Low temps = lower traction, and a low side!

[Paul_F]

Wow, the more this post gains responses the more info and background I have to add for an article I plan to write for the ON. Anything I use in the article from this thread will give credit to the source from this thread. Thanks everyone, I didn't expect this amount of discussion when I first posted this thread.

No, thank you Andy for bringing this to our attention. I had a friend slide out behind me a few years ago. He didn't know what caused his front tire to slide out. It was a very cold day in early April (and wet). After your posting this thread, I'm certain we have finally come across an answer. I often ride into mid-December (not this snowy winter) and I now plan on being much more careful in the cold weather and will use lower tire pressures. Thank you again Andy!

[ANDYVH]

Thanks for the support and comments, all appreciated.

Going back to the initial issue of how the tire could measure colder than the surface it was on, I think "emmisivity" is the answer. Emmisivity is the variance of heat dissipation of different surfaces, think of the heat emmision of a concrete parking lot (where my measurements started) versus the heat emmision of a black tire tread compound. That could easily account for the seven to ten degree temperature difference.

Now, had I been using an actual surface contact style thermometer versus one of those trick infrared digital thermometers the difference may have been minimal.

I can forsee more analysis/data collection coming next spring when I am riding again. Ohh, exciting,...data collection.

I need a life, heh, heh.

[DaveJohns]

Thanks for the support and comments, all appreciated.

Going back to the initial issue of how the tire could measure colder than the surface it was on, I think "emmisivity" is the answer. Emmisivity is the variance of heat dissipation of different surfaces, think of the heat emmision of a concrete parking lot (where my measurements started) versus the heat emmision of a black tire tread compound. That could easily account for the seven to ten degree temperature difference.

Now, had I been using an actual surface contact style thermometer versus one of those trick infrared digital thermometers the difference may have been minimal.

I can forsee more analysis/data collection coming next spring when I am riding again. Ohh, exciting,...data collection.

I need a life, heh, heh.

(I learned of this thread through a post on the Rounders web board, I can't seem to stop my inner geek from spouting something...)

+1 on emissivity. We love to trust what our instruments tell us. IR thermometers and imaging cameras are especially susceptible to giving 'false' readings because of how IR is radiated from particular surfaces. Roughness / distance (think signal noise and 1/r^2 law) / material composition / reflectivity / and even the angle of a surface wrt the camera can all affect how an IR camera 'reads' temperature. You can even see 'reflections' of temperature in a dull but flat metalic or ceramic surface with a FLIR camera - reflections of the heat of other things 'seen' in the surface as if looking in a cloudy mirror, regardless of the temperature of the surface the camera is pointed at.

A quick sanity check:
The first law of thermodynamics says that thermal energy flows from hotter things to colder things. Therefore, the tire cannot be colder than the things around it, unless something colder than the tire (and the environment) is pulling thermal energy out of the tire; pulling it's temperature below that of the environment. That's not likely.

What makes temperature rise? The absorption of energy. What makes temperature fall? Giving up energy. Temperature (change) of a material is inversely proportional to the specific heat of a material and directly proportional to the amount of energy gained or lost. Specifically: delta-T = Q/(Cv *m)... the temperature change in a thing [delta-T] is equal to the amount of heat energy it absorbs (or loses) [Q] divided by its specific heat [Cv] and mass [m]. ...I could continue with this level of analysis if it would be helpful in this forum: to look at the sources of heat generation and heat loss and what can happen to rubber temperature as a function of the environment and the location of the rubber.

Where I run out of expertise is where the rubber meets the road, so to speak. How does the coefficient of friction change as a function of temperature / rubber durometer / ?? / ?? / what else?... Yes, it would be nice if a tire expert could provide some adult-level 'unfiltered' data. Your typical support personnel on the other end of the address 'support@wemightneveransweryou.com' are capable of only parroting back to you information from published data and legal disclaimers. It's hit-and-miss even in the high-tech arena to break through the wall of tech support to get someone who can answer real questions. "I can read the data sheet. If the answer was on the data sheet I wouldn't be calling you..."

