Tire size vs traction 2

[MightyGNU]

I have seen large rear tires on many a (allegedly) high power RWD car, and have always thought they would increase traction. Just now I realized I have no idea why that would be. Given a tire with x cm^3 in contact with the ground and a given vehicle mass, there should be a certain pressure on the contact patch of the tire, if you make the tire twice as big, that pressure should drop by %50, the force remaining the same. Come to think of it, changing tire pressure changes the size of the contact patch, and I know from experience that changes traction, so why is that? Is traction just not linearly proportional to pressure/area?

 

[GregLocock]

The schoolboy model of friction is just one of those lies we tell children.

https://en.wikipedia.org/wiki/Lie-to-children

A bigger tire can be made from softer rubber for a given life, which will increase some of its friction properties. A bigger tire can be run at a lower pressure, which will improve some of its friction properties. People write very long papers about this stuff, which are almost meaningless to me, in the real world, and are meaningless in all senses, to the consumer.

You /can/ model tires, using the brush model. The brush model is great, it seems like a sensible approach, based on physics. It is typically out by 50%. Sensible tire companies who care (I count two) use extremely large FE models. These look fantastic on the screen, but I've never seen a comparison of model to reality. OTOH we've measured thousands of different tires and have a statistically based model of their performance, so in theory I can predict the change in maximum grip for a given change in section dimensions. Of course the problem is that you don't just change one dimension - if you drop the ar to give the same sidewall height alone, peak mu doesn't change as the tire width increases, but the bigger CP allows you to drop the pressure, and your peak grip increases.








Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

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[3DDave]

The nominal friction model assumes the gross shapes and mechanical properties involved do not change appreciably over the range of interest. This is certainly not true when looking at elastomers which are the majority component of a tire.

Increased traction due to lower pressure is usually because the mating surface is yielding, limiting the maximum shear force/unit area and for which the increased area increases the total available force. In other cases, such as dragsters, it can be the opposite as the rubber is being torn loose from the tire due to adhesion. Flood a steel plate with soapy water and watch that increased area yield worse results.

 

[EdStainless]

#DD hit the root issue, when tires are dependent on friction you are out of control. Traction is closer to soil mechanics with a soft compliant surface interacting with a hard abrasive one.
And what part of traction are you interested in, straight line acceleration, turning, turning while accelerating or braking. These all put different forces on tires and bring the entire system (suspension, wheel, side walls, tread) into play.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[NormPeterson]

Is traction just not linearly proportional to pressure/area?

Tire unit contact pressures are far from uniform over the entire contact patch even before realizing that tread features like grooves can't support any load at all, and tread block flexibility likely ruins any notion of linearity on a local level.


Norm

 

[RRiver]

The big rear tire, raked stance of a car was a sign of its time and definitely increased RWD traction when done right. Granted, mostly kids at the time were after the look of a performance car and not an actual performance car.

The big tire strategy was about anti-squat and rear wheel traction. During a hard acceleration start there is a weight transfer and CoG transfer in the car where the rear end drops. Through the suspension and tires the weight transfer can be mitigated. By mitigating the weight transfer wheel spin or burn outs is reduced and potentially removed all together. When a tire is spinning it's power loss to making the car go forward. This is an explanation lite but it's the basic idea.

 

[CapriRacer]

Tires do NOT follow Amontons' Laws of Friction - especially the one about contact area. that's because the road surface has texture and the (relatively) soft rubber penetrates that texture to produce more grip. The max grip occurs at about 15% slip. because the rubber is being torn off.

So more contact area results in more grip - all other things being equal.

Then there is the issue of tread rubber. There is a technology triangle involving treadwear. traction, and rolling resistance. To get more of one property, one or both of the other properties have to be sacrificed. There can be HUGE!! differences in tires just from the tread rubber itself!

 

[BUGGAR]

"Granted, mostly kids at the time were after the look of a performance car and not an actual performance car."
Well, I have to disagree here. We honestly believed forward lean would increase weight transfer and traction. It was only later in engineering school that we learned better. That's why we went to school.
And as for being concerned about looks, I remind you we were the originators of what you now call rat rods.

