461 Algonquin Place, Webster Groves, Missouri 63119
Everyone likes to see the small tractors running around the grounds at thresher reunions. Whether you have a reproduction of an Oil Pull or Mogul, or just parts you put together from the scrap pile, you have a popular display. Kids want a ride and the older folks want to see how it works,
It doesn't matter whether you have a fancy machine shop or just use stove bolts and a 5-pound hammer, some simple rules must be followed in steering and chassis layout. You don't repeal laws of chassis design by ignoring them. If you use these rules, your tractor will be easy to steer, will drive a straight line over rough ground and will not scrub the wheels in a turn.
These geometric rules apply to any vehicle controlled by steering arms and kingpins. Center-pivot tractors like row-crops and most steam tractors do not have the opportunities for steering sophistication (or errors) that steering-arm geometry affords. I have simplified things from what you would need on a 150 miles per hour race car but everything will work fine at 5 mph. These steps are easy. Just lay everything out on the floor before you start to weld.
FIGURE 1: ACKERMANN STEERING
Steering arm layout is very simple but critical. The steering arms and tie rod can be in front of or behind the axle. Wherever they are, the Ackermann relationship must be followed. Lines drawn through the kingpins and the ends of the tie rod and extended must cross in the center of the rear axle as shown in Figure 1.
Make or bend your steering arms so they fit on the extended line. Then make the tie rod long enough to fit. Thread one end of the tie rod to make the length adjustable.
It is almost always easier to place the tie rod behind the axle. Notice that if the tie rod is in front, it will extend into the wheels so the spokes will have to be offset to make room. Either way, the design works and it is 100% a matter of taste and construction difficulty.
THE DRAGLINK, THE PITTMAN ARM & THE STEERING WHEEL
The placement and size of these three components will determine how easily the tractor will steer.
Steering ease is a matter of leverage. The levers are the steering wheel diameter, the Pittman arm length, the steering arm length and the gears inside the steering box. Figure 2 shows the layout.
As a rule, the Pittman arm will be longer than the steering arm. If it is not, you will have a large turning circle and won't be able to make sharp turns. Also, use the largest steering wheel you can to increase leverage.
Length of the draglink is important too. It must be long enough so that the Pittman arm is vertical to the ground when the front wheels are straight ahead, as Figure 3 shows.
Incorrect length reduces the leverage but that is outside our discussion. Just do it right. Also, as the wheels go up and down over bumps, the geometry is changed but that won't be important at the speeds you are going, either.
WHEELBASE AND TRACK
The most important thing about wheelbase and track is to make sure the tractor will fit on whatever you will use to carry it. After that, the wheelbase should be at least twice as long as track. The longer the wheel-base is compared to track the more forgiving your design will be. Figure 4 shows how to measure wheelbase and track.
Short wheelbases are cute but there's not much room for the parts.
FIGURE 5: CASTER
Caster means that the kingpin leans slightly back at the top. About five degrees is OK. This gives the tractor stability to follow a straight course and helps the wheels return to center after a turn just as a car does. Figure 5 shows caster.
Camber also adds to stability. The more camber you have the more the tractor will want to go straight, even over rutted ground. Camber means that the tops of the front wheels are farther apart than the bottoms. The front wheels of a row crop tractor usually have extreme camber, for example. The amount is a matter of taste but do use some. Figure 6 illustrates.
Toe-in is an adjustment for wear and looseness as far as we are concerned. When the tractor is rolling, the front wheels should point just about straight ahead. With all of the looseness in your front-end, if your wheels point ahead while at rest they will spread out when you start to drive. To counteract this, set the fronts of your front wheels about ? inch closer together than the rears, as Figure 7 shows.
When you start to drive, the slack in the steering is taken up and the wheels straighten out. If it takes more than ? inch of toe-in you had better redo that front-end.
If you follow these easy design steps you will have a tractor that your granddaughter can steer with one hand during the parade without frightening the spectators. It doesn't take any more time to lay everything out correctly, so you might as well do it right and have fun.