Dynamic and static friction on tires
Dynamic and static friction on tires
(OP)
In physics and statics they always told us that the friction force F is given by F=µN where N=the force normal to the contact surface and µ=the coefficient of friction. The friction force is independent of the contact surface area. This seems over-simplistic because intuition and observation show that wider tires on cars provide better stopping and starting. What does the friction force REALLY depend on? I know that a bicycle tire width on a car will not stop the car faster than the a wider tire! Somebody correct me if I'm wrong about this.
Tunalover





RE: Dynamic and static friction on tires
RE: Dynamic and static friction on tires
As an example, using a proprietary database of several thousand tires and a tire model of limited ability, a mean tire (Z=0) 26555R17 on an 8.5 rim has a peak longitudinal mu of 1.05, at an FZ of 6750 N, and an OD of 773 mm. A mean tire 24570r17 on 7.5 rims has an OD of 775, and a peak longitudinal mu of 1.062 at the same Fz.
The fundamental reason is that for longitudinal traction a longer narrower contact patch offers a more progressive slip velocity distribution at optimum slip.
We have discussed the mechanisms of tire grip before, I suggest you search this forum.
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
RE: Dynamic and static friction on tires
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
RE: Dynamic and static friction on tires
My guess: Normal pressure, sliding/rolling speed if not static, composition, temperature and surface roughness of both surfaces, cleanliness of the surfaces, nature and extent of any contaminants on the surface.
The physics equation is usually presented as being for "dry" friction. For something like tires, you're intentionally trying to come up with increased friction, so the farther you get from that "ideal" case, the better.
RE: Dynamic and static friction on tires
If your car has typical compliant street suspension which allows body roll, and independent suspension that keeps the wheels more-or-less upright to the body, those wheels will not be upright to the road under body-roll conditions. Skinnies and tires with a rounded tread profile might not be bothered too much by this. Wide, low-profile tires will try to ride up on one corner of the tread, so you are not getting the contact patch that you think you have. Many production cars do this intentionally with the front suspension as one of many factors to ensure that the car understeers.
This situation is far from a simple one.
RE: Dynamic and static friction on tires
I was told by my statics teacher in college that Newton's laws of friction do not apply to tires. He claimed that they vulcanize with the pavement, which is why tires can exceed one 1g acceleration. Does this sound intelligent?
I just Googled "aluminium friction". According to The Engineering Toolbox, aluminium on aluminium has a friction coefficient of 1.05 to 1.35.
--
JHG
RE: Dynamic and static friction on tires
Rubber does mechanically interlock with rough surfaces; the softer the rubber the more it can deform to interlock. Interlocking is what allows steel-on-steel parts to have very high levels of resistance to motion, as screw thread parts demonstrate.
The high friction of aluminum on aluminum is due to galling, where the oxides of aluminum are scrapped off and the crystals in one piece bond with the crystals in the other pieced. It doesn't apply to anodized aluminum, for example. It's a form of welding.
RE: Dynamic and static friction on tires
Tunalover
RE: Dynamic and static friction on tires
As such in general a larger contact patch will tend to have a higher peak friction, because a softer compound can be used, because the shear stresses are lower, for a given life. But a wider tire doesn't guarantee a larger contact patch, it just gets shorter.
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
RE: Dynamic and static friction on tires
The problem is that in going from a thinner tyre to a wider tyre, there are many things going on which affect friction - tyre compound, tyre deformation, tyre deflection (especially if going around corners) and all the things greg mentions.
If you try to build a tyre to handle the same load, but at 25% of the width / contact area, you will have a completely different type of tyre and characteristic, so the width issue is not simple to compare like with like.
Tyre grip or performance varies hugely with surface, dryness, temperature - so many variable and issues that there is no "simple" generalities.
