do they shorten it (which seems counter intuitive, but it might be right)?
Offset seatposts do they lengthen Effective Top Tube or...
- Thread starter MMcG
- Start date
how would it shorten it unless you turned it backward?
I dunno, I think, I thought I read that it actually slackened ST angle and that can actually cause a shorter ETT the further the seatpost goes up or somethign like that?
Just trying to get a sense of what's what, because a web retailer has a good price on Race Face XY seatposts and I need a long 31.6mm post for my frame.
Just trying to get a sense of what's what, because a web retailer has a good price on Race Face XY seatposts and I need a long 31.6mm post for my frame.
Nope, they'll only shorten it if you turn it backwards, as BS said 
The slacker the angle, the longer the effective top tube when you put it up. That applies for seatposts or seattubes.
The slacker the angle, the longer the effective top tube when you put it up. That applies for seatposts or seattubes.
Okay, I think that rules out the RF XY then.
BS is right; unless you install it backwards (and w/ the seat clamp, i'm not sure that's even possible) an offset post will always lengthen the effective TT; that's why people run them.
Thanks everyone. Off to look for a good long 31.6mm straight seatpost now.
If anyone has a spare, let me know!
If anyone has a spare, let me know!
MMcG said:
why what are you trying to do ?
Just trying to buy one and get the best bang for the buck splaticus.splat said:
Jenson USA has some NOS Race Face XY posts for $29.00 or thereabouts - I think they sold for around $95 or so brandy new back in the day. They are strong and light, but they also have a lot of offset to them.
so why does it rule it out ?
They do neither. The effective top tube length is the distance from the center of the head tube to the center of the seat post. Whether you use an offset seatpost or not is a matter of preference, so you are optimally positioned over the pedals.
well I think that a straight post would make more sense on this frame now that I've got the scoop on the offset one.splat said:
Just depends on how you measure. If you measure to the center of the body of the post, it won't extend the effective top tube. If you measure to the center of the post opening, though, which is where the saddle mounts and where the rider sits (so it makes the most sense to measure that way), it'll extend the effective top tube...pdawg said:
If you're shifted backwards and there is more distance between the headtube and the rider's butt, isn't that a lengthening of the effective top tube?
the XY posts have the clamp pressed into the post. I've seen two break at that union, almost as if the ID was undersized or the clamp "plug" was oversized. Spiral cracks in the post both times.
I'm not saying it was a common problem, but ...
I'm not saying it was a common problem, but ...
I guess it would be a lenthening of the cockpit and not the ETT in essence - would that be more accurate?binary visions said:Just depends on how you measure. If you measure to the center of the body of the post, it won't extend the effective top tube. If you measure to the center of the post opening, though, which is where the saddle mounts and where the rider sits (so it makes the most sense to measure that way), it'll extend the effective top tube...
If you're shifted backwards and there is more distance between the headtube and the rider's butt, isn't that a lengthening of the effective top tube?
MMcG said:
so that would be cock pit length??
:wonky2:
I would stay away from the XY post if possible. I had one of those and it seemed like no matter how hard you clamped it down, the saddle would always tilt if you happened to hit it hard. The only way to prevent this was to tighten it to the point where that little aluminum cylinder stripped out. Race Face was good though and sent me a new one, but I would still search for a different post.
back of your ass to the tip of your hands. So stem length and seatpost setback would impact "cockpit length"douglas said:
Um, effective top tube is a frame dimension measured along a horizontal line, to give a better idea of what the top tube would be on a sloping tube frame if instead of a sloping tube it was horizontal. (I know you know this.) Raising or lowering your seat will completely change the distance if you measure it as you suggest. It's fine if you just want to know that measurement, but then you're not really talking about a frame dimension as much as a personal set-up dimension.binary visions said:Just depends on how you measure. If you measure to the center of the body of the post, it won't extend the effective top tube. If you measure to the center of the post opening, though, which is where the saddle mounts and where the rider sits (so it makes the most sense to measure that way), it'll extend the effective top tube...
If you're shifted backwards and there is more distance between the headtube and the rider's butt, isn't that a lengthening of the effective top tube?
Hmm... I always thought "effective top tube" was the top tube length that occured when you you took seat tube angle into consideration as well.OGRipper said:
I understand it's not a frame dimension, but I had always thought that the effective top tube WAS a "personal set-up" dimension to a certain extent. Aren't most ETT measurements taken in a horizontal line from the headtube to where the seatpost would exist if you had a seatpost in the frame? Thus taking things like slack seat tubes into consideration?
Here, look at this:
That measurement is taken to include where the seatpost would be if there were one in the frame. A setback post would increase that measurement.
That measurement is taken to include where the seatpost would be if there were one in the frame. A setback post would increase that measurement.
