New Age Bike Sizing
01-01-24, 01:21 PM
#26
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But reach doesn't tell you how long the frame is - it tells you how long the frame is at a relatively arbitrary point that intersects the steerer tube - and not a point where you can mount the stem. The reality is that we have always treated the "reach" to the bars as a function of the distance between two roughly parallel lines - seat tube and head tube, and that worked because of how we raise and lower stem and seat. But suddenly we are measuring the distance between a vertical line and a 73 degree line - and then documents where the headset bearing goes. It makes little sense.
As long as you correct for variations in seat tube angle, ETT always works - if you know that you need around a 55 TT off a 73 STA, you can compare multiple geometries simply and easily, and not need to adjust for stack. All you need to know is to add or subtract 1cm of TT for each degree of STA difference.
As long as you correct for variations in seat tube angle, ETT always works - if you know that you need around a 55 TT off a 73 STA, you can compare multiple geometries simply and easily, and not need to adjust for stack. All you need to know is to add or subtract 1cm of TT for each degree of STA difference.
So with reach you get the distance from the BB directly to the place where you start building the cockpit. At the most you'll need to account for the spacer stack. But you'd need to do that with ETT too.
So if you compare frames with the same stack but varying reach, each 10mm difference in reach corresponds directly to a 10mm change in stem length. No matter how the frames are constructed, what the headset stack height is, what the seat tube angle is etc.
On the other hand using ETT doesn't make any sense for a multitude of reasons. Firstly, you need to correct for seat tube angle. That "add or subtract 1cm" thing doesn't work, or at least it's not accurate. Personally I like to be within a millimeter when considering a frame.
Also depending on manufacturer ETT is either to top of head tube (sorta makes sense) or to middle of top tube / head tube intersection. The latter is just completely useless, since the top of the headset can be several centimeters above and a few cm behind that intersection depending on how the frame is constructed and what the headset stack height is.
Then you also need to control for seat tube angle headtube angle, frame construction method (ie. How far the top of the head tube protrudes from the top tube head tube intersection) and headset stack height. And only then you get a comparison value that gets you the length from an arbitrary point along the seatpost (not the seat, mind you!) to the place where you can start building the cockpit.
Dang that was a confusing writeup. But that's ETT for you.
01-01-24, 03:21 PM
#27
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Reach tells you clearly how far horizontally the top of the headset is from the bottom bracket. It takes the seat tube angle out of the equation, which is smart as that's a useless metric in frame length.
So with reach you get the distance from the BB directly to the place where you start building the cockpit. At the most you'll need to account for the spacer stack. But you'd need to do that with ETT too.
So if you compare frames with the same stack but varying reach, each 10mm difference in reach corresponds directly to a 10mm change in stem length. No matter how the frames are constructed, what the headset stack height is, what the seat tube angle is etc.
On the other hand using ETT doesn't make any sense for a multitude of reasons. Firstly, you need to correct for seat tube angle. That "add or subtract 1cm" thing doesn't work, or at least it's not accurate. Personally I like to be within a millimeter when considering a frame.
Also depending on manufacturer ETT is either to top of head tube (sorta makes sense) or to middle of top tube / head tube intersection. The latter is just completely useless, since the top of the headset can be several centimeters above and a few cm behind that intersection depending on how the frame is constructed and what the headset stack height is.
Then you also need to control for seat tube angle headtube angle, frame construction method (ie. How far the top of the head tube protrudes from the top tube head tube intersection) and headset stack height. And only then you get a comparison value that gets you the length from an arbitrary point along the seatpost (not the seat, mind you!) to the place where you can start building the cockpit.
Dang that was a confusing writeup. But that's ETT for you.
So with reach you get the distance from the BB directly to the place where you start building the cockpit. At the most you'll need to account for the spacer stack. But you'd need to do that with ETT too.
