Frame Fit: Sit-Bones, KOPS, Saddle Setback, Stem Length

[Petecresswell]

The dilemma:

- I'm what somebody referred to as a "statistical outlier":
6'5" tall, 37" inseam, size 15 feet....and so-forth

- I've got two bikes that I consider TB dialed in and
almost-dialed-in.

- Both of those bikes have seemingly-ludricous saddle
setbacks. If you drop plumb lines through the centers of
the saddle clips, they're
12.75 and 13.5 inches behind the center of the BB spindle
respectively.

- Both bikes work in that the dents made by my sit bones on
the saddles are in the right place on one (the 13.5"
setback...) and pretty close on the other.

- I'm in the process of getting a custom frame to replace
the 12.75" one - on which I achieve that setback via a 2"
setback seatpost....

The framebuilder, however, is at odds with my numbers. He
says 10" is more like it considering my body dimensions.
He definately knows what he's talking about and I
definately don't - except that I *do* know where my butt
bones wind up....

Two things that occur to me:

- Apples and Oranges: My measurements and the builders are
taken/computed differently. Low probability, IMHO.

- The builder is moving my body forward by extending the
handlebar stem.

My problem is that it seems to me like cockpit length (and
that's what it seems to me like the stem's length affects..)
is unrelated to where my butt winds up on the saddle.

Seems to me like if I'm spinning along, upshift a few gears,
increase my effort until my butt is just starting to rise
off the saddle..that the fore-aft position of my ichial
tuberosities is determined by other factors - like my foot
length, my femur length, and so-forth.

Maybe I have to go out and buy a 4" stem and try it out, but
it seems like that's just going to make me more aero and
that my sit bones are going to find their same old place.

Agreements? Disagreements? Reasons why?

 

[santa]

Hi,

I'm similarly built, and I'm also in the market for a custom frame. I wonder what builders you are considering, and I would love to compare notes. Can you Private Message me to discuss it? I think you may have to join to do that.

Thanks.

 

[Petecresswell]

> I'm similarly built, and I'm also in the market for a
> custom frame. I wonder what builders you are considering,
> and I would love to compare notes. Can you Private Message
> me to discuss it? I think you may have to join to do that.

The bike that works is a hardtail from Curtlo.

I spec'd the saddle setback by quoting the setback I
achieved on the FS by adding a 2" setback post. Curtlo then
suggested moving it back a little further - since I had the
saddle all the way back - and that's how I wound up with
13.5" - which, to me, feels dialed in.

I'd recommend Curtlo for a hardtail. A couple of people who
know looked at the frame and volunteered that he did a nice
job with it.

I'm working late tonite - and don't want to splash the
current builder's name around until this thing is settled
one way or another.

Flip me an email to "PC".

The domain is Fatbelly.

 

[Bruce Dickson]

[email protected] (PeteCresswell) wrote in message news:<[email protected]>...
> The dilemma:
>
> - I'm what somebody referred to as a "statistical
> outlier": 6'5" tall, 37" inseam, size 15 feet....and
> so-forth
>

You might want to talk to Lennard Zinn(www.zinncycles.com).
He's tall and is experienced in making custom bikes for tall
riders. I've never dealt with him myself though.

Bruce Dickson

 

[Jeff Wills]

[email protected] (PeteCresswell) wrote in message news:<[email protected]>...
> The dilemma:
>
> - I'm what somebody referred to as a "statistical
> outlier": 6'5" tall, 37" inseam, size 15 feet....and
> so-forth
>

I'm on the same side of the bell curve: 6-foot-4, 36"
inseam, size 13 feet...

<snip>
> The framebuilder, however, is at odds with my numbers. He
> says 10" is more like it considering my body dimensions.
> He definately knows what he's talking about and I
> definately don't - except that I *do* know where my butt
> bones wind up....
>
<snip>
> Agreements? Disagreements? Reasons why?

I just measured my good-fitting 'cross bike, and it's got
about 10" horizontally from the center of the seat rails to
the spindle. It's a stock frame. I suspect you're both
right: "standard" dimensions would dictate 10" of setback,
while *your* particular preference (weight distribution, hip
flexibility, cadence, whatever) allows you to move your seat
further back than "normal". How's that for wishy-washy?

Jeff

 

[G.T.]

PeteCresswell wrote:
> The dilemma:
>
> - I'm what somebody referred to as a "statistical
> outlier": 6'5" tall, 37" inseam, size 15 feet....and
> so-forth
>
> - I've got two bikes that I consider TB dialed in and
> almost-dialed-in.
>
> - Both of those bikes have seemingly-ludricous saddle
> setbacks. If you drop plumb lines through the centers of
> the saddle clips, they're
> 12.75 and 13.5 inches behind the center of the BB spindle
> respectively.
>

I don't know. I started ignoring KOPS and setback on my mtn
bike 10 years ago. The only things I keep track of these
days are nose-of-saddle-to-handlebar distance, rise of saddle-above-
handlebar, and saddle-rail-to-spindle distance.

