CypherNinja

Look in the wheels and tires section. (After drooling over the rest of the car) And, remember, the wider tire will be running a LOWER pressure due to its increased volume. Thus having an even larger contact patch under real world conditions.


EDIT: Dammit, a link helps doesn't it?

http://hem.passagen.se/hemipanter/


Oh yeah, and FEAR THE HEMIPANTER!!!

LOL, I go to that site every few weeks just to drool, that guy is my hero.

SleepR1

The slightly larger contact patch for the rear 13-inch wide tire (340 cm^2) is probably due to the slight rear weight bias of the Pantera (60% rear)? FWIW, this is an excellent example of what wider wheels/tires do for contact patch. The 13-inch wide rear wheel/tire elongates the patch into a more "landscape" strip shape rather than a squarish, "portrait" shape which is seen in the 10-inch front.

If we were to mount narrower wheels/tires up front, and wider wheels/tires in the rear on a nearly 50/50 weight distribution, FD Rx7, you'd see the static contact patch area to be about the same.

It's interesting to note that eventhough the Pantera's tires are so wide, the total square inch area of the Pantera's static contact patch is LESS than the total square-inch area of a 3 x 5 index card. The Pantera only weighs 2970 lbs, so it's no surprise it has such a small static footprint.

3 x 5 index card = 15 in^2
308 cm^2 = 12.13 in^2
340 cm^2 = 13.40 in^2

Of course all this goes out the window, once the Pantera tries to hook up 500 hp to the rear wheels. The dynamic rear contact patch will approach 1.5 x rear static contact patch, or 20 in^2 under WOT longitudinal acceleration (assuming the driver is within 15% longitudinal tire slip). The dynamic rear contact patch could approach as much as 1.8 X static rear contact patch with additional downforce under WOT forward acceleration.

Therein lies the reason for wider wheels and tires on a light car. It boils down to matching available grip potential to the car's total performance potential. IOW, it would be pointless to put 10 and 13 wide wheels/tires on a 235 hp Rx8

DamonB

Sleep, this cannot be so. If the contact patch were to stay the same you are again saying that increased tire width does not equate to increased grip (due to increased contact patch). A wider tire has a bigger contact patch every time! If it didn't there would be no need to run wider tires! If you insist that a wider tire has no bigger contact patch than a narrower one then the wider tire would not offer increased grip in ANY condition because all reactions occur solely at the contact patch. It can never be said that the contact patch area does not increase and yet also say that cornering force does increase. Weight transfer is solely a function of cg height. Regardless of the sizes of the tires they are going to see the same weight transfer. Since weight transfer is thus constant for every vehicle wider tires must offer increased grip solely because of increased contact patch. The increased grip is not due to any other variable.

Due to the fact that tires themselves are elastic as well as the air inside them the wider tire will always have more contact with the ground than the narrower one! The wider tire holds a greater volume of air and thus does not require as much air pressure to support the weight of the vehicle. Even if the wider tire were left at the same air pressure as the narrower one the wide tire would STILL have more contact patch because car tires have flat tread faces.

All these numbers about slip etc are valid, but the argument was that wider tires are only better than narrower ones in turns. I say that's not true. Tire width does not only improve cornering, it improves every aspect of vehicle reaction to the ground.

redrotorR1

Note that I edited my post to remove the weight factor of the truck and RX-7 from the equation. I challenge you to proof otherwise with my Camaro comparison ... or other version of it.


Meanwhile ...

I cannot more wholeheartedly disagree with this statement. I must ask where you got this information from, as I'd like to know how this calculation was made. And WRT corner exit, I can be MUCH, MUCH sloppier on entry and exit position with racing tires compared to street tires. Your comments here completely refute ALL of my racing experiences .... whether it be on a road course or in a parking lot.

redrotorR1

Foregoing the current argument on static contact patch, here you detail the dynamic contact patch. Under load, the tire deformation will inherently increase contact patch size .... therefore exemplifying the reason to run wider tires. And this is just straightline acceleration. Now figure in negative camber gain during mid-corner or during transition. Sooooo ... why are you arguing that contact patch doesn't increase with a wider diameter tire???

