Geo question regarding setback...

[SoCalSteve]

Hi all,

I am getting MUCH better at this geometry thing...But,

Just curious, what is setback and how is it measured?

Please, please respond in simple terms and ideas as you are dealing with a simple person with a simple mind.

And, I beg you, no geometry or algebraic formulations or calculations. (I flunked ALL those classes).

As alway, thank you all in advance,

Steve

[swoop]

setback is your bank balance minus what the frame cost.



if you look at a geo chart at some place like competitive cyclist.. you can see the lines they use to relate to the measurement. seeing it makes it easy to understand...

draw a virtual line north from the center of the bb. measure back from that line to the center of seat tube (extended) from the level top tube. that measurement is setback.

f in the diagram.

[dave thompson]

From BikeCad:


Setback is the horizontal distance between the bottom bracket and the seat tube - top tube intersection.

Note that for sloping top tube bikes, BikeCAD measures setback according to the effective seat tube length. This may not be in agreement with everyone's definition of setback.

[chakatrain]

Doesn't the seat tube angle alway correlate with the setback? I'm speaking from ignorance here, but it seem that certain seat tube angles will always produce certain setback. Is that right?

[BdaGhisallo]

Yes that is correct. For a given seat tube length, a particular seat tube angle will always give the same setback. It is simple geometry. The setback is the seat tube length X Cosine(seat tube angle). Sorry Steve - he set me up!!

[OldDog]

Yes that is correct. For a given seat tube length, a particular seat tube angle will always give the same setback. It is simple geometry. The setback is the seat tube length X Cosine(seat tube angle). Sorry Steve - he set me up!!


The setback will increase as the frame gets larger...no?

[Ray]

Yes that is correct. For a given seat tube length, a particular seat tube angle will always give the same setback.
Yeah, exactly. Which is why I always find seat tube angle to be a much more useful measure than setback. Since there are about 12 ways you could measure setback and it will vary from frame size to frame size, it's tough to determine which frame will allow you to get your seat far enough back (or forward) based on a setback measurement. If you know what seat tube angle allows you to get your saddle positioned where you like it, you're good - only ONE way to measure it! I've never understood why setback is even used in geo charts.

-Ray

[catulle]

And there is saddle setback. Which is the distance between a plump line from the saddle tip to the center of the BB.

[nicrump]

And there is saddle setback. Which is the distance between a plump line from the saddle tip to the center of the BB.

the only one that matters. contact point.

[Dave]

Doesn't the seat tube angle alway correlate with the setback? I'm speaking from ignorance here, but it seem that certain seat tube angles will always produce certain setback. Is that right?

As someone else already noted, setback is the c-c frame size times the cosine of the STA. With the same STA, a larger frame will have more setback, because the c-c frame size is larger. With a given saddle height however, the saddle would be setback the same amount. That's a common mistake to think than a larger frame will somehow increase setback of the saddle, even if the frame has the same STA.

Most buyers do find the STA to be more informative. Beware however, that there is one brand (Cervelo) that advertises their frame geometry as if all sizes have the same 73 degree STA, when in fact, they may not. You have to trust that they have properly accounted for the actual STA when comparing the frames. The only problem is that you don't really know what seatpost offset might be the best if the STA is really not 73. You get to find out when you build up the bike.

Some frames may have seat tubes where the centerline does not intersect the centerline of the BB. That's another case where you hope the manufacturer properly accounted for the offset. If the actual STA is reported with a forward offset ST, then the reach will be longer than expected compared to a non-offset ST, once the saddle is in the same position, relative to the BB.

[Avispa]

And there is saddle setback. Which is the distance between a plump line from the saddle tip to the center of the BB.

I have a Legend and an Ottrott, which (according to the charts provided), supposedly have the same geometry: TT: 54.9, STA: 73.7, HTA: 73. The difference is the TT slope. The Legend is at 6.40 degrees and the Ottrott at 6.95. I am using identical components of both bikes...

Now the million dollar question! :confused:

Then, why must I have the saddle forward about 6mm more on the Ottrott than on the Legend*, to achieve the same "tip of saddle to center of bars" (56 cm), or saddle setback (6.2 cm) measurements???

