Tire footprint

Something I just posted on the Samba- applies more to there than here, with all the small skinny front tires the Cal Look guys use, but some of you may find it mildly interesting- 

https://www.thesamba.com/vw/fo...hp?p=8970896#8970896

"older Intermeccanica Speedster (still under wraps in the garage) a pic wouldn't show much,what with all the junk piled on it..."

 

On a lifelong mission (much to my wife's dismay) to prove that immaturity is forever!

Original Post

I don't know much about tires or how their sizes relate to each other but your Samba piece has me confused. You refer to footprint...that would be the area of rubber on the road measured in square inches ? Your second sentence includes the word ....measure....  Do you mean to really say  .....compare....in this case instead ? How can the depth ( or height ) of the sidewall relate to the "length" of rubber on the road ? Tread width is obvious. Your method of comparing one tire to another is consistent but again I can't see how sidewall height relates to the length of the rubber on the road. Thanks. ( stunned in Ottawa )  

David,

From my understanding, as the aspect ratio of a tire increases, i.e., sidewall measurement gets larger, the contact area gets longer and narrower, which gives a softer ride.  As sidewall measurement decreases, the contact area shape changes to shorter and wider, giving better performance on cornering, but at the price of comfort.  Even if the area of the contact patch remains the same, the shape of l x w has changed, which accounts for differing values in traction.

Just as with most suspensions, most street tires are a compromise between performance and comfort.

I seem to recall reading long ago that the contact patch or footprint is related to tire pressure and load on the tire.   With the same pressure and load, the area of the patch would be essentially the same for different tires but the shape would be different.

Maybe a wider tire would allow you to use a different pressure and maybe it would behave differently with the car in motion.

@David Stroud and @Jim Kelly- I'm going to preface this by saying that my little "list" was originally created to show Beetle owners that there are more (and better) choices than 135-15's for front tires on cars that are "driven". I'm hoping that it creates a dialogue on the Samba, where people can make more informed choices.

David- yes, the word should have been compare (I have a cold and the head is a wee bit foggy).

"How can the depth ( or height ) of the sidewall relate to the "length" of rubber on the road ?"

I could have just multiplied the tread width by the tire diameter (bigger diameter means longer contact patch) to get the values I was looking for, but the numbers then get rather big, and a taller sidewall equates to bigger diameter (and longer patch) so they end up being the same thing, just with smaller numbers where it's easier to see percentage differences. Then, when I figured out how to manipulate the (rather rough) data to turn the figures into actual percentages I was off and running! Again, all I was looking for is a rough comparison of contact patches- the most significant (to me) being that 135's (a popular front tire choice with the Cal Look crowd) has only 65% of the rubber on the ground that stock 165's do. 

My late teens/early to mid 20's was spent driving a Cal Look bug with 135's (and Ghia discs) on the front and 185/70's (and stock Beetle drums) on the back. My experiences mirrored a lot of other guys running the same set up- the 135's weren't much for handling, the discs locked up and the front end would slide at the drop of a hat (and you had to be even more careful if there was dust/dirt on the pavement or it was raining). If you weren't paying attention 100% of the time it could be downright scary. The answer for me was to install type 3 brakes on the back of the car, which went a long way to balancing the front/rear braking bias. Didn't do anything for the handling though...

Jim- You make good points- there is definitely more to it than just contact patch size. I'm trying to keep it simple, though and my little set of figures are a rough comparison of the amount of contact patch on the road (my apologies- I keep repeating myself), given all the different sizes that people are using today. My point is- you're not going to get the VW crowd away from little tires (essential when dropping the front end), but if a guy uses a tire with a similar diameter to a 125,135 or 145 but is wider he'll get better handling/braking and be safer. I don't expect every Cal Look owner to see the light (to some it isn't Cal Look if it doesn't have 135's) and if all your building is a boulevard bruiser to be feared at the local burger joint then whatever, but if you want the car to be more fun to drive and be (somewhat) safe there are choices.

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