Mountain Bike Reviews Forum banner

1 - 20 of 21 Posts

·
Registered
Joined
·
101 Posts
Discussion Starter #1
i want to verify my custom frame seat tube angle, anyone knows how can i do that?
thank you
 

·
Registered
Joined
·
183 Posts
hang a string from the top of the seat tube, better yet a plumb bob. Use a protractor, or a ruler and do the math.
 

·
BOOM goes the dynamite!
Occasionally Cranky
Joined
·
4,868 Posts
hang a string from the top of the seat tube, better yet a plumb bob. Use a protractor, or a ruler and do the math.
Smartphones also have apps (or built in capability) to measure angles to avoid the plumb bob/ruler.
 

·
Registered
Joined
·
101 Posts
Discussion Starter #4
thank you my friend, i need more instruction please.. what do you mean by hang? i need to hold the plumb bob align with the seat tube no? i found this attached photo on the internet, what does it mean there : "frame size point" what does that mean? thank you
 

Attachments

·
Registered
Joined
·
33 Posts
Depends on the tools you have available.

If you have a protractor, then yes, you can just let the string hang vertical (with the bike level) then measure the difference. Depending on if you want the actual angle, or effective will dictate the point you measure to.

If all you have is a tape measure, the screenshot you posted will also work. You're basically just making a triangle in that case. If you know the lengths of the legs, it's just simple trigonometry to get the angle.

In both cases, you'll need to subtract your result from 90 to get the measurement we use for bikes, which is off of the ground.

Hope that helps / makes sense.
 

·
Registered
Joined
·
33 Posts
Also, we talking about a road bike here? I think the "frame size point" may be referring to the top tube length measurement point. Just my guess
 

·
Registered
Joined
·
101 Posts
Discussion Starter #9
Depends on the tools you have available.

If you have a protractor, then yes, you can just let the string hang vertical (with the bike level) then measure the difference. Depending on if you want the actual angle, or effective will dictate the point you measure to.

If all you have is a tape measure, the screenshot you posted will also work. You're basically just making a triangle in that case. If you know the lengths of the legs, it's just simple trigonometry to get the angle.

In both cases, you'll need to subtract your result from 90 to get the measurement we use for bikes, which is off of the ground.

Hope that helps / makes sense.
thank you ben, to be honest-it doesent make sense to me- i dont know the basics.. ive tried to find video online but didnt find nothing..
from where im holding the plumb line? the middle of the seat tube?
what does it mean by triangle? the lengths of which legs? sorry
 

·
Registered
Joined
·
101 Posts
Discussion Starter #10
Also, we talking about a road bike here? I think the "frame size point" may be referring to the top tube length measurement point. Just my guess
no, its my travel bike, custom build, i feel that the angle is steeper then what we agreed on..
 

·
Formerly of Kent
Joined
·
12,197 Posts
If you don’t understand how to measure an angle, how is the information gained by doing so going to help you?


Sent from my iPhone using Tapatalk
 

·
Registered
Joined
·
19 Posts

Put a drywall square on the ground, line it's edge up with the center of the BB (this ensures that there's less error due to uneven ground). Select some point up from the center of the BB (e.g. 20cm). Now measure over to the center of the seat tube. This gives you 2 sides of a triangle and an angle (90 degrees)

Plug those into the triangle calculator:
Side B = 20cm
Angle C = 90
Side A = 6.12cm

That gives you
Angle B = ~73 deg
Side C = ~ 21cm
Angle A = ~17 deg


If you use an angle finder or plumb bob there's a possibility of error because your floor might not be level. Seeing as you're worried about STA as opposed to being able to put your saddle in the right place, I'm guessing you want to be as accurate as possible (this isn't a criticism)
 

·
Registered
Joined
·
101 Posts
Discussion Starter #14

Put a drywall square on the ground, line it's edge up with the center of the BB (this ensures that there's less error due to uneven ground). Select some point up from the center of the BB (e.g. 20cm). Now measure over to the center of the seat tube. This gives you 2 sides of a triangle and an angle (90 degrees)

