Hermit wrote:
Hey Tommy, yep it’s Clint.
My planned use is circuit racing of which my local track is relatively short 3km circuit with a 600meter straight. Possibly also some tarmac rally.
Is it true that a larger overall diameter will give a larger contact patch?
Is there a calculator to work out the ideal tyre width for a given weight of car?
One point that needs clarification here is that you can't change contact patch *size* with any variable other than tire pressure. Contact patch *size* is very simply the vehicle weight divided by the tire pressure. (units and all that, of course)
By manipulating the diameter and width you are changing the *shape* of the contact patch. That has a huge effect on handling based on how much of the contact patch can stay 'stuck' with static friction as the tires operate with increasing slip angle before the whole thing is sliding. Basically a longer, narrower contact patch will progressively 'break free' from the pavement at lower slip angles but also slightly lower grip levels than a wide, short patch.
You might think that the answer is to go to a bigger tire and then lower your tire pressure to get a bigger contact patch which is true. However, keep in mind that most modern tires are designed to work in a pretty well established 25-35psi cold pressure kind of domain and don't tend to work great down where our cars would probably like to run a big, wide tire.
I know it's counter intuitive that wider or bigger tires don't put more rubber on the pavement but it's the case. There's a great book by Paul Haney called 'inside the racing tire' that's outstanding for this and many, many more fascinating aspects of tire design and performance.
As for our cars, the biggest knobs you're turning with wheel and tire sizes, in order of importance, is tire availability (yes, most important), tire inertia, and vehicle mass.
I say availability because the biggest variable in this whole thing is what actual tire you use. If there are only 1 or 2 choices in the size you want that's probably going to end up being further from your ultimate goal than compromising on size but having dozens of options for brands, compounds, and categories of tire.
Here's a chart I put together when making this decision myself. I made a small mistake on the math for 16" inertia but I can only find this picture and not my original file so ignore those couple cells.
So, to explain the chart, my Stalker kit was designed around the 17" wheel which looks way too big IMHO but is common on Miatas and has tons of tire availability. I would have loved to go to a 225/40/15 where there are 50 something tires available but the diameter is so far off the design that they look funny in the wheel wells. These were my four options for similar overall OD. If you drop the OD the inertia changes are more dramatic so that's really the way to go if you're not stuck with existing wheel openings.
The take away is that by just changing wheel OD you really don't affect overall mass that much or 'first gear effective mass' which is the mass of the vehicle plus the inertial effects of the engine (BMW 6 in my case but they don't vary a TON), trans, diff, and wheels/tires. Basically going down from the 17's all the way to 15's only saved 60 lbs from the first gear apparent mass, not too significant. Plus, that drops with gear ratio so it just gets closer in higher gears. I went with 16's in the end just due to appearance and a screaming deal on wheels.
Alex
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