Square is out!
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