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Low temps = lower traction, and a low side!

I'm a product of the Tennessee public school system, so this could be all wrong.

Could the temperature differences between surface, air and tire be due to wind chill?

No, wind chill does not affect tires, machinery, ect.
Windchill factor is what the skin feels.
If you take a temp. reading of a piece of machinery in calm air at -10 deg. the reading will be -10 deg.
If the wind is blowing and the temp. is -10 deg. the temp. of the piece of equipment is still -10 deg.
 
Right, Wind Chill is what humans and mammals feel due to the cooling effect of the moisture in the air, on our skin, and how the wind has the ability to strip away the boundary layer of warm air just off our skin, of trapped in our clothing.

But it still baffles me that my tires were considerably cooler than the air temp. Even after sitting for maybe 45 minutes after my low-side ride. Assuming my tires had warmed (on the surface at least) to about 55 or 60 degrees, within those 45 minutes the front tire surface had cooled to less than air temp. That leads me to think the air in the tire, was never as warm as the tread surface aven after 20 miles of riding. After the ride, the thin heated layer on the tread surface was quickly displaced through conduction of the colder temperature inside the tire.

Plus the cooler air volume/tire surface area inside the tire is much greater than the much smaller surface area on the center of the tread. The more I think of this, the more I think it takes much longer than we think to totally warm up cold tires. The other correlation the tire makers may help with is how long it takes for the tire pressure to increase a few degrees as the tire totally warms up.
 
The other correlation the tire makers may help with is how long it takes for the tire pressure to increase a few degrees as the tire totally warms up.

I live approx. two miles from work and I can see a one to two PSI increase on my car tires with the TPM display.
 
Andy, thanks for being willing to share your experience for the benefit of others.

As for the temperature discrepancy, it will be interesting to see how your tests of the thermometer turn out. The specific heat of air is very low compared to rubber so I doubt if the air in the tire was much, if any, cooler than the rubber....unless the rubber wasn't heated all the way through. Still, that wouldn't explain a tire temp less than the ambient air and pavement around it.

Anyway, thanks again for sharing and I agree with the posters who'd like to see this in the News.
 
I'm inclined to agree with piperjim (post #21) and disagree with Lee (post #22.)

Certainly, if you took a reading of anything inanimate which isn't being heated, the temperature should be the same standing still and moving 60 mph. But cold weather or hot, tires ARE warmed up by friction with the road and also flexing as they respond to a less than perfectly smooth road surface.

So it makes sense to me (assuming you started with the same "cold" tire pressure at both 40 degrees and 80 degrees) that the friction with the road and flexing should be very similar. The reason tires are slower to warm up in cold weather - and I would guess NEVER reach the same rubber temperature as in summer - is due to both the cold surface of the road AND that cold air moving past the tires. I don't think we should be too quick to dismiss "wind chill" as a factor. And then, another disturbing thought occured to me.

In Canada there has been a lot of publicity encouraging people to get winter tires for their vehicles, even in areas where snow is rarely a problem. (US too?) The reason given is that "all-season" tires have much reduced traction, even on bare dry roads, when the temp gets to about 40 F. The tread gets too hard and doesn't have enough "squirm" to give you nearly the control you had for stopping or turning in higher temperatures. "Winter" tires not only have the lugs for gripping snow; they also grip the road better in wet and/or cold or icy conditions.

Could this apply to bikes too, especially "rounders" who do not fear rain or cold?

To the OP - thank you. You have opened up a much wider discussion than you could possibly have imagined. There is clearly a lot to be learned about cold weather traction. Maybe your submission - and all the responses it has and will receive - will help a number of riders avoid an accident. And I agree you should write a motosafe article for the ON. Get more reaction to this thread first before you write a column.

Then do it!
 
Tire/traction/temperature info for Police Bikers

I found this posting on a Suzuki forum, and it was credited to a police bikers forum.
:thumb
"By Lt. L.P. Walker

This is a notice to all motor officers, and especially those who may not have experienced much cold-weather riding. Motorcycle tires and pavement interact differently at colder temperatures giving a rider less traction than he would have in warmer weather. ItÔÇÖs good to remember that riding in cold weather is roughly the same as riding in wet weather.

Colder temperatures affect the rubber compounds in motorcycle tires by making them more rigid and less flexible than they are in warmer weather. This means that the tire has a weaker grip on the roadway surface. Of course the center part of the tire will heat up after a few minutes of riding and give more adhesion, but the edges of the tire that do not contact the surface during normal straight-ahead riding remain colder than the rest of the tire. During tight turns, these cold edges contact the pavement and can lose traction against the cold pavement and cause a front wheel slip that can be disastrous to the rider. In temperatures in the 30ÔÇÖs and below, this becomes much more pronounced.

