Help me understand ST angles

[bob59]

All my bikes fall into the 73-74 STA range and after looking in the classified here and others, 73-74 seems to be the norm.

However, I most recently almost bought a Kirk here after seeing the 54.5TT (my size) and thought I hit the lottery....however a pal here pm'ed me as we have swapped several bike over the last 20 years and are the same size and preferences, said to look at the STA = 72....I would have been disappointed if not caught by someone smarter than me as the tip of saddle to center of bar was + 2cm from my 54.5TT bikes with 73.5-74.0 STA, had to pass.

Now another bike that I like and numbers are in the ball park, but the STA = 75.5. What is the reason for such steep numbers on a clean sheet build??

How does this affect ride ability?

Thanks in advance!

[fa63]

For road bikes, some people require (or prefer) less setback to get their saddle where they want it, in which case a steeper STA is good (or even necessary). For others like people with long femurs, a shallow STA (plus a setback seatpost) might work better.

If I were designing a frame from scratch, I would want to design it such that it puts my saddle where I need it with a ~25mm setback seatpost for maximum comfort. Generally speaking, setback seatposts flex more than straight ones, though there are certainly straight post that are comfort oriented (like the Syntace P6 HiFlex). That would most likely give me an STA around 73.5 degrees.

For mountain bikes, it is a different story

[Dave]

These days, stack and reach determine the fit and the STA affects the seat post setback needed. With a 72 STA a zero offset post might produce the desired saddle setback from the center of the BB.

The small frames I ride usually have a 74.5 STA and the smallest a 75. My new Cervelo Rouvida only comes in 4 sizes, so I'm on a small with a 74 STA. They put a zero setback post on them, which is silly, with a tiny 0.5 degree change in the STA. I still need a 25mm setback, so I had to buy one and sell off the original post.

[MikeD]

Seat tube angle (road bike fit) always puzzled me. Some say that a slacker angle makes for a more comfortable ride. I think that's bull. IMO, the optimum seat tube angle is one where your saddle is at the proper fore/aft position, no setback post, center of the saddle rails. Of course, + or minus .5 degree from ideal probably doesn't matter if you can achieve the proper fore/aft position with a setback seat post. Of course saddles are not standardized to put the fore/aft position in the same place either. Some people don't believe in the KOPS method as well. YMMV.

[Peter P.]

Some builders and OEM frame designers believe thigh length decreases disproportionately with height decrease, necessitating an increase in seat angle so a rider can get their fore/aft into a reasonable position.

Others don't believe this (Zinn and Cervelo are two I can think of) and build their entire range with a 73 degree seat angle.

Gunnar and Ritchey on the other hand, design/ed with considerably steeper seat angles in their frame sizes.

But I also tend to believe seat angles have landed where they are when used with setback seatposts. Using a zero setback post changes adjustability considerably and even Speedvagen mentioned designing their road frame seat angle with a zero setback in mind.

To me, as long as there is enough fore/aft adjustment to get the saddle where you want it, most people can find a stem that results in the same fit despite a change in seat angle. It's just that the body gets used to a position and any deviation feels yucky.

[bob59]

All interesting.....I hate the aesthetics of seeing bikes with seats slammed forward, and less aft. I guess it's vanity, but have purged several nice bikes that a fitter made them work, but could not adjust to seeing the seat slammed in the forward position.....too each is on. Anyone else with thoughts?

The other aesthetic item for me is I don't like the looks of setback posts (espically Thomson).....if you are having a custom bike built, why would it be designed to require a setback post??

[Doug Fattic]

The seat tube angle is related to where a person's butt needs to be placed and that is determined by where their hands are positioned. The kind of bike and riding and body type determine what kind of handlebars are used and where it is positioned.

There are 2 kinds of frame design. One is for production purposes that many riders will try to accommodate to. The other is a custom design where a rider's seat, handlebar and pedal position will determine where the frame tubes belong. I'm a custom builder and fitter (going on 50 years now) and a custom design is all that I do (usually except when I designed our Dutch style Ukrainian bikes for our charity). So in theory I'm starting with a fresh piece of paper for every design. Expect that my fixture is what I design on instead of paper. Production designs are ruled by liability and as a result will have a steep seat angle to get rotating toes out of the way of a front wheel. That probably is not where the seat angle should be for a comfort fit but at least the company will not be sued. Tough luck about comfort.

Of the 3 elements that determine fit - aerodynamics, biomechanical efficiency and comfort, my customers/students favor comfort. People that have gained weight and lost flexibility as they age like to sit more upright. Ordinarily this means their butt needs to sit further back than what a typical 73º seat angle will allow. That is probably why the Kirk you passed had a 72º seat angle.

And if a bicycle uses upright swept back "north road" handlebars (like what are used on a typical 3 speed) the seat angle has to be even further back. Dutch bikes have very relaxed seat angles for this reason.

[echelon_john]

Steep seat tube angles on smaller sized production bikes are primarily to help address shoe/front wheel overlap that results from a proportionally short top tube.

Steepening the sta for a given tt length can result in a front center that’s 2-3 cm longer than it would be for the same tt length with a slacker sta, keeping overlap to a minimum.

