wider is better? or not? size of contact patch arguement 8

[steved33]

In a very heated debate on another forum, some associates and I have been debating whether or not wider is truly better. So I address this forum...(it's been forever since I posted last)

The basis is that for the same compound of tire, running in it's proper temperature zone, that wider is not always better.

On one side of the arguement, the insistence is that as the area of the contact patch increases (you go to a wider tire), that increase in area is what gives you more grip/traction.

On the other side, grip/traction is controlled by the coeff. of friction x the normal force, and width really doesn't matter.

really the question is...Does the size of the contact patch matter?

Apply this to an autocross or road race car that could be setup on anything from a 195 to a 275 width tire.

I can supply a pointer to the arguement if needed.

steve

 

[NormPeterson]

On one side of the arguement, the insistence is that as the area of the contact patch increases (you go to a wider tire), that increase in area is what gives you more grip/traction.

and

On the other side, grip/traction is controlled by the coeff. of friction x the normal force, and width really doesn't matter.

It's inaccurate to assume that as the section width or tread width increases that the contact patch area also increases. Nor does it remain constant as [Vertical Force] / [Constant Inflation Pressure] suggests. Conceivably, it could decrease depending on the relative vertical stiffnesses. But what will change is the shape of the contact patch, becoming shorter in the longitudinal direction as it is made wider (assuming a constant load), and this may be partly responsible for more uniform unit loading over the area of the contact patch. All else equal, integrating a more uniform unit load and associated grip coefficient function over the entire contact patch should give more total lateral grip. The wider tire probably also has higher vertical and lateral spring rates, meaning less camber loss and lateral distortion in actual driving respectively.

BTW, the drag strip crowd tends to run tires that give relatively longer contact patches . . .

Mostly out of curiosity, but what is the other site that is currently discussing this?

Norm

 

[GregLocock]

Also don't be misled by the coefficient of friction thing. It does not apply to tyres in any useful fashion. Wider tyres do have more lateral grip for a given vertical load. Reducing the tyre pressure also increases the lateral grip, up to a certain point, then it falls off as the contact patch distorts.



Cheers

Greg Locock

 

[evelrod]

Steve33, since you reference all this to "autocross and road race" I'll add my two cents worth.

In years past I did some tire testing for Hoosier tires as mounted to an SCCA GT-4 car and, relying on that experience I mounted a "tire test" on my vintage racing Austin Cooper this past June. The object of the test was to determine if the current crop of Yokohama radials or the Hoosier TD's were best for my car. The Hoosiers are racing specific bias tires (used by vertually all my competitors) and the Yokos are radials. Both tires read 57/58 on a durometer and both tires are approx the same size. The big difference is that the Hoosiers are 1 1/2 inch wider.
All suspension settings were set specific to each tire, the radials requiring substantially more negative camber settings front and rear. The results after the three day session and some 12 tires was in favor of the narrow radials by about 1 to 1 1/2 seconds per lap. Tire temps were also a bit lower on the radials. The BIG surprise was that after 4 hours of track time the radials still look "as new" and ready for more racing. The Hoosiers were "pretty well used up" with the same track time.
These tests were for our team and for this ONE track. Results from subsequent races at other tracks have not been conclusive except in the area of "wear" and that is still exceptional. Heat cycles also do not seem to effect the durometer readings adversely as yet but, one would expedt to see some significant hardening eventually. I plan to "soak"(chemically treat) four tires for a few weeks and try them at some future event. I hear a lot of talk about tire soak but, nothing definitive. I am not bound by any rules restrictions so it sounds like an idea for the time. All this points to the main factor in performance is most likely the tire type and construction and less likely the tire size as a determining factor. Yes, the bias racing tires are most probably 30 year old technology but, as I pointed out, most all that race in my class of car in the U.S. use the bias tire. SCCA smaller GT class cars are universally using these bias racing tires. They get wider and wider each season---my contention is that they are going in the wrong direction. Time will tell but, I see a time when all these ancient tech bias tires will be obsolete. That's my opinion, anyway.

Rod

 

[steved33]

http://forums.bimmerforums.com/forum/showthread.php?t=245422

for the curious, namely norm.

This all originally started with a post "I need more grip, how wide should I make my tires"...and has spooled out of control into "wider /= more grip", and in fact, can reduce grip in certain situations.

It's pretty funny reading, and the discussion shouldn't be taken seriously.

sjd

 

[GregLocock]

Rod, I think you were comparing radials vs crossplies, rather more than evaluating tyre width, but that's fair enough. I've always liked Yokohamas when they were fitted to our sporty derivatives.

