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PostPosted: Sat Apr 13, 2019 3:41 am 

Joined: Sat Mar 31, 2012 7:34 am
Posts: 60
Location: Mount Maunganui, New Zealand
Much has been written about the Leyland Leopard. Nonetheless some aspects of its earlier history, through to 1970 or thereabouts, appear to have been less well covered. Included in this group are the change from the O.600 to the O.600S engine, why the original L model was never offered (or built) with the Pneumocyclic gearbox, the relationship between the L, the Royal Tiger Cub RTC1 and the PSU3, particularly in respect of chassis details, the early air suspension option, and the 17’6” wheelbase option. Whilst the intent of this posting is to cover these and some other points of detail, to better put then into context, I’ll include them in a chronological overview as they occur. This does though require some repetition of “known” history. As appropriate I’ll use “Commercial Motor” (CM) magazine articles and items as chronological markers. The CM archive is available at:

The original Leopard L appeared in the third quarter of 1959, when it was described (CM 1959 August 21) as being basically a Tiger Cub PSUC1 fitted with the larger O.600 engine, set to 125 hp at 1800 rev/min for both bus and coach applications. This was the original O.600 engine, as the spheroidal version was still in the future. It had the same basic dimensions as the Tiger Cub, including its 16’2” wheelbase for nominally 30 ft bodies. It also had the same 8-stud wheels and the same diaphragm operated air brakes. The Tiger Cub had introduced diaphragm operated air brakes to Leyland practice, but it took some time before it spread across the range. I think the last major “conversion” involved the Super Beaver and Super Hippo truck chassis, wherefor the change was made with the Power-Plus models introduced in the third quarter 1962, as delineated in CM 1962 August 24. (The BUT ETB1 trolleybus chassis, of Leyland design, was never changed over, and the final production in 1964, for Wellington, NZ, had piston-type brake actuators.)

The Leopard L clutch was mechanically operated, and the gearbox was the “large” 4-speed synchromesh unit, similar to that previously used on the Royal Tiger OPSU series, but at the time nominally with synchromesh on 3rd and 4th gears only, rather than on 2nd, 3rd and 4th, and with constant-mesh rather than sliding mesh 1st gear. However, I understand that this was very nominal, and that there were variations from this. The same gearbox had become an option on the Tiger Cub from around the third quarter of 1958, when it replaced the previous 4-speed constant-mesh unit that had been derived from that fitted to the OPS1 series. Also available on the Tiger Cub at the time were the Albion GB236 5-speed constant mesh gearbox, which had become the standard fitting, and the Pneumocyclic gearbox. The GB236 remained standard on the Tiger Cub right through to the end of production in 1969, even though it had been superseded by the GB241 in all other applications. I suspect that the GB236, with its side-entry shifter, was better suited for “robust” remote control than the GB241, with its shifter entry on top of the bell-housing That said, the GB241 was used on the final iteration of the Albion Nimbus, the NS3A, albeit with what looked like a simple remote control that might well have suffered from quite a bit of lost motion in the lateral direction. The six-speed overdrive version of the GB236, namely the GB240, was never offered on the Tiger Cub, possibly because its extra length would have required undue shortening of the tailshaft. But as best I can determine, the GB240 was used only on Albion bus and truck chassis (including the Aberdonian MR11), but never on any Leyland bus and truck chassis. Also late in 1958, the then-new O.375 engine, which was the standard fitment on the Leyland Super Comet 14SC and Albion Clydesdale CD21 truck chassis, both announced in 1958 September, had become optional to the O.350 Mk III on the Tiger Cub.

The standard rear axle on the L was essentially the same spiral bevel unit that had been used in the Royal Tiger OPSU, in a similar three-piece casing. Optional was a two-speed axle with an Eaton spiral-bevel head inside a Leyland single-piece casing. This had electric shift, as on the Tiger Cub. The latter had introduced this feature to Leyland practice, but it was 1958-59 before electric operation replaced vacuum operation on the Comet range, both passenger and goods.

GVW of the Leopard L was 11 tons, so somewhat higher than that for the Tiger Cub, which was 9 tons for the bus and 9.5 tons for the coach.