So if you compare frames with the same stack but varying reach, each 10mm difference in reach corresponds directly to a 10mm change in stem length. No matter how the frames are constructed, what the headset stack height is, what the seat tube angle is etc.
On the other hand using ETT doesn't make any sense for a multitude of reasons. Firstly, you need to correct for seat tube angle. That "add or subtract 1cm" thing doesn't work, or at least it's not accurate. Personally I like to be within a millimeter when considering a frame.
Also depending on manufacturer ETT is either to top of head tube (sorta makes sense) or to middle of top tube / head tube intersection. The latter is just completely useless, since the top of the headset can be several centimeters above and a few cm behind that intersection depending on how the frame is constructed and what the headset stack height is.
Then you also need to control for seat tube angle headtube angle, frame construction method (ie. How far the top of the head tube protrudes from the top tube head tube intersection) and headset stack height. And only then you get a comparison value that gets you the length from an arbitrary point along the seatpost (not the seat, mind you!) to the place where you can start building the cockpit.
Dang that was a confusing writeup. But that's ETT for you.
In reality, the fact that the headtube angles back at 17 degrees means that moving the stem up or down rarely requires changing stem length, because our reach extends as we bend down and retracts as we sit up. Treating reach as distance to a vertical line ignores how bikes are fit and used, and complicates every calculation from there. Stems, spacers, saddle height are all measured at that same angle.
01-02-24, 08:08 AM
#28
Senior Member
In reality, the fact that the headtube angles back at 17 degrees means that moving the stem up or down rarely requires changing stem length, because our reach extends as we bend down and retracts as we sit up. Treating reach as distance to a vertical line ignores how bikes are fit and used, and complicates every calculation from there. Stems, spacers, saddle height are all measured at that same angle.
Reach tells you on thing. How far the point where you start to build your cockpit from is from the bottom bracket. It eliminates everything that happens behind the BB as that is irrelevant when considering frame length (with the assumption that the STA is within normal parameters). Reach doesn't work in isolation. It requires stack to work. Luckily stack is also as simple a measurement as reach. But to make ETT work as well as reach works with stack, you need to factor in fork length, fork rake, head tube length, headset stack height and head tube angle
01-02-24, 08:25 AM
#29
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I have no idea what you're trying to say here.
How bikes are fitted and how raising the stem relates to the rider sitting up or down doesn't aid you in comparing frame sizes. The effect raising or lowering the stem has on reach retracting or extending also depends on torso angle and a multitude of other factors I'd not want to consider when I'm choosing a bike.
Reach tells you on thing. How far the point where you start to build your cockpit from is from the bottom bracket. It eliminates everything that happens behind the BB as that is irrelevant when considering frame length (with the assumption that the STA is within normal parameters). Reach doesn't work in isolation. It requires stack to work. Luckily stack is also as simple a measurement as reach. But to make ETT work as well as reach works with stack, you need to factor in fork length, fork rake, head tube length, headset stack height and head tube angle
How bikes are fitted and how raising the stem relates to the rider sitting up or down doesn't aid you in comparing frame sizes. The effect raising or lowering the stem has on reach retracting or extending also depends on torso angle and a multitude of other factors I'd not want to consider when I'm choosing a bike.
Reach tells you on thing. How far the point where you start to build your cockpit from is from the bottom bracket. It eliminates everything that happens behind the BB as that is irrelevant when considering frame length (with the assumption that the STA is within normal parameters). Reach doesn't work in isolation. It requires stack to work. Luckily stack is also as simple a measurement as reach. But to make ETT work as well as reach works with stack, you need to factor in fork length, fork rake, head tube length, headset stack height and head tube angle
01-02-24, 11:37 AM
#30
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01-02-24, 06:21 PM
#31
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01-02-24, 09:32 PM
#32
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No no, he has a point. Your comment was really unclear. Now that I get what you mean, it only applies if the STA and HTA are the same. But even then you end up with a value that's basically reach and the distance to a random point of the seatpost. When it could just be reach. You eliminate everything that's behind the BB.
