# Bicycle Mechanics - Setback or Straight Seatpost

Bikeforums.net is a forum about nothing but bikes. Our community can help you find information about hard-to-find and localized information like bicycle tours, specialties like where in your area to have your recumbent bike serviced, or what are the best bicycle tires and seats for the activities you use your bike for.

View Full Version : Setback or Straight Seatpost

Lexi01
07-19-11, 07:09 PM
Hi Guys,

I'm a tall rider and have found myself needing an extra little bit of height since moving from platform pedals to clipless.

I'm 6'5" and have a XXL bike with a 873 standover and 620mm top tube and I have a 400mm seatpost extended to the limit (in fact about 5mm past the limit).

I noticed that thomson make a 410mm post and was going to get one of those...then I saw they come in a straight and a setback. I'd like to go for the setback but had one question:

When they're measured flat (top to bottom with a straight ruler or something) would they both be exactly the same height or would the setback be a little shorter due to the slight angle at the top?

Retro Grouch
07-20-11, 07:55 AM
My GUESS would be they would measure the same. Why don't you email Thompson to ask them and be sure?

FBinNY
07-20-11, 08:17 AM
You do lose a bit of height, but only a tiny bit, in the range of a few millimeters.

Thomson uses the same post, and makes a bend of 10-15 degrees or so (never measured it) 4" or so from the top, so there will be a drop, but it will be less than 0.10" (cosine of the angle x 3").

Hopefully, you're not at a height where that would make any practical difference.

BTW- if the math is an issue, ypu can sketch it out. Use a ruler and make a mark 4" up the edge (height of straight post to bend). Draw a reference line inward & make a mark 16mm (3/4") for the offset. Draw a line from there back to the bottom corner to make a triangle. Measure 4" up the new line, and see how far short of the reference line it comes. That's the drop.

well biked
07-20-11, 08:27 AM
One other note: Thomson setback seatposts only have 18mm of setback, even though the bend in the post looks pretty dramatic. Most conventional setback seatposts have 20-25mm of setback.

fietsbob
07-20-11, 08:45 AM
Setback in regards to the bike has a plumb line thru the BB axis, as a zero point .
so a tall post, long way from the crank axis , first off will be quite a ways apart
at the top of the angle .

the Knee over the pedal measurement is, to some extent, dependent on dimensions,
length, of thigh and foot of the rider, plus setback .

Get on the Bike and see how it all sorts out..

Booger1
07-20-11, 09:35 AM
^ We have a winner,no numbers/math involved....

Lexi01
07-20-11, 08:29 PM
Hopefully, you're not at a height where that would make any practical difference.

Unfortunately I am. I have a 400mm post now that's sitting 10mm past the limit line. This Thomson post is 410mm...so an 1/8 of an inch may make a difference to me...hence this question.

I actually found a BBB post called the Longscraper that's 450mm. I've ordered one from my LBS. That should be plenty. It's not a setback, but at 6'5" with an exceptionally long inseam I don't think I can afford to be choosy - my knees are killing me!