When I'm back in the office I'll have a look and see if I have any objective data on this, although I can foresee a couple of problems - for a constant aspect ratio the wider tyre will have taller sidewalls, so I really need to compare tyres of the same sidewall height but different width. Also they tend to make the wider tyres with a different construction to the narrow tyres.

I'm sure Michelin have published some stuff on this.

In theory for a given tyre wear you could have softer rubber on the wider tyre which should improve the dry weather grip. I suspect this is a small effect.

Cheers

Greg Locock

 

[patprimmer]

I am not the most qualified to comment on this, but in my experience and from what I have gleaned from more qualified people, wider is generally better re grip, but there are several qualifiers.

1) Wider tyres are more prone to aquaplaning.
2) Wider tyres are more sensitive to camber control.
3) Wider tyres on the same rims might not perform as expected, as rim width relative to tread width can have a significant effect.
4) It is virtually impossible to get a result with all other things being equal as there are a lot of variables.

Regards
pat pprimmer@acay.com.au
eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[ZoRG]

Having a bigger tire, and or bigger rim, also adds more weight.

 

[Xwheelguy]

Having a bigger tire, and or bigger rim, also adds more weight.

That is relative to the design of the wheel & the application in which the wheel is being applied. A 16 inch passenger car wheel is more likely to be lighter than a 15 inch truck wheel due to the difference in maximum wheel load.

Also, let's not forget that roll formed rims are now becoming more popular, so it is possible for a rim to be an inch wider but lighter than one that is not roll formed due to the difference in the rim's wall thicknesses.

Tim Flater
Senior Designer
Enkei America, Inc.

http://www.enkei.com

 

[ZoRG]

I have a feeling the original poster already has lightweight rims and won't be trying out truck tires

 

[Xwheelguy]

Yeah, I kinda had that feeling too as it was rather obvious. On the same note, the mention of a truck rim was what I felt an obvious example as well.

Just to make sure you're feeling my meaning, bigger is not necessarily heavier. That's all I was saying.

Tim Flater
Senior Designer
Enkei America, Inc.

http://www.enkei.com

 

[GregLocock]

I just went to a presentation by a big tyre manufacturer, they are very insistent that in the dry, for high performance handling (ie grip vs wear, basically), wider=better.

All tyre design is a compromise, the more real estate you've got to play with, the better that compromise will be.



Cheers

Greg Locock

 

[steved33]

could that just be that the breakaway is more pronounced at the limit?

you have grip grip grip grip nothing...as opposed to a skinny tire which is grip grip give give slide nothing...it's what made 911 turbos such bastard cars (and all the weight in back) as soon as the tires gave, you were done... the early cars are brilliant...esp the 356's and 912's...

sjd

 

[sreid]

I think that it is clear that wide tires generate more grip if the car is set up to match the tires. My basic reaoning is that no one races with skinny tires and racing regulations have rules to limit tire and wheel widths.

One reason (amomg others) is for a given weight/side load, a bigger contact patch results in lower unit area loading. This decreases the shearing force per unit area that's trying to break the adhesion between the rubber and the road.

 

[steved33]

so how does area fit into the equation?

friction force = (coeff of fric) x (summation of tire forces (Fx, Fy, Fz)

or is there more to grip than friction force?

sjd

 

[NormPeterson]

so how does area fit into the equation?

Probably less directly than uniformity of the unit normal loading over that area in its deformed shape. It's entirely possible for unit loading to vary up to 2:1 over the contact patch.

Norm

 

[GregLocock]

steved33. That formula bears very little resemblance to any useful data for automotive tyres.

Try a google search for Pacezka, or Magic Formula, or even just a search in this forum.

As a rule of thumb the maximum lateral or longitudinal force available froma tyre is roughly proportional to (the vertical weight on that tyre)^0.7



Cheers

Greg Locock

 

[steved33]

thanks...took a look...it was a very interesting read. I might have to pick up one of his books on tire dynamics...

sjd

 

[shanba]

Remember too that there are three tire/road interactions that generate total tire grip, it is not just the coefficient of friction * normal force. There is material deformation over the road surface at the localized level (softer rubber = more grip), and hysteresis of the compound as it slides across the road surface.

 

[steved33]

right, it's how dragracers can get a coeff. of friction greater than 1.

still nothing about the area in contact with the road is ever mentioned. the material deformation is due to the compound, and the force provides the grip, that's why racecars have downforce, more grip... nothing is ever mentioned about the area/size of the contact patch except in our breakaway characteristics and our slip angle arguements.

sjd