Dumb Tire Question.

[Serious Eric]

Can some of you tire experts out there tell me exactly why we need our huge 335s as opposed to say 300s or even 275s? Is it for show or go?

Here's the basis of my question -> I'm pretty sure that for any given tire pressure, different width tires will all produce *nearly* the same area of contact patch. Is this not true? My reasoning is thus. The corner weight is transfered to the ground as pressure (force per unit area) over a given area of contact patch. The pressure over this contact patch should be equal to the internal tire pressure (minus some second-order effects of sidewall and tread stiffness). So for two equally loaded wheels, mounted with different sized tires filled to the same pressure, the pressure exerted at the ground will be the same, thus the contact patch must be the same area (although not necessarilly same shape).

I can think of really only one reason why the larger width tire would be superior and that has to do with tire deformation and longevity more than anything else. It seems to me that for a smaller width tire to achieve the same contact patch as a larger tire, it must deform more longitudinally (in the direction of roll) since the contact patch width is constrained by the tire width. I imagine that this degree of deformation is something you want to minimize to reduce heat and tire wear.

Other than this, what real good are these monsters doing for us?

 

[Frank Parise]

I believe you are correct. If the weight on a tire is, say 900 pounds, and the tire is inflated to 30 PSI, then the total area of the tire contact patch should be 900 divided by 30 = 30 square inches.

However, different shaped tires will result in different shaped contact patches whether the car is standing still or in motion.
Different shaped tire contact patches will affect handling.

I suspect the Michelin engineers worked with Team Viper to determine the optimal size tires from an overall handling and braking performance standpoint.

Michelin still believes, for example, that the 275 is the optimal size for the fronts, even for track use. They think we are slowing ourselves down by using 305s, 315s, 335s up front. Their professional drivers apparently used the standard 335 on rear and 275 on front to win the One Lap of America competition in a Michelin-prepared Viper.

 

[Chuck 98 RT/10]

Eric, you think too much.

 

[scottgf]

I put 315's on the back of my 94, on the stock rims...it looked terrible and handled worse. I am no tire expert but I sure didnt like the "feel" of them.
I ended up taking them off and putting them on the front and buying 335's for the rear.
Cheers,

 

[MtHam ACR]

The width is required to put the power to the ground.

A 335mm tire is a bit over 13" a 275 tire is is 10.8", or about a 20% difference in contact patch assuming the same pressure and circumferenece.

There is a reason powerful cars (any exotic for example) run fat tires.

Eric
01ACR

 

[Makara]

<BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR>Originally posted by MtHam ACR:
The width is required to put the power to the ground.

A 335mm tire is a bit over 13" a 275 tire is is 10.8", or about a 20% difference in contact patch assuming the same pressure and circumferenece.

There is a reason powerful cars (any exotic for example) run fat tires.

Eric
01ACR
<HR></BLOCKQUOTE>

I am rather sure that the contact patch would be the same size for a 275 and a 335. The major difference is the shape of the contact patch.

 

[Serious Eric]

Thanks Frank, Makara et al.

I suspected that "common knowledge" was wrong in this case. Wider ain't always better.

Chuck,

You're right, but as they taught me at Faber ... "Knowledge Is Good."

 

[Chuck 98 RT/10]

<BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR>Originally posted by Eric Schieve:
as they taught me at Faber ... "Knowledge Is Good."

<HR></BLOCKQUOTE>

Animal House?

 

[MtHam ACR]

Makara, Thanks for pointing out my lack of clarity. You're right, it's the shape of the contact patch I was attempting to point out, albeit, lamely.

I'm still arguing wider will do a better job of transfering power into forward momentum due to the width of the patch perpendicular to the direction of travel. As you all point out, that 30 sq in can be defined as 1X30" or 30x1" or 5.5x5.5" but bolt up some skinny tires and see how much easier it is to spin them compared to something considerably wider, especially coming out of a corner.

Is there an arguement to the contrary?

I agree there is a balance because not all forces are 100% acceleration or braking. The point I was making is you're going to see beefier tires on a 550 Marenello than on a tercel. There's a reason for that beyond aesthetics.

