[Closed] Help - Dropper post maths.

a cheap seatpost to measure it up

better yet borrow one

I did think about that, I wanted to just get it ordered though.

I'd want to get some accurate measurements if I were you as it will be close. Close enough that what cranks, pedals and saddle you use may make the difference between a 125 and a 150

Ok, would one definitely be fine and one not?

Should I air on the side of caution and go 125mm?

but FWIW I'm 5'8" on a Bird Zero with 480mm seat tube and I just about get away with a 125mm reverb

125mm would be safe but you may forever miss that extra inch

If you're anywhere near southlakes i've got a 31.6 you can borrow, pretty sure if you stuck a thread up you could have it sorted by tomorrow

If your inseam really is 31" think the 125mm will be too long.

I'm just under 6'1" and had a 450 seat tube on my old bike, and had 40mm+ of exposed post using a 150 drop reverb.

So by my maths you're definitely Ok witha 125 and probably just about ok with a 150.

I'm having to go off my bike, but i'm 6ft, 31" inseam, 175mm cranks and have a slackline with a 457mm seattube.
I run a 150mm KS post with about 30-40mm out of the tube (haven't measured exactly but it's somewhere between the two).
Fag packet maths gives you a smidge to work with if you use a 150mm

I did run a 125mm in the frame with no issues

I appreciate all the answers chaps.

I'm properly rubbish at working this stuff out.

So. The general consensus is 125 would be fine but 150 may have the seat too high?

I want the full extension of the dropper to be my ideal ‘XC’ pedalling height.

I am wanting to be able to drop the post as much as possible.

Surely this means that you'd have to order the 150mm drop?

Be careful though as I find too much drop a bad thing.

I'm thinking a 125 should be ok. Any reason why it wouldn't be?

What length cranks?

All you need to do is add up cranks, seat-tube, saddle and reverb collar; then work out how much is left.

Or just build it up and get someone to measure the 'gap'.

175mm cranks

Your legs are shorter than average for a 6' person at 780mm
Minus seattube (480mm) =300mm
Minus cranks (175mm) plus pedal and shoe stack height (20mm) = 145mm

From that 145mm you need to take away the height of the bottom collar and top clamp of the post, plus the stack height of the saddle, plus a factor for the extension of your legs and ankle.

I'm guessing you might be left with 100mm if it is a stealth cable post, of course this is only as reliable as your leg measurement.
I would want to use a post to test things out before i buy.

My 150mm reverb is 260mm from seatube to top of saddle when fully inserted so 260+175+480=915mm minimum height (pedal axle centre to top of seat)

Sums above are assuming that the foot remaind flat and level in a pedal stroke, which it doesn#t

Last 3 posts confuse the life out of me!

Sorry for being dumb

Have you followed the guide on the Fox website?

http://www.ridefox.com/help.php?m=bike&id=802

I believe KS Levs are slightly shorter for their length than the reverbs if that helps.
Edit. Which having read your OP properly is no help at all. Soz!

Guide on the Fox website assumes you can measure current post.

Ok, so by using my diverge as a guide, (that has a 490mm seat tube). I have 230mm from seat collar to saddle rails.

So, 10mm shorter seat tube would give me 240 mm collar to rails.

According to the Fox guide, that tells me to order a 150mm drop post.

Am I miles out? Am I being an idiot?

Ok, so what measurements could I give you guys from my gravel bike to help us work this out a bit more accurately.

BB to seat rails?

PEDANT ALERT... bit late to the forum but it's err not air 😆 i'll get my coat......

My bad, I was never the academic. Hence the thread needing help with basic maths.

BB to seat rails is the one. Never trust seat tube numbers on charts!

Ok, I will measure the bb to seatrails on my current bike.....

Hold tight!

Ok, bb to Seat rails on my Diverge is 710mm

Cranks are same length
BB to seat clamp is 490
New frame is 480
Seat clamp to seat rails is 220mm on the Diverge

I reckon that means I need a 150mm drop transfer post as per the Fox guide a few posts up.

Can anyone confirm I'm not an idiot.

No

Your rails to BB is 710mm. Your seat tube will be 480mm. That leaves 230mm of exposed post. With an internally routed Transfer you're definitely fine. An externally routed one may be ok but it's a very close call (seat post clamp usually adds some height). I know the newest BFe has internal routing so I hope it's one of those!

.

Chief, it's an internally routed BFe, so the 150mm drop is fine?

I thought so. Thanks very much for your hep mate. It's appreciated.

Ok, bb to Seat rails on my Diverge is 710mm

Granted, the 720mm is my gravel bike with SPDs and my BFe will have flats but it seems like there will be a little bit of room to play with. I may even play it safe and order a 125mm drop post.

I really appreciate everyone's help.

I haven't read through all the above comments.... but it is defintgely very important to get the measurements right.
I've previously owned a Crank Bros Kronolog which was too high when fully inserted into the seat tube. So I always had to drop the post about 10mm to get my perfect pedaling height.
And then I changed bikes and got a KS E-ten which ended up being about 5mm too low when inserted to the minimum line. Which meant I couldn't get it to the perfect pedaling height.
As a result, i have just bought an X-Fusion Hilo SL which I can adjust to exactly the right height for my perfect pedaling height, and still get the full 125mm drop available on the post.

FWIW I run a 100mm drop and find it enough to be honest. I'm limited by a ridiculously long seat tube, might be able to get away with 125 but 100mm is enough for me anyway.

Might want it to go lower if I was jumping a lot maybe.