Reach vs ETT?

[Nomadmax]

Pictured below is a comparison of the same frame in two different sizes. The "reach" figure is 5mm shorter on the M as compared to the M/L. Seat tube angles are the same @ 73.5. The HTA on the M is 72 and 72.5 on the M/L.

My question is, if reach is only 5mm shorter on the M v/s the M/L, how does one reconcile the 13mm difference of ETT? Using the same saddle height/setback, do you increase stem length by 5mm, 13mm or something else to get the same measurements (saddle tip to bars, hoods & drops)?

I should mention there's 25mm of stack difference, my questions are based on the idea that stack difference will be made up with 25mm of spacers.

[marciero]

I am only seeing the actual top tube lengths. When you take into account the different tt slopes the effective tt lengths are about 557.5 and 546.3.

[Mark McM]

ETT (Effective Top Tube) is measured parallel to the ground from the top of the head tube to a axis of the seat tube. Another way to look at it is that the ETT equals the frame Reach (horizontal length from head tube to BB) plus the frame Setback (horizontal length from BB to seat tube axis). If the frame stack of the M/L is 25mm higher, the the frame Setback increases by 25mm x cotangent( 73.5 ) = 7.4mm. Combined with the 5mm greater Reach, the ETT would increase by 7.4mm + 5mm = 12.4mm.

But ETT has no direct effect on stem length, because saddle setback is adjusted independently of frame Setback and ETT - as the frame Setback changes, we adjust the saddle fore and back on its rails so that the saddle is in the same horizontal position behind the BB. So changes in stem length are only determined by the Stack and Reach.

We can look at adding stem spacers as changing the Effective Stack of the frame. Because head tube angles backward, increasing the Effective Stack with spacers will also decrease the Effective Reach. The change in Effective Stack and Effective Reach will be the spacer height times the sine of head angle, and the spacer height times the cosine of the head angle, respectively. So for a head angle of 72 degrees if you added 25mm of spacers, the effective Stack of the frame would increase by 25mm x sine( 72 ) = 23.8mm, and the effective Reach would decrease by 25mm x cosine( 72 ) = 7.7mm. So adding 25mm of spacers would change the effective Stack and Reach of the M frame from 565mm/375mm to 588.8mm/367.3mm. So for the same fit on the M/L frame (Stack/Reach = 590/380), you'd need a stem that was 12.7mm shorter.

[marciero]

also keep in mind that reach is measured horizontally forward of the bb. The portion of the tt that doesnt count because it is behind the bb is approx 174.8 and 167.4 in each case. So taking effective top tubes minus portion that doesnt count gives you 382.7 - 377.1 = 5.6. This puts you in the ballpark of the stated difference. There is also the portion of the head tube above the tt which affects things. With different ht angles this will be different but very minimally so. More of course if you add spacers but not as far as the stated measurements

Edit: Mark Mcm beat me to some of this

[RoosterCogset]

What was said already, and a link to a calculator that yields some of the effective numbers being discussed:

https://www.bikegeo.net/

[Dave]

I've purchased a lot of frames in the last 6 years. I only look at three things - stack, reach and seat tube angle. Seat tube angle is just to be sure that I can get the saddle in my desired position relative to the BB with the seat post setback that's available. If a frame has a proprietary seat post with only 15mm of setback, it needs a more slack STA to make up for it. My latest Cervelo came with a zero setback seatpost, but it was a common 27.2 mm diameter. The STA was not nearly relaxed enough to justify that seatpost. Dealers won't swap the post for a 25mm setback either. I bought a 25mm setback post and sold the original.

I just look for a reach of around 370-375mm and stack of 525mm. Stem angle can also affect my selection of stack height. The lowest I've brought was 505mm, with 20mm of headset top cover and spacer, with a -7 degree stem angle on provided integrated handlebars. Another bike had a 383mm reach, so it needed a 100mm stem.

A common mistake is comparing the reach of two frames with different stack heights. If one frame has a 20mm shorter stack, subtract 6mm from the reach, assuming you'll put 20mm of spacer on it to get the same stack height.

