Effect of seat angle on top tube requirement

[saab2000]

So I am looking at a bike.... My CIII has a seat angle of 73?. The top tube is 58 cm.

The bike in question has a seat angle of 72.5? and a top tube of 58 cm.

Will one have more 'reach' than the other, assuming everything else is equal?

[Climb01742]

is the seattube length the same? that would impact reach, too, yes?

[saab2000]

is the seattube length the same? that would impact reach, too, yes?

The seat tube is 1 cm shorter. And the head tube has about 1 cm more length than my CIII, above the level top tube.

I know it's not much, with just a 1/2 degree in question. But my CIII is about at the maximum of the reach I want (though part of that is in the bars and could be dealt with with a new set of shorter reach bars).

[swoop]

1 degree sort of equals 1 cm in either direction.so my 75 degree sta bike has a 53 tt while mt 74 is 54.

[saab2000]

1 degree sort of equals 1 cm in either direction.so my 75 degree sta bike has a 53 tt while mt 74 is 54.

So am I to understand that my reach to the bars would be .5 cm less on the bike with the 72.5? seat angle with the same bars and stem? I guess it would require the seat to be in the same place relative to the BB.

[swoop]

it depends where you put your saddle. so it may or may not bring a change in seat post setback.

[labratmatt]

So am I to understand that my reach to the bars would be .5 cm less on the bike with the 72.5? seat angle with the same bars and stem? I guess it would require the seat to be in the same place relative to the BB.

Yes, this is about right. Picture it in your head or draw it out on paper:

The thing that you want to keep the same is how far you are behind the bottom bracket. So, as you increase the seat tube angle, you're pushing the handle bars away from you the bottom bracket (and you're obviously pull them closer to you as you decrease the seat angle).

Matt

[djg]

So am I to understand that my reach to the bars would be .5 cm less on the bike with the 72.5? seat angle with the same bars and stem? I guess it would require the seat to be in the same place relative to the BB.

yeah, give or take, and that's ALL else equal (same saddle, same position relative to bb, same head tube angle, same stem and stem setup, same head tube, same bars, etc.) You could work it all out with some junior high trig or just go by the rule of thumb he gave ya and match your contact points with a tape measure -- I dunno how the new bike will ride, but if it's the same tt and just a half a degree change at the sta, you're starting pretty darn close on what some folks call "effective top tube" and can probably match the contact points pretty well just by thinking about the bar selection a bit.

[Fat Robert]

you have to add HT angle into the mix

steep hta effectively lengthens the reach

slack hta effectively shortens the reach

ex.

my bikes are 57x57 73/73, 130 stem. same saddle setback and reach were achieved on a bike that was 73.5/73.5 56tt with a 120 stem


what kind of bike is it?

why don't you jut buy that fuji cross pro in our size i can hook you up on?

[Dave]

Using the 1cm per degree rule is accurate enough for most comparisons, but the accuracy suffers a bit when you're comparing frames that are a different size. Any head tube angle difference can also be take into account, but the impact is much smaller, on the order of 1-2mm per degree.

Roughly, the new frame will have about 5mm less reach, but if the HTA is much different, then you might have to add or subtract from the 5mm value.

The 1cm per degree rule assumes that the rider wants the saddle in the same location relative to the BB.

The basis for the rule is simply the equation (cosA-cosB) times the c-c frame size. For example, with a 54cm c-c frame and angles of 73 and 72.5 degrees, you get (cos72.5-cos73) x 54 = .45cm. The c-c frame size is always assumes a horizontal TT, whether real or hypothetical for a sloping TT frame.

[BdaGhisallo]

Seat tube length doesn't really matter when thinking of reach in this sense. That's because it is implied that you will be putting your saddle in the same position on either frame. With a given seat tube angle, it doesn't matter if the distance along the seat tube angle is 90% frame and 10% seatpost or 70% frame and 30% seat post. You'll still have the saddle in the same spot.

And Fat Robert, how does hta affect reach, if you are thinking of reach the same as I am in this example? To me, reach is the distance between the vertical line drawn up through the bb and intersecting the top tube and the vertical line passing through the centerline of the head tube where the top tube intersects it. The hta will affect the front center, wheelbase and trail though.

In Saab's example, the frame with the 72.5 deg sta will have more of that 58cm top tube behind the vertical line drawn through the bb and less in front of it. Assuming a 58cm seat tube length, the 72.5 will have a setback of 17.44 cm and a reach of 40.56 cm. The 73 deg sta will have a setback of 16.95cm and a reach of 41.05 cm, so the difference in reach between the two will be 0.49cm. As said before, for these numbers to have any meaning, one must assume that Saab will put his saddle in the same spot in relation to the bb on each frame.

[chrisroph]

So am I to understand that my reach to the bars would be .5 cm less on the bike with the 72.5? seat angle with the same bars and stem? I guess it would require the seat to be in the same place relative to the BB.

yes

[Grant McLean]

To me, reach is the distance between the vertical line drawn up through the bb and intersecting the top tube and the vertical line passing through the centerline of the head tube where the top tube intersects it.

bingo.

