Measuring seat angle - correct tool?

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horizon
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Measuring seat angle - correct tool?

Postby horizon » 26 Aug 2017, 11:10pm

Does anyone know a simple method of measuring seat angle? I was looking at one of these:

http://www.screwfix.com/p/trend-digital ... UQodLK0OtA

Will it do the trick? Or is there something about measuring a seat angle that makes it difficult to be accurate (the difference is usually just one or two degrees)?
I have two doctors, my left leg and my right leg. (G. M. Trevelyan)
PS I always wondered why the YHA HQ was called Trevelyan House. :)

JohnW
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Re: Measuring seat angle - correct tool?

Postby JohnW » 26 Aug 2017, 11:52pm

horizon wrote:Does anyone know a simple method of measuring seat angle? I was looking at one of these:

http://www.screwfix.com/p/trend-digital ... UQodLK0OtA

Will it do the trick? Or is there something about measuring a seat angle that makes it difficult to be accurate (the difference is usually just one or two degrees)?


No - I don't think it will - that tool needs to get right into the angle, and that will be impeded by the lug, weld, stress-thickening. The tool I use is a draughtsman's adjustable set square.

https://www.cassart.co.uk/drawing/ruler ... set_square

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horizon
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Re: Measuring seat angle - correct tool?

Postby horizon » 27 Aug 2017, 12:11am

JohnW wrote:
No - I don't think it will - that tool needs to get right into the angle,


Which angle? The seat angle has AIUI two parameters - the seat tube and a theoretical horizontal line - there's nothing to put a tool into, that's the problem.
I have two doctors, my left leg and my right leg. (G. M. Trevelyan)
PS I always wondered why the YHA HQ was called Trevelyan House. :)

Brucey
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Re: Measuring seat angle - correct tool?

Postby Brucey » 27 Aug 2017, 12:24am

the seat angle you are really interested in is not the one vs the top tube (which can slope slightly even on frames that have nominally horizontal top tubes) but is that one that is vs a truly horizontal line in space. A problem with this is knowing when the bike is sat on a truly horizontal surface.

Fortunately this is accommodated by taking an average of two readings with the bike's wheels sat on the same spots whilst facing in opposite directions.

A simple way of finding the exact angle (with no special tools) is to dangle a plumb line from the top tube, so that the pointer is just touching the seat tube and then to do a little trigonometry on the resultant triangle. The greatest accuracy comes from a measurement of the shortest side of the triangle and the hypotenuse; divide the former by the latter and then use Inv Cos to get the seat angle directly.

An error of 1mm in measuring the length of the short side of the triangle will give ( for a ~20" or ~500mm hypotenuse at about 73 degrees) of ~0.12 degrees. So it is about 8mm per 1 degree. Even a fairly clumsy measurement gets the true seat angle within 0.5 degrees.

hth

cheers
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horizon
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Re: Measuring seat angle - correct tool?

Postby horizon » 27 Aug 2017, 12:28am

Brucey wrote:the seat angle you are really interested in is not the one vs the top tube (which can slope slightly even on frames that have nominally horizontal top tubes) but is that one that is vs a truly horizontal line in space. A problem with this is knowing when the bike is sat on a truly horizontal surface.


cheers


Yes, that's exactly what I meant. Which is why it seems a bit impossible to measure. I'm working through the rest of your post ... !
I have two doctors, my left leg and my right leg. (G. M. Trevelyan)
PS I always wondered why the YHA HQ was called Trevelyan House. :)

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breakwellmz
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Re: Measuring seat angle - correct tool?

Postby breakwellmz » 27 Aug 2017, 3:55am

You just need to use it with a spirit level. :roll:

tim-b
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Re: Measuring seat angle - correct tool?

Postby tim-b » 27 Aug 2017, 6:46am

Hi
It's difficult to do
Which two of a saddle's many contours do you place a straight edge on? Do they change when you're sitting on the saddle?
I have a known level piece of garage floor, a level line above that on the wall and I sight it by eye from a few metres. Facing the bike in both directions and sighting evens out any errors
Regards
tim-b
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breakwellmz
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Re: Measuring seat angle - correct tool?

Postby breakwellmz » 27 Aug 2017, 7:56am

The spirit level holds one arm of the device horizontal `in air` whist the other arm is put against the seat tube.
I was going to get one of the Lidl ones but they had sold out.I would normally use a piece of cardboard, spirit level, pen and protractor in the kitchen against the wall.
Keep it simple chaps!

alexnharvey
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Re: Measuring seat angle - correct tool?

Postby alexnharvey » 27 Aug 2017, 8:06am

Could you use a smart phone bubble level app? Measure the surface the bike is on, then the seat tube.

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531colin
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Re: Measuring seat angle - correct tool?

Postby 531colin » 27 Aug 2017, 8:09am

Simple?
Google "clinometer app"

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robgul
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Re: Measuring seat angle - correct tool?

Postby robgul » 27 Aug 2017, 10:19am

The method I use is :

A piece of MDF board, rectangular shape slightly bigger than the saddle area
Place the board on the saddle, by default for most saddles (especially Brooks) it will sit on the 3 points of the saddle
Spirit level on the board aligned with the toptube fore and aft

That'll give you the bubble which you can then set as level or at whatever minor variation that you might like.

Works for me - but some "razor blade" saddles may be too narrow across the rear end to get the board level

Rob

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horizon
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Re: Measuring seat angle - correct tool?

Postby horizon » 27 Aug 2017, 10:25am

Thanks for all the replies (I'm still digesting them but keep them coming!). I'm relieved that I hadn't missed the obvious and that suggestions are somewhat Heath Robinson. Whether I can achieve a result I am not sure yet.
I have two doctors, my left leg and my right leg. (G. M. Trevelyan)
PS I always wondered why the YHA HQ was called Trevelyan House. :)

reohn2
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Re: Measuring seat angle - correct tool?

Postby reohn2 » 27 Aug 2017, 11:02am

531colin wrote:Simple?
Google "clinometer app"

:wink:
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JohnW
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Re: Measuring seat angle - correct tool?

Postby JohnW » 27 Aug 2017, 4:26pm

horizon wrote:
JohnW wrote:
No - I don't think it will - that tool needs to get right into the angle,


Which angle? The seat angle has AIUI two parameters - the seat tube and a theoretical horizontal line - there's nothing to put a tool into, that's the problem.


Yes - quite so - I've only ever had traditional classic diamond-frames with horizontal top tubes - and I wasn't thinking 'out of the box'. The horizontal is fairly easy to ascertain using a ruler and a spirit level. Now I think about it, I think that I remember seeing something like the piece of equipment that you've illustrated which incorporated a spirit level in one leg. If I can search out and confirm that, I'll let you know

robc02
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Re: Measuring seat angle - correct tool?

Postby robc02 » 27 Aug 2017, 4:52pm

For years I used the plumb bob / bit of trig. method. Then I decided to make my own adjustable protractor - one edge placed against seat tube the other (top) edge levelled with a spirit level. I always conduct the measurement on the same bit of floor, that I know is virtually dead level.