Bike Fit

idea one-the seat post secured at the saddle rail's mid point should
position the saddle so as your thighs will clear the seat when in a
seated climbing power push forward with the legs riding mode.
which is a bit further back with the knees than the drop a string/plumb
bob from the knees and have the bob touch the - is it the pedal shaft ?

Scott Morrison said:

How do I determine the appropriate top-tube length for a road bike?

There is no formula or exact method for determining top tube length.
Fortunately, changing the handlebar stem extension can compensate for a
less than ideal top tube length as long as you stay within reasonable
limits.

For a classic road frame with a level top tube, the top tube length
will be roughly proportional to the seat tube length. So a bike with a
60 cm seat tube will likely have a top tube of about 59 cm. For a given
"frame size" (i.e., seat tube length), you probably won't find more
than about 1 cm variation in top tube length from one model to another.
Choose a slightly longer top tube if your arms/torso are propotionally
longer than your legs, or if you like a stretched out position.

The handlebar stem extension is also usually proportional to frame
size. A 50 cm frame might have a 90 mm stem, and a 62 cm frame might
have a 130 mm stem extension.

When you get into "compact" frames with sloping top tubes, huge seat
posts, and S, M, L sizing, the whole concept of top tube length gets
really murky.

Bottom line: First determine how stretched out you want to be on the
bike. Then choose a top tube length that will get you in the ballpark.
Finally, fine tune the reach by choosing the right stem.

Test ride the bike before buying, and don't accept it until it feels
right.

Art Harris

Scott Morrison said:

How do I determine the appropriate top-tube length for a road bike?

The best way is to compare the bike to the one you currently ride. If
your current bike fits properly, you'll just need to compute the stem
you need to replicate the saddle to bar "cockpit length" once the
saddle is in the same relative position over the cranks.

To start from scratch with just body dimensions is a lot more
complicated, and beyond the realm of generalizations.

Quoted message said:

I
know that there are a variety of "rules of thumb" for chosing the
"correct" seat tube length.

The seat tube length is relatively unimportant. The only real "fit
parameter" it affects is standover clearance. That is, if you have
two bikes that put the bars, saddle and crank in the same relative
position, they "fit" the same, regardless of the seat tube length.

Also, beware of the fact that bikes are measured one of three ways -
center to top (of top tube), which is the logical one, since it tells
you where the point of human-bike contact will be. Center to center
is nice for the builder, but is slightly less informatic. And some
manufacturers/builders use center to top of seat tube, which never
made any sense to me at all since a bike of the same "size" can vary
by several cm's depending on the amount of seat tube extention the
designer chooses.

Mark Hickey
Habanero Cycles
http://www.habcycles.com
Home of the $795 ti frame

Mark Hickey said:

Scott Morrison said:

How do I determine the appropriate top-tube length for a road bike?

The best way is to compare the bike to the one you currently ride. If
your current bike fits properly, you'll just need to compute the stem
you need to replicate the saddle to bar "cockpit length" once the
saddle is in the same relative position over the cranks.

To start from scratch with just body dimensions is a lot more
complicated, and beyond the realm of generalizations.

Quoted message said:

I
know that there are a variety of "rules of thumb" for chosing the
"correct" seat tube length.

The seat tube length is relatively unimportant. The only real "fit
parameter" it affects is standover clearance. That is, if you have
two bikes that put the bars, saddle and crank in the same relative
position, they "fit" the same, regardless of the seat tube length.

Also, beware of the fact that bikes are measured one of three ways -
center to top (of top tube), which is the logical one, since it tells
you where the point of human-bike contact will be. Center to center
is nice for the builder, but is slightly less informatic. And some
manufacturers/builders use center to top of seat tube, which never
made any sense to me at all since a bike of the same "size" can vary
by several cm's depending on the amount of seat tube extention the
designer chooses.

Yeah, I got hit by that on my latest bike. I pay attention to
center-to-top so I know how much seatpost I'll have extended. I figure
the less seapost extension the less chance I'll break a seatpost.
Seatpost's are one bike part that I have broken on more than one
occasion. But on my Soma there is very little seat tube extending above
the top tube. The bike fits perfectly so I'm not that concerned, I
have plenty of top tube clearance but if I want to race 'cross I don't
have a lot of clearance for a foot forward dismount.