The CM road-test of the L was published in the 1959 December 25 issue. From the chassis diagram therein one may see that it retained the Tiger Cub features of a slight chassis arch over the rear axle, and a slight right-hand offset of the crankshaft centreline. Here is the chassis diagram from Leyland sales brochure #776a of 1960 August:

Leopard L1 Chassis from Leyland #776a 196008.jpg
Leopard L1 Chassis from Leyland #776a 196008.jpg [ 536.88 KiB | Viewed 2229 times ]

The rear axle chassis arch had also been used on the Royal Tiger OPSU1 family, so appears to have been carried over to the Tiger Cub and thence to the Leopard L. The Royal Tiger Worldmaster RT-series had arches both front and rear, this I think being to allow for the lower chassis height of the CRT version, which used the same chassis frame as the ERT. The RT had the engine crankshaft on the centreline, though. One has the impression that the L was developed on a minimum-change, minimum-effort basis. From the Tiger Cub onwards, the international standard chassis width of 34 inches was used. The Royal Tiger had had a 41 inch wide chassis, as shown in this diagram from sales brochure #668 of 1950 September.

from Royal Tiger #668 195009.jpg
from Royal Tiger #668 195009.jpg [ 445.66 KiB | Viewed 2229 times ]

As mentioned, the Pneumocyclic gearbox was not offered as an option on the L. I have never seen a reason given for this. Perhaps demand was seen as being limited, or perhaps there were engineering constraints. Reading back from the later PSU4 model, one might infer that fitting the Pneumocyclic gearbox to the L would have required use of the close-coupled version. Hitherto Leyland had evidently preferred the island-mounted version for its underfloor-engined bus chassis, and had used it on all such applications. In the case of the Tiger Cub, it had adopted a more forward engine position (as compared with the standard version) in order to make room for the Pneumocyclic gearbox. In fact that idea was not new, having been used previously on the Sydney and Auckland special versions of the Royal Tiger OPSU1 in order to accommodate an island-mounted AEC preselector gearbox. The Worldmaster used an island-mounted gearbox on all variants, including the short, RT3 version, whose wheelbase, at 16’3”, was only an inch longer than that of the L. But the Worldmaster had an underslung worm final drive, whose entry flange was less forward of the rear axle line than that of the L with its spiral bevel axle. Perhaps that difference allowed the “big” engine and island-mounted gearbox to be “squeezed” into a 16’3” wheelbase. The RT3 chassis diagram suggests that the gearbox and rear axle were probably as close as they could be, possibly right on the edge of acceptable driveline angles. The same engine and gearbox positioning relative to the rear axle was used throughout the RT range.

Leyland Worldmaster RT3.1 19570903.gif
Leyland Worldmaster RT3.1 19570903.gif [ 450.34 KiB | Viewed 2229 times ]
Leyland Worldmaster ERT2.1 19570903.gif
Leyland Worldmaster ERT2.1 19570903.gif [ 427.97 KiB | Viewed 2229 times ]

By the time the PSU4 arrived, Leyland had necessarily used the close-coupled version of the Pneumocyclic gearbox on the Panther and Panther Cub, so perhaps by then it was less averse to the idea.

By way of a comment on designations, the earliest Leyland Leopard sales brochure that I have, #776a dated 1960 August, refers to the three variants then available as L.1, L.2 and LHL. Consistent with the Leyland system of the time, this suggests that the basic chassis designation was simply “L”, with the “LH” prefix for the left-hand-drive version, and that the “.1”’ and “.2” were simply suffix numbers to denote lesser variations, and not part of the basic designation. Over time though, it seems as if they migrated to become part of the basic designations, as “L1” and “L2”, whether because Leyland made the change or by common usage.

The next major event in the Leopard family history was the release of the Royal Tiger Cub RTC1, essentially an export model, although the available histories seem to dwell upon the miniscule domestic sales. It was announced in CM 1960 April 29, wherein it was described as appearing to be a heavy-duty version of the Leopard L.

