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

I put this up on rec.bicycles.tech yesterday, but in retrospect it seems more
MTB-specific because of the different style of riding.
---------------------------------------------------------

The dilemma: A totally-competant framebuilder, using a CAD program, has come up
with one saddle setback distance; I, using the dents made by my butt, have come
up with another; and there's no way in hell I'm going to drop the kind of money
a custom FS frame costs without thoroughly resolving that difference.

"Resolve" could include making me understand where I've gone wrong.

To wit:

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

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

- Both of those bikes have seemingly-ludricous saddle setbacks: huge, and
humongous.

If you drop a plumb line through the center of each saddle clip, they're
11.25 (the FS) and 13.25 (the HT) inches behind the center of the BB spindle.

On the hardtail, it's actually even more ridiculous in that the ThudBuster post
I'm using gives about 7/8" more setback when compressed by the rider's weight.

- Both bikes work in that my butt bones are no longer riding on the rear rail of
the saddle. On the FS, they're sort of up against the rear rail, and on the
HT, they're not touching it any more. Same saddle, adjusted 100% back both
cases.

- The FS with a straight post (9.75" setback) did not work for the first
1,000-or-so miles. During that time I went through a series of
seemingly-too-hard saddles until it finally dawned on me that I was sitting on
the rear rivets of the saddle. Then I went to a setback post and things got
better with the setback increased to 11.25".

- I'm in the process of getting a custom frame to replace the FS
and, I thought, make my FS more like the hardtail that seems to fit so well.

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.


Some things that occur to me:

- Apples and Oranges: My measurements and the builders are taken/computed
differently. Previous measurements by Yours Truly have already been revised by
a quarter inch in one case.

- With my preferred setup, I'm 'way behind KOPS - as much as 2-3"...but
that's a position my body found all by itself.

- Stem length is a player here and my previous problems - that drove me to move
the saddle backwards - were really a reflection of handlebars not being far
enough forward.

- My size 15 feet aren't in the builder's equation.

- There's something missing from the builder's algorithm - like ratio of thigh
to shin length.



I hope to test the builder's proposal by setting up my current FS to match his
setback/cockpit length/handlebar height and then riding it a hundred or so
miles.

My problem is that I need to resolve the (very large?) discrepancy between the
framebuilder's number (10") with my number (13.25").

Again, "Resolve" could include making me understand where I've gone wrong.
--
PeteCresswell

"(Pete Cresswell)" wrote in message
...
I put this up on rec.bicycles.tech yesterday, but in retrospect it seems
more
MTB-specific because of the different style of riding.
---------------------------------------------------------

The dilemma: A totally-competant framebuilder, using a CAD program, has
come up
with one saddle setback distance; I, using the dents made by my butt, have
come
up with another; and there's no way in hell I'm going to drop the kind of
money
a custom FS frame costs without thoroughly resolving that difference.

"Resolve" could include making me understand where I've gone wrong.
snip
Again, "Resolve" could include making me understand where I've gone wrong.
--
PeteCresswell

Pete,

I'd get a builder who did a better job and didn't leave me more confused
than when I started.

That's probably harsh because bike fit can be really tough. That said, why
is the guy not working for you to resolve the difference instead of the
other way around? Why isn't he explaining to you how he arrived at his
numbers and whether or not your size 15 shoe has been taken into
consideration?

My suggestion is to contact another frame builder and get a second opinion.
What you are saying, if I can condense it down to the lowest common
denominator, is that you are not comfortable with the product this builder
has provided you. Leave it at that and educate yourself further by
discussing it with others in the profession.

When you find the right situation you'll know and be happy with it. I've
heard good things about Strong Frames and Zinn apparantly specializes in
frames for tall people so he should be familiar with the issues you have.
Talk to them both and tell one of them no.

Good Luck.

In article ,
(Pete Cresswell) wrote:
I put this up on rec.bicycles.tech yesterday, but in retrospect it seems more
MTB-specific because of the different style of riding.
---------------------------------------------------------

The dilemma: A totally-competant framebuilder, using a CAD program, has come up
with one saddle setback distance; I, using the dents made by my butt, have come
up with another; and there's no way in hell I'm going to drop the kind of money
a custom FS frame costs without thoroughly resolving that difference.

"Resolve" could include making me understand where I've gone wrong.

I think that personal preference plus being a "statistical outlier"
plus mtb vs road riding could account for the difference.

I like having my saddle slightly farther back for mtb riding, since there's
more low-rpm seated climbing and less high rpm spinning than road riding.

CAD programs or "fit kit" are no more (and no less) accurate than
anything else; the data they work off is the same body of experience
that an experienced "by eye" fitter would use.

