Could my XJ900’s tyres be causing a shimmy?

Q. I ride a 1993 Yamaha XJ900 with about 24k miles on the clock. I had new Avon tyres fitted at the start of the year and have covered about 2500 miles since.

During the past month I have noticed a shake through the forks at certain speeds. It appears mainly between 40-50mph and seems to smooth out above that speed. I first noticed it when decelerating gently.

Recently it has been noticeable when accelerating on a light throttle.  It is not severe and does not affect cornering stability – yet. I have not skidded or worn flats on the tyres and the wheels appear to run true.

Could the wheels require rebalancing now the tyres have worn? Could the tyres themselves be wearing out of true? Could the head bearings have worn and allowed some vibration?

The forks themselves were rebuilt with new oil and seals at the same time as the tyres were fitted. I would really appreciate your advice on a cure.

John Kennett, Wrexham
 
A. It sounds like you have something called a low-speed shimmy – a left/right vibration, or a headshake that occurs when decelerating.

A low speed shimmy (there’s also something known as high-speed shimmy) usually appears when decelerating between 50–30mph.

It can appear when riding in straight lines and corners. Frequency (how fast it vibrates) is about 5-10 Hz (cycles per second) and the amplitude (how big the shake is) can vary but can get worse if you don’t regain control of the handlebars.

It also manifests easier when the tyre is cold. It can also be more evident with a heavier load. Not all bikes suffer from shimmy and some bikes only suffer shimmy when fitted with certain types of tyre.

Depending on the bike/tyre combination it can manifest naturally, which can be seen when you simply let go of the handlebars and let the bike coast down through 50–30mph, or it can sometimes be induced by letting go of the handlebars at the same speed and striking one of the grips from behind.

Tyre related contributing factors can include inflation pressures, how worn the tyres are, tread pattern design, compound characteristics and casing design and stiffness. Mechanical related contributing factors are tight or worn steering bearings.