Eric

 

[Serious Eric]

<BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR>Originally posted by Chuck 98 RT/10:
Animal House?
<HR></BLOCKQUOTE>

Yep ... Delta house. 1.7 GPA. Head of my class. Fat Dumb and Stupid may in fact be a great way to go through life.

 

[Makara]

<BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR>Originally posted by Eric Schieve:
Yep ... Delta house. 1.7 GPA. Head of my class. Fat Dumb and Stupid may in fact be a great way to go through life.

<HR></BLOCKQUOTE>

I tried to get into Delta but Dean Wormer said that no new members were allowed because of double secret probation.

 

[jgfurr]

I had no GPA. All classes were incomplete.

 

[Chuck 98 RT/10]

6 years of college down the tubes.

 

[PMUM]

<BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR>Originally posted by Frank Parise:

Michelin still believes, for example, that the 275 is the optimal size for the fronts, even for track use. They think we are slowing ourselves down by using 305s, 315s, 335s up front. Their professional drivers apparently used the standard 335 on rear and 275 on front to win the One Lap of America competition in a Michelin-prepared Viper.

<HR></BLOCKQUOTE>

Frank, don't you have 335s on the front of your car? Are you trying to trick everybody into thinking 275s are quicker? You truly are the "wiley veteran" of Viper Days.

The single biggest performance advantage I EVER did to my car was going from 275 fronts to 305s. I'd like to see that Michelin-prepared Viper come to the Open Track Challenge (basically One-Lap of the West Coast tracks in May) and try that on the West-Coast tracks. Somebody should lob them an invite.

I think they used 275 because Michelin doesn't make a 305 or 315 front for the 18" rims on an ACR. Don't believe the hype!

 

[K Adelberg]

Frank, you make some great points. For ideal handling there are also other factors such as slip angles to consider.

PMUM also is dead on! I too feel there is no comparison between a 275 front and a 305 or 315. This eliminates much of the oversteer and improves braking. Also generates more heat to the brakes, another story.

We all remember Michilin's Club Slick crap! "Club Chic's", "Club Sh**'s" etc... Many ran the 335 up front because there was nothing but a 275 or 335.

 

[GTS Dean]

<BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR>Originally posted by Frank Parise:
... Michelin Club Slick... but they were shredded down to the cords within the first day, sometimes within the first run session.
<HR></BLOCKQUOTE>

I still disagree. The only set I ever had got me 2 full days and nine heat cycles. They *could not* be driven like Hoosiers or R1s and were somewhat more fussy about alignment.

<BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR>
They obviously had a technical problem that they refused to admit.
<HR></BLOCKQUOTE>

"They" wasn't necessarily Michelin, IMO. The tires had a lot of sidewall flex and not much front footprint. The answer was to turn in and then rear-steer the car - *not* to put in more steering angle and fold the front tire under.

 

[K Adelberg]

Dean, you are right on with regard to the "Club Chic's" except for the wear life. This is where the name "Club Chic" came from, sort of a one time use... use your imagination here. The tire came from the original Viper Tire, I was told and they just put a sticky rubber surface on. Seems they forgot all about the side wall. Driving on them, felt like the car was on silly puddy. No contest to a Hoosier. If only Hoosier and Yokahama could get together and develope a "Hoosier-mama" tire...

Frank, I too agree with the 335 up front. Makes the car oversteer too easily.

 

[Jon 98GTS]

Sorry Frank, but I have to disagree with you here:

"If the weight on a tire is, say 900 pounds, and the tire is inflated to 30 PSI, then the total area of the tire contact patch should be 900 divided by 30 = 30 square inches."

Uh.... does this mean that if I put in 1psi that I will end up with 900 sq inches of contact area?!? That would be a neat trick, considering that this is more than the total surface area of the entire tire...

I do believe that pressure does have a small influence on the amount of contact area -- I just don't believe it is a simple, linear relationship.

But tell us more about the Michelin engineers believing that 275 is the "optimal size for track use"! I guess I always assumed that 275 was chosen as an optimum for performance vs "streetability." (ie, if you put REALLY wide tires on a car, some idiot will lose control, hydroplaning out of a car wash. And you know what that means -- lawsuits!) It's kinda like the rationalization I came up with for why we have such wimpy stock rear brakes ("Fronts must lock first, even in monsoon conditions").