SleepR1

Contact patch is a function of tire load. Wide wheels/tires by themselves will not increase contact patch. You must put loads on the tires to increase contact patch. Therein lies the rub. You must match the wheel/tire width to the car's performance potential. Lightweight, low hp-to-weight ratio cars can't make the best use of wide wheels/tires, while high hp-to-weight ratio cars can, especially if the high hp-to-weight ratio cars have aerodynamic aids. Look at the Lotus Exige versus a Dodge Viper. Compare the wheel/tire widths between the cars. Low hp-to-weight ratio cars with really wide wheels/tires have a hard time overcoming tire rolling resistance, and dealing with the extra unsprung weight. The gain in available grip (assuming similar tire compounds between tire sizes) isn't maximized because a low hp car cannot load the tires enough to increase the contact patch size available with wider wheels/tires.

DamonB

That's true, but tire load is not the ONLY reason.



This is not true.

This is not true. The contact patch is already larger given that a wider tire is on the car. The fact that the car doesn't have huge horsepower and neck snapping acceleration (and therefore big weight transfer) does not mean the increased tire width is not put to use.

I will repeat myself from earlier:

The rate at which the coefficient of friction rises due to download is not constant; the rate tapers off as download increases. This does not mean total traction doesn't increase with incrased download, but it does mean that a 50% increase in download does not equate to a 50% increase in grip; it may only translate to a 25% increase (or less!) in grip for instance. The relationship between grip and downforce is an increasing one, but not a constant one. The converse of this explains why all you need to increase grip is a wider tire.

The statement says that increased downforce (or weight transfer) does bring increased grip. BUT the relationship between the two is not linear; the increase in tire grip accrues at a rate slower than the increase in download on the tire. This is a god given fact about any tire. Since this is a fundamental property of tires we can turn it around backwards and say that a decrease in downforce will also result in less tire grip, but the rate of decrease in downforce will be steeper than the rate of decrease in grip.

If we take that to an extreme we can say there is NO downforce and find that even with no downforce (or weight on the tire) the tire does in fact have grip and that grip is solely based on the size of the tire's contact patch.

Here is the fundamental experiment that proves all you have to do is increase the tread face to increase grip. We did it in high school and it can be easily replicated by anyone with some rubber and a spring scale. Please note the results are particular to rubber. A hard material would have different results.

Take two pieces of the same type of rubber; one piece 3"x12" and another 6"x12". We will make the 3"x12" piece of rubber twice as thick so it weighs the same as the 6"x12" piece. Even though the two pieces of rubber have the same weight the contact patch of the 6"x12" piece is twice as big (does anyone disagree that the bigger piece of rubber has a greater area of contact?). If you were to place these pieces on a surface and measure their coefficient of friction you'd find that the 6"x12" piece has a higher cf in any direction you choose to measure. Even though the two pieces weigh exactly the same the 6"x12" piece has twice the contact area which results in it's increased coefficient of friction. This experiment is fundamental and irrefutable. No matter what other forces (tire pressure, weight transfer, download etc) you choose to add to the experiment you will find that under the same conditions the larger piece of rubber will have a higher coefficient of friction in any direction you choose to measure every single time! This is soley due to its increased area of contact!

The fact that you put a wider tire on a car automatically means the contact patch has increased and therefore the coefficient of friction between the car and road has also increased! Weight transfer, tire pressure, tread compound etc all play roles in overall grip, but since a wider tire experiences the exact same forces a smaller tire does it will ALWAYS have more grip in ANY direction than the narrower tire because the wider tire will ALWAYS have a larger contact patch than the narrower tire. In fact you could make the tire so fricken wide that the car wouldn't have enough horsepower to overcome the road friction and the car would slow down. Because the car has slown down does not mean that the grip did not increase! The grip increased so much the motor could not overcome the rolling friction!

