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Old 09-07-2017, 09:20 AM
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Seat tube angle, reach and performance

being a short person I'm always searching for the frame with a short reach, around 360mm. on a custom built frame I'm able to do this by adjusting the seat tube angle and reducing the top tube length to less than 52cm. on a recent Lynskey I purchased the top tube is shorter the 51 and the seat tube angle is near 74 or so. While looking at Seven's geometry page SSP I noticed the same thing, the shortening the top tube length in order to get to the 36cm reach. How does seat tube angle affect performance? on frames with 74.5 -75 STA I've had to use posts with lots of setback, Easton ec70, fsa . on the customs any post would work. any idea why? opinions?
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Old 09-07-2017, 09:46 AM
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It's just basic geometry.
A steeper seat tube angle will bring the saddle forward, so you would have to compensate with more seatpost setback. It doesn't affect the ride.

I have relatively short legs, so I like a steeper seat tube angle (~74) because it gets the saddle where I want it with a setback post. That said, I could ride a 72sa with a straight post and not feel any different as long as the top tube is lengthened appropriately.
My frame fit strategy these days is:
1. will the saddle be in the right spot?
2. are the stack and reach measurements in the ballpark?
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Old 09-07-2017, 06:57 PM
Peter P. Peter P. is offline
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First you have to prove that a steeper seat angle is what you need.

Let's use the knee over the pedal spindle (KOPS) as a starting point/reference.

I'd want to determine whether your seatpost of choice (setback/zero setback) will provide you with a KOPS position with the saddle centered on the rails. That will give you sufficient adjustment fore/aft to deviate from KOPS for whatever reason.

THEN I'd set the reach.

Literature I've read from Lennard Zinn and Cervelo suggests shorter riders are merely scaled down versions of taller riders, and a 73 degree seat angle works in most cases. The reason manufacturers use steeper angles in shorter frames is to meet CPSC requirements of no toe overlap. So they steepen the seat angle and shy away from shorter top tubes to meet the requirements.

You could avoid that problem with a custom frame if the builder can still meet your fit specs, or go to a 650c wheel size.
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Old 09-07-2017, 10:40 PM
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the closer the STA is to 73 the better KOPS positioning. with a 73 STA i can use most seatposts with some setback but on anything with 74 or more and i'll need a lot of setback. fuji, bianchi, colnago, specialized, cannondale all need lots of setback. the cervelo i have doesn't need much. this is what made me notice STA and reach. Most makers chose steeper 74+ STA and more reach in the smaller sizes. not sure why.
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Old 09-07-2017, 11:53 PM
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Originally Posted by cmg View Post
Most makers chose steeper 74+ STA and more reach in the smaller sizes. not sure why.
short femurs. long toes.

Steep STA shortens the ETT and appeals to the people who don't get stack/reach.

Compensate for short reach (toe overlap) by slacking the HTA and you get some sluggish trail and a shorter ETT. Compensate for sluggish trail with more rake and you get more wheel flop.

Also there's this whole thing about mass producing bicycles to a standard so that people who turn the handlebars instead of leaning while having the wrong foot forward don't lose teeth . . . because too many dentists already own nice bikes.

ISO 4210-2: 4.13.2.2

Bicycles shall have at least C clearance between the pedal and front tyre or mudguard (when turned to any position). The clearance shall be measured forward and parallel to the longitudinal axis of the bicycle from the centre of either pedal axle to the arc swept by the tyre or mudguard, whichever results in the least clearance (see Figure 5). The values are given in Table 11.

W/out foot retention:
City and trekking bicycles 100
Young adult bicycles 89
Mountain bicycles 100
Racing bicycles 100

w/ foot retention:
89mm for all.
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Old 09-08-2017, 02:37 AM
macaroon macaroon is offline
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Ignore KOPS and femur length and whatever else.

Seat tube angles are steeper on smaller frames because the people who ride them are smaller! They don't need as much setback to get a comfortable, balanced position.

Short legs/long torso should mean MORE setback/a slacker seat tube, not less. Otherwise you won't have space to stretch out into. And likewise, longer legs/short torso will probably mean you want a steeper seat tube.

It depends on the bike and the fit though; a head down race bike and you'll probably want the saddle further forward than a more relaxed sportive machine.
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Old 09-08-2017, 11:59 AM
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Originally Posted by macaroon View Post
Short legs/long torso should mean MORE setback/a slacker seat tube, not less. Otherwise you won't have space to stretch out into. And likewise, longer legs/short torso will probably mean you want a steeper seat tube.

What? No.
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Old 09-11-2017, 03:23 PM
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Originally Posted by Peter P. View Post
Literature I've read from Lennard Zinn and Cervelo suggests shorter riders are merely scaled down versions of taller riders, and a 73 degree seat angle works in most cases.
Due respect to Zinn, I still distrust what a tall person might generalize about short people.

Maybe there is data behind this... it better not start with accepting a fugly zero-setback post.
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Old 09-12-2017, 10:44 PM
giordana93 giordana93 is offline
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Originally Posted by sparky33 View Post
Due respect to Zinn, I still distrust what a tall person might generalize about short people.

