[Mykl]

Check out this handy little tool...

http://www.enginuity.org/springfreq/

[SWortham]

Cool, nice find. Unfortunately I don't know what the unsprung weight numbers should be so I left it as is.

I put in 3,600 pounds (very rough estimated wet weight with me in the car). And I specified a 58/42 weight distribution and the stock 224/194 spring rates.

And the calculator came up with a spring frequency of 1.55 Hz (93 cpm) front and 1.72 Hz (103.2 cpm) rear.

For what it's worth, Smithees suggests a spring that'll produce 120 to 130 cycles per minute for a road racing application in order to obtain maximum grip on DOT-legal race tires:
http://www.smithees-racetech.com.au/...cefactors.html

And they say that a typical road car will fall between 80 and 100 cpm, so the STI is obviously on the upper end of that range to begin with.

But the bare minimum requirement to achieve 120 cpm up front would be a spring rate of 385 lbs/in and for the rear a rate of 271 lbs/in. Granted all of these numbers are very rough estimations and don't consider the effect of bump stops on actual spring rates. And there are still a bunch of unknowns that prevent any of this from being an exact science. It's good food for thought though.

It seems that 400/350 rates would be good to have, perhaps on the RCE T2's and R-compound tires.

[Mykl]

That's just the thing... but what about the guys running the monster 275+ width race tires? Not only that, but even on street tires it seems like some STi owners insist on running 600+ lb/in springs.

I chose 450/400, which is at the upper limits of what I should have (according to some, others don't agree), but if I go wider and stickier than 245/45-17 Azenis, I'm going to need more to keep the tires off the fenders. Either that or I'm going to have to compromise tire clearance by running longer springs, or raise the car and my CoG with it.

I guess the simple answer is to run big-ass bars. But I'm not entirely sure those are without their own compromises.

[SWortham]

Yeah, you'd certainly need a very stiff setup if you want to make the most of tires like that.

Did you realize that a Ferrari F430 only has 225mm wide front tires? For me that puts it all into perspective how crazy 275's are on the STI. It sure will be grippy though.

[Mykl]

Originally Posted by SWortham

Yeah, you'd certainly need a very stiff setup if you want to make the most of tires like that.

Did you realize that a Ferrari F430 only has 225mm wide front tires? For me that puts it all into perspective how crazy 275's are on the STI. It sure will be grippy though.

heh, I guess autocross has really screwed with my perception of what fast is.

Next fall I'll be making a transition to doing more track days, so maybe I'll just leave my setup alone.

[stretch]

I'm pretty sure Ferrari (and most supercars) use smaller front tires to maintain good steering feel and to keep tire temperatures similar front and rear.

Remember that wider tires do not necessarily increase the amount of rubber you put on the ground. Your air pressure primarily dictates that. Wider rubber changes the shape of your contact patch, adding weight and wind resistance in the process. Wider certainly isn't always better, especially if you aren't increasing your wheel width proportionately too (which seems to often be the case, but wheel width is another weight-gainer).

[polyol]

Originally Posted by stretch

Remember that wider tires do not necessarily increase the amount of rubber you put on the ground. Your air pressure primarily dictates that. Wider rubber changes the shape of your contact patch, adding weight and wind resistance in the process. Wider certainly isn't always better, especially if you aren't increasing your wheel width proportionately too (which seems to often be the case, but wheel width is another weight-gainer).

Wider is always better for autox...but you are right...when you choke up a tire on a small rim you do not fully use all of the tire.

[Mykl]

Originally Posted by stretch

I'm pretty sure Ferrari (and most supercars) use smaller front tires to maintain good steering feel and to keep tire temperatures similar front and rear.

Remember that wider tires do not necessarily increase the amount of rubber you put on the ground. Your air pressure primarily dictates that. Wider rubber changes the shape of your contact patch, adding weight and wind resistance in the process. Wider certainly isn't always better, especially if you aren't increasing your wheel width proportionately too (which seems to often be the case, but wheel width is another weight-gainer).

