I assume you have a scrap of wood, two small nails, pins, or needles, a straight edge, a ruler, some nuts and bolts, screws, or nails, something with straight edges and two right angles such as a framing squares, right triangles or precisely cut pieces of wood or cardboard, an of 8 ½ by 11 sheet of paper or cardboard, a pencil, and a calculator (or computer with a calculator program or Excel, or if you have any antique trigonometry books or math tables, some trig tables.) A clamp or locking pliers would be nice. A protractor might be nice for verifying your measurements the first time. A flat surface is needed for measuring camber and caster. I use two framing squares, so I’ll refer to them in the discussion. I splurged and bought two shiny new framing squares at Sears for $6 each so my total cost for the alignment tools was $12.00. I could do everything with just one, or none, but I find using two framing squares more convenient.
Why would someone want to know the toe-in, camber, and caster? If you hit a large pothole or have a collision, it might be nice to know what the settings are supposed to be in order to know if the front-end geometry was messed up. As parts wear and loosen, you might want to increase your toe-in. Just as with the front wheels of a car, there are three major specifications for the steering in your tadpole trike: the amount of toe-in (or out), i.e., how much you have your wheels non-parallel, the camber, and the caster. The caster mostly influences how your steering handles when you travel in a straight line. Yes, I know there are other specifications, but I want to focus on these three. On my trike the only setting I can easily change is the toe-in.
Toe-in, or perhaps toe-out, will help prevent wandering caused mostly by play in the wheel bearings and steering mechanism. On a car such looseness can also cause a shimmy, which might be curable by increasing the toe-in. If both wheels are toed in a little, the wheels will want to roll in a straight line as you move along, so the play is taken up by the wheels wanting to straighten out as you roll along. There are exceptions. Some cars use no toe-in, and some cars use toe-out, which in theory should also work
The camber refers to how the wheels, aimed straight ahead, are tilted as you view the trike from the front. If the tops of the wheels are tilted in toward the center of the trike, you have negative camber. Suppose a car with negative camber is going fast and making a sharp right turn. The car wants to tilt and drift to the left. Think of the tilted left wheels propped against the car trying to hold it in place. On a car, incorrect toe-in (or out) or incorrect camber can cause excessive tire wear.
The caster refers to the tilting of the steering axis (centerline of the kingpin on my trike) forward or backward from the vertical. On my Ford, which has front struts, it is the inclination backward, about two degrees, of the strut piston centerline. A backward tilt is called positive. If a wheel has positive caster, then the weight of the vehicle and rider(s) tend to keep the wheel aimed straight ahead. It also helps to you to come out of a corner and does other things, especially in a car. If you have a bike you will notice that the front wheel has a lot of positive caster. This is an imperfect analogy because there are other considerations such as increased suspension due to the front fork and frame flexing, but think of the difference in the steering characteristics of a long wheel base ‘bent with more caster versus the steering of a short wheel base ‘bent, or the difference in steering between a touring upright bike with a lot of caster versus the quicker, more responsive steering in a criterium or track bike with less caster. Having two wheels in front is different than having one wheel in front, and you can’t have as much caster with two wheels in front than with a single wheel. This partially explains why a long-wheel-base recumbent should be less twitchy than a trike on a fast downhill, assuming the LWB has more caster. The same trike, should become more stable (less twitchy) as you make the caster more positive, up to a point. Also, with positive caster when you take a corner fast, the increased tilt of the wheels should help. But there is a trade-off. Too much of a good thing can be bad: Too much caster can cause “bump steering” and make cornering harder and the steering wouldn’t feel as responsive as you would have with small or no caster. After all, you don’t want each front wheel trying to follow the road so much that it has a mind of its own and is not working in tandem with the other. In an extreme case, you can get wobbling and shaking. To visualize a bump steer, think of riding a bicycle with a ridiculously large caster, say with the steering axis tilted back 45 degrees, and approaching a mound on the right side. If you went over the mound on the right side, your wheel would be pulled to the right. But there is more to this than measuring angles. To one rider what is lively and quick steering could be twitchy and erratic to another rider. Not only that, as people have observed, what might start out as feeling twitchy could after some time feel stable and responsive. So what’s the best setting for caster? This is not a mathematics problem. The answer is whatever feels best to you. I would defer to one of the good builders, who, it’s a sure bet, have already experimented with the various settings and finally settled on what they prefer. There are many more issues with steering than what I just mentioned.