Greg

--
"All my time I spent in heaven
Revelries of dance and wine
Waking to the sound of laughter
Up I'd rise and kiss the sky" - The Mekons

Scott Morrison said:

How do I determine the appropriate top-tube length for a road bike? I
know that there are a variety of "rules of thumb" for chosing the
"correct" seat tube length.

Scott,

Lay your forearm along the top tube
so your elbow is backed up against
the seat post, and extend your fingers.
The distance between your longest
finger and the head tube should be
about 3-3.5 inches.

Quoted message said:

If you are a pro please indicate how where your info has come from. Year
of experience is a good source.

That's what I was told when I was
a young grommet, and it definitely
works out for me and for my particular
forearm.

Robert

To fit a bike properly you are really just locating your seat relative
to the bottom bracket and your bars relative to the seat... and there
are a wide range of options that can achieve that quite well. Other
dimensions like top tube length and seat angle are secondary... ie they
are usually important only if they *prevent* locating the bars and seat
in the right place.

There are several fit calculators online that take into account
detailed body measurements, and allow you to estimate the proper
dimensions... it is a good place to start.

I was thinking about the murky world of seat-tube angles, seat-tube
lengths, top-tube lengths and angles, etc... and it occurred to me that
there are two frame dimensions that seem most important for sizing a
frame.

The first is a "reach" dimension that can be universally applied...
unlike the top-tube length that varies with seat-tube angle, and
top-tube angle as well if the frame is "compact". I propose that a
plumb line be projected upward from the center of the bottom bracket,
and the horizontal distance from there to the top-center of the head
tube would be the "reach" of the frame. This a dimension that would
make it easy to compare one frame to another... provided of course,
that you knew what the value was!

The other is a "height" dimension that can be universally applied... as
in the height of the center of the steerer tube just above the headset
(relative to the BB), which directly effects the height of the bars
and/or what you will need to do in order to get the bars at the proper
height relative to the seat.

Now, you just need to determine the proper seat height and setback,
figure the reach and height that would work best for you and you'd be
able to select the equivalent size for any model of frame.

Ron Ruff said:

... and it occurred to me that there are two frame dimensions that
seem most important for sizing a frame.

The first is a "reach" dimension that can be universally applied...
unlike the top-tube length that varies with seat-tube angle, and
top-tube angle as well if the frame is "compact". I propose that a
plumb line be projected upward from the center of the bottom bracket,
and the horizontal distance from there to the top-center of the head
tube would be the "reach" of the frame. This a dimension that would
make it easy to compare one frame to another... provided of course,
that you knew what the value was!

The other is a "height" dimension that can be universally applied... as
in the height of the center of the steerer tube just above the headset
(relative to the BB), which directly effects the height of the bars
and/or what you will need to do in order to get the bars at the proper
height relative to the seat.

Now, you just need to determine the proper seat height and setback,
figure the reach and height that would work best for you and you'd be
able to select the equivalent size for any model of frame.

Great minds think alike! Dan Empfield has come to exactly the same
conslusion, only you wrote it in far far fewer words. He calls these
two frame dimensions "stack" and "reach". See his thoughts at:
http://www.slowtwitch.com/mainheadings/techctr/stack.html

Quoted message said:

Great minds think alike! Dan Empfield has come to exactly the same
conslusion, only you wrote it in far far fewer words.

But he was much more eloquent...

I thought of this when I was researching my position, and noticed that
it was common for road bikes to have slacker seat tubes as you went up
to bigger sizes... but my seat belongs in a particular spot... I'm not
going to vary it just because the seat tube angle changed. After I did
some calculations, I realized that sometimes the *smaller* frame would
have the longer "reach"... in other words they increased the top tube
length on the bigger size, but not as much as slackening the seat angle
increased it!

Another thing... why do road bikes slacken the seat angles and steepen
the head angles as you move up in size, but MTBs are all the same
(usually 73/71)? OK... I can understand some reason for the seat tube
angle changing since KOPS is normally used, and crank length varies
little. But why should a 50cm frame have a 72 deg head angle, while a
60cm frame is 74? That is a fairly big difference in handling. I
realize that toe overlap is a problem on smaller frames, but wouldn't
it make more sense to use a longer TT and a short stem to solve that?

