Impact of slacker seat tube on saddle position

[biwa]

Am considering a new bike with a slacker seat tube angle, going from 73.5-deg to 73-deg. Assuming the saddle and the crank are the same, what effect will it have on the saddle height (measured from BB along the ST to the top of saddle) and the saddle fore-aft position?

Related, when replicating the fit between two bikes with a different ST angles, should we keep their saddle height (as defined above) constant or should we keep the perpendicular distance between the top of the saddle and the BB constant?

I think the saddle height (not the perpendicular distance) should remain constant and saddle should move forward (reducing the setback) due to the slacker angle. Can someone confirm?

[mattr]

It'll make no difference. The setback (Horiz) and height (Vert) is set from the bottom bracket. Not the seat tube angle (or length).
Trying to measure along the axis of the seat tube is pretty pointless (and pretty tricky to make it accurate or useful!)

[Calnago]

You want the vertical distance from the BB to remain the same as well as saddle setback from the vertical line through the BB. You want the saddle to be in the same place in space, relative to the BB, at least as a starting point on the new bike. For ease of setup going forward you will then likely have a new, slightly longer saddle height “measurement” number, simply because you are now measuring along a longer (slacker) line. The clamp on the saddle rails, assuming the same seatpost offset and clamp type as before, will be slightly further back than it was before. Yet, the saddle position itself will be in exactly the same place relative to the BB as your old bike.
But even though you get your saddle relative to the BB in exactly the same place, the new frame’s geometry itself, if different, may warrant slight changes in saddle position, which you have to work out by trial and error on the new bike. Different wheelbases, different relationships between chainstay lengths and/or front center distance, etc., may result in a different center of gravity which you may want to account for in your seated position. It’s a process.

[wingguy]

Also if you are changing saddle setback from the BB, find a consistent spot on the top of the saddle to measure height to (usually the centre) not just along the line of the ST. Then you won’t have to use trigonometry to figure out a compensation, you’ll just measure the distance like normal.

[Bigger Gear]

Everyone has made good points, I'll just add that if the effective/horizontal top tube length is the same on each bike, and you keep the saddle in the same spot relative to the BB then the bike with the 73 degree seat tube will have a reach reduction of ~ 5 mm. Think of it as "more top tube is under the saddle" with the slacker seat tube angle.

[TobinHatesYou]

Everyone has made good points, I'll just add that if the effective/horizontal top tube length is the same on each bike, and you keep the saddle in the same spot relative to the BB then the bike with the 73 degree seat tube will have a reach reduction of ~ 5 mm. Think of it as "more top tube is under the saddle" with the slacker seat tube angle.

That’s why we just ignore eTT and HT these days and look at reach/stack instead.

[Bigger Gear]

That’s why we just ignore eTT and HT these days and look at reach/stack instead.

Some of us older types still like the "real" numbers and doing our own math.

[bikeboy1tr]

Does no one use a plumb bob anymore. Its so simple to get a good starting position as to how far your seat is behind the BB once you have a reasonable seat height measurement.

[TobinHatesYou]

That’s why we just ignore eTT and HT these days and look at reach/stack instead.

Some of us older types still like the "real" numbers and doing our own math.

Nonsense.

You’re still going to need to take a real measurement either way if you don’t trust geometry charts. And in that case you may was well line the bike perpendicular to a wall and take two horizontal measurements: BB to wall minus top of HT to wall.

Or if you want saddle to bar reach you can do similar with the appropriate coordinates.

[bikeboy1tr]

I know I was having a hard time believing the Colnago charts for the V2R in a 54 having a ST of 73.1 when all other charts were saying 72.5. But after I got the bike set up I think the Colnago chart was correct and everyone else was Wrong. It all worked out in the end especially with the SMP saddle.

[Bigger Gear]

Nonsense.

You’re still going to need to take a real measurement either way if you don’t trust geometry charts. And in that case you may was well line the bike perpendicular to a wall and take two horizontal measurements: BB to wall minus top of HT to wall.

Or if you want saddle to bar reach you can do similar with the appropriate coordinates.

So do you trust published stack and reach measurements? Don't fret it Tobin, I've been at this for over 25 years and I've got my reliable methods, but I"m sure you would be able to find flaws

[Bigger Gear]

Does no one use a plumb bob anymore. Its so simple to get a good starting position as to how far your seat is behind the BB once you have a reasonable seat height measurement.

+1 on the plumb bob. I have a marked level spot in my basement where I use a plumb bob, tape measure and a couple of lasers to quantify all of my bikes.

[ChiZ01]

That’s why we just ignore eTT and HT these days and look at reach/stack instead.

Some of us older types still like the "real" numbers and doing our own math.

eTT and HT measurement means very little, two frames with the same eTT and HT but different BB drop, HT angle, and ST angle will fit very differently

[TobinHatesYou]

So do you trust published stack and reach measurements? Don't fret it Tobin, I've been at this for over 25 years and I've got my reliable methods, but I"m sure you would be able to find flaws

I trust them as much as I trust any on-paper measurement. Guess what, no two frames are exactly the same. I've have two Emonda SLR Discs, and their eTTs, TT, reach, stack, etc. are off by 3-7mm. It's utter nonsense to trust one value over the other. If you are that particular, then it's necessary to just whip out the measuring tape.

[numberSix]

reach reduction of ~ 5 mm. Think of it as "more top tube is under the saddle" with the slacker seat tube angle.

The math:
A 0.5° Seat tube angle difference and a saddle height of 725mm
Sin(0.5) x 725 = 6.33mm.
Where saddle height is from the BB c/l to the saddle horizon along the seat tube.

So, for frames with the same ETT and a seatpost with the same setback, the saddle will be 6.33mm further back on the 73° STA frame. Plan your seatpost choice accordingly to get the same x,y saddle position relative to the BB.

Whether the geo tables are accurate is another question.