Some points to consider:
- The TT steers very fast at high speed; some might even call it oversteering. This takes some getting used to. But this is not an instability, in my opinion. It means, though, that you need to steer gently.
- I don't have much of a problem with bump steer, even when going quickly. Brake steer is always a problem, but I cannot see how this can be avoided with independent braking.
- There is a significant pedal steer that induces a noticeable wobble if 1) the trike is moving fast (> 35 mph) and 2)the cadence is high (> 100 rpm). However, what the left foot does, the right foot cancels, making the average effect a straight line. I have had a problem a couple of times when I deliver a strong push with one leg, and then stopped pedalling. In general, when the trike is in condition 1, I stop pedalling before I get to condition 2. (This wobble has been significantly reduced by replacing the Comet Primos with the larger Schwalbe Marathons.)
- I have a measuring procedure now that I think is the best I've come up with yet.
- There is one very important factor in making alignment measurements that I have never seen mentioned. Because the wheels have a non-zero caster, that is, the tops of the wheels are closer together that the bottoms, it is critical that the front and rear measurements be made at the same height from the ground.
- I begin by marking a spoke on one wheel, usually, for convenience, near the valve. Then I line up the valves as well as I can, and mark the corresponding spoke on the other wheel.
- Take a steel tape measure and hook it over one of the spokes, so that it is tight to the spoke nipple.
With the ruler I keep the spoke 1 foot above the ground. - Then I pull the tape to the other spoke. You will see from the picture that light reflected off the curved surface of the spoke creates a distinct silver line, which I use for taking the measure. There doesn't seem to be too much of a parallax problem. I raise the tape on this end a bit from the top of the nipple so that the silver line along the spokes is visible on the same edge of the ruler that has the marks.
It is simple to demonstrate at this point how important it is to make the measurements at the same height from the ground. Keeping the tape measure taut, roll forward and backward, and observe the change in the measurement. - I make the measurements while I am seated on the trike. I make a rear measure, then I roll the trike forward so that the marked spokes are at the front. After ascertaining that the spokes are the same distance from the ground as they were when I made the rear measure, I make a measure on the front. This is simple to do while seated.
editor's note: I don't know whether it fits this technique, but Giles from the trikes@ihpva.org mailing list suggests bungy cording across the back of the two wheels, to take up any play in the steering. If you were to do so using Timothy's technique presented here, it seems that you would consistently want to bungy the back of the wheels before each measurement (front or back).
- After I have the wheels parallel, I make further adjustments on just one tie rod at a time. In fact, I have found that if the nut is loosened on the end of the tie rod that is closer to the wheel, the tie rod and the second nut can be turned together with a wrench, so that it is not necessary to loosen both nuts. I figure that every loose nut is an opportunity for something to move inadvertently when I tighten up.
- Be sure when you make the measurement that the tape is perfectly straight: not twisted or bent over a cable or other obstruction. I have found that a position 10-12 inches above the ground gives a straight, unobstructed path between the wheels.
- This technique is a good way to make accurate measurements of the wheel alignment. However, it does not answer the question of what toe-in or toe-out will suit your weight, seat position, and tires. I weigh ~185 lbs, and my seat pin goes through the most rearward hole in the seat, and the second to the most rearward hole in the frame. I found that a toe-in of ~1/16" worked well with the Comet Primo 20x1.35 tires that came with the TT; and a toe-out of ~1/32" works with the Schwalbe 20x1.5 Marathons. You will need to hunt for the alignment that is right for you. Start with the wheels perfectly parallel, and then, assuming the trike has the Primos, add toe-in by turning one or the other (but not both at the same time) of the adjustment rods by 1/4 of a flat or less at a time. It is necessary to get up to ~15mph before you can judge how well a particular alignment works.
- I hope this helps. I have spent a good deal of time agonizing over the handling, and wondering seriously whether it was possible to get stable handling at high speed. It is also the case that while I have been working on this problem, I have acquired a good deal of trike experience, which I did not have before; no doubt my experience in handling the trike has produced some improvement in my feelings about it. However, I think that the alignment is really the key factor.
- I'd be interested to know how you make out.