ST/HT Angle Measurement

[keno]

Is there a way to measure ST and HT angles very accurately with a bicycle shop tool and without taking any components off?

keno

[dbrk]

Seat tube angle is relatively easy. Start with the set back number. Drop a plomb-bob from the top tube through the bb center. Measure from the point on the toptube back to the center of the seat lug. This is the setback. Measure the seattube center to center. Divide the seattube by the setback and take the inverse of the cosine. Voila, this produces the seattube angle. Easier with a scientific calculator unless you have your trig well under control!

Head tube angles are more mysterious to me.

dbrk

[Kevin]

Keno,

With a traditional frame you can do it with a digital level. I have a digital level made by Smart Tool. I ordered it through Amazon. Just set up the bike on level ground so that the TT is level and then place the digital level on the ST or the HT and the digital readout will be the angle. The digital level is great, you can also use it to level your saddle.

Kevin

[DfCas]

On a compact frame?Or must you level the bike,find the center of the TT on the seatpost,and then measure?

As far as head angle,if you know the rake,you should be able to measure trail and use on of the calculators online to figure out the head angle.

I have trail finder bookmarked somewhere and I'll try to find it
.
http://www.kreuzotter.de/english/elenk.htm

The hardest part for me is measuring.Three times and 3 different numbers....

Dan

[dbrk]

On a compact frame you need to measure level to the intersection...at least I think so and this will definitely work...to get the proper setback.
Any scientific calculator should give you the inverse of the cosine for the angle.

dbrk

[DfCas]

The inverse of the sine seems to do it.I get ~17.5,which subtracted from 90 gives a good number.

I found a scientific calculator in my computer!In windows XP,its under all programs,accessories,calculator,choose view or edit to option for scientific.

Thanks to dbrk for the help.

Dan

[Needs Help]

Divide the seattube by the setback and take the inverse of the cosine.

Hmmm...that's not what I'm seeing. Or, maybe I'm unable to correctly interpret what you're saying? Here's a picture:
http://www.pbase.com/image/26640322.jpg
The angle between the horizontal green line and the red seat tube is the ST angle--marked theta in the diagram(the circle with the horizontal line through it). But, theta is the same as the angle formed by the top tube and the seat tube--also marked theta in the diagram. If I remember my high school Trig correctly, that comes from a theorem that goes something like this: when you have two parallel lines and another line intersects them, the alternate interior angles are equivalent.

Since the cosine of an angle in a right triangle is the adjacent side over the hypotenuse

cos theta = a/b

If you measure a and b as dbrk instructed, then you can go to a cosine table like the one here:

http://wright.nasa.gov/airplane/tablcos.html

and look at the angle that corresponds to the number you came up with. Or, you can you can use the scientific calculator here:

http://www.creativearts.com/scientificcalculator/

and divide a by b, then click on the green INV button, and finally click on cos, which like the table will give you the angle whose cosine is a/b.

The head tube angle can be calculated in a similar fashion:
http://www.pbase.com/image/26640953.jpg
However, I think the error of making those measurements is going to be relatively high, so you won't necessarily come up with the same thing as your manufacturer. If you are going to try it, I would suggest putting a piece of scotch tape on the spot on the tubes you are going to be using as a reference point and drawing a small dot on the tape at the precise spot of the center of the tube juncture.

does the setback measurement work
On a compact frame?Or must you level the bike,find the center of the TT on the seatpost,and then measure?

You need to form a right triangle, so you need to measure horizontally from the vertical plumb bob line to the seat post and mark that spot. Then, measure from the spot on the seat post to the center of the BB.

[Too Tall]

Needs (no) Help, will you be my teacher? Not to get off topic but The simple diagrams are clean. What are you using? How very ungeekish of me not to know. The shame.

[Kevin]

I am surrounded by a bunch of frustrated engineers who were probably on the AV squad in Junior High. Speaking on behalf of the intellectually challenged, the digital level is much easier.

Kevin

[flydhest]

Needs Help,

Just because you're right doesn't give you any privileges on my geek turf. Look out, or else it's going to be slide rules at twenty paces at dawn :mad:

(muttering to himself) "darn, I gotta learn how to make those nifty pictures . . . of course, I would have used a lower case theta to denote an angle . . . "

[keno]

I went looking for my K&E Log Log Duplex Decitrig slide rule as I was inspired by some of the responses. But when it comes down to my reality, the digital level would be where it's at. How far is your Long Island from my New Jersey, Kevin? I'll leave the globe and the navigating instruments in the closet and go to map quest.

keno

BTW, my quest relates to HT angle, rake and trail and bike stability. Something isn't adding up as I compare my Serotta and Spectrum, the latter being more stable yet having numbers which would imply the contrary. One consideration would be that the HT angle on the Serotta might be different than given.

[Ken Robb]

I was wondering how to get the angles on my "Stelvio" this week. The owner of my LBS said he has devices to measure them so since I am math challenged I think I'll ride over to the shop--always a fun thing to do anyway. This bike looks like it could have a HA of 71 it is so much more laid back than my CSi or Rivs. It's also the easiest to ride no-hands.

[davep]

Needs Help,

Your diagrams and expanation are very clear and concise, and drawings are great. But, it is still an inverse cosine relationship. In your example, assume a/b = .3090. If you go to the table you reference and look up .3090 you will get 72 degrees. However, you have just looked up the inverse cosine of .3090. The cos 72 degrees = .3090, therefore the inverse cosine of .3090 = 72 degrees. Using a scientific calculator you would use the inverse cosine function. So Dbrk's formula should be, divide the setback by the seattube and take the inverse cosine.

I also agree that it would be very difficult to make these measurements with sufficient accuracy to assure an accurate answer.

And that's the first time I've thought about inverse trig functions in over 20 years.

[Needs Help]

Thanks for the kind words everyone.

Too Tall,

I did the diagrams in Photoshop Elements 2. They can be done in MS Paint too, which should come with most MS operating systems.

Kevin,

the digital level is much easier.

And, I imagine much more accurate. You might want to post how much they cost so that other people will know whether it's worth it to them.

flydhest,

Changes dully noted

davep,

I added a link to a scientific calculator as well.

[Kevin]

Needs Help,

The digital level cost about $90. The link is http://www.amazon.com/exec/obidos/tg/detail/-/B0000225AL/qid=1078533091/sr=1-3/ref=sr_1_3_etk-tools/104-6551461-6610328?v=glance&s=hi&n=228013

The ability to ensure a perfectly level saddle is priceless.

Kevin

[Needs Help]

Take a digital picture or photograph of your bike, and make the measurements on your image.

[Needs Help]

Originally posted by keno
BTW, my quest relates to HT angle, rake and trail and bike stability. Something isn't adding up as I compare my Serotta and Spectrum, the latter being more stable yet having numbers which would imply the contrary. One consideration would be that the HT angle on the Serotta might be different than given.

This short article on trail and rake and handling mentions several different tricks manufacturers use for "offsetting the hub forward". Maybe it will help?

http://www.phred.org/~josh/bike/trail.html

[keno]

Very nifty thought on the picture solution to get to angles. I'd need to be led through it, but very clever.

I had read the site you referred to and had concluded that a new fork would be the answer, if I am going to attempt to solve this problem. Each solution he provides requires a new fork, as best I can tell.

Thanks,

keno

[Needs Help]

See Dave Kirk's post on how weight distribution also affects handling:

http://forums.thepaceline.net/showthread.php?s=&threadid=1304&perpage=15&pagenumber=2