 

[RRiver]

I'm not sure what you learned in school but Jim Thornton who the NHRA credits as the inventor of the funny car and torque cancelling chassis would disagree along with a whole lot more people. That fact that his designs and principles are still SOP today supports this. I don't think I could count the number of kids I knew in the 60's that put big rear tires and traction bars on their cars while never once pulling out a book of any kind. I don't think "We honestly believed," qualifies as actually pursuing performance. Building performance cars requires just as much if not more work with clean hands as dirty hands. BTW, Thornton was also the head of Chryslers racing department and the person who conceptualized the Daytona and Superbird. He was also the driver for the Chrysler Ram Chargers and still holds records today that have stood since the 60's.

 

[3DDave]

I give - what is a torque canceling chassis. I see no references via Google except that Jim Thornton invented it.

 

[GregLocock]

Best I can find is "Thornton was also responsible for fine-tuning Dodge’s leaf spring rear suspension. By juggling the number of leaves, adding half-leaves to stiffen the forward segment like a traction bar, and adjusting overall arch, Thornton successfully cancelled the effects of engine torque to evenly load both rear tires for optimized traction off the starting line. "

So with asymmetric anti squat he evened up the tire loading instantaneously, perhaps.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

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[RRiver]

Greg, I'm sure this was part of where he ended up but the "Torque Cancelling Chassis" isn't a thing. It's a design process. I was given this information secondhand so I can't verify it's accuracy but I was told it was something he came upon while reading about the early days, late 50's - early 60's, of high speed train development and problems the engineers were dealing with. I've got some more information about this around somewhere. It's been almost 30 years now but I'll see if I can find it.

 

[BUGGAR]

The torque cancelling chassis is as mentioned. Back in our day, big cube Detroit iron would spin the right tire due to driveshaft/engine torque trying to twist the solid axle and lift the tire. So preload springs diagonally to compensate.

 

[BUGGAR]

I forgot to add that tire spinning was not the problem (locked diff's) but equalizing the vertical loads on the tires was thought to be good because tire traction varied with vertical load.

 

[RRiver]

The convential wisdom for dealing with big power engine torque at one time was stiffer frames. This added weight but it worked to a point. Stiff can be too stiff though. A common example of too stiff in the early 60's was the drag racer that went into the wall at lauch. As stiffness goes up the ability to control the car goes down. The same thing happens with suspension springs. Too stiff and under and oversteer is adversely effected.

Another transportation story: As trains got faster and faster the resonace vibrations between the track and the car wheels made the passengerr ride unbearable. Rather than solve the issue with resonance they solved the issue of passenger comfort. IIRC there are 17 identified degrees of freedom with the rsonance but only one effected passenger comfort. Resonance and vibration is directly proportional to the speed of the train. The solution was a secondary suspension for the cars that kept the natural frequency of the cars at a constant. All that was required was to solve for the lateral resonance of the cars for passenger comfort.

This is what Thornton did. He broke the torque issues down to what were problems and discarded the ones that weren't. There's also no universal solution to torque problems. The shortened wheelbase funny car, rethinking the design of the torque box and torque tube, the concepts of the symmetrical and assymetrical frames or subframes were re-thought, etc. The torque cancelling chassis was all about solving the problems and not the source of the problems. It all started with performance vehicles but it has evolved since in to the designs of the daily driver.

Thornton didn't leave Chrysler on good terms and Chrysler has almost completely written him out of their history. They don't recognize the man that built much of the reputation they enjoy and capitalize on today with performance. Read the full history of the "Little Red Truck" sometime. It was an idea that started in the garage of a couple of engineers but they got finacially in over their heads. Thornton fought with Chyrsler about taking over the project and only got them to support the project if he could find someone else to take it over. He did. Read Chrysler's history now and it reads like the truck was managements idea all along. There's no mention of Thornton. I even saw a "Graveyad Cars" episode that parrots the Chrysler story. Surprising considering the authority they pass themselves off to be. The full story is available. Anyway, this is why I imagine there's not a whole lot available on the torque cancelling chassis beyond some limited examples on how it's been applied.

 

[NormPeterson]

RRiver - I suspect that most people in the 1950's and 1960's got their drag racing "book learning" from the illustrated pages of Hot Rod Magazine and similar magazine titles without ever being aware that more scholarly information existed. We didn't have Google back then to help find it anyway.

People still look at things like this and confuse visible consequence with cause.


Norm

 

[RRiver]

Norm, I couldn't agree more.