Just curious...why did oldsmobile have 15" tires on a 56 88 but reduce to 14" in the 60's. Cars were faster, pushed harder and were bigger in the 60's but olds went with a smaller tire. Any thoughts..

 

Back in the early days tires were 21, 18 and lots of other sizes. In the 40's the popular tire was 16 inch then there were 15's then 14's in the 60's. I would guess they were looking for the lower ride height than previous years...

 

Yes, the aspect ratio stayed more or less the same from the forties through the sixties, at about 78%, which was down from probably 100% in the '90s through the early '30s.

As cars got lower, with lower ride heights and ground clearances due to improved roads, the logical thing was to make the wheels smaller. They didn't come out with lower aspect ratios for production cars until the late sixties (75%, then 70%), which allowed larger rim sizes with the same external circumferences.

Even then, tires that were larger compared to rim diameter were considered to be more desirable, as they provided extra ride cushioning. Turning in any way that made you feel any lateral force was considered unsafe and wrong, so lateral stability in hard cornering was never considered by most people.

It wasn't until the high-end sports cars of the late seventies that lower aspect ratios (60%) and their effects on handling entered the public consciousness and began to seem desirable, and not until the nineties that suspension engineering advanced to the point of being able to get a smooth ride out of them.

Watch the car chase scene in Bullit sometime, or better yet, watch the real scene and the scene remade with modern cars, and you'll see what was considered high-performance handling at the time.

- Eric

 

Wheels reduced in size as technology improved to produce smaller diameter tires that could handle higher speeds. This reduced unsprung weight as well as desirable features such as lower ride height and better handling.
In the latter part of the '60s the trend started to reverse, 15" 60% aspect ratio tires are virtually the norm on modern European market cars now, a Ford Focus has bigger, lower profile tires than the XJ Jaguars did when they were launched in 1968.
Also suspension has improved, so an acceptable ride goes along with cornering ability and handling that exotic supercars would have aspired to thirty years ago,
One downside to ever lower profile tires is reduced life, particularly if the cars alignment isn't spot on.


Roger.

 

The main reason for the change to larger diameter WHEELS (as opposed to tires) is to allow room for larger brakes.

Keep in mind that wheel diameter is completely unrelated to tire diameter. Most tires available today are lower profile than in the 1960s, so the tire outside diameter is actually smaller, despite using a larger wheel diameter. That's a major problem today - people who want 16" or 17" wheels on their classic musclecars don't realize that the tires for these rims are usually around 25" - 26" in diameter and look lost in the wheelwells of older cars that ran 27" or so tires. You can find taller tires for larger rims, but it takes some careful tire size selection to look right.

 

Pontiac, Olds and Chevrolet all went to 14" tires in the late 50s thru about 1969. Buick and Cadillac stuck with 15". Maniac hit on it- lower, longer, wider was the style back then.

You could also easily get large diameter 14" tires back then- up to 9.50-14 in some cases, and J or L 78-14's after the switch to alphanumeric sizes. Those tires were load rated for the big heavy cars they went on.

 

Thats why this site is so cool...I learned more in 5 minutes on this site then in any tech school could do in a day. I am sending this post to friends, I have heard this question asked a dozen times. thanks everyone...great stuff.

 

The problem now is that rubber for 14's is less available than 15's. Fortunately, aftermarket rims are around for 15s, and lots of tires. Running stock wheels restricts your tire size a little, but for stock applications, there's plenty.

 

Improved roads is the main reason that wheels and tires kept getting smaller up to the 60s. Look at the 1911 Olds Limited. It rode on 42" wheels! That's what you needed if you wanted to drive wooden-spoke wheels on unimproved, dirt roads at 70 mph. Even as late as the 50s, much of America lived on dirt roads. Every bump looks smaller to a larger diameter tire, and the extra rubber and air provides more cushioning, too.