Compared with the L, the wheelbase was extended to 18’0”, although the front overhang was about the same, and it was intended to accommodate 33 ft bodies. GVW was 13 tons, and it had 10-stud wheels, although retaining the same brake dimensions as the L. It had the O.600 engine set for 125 hp at 2000 rev/min (not 1800 rev/min), at least according to a 1961 May specification. Clutch and synchromesh gearbox were the same as for the L, as were the single-speed and two-speed rear axle options. The chassis retained the rear axle arch and the slightly offset-from-centre engine crankcase line as had been used on the L and the PSUC1 before it.

The Pneumocyclic gearbox was an option from the start. And I should imagine that it was island-mounted. I have a chassis diagram (dated 1960 April) for the LRTC1/1 variant, with synchromesh transmission. I’d say that there was enough room between the engine and the rear axle to fit an island-mounted Pneumocyclic gearbox, but if not, there was room to move the engine forward somewhat.

Royal Tiger Cub LRTC1-1 196004.gif
Royal Tiger Cub LRTC1-1 196004.gif [ 203.4 KiB | Viewed 2229 times ]

The RTC1 filled what was a gap in the Leyland range. In particular AEC offered its export version of the Reliance, the HMU (and the following 2HMU) with a 17’6” wheelbase to accommodate 33 ft bodies, with a GVW of 11.5 tons. (The HMU and 2HMU were also available with a short, 16’0” (not 16’4”) wheelbase for 30 ft bodies, something that seems to have been overlooked in the available histories.) The 33 ft length remained important in some export markets, such as Australia and New Zealand. With the RTC1 Leyland was offering a chassis of comparable size, but with a bigger and better engine and a higher GVW than AEC. AEC caught up somewhat with the 17’6” wheelbase version (export only) of its AH590-engined Reliance 2U/4U in 1961, but this model was viewed as being an example of British undercooling at its worst.

Leyland’s choice of the 18’0” wheelbase for the RTC1 may have stemmed from the fact that this had been used for the Worldmaster ERT1 core model. But the latter was intended for 34 ft bodies. The 18’0” wheelbase was also used on the export, long version of the Albion Viking that was intended for 33 ft bodies. Given its relatively short front overhang, the RTC looked to be more suitable for suburban and country use than for urban use, and I am not aware that it found its way into very many major urban fleets.

Again it looked as if Leyland had done a minimum-change job to get from the L to the RTC1. But that did include some of the re-engineering that would be required for the later PSU3, including 10-stud wheels, springs for 13 tons gvw, and fitting of the Pneumocyclic gearbox.

The previously-mentioned Leyland brochure #776a of 1960 August included the comment: “All models can be supplied with a combined air-steel suspension system on the front and rear axles.” This I think referred to the early Leyland system that combined conventionally-mounted leaf springs, albeit of lower rate, with air bags, two at the front and four at the rear. There was a brief description of this system in CM 1961 February 17, in an article “Air Sprung Leopards for Ribble”. According to CM 1960 June 24, this system had seen prior use on a Tiger Cub for Western Welsh. And it was also used on the front axle only of some Ribble Atlanteans (CM 1960 August 12).

Whilst eventually superseded by Leyland’s definitive air suspension system, this original air-leaf system continued to be used for the front axle of air-sprung Panther buses, although not on the coaches, which had the new system front and rear.

The next major step for the Leopard was the introduction of the PSU3 model in 1961. It was announced in CM 1960 October 06, and described, along with the existing Leopard L, in sales brochure #820 of 1961 October.

The primary raison d’être for the PSU3 was the change in the UK regulations in which the allowed the maximum length for buses was increased from a rather restrictive 30 ft to 36 ft, more in line with worldwide practice at the time. However, the UK turning circle requirements remained restrictive, and so a shorter wheelbase than had hitherto been associated with export 36 ft underfloor-engined models was required. The long version of the Royal Tiger, the OPSU2, had had a 20’4” wheelbase whilst the long Worldmaster, the ERT2, had 20’0”. Daimler chose 20’4” for the long version of the Freeline. AEC was late in introducing a long version of its Regal IV, and chose a 19’6” wheelbase. (Guy did not offer a long wheelbase version of its Arab UF.) Nonetheless, the 18’6” wheelbase had seen prior use on transit type chassis, being offered on the Sunbeam MF2B trolleybus chassis from the start, and becoming an option on the BUT ETB1 from c.1950.

