Javelin gearing

[Keith Clements]

For the Southern sction zoom meeting I did a bit of research on JowettTalk and Jowett Archive.

April 1962 Jowetteer page 68 has page 14 has a detailed 4 page article by Keith Rumsey for fitting a Type A Laycock unit.

For Jupiter owners the January 1983 By Jupiter page 8 has two articles on two different conversions.

[Nick Webster]

The article about various overdrive ratios recently posted in the JOAC section was particularly interesting. It was good to see the analysis of how overdrive gearing fell between the gearbox ratios. It is obvious that even though there are various different overdrive units that might be used, ratios fitted might differ depending on their origin. An indication of what to look for is very useful. I was hoping the calculation might re-run for a Javelin and posted here.

Nick

[jowettgeoff]

Overdrive specialists OD spared are now able to supply a self-contained D Type as a single unit with input and output flanges. I've done a few measurements, and I reckon it could be slotted into the Jup chassis frame with just a small amount of 'surgery'. The input/output couplings would obviously have to be sorted, but I'm sure some of the clever Jupiter owners could sort this out! Looks like a nice solution. Price is around the £1k mark.
See:-

http://www.odspares.com/phdi/p1.nsf/sup ... ent&part=4

[jowettgeoff]

Correction .. should read 'OD Spares'.

[Keith Clements]

Yes Geoff. That is my preffered solution. I think it could replace the front prop in the Jup and would fit in the JCL location in the Jav.
We should be able to use the engineering drawings of the Jup to prove this before trying in the car.
OD spares made one up for me which I have used for decades but I fitted in place of rear prop.

[jowettgeoff]

I took a few measurements from their dimensional data sheet to check it out. It's just a bit too big to be a 'slot-in' solution, but nothing that a big hammer and a hacksaw wouldn't cure! Might give one ago when son Richard removes his SC from my car lift (if ever ...).

[Nick Webster]

It seems Jowetts were perhaps a little bit obsessed with ground clearance. The JCL drawing of the Javelin overdrive layout shows it mounted quite high so that both front (in particular) and rear shafts are sitting at a very undesirable angle. Getting the couplings at a good angle and length must be a prime consideration. The A series Laycock overdrive is physically large and able to handle even tuned up Jowett power with one hand behind its back. Better to seek out a smaller unit and mount it lower. I have not yet had the pleasure of squinting under an overdrive equipped Javelin to see how others have done it, but since I hope to install a mid mounted unit of some sort in my Javelin I'm following this with interest.

Nick

[PJGD]

Nick,
Having generated the spreadsheet with the Jupiter ratios, it is easy now to do the same for the Javelin. Here I have plotted the theoretical road performance for the Javelin with the standard 4.875 rear axle ratio (RAR) and 22% overdrive applied to the wide and the close ratio gearbox. I have assumed that the wheel rolling radius and thus the wheel revolutions per mile are the same as JCL claimed for the Jupiter:
Tire Diameter 26 inches
Rolling Radius 13 inches
Circumference 6.81 feet
Rotations/Mile 775.7
However I don't know if that is a correct assumption as there may be a slight difference between the 5.25 x 16 vs 5.50 x 16 tires.

From these graphs, it is evident that the wide ratio box gives nicely spaced ratios so that all gears have a useful range, whereas with the close ratio box 3rd OD is quite close to 4th gear, so not as useful. Either way, cruising at 70 mph would be at 70/20.34 = 3,344 rev/min rather than 4,410 rev/min.

The Laycock-De Normanville overdrive that I have is a J-Type which I believe came from a Volvo and so should be plenty strong enough to handle the puny Jowett torque. I think (but don't know for sure) that the ratio is 22%. If you need me to plot the graphs for other OD ratios or RAR ratios, let me know.

