More on Traction for Motorcyclists
Friction ReviewFirst, let’s do some simple experiments to get an idea of what affects traction. Take an empty cardboard box, slide it across the hardwood floor toward the bookcase, and fill it with books. Now slide it toward the door. You’ll notice it now takes a lot more effort to make it slide. If your spouse is sitting on the filled box while you try to slide it, it’s harder yet. You write in your lab notebook that weight is one thing that affects friction; specifically, more weight produces more friction.
When you repeat the experiment in the office with the carpeted floor, the same box is still harder to slide. Your second entry is that friction is affected by the nature of the surfaces which are sliding; for instance, hardwood floors are slicker than carpeted floors.
To try to quantify your discoveries you tighten up the experiment. You take a brick-shaped block and glue a piece of rubber tire to the bottom. You weigh the block, attach a fishing scale to it, and measure the pull required to slide the block along a table. Two interesting observations come out of this experiment: First, it takes more pull to start the block sliding than to keep it sliding. You write in your lab notebook that static friction is greater than sliding friction. Second, your earlier “more weight produces more friction” observation can be made more precise, because you’ll find that friction is proportional to weight: Twice as much weight produces twice as much friction, when everything else is kept the same.
Finally, you explore the other obvious factor affecting friction, namely the surface area in contact. Using the brick-shaped block above, you measure the force required to start the block sliding on its large side, its smaller side, and its smallest end. To your astonishment, you find that the surface area in contact does not change the friction. Amazed, you check your college physics textbook and see that this fact has been known since 1699.
So here's what we know: Traction is the friction of tires on roadway, and there are just two variables affecting our traction. One is the force which is pressing the tire into the road. Except for transient conditions, like momentary unweighting as you go over a rise, this is just the weight of the bike and rider. The other is the “stickiness” of the tire and the pavement. That factor is called the coefficient of friction and is denoted by the Greek letter μ, pronounced “myew”. It varies with the pavement, the type of rubber, its temperature and age, road conditions such as the presence of water and sand and leaves, and whether the tire is sliding or rolling. And the relationship between friction and weight and μ is F = μW: friction is equal to the product of weight and coefficient of friction.
Our fishing scale will give us the value of μ in our tabletop experiment. For a modern tire on clean dry pavement, we’ll find values of μ for static friction in the neighborhood of 1.2, meaning that if your block weighs 5 pounds you’ll have to pull it with about 5*1.2 = 6 pounds to start it sliding.
Theory or practice?Our experiments, while instructive, may not accurately reflect real-world conditions of a rolling tire, flexing sidewall generating heat, slight variations in the load as the suspension tries to smooth irregularities in the road surface, etc. A paper published by the Society of Automotive Engineers on 4/12/2010 by Lambourn and Wesley addresses some of these deficiencies. They tested three motorcycle front tires from Bridgestone, the BT003 (race), BT014 (sport), and BT021 (sport touring), in size 120/70ZR17, on two different types of pavement, under much more realistic conditions. Their test setup had the capability of testing wet-pavement traction as well. So what did they find?
I’ll discuss only the BT021 sport-touring model. The value of μ on dry hot rolled asphalt (HRA) varied to a surprising degree from trial to trial, from about 1.1 to 1.4. The average was about 1.25. On a different type of pavement, dry stone mastic asphalt (SMA), the variation was slightly less, from 1 to 1.2, with an average just under 1.2. And when water was added just in front of the tire to an approximate depth of 1mm? On wet HRA the variation increased but so did the average! On wet SMA the average decreased slightly to about 1.0.
μ increased from 1.25 to 1.3 on wet pavement??!!The increase in traction on wet HRA is so surprising that it makes me wonder whether the authors failed to control some important variable, perhaps tire temperature. They noted that the ambient temperature throughout the tests was about 59F but made no mention of tire temperature; nor did they make any remark which would indicate understanding that tire load and run time affect tire temperature, which affects traction. The lead author, the only one for whom contact information was provided, did not respond to an inquiry.
Ok, so what does that mean? From the data presented so far, despite the questions which remain, I think we can conclude two things. First, the spread in the data values indicates that traction will vary somewhat even in conditions that are apparently identical. And second, on wet but clean pavement, traction may decline as much as 20%, though it’s still better than anyone could have hoped for just a few years ago.
What does a coefficient of friction of 1 or 1.2 mean? Tires with μ=1 will support a lean angle of 45 degrees, or a braking distance of 120 feet from 60mph*. (The equations for determining these facts can be found at www.stevemunden.com/leanangle.html.) A value of 1.2 will support a lean angle of 50 degrees or a braking distance of 100 feet from 60mph*. So if you’re in the habit of leaning more than 45 degrees in the dry, better pull it back to a little less than 45 degrees when it starts to rain! And, of course a clever motorcyclist will be scrutinizing a wet surface for slick spots, choosing cornering lines that cross the most tractable surfaces, and transitioning smoothly from throttle to brakes.
