Reach vs ETT?

[Dave]

An online calculator will convert TT length to reach. After that, life is much simpler.

[unterhausen]

Bikecad will tell you both, I'm just really unmotivated. I should see if there is a newer version to d/l.

[NHAero]

I think there is key guidance in how to think about this in Mark's post (#3).
- ETT is less useful when you start with the same saddle setback on each frame.
- Reach needs to be assessed based on where the stem sits vertically, because if one aims for the same fit/contact points then more spacers are needed on the smaller frame, which as Mark points out reduces reach if spacers are used to achieve the same stack. So Mark's example shows that when achieving the same stack on the two frames, the reach difference is not 5mm but rather 12.7mm.

If one isn't concerned about style points it’s not hard to make the smaller frame fit if it doesn’t require excessive spacers (safety-wise) to do so. I have an upward-angled stem with some spacers to get the right fit on my 54cm Supersix Evo Hi-mod. Its stack is IIRC 40mm lower than my custom Bingham but the contact points are identical. The Bingham looks nicer with a stem parallel to the TT slope, but when I'm riding either I don't notice

[marciero]

When you start flipping stems you can change reach pretty dramatically, as well as stack. Nomadmax touched on this. You need a longer stem to achieve the same reach. For example, a 100mm, -17 degree stem with HTA 73 degrees would be horizontal (taking the convention that a stem perpendicular to the head tube is 0 degrees, and stem angle measured negative downward.) If you flip that your stem angle is 17 degrees and you would need stem of length over 120mm to achieve the same reach (100*cosec(HTA - stem angle). On the other hand, with the same flipped 100mm stem the contribution to the reach decreases from 100mm to about 83mm, (100*sin(HTA-stem angle)).

Unless your stems are horizontal, changing stem length or angle changes both stack and reach. In the OP's case the, to precisely determine the number of spacers and stem length achieve the same stack and reach with the two frames you would need to know the stem angle. In fact with the three variables- spacers, stem length, stem angle- you have an underdetermined system so you could make choices - fix any one of them for example. Of course stock stems only come in a few different angles, but I went through all of this when I had a custom stem made.

[whatshubdoc]

Use this tool. It is the best one I've come across.

Caveat: You need to know your fit, down to the millimeter, to have a solid baseline.

I approximated your saddle height based on the bike size, and arbitrarily used 90mm as your saddle setback. Saddle selection is Specialized Power (length and midpoint).

It looks like between the two bikes, assuming the larger one uses 20mm spacers, you will need to use 43mm of spacers on the smaller bike, and add 14mm to the stem, in order to have identical fit. The numbers we are trying to match here are the saddle center to bar center (694mm) and the saddle to bar drop (21mm). Both numbers are located near the top of the schematics.

What you want to preserve is the saddle position relative to the BB, and then adjust the front end to let your hands rest at the target position. Your questions of "what else do you do": Since there is no such thing as a 114mm stem, you can run a 110mm stem with a 5mm longer reach bar, or a 120mm stem with a 5mm shorter reach bar, so that the hoods arrive at the same position in space.

Then you just make the selection based on HTA and wheelbase to match your riding style and intended purpose.

Given the partial geo chart, I also assumed 80mm BB drop and 380mm fork length, amongst other assumptions.

The first picture shows that because of the +0.5mm HTA angle difference, as you "go up", you "gain" ~8mm of top tube length going from a 148mm head tube to a 172mm head tube. Add that to the reach difference of 5mm and you'll get your 13mm TT difference. Purple > Yellow > Green.
(Conversely, if you compare two frames with identical stack/reach/HTA, but with a 1 deg STA, you'll find that the slacker STA bike will have ~10mm longer TT for every -1* difference. ie. 72 deg vs 73 deg STA, 72 bike will have 10mm longer ETT.)

Second pic is your first bike.

Third pic is the smaller of the two bikes.

Anyway, all those geo charts suck because they do not take into account headset stack, and steerer length, which ultimately determines where your stem will sit in the x/y plane. Comparing stack/reach/TT without factoring in steerer length assumes you are riding slammed on all bikes. Also, with the HTA difference, you can see how half a degree can throw you off. I use this tool for ALL my bikes and ANY potential bike purchase.

Hope this helps.

Bike 1: https://www.bikegeocalc.com/#6Bike+1...2F172.5G30H30Z

Bike 2:
https://www.bikegeocalc.com/#6Bike+2...4F172.5G30H30Z

[slowpoke]

Originally Posted by whatshubdoc

Anyway, all those geo charts suck because they do not take into account headset stack, and steerer length, which ultimately determines where your stem will sit in the x/y plane. Comparing stack/reach/TT without factoring in steerer length assumes you are riding slammed on all bikes.