Maybe I should just return to the simpler matter of getting the trike’s vital alignment measurements. It’s probably best to have someone weighing about the same as yourself seated in the trike when you take the measurements, in case it matters. Measuring toe-in-out (the difference in distance between the leading and trailing edges of the front tires) : I admit this is overkill, but I thought you might want to see my latest toe-in gauge (trammel would be the technical jargon.) You really don’t need a special tool to measure toe-in. All you need to do is mark two spots in the centers of the tires at axel height in back, hold the edge of a piece of cardboard, with a notch cut out for clearing things in the way, and mark that distance. Then rotate the tires so the two marks are in front at axel height, line up one of the marks and mark the other spot on the edge of the cardboard, near the other mark. If the distance between the second marks has decreased by 3 mm, for example, then you have a toe-in of 3 mm. The toe-in-out gauge: You need a long scrap of wood, at least as long as your trike is wide, and two shorter scraps of wood, four fasteners such as nuts and bolts, screws, or nails, and two nails, needles, or pins. I made mine extra long so I can use it on my car by moving and reattaching one of the short pieces further down the long piece to match the width of the car tires. Attach two short pieces at each end at right angles. My short pieces are about 14 inches long, but that’s not important. Precision is not necessary, but rigidity is. Attach them so that the midpoints are about the same distance apart as are the tires at the centers of the treads. Drive in two needles or sharp pointers into the short pieces. I took two small nails (wire brads), cut off the heads, grabbed the nails with locking pliers, and drove the headless nails into the short pieces so that I have two sharp points about the same distance apart as the tires on my trike are. What you now have is a low-budget toe-in gauge. Here is a picture of my toe-in gauge:
Next, cut two strips of cardboard or heavy paper wider than the tires, make a horizontal line in each:
Wrap each strip tightly around a front tire and tape it in place. Rotate the tires so the horizontal lines are at the same height as the axel in back. Now puncture each strip, being careful not to poke the tire, and mark the spot the needle or nail made. You might have to relocate one of the needles or nails so you make your marks near the centers of the treads. I did.
Now rotate the tires so the strips have their horizontal lines in front at the height of the axel. Line up one of the needles with one of the marks and make a new mark near the second mark. Now measure the distance between the two second marks. If you measure in inches, you need to multiply by 25.4 to convert to mm. The same methods work for a car. Just use a piece of tape across the treads. The method I described is a standard old-fashioned way to measure toe-in.
Measuring camber (the amount your wheels, aimed straight ahead, are tilted at or away from the trike) : Aim the wheels straight ahead. Hold a framing square, vertical edge against the wheel but keep the horizontal edge on the ground surface. Using your other framing square as a guide, have your framing square perpendicular to the surface in the side-to-side direction also. If your trike is like mine, you will see that the wheels are tilted with the upper part inward so I have negative camber on my trike. Now use a second framing square and rotate the vertical edge slightly so it is flush with the wheel. Back up the first square, held vertical, until the corners are aligned. If you happen to have a protractor, you can measure the angle directly, but not very accurately unless you have a very special protractor. Now measure the angle between the vertical edge and the nearly vertical edge. In the photo the forward one is perpendicular to the table and the one in back has its edge against the wheel. I have the two bottom corners aligned, and I used a small clamp to lock them in place so I wouldn’t change anything while measuring. I got as near the center of the wheel as I could until I started bumping into spokes. If you are worried about variations in the rim and tires causing errors, you could take several readings using different spots on the tires, and at different places on your level surface. I look at the long thin triangle formed by the two edges. For example (these are made-up numbers), if when I go out 16 inches I go up 1 inch, then the angle would be the inverse tangent of 1/16, which in degrees is about 3.6 degrees. The top of the wheel is tilted in, so you would have 3.6 degrees of negative camber. If you use a calculator, be sure you are in degree, not radian, mode if you want your answer in degrees. In case you’re wondering, the wheel is on my beloved Club Sport, essentially a Windcheetah with some different parts to save on cost, made by AVD and other neat vehicles.
