Onslow-Bartlett’s V-twin, Rear-engined Air-Cooled Trials Special
First Published Description of a Daring but Successful Experiment BF,FORE the war that colourful per sonality, L. •Onslow-Bartlett; started his trials’ driving career with a modified, golden-hued, M-type M.G. Midget known. as “The Nugget.” From this he graduated to other 111.G,s and after the war resumed competition motoring with the V8 Mercury Special, with which he swerved the curves with astonishing rapidity and brought home much silverware ; his wife also drove the Mercury effectively. Perhaps sensing the tiegat ive future for big trials ears. Bartlett then built his mid-engined Ford Ten Special, driven today by J. C. Smith, and from this it was hut A step, albeit a big and ambitious one, to putting the engine at the extreme back of his next trials car. However, the man in the vivid open-necked shirt did not stop there—he decided to get real power by
using an air-cooled V-I win engine. His car is not the first air-cooled trials car—I recall an A.11.C.-engined Austin Seven in a Bristol trial a few years ago— but it is surely the first successful one.
The resulting highly-ingenious vehicle made its debut in the Cheltenham M.C. Trial last November, and at once made a profound impression, by reason of its sound appearance, obvious urge and equally Obvious weight where weight should be. Bartlett could, indeed, lift the front of his latest special clear of the ground without straining his biceps.
In the Cheltenham Trial the BartlettJ.A.P. looked like Winning its class but the scrutineers, admiring the ingenious engine installation, also pointed out the absence of a fireproof bulkhead and disqualified the car. Bartlett’s next onslaught was the Roy Fedden Trial, and in this his reasons for departing radically from the orthodox were prayed sound, for he took home the Daphne Cup for best 1,100-e.e. performance. As a matter of fact, he was using a 996-CM. racing J.A.P. at the time.
Confident that the machine worked, Bartlett entered for the R.A.C. Championship Trial, but the bugs were still breeding—they thrive on experimental projects of this sort—and when lie was level -with Champion Ken ‘Wharton or thereabouts, the engine seized. The same thing happened in the Al .G. C.C. Boxing Day Trial, this time within 100 yards of the finish after a siteeessful day’s slime-storming. Alas, the special test was yet to come and, naturally, the time was bad, Onslow pushing over the final line. Even so, he got. a lirst-class award. In the (lee Trial engine trouble intervendHetlfway round, but in tia,; Southsea Trial on February lit It I his air-cooled special again proved its merit by winning the 1. Cup–it might have won
outright had not the front-isle radius arms sheared, resulting in :t stalden swerve into the bank on Wheat ham, a comparatively easy ” section.”
After these performances, eonvinced that Bartlett ” has something,” and believing that readers will be interested to know what is ” under the cowlings ” of this fearsome-looking and Shelsleysounding little car built essentially for trials, I draft down to Bournemouth on a wild, wet day last. month to enjoy OnSIONV’S f UN?itafiLy and to glean an account, in his ?(‘VII elltIllISiaSt iv words, of liii latest of his many brain-elsildren. The chassis of this remarkable special consists at a Font Vs propeller-shaft tube used as a cent rill backbone. This is forked at t I,;tek I ii -eommodate engine, gearbox and differential housing, arid to it are annelid’ cross-menihers uniting with small Inbular side-members. The two seat s tire set well apart, providing plenty of elbow-room as well as enhancing stability, the latter Unproved because these seats are less than it in. from the ground, actually below the level of the backbone-tube. Like the etigitw. the seats are at the back, tinonly weight forward being that provided by a 12-volt battery and a 41.-gal11(11 fuel tank in the nose. That the car
