Can someone help me get my head around some simple math in reguard to stroke length and shock sizing please . If a frames eye to eye is say 230mm and the bike uses a 62.5mm stroke length which equates to 165mm travel . and 65mm stroke length is 170mm travel . how wrong am I in assuming 60mm stroke is 160 and 57.5 is 155mm rear travel or am I way out
Roughly. Travel divided by shock stroke equals leverage ratio.
Therefore in the case above the leverage ratio is 2.64 to 1.
So multiply the various stroke lengths by 2.64 to give you the travel:
62.5 = 165
65 = 171.6 ~ 170
60 = 157.4 ~ 160
57.5 = 151.8 ~ 150
However this assumes the leverage ratio is linear, which it never is. Most frames are at least slightly progressive, ie the ratio goes up over the travel.
Added to this, manufacturers tend to be inexact with their travel measurements and round up to the nearest 5 or 10mm.
Thanks for laying that out straight for me.. I just coundnt get my head round it
You may also find the way the bike rides is awful with less travel as the ramp up if there is any will be in the wrong place, sag point will be wrong etc.
I mean, this is purely hypothetical yeah? You're not actually planning to buy the wrong sized shock for the frame?
No not at all just theorising 🙂
Wot they said, vis the not quite truly linear nature of the rear geo. But working well on my (theoretical/hypothetical, honest) Ripmo AF, with Jade X. Swapped the 210 x 55 shock for a 216x63 (using one 2mm offset Bush to reduce eye to eye length to 214mm). Travel now bumped from 147 to c.170mm, with a moderate but useful gain in BB height too. Re-used the original 500# x 2.5" LS coil.
Are you able to lay out any more detail on how it rides like that?
I hope chives has checked with the spring off and given the bottom out bumper a decent squatch down, hopefully the tyre doesn't buzz the seattube or the linkage doesn't reach an unexpected limit. That is quite a leap in shock stroke and travel. 😀
- @scienceofficer, it doesn't feel much different to be fair. It's running 170mm up front, with a -1 degree angleset, so more stable on the steeps, but still climbs like the proverbial goat. @noeffsgiven, clear all round at both limits of travel. The only complication was changing to a lighter spring (lost 7kg). Went for a 425-500 progressive, which is available in 55 or 65mm options. Chose the 65, thinking it was closest to my 63mm stroke shock. It was a good inch too long (free length). The 55mm option sorted it, and no, it doesn't bind up at full travel. Dunno how their sizing works mind you..