Of course, this works for some people, but if you ride a 52cm frame but have long femurs for your height, it’s a fit challenge.


*edited to say I saw Doug’s post after I posted but we’re apparently in agreement ; )

[carpediemracing]

Seat tube angle places your saddle fore/aft.

Greg Lemond has very long quads, and his bike line reflects that - his STA are very slack compared to most other bike companies. If I was to make a similar line of bikes, my STA would be super steep, for short quad folks.

For me my ideal seat tube angle is where I don't have to scoot the saddle forward on a zero setback post. That translates (for my short quads, which are short even relative to my short legs) to a 75.5 deg STA for a zero setback post. My cleats are pushed all the way back on my shoes; if I moved them forward then even a 75.5 STA wouldn't be steep enough to place my saddle in a neutral position on the post.

With a 74.5 STA I have to slide the saddle forward on the post. I have to do this on my stock geometry track frame.

Set back is not determined by handlebar position. Instead, handlebar position should be determined by saddle position. BB -> Saddle -> Bars

With the steeper STA (75.5) than a regular production road frame, to get proper extension I need a 56.5 cm TT (probably longer actually) and a 14.5 cm stem to place my bars where I want them to be placed. I may be extending that slightly as I plan on going to narrower bars (to mirror my track bike bars), probably a 16.5 cm stem to go with the 33 cm bars. If I had my druthers I'd probably get a longer frame, maybe a 58-59cm TT.

Originally my frames were designed for regular bars and a 12 cm stem, but with compact bars I needed more reach and more drop (approx 3 cm each) to put the drops where they belong, so I ended up buying a few custom geometry stems.

[fourflys]

so this thread makes me wonder what is considered short or long femurs/quads?

[Mark McM]

Originally Posted by Peter P.

Some builders and OEM frame designers believe thigh length decreases disproportionately with height decrease, necessitating an increase in seat angle so a rider can get their fore/aft into a reasonable position.

Others don't believe this (Zinn and Cervelo are two I can think of) and build their entire range with a 73 degree seat angle.

I suspect that there is a bigger reason for the steeper ST angles on smaller frames and slacker seat tube angles on larger frames . I think it largely comes down to economies of scale of component manufacturers, or more specifically, crank manufacturers.

As we know, humans come in a wide variety of sizes (and leg lengths), but cranks come in only a narrow range of lengths. 68% of humans are between 5' 2" and 6' 1" (+/- 8%), and 95% of humans are between 4' 11' and 6' 4" (+/-13%), but cranks typically only range from 165mm to 180mm (+/-4%).
The shortest riders will be using cranks proportionally longer compared to their leg length than the tallest riders. Saddle setback is usually determined by the riders position/CG relative to the pedal on the power stroke (i.e. when they are applying their weigh to the pedal on the down stroke. Because shorter riders use cranks proportionately longer, their position will be proportionately further ahead relative to the BB than a taller rider who will be using cranks proportionally shorter for their cranks.

If both tall and short riders used cranks that were directly proportional to their leg lengths, there would be much less difference in seat angles between small and large frames.

(By the way, the idea that taller riders have longer quads is mostly a myth - just like the idea that women have proportionally longer legs for their height is also a myth.)

[bob59]

Maybe AI solved the mystery? Got me thinking also:


If you take a tape measure and measure from the hip joint (greater trochanter) to the outside of your knee joint, then divide it by your height and multiply by 100 and get the % of your height. If it's less than 27% you have shorter than average femurs, above 27 and you're above average.

[bikinchris]

In a perfect world, your bike sizing session will identify the prefect seat tube angle based upon your riding style and your upper leg length. The custom bike eill be assembled with a new saddle placed directly in the center of the rails on the seatpost and it will be just exactly right for you.

[Wakatel_Luum]

Originally Posted by Dave

These days, stack and reach determine the fit and the STA affects the seat post setback needed. With a 72 STA a zero offset post might produce the desired saddle setback from the center of the BB.

The small frames I ride usually have a 74.5 STA and the smallest a 75. My new Cervelo Rouvida only comes in 4 sizes, so I'm on a small with a 74 STA. They put a zero setback post on them, which is silly, with a tiny 0.5 degree change in the STA. I still need a 25mm setback, so I had to buy one and sell off the original post.

"The main failing of stack and reach is that they only refer to what is happening in front of the bottom bracket.' 'If two frames have the same measured reach but one has a slacker seat tube angle [putting the seat further back], that creates additional reach, which isn't taken into account."

[BdaGhisallo]

Originally Posted by bob59

Maybe AI solved the mystery? Got me thinking also:


If you take a tape measure and measure from the hip joint (greater trochanter) to the outside of your knee joint, then divide it by your height and multiply by 100 and get the % of your height. If it's less than 27% you have shorter than average femurs, above 27 and you're above average.

There was a discussion of this in one of the books Bernard Hinault wrote - or, rather, had ghost written in his name - back in the 1980s. They determined who had long femurs by looking at the ratio of femur length to that of the tibia, as I recall. The normal ratio was said to be 1.2 and if you had long femurs, your ratio was greater than that.

Moser was one who was cited as having long femurs. I recall that Hinault came in around the average.

I might have a dig later on and see if I can find the book.