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September 20, 2014

Braking performance

Braking performance is degraded when carrying a passenger, mainly because the added weight lengthens the stopping distance. Same with cornering. The extra weight takes up suspension travel and makes the bike less responsive to steering inputs. So take it easy when tow-up and extend those safety margins.

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Tyre Science

Yes, the only thing common between a motorcycle tyre and tyres for other automobile applications, say cars, is that they both are mounted on rims and are made of rubber. Seriously! Of course there’s common construction (cross ply or radials), common terminology (bead, tread and crown) and common elements (bead only here) but a motorcycle tyre is unique in many ways in the automotive world. The loads on a motorcycle are concentrated and intense which calls for a different construction method and design premise. This intrinsic need for such extreme performance makes motorcycle tyres inherently more expensive than comparable car tyres, when compared weight to weight and size to size.

Power Loading: Let us consider a powerful 150 bhp motorcycle and compare the way its tyres are expected to perform compared to a high performance cars’. A 300 bhp car typically has some 90 square inches of contact area or so to put down its power while the motorcycle cited above has (say with a 180 section rear tyre) barely 15 square inches (about 1/5th of that for the car) of rubber to do the same. And the motorcycle tyre could at times be doing this while banked into a turn!

More impressive is this performance when we take braking into consideration. Our 150 bhp scorcher will accelerate from stand-still to 100 kmph within 3-4 seconds. This is a very dramatic loading of the rear tyre contact patch but more drama exists when braking is considered. The same bike has a 140 section front tyre which means it has a smaller contact patch. Braking is negative acceleration and the same bike can brake from 100 kmph to zero on good dry tarmac in less than 3 seconds. Since almost 100% of the braking force comes from a motorcycle’s front brakes (due to the forward weight transfer under braking and thus increased traction), the smaller front contact patch is even more heavily loaded under braking as the same motorcycle is being stopped in less time that it took to speed up! The front 120 section, with its smaller contact patch, is in effect transferring a lot more tractive power to the road compared to the rear tyre.

Ratio of contact pressure vs inflation pressure: There’s another measure of the exceptionally high load concentration motorcycles tires are subject to. This results from comparing the pressure on the contact patch versus the tyre inflation pressure. In simpler terms how strongly must the tyre resist and push against the loads put on it. This load comparison is not in terms of the total weight the tyres carry (the trucks carry hundreds of times a greater weight) but in terms of the loading per unit contact area. For example the contact pressure to the inflation pressure ratio of a motorcycle tyre can be up to 1.4 as compared to 0.8 for a typical truck tyre and around 1 for a passenger car. What this means is that a motorcycle tyre inflated at say 30 PSI and having a contact patch area of about 14 sq inches should be capable of supporting about 180 kgs but in reality we find the tyre usually ends up carrying (180 x 1.4) i.e. almost 250 kgs. Compare that for a truck and for a contact patch spread over 80 sq inches and inflated at 75 PSI, it can support around 2700 kgs but in reality there’s typically not more than 2000 odd kgs on each tyre. Such intense concentration of loads calls for special construction details in a motorcycle tyre compared to other vehicles.

Camber Thrust: This is another aspect where motorcycle tyres differ substantially from other automobile tyres. And can be best understood in direct comparison between how a motorcycles turn compared to a car. You’ve probably noticed that a car’s wheels are turned in a lot more compared to a motorcycle’s front wheel when both negotiate the same curve at identical speeds. The difference exists due to ‘camber thrust’ working for a motorcycle. There’s one major difference in the way a motorcycle corners with respect to a car and that’s in ‘leaning or banking’ into the turn. The other difference lies in the curved profile of the motorcycle tyre compared to flat for a car. A wheel forced to follow a curved path resists this change in direction and wants to continue in a straight line. A car tyre gets turned in while it rolls straight up and vertical and needs some manner of force to counter the centrifugal forces that want to take it out of the curve and follow a straight line at a tangent to the curve. The car tyre does this by inducing a large slip angle (see box) which effectively slides the tyre through the curve and so provides grip through adhesion and hysteresis. (Love the sound of squealing rubber eh!) And a large slip angle can only be induced by turning the wheels more inwards than the curvature of the turn. As if the car driver wants to turn tighter. A motorcycle avoids depending upon large induced slip angles by leaning or banking into a turn. As the bike leans, the curved profile of the tyre makes the contact patch to shift inwards and generates a sort of speed difference between the inside and outside of the contact patch. The inside-most part of the contact patch, being at a smaller distance (radius) from the center of the curve, must move slower than the outside-most part and so tends to pull the tyre into the curve with it. (Much like a cone lying on its side on a flat table and describing a circle when rolled). This ‘Camber Thrust’ takes the motorcycle around a corner without a large slip angle. In fact so strong can be the camber thrust in supper sticky tyres, the sort used in Moto GP sort of events, that the rider needs to turn the front wheel away from the curve to stop the motorcycle from over-steering into the turn. So the next time you see Rossi’s front wheel at a slight opposite lock during a turn, know that he’s not drifting but countering the excessive camber thrust through a small negative slip angle! Things though change at slower speeds – say below 20 kph or so. Low speeds mean that we cannot lean much and so there’s a lot less camber thrust to help us turn the bike. This is why we need to turn the handlebars more when taking a turn at a slow speed.

Camber thrust loads up a motorcycle tyre with a very strong lateral force (many times more than that experienced by car tyres for example) and so its tyre carcass has to be a lot stronger and stiffer to be able to counter that force without succumbing to catastrophic deformation.

Slip Angle

PS: All turns on a motorcycle at speeds above around 20 kph or so are initiated through counter-steering. This counter-steering initiates a lean angle that helps generate camber thrust which in return helps the bike go around a curve.

Slip Angle

Riding a motorcycle is as much about going straight as it is about changing direction. And this must happen under total control of the riders’ inputs. In fact, motorcycle apparently seems to contain most of itself in the way it is turned. Straight line riding is not ‘fun’ after all…is it? To turn the motorcycle, the wheel must also point in that direction. Forget counter-steering for the moment for simplicity and imagine a turn at speeds below 20 kph. The rider turns the wheel left to turn left. The tyre contact patch is sticking to the road and resists this turning, a resistance easily overcome by the rider applying force through the long lever arm that the handlebars provide. Forced to turn against resistance the tyre deforms locally (remember Hysteresis) and also deflects laterally under the side-force generated from the turn. The wheel points towards the turn while the rubber at the contact patch lags behind a little and tends to straighten the wheel. The wheel of course calls the shots (as the rider is holding it forcefully in the direction of the turn) and the contact patch slips into the new direction of travel like a reluctant puppy on a leash. This difference in the direction the wheel is pointing in and the actual direction of travel by the tyre contact patch is the slip angle. The slip angle is zero when the wheel is rolling straight ahead and begins to increase as the motorcycle turns. The lateral force also increases in proportion and is in fact responsible for making the bike follow a curve. This lateral force increases to a maximum when the slip angle approaches 150 – 200. Beyond that of course the rider has lost it! To explain this, remember the contact patch shifting inside when the bike leans into a turn. Since the outside of the contact patch has to move faster than the inside (which in fact is moving at the speed of the motorcycle), it follows that the outside part of the contact patch would be ‘slipping’ to keep up with the inside. As the slip angle increases, the ‘slipping’ of the patch creeps more and more inwards and there comes a time when more than half of the patch is slipping. That’s pretty close to losing it! In fact modern tyre rubber compounding is done so that this inward creeping of the slip or gradual increase of the slip angle is predictable and so provides enough warning to a discerning rider to back off on his traction demands in time before the tyre breaks away into a slide.

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