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Tyres are the only interface between your motorcycle and the road, but they have all too often been seen as a maintenance issue with little realisation of the part they play. Thankfully there's a definite shift in thought in regard to tyres, from a legal requirement that is replaced when the tread is barely visible, to today's thinking where riders are changing tyres to make a big difference to their bike and making their own skins feel a lot more secure.
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There's no better time than when buying a second-hand bike to question the state of your rubber, and it isn't just a matter of whether the tread is deep enough to pass an MoT. Is the rubber cracking with age? Replace it. Is the surface intact? Replace it. How and where is it wearing? If the tread in the centre has been worn flat, either the bike has done a lot of motorway miles or has been running over-inflated: does the owner know what the tyre pressures should be? If the tread is good in the centre and worn at the edges, the tyre has been under-inflated or has an aversion to the straight and narrow. And that's the easy stuff. So, you're going to start from scratch: keep it safe. What do you get? Do you fit a set of OE fitments or do you refit a pair like the ones that came off? Do you buy on price or by tread pattern? It's only a pair of tyres, after all: how much difference can there be? In our ongoing attempt to demystify technology, American-V struck out to Cooper/Avon tyres production facility in Melksham to find out the facts. Tyre Specification: You'd expect sizing to be the simplest possible thing, you'd expect, but as soon as you want to talk tyres, the first thing you need to know is what size the tyre is. You really should stick to manufacturers recommendations for sizes - in some places you are legally obliged to - but if you don't, it helps to know what the rules are, and how tyres are identified. There are many ways to express the dimensions, and these are most commonly now determined using a standard which defines the width of the tyre, followed by the aspect ratio, speed rating, wheel height and finally load. Expressed as something like 150/90-16 58H, you would expect a tyre that is 150mm wide, tall (90% of the width) for a 16-inch wheel: 58H at the back means it will cope with 236kg according to its load index, and is safe to 130mph. Speed ratings show the speeds of which a correctly inflated tyre in good condition is capable, not the speed at which it is normally ridden - and it's easy to equate higher speeds with increased heat to explain why. Most of the alphabet is used but we've listed the common ones because there's no point going much further. A good example of why is seen in the ZR rating, because above 150mph it is all rather arbitrary. Of course, you are unlikely to ride a Harley on the road in speeds in excess of the current base level of "S", but if the recommendation is for a V-rated tyre, you should stick to it because weight is another heat generator, and American V-twins tend to be on the heavy side. " The
weights shown above are the maximum loads that the tyres can carry,
but you will be aware that a back wheel will not be supporting the
entire weight of the motorcycle and rider(s), but although it does
take the heavier load, it will share it with the front - hence the
differential. Straightforward stuff … except … what's
that MT90B16 all about? And before we leave that area, there is a nice little addition that is often overlooked, but is realistically only going to apply to Buell riders: as you go above 130mph / 210kmh on V-rated tyres, the load carrying capacity decreases by 5% per 6mph / 10kmh,and the same applies to ZR-rated tyres above 150mph / 240kmh. That's reference more than anything else, and nice way to scare the pants off your overweight Hayabusa-riding buddies over the bar of an evening. You could be forgiven for thinking that the sizes and profiles would be very similar once the sizing was agreed upon, and you would be close, but there is enough flexibility within the standards for a little artistic license to change shapes and even widths. The one thing you will tend to notice is that touring tyres tend to be less triangular than their Sports tyres, but aside from that each manufacturer's production processes determine their final shape. There is a last number that you will find stamped onto your tyre sidewall, and that is the maximum pressure the tyre will take. It will be so far ahead of the range you're playing with that you shouldn't have to worry about it - just as long as you know that it is the maximum pressure. Profiles It doesn't take a detective to see that the tyrewalls on traditional motorcycles are higher than their more modern contemporaries. Harley have joined in with this game, tentatively, with the Deuce and the difference in handling characteristics between that model and its plainer Night Train and Standard siblings are remarkable. Low profile tyres have the advantage of less flexibility in the sidewall