In defense of stack and reach, they're a good baseline for frame-to-frame comparisons because all the other stuff is variable. I don't fault manufacturers for only sharing that.

Now it would be nice for these fit web sites to allow you to add on steerer length, stem length, stem angle to see where the handlebars would sit (now toss in backsweep, rise...).

And keep in mind, a carbon fork will also limit one to a 30-40mm max height of spacers.

[NHAero]

Originally Posted by whatshubdoc

Use this tool. It is the best one I've come across.

Caveat: You need to know your fit, down to the millimeter, to have a solid baseline.

I approximated your saddle height based on the bike size, and arbitrarily used 90mm as your saddle setback. Saddle selection is Specialized Power (length and midpoint).

It looks like between the two bikes, assuming the larger one uses 20mm spacers, you will need to use 43mm of spacers on the smaller bike, and add 14mm to the stem, in order to have identical fit. The numbers we are trying to match here are the saddle center to bar center (694mm) and the saddle to bar drop (21mm). Both numbers are located near the top of the schematics.

What you want to preserve is the saddle position relative to the BB, and then adjust the front end to let your hands rest at the target position. Your questions of "what else do you do": Since there is no such thing as a 114mm stem, you can run a 110mm stem with a 5mm longer reach bar, or a 120mm stem with a 5mm shorter reach bar, so that the hoods arrive at the same position in space.

Then you just make the selection based on HTA and wheelbase to match your riding style and intended purpose.

Given the partial geo chart, I also assumed 80mm BB drop and 380mm fork length, amongst other assumptions.

The first picture shows that because of the +0.5mm HTA angle difference, as you "go up", you "gain" ~8mm of top tube length going from a 148mm head tube to a 172mm head tube. Add that to the reach difference of 5mm and you'll get your 13mm TT difference. Purple > Yellow > Green.
(Conversely, if you compare two frames with identical stack/reach/HTA, but with a 1 deg STA, you'll find that the slacker STA bike will have ~10mm longer TT for every -1* difference. ie. 72 deg vs 73 deg STA, 72 bike will have 10mm longer ETT.)

Second pic is your first bike.

Third pic is the smaller of the two bikes.

Anyway, all those geo charts suck because they do not take into account headset stack, and steerer length, which ultimately determines where your stem will sit in the x/y plane. Comparing stack/reach/TT without factoring in steerer length assumes you are riding slammed on all bikes. Also, with the HTA difference, you can see how half a degree can throw you off. I use this tool for ALL my bikes and ANY potential bike purchase.

Hope this helps.

Bike 1: https://www.bikegeocalc.com/#6Bike+1...2F172.5G30H30Z

Bike 2:
https://www.bikegeocalc.com/#6Bike+2...4F172.5G30H30Z

Great post!
Bar reach is another variable, as you point out. When I am working to duplicate the same fit I measure to center of the hoods from the saddle nose, once I have the saddle set correctly. I use the same saddle on all four drop bar bikes, but not the same handlebars (but I would if I could now, but too much work to change, and one is 26.0 OD, the others are 31,.8.)

[e-RICHIE]

Originally Posted by NHAero

Great post!
Bar reach is another variable, as you point out. When I am working to duplicate the same fit I measure to center of the hoods from the saddle nose, once I have the saddle set correctly. I use the same saddle on all four drop bar bikes, but not the same handlebars (but I would if I could now, but too much work to change, and one is 26.0 OD, the others are 31,.8.)

i think one is better served duplicating a position going the opposite direction than you are describing. saddle height, and then fore-aft, determine your pedal stroke. from there, measuring forward to a place of your choice (i use the stem clamp) to dial in “reach” makes more sense. to me, at least.

and while i’m posting on a thread about riding position and possibly transferring it from one bicycle to another, it’s important, to me at least, to acknowledge that the same position (contact points) on one may not work well on another because everything else on the another is fixed. in other words, you can’t change the center of gravity. or how far apart the wheels are. or the front center.

ps

arrange disorder

[BdaGhisallo]

Originally Posted by whatshubdoc

with a 1 deg STA, you'll find that the slacker STA bike will have ~10mm longer TT for every -1* difference. ie. 72 deg vs 73 deg STA, 72 bike will have 10mm longer ETT.)

Isn't the Effective Top Tube the length of the top tube forward of a vertical line through the bottom bracket?

