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If a bike has a 35mm bb drop with 160mm forks how much difference will putting 140mm forks have on the bb drop.
I know head and seat angle should be about a degree steeper with the 20mm less travel but low much will it effect bb drop?
Roughly (20 x sin(head angle) x chainstay length)/wheelbase
probably.
how would you measure it - sagged at 25% ?
If so, that's axle-crown height minus 25% of the travel number. The 2 forks may not vary by as much as you think
Unsagged to make it easier
Roughly ([b]12 or 15 (Accounting for sag)[/b] x sin(head angle) x chainstay length)/wheelbaseprobably.
assuming you're between 25% and 40% sagged.
In other words, not a huge amount.
I'm guessing there's not much in it but it's not difficult to do sagged - a-c height for the 2 forks is the bit you need to look up, then you take 40mm or so off the height of the 160 and 35 off the 140. You don't ride a bike unsagged - bb drop in both cases will be more than your estimate but maybe unimportant if you know it rides OK on the 160
I know you don't ride bikes unsagged but if you have unsagged measurements on a bike brands website (like most brands), then it's easier to compare two lots of unsagged measurements.
So what does 20mm less fork travel = ?? Lower bb drop
what make/model are the forks?
some companies' 140 forks may be taller than some other 160s - look up the axle-crown height (or may just be crown height)
So what does 20mm less fork travel = ?? Lower bb drop
Generally BB drops by about a third relative to fork travel not taking into account differences in sag etc.