I am also maybe beginning to understand why you'd think raising the stem has no effect, but it's a guessing game at this point so I'm not sure. But on that point, the rider is already locked on the seat position. Raising the stem effectively brings the cockpit closer. If you have steerer to spare, reach as a bb bound value allows you to play with the effect of raising the stem. You then get a value that's basically effective reach.
Now if I have a frame that's a bit on the low and long side, but have that long uncut steerer and a manufacturer that has no qualms about high spacer stacks, I can raise the stem to bring it closer to my already locked in place butt. A 60mm spacer stack brings the stem back a whopping 20mm.
This of course requires knowledge about what works for me, or the rider choosing the bike. But for a beginner number values are useless anyways.
If you stick with ETT the value and if the STA and HTA are the same the value remains the same even if you try to use some kind of effectice ETT. So the reach to the bars with that value remains the same. It's not exactly helpful.
i had to check just for kicks and the ETT values on bikes seem to be all over the place just like I remembered. Let's take a bike that's pretty much perfect for me (Bianchi Impulso). It has a reach of 392mm and ETT of 595mm. If I compare it to Ritchey road logic, I get a an ETT of 590mm, which seems fine but also a reach that's 409mm, which is waaay too long. If I'd use ETT I'd be riding bikes that are just not the right fit for me.
I am also maybe beginning to understand why you'd think raising the stem has no effect, but it's a guessing game at this point so I'm not sure. But on that point, the rider is already locked on the seat position. Raising the stem effectively brings the cockpit closer. If you have steerer to spare, reach as a bb bound value allows you to play with the effect of raising the stem. You then get a value that's basically effective reach.
Now if I have a frame that's a bit on the low and long side, but have that long uncut steerer and a manufacturer that has no qualms about high spacer stacks, I can raise the stem to bring it closer to my already locked in place butt. A 60mm spacer stack brings the stem back a whopping 20mm.
This of course requires knowledge about what works for me, or the rider choosing the bike. But for a beginner number values are useless anyways.
If you stick with ETT the value and if the STA and HTA are the same the value remains the same even if you try to use some kind of effectice ETT. So the reach to the bars with that value remains the same. It's not exactly helpful.
i had to check just for kicks and the ETT values on bikes seem to be all over the place just like I remembered. Let's take a bike that's pretty much perfect for me (Bianchi Impulso). It has a reach of 392mm and ETT of 595mm. If I compare it to Ritchey road logic, I get a an ETT of 590mm, which seems fine but also a reach that's 409mm, which is waaay too long. If I'd use ETT I'd be riding bikes that are just not the right fit for me.
01-02-24, 11:54 PM
#33
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No no, he has a point. Your comment was really unclear. Now that I get what you mean, it only applies if the STA and HTA are the same. But even then you end up with a value that's basically reach and the distance to a random point of the seatpost. When it could just be reach. You eliminate everything that's behind the BB.
I am also maybe beginning to understand why you'd think raising the stem has no effect, but it's a guessing game at this point so I'm not sure. But on that point, the rider is already locked on the seat position. Raising the stem effectively brings the cockpit closer. If you have steerer to spare, reach as a bb bound value allows you to play with the effect of raising the stem. You then get a value that's basically effective reach.
Now if I have a frame that's a bit on the low and long side, but have that long uncut steerer and a manufacturer that has no qualms about high spacer stacks, I can raise the stem to bring it closer to my already locked in place butt. A 60mm spacer stack brings the stem back a whopping 20mm.
This of course requires knowledge about what works for me, or the rider choosing the bike. But for a beginner number values are useless anyways.
