DamonB

Oh sh*t. Somebody's going to insisit the Viper has bigger rotors, drilled rotors, better pads or a bigger brake pedal or something. Then I will just run around the house in a screaming stupor while I pull my hair out

DamonB

I understand where you are coming from and I tend to agree with you on the strict issue of deformation amount. But remember this argument is over a direct comparison of overall grip. That comparison being that all other things equal a wider tire only generates increased grip laterally and has no impact longitudinally. I insist that not to be true. Deforming the tire and increasing contact patch will certainly add grip to any tire, but again the wider tire already has more rubber in contact with the road to begin with; so after deformation the wider tire is still "ahead" of the narrower tire.

clayne

Well I'm saying the narrower tire WOULD have more longitudinal area if it was able to deform properly.

It's limited by the shape of the tire and sidewall height/stiffness. Perhaps if we were able to install 10" diameter x 6" width wheels with some 205/85s or something we may see differently.

SleepR1

http://www.physicsforums.com/showthr...&threadid=7476

Good stuff in there Clayne, although I don't know if the physicists agreed on whether widers tires were better or no better, and if they were better, whether the increase was from contact area or from the stickiness factor (apparently a function of area).

It's safe to say the phyicists might not necessarily have real world driving experience, but I respect their ideas on basic physical principles of friction, normal force, mass, contact area, etc.

What we have is an extremely complicated problem.

The physics model with rubber patches may not be appropriate because a tire is a donut shape and not a patch of rubber. The donut shape can deform under acceleration, braking, and cornering loads; it would take a supercomputer to calculate the predicted contact patch the tire would make given a certain load.

Take into account the variable of tire aspect ratio, tire section width, the wheel width the tire is mounted, and the differences between rubber compounds, you begin to see that what we're trying to solve is the stuff PhD tire engineers try to solve.

What we need is a simple comparison test of 1/4 mile times with 255/40-17s on 9 wide x 17 vs 225/45-17s on 8 wide x 17 (to leave out the aspect ratio variable). Do a statistically significant number of runs, and take the mean of the 1/4-mile ETs.

Next we'll do the same for an autocross or road course. Compare the tire sizes with a few laps on each width wheel/tire, and take the mean.

All else being equal, we should see some differences. Equal means, same car, driver, and day. We should take care not to heatsoak the car, since we know heatsoaked FDs don't perform as well.

FWIW, I think this has been a great discussion, and sure beats the junk I skim on the FD board LOL

SleepR1

Perhaps a professional expert can shed some light on this controversy.

BTW, Splinemodel. I think you should go with the SleepR1 fitment. Works well @ both road and track. You have the freedom to rotate (given non directional tire pattern), you can carry a full-size spare to open track or autocross events (no worries about having a flat and ruining your weekend), and you preserve the handling characteristics of the FD.

SleepR1 fitment is 9 to 9.5 x 17 (45 to 50 mm offset) with 255/40-17 tires. From my experience, 255/40-17 tire size is all the tire the 2750-lb FD needs for blistering performance. More than that, and you're adding too much weight, rolling resistance, and numbing the steering feel.

Devilish

Come on, all other things being equal - car set up, tire compound, etc. - a car, say an FD, running 265's front, 295's rear, is going to have more lateral grip and will stop sooner than a car running 225's at all four corners. Now I can see where it MAY accelerate slower because there is more rolling resistance, but that depends on power. A 400 rwhp car would easily spin the 225's while the 295's will hook up thus being faster.

Seriously, I don't know how you can argue that. I think it depends though on how big a difference you're talking in tire size - i.e. 205 vs 305. I can see where the difference between a 225, 235, and 245 is negligible (on the street). But I've NEVER heard of a racing team choosing a skinnier tire (except in the WRC).

Great thread though.

SleepR1

Road course and drag strip driving require different performance properties from the tires.

255/35-18 fr and 285/30-18 rr Sport Contact 2s on 9.5 fr and 10.5 rr x18 SSR Comp wheels would be a great open track setup if your FD has the hp, larger brakes, springs, and sway bars to take advantage of such huge rubber.

The narrower wheels/tires up front allow for good longitudinal braking force to help scrub speed effectively (while giving up some front-end cornering grip), while the larger rear tires allow more cornering grip potential for earlier throttle application through the corner.

I'm not sure a set up like the above would be effective for a completely stock FD Rx7; more available rubber may not necessarily decrease laptimes if the car doesn't have the matching performance potential.

Drag racers don't use low profile wide tires like 285/30-18s on 10.5 wides, rather they use something like, 275/45-16s on stock 8-inch wide FD Rx7 wheels.

The nearly 5-inch tall sidewalls and slightly lower tire pressures presumably allow the drag tire to transfer the rearward loads more progressively, for better hook-up at the launch.

Once the traction is hooked, the narrower wheel width limits the rear tire contact patch to tranfer more weight per unit area of rubber contacting the road to help longitudinal acceleration (assuming the driver is staying within the tire compound's 15% longitudinal slip).