Measuring saddle position with different saddles is tricky, since the
place where you sit is not always X% back from the nose or Y% on front of
the tail on one saddle vs another. I try to estimate where the sitbones
will actually reside for 'normal' riding, and then measure from there.

You should never adjust your saddle position to compensate for
stem length. But having found your position, you probably will want
to stick close to it. Trying out the builder's position on your current bike
is a very good idea. He might be right after all. I'd suggest trying it
with a longer stem to keep the reach the same.

Eric

(Pete Cresswell) wrote:
I put this up on rec.bicycles.tech yesterday, but in retrospect it
seems more MTB-specific because of the different style of riding.
---------------------------------------------------------

I went through some of your problematics recently when I got me a new FS. We
thought we had frame and cockpit all dialed in but I wasn't comfortable and
I didn't feel balanced. I felt like I was falling forward onto the
handlebars all the time.
I got myself a setback seatpost from Thomson and a new stem that was 15 mm
shorter, the same as the setback on the seatpost. This made a whole world of
difference. All of a sudden my butt landed right on the saddle and I still
had the same distance to the handlebars. My balance felt much better and my
wrists weren't sore anymore.
It actually felt so good that I started adjusting my roadbike towards the
same geometry with disastrous results. The roadbike is back where I had it
and I don't have the same geometry on them at all. But, I feel comfortable
and balanced on both.
Conclusion: I tried to mentally imagine the triangle that is in contact with
the bike. feet hands and butt. If I move the two upper points back or
forward simultaneously I shift my balance until it is too far back, forward
or just right.

--
Perre

You have to be smarter than a robot to reply.

On Wed, 21 Apr 2004 00:55:19 +0000, (Pete Cresswell) wrote:

My problem is that I need to resolve the (very large?) discrepancy between the
framebuilder's number (10") with my number (13.25").

The one thing that seems immediately obvious, is that you do not seem to
differentiate between saddle setback and cockpit size. I'm assuming, as an
"outlier", you've been taking frames that are on the small side for you
and using the layback of the seat to give you extra cockpit room.

Your figures might be better if you compared headtube to saddle distance,
rather than BB to saddle distance.

A competent frame builder should be able to build in extra cockpit room by
fitting a longer TT, thus giving you a more 'ideal' lower body position,
but maintaining your current upper body posture.

--
a.m-b FAQ: http://www.j-harris.net/bike/ambfaq.htm

a.bmx FAQ: http://www.t-online.de/~jharris/bmx_faq.htm

CAD programs or "fit kit" are no more (and no less) accurate than
anything else; the data they work off is the same body of experience
that an experienced "by eye" fitter would use.

One thing that's occurred to me: if their CAD program includes a
stick figure of me on the bike (which I'm pretty sure it does...)
they're actually a little shy of having the info that the "by eye"
fitter has. Reason: that stick figure's dimensions may or may not be
accurate and, whatever algorithms they use to represent the
articulation of it's joints may or may not be accurate...

RE/
I got myself a setback seatpost from Thomson and a new stem that was 15 mm
shorter, the same as the setback on the seatpost. This made a whole world of
difference. All of a sudden my butt landed right on the saddle and I still
had the same distance to the handlebars.

I suspect that's where I've been with my FS.

Problem is that in doing that, I've defeated the design of the bike.
I can live with the handling, but Tony Ellsworth would probably be unhappy
if he thought anybody was going to judge his design based on the bike as I've
got it set up.

Seems to me like if my additional setback turns out to be valid, that the custom
frame should be designed in such a way that I'm properly balanced with the extra
setback - as opposed to the front wheel being really light. Otherwise, why
bother with a custom frame?


My modification of the Ellsworth weight distribution isn't a total crime against
nature though. Today I took my first ride on the bike set up more-or-less per
the custom maker's spec. Totally different ride. Definately properly
balanced. No more front end popping on the climbs. The downside was that it
took me a couple of dings on my knee to discover that the newly-weighted front
wheel tends to follow ruts more strongly and the newly-lightened rear wheel no
longer climbs obstacles at such an oblique angle as it used to. -)
--
PeteCresswell

RE/
The one thing that seems immediately obvious, is that you do not seem to
differentiate between saddle setback and cockpit size. I'm assuming, as an
"outlier", you've been taking frames that are on the small side for you
and using the layback of the seat to give you extra cockpit room.

Your figures might be better if you compared headtube to saddle distance,
rather than BB to saddle distance.


I've been concious of that - but maybe not 100% competant.

I tend to focus on the distance between either the center of the saddle clip
(more precise but harder to measure) or the back of the saddle (easier to
measure) and a point on top of the bars on one side about the middle of where I
grip them.

That way, the difference in cockpit length due to slight rotations of risers can
be accounted for.
--
PeteCresswell