This one is a little less clear but it does give the impression that it takes the location of the top of the seatpost into consideration. The left hand hash mark is clearly located behind where the seat tube ends.
ETT is a pretty vague measurement... maybe companies just aren't consistant with this and I've only dealt with the ones that measure as I do?
Ugh, I have so much work to do, I really need to stop looking up geometry charts and get back to it
ETT is a pretty vague measurement... maybe companies just aren't consistant with this and I've only dealt with the ones that measure as I do?
Ugh, I have so much work to do, I really need to stop looking up geometry charts and get back to it
MMcG said:
yup
so why not a shorter stem & a set back post?
I actually plan on using a short stem, but I still think a straight post will be a better option.douglas said:
Sorry, but that's really not correct. "Effective" means you don't reference actual tube length. It is the EFFECTIVE length, which would include the offset. That's the whole point of calling it "effective".pdawg said:
You might want to go back to the source of your mistaken belief and correct him or her.
"Optimal position over the pedals" is not the only thing an offset seatpost is for. It's one thing, but not the only thing.
For example, if the bike you like has a too-steep seat tube angle for your optimal effective top tube length, you can run a setback seatpost to gain some effective top tube length. Whether this setback puts you in a good, bad or indifferent position relative to the BB depends on the frame's seat tube angle and your particular body type and your biomechanically optimum position on the bike.
I post this again because I like reposting my art work.....
My concept of a cockpit measurement:
For me the distance between the BB and the top of the Head Tube is the important measurement on a Dh bike. Once you stand up, where your seat is located is not a huge concern.
TT was a good measurement tool on traditional rigid frames...not so much anymore. Especially on DH (slack seat tube) bikes.
Rhino
My concept of a cockpit measurement:
For me the distance between the BB and the top of the Head Tube is the important measurement on a Dh bike. Once you stand up, where your seat is located is not a huge concern.
TT was a good measurement tool on traditional rigid frames...not so much anymore. Especially on DH (slack seat tube) bikes.
Rhino
FYI, most frames are designed to be run with a post setback of 2cm. That is the standard anyways. Zero setback posts like a reg thomson or raceface are the should be the exception, not the norm.
This doesn't really apply to DH/FR bikes though.
This doesn't really apply to DH/FR bikes though.
Yes!!! Do you still need one. I have a Titec from a few years back. Let me know.MMcG said:
BV, I agree it is vague, but it IS a frame dimension, and personal set-up should not impact it. Otherwise, how could a frame manufacturer publish ETT? You are also correct that you measure to where an imaginary horizontal line would intersect a seatpost, so yes seat angle will impact the dimension. But seat HEIGHT should not.
Remember back when all top tubes were horizontal? Nobody needed an "effective" tube measurement because all you had to do was measure the ACTUAL center-to-center dimension, and it was easy to compare different frames.
With sloping tubes came confusion. Different manufacturers use different slope angles, so the actual measurement is less useful for comparing different frames. The "effective" dimension is a way of ignoring the actual configuration and getting an approximation of what the actual dimension would be if the top tube was horizontal.
On the Sunday diagram, yes, it's further back from where the seat tube ends, but it's right where the imaginary seat post intersects a horizontal line from the head tube right? It is also higher than where most people will probably ride their posts on their Sundays - so it's not really about an actual seat height.
Rhino, your measurement is definitely helpful but it's not really an effective top tube.
I think the effective top tube dimension is relatively useless for DH bikes because long travel suspension often come with extremely slack or steep seat angles. I tend to agree with Kidwoo and others here who look at the DOWNTUBE measurement to get a sense of how things are laid out.
Remember back when all top tubes were horizontal? Nobody needed an "effective" tube measurement because all you had to do was measure the ACTUAL center-to-center dimension, and it was easy to compare different frames.
With sloping tubes came confusion. Different manufacturers use different slope angles, so the actual measurement is less useful for comparing different frames. The "effective" dimension is a way of ignoring the actual configuration and getting an approximation of what the actual dimension would be if the top tube was horizontal.
On the Sunday diagram, yes, it's further back from where the seat tube ends, but it's right where the imaginary seat post intersects a horizontal line from the head tube right? It is also higher than where most people will probably ride their posts on their Sundays - so it's not really about an actual seat height.
Rhino, your measurement is definitely helpful but it's not really an effective top tube.
I think the effective top tube dimension is relatively useless for DH bikes because long travel suspension often come with extremely slack or steep seat angles. I tend to agree with Kidwoo and others here who look at the DOWNTUBE measurement to get a sense of how things are laid out.
Isn't seat (and therefore setpost position) dependent on your legs to the pedals rather than reach to the bars?
splat said:
Thomson is goodness.