As far as I can tell the basic design is the same, the T2-2.2 has longer arms to accommodate 29" wheels with tires. Is there any functional reason someone would use a TS-2 over a 2.2.?

Yes, if you never expect to work on 29" wheels with the tires in place and can get the TS-2 at a lower price.

I think that TS-2's are nicer to work with, but nowadays I have just as many 29'er tires as smaller tires, so I'm glad I just bought a TS 2.2

If you go with a TS-2 now, and it turns out that you decide later that you are going to do bigger wheels, Park Tool has a retrofit kit that extends the length of the arms of the TS-2 so you can do larger wheels on the TS-2. It's priced pretty reasonably, but I haven't tried it so I can't tell you how well it works.

As others have pointed out the main difference of the TS-2.2 compared to the TS-2 is the longer upright arms and indicator arms to allow for lateral truing of 29er wheels without removing the tire.

A 29er wheel is the same diameter as a 700c wheel so if the tire is removed then a TS-2 will work just fine. The big advantage to not removing the tire is in a professional shop environment where removing the tire to make minor tweaks to lateral trueness would be time consuming. The tire typically must come off for radial truing. For the home mechanic, you have to weigh which is more advantageous: cost savings vs not having to remove the tire.

One other note: Somewhere along the way Park changed the collars that lock the adjustment of the uprights and added thin bearings to make it open/close a bit smoother. They came out with a rebuild kit for the original TS-2 that included the parts to make this upgrade.

I have an older TS-2 that was given to me as a gift back in 1991...it is 100% stock (I still have its original box) and still going strong.

As mentioned there are upright extensions that can be added to give the TS-2 the extra length the 2.2 has. Also, there is a thru-axle adapter kit for them as well. One can get really fancy and outfit either with a set of dial gauges.

The one accessory that I have for the Park TS-2 series that I really like is the disc brake rotor truing gauge. It helps make quick work of straightening rotors.

-j

Anyome knows the difference between TS-2.2 and TS-4
?

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MODERATOR NOTE: Thread closed. Similar discussion here:
Better truing stand than Park Tool TS-4 or TS-4.2