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New Math (Frame Geometry)

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New Math (Frame Geometry)

Old 07-03-12, 12:06 PM
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Originally Posted by Commodus
Setback is set in stone, or it should be. No matter what purpose you are putting the bike to. Finding this position on my racing bike, after much trial and error and finally getting a pro fit was a complete revelation in both power output and comfort.
Yes and no. Setback should be locked in on bikes of the same type. No question.

A proper TT bike-- *not* riding aerobars on a road bike-- would probably be forward of your road bike's setback. That is basically was Brian said in the first part of the post you're quoting. And then he says the mood thing at the end, which I disagree with, as you have.
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Old 07-03-12, 12:16 PM
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Originally Posted by Brian Ratliff
Think about it. Yes, you probably want the three contact points to be fixed relative to each other; however, there is no reason why you can't rotate the whole constellation around the bottom bracket to get a different position. There are plenty of riders who fit their saddles further back and their bars slightly higher and closer to create more power for climbing. Plenty of riders who have a bike with a longer, lower, more saddle forward position for flat races and sprints. Most time trialists and triathletes move their bars down and away to accommodate the much steeper seat tubes of modern time trial bikes.

Fit is all about getting the constellation of contact points right; position relative to the ground is a different story all together.
Sure, but setback determines the balance of your body on the bike relative to the force you're outputting into the pedals. If I set mine up with too little or too less, I get lesser or greater amounts of my weight on the seat and my hands, shifted from my feet. Even disregarding comfort issues, if you are not balanced on the bike, you can't transition properly - you have to shift your weight to stand for a climb, or a sprint, and then shift it again to sit back down. Impossible? Of course not, but isn't the point of fit to optimize your position on the bike?

Of course if you don't have to transition, this becomes a far murkier discussion - hence your very valid examples of TTs and flat races.
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Old 07-03-12, 12:26 PM
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Originally Posted by ColinL
use them together and you have your handlebar position. very simple X-Y coordinates.
No you don't. Look at my explanation and example above. Two frames with the same Reach can require stems of different length, if their Stack is different. If Reach was defined at a fixed height and not allowed to vary with Stack, then you would use the same stem length with every frame that has the same Reach.
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Old 07-03-12, 12:29 PM
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Originally Posted by rruff
No you don't. Look at my explanation and example above. Two frames with the same Reach can require stems of different length, if their Stack is different. If Reach was defined at a fixed height and not allowed to vary with Stack, then you would use the same stem length with every frame that has the same Reach.
You are hung up on using stack and reach separately to compare frames. A more useful way of using the measures is to use them together to find the position of the top tube of the bike, and figure out changes to stem position from there.
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Old 07-03-12, 12:38 PM
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Originally Posted by Commodus
Sure, but setback determines the balance of your body on the bike relative to the force you're outputting into the pedals. If I set mine up with too little or too less, I get lesser or greater amounts of my weight on the seat and my hands, shifted from my feet. Even disregarding comfort issues, if you are not balanced on the bike, you can't transition properly - you have to shift your weight to stand for a climb, or a sprint, and then shift it again to sit back down. Impossible? Of course not, but isn't the point of fit to optimize your position on the bike?

Of course if you don't have to transition, this becomes a far murkier discussion - hence your very valid examples of TTs and flat races.
Your weight balance does not change significantly until we start talking about massive changes in position; on the order of several inches. A TT position (non-UCI legal, which is typical in the amateur ranks) shifts the center of gravity of the rider forward by over two inches. That kind of shift can affect bike handling, but it certainly is rideable. Smaller shifts of less than an inch can't really change very much. A shift of 10mm in saddle is only a change of 1% in weight distribution between wheels.

As far as standing and stuff on your bike, if little changes affect your ability to move around on the bike, you have issues besides mere bike fit.
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Old 07-03-12, 12:39 PM
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Originally Posted by bored117
In all honesty datlas, I want to see you on bike just to further my understanding of bike fit. When I looked at your bike I assumed you are pretty much all legs... Wondering if my assumption is right
This is correct. If I sit down, I am very short. If I stand up, I am pretty tall.

I am riding this evening with a friend, if I can remember I will ask him to take a shot with my iPhone, and I will try to post it afterwards.
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Old 07-03-12, 12:44 PM
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Originally Posted by Brian Ratliff
Your weight balance does not change significantly until we start talking about massive changes in position; on the order of several inches. A TT position (non-UCI legal, which is typical in the amateur ranks) shifts the center of gravity of the rider forward by over two inches. That kind of shift can affect bike handling, but it certainly is rideable. Smaller shifts of less than an inch can't really change very much. A shift of 10mm in saddle is only a change of 1% in weight distribution between wheels.

