Question for geometry experts - lowering my saddle

[scottcw]

With a 73* STA and HTA, if I drop my saddle height by 1.5 cm, how far will my saddle move forward? Thanks.

[David Kirk]

Not much........about 4.5mm

Dave

[93legendti]

Is there a link to a site to these calculations? My new Sidi Carbons appear to have a 2mm thicker sole since losing the Genius 4's cleat cavity -- at least according to Sidi.

[Needs Help]

if I drop my saddle height by 1.5 cmRelative to what?
how far will my saddle move forward?
relative to the ground: 4.59mm

relative to the BB: 4.39mm

[Needs Help]

Is there a link to a site to these calculations?
If H is the horizontal distance your saddle moves, and D(diagnonal) is the distance your saddle moves relative to the BB, then

H = cos(seat tube angle) x D

If H is the horizontal distance your saddle moves, and V(vertical) is the distance your saddle moves relative to the ground, then

H = V / tan(seat tube angle)

[Needs Help]

Hmmm...I guess your seat post offset would affect the change in seat height relative to the BB, so to take that out of the equation, you should use the change in seat height relative to the ground.

[scottcw]

Relative to what?

relative to the ground: 4.59mm

relative to the BB: 4.39mm

Relative to the handlebars. How much closer will the nose of the saddle move toward the handlebars?

[Ti Designs]

Who takes seat height relative to the ground? That would imply that you lower your saddle if you put on larger tires - I know that's wrong, I'm trying to make a point here. Saddle height should be measured from the center of the bottom bracket to the top of the saddle along the seat tube. From there you can break it down to it's relative X and Y components. For instance

Y0 (original setback) = (COS(STA) x Saddle height) + Saddle offset

Y1 (adjusted setback) = (COS(STA) x new saddle height) + Saddle offset

and the horizontal difference is Y1 - Y0

[BarryG]

Relative to the handlebars. How much closer will the nose of the saddle move toward the handlebars?
Dave already answered it.

[93legendti]

Thanks guys!

[Ti Designs]

My new Sidi Carbons appear to have a 2mm thicker sole since losing the Genius 4's cleat cavity -- at least according to Sidi.


That only holds true for SpeedPlay where you put the cleat right into the insert without an adaptor. That said, 2mm isn't much of a difference, I probably see that much difference in height when wearing three pairs of tights on really cold days. We're talking less than 1mm difference in fore-aft position, I don't know anybody who makes that small a change and notices it.

[terry b]

With a 73* STA and HTA, if I drop my saddle height by 1.5 cm, how far will my saddle move forward? Thanks.

assuming a 75.5 Saddle height (I used mine, 57cm frame CTT)
drop the saddle to 74.0

Setback = Cos(STA)*STL (in this case saddle height.)

Cos(73)*75.5 = 22.1
Cos(73)*74.0 = 21.6

.5cm

[93legendti]

That only holds true for SpeedPlay where you put the cleat right into the insert without an adaptor. That said, 2mm isn't much of a difference, I probably see that much difference in height when wearing three pairs of tights on really cold days. We're talking less than 1mm difference in fore-aft position, I don't know anybody who makes that small a change and notices it.

I know, but after 1/2 hour I felt it in my bad knee (I guess 2 operations will do that!). I have to check the fore/aft again. I used pretty precise calipers and had the LBS check it, but maybe it is off...I hate change!

[Needs Help]

if I drop my saddle height by 1.5 cmRelative to what?Relative to the handlebars.
I'm afraid I wasn't very clear. When you said 'if I drop my saddle height 1.5cm', I was asking in what direction you intended to measure that 1.5cm?

Saddle height should be measured from the center of the bottom bracket to the top of the saddle along the seat tube.If that is the definition of saddle height you are using, i.e. the distance from the bottom bracket to the where the extended line of the seat tube intersects the saddle, and you decrease that distance 1.5cm, then seat post offset is irrelevant. For a perfectly level saddle, the change in horizontal distance would be given by the equation:

H = cos(seat tube angle) x D

where D is the change in saddle height.

However, if you measure saddle height from the bottom bracket to the mid point of the saddle, a seat post with offset effectively decreases the seat tube angle, and as you move the seat up and down, the angle between the bottom bracket and the midpoint of the seat changes slightly. That affects the seat tube angle in the equation above. As the seat tube angle goes down, the cos of the angle goes up.

Note that the horizontal change in the saddle is not the same as the change in distance from the nose of the saddle to the handlebars unless the handlebars are level with the seat top.