Greg

 

[Matt O'Toole]

PeteCresswell wrote:

> The bike that works is a hardtail from Curtlo.

IIRC, Doug Curtis is tall -- 6'4"? So he probably
understands tall rider issues.

Matt O.

 

[x]

RE/
>You might want to talk to Lennard Zinn(www.zinncycles.com).
>He's tall

I gave him a call and he was gracious enough to spend some
time talking with me.

A new issue that he surfaced: crank length. By his reckoning
somebody our size needs 207mm cranks.... which he says would
account for an inch of setback.

OTOH, what I've heard so far is that 10 inches is right on
the money for 6'4"/6'5"... So my probem remains: how to
explain my own 13.5"?

I just set my Isis up to the builder's proposed spec by
replacing the seatpost and rotating the risers forward.
I'll get a copy of the builder's proposed spec tomorrow and
make sure all the other dimensions agree. Feels kind of
weird right now, but maybe I'll warm up to it in a hunded
miles or so.
--
PeteCresswell

 

[Rick Onanian]

On 19 Apr 2004 12:34:06 -0700, [email protected]
(PeteCresswell) wrote:
>- I've got two bikes that I consider TB dialed in and
> almost-dialed-in.
...
>The framebuilder, however, is at odds with my numbers.
>He says 10"
...
>Agreements? Disagreements? Reasons why?

You're comfortable and happy with your existing geometry. Do
not let anybody tell you that it's wrong. You know your
body, riding style, quirks, and comfort; you've (presumably)
put in hundreds or even thousands of miles with your current
setup. You are correct by definition.

While there may be other geometries possible that you could
find comfortable, why open that can of worms? Tell him
you're very comfortable with your existing measurements,
even though you know they are "incorrect".
--
Rick Onanian

 

[x]

RE/
>You're comfortable and happy with your existing geometry.
>Do not let anybody tell you that it's wrong. You know your
>body, riding style, quirks, and comfort; you've
>(presumably) put in hundreds or even thousands of miles
>with your current setup. You are correct by definition.

Thousands, but I don't thing I'm talking "geometry" here -
just setback. The rest is way, way over my head - too many
interdependent factors.

However I know what I've done to the ISIS I'm riding has
pretty much defeated the designer's intent handling-wise.
But not being a very gonzo rider, I'm satisified with the
tradeoffs.
>
>While there may be other geometries possible that you could
>find comfortable, why open that can of worms? Tell him
>you're very comfortable with your existing measurements,
>even though you know they are "incorrect".

Before I do that, I want to make sure that my setback
obsession isn't something that I've created out of my own
ignorance.

"Comfortable" may reflect more on my ignorance, lack of
experience, and lack of appreciation for better things than
on proper fit.

"May" is the operative word there.

Currently the biggest unknown to me seems to be bar
placement. For a given cockpit length, I can envision the
rider's whole posture pivoting backwards/forwards as the
cockpit is moved backwards/forwards by simultaneously
adjusting stem length and saddle setback the same amount in
the same direction. This would appear to vary the KOPS
relationship via the tilt while the cockpit length remains
constant. Extending that image a little further, one might
say that once the cockpit length is right, one's butt will
always wind up on the right part of the saddle and whatever
approximation of KOPS is desired can be achieved by moving
the cockpit fore or aft.

OTOH, in my experience, there seems to be something about
where my butt winds up that's independent of both bar and
saddle placement. The harder I pedal, the less weight is on
my butt and arms until I'm pretty much 100% on my feet -
even though my butt may be grazing the saddle and an
observer would say that I'm still seated on the saddle. At
that point, it seems theoretically possible that I could be
riding with my fingertips just touching the bars enough to
keep me from tipping over backwards or forewards and with no
saddle underneath me. In that state, I think my butt is
where the saddle should be so that when I back off on the
pedaling effort, I come down on a place that's intended to
be sat upon.

With what I currently think is the maker's spec under me, I
start out with my butt on that magic spot, but as I "just
ride" and forget body position my butt seems to find a spot
an inch or two further back - right on the saddle's rear
crossmember in this case.

Just got a hardcopy of the maker's spec today and after I
get a critical measurment to supplement it (saddle-to-where-I-grip-the-
bars) I'll have my ISIS set up bar/cockpit/setback/KOPS-wise
precisely according to his spec.