Any difference between two frames in the head tube angle can be compensated
for by changing the height of the stem. The fore or aft change of handlebar
position from a degree or two of head angle is quite small, a couple of mm's
maybe.

-g

[Fat Robert]

[QUOTE=BdaGhisallo]

And Fat Robert, how does hta affect reach, if you are thinking of reach the same as I am in this example? To me, reach is the distance between the vertical line drawn up through the bb and intersecting the top tube and the vertical line passing through the centerline of the head tube where the top tube intersects it. The hta will affect the front center, wheelbase and trail though.

[QUOTE]


for me reach is nose of the saddle to the center of the stem clamp

i've been told by frame makers on the forum that HTA can change reach -- when you're keeping the same contact points and moving them from one frame to another. maybe maybe not

[BdaGhisallo]

Fat Robert,

I see. That makes sense because when I want to record my the reach my position has on a bike that's what I measure.

Check out the diagram I attached. That 157.3mm is the frame setback and the 382.6mm is the "reach" in terms of this discussion, or effective top tube as it is very often referred to.

I have seen this effective top tube length referred to as the "Z" distance. I saw it in an article in the old Bicycle Guide (a great mag that is no longer with us) that was written by Grant Peterson of B'stone and Rivendell fame. It went with the term "Q" which he also coined, which measures the distance between the faces of the cranks - basically tells you how far apart your feet will be.

It's easy to get confused when everyone uses different terminology.

Cheers Mr Fat,

Geoff

edit: I just noticed that the forum won't let me upload the diagram because I already attached it here:

http://forums.thepaceline.net/showthread.php?p=144729&highlight=geo+position+experts#post144729

[Dave]

From a fit standpoint, it doesn't matter whether you reference the center of the BB or the tip of the saddle to determine reach, as long as you don't change the brand/model of the saddle being used in the comparison.

The reason the HTA makes a small difference in reach is because the pivot point is the intersection of the HT and TT, but the stem is higher. The higher it is, the greater the effect. The top of my stem is about 10cm above this intersection point. The difference in reach is calculated using the same formula I posted earlier, but with 10mm substituted for the c-c frame size. 10mm is obviously a lot less than a typical frame size.

You can define reach as the TT length minus the frame setback, as shown on the diagram posted by BdaGhisallo, but there are limitations to this definition of reach. It's only accurate for one frame size. Larger frames will always have larger setbacks (with the same STA), because the setback is measured at a different point along the seat tube centerline (and vertically above the BB). If you compare the reach of two different sized frames, you won't get the correct answer for the predicted difference in stem length. To get the right answer, the setback and TT length would both need to be measured at the same vertical height above the BB. Then you're comparing apples to apples.

[Grant McLean]

You can define reach as the TT length minus the frame setback, as shown on the diagram posted by BdaGhisallo, but there are limitations to this definition of reach. It's only accurate for one frame size. Larger frames will always have larger setbacks (with the same STA), because the setback is measured at a different point along the seat tube centerline (and vertically above the BB). If you compare the reach of two different sized frames, you won't get the correct answer for the predicted difference in stem length. To get the right answer, the setback and TT length would both need to be measured at the same vertical height above the BB. Then you're comparing apples to apples.

People should really forget about TT length altogether. It just so happens
that most frames have a tube that runs from the seatpost to the stem, but
given that most frames are now sloping, TT length measurement should
really just go away because you also need to know where the saddle and bars
are relative to the bb.

Cervelo has a nice little definition called "Stack". With stack and reach,
you can find the location of the intersection of the headtube and top tube
relative to the BB. This is the most basic measurement that allows you
to compare any two frames, without bias of bb height, seatback, or other
geometry or tubing differences that will mislead the numbers.

-g

[Dave]

People should really forget about TT length altogether. It just so happens
that most frames have a tube that runs from the seatpost to the stem, but
given that most frames are now sloping, TT length measurement should
really just go away because you also need to know where the saddle and bars
are relative to the bb.

Cervelo has a nice little definition called "Stack". With stack and reach,
you can find the location of the intersection of the headtube and top tube
relative to the BB. This is the most basic measurement that allows you
to compare any two frames, without bias of bb height, seatback, or other
geometry or tubing differences that will mislead the numbers.

-g

I agree, but only if you can get all manufacturer's to agree on the standard points for measurement. In Cervelo's charts, they use the top-center of the head tube as the forward point to define reach. Taking the old standard horizontal TT length and subtracting the frame setback produces a measurement that accomplishes the same thing, but doesn't produce the same numeric value.

What reach allows you to do is compare any number of frames that are the same size, but not different sizes. If the stack is different, then the comparison is flawed. An example can be seen between the 51 and 54cm sizes of the R3. The reach difference is only 10mm, but even with the same 73 degree STA and HTA, the difference in the TT length is 15mm. The error comes from the fact that the reference point on the 51cm is 20m lower than the 54cm (20 x cos73) = 5.85mm. If the reach on the 54cm was measured at the same point on the head tube as the 51cm, then you'd get the true 15mm difference in stem length required, instead of a misleading 10mm difference in reach. This assumes that anyone considering these two frames would place the handlebars at one given height, regardless of which frame was chosen. The larger would just require less spacer under the stem.