If you go from a 73 STA to a 72 STA, and the total length of the top tube doesn't change, you will have effectively shortened the ETT by about 1cm because the setback has increased, pulling more of the given top tube length behind the bottom bracket.

[NHAero]

Originally Posted by e-RICHIE

i think one is better served duplicating a position going the opposite direction than you are describing. saddle height, and then fore-aft, determine your pedal stroke. from there, measuring forward to a place of your choice (i use the stem clamp) to dial in “reach” makes more sense. to me, at least.

and while i’m posting on a thread about riding position and possibly transferring it from one bicycle to another, it’s important, to me at least, to acknowledge that the same position (contact points) on one may not work well on another because everything else on the another is fixed. in other words, you can’t change the center of gravity. or how far apart the wheels are. or the front center.

ps

arrange disorder

I think that we're saying the same thing as far as duplicating fit beginning with the saddle placement. I set the saddle height and setback, then the nose of the saddle is the reference (assuming the same saddle) to the hoods, because the hoods is where my hands are, they aren't at the stem clamp. This recognizes that bar reach is part of the chain of dimensional characteristics that relate to where one's butt is relative to were one's hands are.

What you're saying about the other characteristics of the frame design is true, but for me at least, the best starting point on a new-to-me bike is duplicating my previous contact points. It may be dumb luck that duplicating my contact points carefully on all four of my bikes seems to work for me, for reasons I may not understand. The two most disparate bikes are my 1972 Bob Jackson and 2016 Cannondale Supersix Evo Hi-Mod. To the first approximation I wonder if my center of gravity is similar because the Jackson is too big for me and it was made to accept 27x1-1/4 tires plus fenders, so the chainstay length and the front center are each about 30mm longer than on the Supersix. HTA is virtually the same; trail is 8mm more on the Supersix. I run a 60mm horizontal stem on the Jackson to compensate for the too-long TT, and a 100mm +17 stem on the Supersix to get the hoods in the same place. I've had both bikes under me for long days, with lots of steep ups and downs, and they handle fine. I can imagine a case where this isn't true but I'm guessing it might be further out of what is considered "normal" bike set up. I've been told that my short stem doesn't put enough weight on the front wheel, but I've done some math to look at this and a couple of cm in change in center of gravity changes this by a pound or two. It seems to the casual observer that the differences in % weight on the front wheel between a 63cm frame for a big person and a 48cm frame for a small person would be much more than I'll experience with a 2cm variation in stem length, but maybe I'm wrong.

[NHAero]

Originally Posted by BdaGhisallo

Isn't the Effective Top Tube the length of the top tube forward of a vertical line through the bottom bracket? Snip

No, that's Reach.
ETT is a horizontal line between the centerline of the head tube (usually taken at the intersection of the HT with the TT junction) and the centerline of the seat tube or extension thereof (seat post). Look at the drawings posted in Post #20.

[e-RICHIE]

My error.

I missed this key part of your other comment:

once I have the saddle set correctly.

As you were!

Originally Posted by NHAero

I think that we're saying the same thing as far as duplicating fit beginning with the saddle placement. I set the saddle height and setback, then the nose of the saddle is the reference (assuming the same saddle) to the hoods, because the hoods is where my hands are, they aren't at the stem clamp. This recognizes that bar reach is part of the chain of dimensional characteristics that relate to where one's butt is relative to were one's hands are.

What you're saying about the other characteristics of the frame design is true, but for me at least, the best starting point on a new-to-me bike is duplicating my previous contact points. It may be dumb luck that duplicating my contact points carefully on all four of my bikes seems to work for me, for reasons I may not understand. The two most disparate bikes are my 1972 Bob Jackson and 2016 Cannondale Supersix Evo Hi-Mod. To the first approximation I wonder if my center of gravity is similar because the Jackson is too big for me and it was made to accept 27x1-1/4 tires plus fenders, so the chainstay length and the front center are each about 30mm longer than on the Supersix. HTA is virtually the same; trail is 8mm more on the Supersix. I run a 60mm horizontal stem on the Jackson to compensate for the too-long TT, and a 100mm +17 stem on the Supersix to get the hoods in the same place. I've had both bikes under me for long days, with lots of steep ups and downs, and they handle fine. I can imagine a case where this isn't true but I'm guessing it might be further out of what is considered "normal" bike set up. I've been told that my short stem doesn't put enough weight on the front wheel, but I've done some math to look at this and a couple of cm in change in center of gravity changes this by a pound or two. It seems to the casual observer that the differences in % weight on the front wheel between a 63cm frame for a big person and a 48cm frame for a small person would be much more than I'll experience with a 2cm variation in stem length, but maybe I'm wrong.