giving greater stability at speed, and sharper handling. It's not all good news, however, and it offers it at the expense of comfort and load-carrying ability. In terms of rolling radius alone - determined by the circumference of the tyre - a lower-profile tyre requires a larger diameter wheel to maintain overall gearing, and the seventeen-inch wheel of the Deuce goes half-way towards the more radical eighteens on the fattest 250-section tyres available at the moment. To get an indication of the rolling radius, a 250/40-R18 rotates the same number of times per mile as a 170/80-B15 or a 180/70-R16 - actually fewer times than the stock, tall MT90-B16 and so represents a marginal increase in gearing. Wider tyres are easier to sustain on lower profiles and it isn't hard to see why: if you have a sidewall that is nearly as high as the tyre is wide, the potential for the sidewall to flex, or slew would be phenomenal, but it would be a very comfortable ride - you'd barely need shock absorbers, but then you'd be wobbling round potholed corners in a way that would make a hardtail look precise. So the widest tyres live on the widest, tallest rims, and have the shallowest sidewalls - as in the example of the 250/40-R18, which is 250mm wide, should be 100mm high and is destined for an 18-inch rim. Something else that low-profile tyres do - especially when combined with a lot of width - and that is to give a bigger contact patch, spreading the load of the bike. That creates a problem of its own, too, because it means that the fattest tyres will need a stickier compound to provide the same grip as the same bike on skinnier tyres. It's the same principle as wearing snowshoes. On a light bike that might be a bad thing, on a heavy bike it's a good thing - especially when leaving the boggy ground of sodden campsites. It's an odd truth, but the thin wheels of a racing pushbike will leave a deeper rut than a 250-section tyre on a Fat Boy giving the impression that the pushbike weighs more: it obviously doesn't but what little weight it has is concentrated over a smaller area. Radial vs Crossply You'll have noticed a few additional letters sneaking in before the rim size, namely B and R: Belted and Radial. They all have a carcass in common, which is the actual bladder of the tyre with beads at the edges, but they differ in terms of the way in which that is reinforced. Crossply
tyres are the most basic form, and while featuring the strongest
sidewalls, they are looked down upon for modern sports use, and
almost all car use these days. Belted plys of cords are laid diagonally
across each other at about forty-degrees to the centreline, from
bead to bead, providing strong radial and lateral strength across
the entire tyre. The downside is weight and heat generation. They
do still have their uses primarily on cruisers and off-road bikes
where the cushioning effect of the taller sidewall is beneficial. Radials have a simpler construction altogether, with little sidewall strength but with belts running round the circumference of the tyre at zero degrees, with the advantage of extraordinary radial strength that suits any situation where the speed of the bike puts greater forces on the tyre than the lateral forces can exert. They are great for high-speed handling dynamics - it's what they were designed for. The lesser height of these low-profile tyre forms reduces the leverage between the road contact patch and the fixed wheelrim to counteract the reduced sidewall strength. You may recall the public information campaigns as radials became increasingly common on cars, relating to the perils of mixing the two types of tyre. Didn't mention bikes, though, did they, but they should have. It is illegal to fit a crossply, or a bias belted rear tyre with a radial front in the UK at least, and that is not just for fun: it is dangerous! It is not illegal to mix a radial rear with a crossply front, but it worth bearing in mind that all tyres are tested as matched pairs and no account is made for people operating outside that environment. You might get away with it, but the responsibility is yours. I am aware of an example of a Buell-based chop that retained its original rear wheel and tyre but matched them to a 21-inch front. It looked stunning, but it shook its head at speed … well, at high speed. Okay, when accelerating very hard at highly illegal speeds. It was the last thing that was suspected and lots of steering geometry work was done in tracking it down. Compounds Rubber is rubber, is rubber surely? Forget it: we're talking rubber as a generic form here: the stuff that forms your tyre has a high rubber content, but owes more to the chemist's art than the man with a drill and a rubber tree. At the tyre's initial design stage, the manufacturer's chemists are told what the compound should be able to do, and a number of variants are produced and tested with the best one being selected for the final tyre. Compound testing isn't something that is done on a computer simulation, but on the road because nothing else can give the cross-section of conditions that a