Geometry Question

[Quagga]

To All you cleaver hubbers.

 

I am between L and XL 29 Frame according to most manufacturers, and at the moment I am looking at Specialized Epic for excample. Currently on a L

 

I ride a 25mm seatback seatpost and about 20mm saddle H-Bar drop ( I know not flexible.)

 

Hubbers have mentioned that maybe I am on a the wrong size frame (L atm) and that is why I need a 25mm setback seatpost. 

 

I look at the Gemometry for the new Epic L and XL and the seat tube is same angle. So in my mind the ONLY thing that will change is the seat tube lengh between the two bikes and I will still need 25mm seatback in order to achieve my 8 cm behind BB to saddle tip that I am used to and the way my bike setup (done by two different export setup people) was done.

 

What am I missing.

if you look at the graffic the Yellow distance remains the same when the angle it the same between L and XL

 

[Captain Fastbastard Mayhem]

You're not missing anything. That's completely correct. Setback has nothing to do with a frame size, unless there's a difference in seat tube angle. Your seat is still at the same height on the same tangent, so the frame size won't make a difference for that aspect of your setup, IF that is indeed correct for you. 

 

Increasingly, seat tubes are getting steeper, putting people into more of a time trial-esque position than has traditionally been the case. That's to make climbing easier. 

[Quagga]

You're not missing anything. That's completely correct. Setback has nothing to do with a frame size, unless there's a difference in seat tube angle. Your seat is still at the same height on the same tangent, so the frame size won't make a difference for that aspect of your setup, IF that is indeed correct for you. 

 

Increasingly, seat tubes are getting steeper, putting people into more of a time trial-esque position than has traditionally been the case. That's to make climbing easier. 

Thanks CFM

[bertusras]

Well yes. If your setup is done correctly, and your saddle position isn't used to compensate for reach, the saddle setback shouldn't make a difference.

 

It's bizarre how these things are some times. On my road bike I've got a no-offset seat post, with the saddle all the way forward, which just looks bizarre. And I went through the same thought process as yourself. Should I perhaps not be on a smaller frame? But then I realised, that won't make any difference, unless I was compensating for reach.

 

(or I am totally wrong and someone smarter is going to contradict me in 3, 2, 1)

[stefmeister]

Where you might experience a difference is if the actual seat tube angles are different between the sizes. Effective is the virtual line from BB to the height of the head tube, but it's also important to look at the actual angle, especially if you have longer legs.

 

I suspect the XL frame would have a steeper actual STA if the bend (where rocket link is located) in the ST is at the same distance from the BB for all sizes. That means the "yellow distance" would be less given the same seat height on a larger frame.

[Quagga]

I just checked and the 2018 Epic has an even steeper seat tube angle wich means I will NEED a setback post look here

 

 

 

[Quagga]

Where you might experience a difference is if the actual seat tube angles are different between the sizes. Effective is the virtual line from BB to the height of the head tube, but it's also important to look at the actual angle, especially if you have longer legs.

 

I suspect the XL frame would have a steeper actual STA if the bend (where rocket link is located) in the ST is at the same distance from the BB for all sizes. That means the "yellow distance" would be less given the same seat height on a larger frame.

They dont supply the Actual between the sizes for 2018 but they do for the older ones

 

and the XL is steeper than the L so it gets me back to on and XL I might need more setback than on the L to be in same spot in space

[Captain Fastbastard Mayhem]

Where you might experience a difference is if the actual seat tube angles are different between the sizes. Effective is the virtual line from BB to the height of the head tube, but it's also important to look at the actual angle, especially if you have longer legs.

 

I suspect the XL frame would have a steeper actual STA if the bend (where rocket link is located) in the ST is at the same distance from the BB for all sizes. That means the "yellow distance" would be less given the same seat height on a larger frame.

If the bend is the same distance from the BB, and the ST angles (from that bend) are the same, then the frame size won't make a difference at all. 

 

What will make a difference is if the ACTUAL seat tube angle (the angle from parallel to the ground to the seat tube, at the bend point) is different between the 2 bikes, which for the 2018 (i presume, seen in the 2nd pic) is true. Effective ST angle is from BB to saddle height equal to bars. Actual is from bend in the frame. Therefore you may just need more setback, BUT if you don't, you'll have a steeper seat tube angle which is good for climbing. 

[Captain Fastbastard Mayhem]

They dont supply the Actual between the sizes for 2018 but they do for the older ones

 

and the XL is steeper than the L so it gets me back to on and XL I might need more setback than on the L to be in same spot in space

Correct. Actual seat tube angle matters when comparing sizes and saddle position relative to BB. They can't change the position of the pivot relative to the BB, as that'd affect the kinematics. 

[NotSoBigBen]

Correct. Actual seat tube angle matters when comparing sizes and saddle position relative to BB. They can't change the position of the pivot relative to the BB, as that'd affect the kinematics.

'kinematics' ... Really dude? #NotUpToDate

[Captain Fastbastard Mayhem]

'kinematics' ... Really dude? #NotUpToDate

fancy word for what is essentially the way that the suspension reacts to input forces (bumps) and 

 

"move pivot point and the way it squishes changes" 

[DuncanCT]

This is something I would change with modern geo specs. ST angle should be actual angle of seat post and then offset should be added.

 

Effective STA is stupid. 

[Quagga]

If the bend is the same distance from the BB, and the ST angles (from that bend) are the same, then the frame size won't make a difference at all. 

 

What will make a difference is if the ACTUAL seat tube angle (the angle from parallel to the ground to the seat tube, at the bend point) is different between the 2 bikes, which for the 2018 (i presume, seen in the 2nd pic) is true. Effective ST angle is from BB to saddle height equal to bars. Actual is from bend in the frame. Therefore you may just need more setback, BUT if you don't, you'll have a steeper seat tube angle which is good for climbing. 

the furthur forward I go the more my back hurts or is that because I need a longer stem then?

[Captain Fastbastard Mayhem]

Back and core work for you then....

 

What length stem do you have ATM tho? I'd say that on an epic your range should be between 40&80 max (up from my 60/70 max for trail bikes cos it's an xc weapon)

[stefmeister]

If the bend is the same distance from the BB, and the ST angles (from that bend) are the same, then the frame size won't make a difference at all. 

 

What will make a difference is if the ACTUAL seat tube angle (the angle from parallel to the ground to the seat tube, at the bend point) is different between the 2 bikes, which for the 2018 (i presume, seen in the 2nd pic) is true. Effective ST angle is from BB to saddle height equal to bars. Actual is from bend in the frame. Therefore you may just need more setback, BUT if you don't, you'll have a steeper seat tube angle which is good for climbing. 

Kinda what I said, no?

[Captain Fastbastard Mayhem]

Kinda what I said, no?

dunno, tbh. I typed it up whilst neck deep in red wine and meds.