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John’s philosophy on questions: from the old Quantum Mechanics site, now not operating.
SHIFTING OVERDRIVES How to properly shift an overdrive
April 2003 – Topic of the Month – Proper shifting of a Laycock overdrive
OK everybody, calm down out there. We have received some interesting emails over the last few months asking (and telling) us about the proper way to shift in and out of overdrive on the Laycock deNormanville overdrives as used on the British cars (A, J, LH, D and compact A types). We received some spirited responses – use the clutch, don’t use the clutch, etc. I would like to express our opinion on the proper method to shift in and out of these overdrive units.
First let me admit that when I had cars with overdrive, especially Big Healeys, I did not shift the recommended way. It is way too cool and impossible to resist the temptation to pull up next to someone at 50 or 60 MPH and, as you shift into overdrive, accelerate away into the great beyond. This usually gets very interesting and surprised looks from the other drivers and passengers and really impresses the people who are with you in the car. What was that, warp drive? Like, you still have another gear? How many gears does his car have???!!!! It is also way too cool to kick down from overdrive at 50 or 60 and watch the expression on the faces of other people as the engine revs on the downshift. This is especially true on the Big Healey, as the 6 cylinder really sounds great at speed. You get the fantasy of what it must feel like downshifting at the end of the straight at LeMans.
Anyway, back to the proper way to do it. Let’s think for a minute why overdrives were put in cars in the first place. I mean after all, anyone can put in a 5th gear on a transmission, why an overdrive? First of all the British never do anything the way we expect or anyone else does it. This was especially true in the 40’s, 50’s and 60’s and is why we have such interesting cars from that period. Laycock overdrives are really like a manually initiated mini automatic transmission. (I personally think a guy named Rube Goldberg had a major part in their design). The important thing to remember is that it was added as an option on most cars in order to reduce engine RPM (and consequently wear) on the highway and increase fuel mileage. Secondarily it was also used to increase the number of speeds forward, giving a better selection of gear ratios for driving.
Given this objective, we can understand that the proper way to shift into overdrive is to reduce the engine RPM’s, not increase the road speed of the vehicle. In fact, it is somewhat damaging to the overdrive clutch to accelerate under power during the shift process. This is akin to slipping the clutch or power shifting during the normal shifting of a regular gear. It can tend to lead to premature overdrive brake ring and clutch wear and failure. You do not need to use the regular clutch pedal at all. Get to a reasonable speed, say 45 to 50 MPH. Activate the overdrive switch. As the overdrive engages, feather the throttle so that the road speed of the car remains the same and the engine RPM’s are reduced. Voila, you have shifted the overdrive with minimum stress to it and now are cruising at a lower engine RPM. Objective achieved. Now you can accelerate to any cruising speed you desire as the overdrive is fully engaged. It is not recommended shifting into overdrive at too low a speed as this also can cause stress on the clutch and lugging the engine is not a good practice either. In fact the early overdrives had a mini Lucas centrifugal type regulator on the output shaft of the overdrive that would not allow the overdrive to be engaged below a specified speed Shifting out of overdrive is the reverse of this process. Turn the overdrive switch to the off position. As the overdrive disengages back to normal drive, push down on the throttle to keep the car’s road speed the same and increase the engine RPM until the overdrive shift is complete. This again minimizes wear to the overdrive clutch and brake ring. You can now decelerate the car as you would normally. Again, you would not want to shift out of overdrive at too high a speed as you could over-rev the engine and cause damage to it as a result.
As you can see, the basic objective of proper shifting is to reduce the wear on the clutch during the shifting process. This can easily be done with some practice and will significantly increase the life of the overdrive unit. We have determined this shift procedure from observing many overdrive units and the wear on the key overdrive clutch components, as well as just plain common sense. We hope this will clear up any questions on the proper way to shift these overdrives to reduce the wear on them and keep them running longer. Most likely we will generate a new set of questions on the procedure, but that is what it is all about! As always, comments and feedback are welcome! Thanks! John
OIL FOR OVERDRIVES Topic of the month for June 2005 – Overdrive oil recommendation
I want to express our experience and opinion on the topic of the proper oil for use in the Laycock de Normanville overdrive units. We constantly get into discussions( and sometimes heated debates!) with British car owners on this subject. This month’s discussion will invariably get some interesting responses as it seems to be a rather emotional (and not necessarily logical) discussion item for British car overdrive owners.
