Jena XP Tether Car

A tear-drop chassis for an eastern block engine by John Goodall

Researching my book on the Oliver marque started something in me that I had not expected, a keen interest in tether model cars. This was strengthened when I was invited to Sweden for the meeting at Orebro in May 2005. Here I met some really nice and open new friends sharing ideas and helping in ways not asked for, but given willingly to assist a beginner very obviously in need.

I enjoyed the experience that much, I have booked this years trip and managed to persuade John S Oliver, Stuart Robinson and Gordon Yates to join me over there. I hope to run the car which features here, and maybe another if it gets finished in time?

One other thing that astounded me in 2005 was the massive difference between the vintagents, loosely called "Old Timers" (I believe this means the cars?) and the modern cars which are simply so amazingly far removed from them performance wise, as to be from another world! I would like to have the skills and knowledge to own and run a competitive car, but without a track in accessible distance it seems a rather fruitless exercise!

The above raises an important question, why when this country had a track in virtually all large towns back in the early 1950’s about thirty I understand, do we not have a t least one track left in use today? I know that in recent years a few enthusiasts have tried to get sufficient people interested to build a new one, but for various reasons they have unfortunately fallen by the wayside. I would like to help in anyway I can, if sufficient people will come forward to assist. I feel it needs to be centrally positioned, accessible, of "International Standard" and near existing noise generators, to negate intrusion on possible near neighbours. I know many of my aeromodelling friends and model engine enthusiasts would build and run cars if there was a track available. It would also attract foreign visitors without doubt as I found in Sweden, they want a good excuse to come here! Please get in touch if interested in helping, through my business address shown later.

This car project started when I attended the 2005 vintage Models weekend at Old Warden airfield, when I espied an interesting 5cc in line twin cylinder diesel engine which remarkably had two output shafts, one at each end of the crank case and I immediately thought ideal for a model car! This was being sold by Richard Dalby and so a price was agreed.

Over the next few months little more was done except to notice it would hardly turn over after I roughly fitted a propeller in order to check condition. It seemed to have some compression on each cylinder and had reed valve induction, so usefully could run either way. Richard had claimed the engine was a prototype Jena twin made by the factory and it certainly wore what appeared to be standard Jena cylinder assemblies.

The project progressed after I was asked to sell a lot of tether cars, parts and accessories on behalf of a friend through my model business and among the items was a tear drop chassis casting set which I had catalogued and it spent a couple of months on the web site list unsold! I later realised the twin might just fit and a trial showed it would quite nicely and so I bought it myself.

The rear end slenderness of the body presented a problem in that there was insufficient material to tap a thread of enough depth to give good security for the body joining screw, the front end being adequate! I decided to fit a vestigal fin, which would add enough material to overcome the problem and give a futuristic styling feature. The fin was secured to the top half by Technoweld, which works quite well and is easy to apply, if a little brittle! One big disadvantage with Technoweld in my view,is that the colour changes to a duller shade with time. So it is not suitable for visible and untreated repairs on model engines or vintage vehicles. It does however securely join Mazak, which is a zinc based die casting alloy and of course aluminium! Mazak was used in a lot of early carburetters on cars and motorcycles!

Things then moved on a pace with the chassis being bolted together after the mating faces had been filed flat and the exterior joint and contours blended by file and much abrasive tape and elbow grease work had been completed.

An engine mount was made up from some ¼" dural which I had to hand. This was drilled and tapped for 4BA securing screws, chassis mounting holes and the holes transferred to the chassis. A machined pad was milled to allow the mount to sit on a flat area in the pan lower half.

The next area tackled was drive shafts, which posed a small problem in not knowing what taper was on the shaft ends and the fact that each was slightly different in length and so had to be individually made. I had a few attempts at making them in spite of the fact I used to be design draughtsman after serving an apprenticeship as a toolroom machinist, but in defence that was a long time ago. I finally lapped them to fit after getting very close with taper reamers. Eventually they were fully machined along with wheel hubs to carry four tyres that I obtained from Peter Hill.