If you stick with ETT the value and if the STA and HTA are the same the value remains the same even if you try to use some kind of effectice ETT. So the reach to the bars with that value remains the same. It's not exactly helpful.
i had to check just for kicks and the ETT values on bikes seem to be all over the place just like I remembered. Let's take a bike that's pretty much perfect for me (Bianchi Impulso). It has a reach of 392mm and ETT of 595mm. If I compare it to Ritchey road logic, I get a an ETT of 590mm, which seems fine but also a reach that's 409mm, which is waaay too long. If I'd use ETT I'd be riding bikes that are just not the right fit for me.
I am also maybe beginning to understand why you'd think raising the stem has no effect, but it's a guessing game at this point so I'm not sure. But on that point, the rider is already locked on the seat position. Raising the stem effectively brings the cockpit closer. If you have steerer to spare, reach as a bb bound value allows you to play with the effect of raising the stem. You then get a value that's basically effective reach.
Now if I have a frame that's a bit on the low and long side, but have that long uncut steerer and a manufacturer that has no qualms about high spacer stacks, I can raise the stem to bring it closer to my already locked in place butt. A 60mm spacer stack brings the stem back a whopping 20mm.
This of course requires knowledge about what works for me, or the rider choosing the bike. But for a beginner number values are useless anyways.
If you stick with ETT the value and if the STA and HTA are the same the value remains the same even if you try to use some kind of effectice ETT. So the reach to the bars with that value remains the same. It's not exactly helpful.
i had to check just for kicks and the ETT values on bikes seem to be all over the place just like I remembered. Let's take a bike that's pretty much perfect for me (Bianchi Impulso). It has a reach of 392mm and ETT of 595mm. If I compare it to Ritchey road logic, I get a an ETT of 590mm, which seems fine but also a reach that's 409mm, which is waaay too long. If I'd use ETT I'd be riding bikes that are just not the right fit for me.
Those two. bikes with 5mm different ETTs have thw same reach to the steerer tube line in space. The reason their Reach are different is becauae of where on the steerer they are measured, because every 60mm of stack takes off 20mm of reach and they have different stacks. But we don't ride at stack.
You dont attach your stem at the stack/reach point. You attach it at a particular height on the steerer, so knowing where that steerer line is proves more useful.
And no, the ETT difference from having a few degrees difference between STA and HTA doea not change the math appreciably within the range of the stem height.
Last edited by Kontact; 01-02-24 at 11:58 PM.
01-03-24, 01:47 AM
#34
Senior Member
You are so close to getting this!
Those two. bikes with 5mm different ETTs have thw same reach to the steerer tube line in space. The reason their Reach are different is becauae of where on the steerer they are measured, because every 60mm of stack takes off 20mm of reach and they have different stacks. But we don't ride at stack.
You dont attach your stem at the stack/reach point. You attach it at a particular height on the steerer, so knowing where that steerer line is proves more useful.
And no, the ETT difference from having a few degrees difference between STA and HTA doea not change the math appreciably within the range of the stem height.
Those two. bikes with 5mm different ETTs have thw same reach to the steerer tube line in space. The reason their Reach are different is becauae of where on the steerer they are measured, because every 60mm of stack takes off 20mm of reach and they have different stacks. But we don't ride at stack.
You dont attach your stem at the stack/reach point. You attach it at a particular height on the steerer, so knowing where that steerer line is proves more useful.
And no, the ETT difference from having a few degrees difference between STA and HTA doea not change the math appreciably within the range of the stem height.
The steeper STA pushes a larger fraction of the TT over the BB line, effectively making the reach longer. This wouldn't matter if I didin't have a preferred position to mount my seat at. While said point is somewhat malleable, it's not 20mm malleable.
So the seat is at its usual distance from the BB using a setback post and pushing the seat back on its rails. Now I'm left with an on paper short ETT and a cockip that's receding to the horizon, because the moment I mounted my seat where it needs to be, the ETT became useless as the STA has effectively changed for me.
You mentioned that instead of raw ETT with whatever STA people should use a 73 degree normalized STA. But I'm left to wonder how that is actually better than reach, which completely eliminates what'a behind the BB and pretty accurately tells you how long the frame is.