tyres has to survive. It is the success of the chemist that makes one tyre grip when another will be heading for the ditch, and I remember all-too well the reactions of an Asian-built tyre that looked remarkably similar to a Dunlop TT100 in tread pattern when confronted with wet tarmac in my youth. I was working at a British bike specialist at the time, and we referred to them "Kamikaze Ditchfinders", and explained to potential customers looking for a bargain that they may be cheap, but not to expect much from them. That was more than twenty years ago, and the tyremakers' art has moved on greatly since then - but then, so have motorcycles. A single manufacturer will now produce a range of tyres for specific applications with massively different grip and wear rates according to what their purpose will be. Tyres grip better when they are hot, and hot tyres leave some of their rubber behind as they grip hard on the bends. Sportsbike tyres reach their operating temperature quickly and are particularly good at keeping a lightweight motorcycle on the road at impossible lean angles, but they don't last long in relative terms. Casting size and load indices aside for the moment - but purely for the purposes of example - put the same tyres on a heavy American-built V-twin and they would be gone in no time, because they're not designed with a heavyweight in mind: they'd overheat and leave a lot of rubber on the road unnecessarily. We need harder compounds to cope with the weight, and can get away with them because we won't be testing their grip anywhere near as much. We go round corners too, sure, but not as quickly, and with gravity pushing the tyre harder onto tarmac. Go the other way, and put a set of tyres from a cruiser onto a Sportsbike and they'd probably last forever - if only because no-one would want to ride it, and certainly they wouldn't want to ride it fast because it would slide all over the place being neither heavy enough to get enough heat into the tyres, nor sticky enough to keep the lighter machine from sliding. So there's really no point in going for the really sticky compounds that your mate's R1 uses because your bike won't get the benefit - it might even make it worse, and it'll certainly hurt your wallet. Experiment,
by all means, but within parameters. Tyre compounds are developed
for sports, sport touring, touring, heavy touring and utility. Custom
tyres generally use sport touring and touring compounds for the
right amount of grip but allow you to use the radical profiles of
sports tyres without suffering an inappropriate compound. Tyre pressures: Tyre pressures are extremely important, but are possibly the most overlooked part of daily maintenance - especially on the rear wheels of dressers because you can't find them very easily. We've all ignored them until they are actually noticeable either visually or by the change in handling characteristics, but it is worth being aware of the implications of getting them wrong. Over-inflation will wear out the tread in the centre on your tyre quicker than your wallet would like, and prevent the tyre from spreading out on the road as it was designed to do to give you the maximum grip. It is easily done because you put more air in your tyres when taking passengers, or when running loaded, but you seldom take it out again when running empty or solo. Under-inflation is potentially worse, because the tyre will spread out on the road further than it was designed to do. It is sometimes done deliberately to increase grip on soft surfaces, but in those circumstances it would be combined with low speeds - don't think about trying the same trick under any other conditions. Under-inflation makes areas of the tyre flex more than they are designed to do, and can lead to premature failure, as the rolling tyre deforms when the weight is placed upon it, and returns to its original shape a moment later: this constant flexing builds up a lot more heat than is desirable, and can even lead to the separation of the composite layers. Quite apart from that, it will tend to wear out the shoulders of the tyre more than the centre which are designed to take a different sort of loading. Recommended tyre pressures are usually shown on a sticker somewhere sensible, but these refer to the stock OE tyres: if you change your tyres, check the pressures recommended by the tyre manufacturer - you may be surprised by the variation. Always check tyre pressures on cold tyres - as the tyre heats up, so does the air inside it by as much as 5psi. Tube and tubeless tyres The traditional inner tube has largely had its day on modern motorcycles, but still makes a useful temporary repair on tubeless tyres unless they are stamped TLO - or tubeless only - in which case the internal dimensions of the tyre are not suited to the use of a tube, and can be dangerous. Inner tubes were essential in the old days when spoked wheels were much more common, because no matter how good a bead you got on the rim, the air would leak through the spoke drillings. The same is true of a lot of spoked wheels today, but there are now