"Not everything that counts can be counted, and not every that can be counted counts." -Einstein. Very very true. There are many things that can cause a real world system to be different from an 'ideal' analysis. But, before we can begin to understand the general behavior of a system, we must begin at grapling with first principals.

"You can't ride at 10/10ths" is also +1!! advice. You gotta ride within the evelope you're given. Hopefully this thread will help more of us be better informed of the things that CAN pull in the edges of the permissible riding envelope.

I'll shut up now. I have no life.

[ANDYVH]

One thing I do have some basis in is synthetic rubber compounds (our beloved tires are NOT rubber, but a synthetic man-made rubber like product). Rubber duromerter (the amount a specific type and thickness of rubber is deflected per a specific load) is affected by temperature.

As a tire gets colder, the ability of the tire tread material to deflect/displace/and grab into/on the roughness of the road surface is decreased. One way to offest this is to reduce the air pressure in the tire, which allows the tire carcass to deflect more per load. That deflection/distortion creates the heat the tire material needs to warm up and grip better.

Most synthetic materials like neoprene and nitrile (common to tires) get harder and less flexible as they get colder. So the tread surface cannot deflect as easily to grab the road surface. Less grip = more slip unless the speed, side load and lean angle are decreased or the loading transitions are applied much more smoothly like we normally do during rain riding.

[DaveJohns]

Most synthetic materials like neoprene and nitrile (common to tires) get harder and less flexible as they get colder. So the tread surface cannot deflect as easily to grab the road surface. Less grip = more slip unless the speed, side load and lean angle are decreased or the loading transitions are applied much more smoothly like we normally do during rain riding.

Yup... and the tires will never achieve the same running temp in cold weather as they will in hot weather, no matter how long you ride. The rate of heat extraction (cooling rate) is proportional to the temperature difference between the tire and the environment. The temperature stabilizes when the rate of heat generation is equal to the rate of heat extraction. So, if your tires stabilize at 20 degrees warmer than the road in the summer, and the rubber DIDN'T get harder in the cold (the rate of heat energy does NOT change), they'd still only achieve 20 degrees warmer than the road in the winter, which is a lower temperature. But, since the rubber DOES stiffen, and heat is generated at a lower rate when cold, the delta-t is probably lower in the cold, which makes your peak tire temp even colder.

The dynamic coefficient of friction is lower than the static coefficient of friction. Therefore it takes more force to get two surfaces to slip between each other than it does to maintain the slip motion once it is started. That's why 'abrupt' changes can cause slip and a washout. What is an 'abrupt' change? Any change in speed or direction requires an acceleration. Acceleration is a change in velocity with respect to time, where velocity is defined as a vector which contains both speed and direction. So, the more 'abrupt' the change, the higher the brief acceleration. F = ma, so a brief acceleration requires an impulse force. Changing speed or direction slowly will decrease the impulse force required to make the change. But, if that impulse force is greater than the product of the mass of you and your bike multiplied by the static coefficient of friction, the tires will slide, and once they are sliding, it takes less force to remain sliding than it took to start the slide, so nature determines the slide will continue unless you can correct for it and regain static contact with the road.

Side note: In a stable turn, centripital force F = (mV^2)/r acts radially outward horizontally from the COG (this is your side load), and the gravitational force F = mg acts vertically downward from COG to the ground (this force times your static coefficient of friction must be greater than the side load, or a slide will begin). But, in the lean, the resulting vector forces normal to the centerline of the bike/rider system (proportional to Sine(angle) times side load and Cosine (angle) times force of gravity) are equal and opposite. The lean angle always adjusts to maintain that equalibrium. So, when the centripital side load is less than gravity times your mass times the static coefficient of friction, your tires are in static contact with the road (with the higher static friction force). When your side load force exceeds the friction force limit, the rubber slips, and at that moment in time, the lean angle is too acute to maintain equalibrium. The result is that gravity pulls the center of the bike downward, increasing the lean, making the situation worse. It's a positive feedback instability that tends toward the bike laying down on its side. Gravity wins. That's why low sides occur.

Many things will affect the static coefficient of friction: tire durometer, sand, water, oil, road roughness, paint, leaves, tar snakes, snow, ice... the reduction in traction due to the temperature effect on tire rubber durometer is only one variable out of many that affects how hard we can start, stop, and turn.