Are you saying that Michelin believes that a 275 front is faster than a 315 on a DRY track? Hard to believe...

 

[Frank Parise]

Now that I have 18" wheels up front, I run the Hoosier 305. When I had 17" wheels up front, I ran the 315. Purner tells me the footprint is virtually identical on both the 305-18 and the 315-17.

I tried Hoosier 335's up front and went slower. They felt very imprecise in cornering, although I could feel the improvement under braking. I think the trend in the VRL has shifted from 335's up front to 305's.

As for Michelin's recommendation, I suspect they are talking about Vipers that run on Pilots. It would be pretty brave to attempt the One Lap of America on a set of Hoosiers. I think they drive something like 5,000 miles in between events, and they have to race the same tires they drive on the highway for the whole event.

Ken, you are right about the old Michelin Club Slick. It wasn't that bad from a performance standpoint, but they were shredded down to the cords within the first day, sometimes within the first run session. They obviously had a technical problem that they refused to admit.

It will be interesting to see if Michelin really comes through with a true race slick for the Competition Coupe. How else can we keep up with Paul?

Paul, BTW, keep your Goodyears in the trailer on Jan 5-6. I want on a fighting chance!

 

[GTS Dean]

Jon,

I think you're correct about front tire size. The only downsides to the 305/315 fronts are hydroplaning, road debris/paint issues and slightly more rolling resistance.

Frank,

Tire contact patch area and air pressure relationships may be linear, but cannot nearly as simple as 900/30=30. A tire is not a toroidial balloon on a wheel. There are multiple belts of different materials, thicknesses and orientations. Bead/sidewall supports affect the tire's ability to keep the wheel centerline close to the centerline of the tread (a MAJOR difference between the XGT-Z and the MXX3) If the sidewall moves a lot, the scrub radius of the suspension/tire changes too. Multiple rubber compounds within the same tire are not really homogeneous, but a composite - each designed specifically for different areas of the tire. Air pressure keeps the different parts of the tire in proper geometric and heat/performance envelopes.

A tire that is inflated to a given PSI will not have the same contact patch area (or shape) with -2.75 degrees camber as it will with 0 degrees, nor will it perform the same because of camber thrust.

 

[Frank Parise]

Jon, I would think the calculation only works if you have enough air pressure in the tire to support the weight such that the weight of the car was carried by the tire and not the wheel.

Dean, I would have thought that the contact patch would be the same square area regardless of camber, however the shape of the patch would be more longitudinally oriented, say with -2.5 degrees of camber versus 0 degrees. Now you got me thinking about how the internal structure of the tire could alter the area of the contact patch. It seems to me that it would only alter the shape.

Skip tells me that Michelin was emphatic about its recommendation for using 275s. Again, I would think this is for Michelin Pilots only, and it was geared toward the guys running the Michelin Challenge series on stock tires. I don't think Michelin makes a 305 or 315 Pilot, so maybe it was just sales talk.

Don't shoot me, I'm just the messenger.

Dean, you're also right about the different driving technique required to get the most out of the Club Slick. However, 9 heat cycles doesn't cut it in my book. I've had Hoosiers with 30 heat cycles that didn't show a cord. They dried up and lost their coefficient of friction big time, but there was still rubber there.

 

[Henry Cone]

Frank, Michelin does make Pilots in 315 widths. We run 315/35 x 17 MXX3's on the front of both of our cars in Viper Days Super Stock class. They work quite well for the reasons that have been mentioned. They do require a wider (11") wheel and rub at full lock but these are reasonable compromises for us.

Obviouosly running 315's on the front means that hydroplaning is potentially more of a problem. The wider tires can also be worse about tracking grooves and irregularities in the road. So I suspect that for a predominately street driven Viper the stock 275 width is a good compromise as both of these vices are lessened.

Of course ol' Billy P swears that he can go just as fast with the 275's as with 315's so maybe Michelin is right after all....