Could two frames with the same STA have different setback?
Oh no!! :eek: :eek:

*correction!

[BdaGhisallo]

There's got to be something out of whack with the dimensions of one frame. If you have the same saddle, on the same seatpost, and with the rails in the same spot on that same seatpost, and with each seat at an identical height from the center of the bb, and the saddle setback is different on each frame, then it only means one thing: different seat tube angles on the frames.

[93legendti]

I have a Legend and an Ottrott, which (according to the charts provided), supposedly have the same geometry: TT: 54.9, STA: 73.7, HTA: 73. The difference is the TT slope. The Legend is at 6.40 degrees and the Ottrott at 6.95. I am using identical components of both bikes...

Now the million dollar question! :confused:

Then, why must I have the saddle forward about 6mm more on the Legend than on the Ottrott, to achieve the same "tip of saddle to center of bars" (56 cm), or saddle setback (6.2 cm) measurements???

Could two frames with the same STA have different setback?
Oh no!! :eek: :eek:
Seat height is the same on both bikes? When you say "I am using identical components of both bikes..." Does this include the same saddle and same length of stem?

[BdaGhisallo]

Theoretically, yes, everything should be identical to make a comparison. All that I said will impact on saddle setback. Looking at the numbers quoted, the effective reach ( horiz distance from line vertically bisecting bb forward to the bar) is 49.8cm. I know the distances were saddle tip to bar, which would imply a sloped line, but for our purposes here let's assume those are horizontal numbers.

I think it's always helpful to think of reach and setback as emanating from a single source, and that source is the line vertically bisecting the bottom bracket. That is where it all begins. The distance from that line back to the seat tube is the frame setback. The distance forward of that is the effective top tube, and when you throw in stem length you get effective reach.

Now think how the sta will affect that setback. As the sta slackens, the seat tube reaches its intersection with the top tube further back from that source line. If the top tube length does not change, then as the sta decreases, it will pull more of the given top tube length behind the source line and leave less forward of it. It will increase frame setback and reduce the effective toptube. As you go the other way with the sta, the opposite happens. To maintain the same effective top tube when the sta changes, the resultant change to the frame setback must be added ( when sta decreases) or subtracted ( when sta increases) from the stated top tube length to maintain that same reach.

The effective top tube length will not only affect reach ( along with stem length of course) but, combined with the head tube angle and the fork rake, it will have a direct impact on the front centre measurement - the holy grail number for a lot of frame designers.

So in a roundabout way, let's get back to the original question. If I take two identical frames - identical in every way - and take a seatpost, with saddle affixed, and insert it into one frame to a given height, and do the same in the other frame to that same given height, the saddle iwill be in the exact same position relative to the bottom bracket on each frame. If the saddle is not found to be in the same position, then you can only conclude that the frames are not identical and do , in fact, have different sta's.

Now if you establish that the sta's are, indeed, the same, then the remainder of the reach equation must account for different saddle tip to bar measurements.

If the same length and rise stems ( identical stems would totally solve it but equal length and rise angle will suffice ) are on the Legend and the Ottrott and you have to do as you say to equalize the reach measurement( move saddle forward 6mm on seatpost - producing less setback, ceteris paribus) , then the Legend has a slacker sta than the Ottrott, and thus a greater frame setback number. That can be the only conclusion to be made. Furthermore, if the reach is the same on both after pushing the saddle 6mm forward on the post when it is inserted into the Legend, then the Ottrott must have a shorter overall top tube length, even if the effective top tube lengths are identical, which it sounds like they are. ( Equal eff top tube lengths plus lower frame setback = shorter overall toptube length on Ottrott)

I hope you followed that, and I hope my fingers followed correctly what my brain was telling them to type. My head is a bit cloudy after a hard 3hr ride in the first real heat of the year.

geoff

Edit: Corrected reference to Legend's sta. It is slacker than the Ottrott, ie 73 on the Legend vs 73.5 on the Ottrott, for argument's sake.

[mike p]

Geoff,
I think I love you, thats the best explanation I've had on sta,and it's effect on position. Are you married?

Mike

[CNY rider]

Agree with above. That's the clearest explanation I've seen of this topic. Thanks.

