A Behind-the-Scenes Look at the Bridgestone SF325 Tire for the First-Generation Mazda MX-5 Miata

This is the first post in a two-part series about the development of original and reproduction tires for the first-generation Mazda MX-5 Miata.

This January, Bridgestone reproduced the SF325 tire to serve as part of Mazda's restoration service for its iconic first-generation MX-5 Miata. The Bridgestone SF325 tire was original equipment on the MX-5 when it was originally released about 30 years ago. The tire ceased production in 2003. However, Bridgestone has chosen to begin producing the tire once again.

After releasing the reproduction tire, it was discovered that there was a yellow first-generation MX-5 at a Bridgestone's education center in Chiba, Japan. Mr. Takahiro Ikeda, a Bridgestone test driver for the SF325 tire when the MX-5 was in development 30 years ago, enthusiastically wanted this car for training of young test drivers. He persuaded his manager and teammates, took the car to the Bridgestone Proving Ground and began a minor restoration to make the car run again. The car was about 30-year old, but in good shape. So it did not need a full restoration.

Same as Mr. Ikeda, Mr. Tetsu Kasahara was involved with the development of the MX-5 from the Mazda side. Upon hearing from Mr. Ikeda about Bridgestone's yellow MX-5, Mr. Kasahara volunteered his support for the restoration, especially the adjustment of tires and suspensions.

"When designing the MX-5, we envisioned an ideal tread pattern of a tire for a light weight sports car. We went to several tire manufacturers, told them our thoughts, and asked them to develop new tires for our MX-5," recalls Mr. Fumitaka Ando from Mazda, who served as a chief engineer in charge of the chassis for the first-generation MX-5. "Even though we had very little time to spend on the development, I remember going up to people at Bridgestone and asking them the really difficult request of developing a lightweight tire with this tread pattern, which would give the car a nimble handling performance."

The first-generation MX-5 is famous to this day for the exceptional speed of its development, which took only 18 months from the decision to mass produce to the first vehicle sale. Like the body of the car, the tires were subject to short development time. Bridgestone received the request in the fall of 1987 and completed the tire design in about one year. The SF325 tire was fitted to a prototype car in the winter of 1988.The quick development of the vehicle's tires greatly contributed to the performance of the MX-5, which embodies Mazda's Jinba-ittai ideology, or the feeling of unity between a driver and their vehicle. Installation of tires on the mass-produced MX-5 vehicle began in 1989.

Mr. Jun Makino, one of the Bridgestone engineers involved in the project, remembers the tire design process." In most cases, the tread pattern of a tire is usually designed by the tire manufacturer, but because Mazda had such a strong vision, they drafted the original design. We made a drawing based on it. I had been watching my senior colleague work out of the corner of my eye. At that time, we couldn't turn 2-D drawings into actual 3-D tires using a computer like we can today, so he was struggling to translate the design into the physical shape of a tire, which was a considerably difficult work. Even so, I was really impressed by the craftsmanship he showed, creating the tread pattern in one shot through the full use of his experience and ingenuity."
In this way, the passionate commitment and craftsmanship of engineers from Mazda and Bridgestone triggered the swift completion of the perfect SF325 tire for the lightweight sports car. "I thought we were making an incredibly difficult request, but Bridgestone delivered a tire that was exactly the same as our first image," recollects Mr. Ando. Mr. Yasuyoshi Mushitani is a Mazda engineer who is currently in charge of the driving feel of each Mazda car based on the spirit of Jinba-ittai. He also owns an first-generation MX-5, which he purchased when the car was launched 30 years ago. Speaking of a key moment, he says, "At the time we had three tire manufacturers providing tires for the car, including Bridgestone, and I remember looking forward to finding out which tire my car was fitted with when I purchased it."

(Pictured from left: Mr. Mushitani, Mr. Ikeda, Mr. Kasahara, Mr. Makino, Mr. Ando, Mr. Honda)

People related to the first-generation MX-5, such as engineers and owners, can't speak about the car without mentioning the SF325 tire, proving that it is an indispensable part of the whole vehicle. More than 30 years later, the performance of the SF325 tire for the MX-5 still holds true as requests for the SF325 reproduction tire continue to roll in. (To be continued)

*SF325 tire is sold only in Japan.

Here is second post of this article