*The braking distances above assume a very aggressive deceleration rate of around 1g and do not include reaction distance. If you’re over age 50, your reaction time is likely to be 1 second or longer. What’s more, few of us are capable of really aggressive braking. On your best day, on clean, dry, pavement, with warm tires, you might possibly initiate braking within 0.75 sec, and pull a 30 ft/sec/sec deceleration rate. In real world conditions, total stopping distances for even a veteran rider will likely be double or triple the numbers published in motorcycle magazine tests.
The tests were conducted on a test track rather than public road, and I assume — the authors didn’t say — that the surface was relatively free of contaminants such as the oil, diesel, antifreeze, etc., that might be found on an ordinary roadway. If so, the test results represent a best case, a road that had been rained on long enough to clear off such contaminants. A conservative will assume that the traction will decline by more than the tests showed, particularly at the start of the rain. (So will a liberal, if he’s wise.) There aren’t any reliable studies about how much traction decreases during the start of rainfall, how much it can be reduced by contamination, or how long it takes for traction to return to “clean” and wet. Lambourn and Wesley didn’t measure such things. Traction depends on how much contamination there might be on the road, how hard it’s raining, tire temperature, water depth, etc. Veteran riders know that surface traction in the wet can vary dramatically.
But all those caveats shouldn’t hide the real value of the paper: On a clean and wet roadway, using a modern sport-touring motorcycle tire, you can expect traction that’ll give you a lean up to 45 degrees or, for the few who have the skill, a stop from 60mph as short as 120 feet. Back off a little on the lean from that and you should be ok.
Does it matter how much load I'm carrying?Our tabletop experiments, and the resulting equation F = μW, tell us that total traction increases with weight. But the forces which use traction — acceleration, braking, and cornering — also increase with increasing weight. The weight cancels out of the two sides of the equations and you’re left with the same stopping distances and cornering ability.
(In case you’re wondering how that statement squares with the fact that heavy trucks have longer stopping distances, it’s because traction is not the limiting factor in their stops; it’s the ability of the brakes to dissipate the energy. By contrast, the brakes on most modern motorcycles are better than the tires, in that sense. Our stops are limited by the tires, not the brakes — and even more by the skill of the rider and the “user interface”: the lever pressure needed to produce a good stop, the feel of the motorcycle under hard braking, etc.)
What does change on a motorcycle with additional weight is the distribution of the traction between the two tires. Nearly all major additions of weight — fuel cells, passengers, a couple of spare BMW final drives with tools — are placed toward the rear of the bike, weighting the rear and lightening the front. That might affect your stopping technique: In the occasional quicker-than-normal stop when my wife is riding with me, I find myself using more rear brake than usual. Hard cornering with a light front tire might result in understeer or even a low-side crash if the front tire lets go.
My tires are getting a little worn. What happens to traction?Lambourn and Wesley noted that their test tires went from new to bald to cord to rubber below the cord to holed during the course of their tests. Traction did not change until the cord was showing, at which time it declined. Then it increased again, having hit the rubber below the cord, until the tire blew out. So worn tires do not lose traction simply because they are worn. Two cautions, however: First, we’re assuming that the rubber is just worn, not old. Aged rubber gets hard and does lose traction. Second, the lack of tread does increase the possibility of hydroplaning on pavement with standing water.
Speaking of hydroplaning…Hydroplaning is a matter of the tire being lifted off the pavement by standing water. There have been tests on airplane and auto tires, but not much on motorcycle tires. I can identify some of the factors and how they affect the probability of hydroplaning: Deeper water and greater speeds will hydroplane more than less water and slower speeds. Narrow, wedge-shaped tires will hydroplane less than wide flat tires. Deeper tread is better than shallower tread. (Note that narrow wedge-shaped rear tires get wider and flatter as the straight-line miles add up, and the tread in the center of the tire gets shallower. Conclusion: Tires hydroplane more as they wear. Duh.) Some tread patterns are better than others. More weight on the tire will reduce hydroplaning.
How does inflation pressure affect traction?Traction is affected by tire pressure, but only indirectly, via its effect on temperature. Everyone’s heard the advice to ride conservatively until tires are warmed up, and it’s clear that if a tire gets so hot that the rubber starts to melt then it’ll have a bad effect on traction. Tires are warmed primarily by carcass flex, and that’s controlled by pressure. Racer friends tell me that they adjust pressure by half-pound increments to maximize their traction and handling, depending on the track and the temperature. Street tires are designed with a much broader temperature range — just as well, since few riders will stop to adjust tire pressure as the sun rises and sets.
Inflation doesn’t affect contact patch size?Yes, pressure has an extremely minor effect on contact patch size, but remember: Contact patch size doesn’t affect traction. Back in our tabletop experiment, we found that the friction was unchanged whether we tried to slide the block on its large side, its smaller side, or its smallest end.