If it was up to me I think I'd cover the range of sizes with maybe 4
evenly spaced "reach" dimensions, and 2 "height" (head tube) choices
for each one. Seat tube angles can vary as appropriate, but I think I'd
leave the head angle the same.

In article
<[email hidden]>,

Ron Ruff said:

Quoted message said:

Great minds think alike! Dan Empfield has come to exactly the same
conslusion, only you wrote it in far far fewer words.

But he was much more eloquent...

I thought of this when I was researching my position, and noticed that
it was common for road bikes to have slacker seat tubes as you went up
to bigger sizes... but my seat belongs in a particular spot... I'm not
going to vary it just because the seat tube angle changed. After I did
some calculations, I realized that sometimes the *smaller* frame would
have the longer "reach"... in other words they increased the top tube
length on the bigger size, but not as much as slackening the seat angle
increased it!

Another thing... why do road bikes slacken the seat angles and steepen
the head angles as you move up in size, but MTBs are all the same
(usually 73/71)?

Because the wheels do not get larger for larger frames. As
the top tube and seat tube get longer to accommodate a
tall rider the center of mass moves forward relative to
the wheel base axis; and this is a bad thing: too much
weight over the front wheel. Moving the seat tube angle
away from the vertical shifts the center of mass back
again. Also chain stay length increases for larger frames.

I do not know why head tube angles are more vertical for
larger frames. Maybe because they can. A longer top tube
allows for more toe-wheel clearance, so they can build a
quicker handling turning geometry. Increasing the head
tube angle also increases the cockpit length.

Quoted message said:

OK... I can understand some reason for the seat tube
angle changing since KOPS is normally used, and crank length varies
little. But why should a 50cm frame have a 72 deg head angle, while a
60cm frame is 74? That is a fairly big difference in handling. I
realize that toe overlap is a problem on smaller frames, but wouldn't
it make more sense to use a longer TT and a short stem to solve that?

If it was up to me I think I'd cover the range of sizes with maybe 4
evenly spaced "reach" dimensions, and 2 "height" (head tube) choices
for each one. Seat tube angles can vary as appropriate, but I think I'd
leave the head angle the same.

--
Michael Press

Michael Press said:

Because the wheels do not get larger for larger frames. As
the top tube and seat tube get longer to accommodate a
tall rider the center of mass moves forward relative to
the wheel base axis; and this is a bad thing: too much
weight over the front wheel.

Then it would certainly make sense for MTBs to do it... they are much
more susceptible to too-forward weight distribution (technical
descents).

Ideally on a road bike you'd like to have a 50/50 weight distibution...
and I don't think any exceed that on the front except for some TT
positions.

Quoted message said:

Also chain stay length increases for larger frames.

Only because they need to for seat tube clearance, since the seat angle
is slacker.

Quoted message said:

A longer top tube
allows for more toe-wheel clearance, so they can build a
quicker handling turning geometry.

True... but if a steeper angle is good on the larger frames, wouldn't
it also be good on the small ones?

Quoted message said:

Increasing the head
tube angle also increases the cockpit length.

How so? Assuming that other dimensions are the same, that doesn't
follow.

In article
<[email hidden]>,

Ron Ruff said:

Michael Press said:

Because the wheels do not get larger for larger frames. As
the top tube and seat tube get longer to accommodate a
tall rider the center of mass moves forward relative to
the wheel base axis; and this is a bad thing: too much
weight over the front wheel.

Then it would certainly make sense for MTBs to do it... they are much
more susceptible to too-forward weight distribution (technical
descents).

You are the expert on mountain bicycles. Perhaps the
manufacturers just don't care.

Quoted message said:

Ideally on a road bike you'd like to have a 50/50 weight distibution...
and I don't think any exceed that on the front except for some TT
positions.

Quoted message said:

Also chain stay length increases for larger frames.

Only because they need to for seat tube clearance, since the seat angle
is slacker.

Quoted message said:

A longer top tube
allows for more toe-wheel clearance, so they can build a
quicker handling turning geometry.

True... but if a steeper angle is good on the larger frames, wouldn't
it also be good on the small ones?

Toe-wheel clearance.

Quoted message said:

Quoted message said:

Increasing the head
tube angle also increases the cockpit length.

