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The outer tracks of a disk are longer than the inner tracks, and could therefore potentially hold more data. Constant angular velocity puts the same number of bits on every track, which wastes much of the potential capacity of the disk. A solution to this problem is constant linear velocity (CLV) which varies the motor speed such that the head can spend more time on the outer tracks and therefore record more data.

But as the linked article indicates, while this is used on optical disks, it has generally not been used on floppy disks. Why not? They would certainly have gained great benefit from more capacity.

It would have added more complexity to the drive controller, but intuitively, this seems unlikely to add significantly to the total cost.

Wikipedia says 'seek performance would be greatly affected during random access by the requirement to continually modulate the disk's rotation speed to be appropriate for the read head's position'. Why? And how much slowdown? Intuitively I would rather the drive spend an extra tenth of a second seeking, than require me to spend an extra ten seconds manually swapping disks because one disk didn't have enough capacity.

Is there another consideration I am missing?

But as the linked article indicates, while this is used on optical disks, it has generally not been used on floppy disks.

It has. Apple's famous Twiggy drive was one attempt to do so. It featured 6 different zones with 15 to 21 sectors per track. As a result some 120 additional sectors per side (or 100KiB per disk) could be used. To keep the data rate constant rotation varied between 394 (outer tracks) and 590 (inner tracks) RPM.

With the switch to 3.5 inch drives on the Lisa and later Mac this idea was the reason for Apple to develop their own format (*1) using a Sony drive. In fact, controller and parameters kept mostly the same (394-590 RPM), except now it was 8-12 sectors due the shorter track length of a 3.5 inch drive. Set off by using 80 instead of 46 tracks, so a single-sided 3.5 inch drive did hold 400 Kib, while a dual sided Twiggy had ~850 KiB

And then there is one really widespread use of the basic idea, even predating the Twiggy: The Commodore drives (*2) starting with the PETs 2031, but most notably the 1541, all 170 KiB drives used to write different zones of 17 to 21 sectors per track. But instead of running the disk at different speeds the data rate was varied to the same result (AFAIR).

Other companies/developers toyed with the same idea as Apple, for example the Sirius(*3) did so as well, increasing standard 500 KiB (unformatted) capacity per side to 600 KiB. And instead of asking to buy 'special' media, to make it happen, standard diskettes could be used. Then again it's not Chuck Peddle's brain child - the same man who was behind the PET development :))

Why not? They would certainly have gained great benefit from more capacity.

The benefit of some 10-15% increased capacity (*4) got several drawbacks:

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  Track switch speed (as mentioned) will increase whenever a zone border is crossed and the motor speed needs to be changed. While not as much as spinning up from stand, it may take several turns to stabilize. After all, we got real motors and real masses to accelerate or decelerate here.

  
  
  
  
  
  • This got offset a with the introduction of direct drive and smaller disk sizes, but not much.
  
  
  • To counter the cost, not every track was handled different, but zones where used (*5).
  
  
  
  



• Increased cost for motor control on the drive side



• Increased cost for motor control on the controller side


• Introduction of a non standard interface, as the existing interface had no motor control lines beside on and off

Especially the latter was an even greater turn-off than increased cost for manufacturers. Offering more size is only a minor advertisement feat, increased cost may be handed to that (*6), but non-standard interface means hard migration into uncharted territory - even more so being tied to this non-standard manufacturer (of the drives). Nothing CEOs like.

The 3.5 inch drive itself is a major example here, as it only took off after stripping everything 'new' but the size and making it compatible with the existing 5.25 drives.

The alternative to modify the data rate would have offset some of the drawbacks, but required new controller chips and different analogue setup - also more delicate, as data rate and head gap are related.

Intuitively I would rather the drive spend an extra tenth of a second seeking, than require me to spend an extra ten seconds manually swapping disks because one disk didn't have enough capacity.

Sure thing - except, with a gain of only ~15% in size (*4 again), the saving isn't much and you'll swap soon again. With diskette sizes it's much like with CPU speed. Every thing less than doubling is hard to notice.

*1 - Keep in mind, this was before the standardisation of 3.5" drives.

*2 - Thanks to Felix Palmen for reminding.

*3 - Wilson digged out the corresponding patent.

*4 - Just pull out your geometry books and calculate the difference in diameter of a circle with 1.354 inches and 2.25 inches (for 5.25 inch drives) or 0.9719 inches to 1.5551 inches (for 3.5 inch drives) - or be lazy and just divide them to get a factor telling the relative length increase.

Also keep in mind that only whole blocks will work, so only if it's increased by at least ~1/16th for 5.25 or 1/8th for 3.4 a new block can be added to a track.

*5 - Which as well makes sense for blocked structures with fixed block length.

*6 - And soon offset new cheap(er) integrated solutions.

At least one computer does (something close to) what you are describing, and there is a relevant patent.

Wikipedia claims:

But disks made at constant bit density were not compatible with machines with standard drives.

And this is apparently supported by a dead-tree citation. I read the sentence as meaning that the drives need special disks. So my guess is that because these drives never caught on, the disks didn't -- it was a chicken-and-egg problem that caused this implementation of the technique not to gain much market share or traction in the industry.

And perhaps because of the patent, no-one else tried to make a CLV floppy drive as far as I can tell.

During the hay-day of floppy drives the technology needed to do CLV was rather expensive. Writing/reading data at a fixed rate and running a motor at a fixes speed is the cheapest option. Variable speed oscillators were uncommon and not generally available as single integrated circuits.

At the time cost tended to be the most important factor, as computers were still very expensive and many people were still using tapes for storage, a floppy drive was relatively fast and spacious.

The gains were also somewhat marginal. Consider that Apple's expensive "Twiggy" drives. They proved unreliable and could only store 871k of data on a 5.25" disk. Sony had already released its 3.5" format two years earlier, and with double density disks computers were able to store 880k on them. They quickly became popular and cheap.

I'm not convinced about Wikipedia's claim that speed changes would have had a big influence on seek times. Floppy disks have a lot more friction and run at much lower speeds than optical media where these speed changes do reduce seek performance.