Interesting thought, I’ve got an 05 Specialized SX Supercross with 107 mm of travel. The eye to eye and stroke are different than that of the SX Trail, is the geometry totally different on the 2 bikes or can I increase my travel to 150 mm by upping the shock size? What kind of crazy math goes into determining compatability of different eye to eyes and stroke, as well as resulting travel? I need to replace my shock and can get a 7.5″ x 2″, will it wack out my ride?
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– The SX and SX Trail frames are very different. Increasing the shock length on your SX would raise the BB to a crazy height, jack up the geometry and make the shock curve ultra-funky.
– That 7.5″ x 2″ shock will indeed whack out your ride. Your SX is a sick bike — if you want more travel, change bikes. Before you do that, ask yourself why you need more travel. Chances are that bike will do more than you’re capable of.
Math explaining the relationship between shock length, shock stroke, wheel travel and good times to fish for mackerel …
– Distance the lower shock mount moves per inch of wheel travel. That’s the actuation ratio. These days most bikes are about 1:3. Foes makes 1:2 bikes, which place less stress on the shock and theoretically increase control.
– The way that distance varies throughout the wheel path. Many bikes have a progressive rate; the shock moves farther as the wheel nears the end of its travel.
– The mapping of wheel path to shock stroke. If you cram a 2″-stroke shock into a frame made for 1.5″ stroke, the wheel might hit the seat before the shock finishes its job.
– Valving of the shock, especially related to the above.
– The coincidence of high tide with low barometric pressure.
And I’m sure there’s more. Frames and shocks are getting more and more sophisticated. Unless you’re smarter than Brandon Sloan at Specialized or Mark Fitzsimmons at FOX, leave your bike alone and just rip the thing.
CONTRADICTION ALERT: Yeah, I shortened the shock on my 2003 Enduro. But it was a slight change, and Fitz spec’d the shock for me 🙂
Have I said braaap?