ETT Calculation

[bjf]

I have a frame for sale, and a prospective buyer wants to know the ETT. The frame's not built, but everything else is known -- HTA, STA, actual TT, slope in degrees, etc. There must be a way to calculate the ETT. Anyone know what it is?

[ergott]

I have bikecad. If you give me as much detail as you can I can calculate it to the best of the program's abilities

[joosttx]

I have a frame for sale, and a prospective buyer wants to know the ETT. The frame's not built, but everything else is known -- HTA, STA, actual TT, slope in degrees, etc. There must be a way to calculate the ETT. Anyone know what it is?

summon Pythagorean. He will tell you the answer.

A^2 + B^2 =C^2

Or just measure the leveled distance between where the top tube meets head tube and seatpost.

[Black Dog]

I have a frame for sale, and a prospective buyer wants to know the ETT. The frame's not built, but everything else is known -- HTA, STA, actual TT, slope in degrees, etc. There must be a way to calculate the ETT. Anyone know what it is?

Put two wheels on the bike and an old seat post. Then you can measure directly. As long as you have a headset in there you don't need a stem bars etc to make the measurement.

[bjf]

Put two wheels on the bike and an old seat post. Then you can measure directly. As long as you have a headset in there you don't need a stem bars etc to make the measurement.

No headset.

[ergott]

ETT is generally a c-c measurement stemming from the middle of headtube and top tube junction. I can be tough to measure better than +-1cm accuracy. A separate set of hands does help things. Have to avoid parallax when looking at rule.

[cmg]

try using cos *(slope in degrees) = b/c, b is ett and c is the actual top tube length, put in what you know and manipulate/solve for the unknown. what is the unknown and the board will calculate? it will look something like cos 09=52.5/c or c=52.5/cos 09

[ergott]

I'm getting about 53.5cm. Fork length isn't mentioned so I had to estimate based on the slope of TT.

[bjf]

try using cos *(slope in degrees) = b/c, b is ett and c is the actual top tube length, put in what you know and manipulate/solve for the unknown. what is the unknown and the board will calculate? it will look something like cos 09=52.5/c or c=52.5/cos 09

Doesn't the STA matter also?

[cmg]

Doesn't the STA matter also?

no need to . Cos A= adjacent/hypotenuse the A is your slope number.

[bjf]

no need to . Cos A= adjacent/hypotenuse the A is your slope number.

I don't get it. First, angle C is not 90 degrees on a bike. Second, this bike has a headtube that extends above the top tube, so the ETT doesn't meet the point of measurement for the actual top tube.

[Kontact]

I don't get it. First, angle C is not 90 degrees on a bike. Second, this bike has a headtube that extends above the top tube, so the ETT doesn't meet the point of measurement for the actual top tube.

You are correct, it isn't 90°. But you can break any triangle down into a pair of right triangles to make calculation of legs easy since the numbers you do know aren't relative but off of horizontal.


But I don't understand how you have the slope of the TT available but not the ETT. Geo charts always have ETT but almost never have TT slope. Where are your numbers coming from? But if you want to just post the numbers you have I'll do the math.

Another way to level the frame to measure the ETT directly is to use the BB drop. Put the center of the rear dropout the correct number of mm above a horizontal line running through the center of the BB.

[bjf]

You are correct, it isn't 90°. But you can break any triangle down into a pair of right triangles to make calculation of legs easy since the numbers you do know aren't relative but off of horizontal.


But I don't understand how you have the slope of the TT available but not the ETT. Geo charts always have ETT but almost never have TT slope. Where are your numbers coming from? But if you want to just post the numbers you have I'll do the math.

Another way to level the frame to measure the ETT directly is to use the BB drop. Put the center of the rear dropout the correct number of mm above a horizontal line running through the center of the BB.

Here's the build sheet. It seems to have actual TT -- because it says "c-c" -- but not ETT. If you can figure it out, let me know.

[Peter P.]

bjf-have you ACTUALLY MEASURED the on-axis top tube length and compared it to the 53.4cm on the spec sheet?

I would expect most builders to list the ETT rather than an on-axis length, as the latter is meaningless to the customer.

[Kontact]

bjf-have you ACTUALLY MEASURED the on-axis top tube length and compared it to the 53.4cm on the spec sheet?

I would expect most builders to list the ETT rather than an on-axis length, as the latter is meaningless to the customer.

Exactly. If the 53.4 is a sloping TT number, you can confirm that by measuring it directly. And if it is, the ETT is 55.3.

But if the ETT is actually 53.4, then you'll find the sloping TT measures out at 51.5.

So you can confirm the ETT by measuring the real TT and confirming one or the other.



This is a useful tool for stuff like this:
http://www.calculator.net/triangle-calculator.html?vc=&vx=53.4&vy=&va=8&vz=&vb=72&angleunits=d&x=75&y=25

I put in 72°, 8° and 53.4 as two different vertices to get the answers.

[bjf]

Exactly. If the 53.4 is a sloping TT number, you can confirm that by measuring it directly. And if it is, the ETT is 55.3.

But if the ETT is actually 53.4, then you'll find the sloping TT measures out at 51.5.

So you can confirm the ETT by measuring the real TT and confirming one or the other.



This is a useful tool for stuff like this:
http://www.calculator.net/triangle-calculator.html?vc=&vx=53.4&vy=&va=8&vz=&vb=72&angleunits=d&x=75&y=25

I put in 72°, 8° and 53.4 as two different vertices to get the answers.

