Why a high-speed wobble as rear tire wears out?

[Sunbeemer]

Anybody care to conjecture about the physics involved in a high speed wobble (not a real tank-slapper, but a definite instability) as my rear tire approaches the end of its useful life? My rear Mettzler Z6 (same brand on front) has ~ 7000 miles on it and is starting to show slight concavity in the center strip (but no threads showing yet). When I take it up to 90+mph the bike becomes a bit unstable, especially when cornering with a cross wind and under hard braking. It also started when my previous rear Pirelli Diablo Strada had ~ 6000 miles on it. When I swapped that out for the Z6, all instability disappeared! Anybody else notice this?
Could this be caused by using a high seat (since I first noticed this about the time I installed it), and why would that make a difference?

[Ves]

I had Z6's. Had a high speed wobble. More like a low speed oscillation at high speed. I replaced the front, which had cupped, and the wobble went away. At one point when ever I turned right I felt a vibration in the rear. I thought it was my bearing or something. I replaced the rear tire, which still had tread on it, and the problem went away. My guess is when a tire really wears out, the contact patch is no longer uniform, so it's sort of like riding over a grooved surfaceatc. This causes the tire to pull one way then the other, producing an instability and a wobble or vibration. A crosswind, or heavy braking just increase the stress on the contact patch, producing a more noticeable wandering. That's my theory, and I'm sticking with it...

[Hottech]

Thats pretty strange, my Z6's have about 6500 miles on them and I am starting to feel abit of fish-tailing. They look perfectly fine, plenty of tread but the rear center always looked sorry from day one but the outside tread looks like new. The front hasn't changed at all and I am also wondering if I should just change them out. I am not an aggressive rider but I am definitely feeling less bite in the curves.

[riceburner]

When new, a tyre has a perfectly circular cross section.

When worn, the cross-section is no longer circular - if you've been doing a lot of straight-line riding (highway/motorway), or don't corner hard, then the profile will have a (more or less) flat line across it.

When on the road this means that the contact patch you have is not the same shape as it was when the tyre was new.

So, irregularities of the road surface cause the tyre to be affected in different ways as it rolls over them.

The upshot is that the reaction forces from the ground (every action has an equal and opposite reaction), when the bike rolls over something push the bike in a slightly different direction, making the bike feel unstable.

don't ya just love physics.

[iowabeakster]

Hottech,

my 2 cents...

I bet the front is good for a few thousand more miles.

On the rear, 6500 is a good lifespan for a Z6. Some people get more, but mine square off before I get that many. I'd be looking for a new one, especially if you think it is starting to feel squirrelly. The Z6 has a tendency to "suddenly" expose the cords (right up the centerline) when owners think that they have plenty of tread left.



I don't want you to waste money...but the tires are not the area to try to stretch your dollars.

[Sunbeemer]

Thanks very much. I am very relieved that my bike is not unique in having this issue - you start wondering about headpost bearings and telever arm pivots, not to mention rear swingarm pivots - a place I don't want to have to go yet with only 27,000 miles. I'm beginning to appreciate the benefits of a well rounded tire! My rear is definately squared off (and so is the bike's tire ), and the bike is certainly giving more road feedback than I'd like, especially on grooved pavement where it develops a mind of its own . Guess it's time to replace the tire before the cords come shining through!

Thanks again!

[twindave]

I'm no crack expert by any means, but I have found that checking the wheel balance now and then helps. This is something I have done on every bike I have owned in my 30 years of bike ownership.

Most times it's good to go, but I have had to rebalanace the wheel on several occasions.

Keeping the wheel balanced will also help reduce mechanical wear.

my 2cts

[OU812]

When new, a tyre has a perfectly circular cross section.

When worn, the cross-section is no longer circular - if you've been doing a lot of straight-line riding (highway/motorway), or don't corner hard, then the profile will have a (more or less) flat line across it.

When on the road this means that the contact patch you have is not the same shape as it was when the tyre was new.

So, irregularities of the road surface cause the tyre to be affected in different ways as it rolls over them.

The upshot is that the reaction forces from the ground (every action has an equal and opposite reaction), when the bike rolls over something push the bike in a slightly different direction, making the bike feel unstable.

don't ya just love physics.

I concur.

[riceburner]

I'm no crack expert by any means, but I have found that checking the wheel balance now and then helps. This is something I have done on every bike I have owned in my 30 years of bike ownership.

Most times it's good to go, but I have had to rebalanace the wheel on several occasions.

Keeping the wheel balanced will also help reduce mechanical wear.

my 2cts

They should be balanced whenever you replace the tyre. To NOT do so means you're paying too much money....

[Sunbeemer]

I had both tires balanced when originally mounted, but would an unbalanced tire cause a high-speed wobble? I don't feel a cyclical thumping at any speed (unless I'm running at 4000 rpm...and that's just the motor since it goes away if I de-clutch and let the rpm's fall to idle).

[paddle007]

Just replaced my Z6 as it also squared off in the center. About three hundrd miles after squaring off the cords showed their ugly face. Made for a tense ride home. I bled about 10 psi out of my tire to relieve the load from the center of tire and limped home. Part of the learning curve I guess.

[twindave]

I'm not sure of all the physics involved. Like I said, I'm not an expert. I can only say from my experience that I have never had any wobbles or vibration anomalies from new or worn tires. I have the wheel/tire balanced when its new and again around the middle of the tires life. I use the Parnes balancer, and it has never let me down. I can let go of the bars it shoots straight and runs smooth on the streets and highways, even at high speeds.

[Hawk]

For me, 7,000 miles is all I hope for out of a Metzler tire; this was true when I rode Japanese bikes as well. I've heard the Michelin Road Pilots give better mileage, but this is second hand.

I do agree that tires are not the place to save money on a motorcycle.....except if you are changing your own tires and balancing them yourself; that's o.k.