Plug those into the triangle calculator:
Side B = 20cm
Angle C = 90
Side A = 6.12cm

That gives you
Angle B = ~73 deg
Side C = ~ 21cm
Angle A = ~17 deg


If you use an angle finder or plumb bob there's a possibility of error because your floor might not be level. Seeing as you're worried about STA as opposed to being able to put your saddle in the right place, I'm guessing you want to be as accurate as possible (this isn't a criticism)
thank you!
i didnt understand the rule of the drywall square, are you talking about this one?'
' https://www.amazon.com/Swanson-Tool-AD124-Adjustable-Drywall/dp/B000IOGBKG'

and, i measure from the center of the BB to the center of the seat tube and what more?
 

·
Registered
Joined
·
629 Posts
Op, you are going to have a very hard time accurately measuring your seat tube angle. What makes you think the seat tube angle is different than agreed upon? Sliding even a minimalist seat all the way forward or back on the rails can change the effective seat tube angle 1.5 degrees. Is your definition of seat tube angle the same as the builder of your frame?

Most importantly, have you spoken with your builder about this?


Sent from my iPhone using Tapatalk
 

·
Registered
Joined
·
101 Posts
Discussion Starter #16
Op, you are going to have a very hard time accurately measuring your seat tube angle. What makes you think the seat tube angle is different than agreed upon? Sliding even a minimalist seat all the way forward or back on the rails can change the effective seat tube angle 1.5 degrees. Is your definition of seat tube angle the same as the builder of your frame?

Most importantly, have you spoken with your builder about this?


Sent from my iPhone using Tapatalk
yes we have agreed on 76, its seems to me that it is 78(using a app in the phone on level ground)
yes we have spoke about that, i had to use 30mm setback and put the seat all the way back, its not ideal espically when its a custom frame.
 

·
Registered
Joined
·
178 Posts
For a frame already built, I use two ways.

First, assuming that your front and rear wheels/tires are the same diameter, same inflation, and that you don't have any suspension settings to mess things up, you can use an angle finder. I have one that's accurate to 0.1 degrees. You put it flat on the ground, zero it, then stick it on the seat tube. It will give you the angle. I have analog ones that are accurate to +/- 0.5 degrees. I don't know how accurate your phone sensor is, but it sounds like you are trying to do this with your phone.

The second method is to take a photo at the appropriate perspective and use ImageJ to measure the seat tube angle. I have used ImageJ to measure frame angles many times and it gives results that are pretty accurate if you do it right.
 

·
Registered
Joined
·
968 Posts
Angle finders only measure actual not effective. Effective is what you are after. Is there a bend in the ST?

I tried using image apps and between getting the phone completely level and the bike completely level and then marking "exactly" the right points, there was a lot of margin for error.

I have tried to use an app to measure it and I could easily get + or - 2 degrees just with super minor adjustments.

When you say you had to use a 30mm setback and push the seat all the way back..what are you feeling that made you have to do that? What is your experience like with 76 degree seat tube angles?
 

·
Registered
Joined
·
629 Posts
yes we have agreed on 76, its seems to me that it is 78(using a app in the phone on level ground)
yes we have spoke about that, i had to use 30mm setback and put the seat all the way back, its not ideal espically when its a custom frame.
Using an app on your phone is not accurate


Sent from my iPhone using Tapatalk
 

·
Registered
Joined
·
101 Posts
Discussion Starter #20
Angle finders only measure actual not effective. Effective is what you are after. Is there a bend in the ST?

I tried using image apps and between getting the phone completely level and the bike completely level and then marking "exactly" the right points, there was a lot of margin for error.

I have tried to use an app to measure it and I could easily get + or - 2 degrees just with super minor adjustments.

When you say you had to use a 30mm setback and push the seat all the way back..what are you feeling that made you have to do that? What is your experience like with 76 degree seat tube angles?
no, no bend at the seat tube.
my experience with the this seat angle is that im leaning to far forward, i feel more comfortable be far back and shortening the stem.
 
1 - 20 of 21 Posts
Top