Shock Sizes Explained

There is no such thing as a standard size in biking. The best we can do is the size everyone decides to use. The industry is continually beseiged with new standards which are intended to help. Sometimes they do but mostly they don't.
Here is a run-down on historic shock sizes (now called "imperial") and the new (metric) shock sizes.

We used to have about 12 imperial shock sizes. Now with metric we have an additional 5 frame sizes, 10 shock sizes and a huge number of custom strokes inside that.

Historic Imperial Sizes

Historically shock makers would make whatever frame makers wanted. Eventually common sizes were settled on but never formalised. Many of these shocks sizes were very tight for internal space.

Inch	Converted to mm	Notes	Manitou Availability
5.5 x 1.0 inch	140 x 25 mm	Obsolete
6.0 x 1.25 inch	152 x 32 mm	Old and uncommon	Mara Inline
6.5 x 1.5 inch	165 x 38 mm	Common XC size	Inline
6.75 x 1.75 inch	171 x 44 mm	Specialized 2000 Bighit only
7.25 x 1.75 inch	184 x 44 mm	Specialized & Giant size	Use 190 x 50 mm Mara Inline with length reducing spacers
7.5 x 2.0 inch	190 x 50 mm	Common XC/trail bike size	Mara Inline
7.785 x 2.0 inch	200 x 50 mm	Can use a 200x56 with a stroke spacer	Use 200 x 56 mm Mara Inline with stroke reducing spacer
7.785 x 2.25 inch	200 x 56 mm	Common trail bike size	Mara Inline Mara Pro
8.5 x 2.5 inch	216 x 63 mm	Common Enduro size	Mara Inline Mara Pro
8.75 x 2.75 inch	222 x 68 mm	Common DH size, rare Enduro size	Mara Pro
9.0 x 3.0 inch	230 x 75 mm	Common DH size
9.5 x 3.5 inch	240 x 90 mm	Rare DH size, Scott Gambler etc

New Metric Sizes

In the mid 2010's the biggest shock makers all agreed on new frame sizes. These have longer length for more internal room and feature standard length changes of 20 mm with 10 mm steps in stroke.
Sounds great right? But wait.
Now within each frame size they can have travel spacers to reduce stroke in 2.5 mm increments so bike makers can get exactly the travel the market segment desires.
Also, each frame size and stroke can have two different head styles:
• Eyelet heads use the same 1/2" DU bushings that imperial shocks used.
• Trunnion heads make the shock 25 mm shorter and mount with bolts straight into the head of the shock using bearings in the frame.
Bearing options also exist for eyelet shocks. These are all 30 mm wide and take 8 mm bolts.

Trek Example: A 230 x 57.5 mm shock is an eyelet shock in 230 x 65 mm frame with a 7.5 mm travel spacer inside.
Giant Example: A 185 x 52.5 mm shock is a trunnion shock in 185 x 55 mm frame with a 2.5 mm travel spacer inside. Same internals as a 210 x 55 mm shock.

Frame Size	Stroke Options*	Shock Size	Manitou Availability	Notes
35 mm Max	30 to 35 mm	145 x 35 mm Trunnion	Possible but we haven't seen one yet.
35 mm Max	30 to 35 mm	170 x 35 mm Eyelet	Mara Inline 170 x 30 mm 170 x 35 mm
45 mm Max	40 to 45 mm	165 x 45 mm Trunnion	Mara Inline 165 x 40 mm 165 x 45 mm
45 mm Max	40 to 45 mm	190 x 45 mm Eyelet	Mara Inline 190 x 40 mm 190 x 45 mm
55 mm Max	50 to 55 mm	185 x 55 mm Trunnion	Mara Inline or Mara Pro 185 x 50 mm 185 x 55 mm
55 mm Max	50 to 55 mm	210 x 55 mm Eyelet	Mara Inline or Mara Pro 210 x 50 mm 210 x 55 mm
65 mm Max	57.5 to 65 mm	205 x 65 mm Trunnion	Mara Inline or Mara Pro 205 x 60 mm 205 x 65 mm
		230 x 65 mm Eyelet	Mara Inline or Mara Pro 230 x 60 mm 230 x 65 mm
		230 x 65 mm Trunnion	Mara Pro 230 x 60 mm 230 x 65 mm for Trek Remedy & Slash	Trek Special for extra downtube clearance
75 mm Max	70 to 75 mm	225 x 75 mm Trunnion	Mara Pro 225 x 70 mm 225 x 75 mm
75 mm Max	70 to 75 mm	250 x 75 mm Eyelet	Mara Pro 250 x 70 mm 250 x 75 mm

*Stroke is reduced from maximum using internal shaft spacers