Saddle setback compensation for different seat tube angle?

Discuss light weight issues concerning road bikes & parts.
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ak47
Posts: 345
Joined: Thu Aug 30, 2007 2:57 pm

by ak47

How should I count how much further back should I move my saddle when changing seat angles? What formula should I use to count this? Let's say my saddle is 67 cm in height from BB and the seat angle changes from 74 to 75,5 degrees, which means I have to move the saddle back to get it in the same position in relation to the BB.

I am thinking of getting a bike that has 75,5 degree seat angle and rigth now I use bike with 74 degree and a saddle moved fairly back on a 25 mm setback. I am woried it will be impossible to find good position on a 75,5 degree SA bike. Why do some manufacturers make so steep seat tube angles in small sizes? I think it's stupid.

cazone
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Joined: Thu Nov 17, 2011 10:32 pm

by cazone

Trigonometry.
The 1.5deg steeper seat tube will result in a 42/43mm setback (compared to your actual 25mm).
That's a lot.

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roadieboy
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by roadieboy

I ran into this quandary when I picked up a track bike, since I ride it on the roads, I had to compensate for the super steep ST angle, and set it up with the same effective setback as my road bike. I just pulled up the geometry charts of both frames, and used the pythagorean theorem to determine just how much further back on the rails I'd have to slide my saddle.

rijndael
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by rijndael

I use a laser level/plumb and have it intersect the BB, it makes it easy to measure the setback - just put a tape measure off the nose of the saddle. It's also easy to make it consistent between all of my bikes.

Valbrona
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by Valbrona

ak47 wrote:How should I count how much further back should I move my saddle when changing seat angles?


When changing position between bikes I have screwed a simple wooden jig together that uses the centre of the bottom bracket and nose of the saddle as reference points.

Valbrona
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by Valbrona

roadieboy wrote:I ran into this quandary when I picked up a track bike, since I ride it on the roads, I had to compensate for the super steep ST angle, and set it up with the same effective setback as my road bike.
.


And then there are people like me who would say that those two bikes quite possibly require two different set-ups ... depending on original set-up, the rider in question and which track discipline we are talking.

shimmeD
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by shimmeD

Back your bike against a wall (best if wall is straight and verticle, and floor is horizontal & level, and have your bike perpendicular to both wall & floor) and measure horizontal (if you need to, use a spirit level) distances between wall & centre of bottom bracket and wall & tip of saddle. Work out the difference.
Repeat on your new bike on the same spot and using the same saddle.
Adjust till you get the same result. Of course seat heights should both be the same.

Or if you want trigonometry use this site http://www.csgnetwork.com/righttricalc.html
74deg - 185mm
75.5deg - 168mm
move it back 17mm

I also find that steep seat-tube angles does not work for me (my seat height is similar to yours).
Last edited by shimmeD on Tue Oct 07, 2014 8:15 pm, edited 1 time in total.
Less is more.

maxxevv
Posts: 1957
Joined: Wed Feb 09, 2005 4:51 am

by maxxevv

For your 67cm seat height, every 0.5 degrees difference is approximately 5mm difference in setback.

So, for every 0.5 degree steeper, you'll be in the ballpark of moving your saddle setback backwards by ~5mm.

Marin
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by Marin

Or just set it up so that it feels right.

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ak47
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by ak47

Thanks guys for help. Still thinking 75,5 degrees SA is ridiculous.

shimmeD
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Location: eNZed

by shimmeD

10+ years ago, I could find bikes that would fit me viz relaxed seat tube angle and short head tube. They're beginning to make frames with shorter head tubes again but small ones still have steep seat tubes. You can't have proper seat setback without ridiculous seat post. I sympathise with you. I ride custom and still have my Look which I don't ride
Less is more.

eric
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by eric

75.5 is not common these days except on very small frames. Back in the 80s I'd see that on "criterium" bikes.
Now we just ride our regular road bikes in crits.

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Donkey
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Joined: Wed Sep 10, 2014 9:12 pm

by Donkey

75.5 is a common seat angle standard on most *(extra)-small bikes.

For small angles of the order of a few degrees the trig maps linearly. So a 1.5 change in degree approximates to around 15mm in offset.

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