Square is out!

Old Bike Australasia - - LETTERS TO THE EDITOR -

In July my wife Diana and I set off on the Ve­lo­cette “Slow Ride” from Cairns, Queens­land to Ade­laide via Dar­win, Broome, Perth – an am­bi­tious 7,000 mile two-up tour on our 1969 Venom Club­man. We were ac­com­pa­nied by Stu­art Hooper and his wife Marsha on a 1956 MSS and an­other friend, Rob de Jar­lais on his 1986 BMW R80GS. For the first 4,000 miles from the Pa­cific Ocean waters of trop­i­cal Cairns to the In­dian Ocean at Derby Western Aus­tralia, our Velo per­formed ad­mirably, need­ing only rou­tine checks. In Dar­win I fit­ted a new rear tyre, an Avon SM Mk2 of tra­di­tional square sec­tion that I would usu­ally shun, but it was all I could get in 19 inch di­am­e­ter and it seemed well suited to the long straight high­ways of the Aus­tralian out­back. Three weeks later on a long stretch of bi­tu­men spray-sealed high­way about 150 miles south of Broome we suf­fered a rear tyre blowout. With­out so much as a warn­ing wob­ble we were both ejected from the sad­dle to the left and onto the road at 90km/h, while the Velo slid on its left side across the path of on­com­ing traf­fic (for­tu­nately none at the time) and onto the far shoul­der. So why no warn­ing wob­ble or op­por­tu­nity for the rider to wash off speed and maybe guide us to a safe stand­still, as one may ex­pect after a rear tyre blowout? There was a straight skid mark on the road of less than 5m length. At our 90km/h cruis­ing speed that lasted for about 1/4 sec­ond after the blowout. Why did it lock the rear wheel? Be­cause the tyre de­cided to par­al­lel­o­gram to the right and after that sec­tion of flat­tened tyre com­pleted 3/4 of a revo­lu­tion it jammed against the in­ner edge of the right hand swing­ing arm. At this mo­ment, since the skid­ding rear wheel had a con­tact patch con­sist­ing of thick tread plus side­wall on the right and no tyre on the left this caused the bike to lay down on its left side, in­stantly eject­ing its pas­sen­gers. Bang, skid, crash down on the left, all in less than half a sec­ond. I never again want to ex­pe­ri­ence a life threat­en­ing mo­ment like that. Un­for­tu­nately tyre per­for­mance re­views fo­cus on pri­mary safety with lit­tle re­gard for sec­ondary safety; ie, how does the tyre per­form if it suf­fers a blowout at speed? I spoke at length to a rep­re­sen­ta­tive at Avon Tyres tech­ni­cal depart­ment who ad­vised that the only time they see a tyre at fail­ure in their test bed is when de­vel­op­ing a new tyre, in which case the tyre may be run to fail­ure. While he ac­knowl­edged that the be­hav­iour of the SM Mk2 in our case was un­for­tu­nate, caus­ing an in­stant crash, he ar­gued that it may not be­have like that in all tyre blowout sit­u­a­tions. Per­son­ally, I doubt that one of these tyres would ever pan­cake flat giv­ing some chance of crash avoid­ance after a blowout. In an un­be­liev­able co­in­ci­dence Stu­art’s MSS suf­fered a rear blow-out with a Pirelli Scorpion tyre, fit­ted new in Broome the week be­fore. This hap­pened close to the site of our blow-out, but 2 days later. This mod­ern, oval sec­tion tyre pan­caked flat and al­lowed a safe re­cov­ery. Stu­art’s use of 18” rear rim pro­vides a broader se­lec­tion of tyres, a wise and for­tu­nate choice in this case. And no doubt Stu­art’s skill in pi­lot­ing the World’s Fastest Ve­lo­cette to 193 mph on the salt flats means his re­ac­tions are sharp. There is also ev­i­dence that the ag­gre­gate used when they sealed this 600km stretch of high­way gen­er­ates an ab­nor­mal level of heat in tyres – an­other in­gre­di­ent of our recipe for disas­ter. Like all ac­ci­dent in­ves­ti­ga­tions it turns out there were 2 or 3 con­tribut­ing fac­tors. Re­move any one of them and the ac­ci­dent would not have oc­curred. For my part, I will never again use an old style square sec­tion rear tyre such as an Avon SM Mk2 on any mo­tor­cy­cle. I have also told my fam­ily that I would like a mo­tor­cy­cle TPMS kit (Tyre Pres­sure Mon­i­tor­ing Sys­tem) for Christ­mas. I was amazed to find there are a lot of brands avail­able (Garmin, Tyre­dog etc) – an in­ex­pen­sive piece of tech­nol­ogy to pro­vide peace of mind when trav­el­ling on a heav­ily-laden mo­tor­cy­cle. The point of the story is that the shape of tyre and its post blowout be­hav­ior needs se­ri­ous con­sid­er­a­tion if used on a solo mo­tor­cy­cle. The Avon tyre didn’t fail, the tube did, prob­a­bly due to heat rise and car­cass/tube fric­tion on this stretch of high­way. It’s what the tyre did in the next half a sec­ond that I want rid­ers to be aware of. Oval sec­tion rear tyres are far more likely to pan­cake flat and give the rider a fight­ing chance of crash avoid­ance.

John Jennings Aus­tralian VOC National Sec­re­tary, Queens­land.

With­out wish­ing to get in­volved in a tech­ni­cal dis­cus­sion, it would ap­pear that a tyre orig­i­nally de­signed half a cen­tury ago, fit­ted to a heav­ily laden mo­tor­cy­cle and rid­den on an abra­sive sur­face in high tem­per­a­tures pre­sents a harsh set of cir­cum­stances. (After this get-off, it sounds like you’ll be need­ing a new set of rid­ing gear John. There’s a pair of Drag­gin’ Jeans ready to go to you.) - Ed

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