Javelin_Close Ratio_22% OD.png

Javelin_Wide Ratio_22% OD.png

Depending on the availability of matching components, there may be opportunity to achieve the final ratio that you want by selecting different crown wheel and pinions:

Rear Axle Ratios.png

Given your interest, Nick, in PSV's, do you have any insight on the question that I raised in the other thread about the Wilson Self-Changing Gears overdrive which was clearly for light duty cars, not trucks. Had you heard of such a product, or was it just a concept that they never produced?

[Nick Webster]

Philip,

Thanks for producing the graphs for the Javelin.

It would be useful if you can put a date on that drawing, but I must admit I have no knowledge of that design of overdrive (with TWO clutches !) being marketed for cars or indeed anywhere. The first 5 speed over-gear Wilson /SCG box I can think of was the Daimler Freeline of 1951 but since the Wilson box is just a row of epicyclic gear clusters on a shaft I imagine that the overdrive was just slightly longer with one more gear train. All of the PSV designs took advantage of the fact that there was plenty of space and the gears were selected by a contracting brake band, which, with the individual operating levers made the boxes big and heavy.

In PSV design the early 1950s were awash with various ideas for overdrive ratios and I know gearbox manufactures such as Turner Engineering and Moss all tentatively made designs for add on units, though I never heard of any being fitted. Albion motors also came up with a design but almost as soon as they thought of it they were scooped up by Leyland who typically phased out anything actually Albion and used it as a badge engineering alternative brand of their own range. Indeed, Self Changing Gears fell under the Leyland influence from 1951 and total control a few years later. Leyland were keen to gain control because they were having to pay licence fees (some of them even to rivals AEC I believe) and secondly they wanted to develop a gearbox for the rear engine buses they had in the pipeline.

Nick

[PJGD]

Nick,

The Wilson/SCG drawing comes from a patent that they filed in 1964 [GB1014621]. I cleaned it up and colored it.
Of course, just because they patented the design does not mean that they had a customer and tooled up to manufacture it, but I suppose that the obvious customer would have been within the BL organization.

[PJGD]

Two posts above this one, Nick has noted that both Turner Engineering and Moss Gear Co, Ltd. may have had overdrive concepts back in the day. I am very impressed that he is aware of these potential devices which appear to be vapourwear. I was unable to find anything about a Turner overdrive, but I did find a patent for the Moss Gear OD, specifically GB791358 filed in January 1956. In the past, I was never very impressed with Moss Gear products, they somehow seemed a trifle crude and industrial even though they were used by Jaguar and Armstrong-Siddeley. Nevertheless, I really like their OD concept, so I cleaned up the patent drawings and colored the two sectional views [side view and front view] to make them easier to comprehend:

Moss Overdrive Unit_Colored.png

There are several things about it that I like, specifically:

• It is installed between the clutch and the gearbox, not at the back like the Laycock unit.
  It is very compact lengthwise, probably only 30% as long as the Laycock OD [so no changes to the Jupiter chassis crossmembers and it might even work for the Javelin without chassis changes].
  You get OD on all gears, including reverse [OK, not super useful, but fun anyway].
  No issues with OD in reverse - it does not destroy the OD as is the case for Laycock, so no need for interlocks.
  The parts count [BOM] is less and thus potentially significantly cheaper that the Laycock unit.

A brief explanation of how it works: The input shaft from the clutch drives the planet carrier [light green], and the sun gear rides on this shaft driven by the one-way roller clutch. With the sun and the planets locked together, the complete unit including the annulus output all rotate as one unit giving direct drive into the gearbox. When the band brake arrests the sun gear, the epicyclic gear ratio means that the annulus output now rotates faster than the input.

This is such a neat arrangement that I am tempted to walk down to the end of my street to the Lyon Gear Works and ask what it would cost to manufacture something like this.

[Keith Clements]

Very interesting. Indeed this might fit but would require extensive mods to the gear box casting and input shaft.
However combined with a suitable new gearbox might fix another weak point. Many are now fitting new gear sets inside existing Jowett or Meadows casings so modern welding techniques should be able to extend the gearbox casing to accommodate.