Tire width/traction based upon vehicle weight - Exocar build question

[hipowernut]

thread68-102250

I read through the "Wider is Better? Or Not?" thread and certainly gained some insight to my dilemma. However, the question that remains for me is how do I select the best width/size of tire for a custom tube frame exocar that I'm building?

The car will weigh about 1800lbs with a mid-engine setup (40/60) and about 400hp at the rear wheels. Usage will be on the street and some autocross events.

You can see my build thread here -

http://forums.nasioc.com/forums/showthread.php?t=2093090

I have searched and searched all over the internet for a formula that could at least lead me to a size selection based upon my parameters noted above, but I've had no such luck.

Then I stumbled upon the thread noted above and more specifically the member tireman9, with hope of enlightenment to my situation.

So, can anyone help me determine the best size tire for my car?

I already have a set of street wheels and Nitto 555 tires (Front = 17x8; 235/45-17 and Rear = 17x9; 255/40-17).

I am considering an alternate autocross set that could be used for travel to and from the events. Hoosier A6 tires >> (Front = 18x10; 275/35-18 and Rear = 18x12; 315/30-18). Many are commenting that this larger set will not provide adequate traction (heat up properly) due to the light weight of my car.

I am wondering if that will be true considering the use of the lower temp Hooser A6 combined with my low CG (turbo Subaru flat 4), low ride height and wider track with the larger wheels.

Professional insight is greatly needed and appreciated.

Hipowernut

 

[CapriRacer]

Coyote,

I'm going to this post to replacement market tires, because talking about OE tires has its own set of issues.

Remember I said a tire line can have 60 or so different sizes. What typically happens is that a common size (or 2) is selected at the start of the design process. This is compared to a competitor tire (or 2) as a target. Then all the testing is done on that size, tweaks are made, then tested again, until the product is launched.

What that means is that data doesn't exist on every tire size. But because there is a huge amount of experience - and because designing tires is tied to some very fundamental science, there are rarely outliers. This is not to say there aren't ever any outliers, but they are rare - and once researched, this new data becomes part of the desiogn criteria. Let me give you an example:

Groove wander. This is a phenomenon where the tire tries to follow the grooves cut in the pavement for water drainage. It's been discovered that the driving force behind this is when grooves in the tire line up with the grooves in the pavement. To prevent this, the grooves in tires are carefully spaced to avoid this - by comparing to a list of widths of pavement grooving. This means that every size (sometimes over 60 different sizes) of a tire line has to be compared to the list and tweaked to avoid this.

However, there is no standard width between the grooves in the pavement and from time to time, we'll discover a new pattern - which gets added to the list. Usually, this means the particular tire (meaning size and line) will stay in the line until it gets replaced. In extreme cases, the existing tire (size/line) will be redesigned and new molds made with the revised groove widths. needless to say, this is expensive.

But you asked "....How can Joe Public Eng. take the best approach to designing a vehicle suspension system and accurately size the tyres required based on torque and vehicle weight etc? ...."

The answer is that it isn't expected that "Joe Public Engineer" would be doing this. That's the area for "Joe works at a car manufacturer Engineer" - and either this data is supplied as part of the qualification process or tires are supplied so the testing can be performed by the OEM.


But you also said: "....I'd have expected tyre manufacturer to have in-house testing facilities that would be checked by a national overseer - much the same way that petrol station pumps are maintained and run by retailers but legally get checked for accuracy by a central government body....."

That are 2 different issues here: Government Regulations and National Overseer.

Goverment Regulations: Needless to say there are regulations that cover certain aspects of tires - safety being one of them. Safety is obviously an issue that government regulation needs to address.

Another aspect is consumer information. How much is enough? Where is the limit where the additional cost of testing becomes too much of a burden on the manufacturer? This is a constant argument that depends largely on which side of the political fence you sit. We are not going to resolve that here.

National Overseer? All the major tire manufacturers are pretty heavy into ISO certifcation. This requires a third party to certify compliance. The ISO standards don't specify which tests are to be conducted - only that there be a testing procedure and that it be thorough and documented.

But to sum this up. When a vehicle manufacturer is going to buy 10's of thousands of tires, the cost of testing can be spread out. But even at this level, there are going to be things left unexplored. It's a matter of economics. if we are talking about a guy buying 4 tires, the cost of testing is just prohibitive.

On the other hand, tires generally behave similarly regardless of who the manufacturer is. Look at NASCAR. Those race teams have scads of money and yet they go to the race track with a whole trailer full of springs, shocks, and sway bars - and even then , they build in adjustability that they use during the race. Frankly, this is rocket science level and it's more art than science.

 

[coyoteboyuk]

And so the ultimate answer here, as far as I can determine, is that the OP and I are best picking a tyre that's cheap and easy to find (while reasonably suitable performance-wise) from those commonly used in normal road vehicles, then just testing them and adjusting the suspension as needs arise. This still poses big problems in how do I select the "right" width for my torque output, but scanning around at competitive similarly sized vehicles should answer that.