tubeless spoked wheels available. Inner tubes can be repaired, but will never be as strong as they were originally: they are relatively cheap to buy new, and a damn sight easier than discovering that the fix wasn't as good as you thought it was when you put the bike back on the road again. Tyre manufacturers recommend that you replace tubes when you replace tyres, and on tubeless tyres, you should replace the valve. It's a relatively inexpensive exercise and a sensible thing to do. Get a virgin balloon, inflate it and then let all the air out of it again - that's very much thinner than an inner tube, but they adhere to the same basic principles. Balancing Regardless of how much care is taken in their manufacture, tyres are never going to be perfectly balanced, and so weights are attached to the rim of an alloy wheel, or the spoke nipple of a laced wheel. Less critical on a rear wheel than a front, an out-of-balance wheel can lead to anything from "pattering" sensation at the front end to uneven tyre wear at either end. It takes next to no time, and makes life a lot better. Aesthetics Tyres are black because that's how we like them. They don't have to be black, and are only made black by adding carbon to the mix. They'd far rather be buff, which is the colour of the mix before it's messed about with, but no-one wants buff tyres, and motorcyclists of all types are typically too conservative to contemplate blue, red, green or purple - any of which is achievable, but none are available. And if you think about it, it would always be the wrong shade anyway. So black it is. We can and do experiment with whitewalls to conjure up an image of days gone by, but these have a legacy that we prefer not to think about: grip. It is an age-old thing, and something that a lot of people will never come to terms with, but there is no reason whatsoever why whitewall tyres should have any less grip than the plainer offerings. There are tales concerning their original manufacture that suggest that white rubber was bonded to black rubber using a galvanising process, and that galvanising process affected the rubber's grip in some way, but we are given to understand - by our friends at Cooper/Avon - that it isn't done that way any more. I have vague recollections about a new process that Avon started using nearly ten years ago with their "Gangster Whites" which was supposed to resolve all such problems, but no-one's letting on any more so we're going to have to leave it at that. I will note, however, that Rich is absolutely convinced that the whitewalls on the Road King Classic are nowhere near as good as the white pins on the stock model which is, as much as anything, the reason why I'm running the narrow white stripes on my 'Glide - watch this space. Then we get tread patterns and love them or hate them, this is the fashion end of the tyre. We don't need tread to grip because the rubber compound does that for us, as you'll know if you watch any sort of motorsport. We need tread to get rid of the water beneath our tyres when roads are wet, and it is little more than a channel for the water displaced by the rubber to escape through, and the reason why there are laws relating to how deep the tread needs to be is to make sure the pathways are capable of transferring enough water. If the water can't escape, you get a thin film of water between your tyre and the road, and that means you're a boat. The expression is "aquaplaning" but that is more applicable to cars: on a bike your bike is sliding out from beneath you at a rate of knots, with no friction between the rubber and road to reduce the speed at which it does it - and it's touch and go as to whether finding a dry spot of tarmac is a good thing or a bad one: at best you'll regain control if you've not gone too far, but at worst you will high-side the whole plot and do significantly more damage to both yourself and the bike. If anything, the tread reduces the grip of the tyre on dry roads because it reduces the amount of the rubber compound that is on the road, but it is to a minor extent. It also affects the stability, and the current generation of tread patterns go a long way to resolving this with irregular patterns breaking up the natural tendency for a tyre to create its own track. In principle it's not dissimilar to "white-lining" - when you cross overbanding or road markings and notice a tendency for the bike to follow the direction of the ridges - except the ridges are on the tyre not the road and it makes the bike marginally harder to break off its line. It also reduces that annoying road hum that haunts riders of quiet stock bikes. But for all that, there is an element of making a good-looking tyre to compliment the bike that it's fitted to. It's also worth noting that treads are the perfect hiding places for stones: if you see them, get them out before they start worrying at the tyre's carcass: nothing is more soul-destroying than throwing away a set of tyres with loads of tread because they've been damaged in other ways. In Conclusion:
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