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Author Topic: The build of GN Kim  (Read 723 times)

RhysN

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The build of GN Kim
« on: 02 January 2019, 03:11:13 pm »
[ Guests cannot view attachments ] This is the 5th or 6th CycleKart build I have started. Some have been completed, others have been seen and purchased before I got to finish them, to my knowledge the buyer took them no further.
With Kim, the inspiration car was the absolute favourite of Archie Fraser Nash, the N of GN, and in it's first iteration dates from before WW1. As with all the early Gns Kim had a chassis made from ash timber with local reinforcing. This to me was ideal as I wanted to show that it's not needed to have a whole lot of workshop facilities to make a CycleKart.
The full size Kim still exists with V twin GN engine, chain drive and now over 100 years of patina.
The basic construction of  my Kim has 2 ash timber 70 mm by 30mm chassis rails with cross members and the plywood centre section to add huge torsional strength. Local plates of steel are used , bolted at the corners.
« Last Edit: 02 January 2019, 03:20:10 pm by RhysN »

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RhysN

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Re: The build of GN Kim
« Reply #1 on: 07 January 2019, 04:08:47 pm »
Having established that I wanted a bit longer wheelbase than the ones I had previously done I settled on 1900 mm. Yes, this is longer than the Stevenson's suggested 66" (1650 mm approx) and was an informed decision.
The engine mounts to a fabricated steel "cradle" which is bolted into the rear of the ash frame. This has the steel tubes which are the main load bearers, and the engine has two alloy shaped bars bolted to the bottom. These are then able to be readily adjusted along the tubes for chain tension.
Forward of the engine, which has no room for a standard fuel tank, is an alloy bulkhead/seat back. This was folded by me at home between two pieces of angle iron clamped in the vice. It has flanges on each side, and a curved top edge, which is then topped off with an alloy capping with wired edges on both sides.
From the seat back forward runs the plywood box, which has 2 vertical bulkheads, one at the dashboard and on at the front. It also has a horizontal diaphragm bulkhead from the dash to the front, the middle being cut away for access to pedals , etc. This is all built from 6 mm ply. The floor is 9 mm. All this is bonded together with epoxy glue, and double bias glass tape. Through the seating area is a layer of tri-axial glass cloth which also runs up the cockpit sides.
Once the box was bolted and glued between the ash chassis rails, the outside was clad with 0.4 mm alloy sheet, which had been donated.
The front axle, has brackets cut to take the ends of the quarter elliptic springs (as per all GN) above the axle, and a fixing for the Hartford friction dampers, which I make, below the axle. This is what locates the axle, to give the required castor. While the springs are bolted to the frame with a single bolt each side, there are also 2 "U" bolts, these can, in theory, be moved to adjust the spring rate. Steering is simple go kart type, taken directly from the previously mentioned ex hire kart. The steering column is steel tube from B &Q, telescoping. In the middle is an arrangement of telescoping square tubes from the same source which allow for length adjustment.  The top end has one of the readily available quick release steering wheel fittings.

RhysN

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Re: The build of GN Kim
« Reply #2 on: 14 January 2019, 03:42:18 pm »
Next step was to make up the other parts of the body, the nose, bonnet top, scuttle, and tail.
These are all made from alloy sheet, thickness is 0.9 mm and 1.2 mm. What I did not know at the time was how different to work different grades of alloy are, nor did I know how to anneal alloy.
Kim is a pretty simple shape, with little compound. I was able to form the metal around pieces of pipe, use the method of two pieces of steel angle clamped in the vice and g clamps as a folder, and generally shove it around until the shape reflected what AFN did on the original. In other words it's rough, or patina if you prefer) After I had done much of the body, my son gave me a stretcher/shrinker for my birthday. That makes playing with alloy a whole lot more fun, and many new shapes to explore. It also meant I could deal with a couple of crimps that appeared on the lower edges of the tail and tidy that up a lot. I also learned about annealing. Some day I'll write in the "Techniques" section how to do that. If anybody needs to know before I get a round tuit, just ask.
My front springs, being quarter elliptic were made for me, they are each 2 full length leaves of approximately 4 mm, 38 wide, and 400 mm long. A single bolt passes through the spring, and each side has a pair of "square U  bolts" 1/4" material. The bolt stops the spring sliding backwards or forwards, the U bolts can be moved to vary the spring rate.
Castor is currently at approximately 6 degrees, very slight positive camber and 1/8" toe in. I don't pay any credence to ackerman, so if it has any, it's by accident.
The castor is controlled by the arms of the friction dampers which locate below the axle. The dampers can be moved along the chassis rail to fine tune that.
Rear axle, brake, sprocket. All of this was from the ex hire kart, but hubs became a bit of an issue. (Remember I'm trying to do this all without spending?) The hubs use part of the kart parts, and pieces of 5 mm alloy welded to them with bolts directly onto the pitbike wheels. All the pitbike wheels I have seen have a tiny lip which prevents the plates bolting as they should. A flap wheel in the angle grinder deals with that in seconds.
All the bearings, bearing holders and other components, except the brackets came from the kart. I made the brackets from what is called sheet steel in this country, and it's possibly a bit too thin at 1.2 mm. right now I'm remaking them in 2 mm. All of this was cut using a 1 mm thick disc in the 42 angle grinder, super tool you need to own as it will do pretty much all the cutting you need to do. Drop saw un-needed. My biggest mistake was making it so that to change a chain, as I'm using proper kart chain, which is plenty strong enough requires an almost total dismantle of the rear. Another reason for the rework of the brackets.
Next big error when I was rushing to be ready for "The First Great British CycleKart Meet" was that I neglected to tighten all the grub screws in the sprocket carrier, brake caliper, and bearings. This allowed the axle to move sideways, which spat the chain off. Memo to all, spanner check everything, and if in doubt put more clamps on the rear axle.
That will do for this time! I need to do some study for build number 7.

RhysN

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Re: The build of GN Kim
« Reply #3 on: 20 January 2019, 10:51:33 am »
Front axle build.
As with most GN (if not all) and the Fraser Nash which followed, this is hung on quarter elliptic springs poking out from the front of the chassis.
The axle tube is 38 mm by 2 mm wall, all brackets are from 3 mm
The axle "yokes" I cut from pieces of steel angle. They are 5 mm.
As can be seen the sprig eyes go to the top of the axle, the Hartford friction dampers to below the base. The position of the dampers where they are mounted to the frame can be changed backwards and forwards to adjust the castor angle. The U bolts on the springs can also be moved to adjust the spring effective rate. As is they are about as stiff as it can be. There is one bolt passing through the spring and frame rail to fix the spring fore and aft. Clamping with the U bolts also helps with lateral location of the axle.
The white rods in the pics are the steering rods, as they came from the hire karts. Rod ends have left and right hand threads to ease toe adjustment. (Start point is 2 mm toe in)