As the physicists talked about on Clayne's physics forum, friction is not dependent solely on available contact area, but on normal force per unit contact area, and the mu constants of the materials (coefficient of friction between rubber and road).

Very complicated stuff (sigh)!

SleepR1

I recommended my fitment because I assume your car is also street driven with stock, or slightly more than stock performance potential. Tell us more about your car, and perhaps we can try to get back on topic with this thread LOL

DamonB

I will once again try to explain this. Now it is being said that all things being equal a wider tire only gains lateral grip compared to a narower tire; the wider tire doesn't gain grip in acceleration or braking. I see two reasons being offered: First, it is said that since the wider tire has less pressure distributed over its footprint it is not mushed into the road as hard as the narrow tire and therefore the wide tire offers no gain. Second is the belief that since the contact patch is now wide and shallow it only offers increased grip in the "wide" direction (cornering).

Here's the BIG issues with that. You are saying the wide tire has less pressure over its contact patch and therefore less grip until under cornering some weight transfer comes along and loads the tire; increasing its grip over a narrow one. If you believe that pressure over the contact patch is the main ingredient for grip you just completely contradicted yourself. Weight transfer is constant on any given car regardless of the tire size. The narrow tire will ALWAYS have more pressure exerted over its contact patch due to its smaller area, but yet you're saying the wider tire will somehow have more grip in cornering when it's loaded with the same amount of weight transfer, even though the wider tire has less force over it contact patch in this instance. Then you turn around and say this "magic" doesn't apply to travel in a straight line because the wide tire can't be loaded enough or its contact patch doesn't face the right direction.

You can't have it both ways. You can't say grip is mostly dependent on the pressure exerted on the contact patch and then say the wider tire will offer more grip in cornering solely due to loading. The wider tire will always have less pressure on its footprint due to the fact its contact patch is bigger and the weight transfer will be exactly the same amount as the narrow tire.

Again it's already been proven earlier that the rate of increase in pressure on the tire's contact patch is not linear to the rate of increace in grip! A large increase in tire load brings a small increase in grip! If it didn't work that way a car would NEVER slide in a corner or under braking or acceleration because the grip would just continue to increase as more and more weight transferred onto the contact patch. Do you truly understand what that means?

As for contact patch shape this does account for the "manners" of tires. A wide, shallow contact patch is more difficult to drive well. This does not mean the tire doesn't still have increased grip in every direction though!

You guys can't see the forest for the trees. Think about these examples that your theory insists to be true:

-Increased tread width is wasted in a straight line because increased tread width has less pressure above it mashing it to the road surface. This means dragsters have no need to ever increase tread width (unless you make the car heavier or increase downforce).

-Increased tread width will not offer increased braking capabilities. Front tire width is strictly for cornering. Tire width offers no gain in braking.

-You're saying the acceleration and braking traction for a given vehicle is soley based on tire compound or inflation pressure or some other thing; width doesn't matter.

-You're saying any car driven at the dragstrip would have the same 60' times with 6" wide tires as it would with 10" wide tires.

-You're saying that since wider tires only offer increased cornering grip that your increase in exit speeds at the track on wide tires is soley due to your increased cornering speed. You have no ability whatsoever to accelerate harder out of the corner or brake harder entering the corner.

-You're saying the transition from braking to turning and then to acceleration would be hairy at best because the tire has shitty grip in braking, then magically has huge cornering grip and then becomes shitty in acceleration again. The transition from shitty to cornering to shitty would be very quick since the tire can only make increased grip when it's contact patch is loaded in a certain direction.

Everyone is too caught up in trying to prove to themselves the above examples are true! You're letting your science fool your common sense! Take a step back from your "facts" and think about it with your common sense!

DamonB

Alright, here's another one for the tire experts.

Why do the majority of racecars need wider tires on the rear wheels?

SleepR1

No one's saying that the braking and accelerating goes to **** with wider tires all around.

What we need is for someone to try the experiment I mentioned a couple posts ago

Hey Splinemodel, are you confused yet LOL

SleepR1

I'm no expert but here's my answer

DamonB

So by your description the rear wheels need to be wider to make more cornering grip, but the front wheels don't need to be wider to make the same cornering grip. How is that so?

But you are saying braking and acceleration don't increase. If they don't increase then the tire will act exactly as I described: Wonderful pure cornering grip and crappy transitional or acceleration grip.

Read the following to yourself very slowly and don't post a reply for at least 15 mins

A large increase in tire load brings a smaller increase in grip! If it didn't work that way a car would NEVER slide in a corner or under braking or acceleration because the grip would just continue to increase as more and more weight transferred onto the contact patch.

This means that the prime determining factor in grip is NOT load on the tire.