Measuring caster, the amount the kingpins are tilted forward (negative) or backward (positive) : The idea behind the method: I don’t see a surface on my trike that I am sure is exactly parallel to the kingpin so I don’t see an easy way to take a direct measurement, except perhaps by temporarily replacing the kingpins with long straight dowels and taking direct measurements off the dowels. The following method will give an approximation by measuring the angle indirectly. Think of the following analogy: Hold a coin vertically, and imagine a tilted axis of rotation though the center of the coin. For example, hold the coin at 2 o’clock and 8 o’clock at the edges and with those two points fixed, rotate the coin with the top moving toward you, say 45 degrees for example. You will see the uppermost point on the coin move a certain distance toward you. If you repeat the experiment holding the coin at 1 o’clock and 7 o’clock, the distance traveled will be less. Of course, with a larger coin the uppermost point will move further when you rotate through the same 45 degree angle. If you measure the angle that the uppermost points sweeps out created by the thinking of the moving point and the center of the coin connected by a moving radius, then that angle is independent of the length of the radius. So we will measure angles in order to have a method that works for any sized wheel. If the coin is not vertical but instead has the top tilted toward you, if you just measure the angle the top of the coin makes with a vertical line after a rotation, you will get too large of an angle to use for computing your caster. You could subtract the camber, but an easier and more accurate way would be to rotate in both directions and use the difference of your two measured angles. This would compensate for the camber. You could think of the coin as representing the left front wheel of your trike as you look at the trike from the left side. The steering axis from 1 to 7 o’clock would represent an unusually large positive camber of 30 degrees. The standard way to measure the caster of an automobile wheel is to measure the difference in the amount of tilt from vertical that a wheel makes when turned 20 degrees in and 20 degrees out and multiply that difference by 1.5. Details on determining if you have positive or negative caster will come later. This only gives an approximation to the amount of tilt forward or back of the steering axis, but it is a reasonable approximation and people take that number to mean the caster, and the specified caster and tolerances for a car are based on this method of measuring. For a small caster, using 1.5 gives you an overestimate of the actual tilt forward or backward of the steering axis, and for a very large caster using 1.5 as the factor will give you an underestimate of the actual angle. 1.5 is the accepted compromise factor, and so although it doesn’t give you exactly the angle of tilt forward or backward of the steering axis, I’ll take the standard way of measuring as the definition of caster. For the engineers, take it as a challenge to figure out where the number 1.5 came from. It’s a nice trigonometry problem.
Now let me first describe an old-fashioned way to measure caster with a bubble-alignment tool so you will see where the method using framing tools comes from. Here is a photo of such a tool, propped against a plant stand.
The large knob raises or lowers a rocker arm that has a bubble, and the rocker arm is attached to the piece perpendicular to the long vertical piece with dowels for placing against the rim of the wheel of a car (or trike.) The instructions tell you to have your car on a level surface. For example, if the rear wheels were higher than the front wheels, you would have tilted the steering axis forward. The instructions then say to set the wheels straight ahead, then turn the front of the wheel in 20 degrees, set the gauge against the rim in a vertical position, turn the dial to center the bubble, remove the gauge, set the wheels straight, then turn the front of the wheel out 20 degrees and set the gauge against the rim in a vertical position. You now rotate the large knob and count the number of revolutions and fraction until the bubble is centered again. Each full revolution is one degree, so for example two and a quarter revolution corresponds to 2.25 degrees. Then multiply the result by 1.5. If you turned clockwise, the caster is positive. Turning clockwise raises the lever arm, so the top of the gauge was more tilted toward you the second time after the bubble was centered. In other words, if the top of the wheel is tilted more toward you the second time than the first, you have positive caster. This is consistent with the coin analogy. The tool came with a handy little sticker that you could place on top of the black knob with marks indicating eighths of a degree and an arrow indicating 0 degrees, but I don’t have that any more so I just use a marker. I seem to get better accuracy by just using two framing squares and measuring carefully. Somebody else might disagree and prefer using a bubble tool, but you have to admit using framing tools is cheaper. Also, if you use a bubble tool, you need your flat surface to be level, but if you use framing tools you don’t need the flat surface to be level as long as you take vertical to mean perpendicular to the surface. The method to measure caster: Apology in advance: The method is really very simple, but as you know, describing something technical can make things look more complicated than they really are. For example, suppose you had to write a detailed description on how to turn on a computer, write a Word Document, save the document, and write the inside address on an envelope. It’s easy to show somebody how to do it, but imagine describing all the necessary keystrokes in a message like this. Take an 8 ½ by 11 sheet of paper, fold in half so you have a 4 ¼ by 11 rectangle, measure up and down 4 inches from the crease at the right edge, and then draw two diagonals so you have two right triangles with sides 11 and 4 inches. The smaller angles will be very close to 20 degrees. I used a yellow sheet of paper for the photo because I thought it would show better. I actually use the cardboard back of a pad of paper to avoid wrinkling while I use it.