ItÔÇÖs good to remember that cold tires against cold pavement can cause a situation similar to riding on wet pavement. So treat colder weather the same as you would wet weather, and remember to ride safe."
:clap

And, from a moderator on a physics forum:

"Cold tires are stiffer. Thus they do not flex as much as they would when the weather is hot. Less flex when rolling in a straight line means less heat generated in the core of the tire and thus less heat on the edges. So edges remain colder in cold weather not simply because of convective cooling from the cold air, but also due to lack of flexing of the carcass, which is one of the primary causes of tire heating. every time the tire rolls into its contact patch, the tire flattens, and this bends the tire shape from round to flat at that point and causes tire heating. Added to the cold tire stiffness causing lack of flexing is the cold tread temp having much less grip. This will depend on the compound of the tire, with summer tires providing much less grip at cold temps than tires with a more all-temperature compound."

I'm suspect that we could keep a few physicists and chemists busy on this question for the rest of the winter!!!

Indeed, fertile ground for a future ON article!!!

piperjim:usa
 
I don't think we should be too quick to dismiss "wind chill" as a factor.

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.
 
Bottom line: there is a point especially in cold weather (<3C) where there is no snow on the road to give you traction that sand, chemicals, black ice, frost, debris etc when one is taking a risk on two wheels. We all have a different tolerance to that risk but it is real. As someone who has not gone down in one of these situations on a street bike - I make a decision each season when to stop riding because there are far more uncontrollable things than controllable things. I find the science helpful and interesting to know why and how but when certain conditions exist on pavement - one will go down - no matter how expert. I know this may be controversial but cold conditions on pavement and the extreme variablity of weather and road surface issues (most of which canNOT be seen) is truly treacherous and unpredictable.

Cold weather riding on dirt and gravel with a bike tires made for it (including studs) extends the season right through winter most places in the world.

Ride safe.
 
Bottom line: there is a point especially in cold weather (<3C) where there is no snow on the road to give you traction that sand, chemicals, black ice, frost, debris etc when one is taking a risk on two wheels. We all have a different tolerance to that risk but it is real. As someone who has not gone down in one of these situations on a street bike - I make a decision each season when to stop riding because there are far more uncontrollable things than controllable things. I find the science helpful and interesting to know why and how but when certain conditions exist on pavement - one will go down - no matter how expert. I know this may be controversial but cold conditions on pavement and the extreme variablity of weather and road surface issues (most of which canNOT be seen) is truly treacherous and unpredictable.

Cold weather riding on dirt and gravel with a bike tires made for it (including studs) extends the season right through winter most places in the world.

Ride safe.

Very interesting point. Where I live there are two factors that I take into consideration when putting away the bikes. The first is temperature - <3C is just about right (daytime high as this would equal -5C overnight). The second is when they start with the salt and the sand on the roads.

And let's not get started on the roads that are shaded and those that are warmed up by the sun. In cold temps this can prove quite "exciting" especially if there is some water run-off.
 
A friend a long time ago told me that in the old days of motorcycle racing they lowered their air pressure in order to allow the tire pressure to come up to working temperatures and expanding the air inside without risk of decreasing rolling resistance due to having an over inflated tire.

I ride in the GA mtns during the winter and what I do is just that. I ride an '05 GS according to BMW recommended tire pressure is 32 front 36 rear so I lower them 2-3 lbs and find that my bike/tires grip better.

Just my two cents but then it could only be one.

Cal
 
Same thing yesterday

Wish I'd seen this post before doing the very same thing yesterday, it was cold bike had sat in the wind for two hours and I hadn't gone 100 yards when I turned too sharply at 15 maybe 20 mph. Watching your bike slide away from you as you sit on the pavement sucks. I attributed my fall to the still fairly new tires and leaning the bike over to where the contact patch was the edge (chicken strip) portion of the tire that was slick. Roadway perfectly dry, ego and finger bruised. Glad I'm not alone but word to the wise if I had taken a more normal approach (faster-smoother) to the curve I wouldn't be thinking about body work this winter.
 
Convective heat transfer vs. windchill

Yes, my use of the term "windchill" was incorrect. I should have used
"convective heat transfer" to describe the effect of air movement on heat loss from the tire. While similar, the two terms are not identical.

As I said, I'm a product of the public school system:cry

piperjim
 
Well I'll see if Avon Tire is interested in participating in the discussion. I just sent an email to their "Contact Us" section on their Avon USA website. I'm hoping to get some relative data on cold tire grip that I could use for an article submisson to the ON.
 
I did get a response from Avon Tire, though not very detailed and noncommittal (as expected). But the response did details that Avon and most tire makers recommoend 15 to 20 miles of riding until the tires are sufficiently warmed. It also said the cold air does have a cooling effect on the edges of the tire even when the center of the tire is warming up. So the edges take langer to get up to temp.
 
