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Background — Canadian Origins of the MOSV? —
CL-91 Dynatrac |
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As would be expected with a complicated component like the new articulating transfer joint, parts were made to
fine tolerances. This was nothing new to an aircraft maker like Canadair. The designers also chose to abandon the RAT-
style band tracks in favour of individual track links forged from aluminum. The cab bodies also employed aluminum -
in double layers sandwiching a balsa wood core. Weight control was the goal in order that the CL-91 be readily
air- portable in US Army CH-47 Chinooks or CV-2 Caribous
as well as maintaining a high 'floatation' over soft terrain.
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A less obvious change between the RAT and CL-91 was that, in the latter, the engine was moved forward in the front
cab freeing space for troops. Compared with the RAT, the Dynatrac's drive sprocket positions were
also reversed. This placed the sprocket close to the articulated joint allowing both quick disconnect and an
optional third powered cab – in place of an unpowered trailer.
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The CL-91's drivetrain arrangement was meant to maximize usage flexibility but also made the vehicles more
complex (with the added disadvantage of a shallower floor in the rear components ). Nonetheless, the US Army was
sufficiently impressed by trials at Aberdeen Proving Ground in Aug. 1962 to place orders for an initial
batch as the XM-571 Carrier, Articulated. [4] Further tests with the first 7 XM-571s resulted in minor changes
mainly to improve cooling – the US Army was looking for vehicles suited to use in Vietnam,
necessitating a larger cooling fan for the XM-571. The Dynatrac body construction made the XM-571 naturally
buoyant, tracks propelling the vehicle in water. But, it turned out, XM-571 were little used in Vietnam.
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Canadair Dynatrac
Specifications
| Crew: |
up to 10 (tow 12 skiers) |
Size:
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Length: 6.09 m, Width:
1.58 m, Height: 1.52 m |
Powerplant:
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65hp 2.3L GM Corvair
air-cooled
'boxer' 6-cyl
Dana 4-speed trans.[5] |
Speed:
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Over snow: 50 km/h, in
water: 3 km/h
(loaded) |
| Weight: |
GVW: 3300 kg [6] |
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Their use of aircraft-quality materials came back to haunt Canadair. The US Army did buy 57 trial XM-571s
[7] but further CL-91 sales were restricted to two each for the Canadian and British armies. No one,
including Canadair brass regarded that as a success. Vietnam- based XM-571s were used as utility carriers
with some success but never in their intended role as ammuntion carriers with the frontline artillery units. The
CL-91 was simply too expensive. The Australian Army wanted 384 CL-91s [8] but this 'small
order' was rejected by the Canadian government. Canadair pressed on with a Dynatrac II but, by then, Canadair
brass had lost patience [9] with its vehicle division which closed down in 1976.
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[1] Other anticipated roles were Electronic Countermeasures and reconnaissance (rear-mounted M40 106mm recoilless
rifles being trialled).
[2] This followed the RAT approach but, for the CL-91, GM specially adapted an industrial model of Chevrolet's
Corvair flat-6 engine for Canadair. As expected in an industrial powerplant, these engines have
heavy-duty features – including much enlarge oil pans, fans, etc.
[3] For the Canadian patent Articulated Joint Assembly (CA 713891, issued 1965-07-20), Thomas is
listed as inventor, Canadair as owner.
[4] The XM-571 is also referred to as an Articulated, Utility Carrier or even as the XM-571 Carrier, Utility,
F-T [Fully Tracked], Articulated.
[5] According to Corvair specialist Dave Newell, the first 10 CL-91s had 4-spd Corvair van
transmissions (GM also provided the two-spd transfer case). Other changes to the Dynatrac's Corvair
engine included type-specific cooling shrouds, exhaust manifolds, oil filter, deep- sump oil pan, and 24-volt
military generator. Engines for the CL-91 Dynatracs were all built at the GM Powertrain Plant in Tonawanda,
NY.
[6] Curb weight for the CL-91 Dynatrac was 2215 kg (4870 lb). Payload was 900 kg (2000 lb) – or
1365 kg (3000 lb) in overloaded condition.
[7] At Aberdeen, the CL-91 tested successfully against 22 competitors. There followed mobility tests at Fort Bragg
NC, then cold-weather trials in Alaska. Overseas trials were conducted first in Thailand in 1966, then Vietnam in
1969 (with elements of the 1st and 25th Inf Divs).
[8] Australia wanted 500 CL-91s but the Army could only afford 384 vehicles at first. Canadian trade representatives
decided that an order for 384 was insufficient to warrant full-scale production. This was rich considering
that DND had only ordered two Dynatracs! However, at that time, such decisions were made by the
Ministry of Defence Production – other federal government involvement being the Defence Research Board,
and the US/Canada Defence Production Sharing Agreement (1956) and Defence Development Sharing Agreement
(1963).
[9] The Dynatrac II was a product of Calgary-based Canadair Flextrac Ltd (formed with personnel from
Flex-Track Nodwell ). Three proto- types of this larger Dynatrac IIs were built but they showed no improvement
over existing FN designs. Canadair Flextrac was sold in 1976.
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The Canadair CL-91 / XM-571 Dynatrac Articulated Amphibious Carrier
as a more capable all-terrain carrier for the US Army. The CL-70s were to fulfil multiple roles with all
specialist gear fitted in the rear section. [1]
In scaling up their concept, Canadair also switched from VW to a larger air-cooled Corvair engine.[2] At
the same time, the original articulated joint (devised by Andy Nowakowski) was
revised by Ian A. Thomas.[3] The RAT's transfer joint had proven fragile under certain
conditions while its 1.1L VW flat-4 engine tended to over- heat under load. The CL-91 drivetrain
was to overcome such woes.