The rear wheels were made up using a single ball race trapped between the two halves, screwed together like the fronts, with 6BA cap screws and fitted to stub shafts silver soldered to the stainless steel rear axle. The axle was cut with bandsaw from some 2mm SS sheet with two mounting holes to fix to a machined pad again on the lower chassis half. I now had a rolling chassis. The rear tyres being identical to the fronts looked too heavy, so I machined slivers of rubber off each side of them in the lathe using a knife tool ground from an old parting tool blade. I am not too happy with them even so and I may replace with proper knife edge tyres later!

Whilst all the work was progressing, I realised that the original carburetters would not be suitable to run the car because the fuel draw would be too high relative to the fuel tanks, in fact well above the body top surface, so new carburetters would be required bringing the intakes down within the body.

Fuel systems might also pose a few difficulties and I contacted John Oliver for advice on how he tackled the twin he and his father made back in the 1950’s, realising that fuel flow would need a well thought out approach. I decided to use a twin tank installation which although a little more involved would allow individual adjustment if required. The position and balance to each carburetter was set to as near ideal as possible using a wooden dummy before metal was cut and soldered for the tanks proper. I made up spring loaded ball filling valves based on my old Dick Edmunds inspired CL team race fuel tank practise.

The carburetter mounting plates were made of brass so that the inlet tube made from bundy tubing, could be silver soldered securely to them. Bundy tube is a ductile but strong brass alloy used for pressure gauge pipework which bends easily without kinking and they were bent to angle the intakes down to the fuel tank centre line.       The original spring steel valve reeds were re-used and the system completed with Os Pet needle valve assemblies which have a nice fine thread for accurate adjustment.

The intake tubes project through slots cut through the upper body half and did not look too pleasing and ditto the exhaust slot, which needed some sort of cover? I decided to make a cabin shaped streamlined shape in panel beaten 1/16" thick aluminium to hide them both.

I made a hard wood block to the inside shape needed and initially bent the alloy over it, clamped the sides in the padded jaws of the vice and started hammering away at the rest with the flat side of a ball pein hammer. I could possibly do an article on this if there is enough interest? The rough cover was then filed up and polished with emery and the mating face filed to the body shape to a good fit all round.

Whilst this was offered up for the mounting plates to be Technowelded in position, I realised I could add another useful feature, a push stick hole for starting the car. Three useful additions in one item is a designers dream! I made a dural block drilled and tapped for mounting to the top body half and slid this in from the front, I think the overall result is quite pleasing?

I learnt the skill of panel beating incidentally , whilst building a control line model in my midteens called the Harlequin if any one remembers it? I was serving a five year engineering apprenticeship at the time and felt fairly confident in tackling most things in those days! The impetuosity of youth? The engine an ED Racer was housed in a nicely shaped hand beaten aluminium cowl. It turned out reasonably well, as orders for the cowl came in good numbers from fellow club mates and no I will not be making any more!!

The fuel cutout was another difficulty and I could find nothing available that would do the job. I studied many types before deciding to make one similar to the Healy example I had obtained some time previously. A twin version of it could be ganged together side by side to simplify machining and operation and it proved easy to make and gave a neat installation with positive action.

Body finish chosen was red enamel basically to cover the Technoweld and give a surface which should stand up to the rigours of diesel fuel and rough use and was applied by my local company Dumelows of Burton upon Trent, who also carry out some of my vintage motorcycle enamelling work. The name of Jena XP is from the engine, which now I have had time to look inside and believe from this that it was an experimental engine (hence XP) rather than a prototype! I say this because the transfer passages cut in the crankcase are differently made for each cylinder. I theorise that this must somehow have been to experiment with and compare different porting arrangements, a prototype engine intended for production would almost certainly have had identical porting?

All that now remains is to see if the car works in practice.

Shall we see you in Sweden, you are guaranteed a warm welcome?

John Goodall

Please email me at J.goodall@bamopro.co.uk

©copyright2006