As far as standing and stuff on your bike, if little changes affect your ability to move around on the bike, you have issues besides mere bike fit.
I think it's fair to say that long term comfort can be affected by a change in balance and weight distribution, even a small one. The same goes for rider power output. Higher power riders can support a forward position much better than lower power riders.
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Old 07-03-12, 12:49 PM
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Originally Posted by Brian Ratliff
Your weight balance does not change significantly until we start talking about massive changes in position; on the order of several inches. A TT position (non-UCI legal, which is typical in the amateur ranks) shifts the center of gravity of the rider forward by over two inches. That kind of shift can affect bike handling, but it certainly is rideable. Smaller shifts of less than an inch can't really change very much. A shift of 10mm in saddle is only a change of 1% in weight distribution between wheels.

As far as standing and stuff on your bike, if little changes affect your ability to move around on the bike, you have issues besides mere bike fit.
Based on my own experiences, my researching regarding fit, the endless 'favourite saddle' threads here on BF and what I've seen on various events and group rides, these 'little changes' affect everyone.

But if you don't think so, I can certainly understand your indifference toward STA, and bike fit in general.
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Old 07-03-12, 01:35 PM
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Originally Posted by Commodus
Based on my own experiences, my researching regarding fit, the endless 'favourite saddle' threads here on BF and what I've seen on various events and group rides, these 'little changes' affect everyone.

But if you don't think so, I can certainly understand your indifference toward STA, and bike fit in general.
Well, okay.

People are in the mindset that each contact point is independent. I'm saying they aren't. You want a lower position but you don't want to affect your power production, you don't just move your bars down. That would close your hip angle and affect your power. You increase the stem length and move your saddle forward and (slightly) up at the same time so your hands, feet, and saddle say in the same position relative to each other. Your power production says the same but now you are in a lower position.

Yes, your hands have more weight on them. True. But this is not a fit issue, this is just a consequence of gravity. Fit is about biomechanics. Gravity is gravity. Rotate the position up if you want less gravity on your hands.
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Old 07-03-12, 01:42 PM
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Originally Posted by Brian Ratliff
People are in the mindset that each contact point is independent. I'm saying they aren't. You want a lower position but you don't want to affect your power production, you don't just move your bars down. That would close your hip angle and affect your power. You increase the stem length and move your saddle forward and (slightly) up at the same time so your hands, feet, and saddle say in the same position relative to each other. Your power production says the same but now you are in a lower position.
Uh, how did your feet move? They didn't.

That's the reason we don't touch saddle setback. Even if you move your cleats, this changes the fulcrum on your foot, which can not only affect power but also comfort and even cause problems with your plantar fascia or achilles.
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Old 07-03-12, 01:53 PM
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Originally Posted by ColinL
Uh, how did your feet move? They didn't.

That's the reason we don't touch saddle setback. Even if you move your cleats, this changes the fulcrum on your foot, which can not only affect power but also comfort and even cause problems with your plantar fascia or achilles.
Your feet go around in a circle. A circle has no top or bottom. Why is this hard?
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Old 07-03-12, 01:57 PM
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Originally Posted by Brian Ratliff
Your feet go around in a circle. A circle has no top or bottom. Why is this hard?
Cuz this particular circle does have a top and a bottom, and more importantly for cyclists, it has a front and rear.
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Old 07-03-12, 02:00 PM
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Originally Posted by Commodus
Cuz this particular circle does have a top and a bottom, and more importantly for cyclists, it has a front and rear.
So then, do you make position adjustments when the ground slopes upward? However do you manage if you have to get off to change your saddle setback every time you go up a steep hill? Does your circle-with-a-top-and-bottom crankset reorient itself when you get out of the saddle? However does it know when to do this? You push a button or something?
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Old 07-03-12, 02:12 PM
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Originally Posted by Brian Ratliff
So then, do you make position adjustments when the ground slopes upward? However do you manage if you have to get off to change your saddle setback every time you go up a steep hill? Does your circle-with-a-top-and-bottom crankset reorient itself when you get out of the saddle? However does it know when to do this? You push a button or something?
No, it all happens quite automatically. The angle of my top tube hardly ever changes, and the front wheel of my bicycle remains at the front no matter how hard I accelerate.
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Old 07-03-12, 02:20 PM
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Originally Posted by Brian Ratliff
Your feet go around in a circle. A circle has no top or bottom. Why is this hard?
really? take 3 pics of yourself on your bike with your saddle where it is now, and then with the saddle bottomed out fore and then aft on the rails with your feet at the 3 and 9 position. it will change your knee angle tremendously.