Well... it seems as though on any time of timed course on a paved surface (with a few exceptions), more rubber to the ground is always faster. But the question is... for most of us, how fast do we really need to be?

Obviously if we're racing our cars in timed events we want modify our cars to the extent of the rules. But for those who are just doing track days having a car that behaves in a way they find comfortable to drive is really more important than the grip that really wide race tires will give you.

*edit* for clarity

[stretch]

Originally Posted by polyol

Wider is always better for autox...

A generally agree with this, but never say always! Wider tires demand a car with camber under better control. Wider tires may not heat up sufficiently or quickly enough, may break away too suddenly (usually a driver problem, but not always on a short-wheelbase, stiff car), or may not make sense on both ends of the car due to inconsistent slip angles or heat-up rates. I think spazegun2213 has it right with his staggered setup, and going wider in the rear probably wouldn't decrease his lap times.

I furthermore think the difference between tire widths is a little overblown- there are diminishing returns (based in part on softness of the rubber and tread depth), and differences in tire compound are much more influential on grip than is width (who wouldn't rather have 225 V710's instead of 255 Azenis?). Before I'm called Captain "Duh", I mention this because I think people associate the added grip from R-compounds due to their width, when it is almost entirely from the softer rubber compound.

So yeah, I agree, wider is better. Just not always. Some (and I admit, few) autocrossers notice performance gains by going with lighter, smaller tires and correspondingly narrower rubber since the weight savings is worth more than the diminishing returns of wider rubber. These tend to be the lower horsepower cars, but it still goes to show that these slowmobiles weren't gaining much from wider tread even when cornering speed is most important. I guess my point is that if I were racing somebody on similar tires but my tires were slightly narrower, I wouldn't blame the tires for the race's outcome.

Well, I was about to start talking about deformation of the contact patch when I found this, which explains things pretty darn sufficiently:

If looking in general at handling balance in the case of tyres, the cof drops off as you increase the weight on the wheel and you can tune a car's handling by adjusting the anti-roll bar.

The bigger the contact patch the more grip you can get. In a drag race, dropping the tyre pressure increases the contact patch area and increases grip.

When you look at lateral grip other factors start to matter. The tyre develops side force because of the slip angle between the tyre and the road. This slip angle means the tread is being pulled sideways by the road surface. At the front of the contact patch the deflection is relatively small. As you move back along the contact patch the deflection increases steadily. At some point, the sideways forces in the tyre exceed the friction between the tread and the road and the tread starts to slip relative to the road. When the tread is slipping like this it produces less grip on the road. As the slip angle increases the sideways deflection builds up quicker so the front of the contact patch works harder. But more and more of the back of the contact patch is sliding and losing grip. At some point you reach a maximum point where more slip angle means less side force because you are losing more grip at the rear of the contact patch that you are gaining at the front.

The longer the contact patch is, the more gradually break away occurs. If you shorten the contact patch, the break away occurs more abruptly but you get more absolute grip at the peak as there is less variation in sideways distortion between the front and back of the contact patch, more of the contact patch reaches maximum grip and starts to slide at the same point.

When you fit wider tyres the contact patch wider and shorter for the same tyre pressure. This means you get a more abrupt breakaway but more grip right on the limit.

If this logic is correct then increasing pressure in the tyre further improves grip, since more pressure = less contact patch area = shorter contact patch = better grip?

However more rubber on the road does help grip due to the hysteresis properties of rubber. As rubber expands to fill a depression in the road, it takes some time to do so. When a tyre is sliding (and due to the slip angle, the rear most portion of the contact patch slides at even low cornering forces), this means that the upward rise of the depression to which the tyre is moving has more rubber acting on it that does the upwards rise on the other side. This allows a pressure differential in the lateral plane, providing frictional resistance over and above that offered by simple friction. As the tyre vertical load increases, the rubber is forced more fully, and more quickly into the depressions, overcoming the hysteresis and reacting on both sides of the upward rise from the depression more evenly – giving less pressure differential and less grip.

