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Old 09-07-2017, 10:20 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, 10:46 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, 07:57 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, 11:40 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 09-08-2017, 12:53 AM
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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, 03:37 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, 12:59 PM
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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 09-08-2017, 03:00 PM
Bonesbrigade Bonesbrigade is offline
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Quote:
Originally Posted by cmg View Post
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?
For my custom frame, we went with a rather steep 74.5 STA in order to get the front centre/HTA/Fork rake/BB drop numbers correct. It's not really a big deal, I'm 5'8 and like to have the tip of my saddle 6cm behind the BB - this requires me to use a setback post. No big deal IMO.

Edit: My stack and reach are 551mm and 385mm based on using a 90mm stem and 165mm cranks and a 5mm spacer under the stem.

Last edited by Bonesbrigade; 09-08-2017 at 03:03 PM.
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Old 09-08-2017, 03:48 PM
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"My stack and reach are 551mm and 385mm" How does this relate to what you would find on a 52-53cm bike frame? Is the reach/front center shorter on your custom?
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Old 09-08-2017, 06:53 PM
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Quote:
Originally Posted by cmg View Post
How does this relate to what you would find on a 52-53cm bike frame? Is the reach/front center shorter on your custom?
Bruh:
Most major MFGs now provide stack reach for their bikes. Move those fingers over to the Google Box and type something.

Reach:
Domane is 371mm for a 53 ETT
Tarmac is 380mm

Stack varies depending on the intention.
Venge + Madone will have lower stack than a Diverge or Domane Gravel.



FWIW, reach in the 360mm range sounds rather short and a recipe for toe overlap even with 165mm cranks and size 40 shoes (yeah, I made a drawing to check)

Even the smallest Tarmac has 375mm reach and with a ETT 508 and 63mm trail.
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Old 09-08-2017, 11:23 PM
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Originally Posted by cmg View Post
....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?
1. Reach is Reach. ETT is ETT. They might correspond but I bet I could get you to the same (short) TT length with 2 very different reach measurements.

2. STA. I think it does make a difference but maybe not in the way you're thinking. I think that having a seatpost with more setback reduces some road vibration and, on longer days, that's really nice. But the closest I've come to testing it is going between my Serotta + Ritchey -- the FT is almost the same but the RT has different tubing profiles.

3. Big brand bike designers aren't complete idiots and decent bike design has been around for years. So if you're using a seatpost with an abnormally large amount of setback then I'm guessing you're part mutant, not sitting on the bike right and/or on the wrong size bike.

But maybe I'm wrong.

Also, I've been coughing so I need to check WebMD to see if I have the black lung.
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Old 09-09-2017, 06:55 AM
Bonesbrigade Bonesbrigade is offline
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Quote:
Originally Posted by cmg View Post
"My stack and reach are 551mm and 385mm" How does this relate to what you would find on a 52-53cm bike frame? Is the reach/front center shorter on your custom?
My reach on my custom build is a bit longer compared to a typical 52cm that I'd normally ride. This was to create a little longer front centre - hence the 90mm stem speced. I normally use a 100 or 110mm stem for stock frames, depending on the reach of course.
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Old 09-11-2017, 04:23 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, 11:44 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, 04:48 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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