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How To Maximize “Street Tire” Grip on Dirt

 

No matter what type of racing you are involved in, tires play a huge role in the overall handling of the car. The tread of the tire is the only part of the car that touches the track. Tires act as part of the suspension and absorb many irregularities that the typical dirt track has to offer. They are relied upon to keep the car traveling in the direction it is pointed and are the only means to transfer power to the track. Anyone who finds victory lane has understood how important it is to maximize the amount of grip they get from their tires.

A lot of assumptions are made about tires, but are they really true?

At the beginning of the 2010 season I was in search of a new “race” tire. The tire I had been using was discontinued in 2009 and I needed a replacement. My friends at Rubidoux Tire made a recommendation and after a little research I decided it was a good choice. I have been using the MasterCraft Avenger GT ever since and I am satisfied with the tire. It seems to be durable, offer decent grip, and although tire prices seem to be skyrocketing, it is relatively inexpensive. However, while trying to determine which tire to use I found some conflicting information and made several assumptions. I had questions about whether the information I used to make my choice was valid or not. I decided to do a little more research and a few simple experiments to find out.

There were three questions I wanted to settle:

Should older tires be discarded simply because they are old and do they get harder and lose grip as they get older?

Does a lower treadwear rating on the side of a street tire indicate a softer compound of rubber thereby increasing grip?

Finally, does grinding street tires make them softer and increase grip?

The Tire’s Durometer

Durometer is the measurement of hardness for a material. There are 12 scales of durometer used for materials with different properties. The two most common scales are the A and D scales. The A scale is for softer rubbers (like car tires) and plastics, while the D scale is for harder ones (like a hard hat). The method is spelled out in the ASTM D2240 testing standard.

I only needed a couple of tools for this project: A grinder fitted with a sanding wheel and a Tire Durometer.

 

The basic idea is that the tire durometer is pressed against the material to be tested until the presser foot is firmly against the test surface.  A spring loaded pin (indenter) protrudes .100 inches from the bottom of the Tire Durometer and as it is pressed against the surface, the indenter is pressed into the material, making an indentation.

The durometer reading is simply measuring how far the indenter is pushed back into the tire durometer by the material.  If the indenter is pushed in .020 (.080 still sticking into the material) then the durometer reading is 20.  If the indenter is pushed in .065 (.035 still sticking into the material) then the durometer reading is 65 (typical reading for a street tire).  Lower durometer numbers indicate a softer material.  The standard specifies that the sample should be at least a quarter-inch thick and should be supported by a solid surface.  It also specifies that readings should be taken at least a half-inch away from the edge of the sample.

The indenter of the tire durometer is pressed into the rubber until the foot sits squarely on the surface. The further the indenter extends into the rubber the softer it is.

Does Age Matter ?

I learned that rubber doesn’t have a set shelf life. When stored properly, tires (rubber in general) can last a very long time. Tires can degrade over time but what degrades a tire the most is exposure to heat and light. So if a tire is stored in a dark, cool place it shouldn’t show much signs of aging.

I have been told for most of my life that “Tires get old and should be replaced every five years no matter how they look and how much tread is left.” I have also been told that as a tire ages it gets harder and looses traction. These two things don’t necessarily jibe with what I learned to during my research. Confirming or dispelling what I have been told for so many years seemed relatively simple.  Measure the hardness of older tires and compare them to newer tires.

These are the “test” tires. One of the tires is old enough to be in kindergarden

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No matter what type of racing you are involved in, tires play a huge role in the overall handling of the car. The tread of the tire is the only part of the car that touches the track. Tires act as part of the suspension and absorb many irregularities that the typical dirt track has to offer. They are relied upon to keep the car traveling in the direction it is pointed and are the only means to transfer power to the track. Anyone who finds victory lane has understood how important it is to maximize the amount of grip they get from their tires.

A lot of assumptions are made about tires, but are they really true?

 

At the beginning of the 2010 season I was in search of a new “race” tire. The tire I had been using was discontinued in 2009 and I needed a replacement. My friends at Rubidoux Tire made a recommendation and after a little research I decided it was a good choice. I have been using the MasterCraft Avenger GT ever since and I am satisfied with the tire. It seems to be durable, offer decent grip, and although tire prices seem to be skyrocketing, it is relatively inexpensive. However, while trying to determine which tire to use I found some conflicting information and made several assumptions. I had questions about whether the information I used to make my choice was valid or not. I decided to do a little more research and a few simple experiments to find out.

