Derby Driveline Choices

Observations on Driveline Choice for your Derby Bentley

The Chairman of the Foundation’s Trustees, Ken Lea, discusses the history and technical background of Derby cars, considering the driveline options through the years, and citing various proven methods and personal experiences


Short historical review - Derby Bentleys

The Derby Bentley car was announced in October 1933 being the first model of the new Company, Bentley Motors (1931) Ltd.

The model was built as an ex-works chassis with bodywork supplied by a myriad of traditional coachbuilders operating at the time for the luxury car market. So it is acknowledged that the chassis had to cope with a very wide range of weight and wind resistance, a problem common to all chassis builders. This leads to a number of design parameter choices - and the result is always a compromise, with success being determined by its customer’s requirements.

This car first launched in 3.5 litre form with a direct drive top gear four speed box. It had instant appeal for its sure-footedness, its servo driven brakes and not least its ability to maintain high average speeds on the open road.

        Derby Bentley - a perfect example of a light Derby body well suited to and fitted with overdrive

              A perfect example of a light Derby body well suited to and fitted with overdrive.


Axle Ratios

However, the different markets, though the majority market was in the UK, also had a significant effect on the commentary. The standard axle ratio at 4.1:1 was chosen to meet the largest market conditions of having to cope with traditional twisting roads and a need for good performance in town work. For those markets where more open road conditions were prevalent or very light bodies were envisaged, the chassis could be fitted with a slightly higher ratio of 3.9:1.


A reputation established - and further improvements

Both the public and the Motoring Press hailed the car as a success. The record shows that although a number of running improvements were made to chassis torsional stiffness and to the introduction of variable damping on the CW series chassis, the running gear and engine remained consistent - until the inevitable grandiose body styles added weight. This scenario again an irritant to the great man (WO) himself.

This led to a number of power enhancement projects culminating in the introduction of the 4.25 litre engine on GA series cars in 1936. Significantly the project improved overall driveability but the top speed was unchanged and the gearing remained unchanged.


Open Roads and Driveline Considerations

The advent of the autobahn and autostrada roads showed an increasing incidence of big end failure which was eventually solved for the 4.25 engine itself and is well documented elsewhere. The traffic and road conditions in the UK were relatively constant but in parallel to the work on the PIII, the new concept of the Rationalised Range. The first example was the new Bentley V, which embodied the requirement of longer gearing and higher cruising speeds.

This led to the concept of an overdrive gearbox with a 16.5% over-gearing in top gear and a direct third gear. The overall driveline for the MkV was of a 4.25 litre engine, much in common to its predecessor, coupled to the new “universal” gearbox and a slightly lower axle ratio of 4.3:1 being adopted giving an equivalent gearing of 3.595:1.


      Black Derby Bentley

             View of a 4.¼ litre Bentley in current form with original bodywork

Developing the M Series

However, market demand for overall improvement married to the design and development demands made by By (Robert Harvey-Bailey) determined that an updated 4.25 traditional chassis was urgently required as a holding exercise and this driveline design was read back onto the then current model together with other enhancements. The engine performance was also adjusted to better utilise the overall experience but the gearbox was a derivation of the standard box and did not feature synchromesh on second as designed into the new universal box. The over-gearing was reduced slightly gives an equivalent ratio of 3.655:1.

The M series cars, whose survival rate is quite remarkable, are testament to By’s insight into what was required. Sadly the war precluded the advent of the Bentley V save in very small numbers and the concept was changed back to a direct drive box for the Mk VI although the prototype R-type Continental, or Corniche II as it was first called, continued the concept in development (but that’s another story!)

However, it is against the M series cars that one should judge the alternatives available today because the driveline ratios in the earlier chassis series spoil, through under-gearing, the earlier car’s ability whilst still meeting so many other facets in modern driving conditions.

In summary for the M series, the engine’s high torque at low engine speed gives good performance in second on the worst of hairpin steep gradients, yet a comfortable 75mph plus can be safely achieved with cars in good condition.

The 4.25 cars series G to L with equivalent overall gearing still outperform the M series because the later power /torque adjustment was quite deliberate to keep customers further away from the dreaded 3rd order torsional vibration!


Decisions for today’s Driving Conditions

As with the original design compromise, there is no prescriptive recommendation and the design alternatives available need personal consideration of:

  • the style of driving
  • the car’s likely use
  • the road/traffic conditions
  • body weight and design

All just as with the original! The objective is to enhance cruising speed/ acceleration whilst not spoiling the original delightful characteristics found overall.

