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Impact of 18mm?

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Impact of 18mm?

Old 01-22-23, 08:47 AM
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Zaskar
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Impact of 18mm?

Yesterday was ride #1 on the new bike. I was fitted professionally a long time ago and have transferred those numbers to several bikes. All was good. I thought (thought) I did that with the new bike. In an oversight, I got the saddle fore/aft off by 18 (ish) mm. It's further back than my other bikes. I couldn't feel this just riding. Probably because there are some other things to address on the new bike - the steerer still needs to be cut and two spacers to be removed. And the 42cm bar will be swapped for a 44cm. This will happen next week.

On my ride yesterday with four guys I ride with a lot (!), I felt, well... slow. They even kidded that I was off. After 45 miles, my quads were spent. So my question: Can 16-20mm of fore/aft saddle movement have that much impact? Or...do I have this backward - that moving forward loads the quads?

And, there could be too many factors to put it all on the setback. The narrower and higher bar (for now) and the 71 (new) vs 73 (old) degree head tube angles.
Correction: Seat and head tube angles are both 73 degrees.

Last edited by Zaskar; 01-27-23 at 07:12 AM.
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Old 01-23-23, 10:13 AM
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Yep. For and aft of the saddle affects your position over the BB. Up to a point, you get better transfer of power to the pedals as you move your saddle forward.

Also if the new bike is a different seat tube angle along with what the numbers are you are transferring to the other bikes, then you might be well off from what that original professional fit was on that first bike long ago.

Other things about the geometry can also make just transferring saddle to bar measurements and saddle height not always give the same feeling of fit even though you are in the same position.

Last edited by Iride01; 01-23-23 at 10:16 AM.
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Old 01-23-23, 01:34 PM
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My WAG is that your current position is a little different, and different in such a way that some muscles that you used to use aren't getting hit and thus you transfer the load to your quads. For me, more hip angle means more hamstring and glute use, less quad use. But I always fall back on "everyone's different."

BTW, the only accurate way to measure saddle set back (using the same saddle as on previous bike) is to drop a plumb line from the saddle nose and measure the distance to the CL of the bottom bracket.
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Old 01-24-23, 06:39 PM
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Yeah... It seems a refit is in order. That's on the schedule for Friday.

Carbonfiberboy that's the method I use - plumb line to the chain stay, measure from that point to the spindle.

Thanks guys
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Old 01-26-23, 05:34 PM
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Originally Posted by Zaskar View Post
Can 16-20mm of fore/aft saddle movement have that much impact? Or...do I have this backward - that moving forward loads the quads?

.
Absolutely. An 18 mm change in setback is nearly as consequential as an 18 mm change in saddle height. When I switched from a Giant TCR (72.5 degree seat tube) to a Trek Madone (73.0 degree seat tube), my setback crept back a couple millimeters.
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Old 01-27-23, 06:50 PM
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Originally Posted by Carbonfiberboy View Post
My WAG is that your current position is a little different, and different in such a way that some muscles that you used to use aren't getting hit and thus you transfer the load to your quads. For me, more hip angle means more hamstring and glute use, less quad use. But I always fall back on "everyone's different."

BTW, the only accurate way to measure saddle set back (using the same saddle as on previous bike) is to drop a plumb line from the saddle nose and measure the distance to the CL of the bottom bracket.
how about having your rear tire against a wall and measuring from the wall to the back of the seat? I am asking not saying. I am getting ready to swap saddles and thought this would be a way to do it. thanks
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Old 01-27-23, 07:45 PM
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Originally Posted by jadmt View Post
how about having your rear tire against a wall and measuring from the wall to the back of the seat? I am asking not saying. I am getting ready to swap saddles and thought this would be a way to do it. thanks
If you're swapping saddles on the same bike sure, but if not, not.
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Old 01-29-23, 04:36 PM
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Originally Posted by Carbonfiberboy View Post
My WAG is that your current position is a little different, and different in such a way that some muscles that you used to use aren't getting hit and thus you transfer the load to your quads. For me, more hip angle means more hamstring and glute use, less quad use. But I always fall back on "everyone's different."

BTW, the only accurate way to measure saddle set back (using the same saddle as on previous bike) is to drop a plumb line from the saddle nose and measure the distance to the CL of the bottom bracket.
I'm a little unconvinced that the direction of the center of the earth plays a major role in how muscles are used in cycling. "Over the crank" is a direction, not a physiological relationship.

Plumb bobs are excellent - as long as both bikes are measured in the same place on the floor, or the floor is shown to be perfectly level. Otherwise you'll have an error of about 1cm per degree of slope change.
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Old 01-29-23, 06:18 PM
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Originally Posted by Kontact View Post
I'm a little unconvinced that the direction of the center of the earth plays a major role in how muscles are used in cycling. "Over the crank" is a direction, not a physiological relationship.

Plumb bobs are excellent - as long as both bikes are measured in the same place on the floor, or the floor is shown to be perfectly level. Otherwise you'll have an error of about 1cm per degree of slope change.
1) As has been pointed out before, using a plumb bob is a method of transferring measurements and has nothing to do with pedaling.

2) As has been pointed out before, floors are vastly more level than 1°, at least 10 times more level if not 100. And as long as both measurements are taken at the same spot, bikes pointed in the same direction, level doesn't make any difference.

3) And as accurate as this method is, it only works if the two saddles are identical. Because of the variations in the shapes of modern saddles and bikes, the only real way to determine saddle setback with different saddles or on different bikes, is to get on the bike and ride, adjusting until if feels right. Dropping a plumb bob is a good start, but not a guarantee of perfection.
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Old 01-29-23, 07:15 PM
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Originally Posted by Carbonfiberboy View Post
1) As has been pointed out before, using a plumb bob is a method of transferring measurements and has nothing to do with pedaling.

2) As has been pointed out before, floors are vastly more level than 1°, at least 10 times more level if not 100. And as long as both measurements are taken at the same spot, bikes pointed in the same direction, level doesn't make any difference.

3) And as accurate as this method is, it only works if the two saddles are identical. Because of the variations in the shapes of modern saddles and bikes, the only real way to determine saddle setback with different saddles or on different bikes, is to get on the bike and ride, adjusting until if feels right. Dropping a plumb bob is a good start, but not a guarantee of perfection.
1. I wasn't talking about the plumb bob, but the way you described the change in muscle use based on set back. Set back is little more than an angle from the crank to the hip.

2. Many basements, garages, decks and older home floors aren't very level. I'm not discouraging anything, just making people aware. If you use the same spot for both bikes it doesn't matter if the floor is level.

3. Yup.
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