Maybe there is data behind this... it better not start with accepting a fugly zero-setback post.
agreed. there is a reason for the steeper sta for smaller bikes and it is what was stated above. my own data set, from personal fit and quite a few folks I know, shows that it is a rare, rare bird that rides a 52 and needs 73 sta. I thought I was one (fairly long femurs, a long torso to cantileverover the tt) and gave it a shot....nope. and a 75 is too steep, so 74 seems best in my case. ymmv of course

and for short legs long torso (me) more setback with slacker seat tube does not give more reach; it is the exact opposite. this comes up in discussions pretty often, but again is basic bike fit geometry. a 52 cm top tube on a steep bike will ride LONGER than on a slacker angle, given the same rider setback. do the math, make yourself a drawing, read another one of the countless threads we've had on it. putting the starting point of the top tube back a cm or so on the slacker st means it ends a cm or so sooner. it will fit shorter & have less room to stretch out, period. which doesn't mean it won't work for some riders. but cervelo's line about everyone needing 73 sta no matter their size was a nice theory but more marketing bs in the real world. I've seen many jacked up fits because of it, and to put it gently, they are not known as the best handling bikes out there

so here I go, spending too much time on this, but I often refer people to dave moulton on this stuff. I guess there needs to be a grain of salt, evolving hand positions with modern equipment, etc, but the following are good starting points:


http://davesbikeblog.squarespace.com...for-women.html


http://davesbikeblog.squarespace.com...me-sizing.html
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Old 09-14-2017, 03:48 PM
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The funny thing about Zinn is that as he goes up to XXL, XXXL and 4XL sizes he offers a slacker seat tube. IIRC down to 71.5*

XL = 73*
XXL = 72.5*
3XL = 72*
4XL = 71.5*

But when you go small you don't need to steepen the STA?
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Old 09-15-2017, 09:41 AM
weiwentg weiwentg is offline
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Quote:
Originally Posted by Peter P. View Post
...
Literature I've read from Lennard Zinn and Cervelo suggests shorter riders are merely scaled down versions of taller riders, and a 73 degree seat angle works in most cases....
The way you phrased this is likely putting people off. I think you mean that the literature suggests that there's no evidence that the average leg/torso proportions differ by height. So, all else equal, 73 degrees might sit right at the middle of the distribution of leg/torso proportions, so it would fit the most riders across the height spectrum. If the literature is correct, that is. That is something I could believe.

Personally, when I got into riding, I found myself sliding all the way forward on the saddle all the time. I had a small Giant TCR with a 74 STA and a Thomson post, and I put my saddle all the way forward to get comfortable. So, clearly I prefer riding quite far forward. I now have a 76 STA, no setback post, saddle in the middle. This is pretty comfortable for me. So, I'm on the short side of the height distribution, and I'm simultaneously on the short leg side of the distribution. So, you can say that 73 STA will work for most people, and I can believe you, but I can still believe that it doesn't quite do it for me.
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Old 09-15-2017, 01:24 PM
Bonesbrigade Bonesbrigade is offline
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Originally Posted by weiwentg View Post
The way you phrased this is likely putting people off. I think you mean that the literature suggests that there's no evidence that the average leg/torso proportions differ by height. So, all else equal, 73 degrees might sit right at the middle of the distribution of leg/torso proportions, so it would fit the most riders across the height spectrum. If the literature is correct, that is. That is something I could believe.

Personally, when I got into riding, I found myself sliding all the way forward on the saddle all the time. I had a small Giant TCR with a 74 STA and a Thomson post, and I put my saddle all the way forward to get comfortable. So, clearly I prefer riding quite far forward. I now have a 76 STA, no setback post, saddle in the middle. This is pretty comfortable for me. So, I'm on the short side of the height distribution, and I'm simultaneously on the short leg side of the distribution. So, you can say that 73 STA will work for most people, and I can believe you, but I can still believe that it doesn't quite do it for me.
Wow, that sounds pretty far forward. I'm curious what your saddle height is? and how far your tip of saddle is behind (or forward!) of the BB?

Also, is this a setup you use for a road bike?
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Old 09-15-2017, 02:10 PM
weiwentg weiwentg is offline
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Originally Posted by Bonesbrigade View Post
Wow, that sounds pretty far forward. I'm curious what your saddle height is? and how far your tip of saddle is behind (or forward!) of the BB?

Also, is this a setup you use for a road bike?
Yep, it's a road bike! Saddle height is 640mm (center of BB to top of saddle), and I'm not sure how far the saddle tip is behind the BB! The original fit diagram shows it right in line with the BB. Is that normal? Abnormal? Highly abnormal?
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Old 09-15-2017, 05:08 PM
OtayBW OtayBW is offline
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Originally Posted by weiwentg View Post
Yep, it's a road bike! Saddle height is 640mm (center of BB to top of saddle), and I'm not sure how far the saddle tip is behind the BB! The original fit diagram shows it right in line with the BB. Is that normal? Abnormal? Highly abnormal?
I think maybe it's a little Abby Normal....https://i.imgur.com/cZISZjC.gif
Actually, most folks that I know of tend to have at least a few cm of setback.
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Last edited by OtayBW; 09-15-2017 at 05:11 PM.
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Old 09-15-2017, 06:03 PM
Bonesbrigade Bonesbrigade is offline
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Originally Posted by weiwentg View Post
Yep, it's a road bike! Saddle height is 640mm (center of BB to top of saddle), and I'm not sure how far the saddle tip is behind the BB! The original fit diagram shows it right in line with the BB. Is that normal? Abnormal? Highly abnormal?
Setback is proportional to saddle height in general. People that run higher saddles tend to have more setback. Your saddle is quite low, so you likely prefer less setback. I'm not tall - 5'8 and I run approx 6cm of setback with a normal length saddle model (selle Italia slr). That's pretty average for my height.

It sounds like you are on less than normal setback, but if that's what you feel comfortable then all is good. Have you ever experimented with more setback? If you do experiment, make sure you lower your saddle height to match your preferred leg extension. You're also going to want to shorten your stem and bar height proportionately if you want to make a true comparison.

Last edited by Bonesbrigade; 09-15-2017 at 06:19 PM.
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