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Answers and comments to Why were 5.25" floppy drives cheaper than 8"? suggest some reasons why floppy disks moved from 8" to 5.25"; basically it seems the smaller size reduced engineering difficulty and thus cost in a number of ways.

Given that, why not make the drives even cheaper by jumping straight to an even smaller form factor such as the 3.5" that was eventually settled on?

I can think of three possible reasons:

1. Reduced size means reduced capacity! 5.25" was a trade-off between the desire to make the drives smaller which would indeed reduce cost, and the desire to preserve capacity.




2. On the contrary, beyond a certain point, miniaturization becomes difficult and adds cost. With late seventies technology, 5.25" was the optimum balance between the cost of a larger mechanism and the cost of a smaller one.




3. As it turns out, smaller was indeed cheaper, but that would've been too much of a leap into the unknown at the time, for an industry that as yet had no experience with disks smaller than eight inches.

Was it for one of those three reasons, or something else?

[W]hy not make the drives even cheaper by jumping straight to an even smaller form factor such as the 3.5" that was eventually settled on?

Because that needed a new technology. The move to 5.25 inch didn't change any technology involved. Everything stayed the same:

• Drive design


• Drive mechanics


• Electronics (including analogue)


• Material of floppies stayed the same


• Manufacturing of floppies

The only difference was in size, a linear shrink with a factor of about 1.5 (*1), which means the needed surface and thus size shrunk in half (*2). All without any basic change, just relative minor adaptions.

It's a bit like the Tick part of the often cited Tick-Tock strategy Intel follows for CPUs. A shrink of an existing design just in scale, not design or function. The following Tock was then again a step of design changes (*3).

(Beside, a 3.5 inch drive in an 8 inch bay would just look ridiculous :))

*1 - It's sufficient close to square root of 2 (1.41)

*2 - drive height was kept the same thus half volume. Later developments cut the height as well in half, resulting in 1/4th volume of a slim line 5.25 compared with a full height 8 inch

*3 - In fact it was a competition of many designs: IBM's 4", Sony's 3.5", Matushita's 3", Mitsumis 2.8", Sharp's 2.5" and Fujitsu's 2". Not to mention spiral formats like Sony/Canon's 2" VideoFloppy

As Albert Einstein allegedly said “Everything should be made as simple as possible, but no simpler.

The same applies to floppy drive size and many other things. There is a big difference between 8" and 5.25" in both drive size and disk size.

• Drive size: 8" drives as part of an integrated system really limits your form factor choices. You can have the drives integrated with the monitor and everything else, like the TRS-80 Model II and then it doesn't seem so bad. But if you are making a smaller machine - e.g., Northstar Horizon, or a machine where the floppy drives are separate - e.g., Apple II, Atari 800, then 5.25" gives you a lot more options on how/where to place the drives.




• Disk size: 8" disks require a large envelope (e.g., 9" x 9" or more typically stick them in a 9" x 12") to mail, and can only be stored one per page in a typical letter-size looseleaf binder. 5.25" disks can be sent in a smaller envelope and stored 2 per page in a letter-size looseleaf binder. They also work well with smaller software manuals - e.g., ~ 6" x 9", one per page.

However, jumping in the 1970s, to a smaller size would have resulted in either significantly reduced capacity (as already noted, if using the same track density and other parameters as 8" and initial 5.25" drives) and/or significantly increased costs due to more expensive (at the time) integration of electronic circuitry. So 5.25" gave the desired advantages - space, weight, cost - without going "too far".

When 3.5" did become a real thing, it came with a significant change - the hard plastic case. This brought in a new advantage of durability. At the same time, the technology for the necessary circuitry had advanced by that time enough to provide a higher capacity (720k and up) without a higher cost. This was also the era where the rest of the computer had shrunk enough to start producing laptops, where the size advantage of a 3.5" drive was critical. In the late 1970s and early 1980s, a typical motherboard the size of today's (or even 1990s) laptop motherboards, didn't include floppy drive controller, hard drive controller, video card, etc. So there was no practical reason to make the drives that small.

The main issue was the limitations of available stepper motors and control hardware for them, and the sensitivity of the read/write head.

In order to read/write information from the disk the read/write head has to be positioned over the correct area. Then the head itself needs to either sense changes in magnetic flux (read) or alter the magnetic flux (write).

So each track needs to be wide enough that the stepper motor can reliably position the head over it. Unlike a hard drive where it's always the same stepper motor, floppy disks have to work with the motors in many different drives so the tolerances have to be a lot lower.

5.25" was as small as the could go while keeping the technology somewhat affordable and reliable at the time. Later Sony improved tracking and better motors were available, so 3.5" disks became commercially viable.