Sprite 500, rear tyre size

[Sprite_500]

Hi all

The rear tyre on my Sprite 500 (Triumph engine) rubs the chain. I'll need to check alignment, the tyre is 120 90 18, does this size sound right?

[80-85 husky]

close to 5 inches (said the barmaid to the bishop)...., may be a tad big for the era?

[FourstrokeForever]

A 120/80/18 section tyre is what I run on my B50 and is the only tyre size/make I have found that fits. It's hard to find a modern tyre that fits in the early frames without chain interference. Different manufacturers have different size interpretations. Mine is a Michellin. You can also cut the knobs on chain side for clearance. Something I hated doing to a new tyre but it needed to be done

[tony27]

A 120/80/18 section tyre is what I run on my B50 and is the only tyre size/make I have found that fits. It's hard to find a modern tyre that fits in the early frames without chain interference. Different manufacturers have different size interpretations. Mine is a Michellin. You can also cut the knobs on chain side for clearance. Something I hated doing to a new tyre but it needed to be done

Michelin measure across the knobs from memory so gives a truer indication of size & the number is a higher number than the same size from most manufacturers who I think measure across the case eg, Michelin 140/80 is the equivalent of a 120/100 for others, think a 120/90 is most others 100 or 110/100
Hope that makes sense

[Sprite_500]

Thanks for the excellent and quick replies.

The pre-65 rule book (UK) states:

26.5 Any motocross tyre may be fitted max 4.5" tread width.

so it is likely that my rear tyre is larger than original. I've checked the alignment of the rear wheel, it sits at an angle to the swinging arm, pointing slightly away from the line of the chain. So if I align the rear wheel correctly to the swinging arm then it will increase the likelihood of the tyre hitting the chain. But perhaps the swinging arm is distorted?! I'll need to spend a bit of time with tight lines and a tape measure.

The tyre is a Michelin and it measures between 122mm and 123mm at the widest point, although it is pumped up to 25PSI, which is more than I used to run many years ago. I'll drop it 15 PSI and recheck.

[tony27]

120 section will be more than your allowed 4.5 inch width, it won't change much when you let some air out

[Sprite_500]

You're 100% right Tony, no discernible reduction in width. It appears that most Brit pre65 events disregard the max width limit of 4.5". I might have a go at modifying the swinging arm by 10mm to give some clearance. I've found some trail bike tyres that fit, but they'll be hopeless on a muddy MX track.