Tethered car build

The first task is to get the new Picco engine in, which has a much stiffer front end. You will see in the picture that the old engine has been flexing and has put a dent into the pan. To get the engine in was a straight forward milling job to remove metal from the pan so that the front housing of the engine had clearance all round it.


Engine flex witness marks in pan	Old engine left, new stronger cased version right

We had our annual meeting at David Giles' house down in Bristol, the only modern tether car racer missing was Stuart who was away at the Hydro AGM. A good day was had by all. Part of the day is 'show and tell'. Dave Cunliffe had a nice copy of a Oliver twinshaft he had built and we saw progress on David G's steam locomotive. I took down my part completed Dooling Arrow car.

I came away with a 1.5cc SMRU car from the 70’s that David had built for his son. This car was developed by the Swede’s as a beginner’s class for the youngsters. I am led to believe that these cars were fiercely contested by the fathers in the sons /daughters name.

The class is still raced in Sweden and I had been looking for a chassis but David came up trumps. It needs a bit of an overhaul and then it will be ready to run. Maybe in the UK.

I also came away with a new job. I am now the UK FEMA representative.

Back to the 10 car. You will see that my old engine has had a piece cut out from one of the strengthening webs. This cannot help the stiffness of the front of the engine, so a new tether arm has been made out of 3/16" gauge plate.

It has a much higher bridge to go over the front of the engine without mod’s to the engine. The arm is an upside down aerofoil to give down thrust to help improve traction and keep the car on the track. The aerofoil shape is done partly by milling, then roughed to shape with an angle grinder and finished by filing and emery. The final finish was with a mouse, detail sander.

You can see my smoke lamp, and in the next picture the area where the tether arm fits has been smoked after fitting of the arm. You can see shiny (white) areas where the soot has been removed. This shows where the metal needs removing from the end of the arm. The last picture shows the engine and arm in place.

Next is the pipe. I got carried away with shortening and it has ended up too short, now needs a piece putting in.

The pipes we use are adjustable for length and the stingers are different diameters. These vary in 0.1mm steps and the pipe length is adjustable by 5mm from memory. In the other picture, engine is set up with exhaust valve and pipe for measurement. Making the extension piece for the pipe proved to be a challenge I am not the best at screw cutting, especially internal threads. The 0.5mm pitch was very fine. My lathe is imperial, and pitches are set by change wheels. Set it up, cut the thread, the adaptor did not fit. Two more attempts, no joy. After a few chats and a search on the internet we decided the lathe did do the pitch conversion very well, because it almost fitted.

Needed a change from screw cutting so I stripped and cleaned the 1.5cc car and engine, it is not a twinshaft engine as only one wheel is driven, the other freewheels on an axle built into the engine back plate. On the original SMRU car drawings the axle is fitted to the pan.

The hole in tank was soldered up. The cut off is a simple tube crusher. Just need to find some thin wall diesel fuel-proof tubing as all I can find at present is thick translucent stuff. The other problem is diesel fuel. Nobody seems to stock it anymore, or the additives so you can brew your own. Any help appreciated in locating these things.

Back to screw cutting. Put my glasses on, looked at the plate showing which gears to use, yes the main ones were correct but I had not seen in the small print that I needed to change to other gears. Cut the thread again and it fitted perfectly. Finished the adaptor piece, just needed silver soldering to the pipe.

Left: The pipe prior to soldering. The white band is tipex this stops the solder going where its not wanted the other is easyflo flux. The secret to success with silver soldering is enough heat and all the parts to be joined must be clean. Centre: The pipe set up for soldering and Right: The finished job fitted to the engine.

The track at Lyon is a bit bumpy and I always manage to break something. This time it was the needle valve and the support for the rear of the tuned pipe. This is a fairly straight forward turning and milling job.

Broken ring on the right. Break is close to the lug on the left and the new ring part way through the process.

The dampers used on the car are much modified r/c car ones. The plastic piston and circlip have been replaced with a brass piston on a screwed rod. Seals are held in place with a gland nut similar to a steam engine, and the plastic ends have been changed to aluminium. Not much left of the original. All the modifications have been done following failures on track.

I modified the car from the original friction type dampers to the current springs and damper system used now. It has stopped the car flying like it used to at Lyon the first time I ever ran it. The horser used to do aeroplane impersonations when it was running, and Horst Denneler told me the suspension needed attention.

Gear box in place with the spring damper attached the aluminium construction is basically a 90º bell crank. The gearbox has a ball on the end which works in a socket to allow the up and down movement of the axle.