Worn Tires

[SliderWV]

I've noticed over the years that when my tires are really wearing down and getting flat across the bottom the bike starts to handle in a totally different way, particularly at slow speeds.

 

What I notice is that the bike seems to steer all by itself when you lean the slightest bit. At slow speeds I feel it most - the wheel wrenches the bars in the direction of the turn. At higher speeds it seems the bike wants to remain more upright and if I am lazy with my body I have to hold bar pressure throughout the turn.

 

Why exactly is this? What is happening, in the sense of physics?

[PorradaVFR]

Worn tires have a different profile, different grip characteristics and (if not properly checked) may well have lower pressures to boot. Each time I put on new rubber my bike feels substantially different. My personal take is that once my tires create doubt in my mind as to their predictability I change them. Never quite understood the heroes that ride to the belts or until there is NO tread left....why risk thousands worth of bike and injuries over a couple hundred worth of tire?

[SliderWV]

Certainly the change in profile is responsible for the change. But more specifically why does the flattened profile cause the wheel to turn in with the slightest bit of lean, whereas when the tire is round you feel the tendency for it to countersteer? Seems it would work the other way around but it doesn't.

[racer]

Hey slider,

 

It took me a while to sort out that you meant different things by "steer" and "lean".

 

The short answer to your question is that the flatter the tire profile becomes, the more like a car tire it becomes, the more it needs to turn to track at shallow lean angles.

 

Until it leans past the sharp transition to the more rounded and less worn portion of the tread. That transition can be a bit sudden and a surprise as the reason it exists is because the rider does not go beyond that point under normal circumstances.

 

My own perception is that the motorcycle becomes more difficult to steer or lean over with worn tires. It seems to require more force applied at the handlebar.

 

Consider that the reason a motorcycle is able to track a curve with two wheels is that the front and rear wheels are askew and slightly out of line with each other while occupying two discrete and different points along a curved line.

 

Imagine that the front tire has a completely square profile like a car tire. If forced to lean, it will deform and ride a narrower and narrower part of the tread until it reaches the critical point and centers on the corner or edge between the sidewall and tread. Something similar happens on a worn motorcycle tire between the more worn and less worn portions of tread. Effectively, there is a comparative ridge worn into the tire that resists leaning and then leans really fast if you cross it. Combine more steering input force with a sudden reduction in force required and less turn angle required and you are in for a whoa whoa as the bike leans in suddenly before you can reduce pressure on the bar.

 

Anyway, I'll try to edit this for clarity later; but, I have posted about this particular subject in depth and detail last year. You might try a search in the meantime.

 

Cheers