Then we'll *really* find out about my self-positioning butt
theory....
--
PeteCresswell

 

[G.T.]

(Pete Cresswell) wrote:
> RE/

>
> Currently the biggest unknown to me seems to be bar
> placement. For a given cockpit length, I can envision the
> rider's whole posture pivoting backwards/forwards as the
> cockpit is moved backwards/forwards by simultaneously
> adjusting stem length and saddle setback the same amount
> in the same direction. This would appear to vary the KOPS
> relationship via the tilt while the cockpit length remains
> constant. Extending that image a little further, one might
> say that once the cockpit length is right, one's butt will
> always wind up on the right part of the saddle and
> whatever approximation of KOPS is desired can be achieved
> by moving the cockpit fore or aft.

That's the theory I subscribe to since going KOPS-free. I
might worry about KOPS a little more on a road bike but on
my two mtn bikes which have entirely different KOPS
measurements I have no problem finding my comfort zone on
the saddle when sitting back down.

Greg

 

[B.C. Cletta]

[email protected] (PeteCresswell) wrote in message news:<[email protected]>...
> The dilemma:
>
> - I'm what somebody referred to as a "statistical
> outlier": 6'5" tall, 37" inseam, size 15 feet....and
> so-forth
>
> - I've got two bikes that I consider TB dialed in and
> almost-dialed-in.
>
> - Both of those bikes have seemingly-ludricous saddle
> setbacks. If you drop plumb lines through the centers of
> the saddle clips, they're
> 12.75 and 13.5 inches behind the center of the BB spindle
> respectively.
>
> - Both bikes work in that the dents made by my sit bones
> on the saddles are in the right place on one (the 13.5"
> setback...) and pretty close on the other.
>
> - I'm in the process of getting a custom frame to replace
> the 12.75" one - on which I achieve that setback via a
> 2" setback seatpost....
>
>
> The framebuilder, however, is at odds with my numbers. He
> says 10" is more like it considering my body dimensions.
> He definately knows what he's talking about and I
> definately don't - except that I *do* know where my butt
> bones wind up....
>
> Two things that occur to me:
>
> - Apples and Oranges: My measurements and the builders are
> taken/computed differently. Low probability, IMHO.
>
> - The builder is moving my body forward by extending the
> handlebar stem.
>
>
> My problem is that it seems to me like cockpit length
> (and that's what it seems to me like the stem's length
> affects..) is unrelated to where my butt winds up on
> the saddle.
>
> Seems to me like if I'm spinning along, upshift a few
> gears, increase my effort until my butt is just starting
> to rise off the saddle..that the fore-aft position of my
> ichial tuberosities is determined by other factors - like
> my foot length, my femur length, and so-forth.
>
> Maybe I have to go out and buy a 4" stem and try it
> out, but it seems like that's just going to make me
> more aero and that my sit bones are going to find their
> same old place.
>
> Agreements? Disagreements? Reasons why?

did this guy see you ride or is he looking at a
spreadsheet?

 

[Rick Onanian]

I wrote:
>>You're comfortable and happy with your existing geometry.
>>Do not let anybody tell you that it's wrong. You know your
>>body, riding style, quirks, and comfort; you've
>>(presumably) put in hundreds or even thousands of miles
>>with your current setup. You are correct by definition.

On Thu, 22 Apr 2004 00:19:17 GMT, "(Pete Cresswell)"
<[email protected]> wrote:
>Thousands, but I don't thing I'm talking "geometry" here -
>just setback. The rest is way, way over my head - too many
>interdependent factors.

Setback is certainly a component of geometry. The rest is
not way over your head unless you are looking for a change;
otherwise, you'll want the new frame to match the old one.

>However I know what I've done to the ISIS I'm riding has
>pretty much defeated the designer's intent handling-wise.
>But not being a very gonzo rider, I'm satisified with the
>tradeoffs.

If you'd like to improve handling, I think (somebody else
will know better than me) that you would want to keep all
the body-bike interface points the same, and that whole
set of points the same in relation to the ground; the
wheels' positions and angles of tubes will change to help
handling while leaving your connections to the bike all
laid out the same.

>>While there may be other geometries possible that you
>>could find comfortable, why open that can of worms? Tell
>>him you're very comfortable with your existing
>>measurements, even though you know they are "incorrect".
>
>Before I do that, I want to make sure that my setback
>obsession isn't something that I've created out of my own
>ignorance.
>
>"Comfortable" may reflect more on my ignorance, lack of
>experience, and lack of appreciation for better things than
>on proper fit.
>
>"May" is the operative word there.