Let’s start out with history. Historically even the recommendations from various car manufacturers are confusing. Some say to use multi grade oil (MG manuals), some hypoid oil (Triumph manuals), some non detergent oil and some even recommend automatic transmission fluid! No wonder everyone is so confused!
Many years ago (in a land far, far away (OK it wasn’t that far away or even that long ago)) we ran an experiment on overdrive oil. We rebuilt an A type overdrive unit and initially ran it with 30 weight non detergent motor oil. When spun up on our test bench at 1,000 RPM, it reached a normal pressure of 400 PPSI. When shifting the pressure dropped to 300 PPSI and quickly recovered to 400 PPSI. All was right in the world of overdrives.
We then drained the oil and replaced it with 10W30 multi grade oil. When spun on the test bench, initially it tested fine. However, after a few minutes of running, the oil pressure dropped to 300 and when shifting, to 200. Upon observation of the internals of the operating overdrive we found bubbles developing in the oil pump body and oil pump output passage. We surmised that the detergents in the oil were causing the oil pump to cavitate, and develop air bubbles as it pumped.
We then drained the oil again and replaced it with 90 weight hypoid oil. This time the oil pressure jumped to 600 PPSI! When shifted, the pressure dropped to 450 PPSI, which made the shift immediate and harsh. After a few minutes of running the oil pressure actually began to climb even higher. (Which made no sense since we thought the oil would thin out and the pressure would drop). We finally shut it off at 750 PPSI as we did not want to damage the unit. Even though the overdrive unit was now in the non overdrive position (solenoid disengaged), the overdrive was now stuck in overdrive and would not come out. The higher pressure had driven the sliding clutch member so hard into the brake ring that the clutch return springs could not return it to the non overdrive position. A tap on the brake ring with a hammer (the universal overdrive release tool), shifted it back into the direct drive position. After running a number of these test with the same result we found what was happening. The oil holes in the accumulator sleeve are very small. The 90 weight oil was so heavy it could not escape from the accumulator chamber as fast as the oil pump could pump new oil into it. So even though the accumulator piston had passed the oil hole relief position, the pressure continued to build up because the oil could not leave the system as fast as it was being pumped in. The accumulator piston actually bottomed out in the sleeve (similar to coil bind on valve springs). When removed we found the accumulator spring had been compressed and was no longer useable.
After replacing the spring, we then tried automatic transmission fluid. We saw the same results as we did when we used the 30 weight non detergent oil.
We then tried synthetic oil and the unit also worked OK although it began to leak from all sorts of places it had not leaked from before.
Based on these tests we have since and continue to recommend the 30 weight non detergent motor oil as the best oil to use in the overdrives.
Some other experiences with customer overdrives over the years have reinforced this choice. For example, we found a Jaguar compact overdrive with a broken accumulator piston and bent spring when it had been used with 90 weight oil by the owner. When the piston and spring were replaced and the unit filled with 30 weight non detergent oil, it functioned normally. A customer LH overdrive unit that was filled with 90 weight oil "pulsed" between direct drive and overdrive without even being switched on electrically. When the oil was flushed and replaced with 30 weight non detergent oil the unit worked normally.
Other noted problems with overdrives filled with 90 weight are excessive wear on the oil pump plunger wheel and the eccentric cam, probably due to the higher pressures developed. Also the clutch lining seems to be more deteriorated in units with 90 weight oil than those with 30 weight oil.
The use of non detergent 30 weight oil does not seem to affect the transmission parts or function. Bearings, synchros, gears and hubs do not seem to function any less effectively with the non detergent 30 weight oil as with 90 weight gear oil.
So there it is gang, our scientifically based rationale for using and recommending 30 weight non detergent oil in Laycock overdrives. I welcome further discussion and feedback on this topic by anyone interested. As always, thanks for reading this article!