Wider tires absolutely positively have larger contact patches than narrower tires and therefore they absolutely positively have more grip in every direction at all times!

If anyone believes differently please perform the experiment I described.

DamonB

You've got it backwards. Slicks do have highest grip at lower slip angles than street tires, but the grip of the slicks also falls at a much faster rate once you reach maximum grip than a street tire does. Street tire grip does not fall nearly as quickly as race tire grip once the maximum grip level is acheived. This is why it takes a finer driver to drive on slicks: Once slicks reach their maximum potential their grip plummets. A street tire is more forgiving because once it reaches maximum grip it does not completely break away as quickly as the slick.

If you think about it it makes perfect sense. When you reach maximum grip on a racing slick and then begin to exceed it the car will immediately wish to exit the race track as it begins sliding. The street tire can be happily driven sideways and recovered much more easily than the racing slick because it's grip level doesn't drop as quickly once "maximum stick" is obtained.

ArcWelder

I seem to remember from the Skip Barber book that the slick had a "peakier" CF curve but the street tire actually fell off quicker at higher slip angles than the racing slick. I'll have to double check though.

Mark

DamonB

The graph you refer to is on page 198. At very high slip angles the street tire's grip does indeed fall more rapidly than the race tire because the street tire is of a harder compound. If you concern yourself with the actual usable grip of the tire (which anyone but an extremely poor driver would use) you can see that the rate of decrease after peak grip is much higher for the slick than the street tire. You have to get much more out of shape with the street tire before its grip drops rapidly, so overall the street tire is more forgiving. That graph is also of a shaved street tire. A real tire you would drive on the road would be even more forgiving than the shaved tire since with tread it would be able to absorb even more slip angle without breaking completely away.

By your description a "peaky" tire is going to have steeper slopes on each side. That means the slick gives up faster.

SleepR1

Going Faster Mastering the Art of Race Driving, by Carl Lopez, Fig 13-5, p. 198. Street tires CF plateu @ 1.2, but its broader usable slip angle (from 2 to 9 degrees, CF of ~1) is what most drivers perceive as forgiving...up to a certain point. When you exceed the street tire's usable slip angle (and thus grip), the traction falls away sharply--slip angle from 9 to 11 degrees yields a drop from 1.25 to 0.5 CF. Yes the tire is probably shaved to 5/32nds. This is to minimize overheating the tire compound. Full tread depth tread blocks are susceptible to hysteresis, and thus excess heat generation from each tread block's deformation under track conditions. Overheating the tire would yield lower CF, but I'm not sure how overheating affects the street tire's slip angle range.

The racing slick reaches its CF peak @ 5 degrees slip angle. With slip angles from 6 to 11 degrees, CF falls from 1.5 to 1.2. Very forgiving drop in CF eventhough you've slid the tires 5 degrees past its optimum CF/slip angle range

To get the most out of racing tires, the driver should stay at that narrow range of 5 degrees (+/- 0.5 degree)--and this is where the driver skill comes in. Not enough slip angle, or too much slip angle, and you're wasting the tire (by not using the racing tire's maximum grip potential).

IMHO, if you're not competing for trophies, there's no point in wasting money on racing slicks @ open track events. The point of open track is to have fun, and street tires are undoubtedly more entertaining with ~8 degrees of usable slip, while staying in the 1 CF region

As stimulating as this information exchange has been for us veterans, I'm not sure we've actually answered Splinemodel's question?

Devilish

I love all this technical stuff. To answer the original post, I think the most important thing to consider is WHAT KIND OF RACING will you be doing? Look at the rules and see what is allowed. Will it be casual track days or professional-rub 'em-out racing?

What size brakes will you be running - 13", 14"? What tires are you running - Hoosiers you can order from Tirerack or real, real race rubber (not to disparage Hoosiers)? I remember a while back, Hoosier supposedley used a stickier compound for their 18" tires though it may have changed now. Do you want to run 305's in the back?

It's all important stuff to consider, but I wish you the best of luck.

clayne

Let's have fun with this topic...


