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Tyre size irregularities


BiKenG

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This is based on observations on my VFR1200, but really apply to any bike.

 

My VFR came with Michelin Pilot Road 2 tyres. I have no complaints, but will need to cover a 2,500 mls trip this summer and as they may not last, I bought some Bridgestone S21s to try. Not only that, but I wanted to try a 200 tyre to see what difference it made. On receipt of the new tyres, the appearance of their size made me do some checking. The new Bridgestone 200 width tyre, sitting beside the bike looked narrower than the Michelin 190 still on the bike. Surely this can't be. Must be an optical illusion.

 

I'm fully aware that tyre sizes can vary and there obviously needs to be some latitude as rubber is not a precision material. But I was surprised by what I found. I didn't use any supper accurate measuring device for this, just a wide caliper that could be set to the overall width and then compared to a ruler. Accurate enough.

 

The 190/55x17 Michelin Pilot Road 2 on the bike = 198 mm width.

The 200/55x17 Bridgestone S21 bare, not on a wheel = 192 width (hence my concern)

The 200/55x17 Bridgestone S21 on the wheel = 196 width

 

So, fitting to a wheel and pumping up the tyre caused it grow in width by 4 mm. But even so, the new Bridgestone 200 width tyre is NARROWER than the Michelin 190 tyre. This seems nonsense to me. Yes we allow some tolerance, but for the smaller size to end up larger than the supposedly larger size makes a mockery of the entire tyre size standard.

 

The front S21 bare is 117 and not fitted yet. But the Michelin front on the wheel =124 mm. So even if the front Bridgestone grew by the same 4mm as the rear (impossible with the much narrower front), it would still end up significantly narrower than the same supposed size Michelin.

 

So...

Michelin Pilot Road 2 tyres seem to be very wide and significantly wider than their quoted size, whereas

Bridgestone S21 tyres are significantly NARROWER than their quoted size.

 

I will say though that the Bridgestone 200 S21 is definitely taller (as it should be) than the Michelin which although also a 55 section, has a noticeably flatter profile. With the BS 200 in the bike, I had to adjust the position of the hugger to stop it rubbing on the tyre. No clearance issues with the swingarm itself of course.

 

Finally then, although you may be aware of a variation in actual tyre sizes, you probably are not aware just how much variation there can be.

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21 hours ago, BiKenG said:

So, fitting to a wheel and pumping up the tyre caused it grow in width by 4 mm. But even so, the new Bridgestone 200 width tyre is NARROWER than the Michelin 190 tyre. This seems nonsense to me. Yes we allow some tolerance, but for the smaller size to end up larger than the supposedly larger size makes a mockery of the entire tyre size standard.

 

Yes, but if it had been advertised as a 190/55x17 you wouldn't have bought it!  Job done, Bridgestone "big-azz" tyre pushers... :wink:

 

Ciao,

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True.  Tyre manufacturers usually give a range of rim widths their tyres are designed to fit, but only one of those rim widths will have been used to establish the "official" measurements. 

 

I should have mentioned that I had encountered this exact sort of issue way back when I was fitting a Rick Oliver VFR750 wheel kit to my NC30, and discovered that actual tyre widths varied much more than one would have expected given the supposed meaning of the tyre numbers (which is pretty clear: section width, aspect ratio, speed rating, rim diameter).  Bridgestones were often the narrowest in a given size, and Dunlops were often the widest.  I even made a web page about this.  But even my very limited data at the time showed that a Dunlop D207 (remember those?) was actually 1mm wider than a 10mm "larger" Metzeler MEZ-2 mounted on a 0.5" wider rim!  (In other words, the Dunlop 160/70-ZR17 I measures was actually 1mm wider than the Metzeler 170/60-ZR17 I measured.)

 

Ciao,

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I did actually fit a 160/70 D207 to my '94 back in 1999 or so, and it turned in a bit better than with the 170/70 D207 (I was pretty used to how the 170 felt).

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