How so? Assuming that other dimensions are the same, that doesn't
follow.

Moving the head tube angle toward the vertical means the
steering tube moves forward.

--
Michael Press

`It was a very ordinary filling station,
in front of a house with a bicycle shop below.'
-- Georges Simenon, L'Auberge aux Noyés.

Michael Press said:

Quoted message said:

True... but if a steeper angle is good on the larger frames, wouldn't
it also be good on the small ones?

Toe-wheel clearance.

That can be achieved by making the TT (reach) longer, and fitting a
shorter stem. Shorter cranks and/or smaller would probably be a better
solution for shorter people.

Quoted message said:

Moving the head tube angle toward the vertical means the
steering tube moves forward.

Unless you are changing the TT length, the bottom of the steerer will
move back. It sounds like you are fixing the front wheel position and
pivoting everything around that.

In article
<[email hidden]>,

Ron Ruff said:

Michael Press said:

Quoted message said:

True... but if a steeper angle is good on the larger frames, wouldn't
it also be good on the small ones?

Toe-wheel clearance.

That can be achieved by making the TT (reach) longer, and fitting a
shorter stem. Shorter cranks and/or smaller would probably be a better
solution for shorter people.

Quoted message said:

Moving the head tube angle toward the vertical means the
steering tube moves forward.

Unless you are changing the TT length, the bottom of the steerer will
move back. It sounds like you are fixing the front wheel position and
pivoting everything around that.

OK, you are the expert. Explain the geometry changes for
larger racing frames.

--
Michael Press

`It was a very ordinary filling station,
in front of a house with a bicycle shop below.'
-- Georges Simenon, L'Auberge aux Noyés.

Michael Press said:

In article
<[email hidden]>,
"Ron Ruff" <[email hidden]> wrote:

Quoted message said:

Quoted message said:

Another thing... why do road bikes slacken the seat angles and steepen
the head angles as you move up in size, but MTBs are all the same
(usually 73/71)?

Because the wheels do not get larger for larger frames. As
the top tube and seat tube get longer to accommodate a
tall rider the center of mass moves forward relative to
the wheel base axis; and this is a bad thing: too much
weight over the front wheel. Moving the seat tube angle
away from the vertical shifts the center of mass back
again.

I think the reason is that the effective seat tube angle is actually
that between the end of the seat post and the pedal axle at the "3
o'clock" position. Since crank lengths vary by only 10% but seat tubes
vary by 30+%, the effective seat tube angle gets steeper in large
frames, so the actual seat tube angle is made shallower to compensate.

Quoted message said:

Also chain stay length increases for larger frames.

Unfortunately, it usually doesn't. It should, to match the greater
front-center distance created by longer top tubes. Wheelbase should be
increased proportionally front and back. Failure to do this makes a good
wheelie bike, and a lot of tall frames fall into this category.

Quoted message said:

I do not know why head tube angles are more vertical for
larger frames. Maybe because they can. A longer top tube
allows for more toe-wheel clearance, so they can build a
quicker handling turning geometry.

Small frames have toe overlap problems. Slacking the head angle helps,
it also allows you to put more offset in the fork, which also helps.

Quoted message said:

Increasing the head
tube angle also increases the cockpit length.

By the tiniest amount.

Art Harris wrote:
---snip---

Quoted message said:

When you get into "compact" frames with sloping top tubes, huge seat
posts, and S, M, L sizing, the whole concept of top tube length gets
really murky.

Not necessarily...Calculate your 'traditional' size, then look to find
out what the 'effective TT length' is on the compact bike you're
considering. This is the length the TT would have were the bike a
'traditional' frame. Thus, as a starting point, this should more or
less correspond with the 'rule of thumb' size for a given rider.
Granted, with so many manufacturers switching to compact sizing to
avoid a full run of sizes (S,M,L), finding the right size *may* be more
of a challenge

---snip---

Quoted message said:

Test ride the bike before buying, and don't accept it until it feels
right.

--/snip---

Excellent advice.

As the resident goofy morphology torch holder for compact bikes (short
legs, long torso), I would be remiss if I didn't urge you to keep them
in mind if you cannot find a traditional frame that fits you
comfortably (as long as you can stomach the aesthetics).

SYJ