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Discussion Starter · #1 ·
I currently ride a TCR Composite XL with a 100mm 6 degree stem flipped up. The HT length is 200mm. On the fork, I have 20mm of spacers.

I am getting a new frame in a smaller size, the TCR advanced. The HT is 25 mm shorter and the TT is 25mm shorter. I am expecting to need to use a 110 or 120 mm stem with the new bike (the seat angle is a little more laid back on the new bike so I am not really losing 25mm in TT length). Assuming I use a 120mm stem with the same angle (buy the same exact stem except bigger), how much additional height will be added by the longer stem? I obviously want to avoid using 45mm of spacers to get the handlebars to the same height, which might also be dangerous with a carbon steerer.

This math calculation is way beyond my capability. Geometry for both bikes below:
 

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Discussion Starter · #3 ·
Thanks

Thanks for the link. It looks like I will gain about 10mm of height so I should be able to eliminate 1 of the spacers. Then I will give lower the bars by 5mm so in total, I am using 30mm of spacers, well within reason. I could also always go with a stem with a slightly higher angle.
 

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a little help....

If you need a 220mm stack height, plus the headset height and a 96 (+6) degree stem then a longer stem won’t help much. A 120mm stem will only add about 8mm to the height, so you’ve got a 17mm deficit to make up.

Before I get into anything else, I’ll note that your idea about the less steep STA is backwards. The 72.5 STA on the new frame makes the reach difference about 6mm greater than the 2.6cm (61-58.4) difference between the TT lengths. The saddle will need to be moved forward at least 6mm on the new frame to position the saddle the same as your other bike. I guess the good news is that a 130mm stem will raise the bars about 11mm, you’ll have less additional spacers to add.

A better solution to the spacer problem is to add more stem angle. A 130mm by 100 (+10) degree should require only about 5mm more spacer. That's where I'd start. Be sure to get a brand than has a 40mm steering tube clamp height (not an ITM Millennium). Measure the height of the handlebars from the floor to the top of the bars on the old bike, so you have a reliable dimension to shoot for.

To make this transition easier, I’d highly recommend using the same saddle. Using a plumb bob, set the saddle tip the same distance behind the center of the BB as the old frame, then measure from the saddle tip to the center of the bars to insure the same reach has been achieved. This assumes that you are not also changing the handlebars or shift lever setup.
 

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sbindra said:
Thanks for the link. It looks like I will gain about 10mm of height so I should be able to eliminate 1 of the spacers. Then I will give lower the bars by 5mm so in total, I am using 30mm of spacers, well within reason. I could also always go with a stem with a slightly higher angle.

that figure is not to scale... I have a hard time believing that going from 100mm to 120mm with the same angle is going to give you 10mm in height. if you increase the angle you'll likely need to go even longer, but remember, stems only come in certain lengths and angles
 

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Discussion Starter · #6 ·
C-40 said:
If you need a 220mm stack height, plus the headset height and a 96 (+6) degree stem then a longer stem won’t help much. A 120mm stem will only add about 8mm to the height, so you’ve got a 17mm deficit to make up.

Before I get into anything else, I’ll note that your idea about the less steep STA is backwards. The 72.5 STA on the new frame makes the reach difference about 6mm greater than the 3cm (58.5 -55.5) difference between the TT lengths. The saddle will need to be moved forward at least 6mm on the new frame to position the saddle the same as your other bike. I guess the good news is that a 130mm stem will raise the bars about 11mm, you’ll have less additional spacers to add. You may find that you’re in between stem lengths, and may have to decide between using a 130mm stem with saddle a bit further back or a 140mm with it more forward, if you want to maintain exactly the same reach.

A better solution to the spacer problem is to add more stem angle. A 130mm by 100 (+10) degree should require only about 5mm more spacer. That's where I'd start. Be sure to get a brand than has a 40mm steering tube clamp height (not an ITM Millennium). Measure the height of the handlebars from the floor to the top of the bars on th eold bike, so you have a reliable dimension to shoot for.

To make this transition easier, I’d highly recommend using the same saddle. Using a plumb bob, set the saddle tip the same distance behind the center of the BB as the old frame, then measure from the saddle tip to the center of the bars to insure the same reach has been achieved. This assumes that you are not also changing the handlebars or shift lever setup.
Thanks for the help but I am little confused by your reply. The TT on the new bike is 25mm shorter than the old one. I use a 100mm stem. I assumed to make up the difference, I would need a 120mm stem more or less. When I met with my fit guy, he stated that because of the angle of the ST, I might need only 110mm. Are you saying that he got it backwards? That I might need 120 to 130?

I have sufficient flexabillity that I can ride with the bars slightly lower.
 

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The head tube and seat tube on the new frame both have a slacker angle by 0.5 degree. TCR 73.5 and 73 vs TCR advanced at 73 and 72.5 It's kind of like moving the the top tube rearward relative to the bottom bracket. This will affect your setback, so you'd have to move your seat forward and the bars forward to duplicate the position from the old frame relative to the bottom bracket. I hope this helps, and doesn't confuse matters.
 

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Counter intuitive

sbindra said:
Thanks for the help but I am little confused by your reply. The TT on the new bike is 25mm shorter than the old one. I use a 100mm stem. I assumed to make up the difference, I would need a 120mm stem more or less. When I met with my fit guy, he stated that because of the angle of the ST, I might need only 110mm. Are you saying that he got it backwards? That I might need 120 to 130?

I have sufficient flexabillity that I can ride with the bars slightly lower.
Yup, your fitting guy got it wrong and C-40 is correct on this. The key piece of information to remember is that the saddle stays constant in its position relative to the BB. Then, with a more slack (shallower) seat tube angle the saddle has to be placed more forward relative to the top tube to keep the position with the BB, effectively reducing the length of the top tube.

If you position the saddle in the place relative to the BB on two frames with identical top tube lengths, using the same saddle and seatpost, but one frame has a shallower seat tube then you will have to move the saddle forward on the seatpost to get the same BB/saddle relationship. Remember, the top tube length in this example is the same, but on the shallower frame the saddle is "over" more of the top tube. If the same length stem is used, the shallower bike will have less reach.
 

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sbindra said:
He is an excellent fit guy actually. We were reviewing the geometrys in the store. Like anyone else, he may just have gotten confused. I am sure he would have realized the mistake.
Unless he was trying to move you back on the bike so the CG would work out right. Might also be he thought the slack angle was more appropriate for you, due to relative femur length or certain other factors. In either of those cases, his directional sense was right. Because the saddle's above the TT, the slack angle will move it back a bit more than the TT length alone change would suggest, and so you'd need a shorter-than-expected stem.

But if he was trying to duplicate your position with the new frame, looking only at reach and your absolute position relative to the BB, then he got crossed up somewhere. Going back and forth between charts, it's easy enough to do.
 
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