Rub­ber wor­ries

Old Bike Australasia - - LETTERS TO THE EDITOR -

Com­ing back from the Ral­ly­man Rally on the trusty Comm-n-go pretty well com­pletely wore off all the tread on the rear Dun­lop TT100/K81. I’ve had this bike for 38 years and have done all the tyre changes my­self. I have the orig­i­nal 19” front rim and many years ago I changed to an 18” rim on the rear as the choice of good 19” rear tyres dried up. I’d set­tled on and was very happy with the orig­i­nal fit­ment K81 410 X 19 front and sim­i­lar K81 4-10 X 18 rear. Need­ing new hoops I dropped into Graeme Mor­ris Mo­tor­cy­cles in New­cas­tle where I’ve been buy­ing them for years for an­other pair. I started with the rear. Off with the old hoop. The new was a bit tight to get the first bead onto the rim, but I just dis­carded that as be­ing new and tight. Next, fit the tube. Try­ing to poke my fin­gers un­der the bead to stick the tyre valve through the hole in the rim proved im­pos­si­ble. I was re­ally do­ing some dam­age to the skin on the back of my fin­gers. The tyre walls were as stiff as. It wasn’t work­ing! Tore the tyre back off the rim and put it side by side with the tyre I’d taken off. De­spite them be­ing both marked as Dun­lop TT100 K81 4-10 X 18, they were dif­fer­ent. The new tyre is smaller in cross sec­tion. The tyre I took off was made in Ja­pan. The tyre I’d just bought was made in In­done­sia. Now, as long time buy­ers of these tyres, I’ve bought them Made in Eng­land, France, Spain or Por­tu­gal (I think) as well as Ja­pan, sev­eral dif­fer­ent coun­tries over the years, why should In­done­sia be any dif­fer­ent. I looked at the front tyres, also dif­fer­ent. I took some side­wall mea­sure­ments and the In­done­sian tyre is smaller in cross sec­tion. The side­walls on the In­done­sian tyres are shorter, mak­ing the over­all di­am­e­ter of the tyre less. The shorter side­wall di­men­sion on the In­done­sian tyre mak­ing it im­pos­si­ble to be able to stretch the beads apart enough to get the valve stem into the hole in the rim while do­ing bad things to the backs of my fin­gers. Not to men­tion the ap­pear­ance of the tyre when un­der the mud­guard looks like it should be fit­ted to a 125, not a 750. The next day I got onto the blower to Dun­lop. I was told that Dun­lop had been mak­ing tyres in

In­done­sia for about the past eight years. The per­son I spoke to knew there had been a change in the de­sign spec­i­fi­ca­tions/di­men­sions for the 18” tyre. This had come from JATMA; Ja­panese Au­to­mo­tive Tyre Man­u­fac­tur­ers As­so­ci­a­tion. The guy I spoke to said that JATMA had de­cided the de­sign size of the 4-10 series tyres were al­ways ‘over­sized’ and so they re-de­signed them and re-did the specs so they were now, in fact, smaller in cross sec­tion. So, am I hear­ing this right; the tyre size was al­ways over­size? Al­ways over­size, even ever since 1969 when Mal Up­hill rode his works Thrux­ton Bon­neville to victory with a fastest lap of 100.37 mph dur­ing the 750 cc class of the TT Pro­duc­tion race? Re­mem­ber my 1972 Nor­ton Com­mando was fit­ted stan­dard with K81 4.10-19 tyres both ends. So now with a re-de­sign of tyre specs mak­ing a smaller cross sec­tion and smaller side­walls I was hav­ing all sorts of prob­lems try­ing to fit them to my rim. I called the Aus­tralian Tyre & Rim As­so­ci­a­tion and talked to a lady there who knew that JATMA was the Ja­panese ver­sion of the Aus­tralian Tyre & Rim As­so­ci­a­tion and that’s the sort of thing they did. I have no other tech­ni­cal de­tails or political in­for­ma­tion other than this, how­ever I do know for a fact that the two K81’s I bought were dra­mat­i­cally dif­fer­ent to the tyres I’d been buy­ing and fit­ting for the past 38 years. I took the K81’s back to Graeme Mor­ris Mo­tor­cy­cles and Graeme him­self and his staff were very help­ful in help­ing me go through his large se­lec­tion of stock to find a pair of suit­able tyres. I’ve fit­ted a Bridge­stone Bat­tleaxe BT-45 100/90-19 M/C 57V to the front and a Pirelli Sport De­mon 110/90-18 M/C 61H to the rear. It was more about the ac­tual size of the tyre and the way the ap­pear­ance is as fit­ted un­der the guard etc. Noth­ing worse than look­ing at a bike that looks like the tyres are too small for it, just looks weird. I’d be very in­ter­ested to know if any­one else has ex­pe­ri­enced these is­sues with the cur­rent K81’s.

Bob Davis Warn­ers Bay, NSW Email: bob­[email protected]­pond.com

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