The PSU3 may be seen as having been a direct development of the RTC1 rather than of the L. It had the 10-stud wheels of the RTC1, and was designed for 13 tons gvw, the same as the RTC1, although it could also be sprung for 11.25 tons gvw to suit the less demanding domestic market requirements. So Leyland evidently had the export market in mind when it developed the PSU3. As well as a longer wheelbase, the PSU3 had a longer front overhang than the RTC1. Probably this was necessary in part accommodate a 36 ft overall length on an 18’6” wheelbase, but it also better suited the PSU3 for urban operation.

There was an engine change for the PSU3, to the spheroidal (combustion chamber) version of the O.600 engine, also referred to as the 600S. Two settings were available namely 125 hp at 1700 rev/min for bus applications, and 130 hp at 2200 rev/min for coach applications. The spheroidal versions of the O.600 (and O.680) had been introduced in 1960 September for the reworked heavy truck range (Beaver, Hippo and Octopus), with nominal settings of 140 hp at 1700 rev/min for the 600S and 200 hp at 2200 rev/min for the 680S. (See CM 1960 September 09.) In the truck applications they were referred to as “Power-Plus” engines, but in general that descriptor was not used for Leyland bus applications, the apparent exceptions being the PSR1 Lion of 1960 September and the later versions of the Comet passenger (12C and 13C).

Nonetheless Leyland brochure #820 made quite a feature of the spheroidal engine, without using the “Power-Plus” name. It was not always the case that the change from the 600 to the 600S engine (and indeed from the 680 to 680S) in the early 1960s was marked by a change in power settings. For the Worldmaster and Tiger/Titan, it was in the nature of a “silent” change, retaining the same power settings, for example 125 hp at 1800 rev/min for the 600S in the Tiger/Titan.

from Leyland Leopard #820 196110.jpg
from Leyland Leopard #820 196110.jpg [ 844.47 KiB | Viewed 2229 times ]

Some sources suggest that the 125 hp at 1700 rev/min and 130 hp at 2200 rev/min power settings were available for the Leopard L from the start. Not only do the Leyland sales data of the time say otherwise, but the spheroidal engines were not announced until around 1960 September. And as far as I know, for the original versions of the O.600 and O.680 engines, the highest rated speed was 2000 rev/min. The (nominal) maximum rated speed for the spheroidal versions was increased to 2200 rev/min.

The 4-speed synchromesh gearbox was standard on the PSU3, which had a hydraulically operated clutch. Hydraulic clutch operation appeared to have entered the Leyland lexicon with the Super Comet 14SC truck in 1958 September, and was thereafter propagated slowly throughout most of the range. The PSU3 also had the option of the Pneumocyclic gearbox, the chassis-series specific engineering for which had been done with the RTC1.

A chassis detail change was the elimination of the arch over the rear axle, so that the PSU3 frame was straight throughout in profile. As best I can determine, in the (non-truck derived) bus range, the straight frame had first been used on the Lion PSR1. That model was said to have a Worldmaster-derived frame, so Leyland must have decided to straighten it out. As well, the PSU3 had the engine crankshaft exactly on the chassis centreline, and not slightly offset to the right as on the L and RTC1. These changes may be seen from the chassis diagram in the CM road test, in the 1963 May 10 issue. That showed ample room between the engine and the rear axle to accommodate an island-mounted Pneumocyclic gearbox. Thus one might deduce that the PSU3 retained the same engine position for both gearbox types, although that needs to be verified.

Leopard PSU3 Chassis from CM 19630510.png
Leopard PSU3 Chassis from CM 19630510.png [ 292.77 KiB | Viewed 2229 times ]

Thus the PSU3 may be seen as being more than simply a lengthened version of the L. The sequence of changes was:

L to RTC1: Longer wheelbase, longer overall length, 600 engine running at 2000 not 1800 rev/min, Pneumocyclic gearbox option, 10-stud wheels, gvw from 11 to 13 tons.