There were three questions I wanted to settle:

Should older tires be discarded simply because they are old and do they get harder and lose grip as they get older?

Does a lower treadwear rating on the side of a street tire indicate a softer compound of rubber thereby increasing grip?

Finally, does grinding street tires make them softer and increase grip?

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The Tire’s Durometer

Durometer is the measurement of hardness for a material. There are 12 scales of durometer used for materials with different properties. The two most common scales are the A and D scales. The A scale is for softer rubbers (like car tires) and plastics, while the D scale is for harder ones (like a hard hat). The method is spelled out in the ASTM D2240 testing standard.

I only needed a couple of tools for this project: A grinder fitted with a sanding wheel and a Tire Durometer.

 

The basic idea is that the tire durometer is pressed against the material to be tested until the presser foot is firmly against the test surface.  A spring loaded pin (indenter) protrudes .100 inches from the bottom of the Tire Durometer and as it is pressed against the surface, the indenter is pressed into the material, making an indentation.

The durometer reading is simply measuring how far the indenter is pushed back into the tire durometer by the material.  If the indenter is pushed in .020 (.080 still sticking into the material) then the durometer reading is 20.  If the indenter is pushed in .065 (.035 still sticking into the material) then the durometer reading is 65 (typical reading for a street tire).  Lower durometer numbers indicate a softer material.  The standard specifies that the sample should be at least a quarter-inch thick and should be supported by a solid surface.  It also specifies that readings should be taken at least a half-inch away from the edge of the sample.

The indenter of the tire durometer is pressed into the rubber until the foot sits squarely on the surface. The further the indenter extends into the rubber the softer it is.

 

Does Age Matter ?

I learned that rubber doesn’t have a set shelf life. When stored properly, tires (rubber in general) can last a very long time. Tires can degrade over time but what degrades a tire the most is exposure to heat and light. So if a tire is stored in a dark, cool place it shouldn’t show much signs of aging.

I have been told for most of my life that “Tires get old and should be replaced every five years no matter how they look and how much tread is left.” I have also been told that as a tire ages it gets harder and looses traction. These two things don’t necessarily jibe with what I learned to during my research. Confirming or dispelling what I have been told for so many years seemed relatively simple.  Measure the hardness of older tires and compare them to newer tires.

These are the “test” tires. One of the tires is old enough to be in kindergarden

 

I chose five Kelly Charger tires, several of which are over four years old and one that is nearing six years old.  All of these tires have been stored out of the sun and in a relatively cool place (in the shade).  Other than worn tread and a few battle scars, none of these tires shows any signs of aging.  There are no cracks in the rubber; not between the tread blocks or on the side wall of the tire.  Without looking at the date code you wouldn’t know which of the tires is newest and which one is oldest.  I feel comfortable running any of these tires (which is why I still have them).

However, just because they look fine doesn’t mean they aren’t hard.  I measured the durometer on each of the tires five times to confirm that I was getting consistent reading from each tire.  All five tires measured within two points and the oldest tire did not have the highest durometer.  This indicated to me the age of a tire does not determine its hardness. Of course, you want to be very careful when determining whether to run a older tire or not. You need to examine the sidewall of the tire very carefully for cracks and signs of damage from racing. If something doesn’t look right, just replace the tire! It is cheaper to replace a $70 tire, then it is to repair the car after the right front let go and you smacked the wall.

These results indicate that when stored in a cool dark place old tire do not get harder as they age

Treadwear Rating vs. Tire Durometer

During my tire search I had a theory on the treadwear rating marked on the side of the tires.  A tire with a high treadwear number indicates that the tread should wear slowly and last longer.  According to my theory a tire with a long lasting tread would require harder (higher durometer) rubber.  Conversely, a tire with a low treadwear rating could be constructed of softer rubber that could be tackier.  Since I am not concerned about tread life I concluded that a lower treadwear rating could indicate that I may get more grip from the tire.

The tire durometer was easy to use. It was a matter of pressing the foot squarely on a block of tread and reading the dial. It yielded consistent results.

I measured more than a dozen brands of tires at a local tire shop and at my house.  Most of the tires measured in the middle 60s.  Their treadwear rating varied anywhere from as low as 420 to as high as 700.  I measured some ultra-high performance tires with low treadwear ratings and was surprised to see that several of them had durometers in the 70s. There seemed to be no correlation between treadwear rating and tread hardness.  One tire that did stand out was the Goodyear Eagle GT II.  It was by far the softest tire and measured a full 10 points less than most of the other tires.  Unfortunately, it was also about 50% more expensive than the tire I use.

