bike geometry help from the math guys...

Discuss light weight issues concerning road bikes & parts.
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chiltonp
Posts: 118
Joined: Fri May 01, 2009 8:48 pm

by chiltonp

Hello,

I am currently riding a 2016 M/L TCR Advanced SL, and I am contemplating cutting the seat mast slightly more, (1-1.5cm) to compensate for some slight reach issues i'm having. My question is this, factoring in seat-tube angle, how much will a 1-1.5cm decrease in seatmast height move my saddle nose forward?

youngs_modulus
Posts: 512
Joined: Wed Sep 20, 2006 1:03 am
Location: Madison, WI USA

by youngs_modulus

The trig function you're looking for is cosine. If your seat tube angle is 73 degrees and you cut off 1.5 cm, the horizontal location of the seat mast will move forward 0.44 cm. Cos(73) * 1.5 = 0.439 (rounded).

That said, adjusting your reach by changing your saddle height is a terrible idea. If you're having "slight reach issues" you need to buy a new stem, not lower your saddle height. This goes double when you're lowering your saddle via an irreversible change (cutting a seat mast).

wingguy
Posts: 3834
Joined: Thu Mar 08, 2012 11:43 pm

by wingguy

I 100% agree with the previous poster. This is a crazy, terrible idea.

chiltonp
Posts: 118
Joined: Fri May 01, 2009 8:48 pm

by chiltonp

Good information. When I had the bike built I intentionally left the seat-mast higher than previous bikes because I was unsure of fit at that point and chopping the ISP somewhat scared me. If I did cut the seat-mast it would only be to the same height of previous bikes.

I do not intend to cut the mast just to fix a reach issue, just curious how much lowering it to previous setup would change things.

boots2000
Posts: 1439
Joined: Mon Oct 15, 2007 9:28 pm

by boots2000

For reach issues you need to address reach- not seat position/saddle setback (unless your saddle position is also off).
For reach you change stem or change bars or both-

chiltonp wrote:Hello,

I am currently riding a 2016 M/L TCR Advanced SL, and I am contemplating cutting the seat mast slightly more, (1-1.5cm) to compensate for some slight reach issues i'm having. My question is this, factoring in seat-tube angle, how much will a 1-1.5cm decrease in seatmast height move my saddle nose forward?

wingguy
Posts: 3834
Joined: Thu Mar 08, 2012 11:43 pm

by wingguy

chiltonp wrote:I do not intend to cut the mast just to fix a reach issue, just curious how much lowering it to previous setup would change things.

Ok, you probably should have said so then! In your OP you said you were going to lower the saddle in order to fix a reach issue, not that you were lowering the saddle anyway. If you'd written your first post properly you'd have avoided scaring the shit out of us :lol:

(BTW, have you been riding the bike with the saddle 1.5cm too high? Because that's massive in terms of saddle height. Are you sure you know that your previous saddle height is correct?)

youngs_modulus
Posts: 512
Joined: Wed Sep 20, 2006 1:03 am
Location: Madison, WI USA

by youngs_modulus

Yes, this is my question too. A saddle 1.5 cm too high on my bike would, I think, lock my knee at the bottom of each pedal stroke. You first posted about your Giant in the fall, mentioning a reach issue but not a saddle height issue.

If you have been riding with a saddle that's too high, why?

I suspect you were thinking that cutting your seat mast by 1.5 cm but keeping your saddle height constant (so that the topper is at the upper limit of its adjustability) would move your saddle forward. It wouldn't. But on the off chance that's what you were thinking, there's no shame in admitting it. If that was your thought process, it would explain a lot.

I do this stuff for a living; I'm a mechanical engineer, and I think Wingguy might be one too. I have a good friend who has to say to himself, out loud, "righty tighty lefty loosey" before he turns any bolt on his bike. He'd be a terrible engineer, so it's a good thing he's a hell of a lawyer. I'm sure I'd be terrible at writing a brief. Like I said, there's no shame in getting this wrong (unless you're a mechanical engineer).

wingguy
Posts: 3834
Joined: Thu Mar 08, 2012 11:43 pm

by wingguy

youngs_modulus wrote:I suspect you were thinking that cutting your seat mast by 1.5 cm but keeping your saddle height constant (so that the topper is at the upper limit of its adjustability) would move your saddle forward. It wouldn't.

I was suspecting the same thing :wink:

Also for the OP, are you aware that you can reverse the two clamping plates on the TCR seatmast to change it from a 15mm layback to a 0mm inline setup?

