Building the GeepStar - Axles, Steering and Motors


The swing axle steering mechanism was chosen for improved traction when running over uneven surfaces.

The system was also modified to include a reduction gearbox and Ackerman angles on the steering arms.

Underside view of front axle

 

 

 

 

 

 

 

 

Front axle and steering.

The steering column was modified to include a reduction gear. This would reduce the direct feeling of the steering geometry by reducing the motion by a ratio of 4:1.

The 4:1 ratio was achieve by combining two 2:1 reductions on a lay shaft. The lay shaft type gearbox was used to ensure the direction of the steering was not reversed.

The gearbox was built from some gears ordered from Technobots. Technobots stock some reasonably priced gears of decent quality. These gears were Mod 1 and were of 40 teeth and 20 teeth (2 of each).

The complete gearbox

 

 

 

 

 

 

 

 

The gearbox had to have the 2 shafts parallel and spaced 30mm apart. So the two sides of the gearbox housing were machined together to ensure they would be the same. The holes for the lay shaft were 10mm diameter and the holes for the steering column were 14mm diameter. This was to allow the steering column, which was made from 12mm threaded bar, to be housed in a 14mm steel tube.

The two sides were mounted on an aluminium block. Two pieces of 14mm tube were inserted into the upper holes and brazed in place. Alignment of these tubes was maintained with a length of threaded bar.

The steering gearbox after brazing

 

 

 

 

 

 

 

 

The smaller gears had 8mm holes in them and so these were expanded on the lathe to 10mm to match the larger gears. All the gears were also cross-drilled and tapped to take 4mm grub screws.

The 12mm threaded rod lengths had the ends reduced to 10mm to suit the gears and all the rods had flats filed on them to help the grub screws grip.

Fitting a gear to the steering rod

 

 

 

 

 

 

 

 

The gearbox was mounted on a block of hard wood screwed to the inside of the body.

Upper and lower mountings were made as shown. They were both made from a larger piece of pipe drilled out to be a snug fit on the 14mm steering column and brazed to a mounting plate.

To finish, the threaded rod was cut to final length and the gears assembled on the shafts. Grub screws were loctited in place.

Steering column in car

 

 

 

 

 

 

 

 

The steering wheel was taken from a child's toy car which someone had abandoned in the street. The wheel was broken on the back and had a squeaker horn in the middle.

Donor steering wheel

Wheels fixed to steering rod

A new metal boss was turned up on the lathe to fit inside the steering wheel boss. This was fixed in place with three M6 screws. The new boss was tapped M12 x 1.75mm match the steering column threaded rod.

 

 

 

 

 

 

 

 

 

 

The complete steering column mounted in car. Click to enlarge.

 

 

 

 

 

 

 

 

 

 

The front swing axle was chosen from the plans so that the Jeep would ride over uneven terrain more easily. However the design was adapted to accommodate Ackerman geometry. Ackerman steering allows the inside front wheel to turn sharper during a turn to reduce tyre scrub.

Setting this up was done by stretching a piece of string from the mid-point of the rear axle across to each steering pivot. The angle formed between the string and front axle is the Ackerman angle and this was transferred to the steering arm before welding (see below).

Calculating the Ackerman Angle

 

 

 

 

 

 

 

 

The other modification to the steering design was to use plain pivot shafts for the steering pivots, rather than the threaded rod. This was because no 20mm threaded rod was available and because a suitable block of steel was available.

The picture on the right shows the components. Each steering pivot was a piece of 50mm steel bar with a section reduced to 20mm to fit in a sleeve on the end of the axle. The part also had a boss turned on one side to locate the stub axle before welding in place.

Steering components

An assembled part is circled in blue.

 

 

 

 

 

 

 

 

 

Finally all the steering parts were assembled to the chassis. The axe was moved 10mm forward of the indicated position so that the front wheels wouldn't rub the wheel arches on full lock with the axle tilted.

The link to connect the column to the steering bar was twisted by clamping in the vise and using a bar on an adjustable spanner to put the required twist in place (see below).

 

 

 

 

 

 

 

 


Rear Axle and transmission.

The rear axle was made exactly to the plans. The parts were assembled to the chassis and clamped in place before welding to ensure that it would be an exact fit. Once tack welded in place, it was removed and seam welded for strength.

Rear axle

 

 

 

 

 

 

 

 

The motor and electronic speed control unit were take from an 8mph scooter. Although the original scooter had two motors, it was decided to use just one for simplicity and to reduce overall weight of the finished vehicle.

The motor was a 250W 24V DC motor with a built in brake.

 

 

 

 

 

 

 

 

To fit the motor into the rear of the jeep, first a new plate was welded to the existing bearing housing. Care was taken to ensure this plate was square to the shaft.

Lengths of M8 threaded rod were used to secure the motor through the housing and the new mounting plate into a double thickness section on the rear inner wing.

The inner wing was slotted to allow some motor movement to tension the chain drive.

 

 

 

 

 

 

 

 

Slotted Motor Mount

The double thickness mounting was used to space the motor inwards by 12mm so that the motor sprocket would be close to the inner wing enabling the wheel to be mounted without sticking out too much.

 

 

 

 

 

 

 

 

A 3/8" chain and sprocket set was ordered from Technobots. The sprockets chosen were 35 teeth and 17 teeth so that the original 8mph scooter speed would be reduce to about 5mph on the new, larger wheels.

One benefit of the steel Technobots sprockets was the large boss on the rear which would allow for customisation.

An additional roller bearing was also purchased to suit the 25mm rear axle. This bearing was 20mm long.

 

 

 

 

 

 

 

 

The purchased motor pinion was bored out to fit the motor shaft (20mm) and drilled to accept two 6mm grub screws. These screws would locate in the shaft key way.

Pinion held in the 4-jaw chuck for accurate boring.

 

 

 

 

 

 

 

 

Machining the wheel sprocket.

The wheel sprocket was bored out to 28mm to be a clearance fit on the shaft. Then the outside of the boss was machined to have a step, which would fit inside the wheel hub without the bearing seal.

On the opposite face another recess was cut to take the new needle roller bearing.

Removing wheel-bearing seal.

 

 

 

 

 

 

 

 

Sprocket welded to driven wheel

The sprocket had the needle bearing Loctited into place. Then the sprocket and wheel were positioned on a shaft to ensure alignment.

After checking the sprocket for true running, it was welded in place.

The final sprocket position was only about 5mm clear of the tyre but this would allow a nice close fitting wheel within the wheel arch.

 

 

 

 

 

 

 

 

 

 

Motor fitted in chassis and sprockets lined up.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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