A friend a long time ago told me that in the old days of motorcycle racing they lowered their air pressure in order to allow the tire pressure to come up to working temperatures and expanding the air inside without risk of decreasing rolling resistance due to having an over inflated tire.

I ride in the GA mtns during the winter and what I do is just that. I ride an '05 GS according to BMW recommended tire pressure is 32 front 36 rear so I lower them 2-3 lbs and find that my bike/tires grip better.

Just my two cents but then it could only be one.

Cal

The question is - what's the temperature when you set the tire pressure? Chances are - by filling to a lower pressure at lower temps, you're actually filling the tire correctly.

I recently set mine at about 45F - to 34F/38R. Next day I took a ride in mid-30's temps. I have a tire-pressure-monitoring system that allows me to monitor not only the pressure (on a real time basis - every second) but also the temperature of the air in the tire. Leaving the garage, the tire temps were low 40's.

After riding for about 45 minutes, moderate speeds (never over 50MPH - it was also very gusty out, with 30MPH gusts, so no highway..) my tire temps had risen to about 60F. Tire pressure had increased to 38F/42R - and the difference was quite noticeable as the pressure increased.

BMW's recommended pressures are at 20C (68F) - any other temperature requires some compensation for the effect of heating or cooling the air in the tire.

In any case - my rule of thumb, below 50F consider traction much less then "normal" riding temps - and ride like it's raining (smooth, conserve traction, etc.)

YMMV (but Boyles law sez it won't..)
 
Don,

What you are describing is exactly what I do. The only exception is I have not quantified with instruments, etc; just my own gut feeling.

Cal
 
Dropping pressure 2-3 psi to increase the contact patch and, therefore, traction makes sense.

I think I will continue to keep my tires at the specified pressure and adhere to the guidelines such as riding like it's wet when the ambient temp is below 50 and knowing that it takes a good 15-20 miles to warm the tires. That said, I wouldn't hesitate to lower the pressures if I were facing a few miles of slippery roads, re-inflating when the hazard is passed.

It seems to me tires should be set at the specified pressure at the current ambient temperature in which the vehicle is operated. On airplanes and cars were are always cautioned in cold weather to check pressures and bring them back up to spec. It has to do with load capacity and energy efficiency.

Don, it looks like you got a 4 psi increase for only a 15 degree temperature rise. Charlie Boyle must be turning in his grave. :) It would be interesting to know how much the tire temp increases with all things equal except ambient temp. Your experiment showed about a 15 degree rise with a 45 degree ambient. Wonder what it would be with the same ride, wind speed, same road surface, same weight on bike, ie, all things being equal, except an ambient temp of 70. It won't be much more than 15. In fact, the inability to control the other variables might make such an experiment invalid. Or maybe what's needed is a designed experiment for a Six Sigma pro.

Good discussion - it will help me be a better rider next spring (tonight's ambient will be about zero F) and it's good mental exercise for this mechanical engineer who likes thermal science.
 
Tire Pressures

I have been running tire pressures in the 40 front and 42 rear range for years now, with no other adverse affects prior to this event. So I won't consider tire pressure to be at fault. At the lowest, my tire pressures "may" get down to the low 30's at worst.

I asked what pressures you are running because I have always understood that tires that are over inflated (within reason) run cooler, even in the summer. I have verified this personally by taking tire temperature readings after riding at different tire pressures to try and find the optimum pressure to run a tire long distances at high speed on my trips over the years. I also have had airhead models that due to thier age lack reliable tire pressure recommendations from BMW because today's tires aren't comparable to those made when the bikes were new. I have always found that the higher the pressure, the cooler they run.

Some of the other posts about lower pressures causing more tire flex and heating while cornering seem to also support this approach. Also, I suspect that the larger contact patch caused by a tire inflated to a lower pressure may help to warm a larger section of the tread. You may want to try lowering the pressures to 36 on the front tire (or whatever BMW has in the manual for your bike) and try the experiment again. Then, try different pressures, maybe even lower.

I suspect that you will find a much wider section of tread being heated and therefore able to get you as much traction as the center of the tire can provide.


Regards,

Greg
 
...............
Some of the other posts about lower pressures causing more tire flex and heating while cornering seem to also support this approach. Also, I suspect that the larger contact patch caused by a tire inflated to a lower pressure may help to warm a larger section of the tread. You may want to try lowering the pressures to 36 on the front tire (or whatever BMW has in the manual for your bike) and try the experiment again. Then, try different pressures, maybe even lower. .....................

Lower pressure increases traction and heat generation. Most people at track days START at 30 psi front AND rear. Some lower, certain Michelins (I believe), recommend starting at 24PSI in the rear!
 
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.
 
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