(In an ideal world you still shouldn't be making drastic saddle setback changes for the purpose of altering reach, but if you've reached the limit on stem length and don't want to get rid of the bike...)

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ms6073
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Joined: Fri Jul 23, 2004 8:24 pm
Location: Houston, Texas

by ms6073

chiltonp wrote:I do not intend to cut the mast just to fix a reach issue, just curious how much lowering it to previous setup would change things.

Not trying to offend, but from what I am reading here, you seem to be failing to understand the consquences of such drastic changes, and I really (REALLY) suggest seek the advice of a professional bike fitter.
Michael - The Anaerobic Threshold is neither...

chiltonp
Posts: 118
Joined: Fri May 01, 2009 8:48 pm

by chiltonp

I appreciate the feedback. I plan to have a professional fitting soon, (possibly this weekend).

I'll admit that when I had the TCR built I had the seatmast left higher than on my previous bikes. I had been contemplating raising the seat-post on prior bikes but never did it and I saw the TCR build as an opportunity to try the higher saddle position before chopping the post. I have never owned a bike with an ISP and feared having it cut too short and then later realizing that I actually preferred a higher saddle. I have been riding the current saddle height for the past few months without knowing if it is correct. I have already take some steps to remedy what I feel is a reach issue, swapped from a 110mm stem to a 100mm, flipped the seatmast topper to the non-setback position and have the saddle fairly far forward in the clamp.

I love the TCR, but have yet to be able to get my fit as dialed in as I felt on past two bikes. Hopefully a professional fitting is the answer.

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ms6073
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Location: Houston, Texas

by ms6073

chiltonp wrote:I love the TCR, but have yet to be able to get my fit as dialed in as I felt on past two bikes. Hopefully a professional fitting is the answer.

Appreciate your position and will now add, if you find the fit/position of your other bikes to be adequate/comfortable, simply take some time to measure the setup of the old bike then transfer those measurements as close as possible onto your TCR. Some will probably not like the advice offered, but you might want to take a look at the article linked below as it has a good foundation for transferring your fit (assumes floor and walls are at 90-degree angles to each other which I found was not the case in my garage :wink:).

Technical FAQ: Setting your bikes up identically

Also, not sure if this is true of current generation TCR's, but when I had one, the frame was supplied with two seat mast toppers, one that allowed for a 1-cm increase in saddle height, and a second that allowed for up to 2.5 cm increase in saddle mast height.
Michael - The Anaerobic Threshold is neither...

wingguy
Posts: 3834
Joined: Thu Mar 08, 2012 11:43 pm

by wingguy

chiltonp wrote:I love the TCR, but have yet to be able to get my fit as dialed in as I felt on past two bikes. Hopefully a professional fitting is the answer.

To be blunt, if you've been riding with a saddle position 15mm higher than normal and it hasn't caused you any problems then it is quite unlikely that your previous bike setups were in any way 'dialled'. Also if you've already got a shorter stem and the 0 setback clamp position and the bike really is too long, then it's quite likely that the answer you get from a pro bike fit will be 'get a new bike'...


What is the saddle height you're running on it by the way?

jeffy
Posts: 1255
Joined: Sun Oct 05, 2014 11:51 pm

by jeffy

Get a fit. Seriously.

If i recall correct from when i did some maths for it (disclaimer; not a mathematician) raising the saddle 3mm would move the saddle back 1mm

chiltonp
Posts: 118
Joined: Fri May 01, 2009 8:48 pm

by chiltonp

Got a fitting yesterday. ISP was cut 7mm and saddle position was tweaked, a good deal of time was spent on my cleat position as well. Fitter also noted that I am riding too wide of a handlebar in his opinion and recommended a 40cm.

I hope to do a hard ride this evening and see how it feels, but so far I like the results. Thanks for the info and insight that you guys provided, I appreciate it.

BdaGhisallo
Posts: 1890
Joined: Wed Mar 17, 2004 1:38 pm

by BdaGhisallo

jeffy wrote:Get a fit. Seriously.

If i recall correct from when i did some maths for it (disclaimer; not a mathematician) raising the saddle 3mm would move the saddle back 1mm


The effect on setback from raising the saddle is related to the cosine of the seat tube angle:

5mm change on a 73 deg STA = 5 X cos73 = 1.46mm

10mm change on a 74deg STA = 10 X cos74 = 2.75mm

It's not hard to determine. The calculator on your pc or phone should have the cosine functionality.

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