larsalan

so, what are they size, purpose, etc.
28mm on 700c rims?

Amani576

They're desinged for more air volume to give a "cushier" ride. Since they're larger they hold more air to let you safely use a lower, less jarring air pressure. Most people who have them use them for touring or commuting. They also sometimes come equipped on some hybrid style bikes.
-Gene-

Road Fan

They also provide more rim protection than 23s or 25s for heavier riders.

JanMM

Or, coming from another direction, they are lighter and possibly faster than 32 or 35, 37, 40 etc.
Probably about the skinniest tires that can be mounted on many typical hybrid rims. It depends on where you are coming from and what you are looking for.

Severian

Tire size choice is personal preference dictated by riding conditions. If I were riding dead smooth (no chuck holes, no cracks) perfect pavement, or on a velodrome track I might consider using 21c or smaller tires. The worse the road gets the larger the tire volume I go for, partially to save my spine, partially to save my rims.

acorn_user

Depending on the tyre, you can ride faster on rough surfaces with 28's than with 23's. A lot of wider tyres also have heavy, unsupple carcasses, but not always. E.g. Challenge and Dugast Paris Roubaix tyres. I'd love to ride those. However, a lot of racing bikes are built with minimal clearance under the fork, brakes and around the rear stays. If you have one of those bikes, you might struggle to fit a wider tyre at all.

Gonzo Bob

Yes, the 28 is the width in mm. The 'C' goes with the nominal rim diameter which you haven't given. 650x28C fits on a 650C rim and 700x28C fits on a 700C rim.

Applehead57

And no, there really isn't all that much of a difference using 23 or 28 width tires on your bicycle.
To the average rider, it's a small difference, a little more cushy perhaps with the 28, but no earth-shattering change.
The pro's stay with a narrow tire and that's why most of us use 22's or 23's.
It's no big deal.

late

I respectfully disagree. When you hit something significant, like a pothole, the
lower pressure takes some of the sting out. If you live where there are crappy roads, that can mean the difference between riding, and not riding, at least for me.

CACycling

^^ +1 ^^
I run 23s on my road bike and 28s on my commuter (also a road bike) and the diference is significant. Even though I run both at max pressure, the 28s are much cushier than the 23s.

AdamD

I commute on a Cross Check with 700x28 tires and love it. I commuted for a few weeks on 700x23's, but since I cross multiple railroad tracks and the overal road quality is pretty bad I wanted something wider/cushier without knobbies or slowing me down too much. The 28s seem to be the perfect combination. I can hang with most roadies but I can also do some light trail riding and don't fear my front wheel getting caught in cracks in the road too much. It also makes me feel superfast when I hop on my road bike when I'm not commuting.

It's just another tool in the toolbag.

caloso

28mm tires are a good choice if any of your ride takes you over dirt or gravel. Since you can run them at a lower pressure than 23mm tires, you'll float over the rough stuff better.

Bill Kapaun

"Fatter" tires also dampen "road buzz" better-

z415

I certainly felt the difference in comfort and pinch flat threshold between 23s and 28s.

I_bRAD

While we're at it, FTW means "for the win"
You wanted WTF.

LesterOfPuppets

FTW also means F*** the World to old bikers (of the motored variety) and Turbonegro fans, but on teh interwebz, ya, see I_bRAD's definition.

28s are SUPER smooth and cushy! Darn near a requirement on road tandems. They're kind of a drag with most caliper brakes I've had as you have to deflate them for wheel removal.

I kinda miss my 28s. I've been thinking about running them on the back if it doesn't adversely affect steering geometry. My fork won't fit 28s, so gotta stick with 25s up there.

DannoXYZ 

On a 3.5 mile downhill near my house with speeds from 10mph switchbacks to 50mph+ straights, I always go for fastest times possible. Braking and cornering speeds on 28mm tyres at 75-80psi is simply superior to 23-25mm tyres at 90-110psi. I'm consistent to 5:12 +/- 5s on this downhill and the 28mm tyres are worth 12s faster than narrower ones. That's about 2.5 football fields ahead...

Mr. Underbridge

Quick release?