some people are super sensitive to fit, but if you really size your bike like that Brian, you are either missing out (as Commodus said before) or you're performing well but strangely insensitive to fit.
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Old 07-03-12, 02:20 PM
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So you are saying the front and back of the circle are dictated by the frame? Are recumbent riders just screwed then? Where is the front and back of their pedal circle?
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Old 07-03-12, 02:23 PM
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Originally Posted by ColinL
really? take 3 pics of yourself on your bike with your saddle where it is now, and then with the saddle bottomed out fore and then aft on the rails with your feet at the 3 and 9 position. it will change your knee angle tremendously.

some people are super sensitive to fit, but if you really size your bike like that Brian, you are either missing out (as Commodus said before) or you're performing well but strangely insensitive to fit.
Wow, so you are saying there is something special about the 3 and 9 o'clock positions of the crank? I usually reference my crank rotation by the point where my leg is fully extended. This is rarely ever the 6:00 position on anyone's bike. More like the 5:00, looking from the driveside. If I am on a driveway slope, do I take the "3 and 9" to be referenced to the ground or the frame? If I stand in the saddle, can the "3 and 9" be just anywhere, or should it be the same "3 and 9" as when I am sitting? I'm really confused.
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Old 07-03-12, 02:28 PM
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are you taking lessons from Campag4Life? let's simmer down.

the 3 and 9 positions are easiest to see the change in your knee bend and the 3 is where fitters look for KOPS if they are so inclined. your knee should also not be fully extended at/near the 6 o'clock position. you should have 15-20 degrees of bend.
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Old 07-03-12, 02:29 PM
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FWIW, depending on how hard I am going, I am regularly shifting between sitting on the nose of the saddle (during hard efforts) and sitting on the tail (climbing). Does the "3 and 9" change depending on how hard I'm going? Because my saddle position certainly does.
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Old 07-03-12, 02:35 PM
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Originally Posted by ColinL
are you taking lessons from Campag4Life? let's simmer down.
...
Just having some fun...

Seriously though, why do you think 3 and 9 positions are important? Where are the "3 and 9" on a recumbent? Everyone agrees that, though they look dorky, the fit ones go quite fast and don't seem to explode their knees or hips even though their saddle setback is, what, like 3 feet.
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Old 07-03-12, 02:44 PM
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Originally Posted by Brian Ratliff
So you are saying the front and back of the circle are dictated by the frame? Are recumbent riders just screwed then? Where is the front and back of their pedal circle?
No, the frame dictates the reference for the positions on the circle. You've said that your position on your saddle changes depending on what you are doing, right? So what if your saddle was positioned 1.5" rearward? The position that you currently favour for very hard, TT style efforts would be in air. Forward? Same problem for your favoured climbing position.

Since your favoured positions are in fact available to you, and you are an effective cyclist, your saddle position must be pretty close.
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Old 07-04-12, 07:16 AM
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OK, I finally got my 10 year old to take a picture of me ON my bike so I can demonstrate how the bike in question fits me.

I had to hold onto the obligatory white garage door with one hand, but I think you get the idea...this is my "JRA" position, what I do 75% of the time. If pulling at the front, I bring my forearms down parallel to the ground.

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Old 07-04-12, 02:58 PM
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Originally Posted by Brian Ratliff
You are hung up on using stack and reach separately to compare frames.
You are damn right I am!!

The only point for coming up with new frame sizing numbers is if they are completely independent of each other and represent what you need to know. Why is that so hard? Stack works... Reach doesn't.
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Old 07-04-12, 03:05 PM
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Originally Posted by Brian Ratliff
You want a lower position but you don't want to affect your power production, you don't just move your bars down. That would close your hip angle and affect your power. You increase the stem length and move your saddle forward and (slightly) up at the same time so your hands, feet, and saddle say in the same position relative to each other. Your power production says the same but now you are in a lower position.

Yes, your hands have more weight on them. True. But this is not a fit issue, this is just a consequence of gravity. Fit is about biomechanics. Gravity is gravity. Rotate the position up if you want less gravity on your hands.
You are completely ignoring the effect of saddle position on the tendency of pedal force application to push you forward or back. If you move your saddle forward it is difficult to use your glutes effectively without pulling you forward on the saddle. That is why many pro TTers do the "carriage return"... and triathletes just sit farther forward and use their quads.
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Old 07-09-12, 10:43 AM
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Beautiful!

Originally Posted by datlas
ok, i finally got my 10 year old to take a picture of me on my bike so i can demonstrate how the bike in question fits me.

I had to hold onto the obligatory white garage door with one hand, but i think you get the idea...this is my "jra" position, what i do 75% of the time. If pulling at the front, i bring my forearms down parallel to the ground.

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