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