Both 3.5 and 4.25 litre cars have a strong engine with good torque output at low speed and over the prescribed speed range but for the special cars similar to B-35-AE or B-27-LE the engine will deliver significantly more torque and power but the engine speed range cannot increase for the reason displayed above. The advent of multi-grade oils and decent filtration together with good inherent cooling means that this can be delivered reliably and durably, of course subject to proper care, maintenance and a measure of driver awareness.

So for standard cars in good condition there is a large measure of comfort that higher cruising speeds are attainable whatever the requirements an individual desires for his machine, but as engine speed is strictly limited the only method open is to increase the driveline gearing or to give more wheel rotation for every engine rotation.

The alternative design offerings are therefore to:

  • increase the axle ratio (i.e. a numerically lower figure)
  • add a two-speed device between gearbox and axle
  • or - a combination of both!

In theory larger rolling radius tyres will also contribute but body clearances and wing sweeps matching the designed tyre size often preclude this.

However, I am not recommending what I might term ‘unauthorized’ concepts as they are generally unproven and can lead to considerable heartache especially if parts of the original are lost or irretrievably modified. Be mindful of the very limited availability of some parts, a situation that will only become more difficult. That said if you have a G0 GET mentality and don’t mind the risks, then the world is your oyster and don’t mind me. Hotrods to the fore!!!


          Derby Bentley back view

                   View of a 3.5 litre Bentley again fitted with high axle ratio


The Options

1.0 Later MkV1 axle
The later MkVI axle (which was the new axle foreseen for the later Bentley Vs and was actually to be introduced on Corniche) has the same track and spring centres as the earlier axle and can be fitted to the car allowing ratios of 3.727:1 (11x41) or 3.416 (12/41) to be used. The propshaft requires changing for a Hardy Spicer unit but the major work is marrying the later brakes to the Derby system. This can be neatly and sensibly achieved and indeed is successful but complex and expensive. However, use of the 12x41 gear set is too great compromise for road use in that whilst top speed is enhanced second gear is too high and too much of a compromise. 4.25 litre cars so modified generally have very lightweight special B-35-AE concept bodies and are used in competition but not for hill climbing! However, the 11x41 giving 3.72 gives a good on road performance but as shown below it can be achieved by other means. The advantages are the elimination of the driving dog and the improved pinion bearing assembly.

2.0 Higher Ratio Gear Set
Some years ago Will Fiennes took the bold step of offering a higher ratio gear set that fits the standard banjo housing and all standard axle parts. This ratio of 3.636:1 gives slightly higher gearing than the MKVI 11x41 axle and is the virtual equivalent of making the overall gearing in 4th of the standard driveline common with the M series. Second gear overall ratios compare as 6.407:1 for the M series to 6.290:1 for standard cars fitted with the higher ratio. See later commentary. The advantage of this scheme is that it retains the maximum originality, does not interfere with the propshaft/chassis installation, benefits from a full overhaul opportunity of the axle. Its disadvantage is it retains the known sensitivities of the earlier axle of driving dogs and the pinion bearing assembly. However oversize dogs are available and the modern bearing does seem to live adequately.

3.0 J type Laycock overdrive unit
The last option is to fit a J type Laycock overdrive unit having a ratio uplift of some 22%. There are two installations on the market, the earlier design being the one available from the Derby Bentley Spares Scheme of the BDC. This mounts the unit midships and is a well-proven design that does not suffer from driveline vibration. The design was subjected to extensive testing before release some years ago. It has to be said its original scope was for special cars but it has been fitted very successfully to all types subsequently. The known drawback is that some coachbuilder’s designs have insufficient space to allow the unit to fit into the “tunnel” and can result in the necessity to modify the floor pan.

The second design mounts by means of a swing link assembly fitted to the chassis banjo cross member just ahead of the axle pinion flange. This leaves a very limited space between the two flanges of unit and axle pinion. The early production utilised a single Hooks joint to couple the driveline system. This is not a satisfactory design and will fail the pinion bearing and the inner pinion roller bearing due to its torque transmission characteristic. The later kits would now appear to fit a constant velocity joint, which like front wheel drive cars gives adequate torque characteristics.

In theory, the unit can be operated in both modes to provide a wider spread of gearing but it is usually limited to top gear only. Intermediate overdrive on 3rd has also been achieved.

