Quote Originally Posted by aaronng View Post
I believe that tramlining is because of the suspension setup. Not enough castor and almost zero toe at the front makes the car follow any groove on the road.
My car doesn't tramline with 205/55R16 tyres on the stock 55mm offset 16" rims but it does with the 225/45R17 tyres on the ten spoke 17" Honda Luxury rims that also have 55mm offset.

I had previously argued that there must be a difference in the suspension geometry of the CL9 Luxury model with 17" rims compared to the CL9 Standard with the 16" rims. Both have the same rim offset but Honda dealers aren't supposed to sell the 17" wheels to people with the Standard model. The difference might be more positive castor as toe is the same. But is it so?

What's causing the problem on my car when I swap between the 205/55R16 tyres and 16" rims and the 225/45R17 and 17" rims? The offset is the same. I have not changed the castor or the toe or any of the other suspension geometries when I do the swap. The tyre diameters are virtually the same but the only difference is essentially the tyre width and therefore the width of the contact patch.

I remember studying coefficients of friction in my 1st year uni physics lectures (that was very hard considering how long ago it was). Having given it some (more) thought, I have concluded that a tyre with a contact patch where the width is near or more than the length of the contact patch is more likely to tramline.

Imagine the side of a wide tyre encountering a grippier surface: The tyre would suddenly engage the higher grip surface on that side and turn the wheel towards that side. As the contact patch is shorter than the width, the "front" of the tyre tread contact patch has less influence on what the rest of the tyre is doing and so allows the tyre to deflect sideways.

What happens when a tyre that has a longer contact patch than it is wide encounters the same higher grip surface on one side? Well, in my opinion, because the area of the contact patch further forward of the lateral centreline is greater than the area of the tyre to side of the centreline of the tyre, the tyre is more likely to just keep tracking straight.

Analogous to this is the polar moment of a car. A car with a higher polar moment (long wheelbase) resists turn-in more than a car with a low polar moment (short wheelbase). Sports cars tend to have short wheelbases so that they can turn-in sharper. GT (Gran Turismo) cars tend to have a longer wheelbase to aid high speed stability (resist turn-in). In this analogy, the long wheelbase car is like the larger diameter tyre with a longer contact patch. The tyre is better able to resist being deflected sideways.

I believe that Honda has countered the tramlining effect on the CU2 by dramatically increasing the diameter of the tyres to elongate the contact patch. The 18" tyre is of even larger diameter than the 17" tyre fitted to the CU2 Standard.