The Inaccuracy Of Tyre Labelling/ The Birth Of The 21″ Wheel

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The term ’20 inch bike’ gets thrown around a lot, usually by douchebag pseudo-journalists like me trying to contextualise their gonzo style drivel about dudes riding or making little 20 inch kids stunt bikes… see, i’m at it again. Cruising on the interwebiverse one day I came across this photo on 22 inch BMX‘s Facebook/Instagram/whatever of Kye Forte’s ride;

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The striking thing about it is the addition of 22 inch forks and front wheel whilst the rear wheel remains 20 inch… yet, somehow, it doesn’t look too odd at all.  While one could argue all day about the benefits or hindrances this set up might provide(a big one being head angle, but I’ll get to that shortly) one thought awoke and wouldn’t slumber; what even makes it a 22 inch wheel? What, for that matter, makes a 20 inch wheel a 20 inch wheel?! I got my tape measure out and measured the outside diameter of my rim; 17″… so it would make sense that the ’20 inch’ label is relative to the tyre, yet when I measured it, for some reason or another, the inflated tyre didn’t measure 20 inches either but closer to 21 inches.  I vaguely remember doing this (odd) exercise when I was younger (as a tweeky kid does when he’s done playing San Andreas…) and it was slightly UNDER 20 inches, this was back when a 2.2″ wide was considered the limit for frame/fork clearance by the way.

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So what changed?  With the emergence of wider tyres becoming the popular choice, the extra material involved in it and the way that a tyre will always protrude to the shape of the tube inside it, a tyre’s diameter will increase with the width.  Not only will the diameter change with the width but the width AND diameter will change dependent on the rim you are using! So the truth is that there is no possible way to tell how a tyre will turn out until you install and inflate it.  This is not a problem confined to consumers/riders but manufacturers too.  It’s common place for a company to make a ‘2.4 inch’ tyre only for it to end up as 2.5″ wide, yet the exact same model tyre in a ‘2.3 inches’ size could end up as small as 2.2″.  It’s a major headache seeing as a tyre mould in one size can cost a company multiple thousands a pop.  The European tyre labelling standard (ISO5775) only requires manufacturers to give approximate measurements regarding width and diameter which keeps things nice and simple for manufacturers and shops alike but not so much for pedantic bike nerds like us.

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To give a few examples; Cult Dehart Slicks, Eclat Fireballs, Volume Vaders,  Demolition Big Rig, S&M Speed Balls, Fly Ruben Ramperas, and Demolition Momentums all have inflated diameters of a full 21 inches in their respective largest sizes and there are plenty more that fall a mere fraction below that according to the measurements on DansComp (baring in mind there’s no word on what rim they inflated/measured them on so this is subject to change) The Fit T/A and Duo High Street are bigger still at 21.2 inches in diameter at their peak size options.  On the opposite side of the spectrum, a 1.95 inch version of the Animal GLH tyre measures at only 19.96 inches in diameter.animal-glh-tyre-295-p1

BMX’s are highly sensitive to the smallest changes you make to them, this is why frames come in top tube sizes of half or even quarter inch differences; a quarter inch here and there feels like a mile and makes an astronomical difference. Even a pair of new pedals with thicker bodies than the last could shift your entire stance up a few millimetres making your bike feel a little twitchier.  Head angle is no different.  When Kye put on that 22 inch front end he slackened his frame’s existing head angle by about 1.6 degrees (according to this article by Mr French) and theoretically made it a tonne stabler at higher speeds but you could slacken or tighten your head angle by an entire degree (that’s a HUGE difference by the way) by merely choosing the right (or wrong…) tyres.  *Edit* BB height would also be affected, but not quite at the same rate as the head angle.

In conclusion I’d say if you have a set of ‘old faithful’s you use; stick to them because a different model with the same measurements as the old ones could still end up feeling totally different.  Yet if you feel you could do with a slacker head angle, go ahead and put a big fat basketball-esq tyre on up front, or on the rear if you wanna get your front wheel game on a bit.  But I think the main lesson in all of this is not to stick your nose up too high at the 22″ BMX’s because in reality, you’re probably closer to riding one than you think…

 

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