Low tyre pressure is better for grip from deformation and hysteresis.
Tuning the pressure is about balancing the contact patch length (which is better as pressure goes up), and the contact patch pressure (which is better as tyre pressure goes down). Even though the optimum grip may be achieved at low pressures higher slip (because the hysteresis element is significant),low pressure increases tyre deflection, which increases heat (less even radius over longer contact patch). It therefore appears that the best way to increase grip is a wide tyre as this gives a shorter contact patch for the same inflation pressure.

However tuning the handling balance using tyre pressures, appears to be a combination of trying to match front and rear slip angles, ultimate grip at the limit, and effect of heat which may effect inflation pressure and tyre compound?

Maybe I'm preaching to the choir here about this tire stuff, but hell, I like reading this stuff again and again anyway. Here's some slip angle math for those capable (not me): http://www.racer.nl/reference/pacejka.htm

[SWortham]

By the way, the reviewers rave about the F430 for its supreme balance and how easy it is to drive at the limit. Of course the car is also a RWD, mid-engined, double-wishbone super car, and a couple hundred pounds lighter than the STI so it's like comparing low-grade Top Ramen to quality Maruchan Ramen (there is no comparison).

But in the STI's case, it's front-heavy, AWD, and needs all the front grip you can get.

[Rootus]

Originally Posted by Mykl

Well... it seems as though on any time of timed course on a paved surface (with a few exceptions), more rubber to the ground is always faster.

How do you put more rubber to the ground? Adding weight to the car?

Wider changes the contact patch shape.

[stretch]

Originally Posted by Rootus

Wider changes the contact patch shape.

I was careful to say primarily in my posts. You're mostly right. I think the quote in my above post explains why (better than I'm about to!), but the advantage of wider tires amounts to two things:

1) The distribution of weight over the contact patch evens out. This is because as the contact patch becomes shorter, the stiffness of the sidewall plays less of a role in supporting the tire. Since the sidewall is circular, it puts the most load on the exact bottom of the tire and much less so towards the front and rear of the contact patch. Larger radius tires actually help here, too.

2) Wider tires can often run lower air pressures, which does increase the contact patch. If you look at a tire compound as being able to support a certain distance of tread deformation at the rear of its contact patch, the wider tire will be able to run a lower tire pressure before hitting this limit.

However, I think there are deminishing returns to both of these advantages, which is why I say the advantages of wider tires becomes quite minor. I'd love to have 255 or 265's on my STI, but wider than that? Only if a sponsor was paying for it!

[SWortham]

More interesting facts...

The first classical law of friction states: “The size of the contact surface does not influence the friction”. Why, then, are tires for the Formula 1 racing cars so wide?

According to the “FIA Technical Commission”, the organisation that decides on the technical rules for Formula 1, the front tires should not be wider than 355 mm (i.e. 14 inches) and the rear tires no wider than 380 mm.

The classical friction laws apply to materials that undergo mainly plastic deformation in the asperity summits. This in not the case for polymers (and elastomers). Polymers display a lower friction coefficient at higher nominal contact pressure. A high friction force can consequently only be achieved with a large surface area.

Other advantages of a wide surface are that the wear is distributed over a larger area, heat development per unit of surface is lower and the result is a larger heat-radiating surface.

A racing tire performs optimally at an operation temperature of 100°C-110°C. During racing, the tire temperature is measured constantly and the data passed on to the driver. If the F1-car is getting ready for the race, the tires’ temperature is raised with tire heaters. A sort of electric blanket is used to pre-heat the tires to 80°C. Why is the tire temperature so important and what would happen if the temperature would become too high?

http://tribology-abc.com/abc/formula1b.htm

[RainMaker]

In the calculator, don't they mean "sprung weight/corner" in the results?

My frequencies were 2.7 front and 2.6 rear

[SWortham]

Originally Posted by RainMaker

In the calculator, don't they mean "sprung weight/corner" in the results?

My frequencies were 2.7 front and 2.6 rear

Yeah, that must be what they meant. I noticed that too.

And dang, what spring rates are you using?