The tires listed above are available in a 15-inch wheel size as well as a variety of section widths and aspect ratios commonly used in the class I race in. There seemed to be no correlation between tread rating and durometer

To Grind or not to Grind

The third piece of information that I wanted to confirm was whether grinding tires increases grip.  I have heard this one for years and seen many teams do this at the track, but does it really provide any benefit? Most of the time, the story is that someone knows a guy that does it or that a lot of racers in the mid-west do it.  This claim is almost always followed by the number of tenths that can be shaved off of your lap times by doing so.  At the same time, almost nobody I know does it.  I know that sprint car and late model teams do it.  I assume they believe it has some affect on the performance of their dedicated race tires.  The question is… does it work on the street tires I have to use?

Grinding tires is fairly simple and takes only a few minutes per tire.

The process was simple.  I selected a couple of tires and measured their durometer.  I then took those tires and ground the tread surface with my 4 inch grinder fitted with a sanding disk. Allowed them to cool, and then remeasured the durometer of the tire.  The idea is to remove the outer “skin” of the tire revealing the soft, “grippy” rubber underneath.  The process was quick and easy.  I could grind an entire tire in about five minutes.

The surface of the un-ground tire was a bit shiny and the edges of the tread blocks had been rounded from sliding across the abrasive track surface. After grinding the surface, I noticed it was darker in color and had lost what little sheen it had. I also noticed that the edges of the tread blocks looked more square. The rounded edges weren’t completely gone but they seemed to be more crisp.

Once the tire surface was ground I again measure the tire’s durometer. In every case the hardness of the tire’s tread dropped.  Grinding the surface of the tire lowered its durometer two to four points. What I don’t know is how long the affects of grinding the tire last. Does it last a few laps, one heat cycle, a whole race?  I also don’t know how much reducing the hardness of my tires by two points would affect the handling of the car.  Could I even feel the difference? These answers will be solved when I get to the racetrack, but regardless the tire is softer after I grind the tire.

Tire hardness dropped in every case after grinding the tire surface using the sanding disk

 

 Tire hardness dropped in every case after grinding the tire surface using the sanding disk

Once the tire surface was ground I again measure the tire’s durometer. In every case the hardness of the tire’s tread dropped.  Grinding the surface of the tire lowered its durometer two to four points. What I don’t know is how long the affects of grinding the tire last. Does it last a few laps, one heat cycle, a whole race?  I also don’t know how much reducing the hardness of my tires by two points would affect the handling of the car.  Could I even feel the difference? These answers will be solved when I get to the racetrack, but regardless the tire is softer after I grind the tire.

Close up pictures of tire tread before (left) and after (right) grinding. The tread block edges are much more crisp after grinding and my test showed that the durometer went down two to four points

 

What This all Boils Down to

 

I set out to answer three relatively simple questions.  With the help of my shiny new tire durometer and trusty grinder I have come to some conclusions.

I believe that old tires don’t necessarily need to be discarded just because they are old.  I also think I dispelled the myth that older tires get harder as they age.  If stored properly tires can have a very long service life.  It is not the age of the tire itself that causes problems but extended exposure to the sun and heat that degrades it.  Certainly, years of exposure to sunlight and blistering temperatures will ruin a tire and cause it to get hard and crack.

I was truly surprised by the treadwear rating/durometer comparison.  I really expected to find a direct correlation between the treadwear rating and the durometer of the tire.  Instead, the results indicated that most of passenger car tires had about the same hardness (with one notable exception) and most of the ultra-high performance tires I measure had a durometer that was several points higher than the passenger car tires.  Luckily, I can now use the tire durometer to measure the hardness of a tire directly and remove all the guess work.

Finally, it looks like grinding the surface of a street tire might actually increase its grip.  It definitely decreases its durometer and the sharper edges seem like they might dig into the track a little better as well. I am still satisfied with the tire I have been using for the last season and a half and I don’t plan on switching brands.  However, I am always on the look out for a better option that is still cost effective and it is always good to have as much information as possible.  I now know to continue storing my tires in a dark cool place to keep them looking and feeling young.  I also know to rely a little less on the writing on the side of the tire and more on my new tool when I go to the tire shop.

http://www.onedirt.com/tech-stories/how-to-maximize-street-tire-grip-on-dirt/

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