RTC1 to PSU3: Still longer wheelbase, still longer overall length, longer front overhang, 600S engine with two settings, straight chassis frame, engine crankshaft centred on chassis centreline, hydraulically operated clutch.

Sales brochure #820 shows that by then the L also had the 600S engine with the choice of the same two settings as for the PSU3. Possibly that had come in before the PSU3 was announced, but my guess is that they were concurrent events. There was no indication that any other changes were made to the L at this time, and for example its gvw remained at 11 tons. The original air suspension system was listed as an option for both the PSU3 and the L.

It is reasonable to assume that the RTC1 was changed from the 600 to the 600S engine somewhere the 1961-62 period, but whether that event was concurrent with the change for the L is unknown. Also unknown is whether it then had the PSU3 settings or it was done as a silent change, retaining original 125 hp at 2000 rev/min setting.

The new Leyland air suspension system was announced in CM 1963 January 18 as an option for the Leopard PSU3. (It was also known to have been offered on the Worldmaster). Leyland issued sales brochure #875 of 1963 November describing the new air suspension as applied to the PSU3. Presumably it was not offered on the L. Whether the RTC1 ever had an air suspension option is unknown.

from Leyland Air Suspension #875 196311.jpg
from Leyland Air Suspension #875 196311.jpg [ 251.83 KiB | Viewed 2229 times ]

In 1964, CIE Ireland placed a large order for air-suspended PSU3 Leopards, including 173 of a specially shortened version intended to carry 30 ft bodies. These were recorded in CM 1964 June 05, wherein it was noted that their 5-speed Pneumocyclic gearboxes were to be direct-coupled to their O.600 engines. This was the genesis of what became the PSU4 model. One may envisage that CIE wanted a short Leopard with the Pneumocyclic gearbox, and as much commonality as possible with its standard-length PSU3 fleet.

At about the same time, Brisbane had placed its first order for the Leopard PSU3. CM 1964 May 22 recorded the order for 40 “36 ft” Leopards. But these were not 36 ft buses. They were a modified PSU3 with 17’6” wheelbase to take 33 ft bodies. Possibly the CM staff had equated PSU3 with 36 ft. This was arguably a more important PSU3 variant than the short version for CIE. Brisbane already had a fleet of the AEC Reliance 4U model with AH590 engines, 17’6” wheelbase and 33 ft bodies, and evidently wanted to keep to these dimensions for its next order. For Leyland, the 17’6” wheelbase implied a non-standard chassis. Whilst the RTC1 was designed for 33 ft bodies, it had an 18’0” wheelbase and a relatively short front overhang. The PSU3 was probably a better starting point from which to arrive at the desired destination.

I suspect that this Brisbane modified PSU3 had an island-mounted Pneumocyclic gearbox. Wellington, New Zealand received 16 modified PSU3s in 1966-67. These had the 17’6” wheelbase, and carried bodies of around 33’6”, and were said to be based upon a chassis recently built for an Australian operator, for which Brisbane appears to have been the only candidate. The Wellington fleet definitely had island-mounted gearboxes. My guess is that the 17’6” wheelbase version of the PSU3 had one foot taken out of the wheelbase between the gearbox and the rear axle, as from memory of looking under a Wellington example, the gearbox was mounted reasonably close to the rear axle.

The PSU4 as such, effectively a redesignation of the CIE short PSU3, and replacement for the L, was announced in CM 1965 August 27. Therein it was stated that the fitting of the Pneumocyclic gearbox to the short Leopard, the most notable of the changes, was made possible by its close-coupling to the engine.

Leyland Sales Brochure #820d of 1966 December described both the PSU3 and PSU4 models. #820b of 1964 March had covered the PSU3 and L. I don’t have #820c, but I imagine that it would have been issued in 1965 to correspond with the release of the PSU4. The PSU4 had the same 13 and 11¼ tons gvw options as the PSU3.

So the main differences between the PSU4 as compared with the L were:

Straight-framed chassis rather than arch over rear axle.
Engine crankshaft on chassis centre-line rather than offset to the right.
Hydraulically operated rather than mechanically operated clutch.
Pneumocyclic gearbox option.
Air suspension option (assuming that the original air-leaf option on the L had lapsed circa 1963).
10-stud rather than 8-stud wheels.
13 (optionally 11.25) tons gvw rather than 11 tons.

