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Surly Bikes is a designer and importer of bicycles, frames, parts, and accessories based in Bloomington, Minnesota, established circa 1998. Surly is owned by and shares facilities with Quality Bicycle Products (QBP).
The company is well known in bicycle messenger and single-speed culture[who?] and helped establish the Single Speed World Championship (SSWC). Surly specializes in steel bicycle frames. Frames are made in Taiwan from 4130 chromoly steel.
Distinctive components made by Surly include the Dingle cog, cranksets with separately detachable spiders, reversible chain tensioner, and the "Large Marge" an unusually wide bicycle rim. Surly also sells lifestyle items such as wool jerseys and flasks bearing the Surly logo.
In 2005, Surly began selling the Pugsley, the first mass-produced fatbike, a mountain bike with extremely large volume tires—up to 4 inches wide—for deep snow and sand riding. The front and rear wheels share a common hub size and can be interchanged, allowing for redundancy and additional gearing combinations. Noted bicycle technical authority Sheldon Brown said, "Pugsley is, in its way, as revolutionary as the original mountain bikes were in the early 1980s." Bicycling Magazine wrote, "It's not ideal for everyday use, but it can handle a wide variety of demands and conditions well."
Other Surly bikes include:
- 1x1: single-speed mountain bike
- Steamroller: single-speed bicycle
- Pacer: road racing bike
- Cross-Check: cyclo-cross bicycle
- Cross-Check SS: a single-speed version of the Cross-Check cyclo-cross bicycle
- Straggler: cyclo-cross bicycle w/ disc brakes
- Long Haul Trucker: touring bicycle
- Disc Trucker: touring bicycle w/ disc brakes, frame based on the Long Haul Trucker
- Trucker Deluxe: a Long Haul Trucker with S and S Bicycle Torque Couplings, allowing it to be disassembled for travel (available as frameset, with 26-inch wheels only)
- Troll: mountain/touring/commuter bike with 26-inch wheels
- Karate Monkey: 29er mountain bike
- Ogre: 29er mountain and touring bicycle.
- Krampus: 29er mountain bike with 3inch tires on 50mm rims
- Moonlander: "fatbike", accommodates wide 4.7" tires on 100mm rims
The following Surly cycles have been discontinued:
- Conundrum: a mountain unicycle
- Instigator: a Hardtail mountain Bike, reintroduced in 2014
- Travellers Check: a cyclo-cross bicycle with S and S Bicycle Torque Couplings.
- Dingle cog: A two-speed track cog for fixed gear bicycles. It was released in early 2007. It is a multi-speed fixed-gear cog that is compatible with standard track hubs and lockrings. Three gear combinations are available, 17t and 19t, 17t and 20t, as well as 17t and 21t. When used with two appropriately sized chainrings, the rear axle's position in the dropouts can remain almost the same and still allow a relatively large difference in gear ratio between the two combinations. Bicycle Fixation's Harold Ikerd wrote, "The ability to change gear ratios while on the trail, road, or lost is heavenly!"
- "QBP Brands Page". Retrieved 2013-02-23.
- The description pages of their frames state that "[this frame] is, like all our frames, made of 4130 CroMoly steel...." 
- Regenold, Stephen (July 2006). "Adventure bikes :: Gear Review :: July, 2006". Retrieved 2008-05-15.
- Brown, Sheldon. "The Amazing Surly Pugsley!". Archived from the original on 2006-04-28. Retrieved 2008-05-15.
- Cassidy, Chris. "Surly Pugsly". Bicycling Magazine. Retrieved 2010-10-16.
- "Surly 2008" (PDF). Retrieved 2008-05-15.
- Ikerd, Harold. "Surly's Dingle Cog". Bicycle Fixation. Retrieved 2010-10-16.