Put the trike on a level surface. Aim the wheels straight ahead. Now turn the front of the wheel in 20 degrees, aligning the wheel with the hypotenuse of your right triangle. Hold a straight edge horizontally along the wheel and sight down and set the wheel so the straight edge is parallel to the hypotenuse of the 20 degree triangle. It is easy to see if two lines near each other are parallel. Now bring a right triangle or framing square and make the “sharp” vertical edge flush with the wheel. Depending on the tilt, either the corner or end of the base edge should be on the flat surface. Suppose for now that the wheel is tilted with the top in. I want to use this angle later, so I slide a shim such as a small piece of wood until it supports the framing square as it is flush against the wheel. I mark where the shim is. If you wiggle the top of the nearly vertical edge from side to side a little, you’ll notice that the location where the shim goes changes, so you want to make sure the nearly vertical side of the square is perpendicular to the table. I use the second framing square for this. That explains the second framing square with the vertical edge against side of the nearly vertical edge of the first framing square. I want to come in at a 20 degree angle, so I put a piece of paper along the hypotenuse of the triangle and keep the square parallel to an edge of the paper. The angles appear a little distorted in the picture, but the edge of the framing square coming out at you is parallel to the piece of paper at 20 degrees. The little scrap of wood painted white on top is my shim.
I experiment a little until I am convinced I have the angle of tilt stored in my framing square with the shim and the location is marked correctly. Now straighten and then turn the wheel out 20 degrees. Suppose again the top is tilted in. I’ll discuss the other case shortly. I repeat the process and mark the location of the shim, which will probably be different. In this example, the caster being measured is small and the camber is negative, large in size, so the wheel is tilted in both times. When that happens, it is a little trickier to measure. It is related to the subtle fact that if you have two numbers close to each other and only accurate to a few digits, say with the first digit or two are the same, you lose significant digits when you subtract. I’ll use some made-up numbers to illustrate the computation. Suppose at the first measurement the inside edge of the shim was 7 and 3/16 inches from the corner and the second measurement gave a mark at 7 ¾ inches. Consider two right triangles. The first has a height of 3/8 and hypotenuse of 7 3/16 and the second has a height of 3/8 and hypotenuse of 7 ¾. The first angle has sine 3/8 divided by 7 3/16 so I want the inverse sine of that quotient, which is 2.99 degrees. For the second angle you get 2.77 degrees. Again, assuming the wheel was tilted the same way both times (turned in and out 20 degrees) take the difference. Multiplying by 1.5 you get about 0.3 degrees caster. An alternative, probably more accurate, would be to place the first square with the shim in the original position and align the corners, and you will see the difference between the two angles and you could measure that angle. Besides, this way you can actually see the difference between the two angles. If you couldn’t align the corners, you could measure with the wheel first turned out and then in. Then I hold a caliper-ruler against the two edges and slide until each edge just touches. It’s hard to tell sometimes, so I tilt everything and let the caliper slide until I feel it touch both edges, and do it a few times until I’m sure I have the location right. That little sticky-outy piece (sorry to throw such technical jargon at you) is 1/16 of an inch, so I add that to my reading on the caliper-ruler and if I had the ruler set for 1/8 inches for example, the distance is 3/16. In other words, I measure with the inside surface of the calipers at the top and the outside at the bottom, and compensate for that.
I see where the two pieces have their edges that distance apart. For example, again using made-up numbers, suppose you get the distance 3/16 apart at 20 inches out. The angle whose tangent is 3/16 divided by 20 is 0.537 degrees. Multiply by 1.5 and it rounds to about 0.8. So in this made-up example, the caster would be about 0.8 degrees. You might prefer to skip using a shim and measure both times with one square against the wheel and the other purely vertical (in the sense it is perpendicular to the table), and then see how many degrees you traveled by turning in and then out. This is also how I would measure if I got the top of the wheel tilted in when turned one way 20 degrees and tilted out when turned the other way 20 degrees. In that case you would add the angles, always taken as positive numbers, and multiply by 1.5. When measuring the angle with the vertical, if the wheel is tilted toward you and you prefer to measure with the base of the framing square on the surface, you can measure on the other side of the wheel to get the angle with the vertical. Now it is time to decide if the caster is positive or negative. If you have a lot of caster, it will probably be obvious by looking at the kingpins. If the top of the wheel was tilted more toward you with the wheel turned out than when turned in, then you have positive caster. To see this, think of the coin analogy or the bubble tool. I would not believe any measurements unless I could repeat them, or to put it another way, I would take the measurements various times on different surfaces, look at all my numbers to try to determine how many digits of accuracy I think I have. If you wanted to do just one measurement, not recommended, you could remove the tires, all three to avoid tilting the trike, true the front wheels, wiggle the front wheels to see if you have loose wheel bearings or suspension affecting your readings, and then take your measurement. I’d rather take several readings, not expecting exactly the same numbers each time due to variations in the wheels, tires, surface, errors in taking the measurements, etc.