is colt 1pact. is evident from its wheelbase and track dittwilS ions, the former 5 ft. ILI in., the front track 3 ft. 9 in. and the rear wheels crabtracked about 3 inches. ‘The front axle is a Ford Ten beans ‘upside down and drilled for lightness. Naturally, lite castor angle and steering geometry have been adjusted to this new arrangement of axle. Suspension is by a six-leaf tralisverse spring spaced front the front. of the ebassis by a dist at in (Mkt to prevent the traversing arms of the rack-: ((Ill steering gear fouling the rad i US arms at full bump. These radius arms are Ford, adapted to the job and centrally mounted on the chassis, and t Steerif IV: is 13..M.W., chosen on ;termini ef the light weight of its alloy easing. It provides a steering ratio of It turns. inek-I ()lock. Motorcycle-size Hartford shoek-absorbers, mounted transversely, complete the front suspension assenshiy, Going to the l’ack of the car we find the 60-deg. Vtwin, air-cooled, o.I I, v. J.A.P. engine mounted high-up. Its ” ceiling -.is deliberate, because its oilpuns’) is II eat vit 111n 1101Vii ansI trials ‘ sections ” are unsympathetic to vulnerable things set underneath a our ! At first Bartlett used a 1930 990-i.e. racing .I.:N.P., the one front t:eorge Hartwell’s Cooper 1,000. Next he tried A modern 1,10o-c.e. racing ;Ma his latest is a prototype I,100-c.e. engine of this kind. The use of an air-cooled t win does not save weight as you might expect. la(ed!, the present engine is 22 lb. heavier Ulan a Ford Ten engine. Its alloy cylinder barrels, for instance, are 2I lb. heavier than .I.A.P.’s former steel barrels, because they are very substantial. But there is no doubt about the gain in respect of power/weight ratio, for the new J:A.P., on Pool petrol and 7.9 to 1 compression ratio, develops 27 b.h.p. at 2,500 r.p.m„ 50 at 4,000 and 38 at 4,800 r.p.m., while the torque figures are 55 ft./lb. at 2,300 r.p.m., 67 ft./lb. between 3,500 and
4,21)0 r.p.m., falling to 60 ft./11). at 5,000 r.p.m. Naturally, the engine vibrates, so that its vitals are wired-up. and onwh experimentation had to he eonducted on earburnt ion–from no-needle track carburetters ()aslow went to the remote-needle type, then to rezitote float-chambers, and has now finalised on one normal Amal per cylinder, each with twin track, type float chambers, standard bodies and track-type intakes. Dual hairpin valve springs are used because eoil springs just died of heat-exhaustion. Tito exhaust. pipes curve over to the (Ill’ side and each doubles back into its own dimpact, laterally-placed Burgess silencer, one above Cother. Onslow points ruefully to dents in the bottom one—and it must be two feet from Mother Earth ! Luckily, the aforementioned oil-pump is-a trifle higher I The exhausts are between 42 in. and 43 in. long, as preferred by this cogille. At first a 7.5 to I eompression-rat ill was used and Bartlett says the magnetos were then merely ornaments, eomplententary instruments, ;Ls it were, to the dieselling within ! A reduction has been made to a 7 to I ratio. Ilse eompression plates under the barrels; are mighty tliick, but a further reduction, to 6 to 1, is contemplated in the interests of smoollter functioning. About one-third benzole is added to WC l’001 pet I’01 does not seem greatly to alleviate pinking. While on the subject. of Hu. .1..1.11., it. may be said that it haS its own mechanical fuel pump on the off side. driven from th h b
e timing gears, tis eing sujiplemented by an electric pump mounted on the near side. Oil (Bartlett uses Notwen castor oil) is carried in a one-gallon tank on the near side of the engine and cireulates On I he ’11’Y-swill’ “Ystelll, with a ‘1″lthle-l’aPacity scavenge pump. !tartlet I lets added Itis own oil-leads to the poi s, conscious that Mr. Prestwich might be a 1)it troubled about the lack of air playing round them. Two B.T.n. magnetos suppl!,, the sparks ;aid K.L.G. plugs arc used, at first waterprooli.d pattern, then pnigressively through F.50 and P,60 to U.70.