Take an assembled 13B-REW engine:

a. Put it on a piece of 1/8" thick ply about 48" x 48" with the oil pan side down. Tape the oil pan to the wood with duct tape.

At the same time have your friend take his own 13B-REW engine and:

b. Put it on a piece of 1/8" thick ply about 12" x 12" with the oil pan side down - approximately enough to sit between the pan and the concrete only. Tape that pan to the wood too.

Ready?
Set?
PUSH!


Damnit, friend B seems to have used the concrete garage floor as a planer for his oil pan, why did that happen?



The larger ply *should* have stuck to the ground *more*, transfering excessive load to the duct tape, causing the block to tear off from the ply, right?

The above is assuming a larger contact patch automatically has a higher cF, as presented in the quote. But we know intuitively that friend B would have had a hell of a time pushing his engine block across the garage floor with only his 1 ft^2 ply spacer AND the duct tape would have easily ripped way before you were half-way across the garage with your 4 ft^2 ply spacer, duct tape still intact.

WHY?

We know in a large amount of cases that frictional force tends to be independent of surface area - in LOW pressure situations. The above example is a very simplistic one - but I'm trying to use it to show that surface area does indeed play a role on cF as the pressure goes up. Naturally, pressure goes up as surface area goes down.

Our cars do not dynamically change their net weight, but they do "transfer" the weight under various cornering and braking conditions.

While it may be reasonable to make the observation that a piece of rubber with more contactable surface area has higher "grip" than an equally weighted piece of rubber with a smaller surface area - that example depends on constant low pressure over the area. This doesn't even take into account distortion of the rubber or increased molecular grip as it is a low pressure example. I think it's safe to assume that the contact patch of a typical performance tire when sized appropriately has a reasonable level of pressure on it - especially do to the fact that it's a round object, not a flat one. It's actual contact area with the surface is lower as well - which ends up increasing the pressure - which ends up distorting the rubber - which ends up changing the shape of the contact patch more in the lateral direction as the tire becomes wider and vehicle weight stays constant.

In order to really have an accurate situation one would need a plot of a given tire's cF vs surface pressure.



----
http://www.physlink.com/Education/AskExperts/ae200.cfm

Question:

As an engineer, I know that friction does not depend upon surface area. As a car nut, I know that wider tires have better traction. How do you explain this contradiction?

Asked by: Mark Secunda

Answer

This is a good question and one which is commonly asked by students when friction is discussed. It is true that wider tires commonly have better traction. The main reason why this is so does not relate to contact patch, however, but to composition. Soft compound tires are required to be wider in order for the side-wall to support the weight of the car. softer tires have a larger coefficient of friction, therefore better traction. A narrow, soft tire would not be strong enough, nor would it last very long. Wear in a tire is related to contact patch. Harder compound tires wear much longer, and can be narrower. They do, however have a lower coefficient of friction, therefore less traction. Among tires of the same type and composition, here is no appreciable difference in 'traction' with different widths. Wider tires, assuming all other factors are equal, commonly have stiffer side-walls and experience less roll. This gives better cornering performance.

Answered by: Daryl Garner, M.S., Physics teacher MacArthur High School, Lawton, OK


Friction is proportional to the normal force of the asphalt acting upon the car tires. This force is simply equal to the weight which is distributed to each tire when the car is on level ground. Force can be stated as Pressure X Area. For a wide tire, the area is large but the force per unit area is small and vice versa. The force of friction is therefore the same whether the tire is wide or not. However, asphalt is not a uniform surface. Even with steamrollers to flatten the asphalt, the surface is still somewhat irregular, especially over the with of a tire. Drag racers can therefore increase the probability or likelihood of making contact with the road by using a wider tire. In addition a secondary benefit is that the wider tire increased the support base and makes it hard to turn the car over in a turn or in a mishap.

Answered by: Stephen Scholla, B.A., Physics Teacher, Vienna, Virginia

ptrhahn

iTrader: (9)

I can tell you that I have as near a scientific experiment as one could find in practical application.

