tire footprint

[ALB]

Every once in a while someone asks about front tire size for a lowered car and the conversation ends up with the Cal Look traditionalists on 1 side arguing that nothing other than 135's (or maybe 145's) is needed, while there are those that believe that there are better choices, especially when going wider in the back as well. After thinking about it for a while I think I've come up with a way to compare how much tire is "on the ground" (or footprint) of all the different sizes, and I've found it very interesting.

It's a pretty simple formula- tread width multiplied by sidewall height (which I would arrive at by taking the tire diameter, subtracting the rim diameter and then dividing by 2). It's a little crude, as all the tire diameter figures and most of the tread widths are taken straight from info on Tirerack, and in a few instances there's tread width and slight diameter variations between manuftacturers, but for the most part it gives a good idea of how much tire is actually "on the ground" for braking and handling.

example- 165R-15- 5.4 (tread width) x 5.2 (25.4- 15 then divided by 2) =28.08. Now, if we multiply that by 3.571 to give it a value of 100, all other sizes can be compared as a percentage of the stock size tire. And note I've added tire diameters-

125-15----- 22.9"- 56.4%
135-15----- 23.5"- 65.26%
145/65-15- 22.5"- 61.5%
145-15------24"-- 74%
155/60-15- 22.3"- 64.%
155-15------24.75-90.4%
165/45-15- 20.84- 56%
165/60-15- 22.8"- 75.2%
165/65-15- 23.4"- 90%
165-15----- 25.4"- 100%
175/55-15- 22.6"- 77.3%
175/60-15- 23.3"- 84.5
175/65-15- 24"--- 91.5%
185/55-15- 23"--- 87.1%
185/60-15- 23.7"- 94.7%
185/65-15- 24.4"- 102.3%
185/70-15- 25.4"- 113%
195/50-15- 22.7"- 91%
195/55-15- 23.5"- 98.6%
195/60-15- 24.2"- 103.2%
195/65-15- 24.8"- 113.7%
205/50-15- 23.1"- 102.7%
205/55-15- 23.9"- 111.2%
205/60-15- 24.7"- 120%
205/65-15- 25.4"- 130%
205/70-15- 26.3"- 141%
215/60-15- 25.2"- 136.5%

Any way, there it is. I included a few back tire sizes in there as well, just because I had the figures. You can certainly see why a bug, when equipped with front discs, 135's and stock drums with 185/70's or 205/60's on the back can be a little "unbalanced" in the braking department.

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 would lock up and the front end slide at the drop of a hat under heavy braking (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 (the only rear discs available at the time were off of a 914), which went a long way to balancing the front/rear braking bias. The front still didn't handle all that well, but this was before all the short/wide tires that are available today.

There is more to it than just contact patch size, but I'm trying to keep it simple, and my little set of figures are a rough comparison of the amount of contact patch on the road, given all the different sizes that people are using today. I don't expect every lowered bug owner to agree with me (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 and at the strip then I get it, but if you want the car to be more fun to drive and be (somewhat) more sure-footed in all traffic conditions there are choices.

As usual, just my 2 1/2 cents (I'm Canadian, eh). Al
_________________
On a lifelong mission to prove (much to my wife's dismay) that Immaturity is Forever!!

[boxer74]

This is a very helpful post. Thank you.
_________________
74 Super
84 Westy

[ALB]

Glad you found it helpful, @Boxer24. I've since simplified/refined the formula and found it more accurate for comparison if you just multiply the tread width by tire diameter. Maybe I'll take the time update the whole thing one day, but since no one seems to give a sh*t, at the moment it doesn't really seem worth the effort.
_________________
On a lifelong mission to prove (much to my wife's dismay) that Immaturity is Forever!!

[slalombuggy]

I know it uses a mathematical formula but owning a set of 165/45/15s on the front of my 57, I can't see how it only has a contact patch that's only 56% as wide as a 165 and narrower than a 135. The tread on a 165/45 is almost 6" wide. Somtin goofy somewhere.

I just about lost my buggy when I forgot I had put 135s on the front to try them out and took a curved freeway on ramp at my usual autocross cornering speed and just about went straight off the road mid corner when the front tires lost traction. 😳 I put the 165/50s back on.😁