My motto: Learn something new every day
Also: There's usually a good reason why everyone does it that way
RE: Dynamic and static friction on tires
http://greglocock.webs.com/vehicledynamics.htm
you'll need matlab or octave
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
RE: Dynamic and static friction on tires
"There MUST be at least some simple, generalities that can be made relating contact area and friction. What are they?"
You can't have it both ways!
It might help a lot to tell us what it actually is that you're actually trying to do with this information. As it is, you basically have an "Explain friction" post, which can generate a lot of answers without giving you the information you're after.
RE: Dynamic and static friction on tires
Other more detailed answers attempting to illustrate the phenomena can be found on the following pages:
http://astro.physics.sc.edu/~rjones/phys101/tirefriction.html
http://newton.dep.anl.gov/newton/askasci/1993/physics/PHY2.HTM
http://insideracingtechnology.com/tirebkexerpt1.htm
RE: Dynamic and static friction on tires
1) There is differing normal force on differing subsections of the tyres contact patch. There are also differing shear forces throughout the contact patch to be transmitted through friction, because rubber is extremely flexible compared to the stiffness of the static friction bond and the ground. Therefore, parts of the contact patch will slip as they have less normal load and more shear forces. So the contact patch is partially static and partially dynamic - this, amongst other more difficult for me to explain reasons, really ruins any simple attempt at a single friction coefficient.
2) Note that there is no rule stating things cannot have a coefficient of friction greater than 1. Most interfaces don't but the 1:1 ration is just arbitrary. I recall a post on this very forum years ago describing an experiment to convince people of such involving a piece of some rubber or another on an inclined plane of glass, and the rubber didn't slip until well past 45 degrees.
RE: Dynamic and static friction on tires
"As such in general a larger contact patch will tend to have a higher peak friction, because a softer compound can be used, because the shear stresses are lower, for a given life." (GregLocock)
Engineering is the art of creating things you need, from things you can get.
RE: Dynamic and static friction on tires
T^2 = 2d\a
T = ( 1320 ft / 32.2 ft/sec^2)^.5
T = (41)^.5 seconds
T = 6.4 seconds = the quickest any vehicle will traverse the quarter mile driven thru its tires if max rubber u = 1.0. (and neglecting aero drag)
That was sometimes heard back in the 50s and 60s.
It took the Top Fuel dragsters until the 70s to defy high school physics and exceed 1 g average acceleration for 1320 feet
RE: Dynamic and static friction on tires
Perhaps the drag boys had suspension problems, or their tire pressures were too low.
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
RE: Dynamic and static friction on tires
"Schiefgehen will, was schiefgehen kann" - das Murphygesetz
RE: Dynamic and static friction on tires
RE: Dynamic and static friction on tires
Last time I checked, top fuelers have about 1000lb of downforce at zero road-speed. This is jet effect from the upswept exhausts. Power has gone up since then so the 1000lb has as well. This is probably one reason they use clutch-slip and ignition retard (not throttling) for low speed traction control - it is important to maintain mass-flow through the engine to generate this DF.
Engineering is the art of creating things you need, from things you can get.
RE: Dynamic and static friction on tires
"Schiefgehen will, was schiefgehen kann" - das Murphygesetz
RE: Dynamic and static friction on tires
RE: Dynamic and static friction on tires
Why do wide tires work better than skinny tires in normal uses?
It is NOT due to friction, it is due to shearing and wear.
When you put the tires to the limit they shred, which is a function of shear strength.
Hardness is proportional to strength including shear strength.
Harder tires have a lower coefficient of friction between the tires and the road but do not shred as easily, so they last a long time.
Softer tires have a higher coefficient of friction between the tires and the road and shred easily.
To have both a high coefficient and strength the area in contact with the road must increase.
Hence the huge tires on top fuel that are really soft, at low air pressure so they deform, are heated by the burnout so they are softer yet and will turn into a glue between the tire and pavement. One other function is if the contact area has time to set with perfect seal between the tire and road, a vacuum will form increasing the normal force but just for the very start of the launch,
Please also note top fuel tires do not last very long…
Hydrae