Setup 1:
Rear: 17x8" SSR integral with 245/40/17 Bridgestone RE71 tires.

Setup 2:
Rear: 17x9" SSR Integral with 275/40/17 Bridgestone RE71 tires.

The difference in terms of straight-line grip, particularly on cold days, and resistance to power-on oversteer on the track cannot be overstated. Its like night and day.

I know there are some subtle differences, like the diameter of the 275 setup is taller, and the slightly wider rim and tire are a couple lbs heavier... however, i'm sorry to say I refuse to believe that I could have acheived the same increase in grip by going with a 245/45/17 RE71 and hanging a pound of extra tire weights on it. How about a 225/50/17?

So, perhaps the contact patch area didn't change, but something else sure changed, and the result was significantly more grip in ALL directions, period.

There's a reason that racing bodies regulate the widths of allowable tires... even in the case of Formula One when engineers could play with virtually every parameter of compound and shape, and the tires are exposed to the airflow and would heavily benefit from reduced frontal area, they make em as wide as possible. Circa 1992, they were 18" at the rear on 1000 lb cars...

There may be a point with our cars where you could mount a tire that is too wide for the suspension geometry to take advantage of, is more than you need for the horsepower you make, won't get up to optimum temp with the given compound (race rubber only) or is detrimentally heavy or has too much rolling or aerodynamic resistance to offset any gain in grip, but i'm pretty sure that tire isn't fitting under the stock fenders anyway.

SleepR1

PVerdieck, seems to have stirred up some controversy. The most interesting reading on this forum to date Great responses. Not sure we'll have a ruling any time soon LOL

xchaos

Hey, I just decided to put 17s on front and 18s in the back. That way I'm on both sides of the controversy!

hehe...and possible irrefutably destroyed the handling characteristics of the car on the track! I'll try it out this summer though! Should look good and be great for the street however.

SleepR1

Hey xchaos,

I think you're starting a trend

I'm thinking hard about 9.5 x 17, 42-mm offset fr and 10.5 x 18 43-mm offset rr SSR Comps with 255/40-17 and 295/30-18 Pirelli P-Zeros.

Seeing that 295/30-18 you tried on your 9.5 x 18 wheel got me all excited about that rear tire size.

Like you, I have no idea how the car will handle, but hey...at least we'll look good LOL

rynberg

Manny, what happened to the 9.5x17 or 18 +47 offset and 10.5x18 +45 offset Volk CE28ns? The Volks would give you more fender clearance for the wide rubber....

clayne

Such as the rubber composition being softer.

SleepR1

Have not actually seen CE28Ns in person. Until I do, the Comps will be favored. You're right about the setup though. All I need is money

ptrhahn

iTrader: (9)

How is the rubber composition of the exact same brand and model of tire softer from size to size?

SleepR1

Well, assuming Pete used RE71s for both tire sizes, one can presume the tire compound should be the same. Perhaps the taller sidewall of the 275/40-17 allowed more deformation allowing more weight transfer and load to the rear tires compared with a lower sidewall 245/40-17 tire in the back?

ptrhahn

iTrader: (9)

Its possible that deformation plays some part, and as stated they WERE the exact same brand of tire.

However, re: deformation, i once again refuse to believe the same result could have been acheived with a 245/45/17 tire (which incidentally has the exact same height sidewalls as the 275/40/17). Obviously if tire patch is indeed constant (and i can't confirm or deny it), then there must be SOME other advantage at play here... in both lateral and longitudinal traction.

clayne

We do not know for a fact that the rubber is of the same composition on the narrower tire. As presented above, one would need a stiffer sidewall and overall harder composition rubber to support the sidewall when narrower. It's quite possible that they alter the composition per the width.

What are the treadwear ratings (this is manufacturer specific, but applicable here) for that tire in both of those sizes, phahn?

SleepR1

Which size is setup 1? 245/40-17 or 245/45-17? You're right that the sidewall is the same height with 245/45-17 and 275/40-17, if indeed 245/45-17 was setup 1's tire size What wheel width did you use with the 245?