The first floppy disks were eight inches. This size was set by IBM; I haven't been able to find any indication of why they chose it, but maybe it was just because it seemed quite small to them compared to the hard disks they were used to.

According to https://en.wikipedia.org/wiki/History_of_the_floppy_disk

In a 1976 meeting, An Wang of Wang Laboratories informed Jim Adkisson and Don Massaro of Shugart Associates that the 8-inch format was simply too large and expensive for the desktop word processing machines he was developing at the time. He argued for a $100 drive.

And that was how 5.25″ came to be.

I can see how the smaller size was considered preferable if e.g. you were hoping to build a portable computer. But why was it cheaper? Is it because of the engineering difficulty of making a mechanism both large and precise?

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    The large footprint for an 8" floppy (especially, as was common, for a pair of them) was a non-monetary cost. It is kind of awkward to sell a "desktop computer system" that would not fit on a large number of desks. – RichF Apr 2 '19 at 14:17
  • The original 8" disks were designed to load firmware into the CPU of the new IBM System 370 mainframe. The disk was able to change the instruction set so that the mainframe could emulate the older System 360 perfectly. The 8" disks needed to be larger to hold the firmware for the mainframe. The smaller disks have a reduced capacity but they are much easier for handling and the 5.25" format lasted until the 3.5" took over. Later mainframes continued to use reconfigurable firmware so allow customers to run any applications desired. Once enough RAM was available this was less of an issue. – Vegan Fanatic Apr 2 '19 at 21:29
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    It's (not so) simple economics. The last CD-ROM drive I ever bought was ~$15. The first one was like +$100. When you assign an arbitrary price point, you need to lower the costs of manufacturing and sourcing parts, all to the lowest bidder who has the most reliable product for the lowest price. Which probably means redesigning it from the ground up. You need Economy of Scale on your side or you won't be able to sell expensive IBM corporate hardware to personal consumers. And if EoS won't even get an 8 down to $100, it's back to the drawing board. – Mazura Apr 3 '19 at 0:07
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    @Acccumulation It has been a long time since I worked with 8" floppies, probably mid-to-late '80s. (It was an old system even then.) So my numbers will be guestimates. The stand-alone dual-floppy drive system approached a 10-inch cube -- one of the dimensions might have been more than 10, another less. It was big, it was loud, it was slow, and the disks did not hold much. One drive held the operating system (maybe cp/m), the other the development system. – RichF Apr 3 '19 at 1:49
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    A Pertec PCC 2000 CP/M desktop had two half-height 8" floppy drives oriented vertically, and a 10MB internal hard drive. It also included the monitor and keyboard in a single case. Although not that large, it was heavy. – rcgldr Apr 3 '19 at 3:31

A few miscellaneous thoughts:

  • Less Materials: On a certain level, a smaller device is cheaper, simply because it uses less raw materials. You need less of the magnetic coating if you're applying it to a smaller disk, there's less metal required for the drive's chassis, etc. The 8-inch drives also used a 50-pin data connector, whereas the 5 1/4-inch drives removed many unused pins to cut that down to 34, so you'd need less wiring too.

  • Simpler stepper motor: Early 5 1/4 floppy drives used the same track spacing (48 tracks per inch) as the older 8-inch drives, so there's no difference in the precision required*. But an 8-inch disk would have 76 tracks, compared to a 5 1/4's 40 tracks. So while you'd need a stepper motor with the same precision, it didn't need as large a range of motion.

  • Cheaper motor: 8-inch floppy drives were designed to rotate the disks continuously, and lower the read/write heads onto the disks when required. 5 1/4-inch drives only rotated the disks when required. As they wouldn't be in continuous use, they didn't need to be engineered to such a high standard to achieve the same operating lifetime. Additionally, as the 8-inch disk was larger, it would require a higher-rated motor to spin it in the first place. 5 1/4-inch drives generally used 12V DC motors, whereas the 8-inch variety used 24V DC, or even mains AC motors.

  • Simplified head loading: As the 5 1/4 drive isn't spinning unnecessarily, there's little danger of tracks being worn away by an idle drive's heads, and some drives omitted the head-loading circuitry completely as a result. Springing the heads down as the drive's latch/door was closed would suffice.