Not mentioned in #820 was the O.680 engine option, which I understand was in place by the end of 1966. Brisbane had a second batch of 40 17’6” wheelbase Leopards, and these were fitted with O.680 engines. The earliest sales brochure I have that mentions the O.680 engine is #1054 of 1968 September. Here it is shown as an option without mention of the power setting, but the main emphasis was on the O.600. Leyland specification #935a of 1968 September, covering the PSU4, did give the O.680 power setting as 155 hp at 2000 rev/min.

After Brisbane and Wellington, Sydney was the next city to order the 17’6” wheelbase version of the PSU3, in 1966 third quarter. At least since WWII, Sydney had standardized on the 17’6” wheelbase for all of its buses. With the earlier underfloor-engined models, this was not a problem as the Royal Tiger OPSU1 and AEC Regal IV had the 17’6” wheelbase, for 33 ft bodies, as standard. With the Worldmaster, a 17’6” wheelbase version of the ERT1 – which normally had an 18’0” wheelbase - was needed, but this would easily have been achieved by taking six inches out of the wheelbase between the front axle and the engine. The Sydney tender that resulted in its first Worldmaster order was issued before that model was formally announced, so I expect that Leyland had included a 17’6” wheelbase version in its design workup, assuming that it was alert to its major customer needs. For its Leopard fleet, Sydney went to a 35 ft body on the 17’6” wheelbase, and for this its version of the PSU3 had a longer front overhang than standard. Another change was the use of a close-coupled Pneumocyclic gearbox, as on the PSU4. Whether the gearbox stayed in the PSU4 position relative to the rear axle and the engine was moved backwards, or the engine stayed in the same position and the gearbox was moved forwards, or they met in the middle I don’t know. But applying Occam’s Razor points to the first of these, in which case the initial Sydney PSU3 was effectively a PSU4 extended forwards ahead of the engine. Instead of the usual Leyland single-speed axle, the Sydney version had an Eaton single=speed driving head in a Leyland single-piece casing. Leyland had already gone this route as standard with the Panther, so why it didn’t standardize the Leopard with the same axle at about the time the Panther was introduced is a mystery.

The initial 232 buses of the Sydney Leopard fleet had the close-coupled Pneumocyclic gearbox, but the remaining 512 of the 744 total reverted to island-mounting, presumably with the same layout as on the Brisbane and Wellington fleets. Why is unknown, but one may surmise that cooling of the fluid coupling and/or gearbox was less satisfactory with the close-coupled arrangement.

Perth acquired 50 Leopard PSU3s in 1968. These had the standard wheelbase of 18’6”, but for weight distribution reasons had the engines moved backwards from the standard position and probably as a consequence of that, also had close-coupled gearboxes. Most likely I think is that the engine/gearbox assemblies were in the PSU4 position relative to the rear axle.

The Sydney Leopard fleet was not completed until 1976 or thereabouts. And a small number (8 I think) of 17’6” wheelbase Leopard PSU3s were supplied to Dunedin, New Zealand in 1975-76, amongst a larger number of the regular, 18’6: wheelbase version. By then the mid-engined urban bus was becoming a rare species for new production.

As best I can determine, all of the 17’6” wheelbase Leopards were air-suspended.

The Royal Tiger Cub RTC1 was withdrawn in 1968, and appears to have been caught by the same rationalization exercise that eliminated the Tiger Cub PSUC1. In the case of the RTC1, the 17’6” wheelbase version of the PSU3 would have been a suitable alternative for customers who wanted a 33 ft or so urban or suburban bus with a commensurate wheelbase. I think that it has been suggested that the PSU4 was the replacement for the RTC1 as well as for the L, but that does not fit either timing-wise or suitability for application. As far as I know, exports of the PSU4 were minimal.