My latest bike had some limitations. The smallest frame had a 524mm stack, but there's only one stem angle of -8 degrees on a proprietary stem. I settled for a little less saddle to bar drop. 6 years ago I had a -17 stem with that stack. Might turn out to be a good thing at age 71.

[avalonracing]

I. Hate. Math.
I'm glad some people like it so we can have nice things.

[Smitty2k1]

https://bikeinsights.com/cyclopedia/top-tube-length

[slowpoke]

Pretty sure OP is looking at a Bike Insights comparison.

I'm surprised there's no effective top-tube length. Is this a "stock" geo, or some custom numbers you've thrown in?

[Nomadmax]

I had to wait until morning to respond, after I've had my coffee I'm always impressed with the mental horsepower lurking around here. The numbers on bike insights are stock Litespeed numbers, nothing custom. After reading, and re-reading these replies I understand now, it's all inter-related. Change stack with spacers, reach is affected (or is it effected?).

In the end, I'd rather have my optimum fit with a frame/fork that requires a 100mm stem and stack to spare as I get older, as opposed to a smaller frame that requires a stack of spacers and a 120mm stem from the beginning. More importantly, with nowhere to go but flipping the stem as the years add up. Even then, if it takes a 120mm stem in the -6 position to make it work from the start, it'll take an even longer one when it's in the + position.

I often wonder how many people are stuck between frame sizes.

[Tandem Rider]

Originally Posted by Nomadmax

I had to wait until morning to respond, after I've had my coffee I'm always impressed with the mental horsepower lurking around here. The numbers on bike insights are stock Litespeed numbers, nothing custom. After reading, and re-reading these replies I understand now, it's all inter-related. Change stack with spacers, reach is affected (or is it effected?).

In the end, I'd rather have my optimum fit with a frame/fork that requires a 100mm stem and stack to spare as I get older, as opposed to a smaller frame that requires a stack of spacers and a 120mm stem from the beginning. More importantly, with nowhere to go but flipping the stem as the years add up. Even then, if it takes a 120mm stem in the -6 position to make it work from the start, it'll take an even longer one when it's in the + position.

I often wonder how many people are stuck between frame sizes.

Why is there an assumption that aging means a position change? This is a real question. I'm in my 60s and so far, my road position hasn't moved a single mm since the 80s. I would buy the bike that fits now and worry about later when it get here.

[Nomadmax]

Originally Posted by Tandem Rider

Why is there an assumption that aging means a position change? This is a real question. I'm in my 60s and so far, my road position hasn't moved a single mm since the 80s. I would buy the bike that fits now and worry about later when it get here.

Because my tolerance for saddle bar drop decreases about every 4-5 years and I intend to ride into my 80s? I'm 65 now and because my position from the 80s HAS changed.

[Dave]

I lowered my bars by 20mm in my mid 50's to get a 10cm saddle to bar drop and even more when I came back to cycling at age 65. I decided not to slam my new Cervelo. It's got a 7mm headset top cover and two 5mm spacers, buy that's only 2mm more than what used to be the standard 15mm headset top cover. The total stack is 541 which is 16mm more than previous bikes. It's the smallest frame size. The bike came with 50mm above the headset top bearing. Flipping the -8 stem would raise the bars by about 30mm.

I rode my newest bike on a couple of 50 mile rides with a stem that was 20mm shorter and 30mm higher. It really wasn't a problem. That makes me think that flipping a stem is really not a big deal. My regular fit is just more aerodynamic.

[RoosterCogset]

The older you get, the more you want to lay down.

[unterhausen]

According to some people, I always rode with my bars a little higher than they should have been, but I always used stems that were a little longer than they should have been as well, so it probably averaged out. But I'm pretty sure my bars are too high now, I just haven't gotten around to removing spacers.

Someday, I'll figure out what reach works well for me, but for now I have to cut the top tube to the length I want, so knowing reach is not as useful as ETT.