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If you don't have brake steer with the 349's, what sort of brake steering response do you have with the 406's? Do the wheels tend to twist when you apply one brake?

I assume that a 355 is 6mm larger than a 349, and thus no where near 18 inches unless you have big apples. but I've never seen them side by side, so how big is the rim size difference? And in your case with the tires on both what is the total difference?

I'm very curious where the center line of the kingpin points relative to the centerline of the 20 inch wheel. Does it intersect the ground at the centerline, or well outside of it?

Sorry no solutions, I just want to learn more.

Thanks Bruce,

My question about 355's was motivated by the goal of putting the correct sized wheels back on my trike, I thought that 349's and 355's were closer in size, and that I could just get 349's from catrike since 355's don't come on their current models. That and I was thinking of scorchers. If the scorcher's don't bring the axel up to 355 levels, I'd better look for 355's.

Yes, If the kingpin axis contact point is at or near the centerline for 406 wheels, it should be well inside the tire contact on 349's and I would expect a strong tendency for steering to twist and pull to the side you brake on. Based on my experience with shimmy, I thought the kingpin might point further outside on the expedition 406, and thus the 349 brings it inward, but not so much as to cause much leverage around the axis when braking. But in that case I think twisting while braking with the 406 would occur. ** just reread your first post and you say that shimmy and brake steer returned when you used 18 inch wheels. I'm not sure how you define brake steer in this case, but you imply that it returned to a condition that you also experience with the 406 wheels.**

But you did say there was a big improvement in steering in general with the smaller wheels.

If you willl permit me to speculate a moment, from your report and my prejudice I assumed that the expedition has NSR steering and the kingpins pointed a bit too far to the outside of the wheels. If this is the case, one characteristic of this geometry that I have not seen discussed and that takes a lot of imagination to visualize, is that correcting forces due to trail in turns will be different for each wheel. In pure center point steering, the contact patch of the tire is directly behind the axis intercept point on the ground. When you turn the wheel, the axis ground intercept is rotated to the side the same distance for each wheel, and a centering force vector develops between the angle of the wheel and the direction of travel. It is the same for each wheel.

But in NSR where both kingpin axies point forward and to the outside, a right turn will move the right kingpin axis further to the right away from the velocity vector thru the tire patch, while the left side kingpin axis will move toward the centerline of the trike and closer to the line of the forward motion of the trike thru the center of the left tire patch. A little hard to visualize, but this must put different correcting forces on each wheel. What I visualize is the left axis point dragging a scrubbing tire along behind it, while the right wheel has a very strong centering force on it. Net effect may not be a lot different, but this can't feel as good as centerpoint steering. If the 349's did bring the kingpin intercepts closer to center point, that may help explain better steering response.

Additionally, and perhaps a larger contributer, smaller wheels will reduce the castor and trail, which reduces the centering force on the wheels in turns. This may be felt as better steering, or at least a lighter touch.

Very interesting, I hope you guys get to the bottom of it with a clean solution.

Michel and I plan to make a day of it exploring bike trails she has never been on. We are hoping the remaining particulate matter in the air disperses more over the next two days, because the areas we plan to ride border on the Santiago fire area of Orange County Ca. If not, we can always ride up the beach.

Thanks,

Jim