*Later versions of each doubled this to 96tpi, but again both were available as 96tpi, so there's no difference in precision between them.

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    @StephenKitt Some 8-inch drives used AC motors, some used DC, e.g. the Perted FD-410 deramp.com/downloads/altair/hardware/8_inch_floppy/… .Though the DC ones tended to run on 24V DC rather than the 12V found in 5 1/4-inch drives. – Kaz Apr 2 '19 at 13:23
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    "Additionally, as the 8-inch disk had more mass, it would require a higher-rated motor to spin it in the first place" - I suspect drag with the sleeve was the real issue here, as that's based on total surface area, which in turn is the square of the radius. Roughly,a 5.25 was rubbing against 40 square inches, while the 8 was about 100. – Maury Markowitz Apr 2 '19 at 21:35
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    I suspect that neither the mass nor rotational inertial are even within two orders of magnitude of the drag. – Maury Markowitz Apr 3 '19 at 13:03
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    The stepper motor of every drive I've seen can rotate continuously in either direction without limit. Typically there would be a piece of spring steel wrapped around a wheel which would be wound or unwound to move the head. The piece of spring steel would need to be longer for an 8" drive, but I don't think that would add complexity if one accepts that the track spacing won't be perfectly even but will get wider as the spring winds around the wheel (the change in spacing is probably smaller than other mechanical tolerances anyway). – supercat Apr 3 '19 at 15:46
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    @supercat I was thinking of the stepper assembly as a whole, and basing my experience on my Olivetti FD-501 drive. An old full-height SSDD 5 1/4" drive, it has a plastic disc with a spiral groove mounted to the stepper motor, with a notch of the head assembly resting in the groove. As the motor rotates, the notch is drawn toward the centre of the spiral (or toward the edge), moving the head along the surface of the disk in the process. This specific method is definitely designed for just 40 tracks: to go any further you'd need a larger plastic disc (maybe larger than one can fit in a 8" bay). – Kaz Apr 3 '19 at 17:07

Pricing doesn't necessarily have anything to do with manufacturing costs. Just look at printers today. You can buy an HP printer for $59 at Walmart, which probably doesn't generate much profit for HP. What makes HP money is the ink you will buy in the years to come.

I don't actually know the answer, because I'm not sure there is one definitive answer. I can suggest some less obvious reasons why 5 1/4" drives might be cheaper:

  1. To increase adoption. Force the 8" drive into obsolescence, and there will only be 5 1/4" drives. That takes time, but in the time it takes for it to happen, you have optimized your manufacturing process and your manufacturing costs are a lot lower. You'll profit more at the lower price.

  2. To sell disks. This is the printer and ink model. You can only sell a drive once, but you can sell a lot of disks to put into it.

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    I don't see #2 explaining why the price of 8" equipment couldn't also be lowered in a similar "printer and ink model". That also only really works when the market is big, and possible markups on consumables are high. Back when 8" floppies were common, the computer market was positively tiny compared to today, and any floppy that met the technical requirements was essentially equivalent to another. Making floppy disks isn't that difficult. – user Apr 2 '19 at 20:36
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    As for the "force it into obsolescence" argument, Did personal computers ever support 8" floppies? looks quite relevant. – user Apr 2 '19 at 20:38
  • @aCVn The 8" floppy WAS forced into obsolescence by the 5 1/4" drive. That's not really in dispute. Even if some pcs supported 8" drives, and I remember the "high-end" Radio Shack models did, but that was the last gasp of a dying format. – Mohair Apr 2 '19 at 21:16
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    The printer ink pricing also only works because ink cartridges aren't standardized, and it's easy to make another style of cartridge completely unusable in your printer. There are also printers and cartridges that have DRM-like restrictions to keep off-brands from working even when the physical packaging is the same. 5.25 floppy drives have none of these restrictions. – Ghedipunk Apr 2 '19 at 21:25
  • @Ghedipunk Floppy disks were patented, so you can do things like license the drives at low cost/free but charge a larger license fee on the media. – user71659 Apr 3 '19 at 4:42

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