Side note: There is a point during a typical 'fishtail' where the sideways motion of the rear wheel stops and swings the other way. If the tire 'grabs' - that is, if it regains static contact w/ the road, the coefficient of friction takes a unit step upward in magnitude. The lean angle required to maintain equalibrium during the slide in the moment prior to the 'grab' is not correct to maintain equalibrium with the higher static friction. The result is that the bike flips forward. The centriptal force wins. That's why 'high sides' occur.

I think we know these things intuitively just from watching the way the world works from the time we're toddlers. But, it's neat to look at the underlying physics of why these things happen.

I hope this has beneficial in some regard.

[ANDYVH]

I understand this one too, as I ride dirt bikes and an ice bike:

"Side note: There is a point during a typical 'fishtail' where the sideways motion of the rear wheel stops and swings the other way. If the tire 'grabs' - that is, if it regains static contact w/ the road, the coefficient of friction takes a unit step upward in magnitude. The lean angle required to maintain equalibrium during the slide in the moment prior to the 'grab' is not correct to maintain equalibrium with the higher static friction. The result is that the bike flips forward. The centriptal force wins. That's why 'high sides' occur."

I have had occassions to get the rear tire to break loose in corners while riding a R1200GS. The rear tire slide was very predictable and easy to control. The key is to NOT chop the throttle closed if rear tire slide occurs. Doing that will cause sudden grip and the bike can high side quickly with terrible results.

[DaveJohns]

I remember the MO DMV manual for motorcycle riders cautioning to keep the rear wheel locked in a skid, and not let off the brake until the bike is stopped. I think the CA manual had the same warning...

The same thing would apply to a power slide - keep it going! It looks cool too, and feels great. Ride on!

[Terry Karney]

Wind chill is all about perceived temperatures. Tires don't perceive! Tires may not get as warm in the wind and they will cool to ambient temperature faster in the wind, but they will not fall below ambient regardless of the amount of wind.

That's not quite what wind chill is. Wind chill (and it's been modified a few times tin the past 10 years, one learns a lot as an NCO in the Army about things like this), is a function of how the wind removes heat.

If the air is still, the warmth of your body heats the surrounding air, and so less energy is needed to keep the body warm. The principle of layering is to keep that air near the body from moving so much, and so keep the radiant energy of the body in a place it can be used.

Wind, of course, removes that layer of heated air.

Tires, not being animate, don't work in the same way. They absorb energy from the surroundings, and radiate it back; because thermodynamics means they will move to stasis with their environment.

That is also why, though I'd have to see the actual conditions, the tires were colder than the ambient. By excluding incoming radiation one can make ice in the desert, merely by digging a hole, and exposing the water to the nighttime sky. It's a quirk of how things work that a the water is trying to heat the whole of the sky, and it can't.

Given the right conditions, the tires, in a colder environment than N. Africa (where I learned of this trick being done), could have been doing the same thing. If they, like water, were shedding heat in a situation where there was no source of incoming heat, to balance them out, they could have been trying to warm the surrounding atmosphere, and so gotten colder than ambient. It's counter-intuitive, but it's possible.

[BCKRider]

Anyone else not understand the last two paragraphs of the above post? I'm not sure if my IQ is slipping, someone can provide a clearer explanation of a real phenomenon, or this is nonsense.

I would have bet my bottom dollar (well, maybe not) that 1. a tire off the ground and out of the sun would eventually reach exactly the same temperature as the surrounding air, and 2. a tire in contact with colder ground than air temperature would be very close to air temperature.

Not sure what this has to do with scary performance of cold motorcycle tires, but it's always good to learn something new. Let the explanations continue.

[E_Page]

Is a simpler explanation like when you take a room temperature bottle of beer and place it into a bucket of ice then spin it around while in contact with the ice thus making the beer cold in a matter of a minuet or so versus leaving it just sit in the refrigerater and the cold air in the frig makes the beer bottle cold but it takes longer?

I think it's more like when you take a really cold bottle of beer, open it and it instantly turns to beer slush.