To Diff or not to diff

That is the question! Or was, in the vintage years. These days, a differential is fitted to every car, is found in limited-slip form on high-performance models and in multiples, often of sophisticated type, on 4WD vehicles. Back in the 1920s, however, this component (which can only be properly understood by making a Meccano model of it) could be dispensed with at the designer’s discretion, on the lighter cars. To some a “solid” back axle was essential for good roadholding and for the defeat of slippery hills. It was also necessary with simple transmission systems, as that VSCC verse reminds us:

Nash and Godfrey hated cogs, Made a car with chains and dogs.

But, I wonder, would it if, They had made it with a diff?

Apart from which, dispensing with the differential saved the small-output manufacturer some £10, and about 40lb of unsprung weight. More weight and expense could be saved with a “diffless” back axle because brake compensating gear was not required, one brake drum sufficing to retard both wheels, or to comply with the legal requirement of two independent brakes, the foot brake could operate shoes in one drum, the hand lever those in a second drum.

Against that, a differential-less car wore out its back tyres more rapidly and some drivers thought it more difficult to steer. Those were the main 1920 pros and cons. I believe that the Trojan, a true utility car had a peg which could be withdrawn, traction-engine fashion, to free one wheel, Which was a simple way of gaining the best of both worlds, if you didn’t mind dirty hands.

Some credence for not bothering with a cliff came in 1922, when the celebrated Louis Coatalen brought out his little 8 hp Talbot-Darracq, which had a solid back axle. But my opinion is that he intended it mainly for the straight roads of France, or was forced into using a simple chassis to reduce purchase price, after giving his new light car an overhead-valve engine; for at £320 it was scarcely cheap. That not withstanding, arguments for and against the diff were widespread, and in 1923 a weekly motor paper conducted an odd experiment.

It took a 12/24 hp Anzani-engined Crouch without a differential, disguised it under a sheet, and asked three experienced drivers to take it out over greasy main and side roads, to decide what sort of back axle it possessed. Mr Aldridge, who had done 17,000 miles recently in his sports Alvis and 20,000 in a Morris Cowley, thought the steering a trifle heavy but failed to detect the solid axle which, however, he thought had a negligible effect, after being told the secret of the camouflaged Crouch. Miss Roper, the well-known Midlands light-car lady who owned an AC, also found heavy steering and less self-correction than on her own car, but she did not detect the reason, nor that both had Anzani engines! Like a woman, when told, she said she had known the reason all the time! A Captain Moss only realised in the last few hundred yards that the Crouch felt like the diffless 8hp Talbot.

None of that was convincing, and it was left to the publicity conscious SF Edge, of AC Cars, to put on an RAC-observed test, to try to prove the benefits a diff gave. He was promoting luxury small cars which had back-axle gearboxes to which a differential added more unsprung weight, so had a case that needed making. A 17-cwt AC was set to run in a 70-foot diameter right-hand circle, steering locked, for two runs, each of 14 miles, at Brooklands, the first sans diff, the next with one. New tyres, at 45 lb sq in, were fitted for each run, the wear rate and weight of lost rubber being carefully measured afterwards. All eight tyres showed wear, the rear pair 1.9mm n/s, 4.4mm o/s with no diff, 1.75mm and 1.5mm respectively, with one. The speed was a mere 11.3 mph, however. One driver sneered at this test, saying he did not wish to drive all day in tight circles and could not do so in a straight line, so the results were pointless.

Capt AF Frazer-Nash and HR Godfrey defended the “solid” axle on behalf of the GN, refusing suggestions that tyre bursts could be dangerous on such cars, while the maker of the Stack cyclecar said the AC test represented 7500 miles of normal road running, in which the AC would have lost most of its rubber! Someone else gave mathematical evidence that the converted distance equalled only 79.4 normal miles, after which the AC would have no tyres left at all, no doubt to the wrath of Mr Edge!

It was thought “diffless” cars were sensitive to correct tyre pressures and to obviate frequent pumping-up, the Stack had durable 26 x 3 Avon Duroliths, the same size as the tyres on the AC, although the cyclecar was 700 lb lighter; there is a link here with those excellent Avon Sidecar tyres, the equivalent of 26 x 3s (or 700 x 80) which are used now on many vintage Austin 7s.

I have owned eight or so vintage light cars, all but two with differentials. My 1922 Talbot-Darracq has a solid back axle and is so heavy to push about that a garage once thought its brakes were binding and slackened them off, which was alarming when I came to use them! Back-wheel tyre wear on this T-D is also quite heavy, new (and now very expensive) 710 x 900 being needed about every two years of ordinary vintage car usage, with no competitive outings.

As for steering-box wear, I know the Talbot’s box was replaced with one from a Morris Minor before a previous adventurous owner took the little car through the 1935 1000-mile RAC Rally (incidentally completing the road-section with no loss of marks), but whether due to wear on the original component, I do not know. All this is rather academic now, but amusing to look back on. WB