Impact of slacker seat tube on saddle position

Discuss light weight issues concerning road bikes & parts.
Bigger Gear
Posts: 476
Joined: Fri Mar 27, 2009 6:58 pm
Location: Wet coast, Canada

by Bigger Gear

TobinHatesYou wrote:
Wed Apr 10, 2019 4:05 am
Bigger Gear wrote:
Wed Apr 10, 2019 3:27 am

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 :roll:

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.
Agreed, I have also had many bikes over the years where tube lengths have been up to 5mm off published values, seat tube angles have been off by 0.5, etc, etc.

The word particular is poignant, because in my younger days I certainly was not particular. I could have 2-3 bikes that were close, but not really that close and hop from one to the other. But as I've aged and my body has started to show the effects of years of abuse (don't let your kids race motocross :) ) I've found that I really want my bikes to be as close in setup as I can possibly get them. But maybe also my OCD is worsening as I age. The funny thing is, even if I get the contact points identical, no 2 bikes will ever feel the same. My gravel/winter bike with long chainstays and slack head tube angle is going to feel so much different than my Moots Compact SL which is pretty steep and short, and that will feel different from my older Trek Madone, that is close in geometry but another material completely.

I'd certainly caution the OP against buying bar/stem/seatpost until you have the frame in hand. I once used published numbers (all of them, stack, reach etc) to calculate my fit and I was convinced I needed a 110 stem. Built the bike up exactly as calculated and found the reach was too short. Frame had a slightly shallower seat tube angle, and eTT was 3mm shorter than published. So....ended up with a 120 stem because even though it measured out a touch longer than I wanted it was better feeling than being 5-6mm shorter.

biwa
Posts: 148
Joined: Fri Sep 21, 2018 8:39 pm

by biwa

numberSix wrote:
Wed Apr 10, 2019 4:41 am
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.
So does this mean the saddle should be move forward by ~6.3mm in order to retain the same horizontal distance in relation to BB?

by Weenie


mattr
Posts: 4673
Joined: Fri May 25, 2007 6:43 pm
Location: The Grim North.

by mattr

If you start from the bottom bracket, no. It'll be in exactly the same location.

6.33mm is the measurement at a saddle height of 725mm, so you'll need to adjust based on your saddle height.

Or just start at the bottom bracket. Which is a fixed point in this context.

biwa
Posts: 148
Joined: Fri Sep 21, 2018 8:39 pm

by biwa

mattr wrote:
Wed Apr 10, 2019 7:08 am
If you start from the bottom bracket, no. It'll be in exactly the same location.

6.33mm is the measurement at a saddle height of 725mm, so you'll need to adjust based on your saddle height.

Or just start at the bottom bracket. Which is a fixed point in this context.
I see. So the vertical distance from saddle top to the BB is fixed here, and from there you work out the new saddle height (measured along the seat tube from the top to the BB) given the new STA.

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