[BdaGhisallo]

Yep, I am hitched - going on five years now, with a little cyclist on the way!

Thanks for the kudos. I find the subject of bike geo to be fascinating one. I am always hungry for information on what the pros ride and what great frame designers have specced for big riders of the past. I couldn't give a turd what frame material they are riding on, but let me see their geos and I am in heaven.

Geoff

[atmo]

Yep, I am hitched - going on five years now, with a little cyclist on the way!

Thanks for the kudos. I find the subject of bike geo to be fascinating one. I am always hungry for information on what the pros ride and what great frame designers have specced for big riders of the past. I couldn't give a turd what frame material they are riding on, but let me see their geos and I am in heaven.

Geoff
their geos are dependent on that they are full time athletes. to
a degree, we shouldn't draw too much from them if we're not
racing and/or logging in 20,000 mi per year. it's all interesting
suff regardless atmo. in a sense, all that matters is 1) what is
your saddle height, where is it horizontally placed (behind the
central movement), and where are the h'bars in relationship
to it? once you know what works for you, and can express it
linearly, you'll never need to know a frame angle again.

[mike p]

"Yep, I am hitched - going on five years now, with a little cyclist on the way!"

DAMN!! Story of my life. How bout you ATMO, your a close second?

Mike

[atmo]

"Yep, I am hitched - going on five years now, with a little cyclist on the way!"

DAMN!! Story of my life. How bout you ATMO, your a close second?

Mike
i prefer rulers and millimeters to protractors and minutes atmo.

[BumpyintheBurgh]

Okay, SoCalSteve said to keep it simple...so we all now know what setback is, how to measure it and that Geoff is married with one in the oven and has a PhD in Bikology...my question is, why does setback measurement matter?

[atmo]

Okay, SoCalSteve said to keep it simple...so we all now know what setback is, how to measure it and that Geoff is married with one in the oven and has a PhD in Bikology...my question is, why does setback measurement matter?
because it is the one true reference point that you,
being the client, can effectively transfer (and or
alter if need be) when setting up a bicycle. sorry
if that's unintelligable; we just had a dinner party
and while the dishes are done, the sugar high lingers
atmo. here's the deal: if your saddle (nose) is 6cm
behind a central movement, that is the baseline
measurement, along with saddle height atmo.

[Avispa]

their geos are dependent on that they are full time athletes. to
a degree, we shouldn't draw too much from them if we're not
racing and/or logging in 20,000 mi per year. it's all interesting
suff regardless atmo. in a sense, all that matters is 1) what is
your saddle height, where is it horizontally placed (behind the
central movement), and where are the h'bars in relationship
to it? once you know what works for you, and can express it
linearly, you'll never need to know a frame angle again.

I totally agree with the atmo, and I will leave this subject as it is for now, with the exception of the following comments and next post, just to clarify things....

I must say that I am a bit disconcerted since this is the second time I have found small flaws on my Serottas! By small flaws, I mean, things where made different than specified.

Do frame makers have/should have a margin of error disclosure as component makers do? For instance, Campagnolo says:

*The nominal weight refers to the lighter specification among the available options. The weight of the hubs does not includes the quick-release. The nominal weight does not take account of the sometimes considerable quantities of grease used in the assembly of the products.

Should frame builders say:

Your bike frame or fork may be off X point X of a degree or centimeter due to the materials used, etc. etc.

I think that will make some of us happy.... Cheers bros! ;)

[Avispa]

Seat height is the same on both bikes? When you say "I am using identical components of both bikes..." Does this include the same saddle and same length of stem?

Bro, this is what I meant:

i·den·ti·cal /aɪˈdɛntɪkəl, ɪˈdɛn-/ Pronunciation[ahy-den-ti-kuhl, i-den-]
–adjective 1. similar or alike in every way: The two cars are identical except for their license plates.
2. being the very same; selfsame: This is the identical room we stayed in last year.
3. agreeing exactly: identical opinions.

The last picture is from Danilo Di Luca's bike... I figure, if he rides with his saddle a bit backwards, I guess my insignificant (cycling) arse can ride with the saddle a bit backwards... on my Legend! ;)

Check it out: