brainache and need help thinking

rickbst170

Sorry. Didn't get much sleep. Quick question is this: does anyone ride and 18" frame and have 32" inside leg? If so, how long is your seatpost?

Long version...

I have a giant reign 2005. Spec here...

https://www.google.co.uk/url?sa=t&sourc ... 33GSc2e7YQ

From the table on page 4, I'm trying to work out if a seatpost which is only 300mm long will go high enough for me. I have an 18" frameset but not built so can't measure/try a seatpost for reference.

If the seat tube is 457mm, And the rear wheel centre is 438mm, the top of the seat tube is 895mm at 73.5 degrees, right?

Then a bit if maths to work out the real straight line length from floor up tells me it's 858mm from floor to seatpost tube top.

So if the top of seattube is 858 from floor, and I have 240mm seatpost (once 50mm is inside the the frame to be clamped - 50mm sounds about right for a minimum? And apply the same 73.5 degree angle), and my inside leg is 32" (813mm), the saddle will be 858+240=1098mm from floor and so 285mm higher than my nutsack so should be fine for me with 170 or 175mm crankarms?

Found a seatpost which ends on egay in 5 hours. Giant frame is 30.8 so funny seatpost size, not found many that fit so don't want to miss this if it'll fit

Ta

JMcP92

You have the bike, sit on it, if the seatpost is in beyond the minimum distance when you're comfortable, measure that, see what kind of length it is, add 70mm for a guess at minimum insertion, and go off that

rickbst170

Cheers matey, but I only have a frame. No bars, wheels, fork etc so can't sit on it. Would have helped if I mentioned that :oops:

cooldad

Not even trying to do the maths for you, but I would suggest a lot more than 50mm in the seat tube. Double that at least.

rickbst170

Cheers cooldad. Missed out on the seatpost, too busy eating dinner

I reckon 300mm is too close, particularly if 100mm is going in. 350 is a minimum. Will just buy an uncut 400 I think. Bloody giant and their ridiculous 30.8 tube. If I can find a 30.9 ream I might open it up.

JBA

rickbst170 wrote:

Bloody giant and their ridiculous 30.8 tube.

Are you sure about that? Bikepaedia states 30.9mm, originally an Easton EA50 or Race Face Evolve.

Kowalski675

JBA wrote:

rickbst170 wrote:

Bloody giant and their ridiculous 30.8 tube.

Are you sure about that? Bikepaedia states 30.9mm, originally an Easton EA50 or Race Face Evolve.

Do 30.8 seat posts even exist? Dunno about in 2005, but Giants are 30.9 these days.

rickbst170

Thanks guys i'll measure it tomorrow. Not sure where I got 30.8 from

Thomson do a 30.8 but it's expensive. Few others around too.

YIMan

rickbst170 wrote:

Sorry. Didn't get much sleep. Quick question is this: does anyone ride and 18" frame and have 32" inside leg? If so, how long is your seatpost?

Long version...

I have a giant reign 2005. Spec here...

https://www.google.co.uk/url?sa=t&sourc ... 33GSc2e7YQ

From the table on page 4, I'm trying to work out if a seatpost which is only 300mm long will go high enough for me. I have an 18" frameset but not built so can't measure/try a seatpost for reference.

If the seat tube is 457mm, And the rear wheel centre is 438mm, the top of the seat tube is 895mm at 73.5 degrees, right?

Then a bit if maths to work out the real straight line length from floor up tells me it's 858mm from floor to seatpost tube top.

So if the top of seattube is 858 from floor, and I have 240mm seatpost (once 50mm is inside the the frame to be clamped - 50mm sounds about right for a minimum? And apply the same 73.5 degree angle), and my inside leg is 32" (813mm), the saddle will be 858+240=1098mm from floor and so 285mm higher than my nutsack so should be fine for me with 170 or 175mm crankarms?

Found a seatpost which ends on egay in 5 hours. Giant frame is 30.8 so funny seatpost size, not found many that fit so don't want to miss this if it'll fit

Ta

It seems a bit flawed to be measuring the distance from the floor? All that is telling you is stand over height and I suggest if you can stand over your saddle with your feet on the floor, the bike/seatpost is way too small.

Assuming it's all in line would be near enough to give you an idea - crank length + seat tube + seat post + seat height minus pedal height?

rickbst170

From what I remember when sizing people up for new bikes, the general rule was to have tiptoes just about touching the floor and when sitting on the saddle, your leg should be straight when the pedal is at it's lowest and your heel is on it. Basically what youvve said above (length minus pedal height). I reckon 300 is too close to the limit so will just look for a 400mm post, 350 minimum. Ta.

Initialised

18" bike, 33" inside leg, 400mm seat post with ~300mm showing.

Bear in mind you need to lose 2-4" (50-100mm) of post for minimum insertion depth.

The Rookie

Frame (457mm) plus 175mm crank, less 10mm for half of pedal thickness, the saddle sitting point will be about 40mm above the clamp on the seatpost, so that gives
Leg length=seatpost exposed plus rest of bits
32" or 813=457+175-10+40 = seatpost exposed=662+ seatpost exposed

Therefore seatpost exposed = 151mm

Seatpost length (min) 251mm for 100mm min insertion

Height above floor is totally irrelevant!

Giant posts are 30.9mm, just get/use a 27.2mm post and a shim (£3.99) and it will be cheaper than buying a Giant specific post.