TYRED OUT

Real Classic - - Letters -

I’ve been an avid reader of clas­sic bike mags since the late 1980s. I al­ways en­joy read­ing about read­ers’ restora­tions and the lengths to which some peo­ple will go to achieve their fin­ished ma­chine. There is a para­graph, how­ever, that ap­pears many times in such ar­ti­cles, and it goes some­thing like this: ‘I sent the en­gine and gear­box to be com­pletely re­built in­cor­po­rat­ing all the lat­est mods, had the chas­sis parts pow­der coated, sent all the tin­ware to our lo­cal coach­works for re-spray­ing and had all the chrome parts stripped, pol­ished and re-plated. The tyres had plenty of tread and ap­peared to hold pres­sure, so I thought there’s no need to re­place them.’ Aaaaaaaaaar­rrrgghh! Let’s get one thing out of the way. On a dry sur­face, tread does not give you grip. In fact it does quite the op­po­site. Grip is a prod­uct of the co­ef­fi­cient of fric­tion of the ma­te­ri­als and the sur­face area in con­tact. Ergo, if you cut slots in the sur­face of a tyre, you are re­duc­ing the over­all con­tact area and, thereby, the grip. Added to this is the fact that the deep grooves al­low the tyre to flex and ‘walk’ when side loads are ap­plied. If you don’t be­lieve me, fit some tri­als tyres to your Tri­ton and see how well it goes round cor­ners.

That’s tread dealt with, but the big­gest prob­lem with old tyres is the com­pound. Any­one who fol­lows mo­tor­sport will be well aware that tyre com­pound is ul­tra-crit­i­cal, and very of­ten af­fects the out­come of a race. Dur­ing the 1980s I was a prod­uct en­gi­neer and had re­spon­si­bil­ity for the man­u­fac­ture and re-cov­er­ing of rub­ber rollers, mainly for the of­fice equip­ment and print­ing in­dus­try. We pro­cured our rub­ber from two of the big play­ers, BTR and Con­ti­nen­tal, and although we weren’t in­volved with tyres, you do tend to learn a cer­tain amount about the ma­te­ri­als that you are us­ing.

The sci­ence of rub­ber manufacturing is an ex­tremely com­plex sub­ject and I am not qual­i­fied to lecture on the mat­ter, but I will try to give a gen­er­alised over­view.

Firstly, what is ‘rub­ber’? Well it’s a bit like ‘plas­tic’. It’s an um­brella term that cov­ers a mul­ti­tude of ma­te­ri­als. Most peo­ple think that all rub­ber orig­i­nates from trees. Not so. Although nat­u­ral rub­ber starts as a gooey sap tapped from trees, many rub­bers use a syn­thetic base, butyls, ni­triles, styrenes, etc; mainly a by-prod­uct of the petro-chem­i­cal in­dus­try. Most of the rub­bers I was in­volved with were syn­thetic, but nat­u­ral rub­ber was of­ten spec­i­fied for some prod­ucts. Which­ever type is used, they all share many prop­er­ties, and top of that list is age­ing and hard­en­ing.

So how is it made? You start with the pri­mary ma­te­rial and add many other com­pounds de­pend­ing on the fi­nal prop­er­ties that are re­quired. These can com­monly be car­bon black (a filler), sil­ica (for wear), sul­phur (es­sen­tial for cur­ing / vul­can­is­ing) de­pend­ing on the man­u­fac­turer’s needs. All these in­gre­di­ents are mixed and then usu­ally rolled out as a sheet (cal­en­dered), ex­truded into tub­ing or pro­files or moulded.

At this point the rub­ber has a con­sis­tency a bit like chew­ing gum, so to turn it into a useable prod­uct it has to be cured, or vul­can­ised (poly­meri­sa­tion, cre­at­ing cross­bonded mol­e­cules). There are nu­mer­ous ways to achieve this, mostly us­ing a heat source such as steam (au­to­clave) or mi­crowaves. In the case of tyres, the rub­ber is cured in the mould. This is car­ried out for a very care­fully con­trolled time to main­tain the re­quired grade of the fin­ished prod­uct. In gen­eral terms, the softer a rub­ber, the greater the grip and the higher the wear rate. The op­po­site ap­plies to harder com­pounds.

Rub­ber hard­ness is mea­sured on the ‘Shore A’ scale with a durom­e­ter. Road tyres can range from low 50s to mid 70s. Tyres are also far more com­plex in that they are con­structed us­ing steel belt­ing, can­vas cas­ings and, very of­ten, a mix­ture of nat­u­ral and syn­thetic com­pounds in the tread and side­walls. What­ever the fi­nal use, they will all de­te­ri­o­rate with age and ex­po­sure to heat and UV. I, for one, would be happy to see durom­e­ters used in MoT test­ing, as, dur­ing its nat­u­ral life, a tyre can harden by up to 15 de­grees.

That’s just a very brief over­view. Sorry to be a killjoy but I feel that re­tain­ing 40 year-old (or even 10 year-old) tyres on your pride and joy is very much a ‘ha’pence of tar’ sit­u­a­tion. When you’re rid­ing your show-win­ner on its fac­tory orig­i­nal tyres (I know peo­ple who have), re­mem­ber that it’s not just the shiny paint­work that’s at risk!

Keep up the good work, great mag. Jerry Al­brow, mem­ber 10,670

Thanks for this, Jerry. I can feel all smug and re­veal that I al­ways re­place tyres when­ever I put a bike onto the road af­ter a year or so’s lay off. It’s not cheap, but I re­mind my­self that the only things be­tween de­light and de­struc­tion are two small con­tact patches. That al­ways flexes the wal­let… Frank W

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