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.

[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?

[vespasianus]

Originally Posted by 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.

His long femur length is legendary and his slack seat angles are one of the reasons why some people did not get along with his line of bikes.

[Doug Fattic]

Most observations/comments about proper saddle setback are based on maximum speed and efficiency. But not everyone that enjoys cycling is a member of the go fast and - maybe with a few tweaks - go even faster club. Some are willing to trade effort for comfort. They are going to raise the tops of their drop handlebars up around level with their saddle. If their seat angle allowed it, they would push their saddle back far enough until their weight was balanced over their pedals. What this does is take the weight of supporting their upper body (most likely heavier than when they were younger) off of their arms and hands. That adjustment can make cycling longer less tiring. This saddle back comfort place can be easily found on a stationary fitting bike (like the old Serotta fit cycle). As they keep going back (maintaining the same reach to their handlebars) they will discover a place where - if they lift their hands slightly off of the bars - they no longer have to strain to hold that position. Their body weight is balanced over the pedals.

Unfortunately many American road bikes (particularly before gravel bikes became popular) have steeper seat angles that don't allow a rider to find this balance point. In many cases this balance point probably requires at least a 72º seat angle and likely even less. And if they push their saddle back they need to correspondingly shorten their stem to keep from stretching out too far. All of these factors work against finding their most comfortable position with a typical road bike - whose design was based foremost on foot clearance with the front wheel. .

Old British frames were designed for get around function with slacker angles. Some touring bikes too were looking down the road instead of millimeters off of another's back wheel was the purpose of the bicycle's frame design.

[benb]

You don't actually need a seat tube on your bike, nor do you need a top tube.

To a great extent we only have those tubes on the bike because it's required by UCI rules.

All changing the STA really does is change the setback of the post you need and change the tube lengths to put the stack/reach where they need to be.

There are a bunch of comments that are incorrectly assuming that the top tube has to stay the same length if you vary the STA. It doesn't. If you want to keep the stack/reach on the bike the same you just shorten the top tube as you steepen the seat tube angle.

I suspect a lot of the current steep seat angle craze across the size range has to do with the fact that it can allow the manufacturer to more easily use the same rear triangle assembly across more sizes. For a carbon monocoque bike this is a big advantage for economy of scale. You make a single model of seatstay/chainstay/dropout for the back of the bike and then bond it onto the 5 different front triangles you produce to make the 5 different frame sizes. It reduces the # of molds you need, and probably eliminates some sources of mistakes that can happen on the production line.

For me being 6'1" and having a lot of leg I did fit well on those old school bikes that were slacker. The new bikes being steeper causes me to basically go down a size in the range cause the steeper STA means I need more setback on the post and I'm further away from the bars. So I need a shorter/taller frame, cause I downsized to get the reach right and now that means the bars are lower.

Where I think it gets really confusing are all the new MTBs that are offsetting the BB from the end of the ST. This ends up creating an effective and actual STA that makes everything a bit more confusing. The TT ends up longer for the same reach IIRC, but the saddle moves backwards like a slacker bike when you raise the seat, so you end up needing less setback on the post.

[Mark McM]

Originally Posted by benb

I suspect a lot of the current steep seat angle craze across the size range has to do with the fact that it can allow the manufacturer to more easily use the same rear triangle assemble across more sizes. For a carbon monocoque bike this is a big advantage for economy of scale. You make a single model of seatstay/chainstay/dropout for the back of the bike and then bond it onto the 5 different front triangles you produce to make the 5 different frame sizes.

I haven't seen this to be the case. While seat tubes have gotten steeper, and frame builders have generally cut down on the number of frame sizes, frame makers typically still vary the seat tube angle from size to size. Since each bike frame size has a unique seat tube length, and therefore unique lengths and angles of seat stays, I think manufacturers still need unique rear triangles for each frame size.

For example, take Trek's lowest cost carbon bike, the Checkpoint SL 5 ($3000). This bike comes in only 6 frame sizes, and each frame size has its own unique seat tube angle (74.1, 73.7, 73.2, 72.8, 72.5, 72.1) Or Giant, who was a leader in adopting sloping top tubes for road bikes, and thus in reducing the number of frame sizes. Their current lowest cost road bike is the TCR Advanced 2 ($3300). It also comes in only 6 sizes, but these sizes use 5 different seat tube angles (74.5, 74.0, 73.5, 73.0, 72.5).

[Wakatel_Luum]

Originally Posted by vespasianus

His long femur length is legendary and his slack seat angles are one of the reasons why some people did not get along with his line of bikes.

Greg says in his 'Cafe' ride on youtube that he was too far back then and no longer uses 175mm cranks so his opinion has 'relaxed' in that ideal.

[BdaGhisallo]

Originally Posted by Wakatel_Luum

Greg says in his 'Cafe' ride on youtube that he was too far back then and no longer uses 175mm cranks so his opinion has 'relaxed' in that ideal.

It was Cyrille Guimard who put Greg in that position. Greg had to raise his saddle an inch and a half when he turned pro with Renault and Guimard changed his position.