I get 51.8 (approximately), so the listed value IS the ETT. Thanks; I hope this guy buys it after all your trouble.

[cmg]

i get 52.8. cos 8*= b/53.4 A is your slope, Intersection of center lines head tube & top tube, C is the actual top tube length , b is the ETT

or take a string, tape it to the headtube at the center line intersection of the headtube and top tube, then put seat post in the frame pull the string horizontal tape to seat post and measure.

[Kontact]

i get 52.8. take a string, tape it to the headtube at the center line intersection of the headtube and top tube, then put seat post in the fram pull the string horizontal tape to seat post and measure.

From what? If you take a triangle that is 72° and 8° in two corners and has one leg that is 53.4, how could you end up with 52.8?

[cmg]

i get 52.8. cos 8*= b/53.4 A is your slope, Intersection of center lines head tube & top tube, C is the actual top tube length , b is the ETT

or take a string, tape it to the headtube at the center line intersection of the headtube and top tube, then put seat post in the frame pull the string horizontal tape to seat post and measure.




I'm wrong,,,

[cmg]

this should be amusing. next guess 55.4 with chicken scratch :):)

[ergott]

Wait, have to fix something

[ergott]

Wait, you sure that 53.5 isn't the level dimension? I'd ask Zank if he listed actual or effective in that sheet.

[ergott]

This checks out pretty well.

https://photos.smugmug.com/Other/Randoms/i-5JQ87Dr/0/6fb91620/O/Zanconato.jpg

[cmg]

i thought the 53.4 was the sloping tube dimension? 3rd guess, ett is 55.2 after correcting for seat tube angle.

[ergott]

Not sure, but you can't just change that without effecting front center and everything else, so what I have jives pretty good with the rest of the dimensions give or take a tenth.

[Kontact]

I'm wrong,,,

You forgot to add the base of the second triangle back in, so you only had part of the length you were looking for.


But if you don't have someone set on Ignore, all the answers are in the preceding posts. The ETT is 53.4.

[bjf]

We can close this thread, unless you number guys want to continue the debate. I told the buyer that the 53.4 is the ETT, which I think has to be right because the actual TT measures 51.8 or so. More importantly, he bought it, so it's going to a new home. Thanks for the help, though.

[ergott]

the actual TT measures 51.8 or so.

That jives with with I have.

[Mark McM]

It sounds like you've managed to come up with the answer, but since I'm a math nerd, I'd just like to point out that the most direct way to calculate the Effective TT length from the Actual TT length plus the angles is to use the Law of Sines (https://en.wikipedia.org/wiki/Law_of_sines).

http://www.mathwarehouse.com/trigonometry/law-of-sines/images/formula-picture-law-of-sines2.png



Using the Law of Sines, the actual TT length is calculated to be 51.76 mm.

[pdmtong]

I thought that llewellyun slant six lugs were 6d slope - so how can the zanc be 8d slope?

[Kontact]

It sounds like you've managed to come up with the answer, but since I'm a math nerd, I'd just like to point out that the most direct way to calculate the Effective TT length from the Actual TT length plus the angles is to use the Law of Sines (https://en.wikipedia.org/wiki/Law_of_sines).

http://www.mathwarehouse.com/trigonometry/law-of-sines/images/formula-picture-law-of-sines2.png



Using the Law of Sines, the actual TT length is calculated to be 51.76 mm.
This tool, which I'm sure uses the law of sines, gets 51.57.

http://www.calculator.net/triangle-calculator.html?vc=72&vx=&vy=53.4&va=&vz=&vb=8&angleunits=d&x=74&y=21
Why do you think your number is different? Did you use 53.4, 8° and 72°?

Same with this one:
https://www.triangle-calculator.com/?what=asa&a1=8&c=53.4&b1=72&submit=Solve

[Kontact]

I thought that llewellyun slant six lugs were 6d slope - so how can the zanc be 8d slope?

Lugs are normally bent by framebuilders to achieve the angles necessary - up to 3° for Henry James cast lugs:

http://www.henryjames.com/bike-tech-info/faqs.html#ic2

[Mark McM]

This tool, which I'm sure uses the law of sines, gets 51.57.

http://www.calculator.net/triangle-calculator.html?vc=72&vx=&vy=53.4&va=&vz=&vb=8&angleunits=d&x=74&y=21
Why do you think your number is different? Did you use 53.4, 8° and 72°?

72 degrees is the head tube angle, which is irrelevant to the calculation of ETT. The correct angles to use are 8 degrees (TT slope) and 73.5 degrees (Seat Tube Angle). Plugging those into the calculator that you linked gives a TT of 51.77 mm.

[Kontact]

72 degrees is the head tube angle, which is irrelevant to the calculation of ETT. The correct angles to use are 8 degrees (TT slope) and 73.5 degrees (Seat Tube Angle). Plugging those into the calculator that you linked gives a TT of 51.77 mm.

Okay, I see what happened. ETT isn't always measured the same way. I used the method you see with a lot of tall head tubes, which is from the collar horizontally to wherever on the head tube. You used the top of the head tube to wherever it hits the seat post. Since the seat tube and head tube are converging at 1.5°, plugging in 53.4 up high makes the real TT length longer than using the same number down low.

Sometimes ETT is measured so it splits the TT, in which case the number would have come out 51.67.