un-reduced (in 1st) and less reduced in 2nd and 3rd, which will be too much for the OD to handle.

Not quite correct. The torque is multiplied by the lower gears according to their respective ratios. At a ratio of 1:1 the gearbox output torque is equal to the engine's torque at the flywheel (assuming no losses).

Hence, the lower the gear, the higher the torque seen by the overdrive unit.

Regards

You are correct off course. My bad. Must have been asleep when I wrote that :-)

[lowact]

If you ignore losses, whatever torque you have at the flywheel can simply be multiplied by the reduction ratio.

Remembering that, in lower gears, what you can have at the flywheel is limited to what will cause the wheels to slip (tyre “chirp”). To calculate you need to work backwards from the wheels, is a function of car mass, tyre friction coefficient and gear ratios (diff, OD, box). Nerd alert, lots of numbers ahead …

In first gear without OD, with 2.88 diff, slip will occur when the engine output reaches 330 Nm (80%). With a 3.54 diff it requires less, only 270 Nm (66%) to lose traction.
With taller diff, drive train (incl. OD) is more highly stressed. Using Marek’s bicycle analogy, with 2.88 diff he is having to stand in the pedals and push hard and slowly, compared to me with a 3.54, sitting back in the seat, little effort req’d despite legs like a thrashing machine.

1st gear ratio is 2.94. With 2.88 diff the torque from the gearbox = 330 x 2.94 = 980 Nm, with 3.54 it is “only” 800 Nm. These are the torques from the gearbox when there is no OD or when the OD is “out”.

Now consider with OD "in", in 1st gear: With 2.88 diff. the flywheel torque required for wheel slip would be 430 Nm which is more than OEM engine is capable of (100% = 410 Nm) so the latter is the max, torque from the gearbox (into the OD) = 410 x 2.94 = 1200 Nm. With 3.54 diff and OD, wheel-slip would still occur, limiting 1st gear flywheel torque to 350 Nm = 1030 Nm into the OD.

Next consider OD in 4th gear. Wheel-slip is not possible, so OD torque is independent of diff ratio, gear ratio is 1:1 so max torque into the OD is the engine capability, say 410 Nm as above.

Putting it all together in a table of OD input torques, the blue numbers define the range of OD input torques occurring in 2.88 + OD cars, e.g. Marek, Peter, while the orange is the range that would occur if I had OD in all gears with 3.54 diff, note the max is only 5% more than what Marek and Peter have already proven:

Difference is that with Marek and Peter, the max is with the OD “out” whereas my max would be with the OD “in”. My suggestion, that with good design this shouldn’t make a lot of difference, is probably BS, more likely the load that ODs can withstand when transmitting at 1:1 via the sprag clutch is a lot more than when transmitting at 0.78:1 via the epicyclic gears.

[christopher storey]

You will just wreck your OD if you persist in your plan. Forget all about wheelspin etc : the torque which the OD will receive is the engine torque multiplied by the ratio of whatever gear is selected, and the Laycock ODs were most certainly not designed to cope with this from a large engine . Even on the TR6s which had OD on 2nd, the capacity was very marginal , and here we are talking about an engine of twice the capacity and a ratio far lower than 2nd on the TR gearbox

PS and by the way, the final drive ratio has absolutely nothing to do with the torque the OD receives : a higher FD ratio merely reduces the torque at the wheels

[lowact]

the torque which the OD will receive is the engine torque multiplied by the ratio of whatever gear is selected

Correct, but what is the magnitude of this torque? It is impossible for it to be more than what would cause wheel-spin. Most people don’t drive in a manner that would cause wheel-spin (heavy throttle in low gears). But with a 3.54 FD e-type, this can happen even when you don’t want it to.

Torque is just rotating force. Go out yr front door, latch it shut and try to push it open, as hard as you can. Then unlatch it and try to push against it just as hard. You can’t push as hard if it moves. Same with an engine. No matter how powerful a car engine, it cannot produce more torque than would cause loss of traction. This is also why, if you enquire about the suitability of a gearbox, the first question you will be asked is not how powerful the car is, but how heavy is it?
It is also why, next time you are down at the dyno-tuners, you might notice they have sandbags or their fattest blokes sitting in the boot. Because you cannot realise maximum torque if there is wheel-spin.

Changing the FD (or any of the gears) changes the magnitude of the engine torque required to cause wheel-spin, i.e. the maximum torque that the engine can realise (in that gear). As per yr above statement, the OD will see this torque multiplied by the gearbox ratio. As per my table, net result is that changing FD from 3.54 to 2.88 will increase the (wheel-spin limited) gearbox output torque from 800 to 980 Nm. What this table doesn’t show is, if I filled my boot with enough sandbags and fat blokes (>900 kg), the maximum gearbox output torque with a 3.54 diff in 1st gear could be a 1200 Nm without OD and a thumping 1540 Nm with OD. With 2.88 diff these numbers would be 1480 and 1900.

OD will be (initially) wired-up for top gear only, as Pete Crespin points out, it is easiest to do. But with bypass provision.
I’m imagining these OD’s wouldn’t break without giving plenty of warning. As well as the sprag clutch and epicyclic gears, there is a spring-loaded brake ring. This brake ring is not used to transmit torque, has two functions: When the OD is “out” (not being used), it provides for engine braking by preventing the sprag clutch freewheeling during times when the wheels are driving the engine. When the OD is “in” the brake-ring is used to hold the sun-gear stationary. This brake ring is a relatively little thing, I imagine so that it can act sort of like a safety valve, if I try to over-load the OD the brake ring will slip, which would be noticeable, telling me to either back off or change up. I’m thinking that, if I can't induce brake ring slip in top gear, it should be ok to try the next gear down, and so on?

[Series1 Stu]

Your maths is still flawed.

An overdrive unit reduces the torque transmitted by the overdrive ratio.

Regards