Leyland Australia offered the PSU3B with the option of a 19’6” wheelbase as well as the standard 18’6”. This was intended to accommodate 37 ft bodies. I imagine that in some states, whilst 37’0” length was allowed, it required longer than an 18’6” wheelbase, but this is unverified. I do not have the timing for this option, but early 1970s seems reasonable.

from Leopard, Viking Aus p.02.jpg
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from Leopard, Viking Aus p.04.jpg
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PostPosted: Sun May 12, 2019 4:11 am 

Joined: Sat Mar 31, 2012 7:34 am
Posts: 60
Location: Mount Maunganui, New Zealand
In the previous post I has noted that as far as I can determine, all of the 17’6” wheelbase Leopard PSU3 variants were air suspended. I should also have added that they were all fitted with Pneumocyclic gearboxes.

A closer look at the data on hand shows that the Leopard L and the Royal Tiger Cub RTC1 had the same engine front position relative to the front axle. Thus in the case of the RTC1, the wheelbase increment of 22 inches, from 16’2” (194 inches) to 18’0” (216 inches), was behind the engine. The RTC1 chassis diagram gives the distance from the engine rear to the centreline of the rear axle as 102.6 inches, which we may round to 103 inches. That I think would have been sufficient to accommodate an island-mounted Pneumocyclic gearbox without any need to move the engine forward somewhat. As a reference point, the same dimension for the Worldmaster was 96 inches. Adding the extra length required for the RTC1 behind the engine meant that the same engine position could be used for both the manual and Pneumocyclic variants, which could well have been the lower cost option for a chassis that would be produced in moderate numbers. It was though a change of direction for Leyland, as hitherto it had used more forward engine positions for the Royal Tiger and Tiger Cub variants fitted with epicyclic gearboxes, as compared with their respective manual gearbox counterparts.

From the RTC1 we can work back to the L, which would then have had (103 - 22) = 81 inches between the engine rear and the rear axle, insufficient for an island mounted gearbox.

On the Leopard PSU3, as best as I can estimate, the engine was positioned about a foot further ahead of the rear axle than on the RTC1, say at somewhere around 115 inches. This put the engine about six inches closer to the front axle than on the L and RTC1. Possibly this rather forward location of the engine was done to achieve the desired weight distribution for British domestic applications, bearing in mind that the PSU3 would normally carry a body with a longer rear overhang than the L, so a bigger proportion of the payload would fall upon the rear axle.

Thus one may see why Perth had a potential problem with the weight distribution of the standard PSU3, whereas the Worldmaster ERT1, with its engine 19 inches closer to the rear axle, had been satisfactory in its standard form.

Given the deduced PSU3 engine positioning, one may then postulate that the 17’6” (210 inch) wheelbase variant was obtained by removing one foot from the chassis aft of the engine, which would have resulted in an engine rear to rear axle distance of around 103 inches, similar to that on the RTC1 and so enough to accommodate an island-mounted Pneumocyclic gearbox. Another subtraction of 16 inches to get down to the PSU4 wheelbase of 16’2” would have reduced this dimension to 87 inches, evidently not enough to accommodate the island-mounted gearbox, and so necessitating use of the close-coupled version. The design work for the 17’6” wheelbase variant would have been done at about the same time as that for the PSU4 (initially as the PSU3 special), so it is not inconceivable that Leyland opted to keep the engine position relative to the front axle constant across the PSU3/4 range and vary the engine-to-rear axle distance. That was quite the opposite of what was done with the Worldmaster, but in that case all variants were designed at the same time. The original PSU3 dimensions were a given, and they were such that the only place where non-negligible length could be subtracted was aft of the engine.

That development pathway for the shorter variants of the PSU3 seems plausible, but needs to be confirmed by actual data.

Looking back to the Royal Tiger, my best estimate is that the engine rear was 80 inches ahead of the rear axle centre-line for all standard variants. The OPSU2 Pneumocyclic had the engine at an estimated 102 inches ahead of the rear axle, or 22 inches ahead of its normal position. The front of the gearbox was in the same position, relative to the rear axle, as the front of the manual gearbox on the standard versions. The same relativity, that is engine 22 inches forward, would appear to have been about right for the OPSU1 preselector, basis very limited photographic and remembered first hand visual evidence. I do not have any quantitative data, actual or estimated, for the Tiger Cub Pneumocyclic.


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