You're absolutely right! Flex is not a bad thing depending on what kind of riding you do. But, I will try to explain how this kind of flex can get you hurt. I hope I can get my point over whithout putting you to sleep. There's a lot of background behind it.
Back when radial tires first came out we had a bad rash of accidents because people were mixing radials and bias tires. One of the features of radials was that they gave a much smoother ride. They did this by their sidewalls giving more and allowing the tread to move around and conform to the road better. Bias ply tires had very stiff sidewalls and didn't give much, so they gave rougher rides. When bias plies were used up front, they reacted quickly to steering inputs, but the radials in the rear were slow to follow. When they did finally move, they moved even more than the bias plies up front. This had the effect of feeding in some more steering than was wanted, causing cars to spin out before the driver could react. Radials required the suspension to be altered to take full advantage of their characteristics, and this went against what the bias ply tires needed. Remember the term "radial-tuned" suspension?
If you will remember the old Uniroyal Tiger Paw commercials, they showed a car going around a mountain curve, with the tires' treads still hugging the road, but the car hanging over the cliff, stretching the sidewalls to do so. Imagine if the front tires were bias plies and didn't allow so much side play as those radials. The rear end swinging out farther than the fronts would add some steering to the situation. Bad combination of tire performances!
Now, consider a banzai run down a mountain road on your 700 at maybe 50+mph. You KNOW that rear wheel will be doing some flexing, and so will the frame. The big problem here is knowing just when will the wheel reach it's limit of flex, and when will the frame? Also, which one will reach it's limit first (we really don't care about that!)? At first they both start flexing together, but one will reach it's limit first, and the rate of flexible movement will change abruptly at that point. Also at that point you are leaning pretty hard against the whole works, hoping that things will continue to hold to the road. When you reach the limits of flex of the wheel or the frame, and you are still asking more from them, it will tend to "kick" the rear end out UNEXPECTEDLY, unless you are extremely tuned into your trike from miles of practicing this type of extreme riding. I would call this a sort of whiplash effect. This is just one dangerous aspect of uneven, unpredictable flexing in the rear end alone.
Now, consider the front end and how it reacts to hard cornering at high speeds. With it's smaller 16" wheels and stiffer framing connecting those wheels, the steering input reaction time is almost nil. So, just like when combining bias ply tires up front and radials in back on a car, there is a delay from the time the front wheels turn and when the rear reacts after going through it's flexing motions. Also, by flexing, it allows the rear end to swing out just a bit, but that little bit adds a bit more turn that was unexpected. If you are doing 50mph down that mountain and there is a car in your lane that you have to avoid, just how quickly do you think you can make that wet noodle trike maneuver around the car? Not me! Give me a Speed anyday!
OK, I know that most folks don't ride like that MOST OF THE TIME. But, we do read about that once in a while, and it's nice to know what you can expect in a trike's performance.
Now we get to the bottom line! WAKE UP!!!!!!
If we can eliminate one of the sources of flexing in the rear end, or seriously limit it, we can get closer to knowing what to expect in a hard, fast corner.
Race cars demand as much stiffness in their frames as possible to make their suspensions do their jobs. With a mushy frame, the suspension does the work sometimes and the frame does it the rest of the time, but this is unpredictable. By removing any frame movement from the equation in a race car, the suspension will work as designed and with predictable and tuneable results. When I first designed my latest dune buggy I made the triangulation braces too long, allowing too much flex and twist in the rear end. My coil-shocks couldn't do their job consistently and reliably in hard turns. The frame would flex, letting the coil-shocks "load up", then they would all of a sudden unload and plant my tire into the ground hard, launching the front end into the air! It's this delay reaction that makes things so unexpected and dangerous. Our trikes need this stiffness as much as possible, too, for the best handling.
I don't mean to dismiss your riding skills, but I will ask, did you try your Speed in that same slalom course as the 700, under the same condidtions? I would bet it would have kicked the 700's butt! Try this; put the rear wheel off of Your Speed onto the 700 and see how well the 700 handles a hard, fast turn!!!
I've ridden and followed and watched both the 700 and the Expo, and I know that they both have severe rear end flex that affects their steering response times. I know that they both can be modified to improve this condition a LOT! Running your rear wheel spoke tension up to the upper limits of the rim will help a lot.
One last thought about eliminating the flex is that ANY flex, anywhere in the trike, will rob you of your pedaling efficiency. This is why it will help in climbing hills, especially.
If your riding style and requirements don't SEEM to be impaired by all of this, then ride on! But, if you want the ultimate in handling, this flexing HAS to be removed to make the trike more responsive. Machining chromoly axle tubes to replace the aluminum ones on Holey Spokes made a very drastic difference in steering response. It eliminated a calculated 57% of the normal flex that aluminum axles allow. They also allowed me to set my brakes a bit tighter without incurring any drag in corners.
My motto is, the more weight and flex you can eliminate from your trike, the better it will handle and the more you will like it!