                  Bentley overdrive installation

    View of the swinging link overdrive installation as originally available and showing the single Universal Joint (Hook's Joint)

The advantage of these designs is to retain adequate performance as standard particularly in second but as demonstrated in section 1.0 the overall top gear is much higher and risks a loss in overall cruising ability with frequent changing required.

Its compromise is that it further loads the existing gearing and does not reduce the sensitivity of the Derby axle. It is often fitted without overhauling the original axle, which is basically unsound practice. Thus in terms of cost it is potentially much higher than option 2.


Personal Experiences

As many will know, I have been involved with the cars for many years since I bought the 4.25 litre B-119-JY in 1972. This was my first experience but since then I have restored a number of cars and carried out major work on many others. I have experienced both systems and compared them to a number of M series cars with different bodies and weights.

B-119-JY carries an original steel bodied Park Ward 4 door close coupled sports saloon on which I carried out major restoration between 1972 and 1975. I must qualify for consideration for Neill Fraser’s award as I did all the restoration from scratch even building a English wheeling machine to shape the new panel sections for doors and boot lid etc. But again that’s another story. When the car was recommissioned in 1975, I still had the 4.1:1 driveline. I believe I was amongst the first to fit the higher axle ratio from Fiennes. The car has also been fitted with 17” wheels for many years which Alec Harvey-Bailey states gives a further 3% in gearing as the 650x17 has a larger circumference than the standard 550x18. Thus the car is effectively a direct drive version of the M series.

I have appreciated the transformation for many years and it is for me the standard by which I compare the options. I have never experienced an inability to climb any tortuous road and hence the slightly higher second gear is not a problem for this or for acceleration. In fact second gear acceleration still startles many! The third gear so nearly equivalent to the M series direct is an absolute delight in my view.

Firstly, is the M series the best available comparator? I have driven a couple of M series cars with traditional coachwork but fitted with the 4.1: 1 gearset and to my mind it is equally acceptable overall but unless there is a need to replace the original, I would not recommend change.

Secondly, for those Derby cars that have become “specials” with very lightweight bodies an overdrive kit is a good solution depending on use but I would not be too prescriptive.

Turning now to 4.25 litre cars, both systems are pleasant with the extra power of the bigger engine able to cope reasonably with the overdrive gearing. It is a matter of personal choice which for me, every time (for cars with original or traditional replica bodies) it is to keep the overall design concept and fit the high ratio gearset.

The more sensitive choice surrounds the 3.5 litre cars. Theoretically, the overdrive system would appear a better match but the higher gearing of the system is pitted against the lower engine power because the high ratio overdrive can compromise overall drivability as, possibly, the car is over-geared for the power available.

           Derby Bentley dressed for a wedding

                        Derby Bentley doing wedding duty

CW series 3.5 litre (H J Mulliner drophead)
My other Derby is a CW series 3.5 litre fitted with an H J Mulliner drophead body. This too was a ground up restoration and in line with my view of keeping traditional features I again incorporated the Fiennes gearset.

I have now covered some 30,000 miles in this car and have developed a deeper appreciation of it by comparing the drivability to B-119-JY. It is fair to say that in terms of cruising speed, as the cars are about the same weight, the 3.5 litre car is comparable on the level and modest inclines. On gradients seen on some trunk roads, the higher gearing slows the car more quickly and 3rd is engaged earlier. In overall climbing ability the car performs well but on very steep hills I engage lower gears early due to the sensitivity of the clutch and my wish only to slip it minimally.

So after much thought I was coming round to the view that an overdrive system would have been a better choice. This view was completely reversed on a recent European trip in Spain and Portugal. On occasion we traveled in my 3.5 litre car in company with another 3.5 litre car fitted with an overdrive and a VDP replica body considerably lighter than the Mulliner. I was surprised to see that the expected “superior” gearing of the VDP car had a detrimental affect under cruising conditions on the open road such that from running together after a few miles we were significantly ahead.

Both drivers have a similar approach to driving style and both wish to cover the ground within the normal reserves of safety. In later discussion with my friend, he stated that for the undulating motorway/trunk road we were using, the inclines made the car overgeared and hence he had to revert to standard 4th gear which slowed him even more as, quite rightly sustained high revolutions do give high oil temperatures etc.



If the result is set into the context of all stated previously, it is again apparent that the choice is very dependent of driving style, average speed, type of country and hence a personal choice. That said, personally I would always choose the higher ratio gearset for my driving and touring requirements.

Ken Lea

Chairman of Trustees

WO Bentley Memorial Foundation(WOBMF)


Further resources:

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