SleepR1

With road course racing, the front tires aren't used as much as rear tires (assuming a well-balanced, high-hp RWD racing car). The front tires are used to scrub speed going into a turn, and for getting the car turned in. Once you've got the car changed direction and turning into the corner, the rest of the corner's execuation comes from the rear tires

Quote:

Again it's already been proven earlier that the rate of increase in pressure on the tire's contact patch is not linear to the rate of increace in grip! A large increase in tire load brings a small increase in grip! If it didn't work that way a car would NEVER slide in a corner or under braking or acceleration because the grip would just continue to increase as more and more weight transferred onto the contact patch. Do you truly understand what that means?

What I understand is that friction is a function of normal force, force per unit contact area, and the frictional mu constants involved with the two surfaces. The variables with tires is that friction actually increases as heat is generated in the tires which one of the physicists on Clayne's forum called, "rubber stickiness". So grip does increase as load increases, up to the point where you exceed the tire compound's ability to generate "stick". Once you overheat the rubber compound, stickiness decreases, and the slip angle increases (slide). In this case, although the load has increased due to weight transfer, the tire cannot make anymore stick because you've over-stepped the tire compound's optimum grip heat range (which is related to the tire's usable slip angle range).

Tire stick is more than just about the contact patch. Tire stick involves many different variables, and it's too simplistic to just say, "bigger tires win"

What physics seems to be saying is that all else being equal, bigger tires don't necessarily win

SleepR1

I don't think anyone here has said load was the prime factor. Load is only one of the variables in determining tire stick.

DamonB

I'm through with this thread and the performance of the experiment because it's been proven nobody will believe the results anyway. Sleep, you already had it right when you said "larger rear tires allow more cornering grip potential for earlier throttle application through the corner". You just said the larger tire has the potential for more acceleration grip out of the corner! You know that tires trade cornering for acceleration and vice versa. If the larger tire did not have a bigger traction budget in both cornering AND acceleration than the narrower one you would not ever be able to come out of the corner with harder acceleration when on wider tires!

I'm through. Everyone enjoy your time at the track; the warm seasons are coming

SleepR1

We agree that wider tires offer more traction budget. We disagree in one place. I posit that the traction budget is biased toward lateral grip with wider wheels/tires. You posit the traction budget is distributed equally between longitudinal and lateral directions with wider wheels/tires. No big deal. A tire engineer can probably settle it, or perhaps a well-designed test

DamonB

You can't have increased acceleration on the car out of the corner unless you have increased grip in a straight line!!! ARRRRRGGGHHHH!!!!! In the aspect of maintaining my mental health I decline further participation

SleepR1

You can IF you make short work of the corner It's getting to 50 F today. I may have to do a few donuts and burnouts to test these theories LOL

DamonB

No, that's SPEED out of the corner. Acceleration out of the corner means the car is able to pull out of it harder with it's engine power and not break the tires loose.

SleepR1

Yeah, but I begin accelerating as soon as I get the car turned into the corner? So for me, acceleration out of the corner begins once I transition from braking/turning to accelerating. From this perspective you're "theoretically" lengthening the straight with larger rear wheels/tires because you can begin accelerating to full throttle sooner, ultimately leading to a higher exit speed out of the turn.

Clearly with road racing the rear tires have a larger probability of breaking loose mid corner than they do accelerating in a straight line out of the corner.

If you're not breaking the tires loose mid-corner you have two problems.

You're coasting through the turn, or your rear tires are not wide enough LOL

Going to lunch now.

Hey where's Arc Welder and Clayne?

DamonB

Digging my grave

SleepR1

Tire Rack tech articles. They're a short read.

This is a nice illustration of how tire section width affects contact patch shape. Notice the landscape vs portrait shape change with wider vs narrower tires/wheels.

Tire aspect ratio plays a significant role.

http://www.tirerack.com/tires/tirete...al/contact.htm

Air pressure plays a significant role in contact patch.

http://www.tirerack.com/tires/tirete...s_wetperf.html

While UTQGs are done by each tire maker, UTQGs do give a rough guide as to how soft (or hard) the tire compound is. The article implies that you can compare UTQGs between brands, but the accuracy is increased when comparing UTQGs within the SAME brand.

Traction is evaluated with straight ahead wet-braking. A specific tire width is NOT mentioned when doing these straight-ahead wet-braking tests.

One could presume that if width mattered in longitudinal traction, there would be a guideline stipulating the width of the tire being tested to qualify the traction rating?

Wet cornering ability is not stipulated in the traction test. Only straight-ahead. Again perhaps wet cornering grip would differ depending on tire section width. If that were the case, testing would prove too combersome, and thus only straight-ahead wet-braking tests (where presumably width doesn't matter) are done?

http://www.tirerack.com/tires/tiretech/general/utqg.htm

ArcWelder

Still here, I'm just sitting back enjoying the debate.

ArcWelder

Nope, not digging your grave. I too had to remove myself from the debate for mental health reasons. Actually the debate has caused me to do some thinking, and that's probably not a bad thing. I do, however, still hold to my initial belief brought up on page 1.

Mark