I suppose that's possible...maybe some dialog here will help
you determine if you're as comfortable as you ought to be.
When you're riding, enjoying the scenery, do you forget that
the bike is under you? Do you not notice the bike?

Those are sure-fire ways to tell that you're quite
comfortable.

I, personally, have not quite achieved those as well
as I'd like.

>Currently the biggest unknown to me seems to be bar
>placement. For a given cockpit length, I can envision the
>rider's whole posture pivoting backwards/forwards as the
>cockpit is moved backwards/forwards by simultaneously
>adjusting stem length and saddle setback the same amount in
>the same direction.

This would result in different weight distribution, which
would strongly affect comfort. As you tilt the whole
package forward, weight shifts to your arms; and obviously,
as you tilt back, weight shifts to your butt. There's a
sweet spot for each person where weight distribution is
optimally split.

>that once the cockpit length is right, one's butt will
>always wind up on the right part of the saddle and whatever
>approximation of KOPS is desired can be achieved by moving
>the cockpit fore or aft.

I'll buy that part, but I'm not sure if KOPS is necessarily
a requirement; especially if you've got experience with
something that is comfortable.

>With what I currently think is the maker's spec under me, I
>start out with my

In general, manufacturers' OE specs means _nothing_. It's a
guess at what most people will fit with the least
modification.
--
Rick Onanian

 

[Petecresswell]

> did this guy see you ride or is he looking at a
> spreadsheet?

Never seen me ride.

I'm pretty sure he's looking at a CAD screen that has the
proposed frame and a stick figure representing my body
measurements.

 

[David Mackintos]

"Matt O'Toole" <[email protected]> wrote in message news:<[email protected]>...
> PeteCresswell wrote:
>
> > The bike that works is a hardtail from Curtlo.
>
> IIRC, Doug Curtis is tall -- 6'4"? So he probably
> understands tall rider issues.
>
> Matt O.

I had Curtlo build me a True Temper S3 road frame (with a
1-3/8" OX Platinum down tube used for a top tube), which
ended up working out great. I would definetely use him
again, best deal in the full-custom business. I'm 6'3" with
an over 38" inseam, so the bike has a long head tube
(240mm), and relatively short sloping top tube (59cm
virtual). My biggest concern was stability for high-speed
descents, and this was achieved with the stiff front end,
73 degree head angle, and relatively long wheelbase (73.5
deg seat tube, 42cm chainstays). I set the road frame up
with KOPS at 8cm setback to the saddle nose (using a
straight Thomson post with clamp near rail center) using
180mm cranks.

As far as mountain bike frames for tall riders go, I would
again recommend keeping the wheelbase relatively long, and
stay conservative with the head angle (fairly slack). This
will keep it stable at speed, and minimize the tendency to
loft the front wheel while climbing. I don't believe most
stock geometry is good for taller riders.

-David

 

[Petecresswell]

> I just set my Isis up to the builder's proposed spec by
> replacing the seatpost and rotating the risers forward.
> I'll get a copy of the builder's proposed spec tomorrow
> and make sure all the other dimensions agree. Feels kind
> of > weird right now, but maybe I'll warm up to it in a
> hunded miles or so.

Been riding it that way since the last post. It's really
obvious now that I had been defeating the designer's intent
with my preferred setup. Doesn't feel nearly as weird now
that my lower brain stem has learned to deal with the diffs.

That's not to say one is all 'good' and the other is all
'bad'... but the the light front wheel is history and
consequently the bike climbs a *lot* better. It also has a
more 'balanced' feel when going over irregular
surfaces/doing sudden out-of-the saddle climbs and so-forth.
There's no diff between my balance on the saddle and off.

I think that what I had heard (or imagined I'd heard)
about reach not relating to butt placement is untrue.
Moving the bars a couple inches forward definately got my
butt far enough forward so I don't need nearly as much
saddle setback.

OTOH, I feel more likely to endo going down steep (for me,
at least...) descents - one's arms are only so long so the
further forward the bars, the less weight you can get behind
the saddle. Also I feel more likely to do a Christopher
Reeves if something goes wrong up front.

Bottom line, though, I think I'm going for the
builder's spec.

My theory is that he has a suite of designs that he knows
work and he picks the closest one and then fits the customer
to it by tweaking seatpost and stem.

I've been through a couple of custom windsurfers and what I
found there is that if it really is a one-off custom design
you're getting what's effectively the first prototype of
something - whereas with a production board you're getting a
more refined end product of many prototypes.

This way, I can ride the builder's spec setup and know I'm
getting the most out of the design - but if I'm still not
totally comfortable after awhile I'll just tweak it until I
am - accepting whatever performance/handling tradeoffs
there are.