For reach I have one number. 400mm. I don't want to go over that with road bikes. With ETT I can't have that same quick to lookup number because I have to factor in the effect STA might have AND the point where ETT is measured IF STA and HTA aren't identical.
If I want to know how tall a frame is I again have one number. 630mm. I don't want to go below that. Without stack I'd need to factor in headtube length, fork length (which isn't often available), fork rake and headset stack height.
01-03-24, 07:35 AM
#35
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You're actually wrong. The difference in reach is due to the difference in STA. I had to go and check after I posted my last message and I actually knew it, but had forgotten since I haven't used ETT in so long.
The steeper STA pushes a larger fraction of the TT over the BB line, effectively making the reach longer. This wouldn't matter if I didin't have a preferred position to mount my seat at. While said point is somewhat malleable, it's not 20mm malleable.
So the seat is at its usual distance from the BB using a setback post and pushing the seat back on its rails. Now I'm left with an on paper short ETT and a cockip that's receding to the horizon, because the moment I mounted my seat where it needs to be, the ETT became useless as the STA has effectively changed for me.
You mentioned that instead of raw ETT with whatever STA people should use a 73 degree normalized STA. But I'm left to wonder how that is actually better than reach, which completely eliminates what'a behind the BB and pretty accurately tells you how long the frame is.
For reach I have one number. 400mm. I don't want to go over that with road bikes. With ETT I can't have that same quick to lookup number because I have to factor in the effect STA might have AND the point where ETT is measured IF STA and HTA aren't identical.
If I want to know how tall a frame is I again have one number. 630mm. I don't want to go below that. Without stack I'd need to factor in headtube length, fork length (which isn't often available), fork rake and headset stack height.
The steeper STA pushes a larger fraction of the TT over the BB line, effectively making the reach longer. This wouldn't matter if I didin't have a preferred position to mount my seat at. While said point is somewhat malleable, it's not 20mm malleable.
So the seat is at its usual distance from the BB using a setback post and pushing the seat back on its rails. Now I'm left with an on paper short ETT and a cockip that's receding to the horizon, because the moment I mounted my seat where it needs to be, the ETT became useless as the STA has effectively changed for me.
You mentioned that instead of raw ETT with whatever STA people should use a 73 degree normalized STA. But I'm left to wonder how that is actually better than reach, which completely eliminates what'a behind the BB and pretty accurately tells you how long the frame is.
For reach I have one number. 400mm. I don't want to go over that with road bikes. With ETT I can't have that same quick to lookup number because I have to factor in the effect STA might have AND the point where ETT is measured IF STA and HTA aren't identical.
If I want to know how tall a frame is I again have one number. 630mm. I don't want to go below that. Without stack I'd need to factor in headtube length, fork length (which isn't often available), fork rake and headset stack height.
If two bikes have a stack/reach of 572/389 and 552/383 - how similar are they, and how much will the stem length need to change? The answer is they would use the same stem length for the same bar height because they both have ETTs of 564. So you know that the reach is the same, the stem is the same, and the only difference between them is where you clamp the stem vertically on the steerer. This is immediately obvious, while with stack and reach number you have to subtract 3mm of reach for every addition of 10mm of stack - and then know what height your stem is going to be mounted at. With ETT it doesn't matter what height you mount the stem since the seat tube to steerer tube relationship is fixed.
And the ETT rule of adding or subtracting 1cm of TT per degree of STA difference is easy and accurate. On a 50cm bike the difference between 74 and 73 STAs is 1.1cm, and on a 60cm is it 1cm - so pretty much the whole normal range of road bike sizes work with an unimportant variation of only 1mm. Variations in HTA are similarly trivial.
So a technique that allows you to quickly compare two bikes without any real math or knowing your stem stack that still yields stem length is more useful to me.