Yes, that does mean that you’d get the same traction from a skinny bicycle tire as from your 180- or 190-section rear tire, provided the same rubber was on the outside. We don’t use bicycle tires for other reasons: With less rubber, that skinny tire would wear very rapidly; it wouldn’t support the side loads in cornering or the acceleration and deceleration stresses as well as the larger tire; it would overheat easily. There are probably other reasons known to tire engineers. But the traction would be the same.
I know, you don’t believe it. I don’t care. I invite you to do the experiments which will prove the scientists and engineers have been wrong ever since 1699, when Guillaume Amontons introduced F = μW to the world. After you’ve proved them wrong, you can laugh all the way to Stockholm to receive your Nobel Prize. Tell em I sent ya.
So you're a dark-sider?(A dark-sider is a person who uses an auto tire on a motorcycle, usually if not exclusively on the rear.)
Lambourn and Wesley must have known you were there. A major point in their paper, as you can read from the abstract at http://papers.sae.org/2010-01-0054 , is to compare motorcycle tires with auto tires. The Bridgestone motorcycle tires were compared at the same time with a Pirelli auto tire, and to the authors’ surprise it performed very similarly. The coefficients of friction were sometimes slightly lower, but very little, and typically overlapped the spread from the motorcycle tires.
This tells us two things. First, for those of you who run an auto tire on the rear of your motorcycle, you’re not sacrificing much if anything in your stopping or turning traction. Of course, this says nothing about handling or fit or the potential for blowout due to a bead coming off — if there is any such potential.
Second, for all of us motorcyclists, if you believe that we can stop faster than cars, you’ll have to look somewhere other than tire quality for the reason. You'll have to look for some evidence supporting that belief, too: Auto tests which record 60-to-0 stopping distances are very similar to those recorded for motorcycles.
What happens to traction with a sliding tire?It decreases; we learned that in our earlier experiments when we noticed that it takes a stronger pull to start the block sliding than to keep it sliding. How much? Lambourn and Wesley measured μ for sliding tires of about 2/3 of the peak value. On average, the values for a sliding tire decreased from around 1.2 to .8. In an emergency, do not lock either wheel!
But how much does that really matter? Will your stopping distance really increase by a third if you lock a wheel? Debatable. If you lock the front tire and fail to ease off within a few feet, you’ll crash; releasing the brake to regain stability and then reapplying will recover the traction of a rolling wheel, though the time required to do that will certainly increase your stopping distance. At μ=1 a perfect stop from 60mph will take 120 feet and about 2.7 seconds. To recover from a front-tire skid, if it takes you half a second to release the brake and then reapply it — quite optimistic in my opinion — that’s about a fifth of the time for the whole stop. Whether that’s a fifth of the stopping distance will depend on whether you locked the tire at the beginning, higher-speed part of the stop, or the later, lower-speed part. Clearly it is much to be desired not to lock the front tire.
If you lock the rear tire you should stay on it (we all know that, right? If not, read up on high-sides here) and the contribution from the rear tire will indeed decline by a third, but so much of the weight, and hence of the traction, has moved forward to the front tire that the contribution of the rear tire is quite low. The Motorcycle Safety Foundation estimates that the rear contributes 30% to your stopping power, but I believe that to be laughable. In a “real world” quick stop, there might be almost 100% of braking on the front, with just enough weight on the rear wheel to keep it on the ground. We also realize that in the real world, it’s not likely many of us would have reaction times much shorter than 0.75 sec, or be able to pull off a 1g stop. Even the best of us should be proud to brake from 60 mph to 0 within a total braking distance of 195 feet, even with ABS and clean, dry pavement.
What happens when you leave the pavement?We’re all aware that traction declines on dirt. Like a roadway contaminated with oil and whatnot, this is an area where any attempt to quantify that observation founders on the multitude of variables. Light sand, deep gravel, wet clay, tree roots, bedrock — the possibilities are endless. And the friction model developed for tires on pavement may not apply well, as you move from traveling over the surface to plowing through it to actually digging it up as you would with knobby tires on dirt.
Speaking of knobby tires…In my limited experience with knobbies on pavement I find that they understeer, that is, they turn less sharply than I would expect, given the lean angle and speed. It feels as if the tires are sliding slightly. I know they aren’t; I am far from having the skills to control a sliding tire around a turn. What’s happening is this: If you push sideways on a knob of a tire, it bends. The tire moves slightly sideways in the direction of the push. As the tire rolls on, the next, straight, knob starts to take the load. It bends, and the tire moves sideways again. Then the next knob bends and moves the tire sideways. So the effect is that the tire creeps toward the outside of the turn.
What determines how much each tread block moves? Longer, softer, and skinnier blocks will move further than shorter, harder, and fatter blocks. And the harder you push them, the further they’ll move, so that harder turns will have more creep than gentler turns.
The same thing happens when slowing or accelerating, but it’s subtle enough that I notice it only on turns. The actual traction of the tire — its resistance to skidding — should be the same as a slick tire with the same rubber.