Help me understand ST angles

[weiwentg]

My custom road frame has a 76 degree STA with a 0-setback post.

Appleman mentioned on the Cycling in Alignment podcast that his personal frames have STAs in the high 60s.

People come in a range of sizes.

[Dave]

Originally Posted by Wakatel_Luum

I am 179cm tall, I require a TT of at least 56cm CTC and a ST length of approximately 52-53cm CTT with a sloping TT. I use a 120mm stem currently with a drop from the saddle to the top of my handlebars at about 130mm.

I have a long upper body with shorter legs. The reach on the Factor Bikes geometry chart for example does not provide a TT length which I require. A ST/HT angle slight variation is fine. Reach only provides a portion of the space I require to feel comfortable on a bike otherwise I have to compensate with a longer or shorter stem..

You've proven my point, still looking at TT length. Find a frame from any brand that has what you consider a proper TT length and STA, then note the reach. After that you can quit looking at TT length. I've pay absolutely no attention to it. Also note that reach can only be compared at the same stack height. If you're trying to decide between two sizes of the same frame and one has a 20mm shorter stack, subtract 6mm from the reach of the smaller frame before comparing.

Some brands may indeed have a combination of reach and stack that won't work as you want. Try another brand.

[prototoast]

If I may be the contrarian here, frame designers often choose seat tube angles for a reason, and a rider should not necessarily approach a frame with a 72° STA and a frame with a 74° STA with the goal of having the saddle end up in the same place with respect to the bottom bracket.

[BdaGhisallo]

Originally Posted by prototoast

If I may be the contrarian here, frame designers often choose seat tube angles for a reason, and a rider should not necessarily approach a frame with a 72° STA and a frame with a 74° STA with the goal of having the saddle end up in the same place with respect to the bottom bracket.

Shouldn't the bike fit the rider, rather than the rider having to fit the bike?

My saddle position is pretty fixed because I have figured out what works for me. I am not going to adopt a whole new saddle position because some builder uses quirky STAs as something to distinguish their product.

[Dave]

Originally Posted by prototoast

If I may be the contrarian here, frame designers often choose seat tube angles for a reason, and a rider should not necessarily approach a frame with a 72° STA and a frame with a 74° STA with the goal of having the saddle end up in the same place with respect to the bottom bracket.

If I look at stock frames with the stack and reach that fit me, the STA is always between 74 and 75 degrees on road bikes, but I've never bought one with a 75. I now have one with a 74 instead of the most common 74.5. You won't find any stock frame that differs by two degrees from the norm.

What comes from a custom builder is another story.

LOOK used to have a 72.5 STA in all sizes. At my 68cm saddle rail height, a 2 degree more relaxed STA requires 23mm less setback, so a zero setback would work instead of a 25mm setback. Seatpost setback=cosine 72.5 x saddle rail height.

[prototoast]

Originally Posted by BdaGhisallo

Shouldn't the bike fit the rider, rather than the rider having to fit the bike?

My saddle position is pretty fixed because I have figured out what works for me. I am not going to adopt a whole new saddle position because some builder uses quirky STAs as something to distinguish their product.

To take two examples on fairly extreme ends of the spectrum, the Rivendell A. Homer Hilsen and the Santa Cruz Stigmata, these are both broadly speaking all road/gravel bikes, but they are designed for a rider to be in very different positions. The Rivendell bike has a very slack seat tube angle, paired with long chain stays, a short reach, and a short front center. In contrast, the Santa Cruz bike has a steep seat tube angle, short chain stays, with a longer reach and longer front center.

Trying to put the rider in the same position with respect to the bottom bracket on these two bikes would result in a bike that responded very poorly, because the weight distribution between the front and rear wheels would be off. A rider would have difficulty controlling the front wheel on the Santa Cruz with a massive setback seat post, or would have difficulty maintaining traction on the rivendell with far forward position. The bikes just wouldn't handle right that way.

In the case where you're talking about two road bikes that have otherwise identical numbers except for a half a degree of seat tube angle, it's fine to move things around a little bit, but bike geometry is about balancing lots of different dimensions. I think we would all intuitively understand that it would be silly to tell someone to buy a bike based only on the seat tube angle, because you can just control for the reach and stack with different stem lengths and angles. A bike designed around a 120 mm stem might not handle well with a 50 mm stem, and vice versa.

[benb]

Originally Posted by prototoast

To take two examples on fairly extreme ends of the spectrum, the Rivendell A. Homer Hilsen and the Santa Cruz Stigmata, these are both broadly speaking all road/gravel bikes, but they are designed for a rider to be in very different positions. The Rivendell bike has a very slack seat tube angle, paired with long chain stays, a short reach, and a short front center. In contrast, the Santa Cruz bike has a steep seat tube angle, short chain stays, with a longer reach and longer front center.

Trying to put the rider in the same position with respect to the bottom bracket on these two bikes would result in a bike that responded very poorly, because the weight distribution between the front and rear wheels would be off. A rider would have difficulty controlling the front wheel on the Santa Cruz with a massive setback seat post, or would have difficulty maintaining traction on the rivendell with far forward position. The bikes just wouldn't handle right that way.

In the case where you're talking about two road bikes that have otherwise identical numbers except for a half a degree of seat tube angle, it's fine to move things around a little bit, but bike geometry is about balancing lots of different dimensions. I think we would all intuitively understand that it would be silly to tell someone to buy a bike based only on the seat tube angle, because you can just control for the reach and stack with different stem lengths and angles. A bike designed around a 120 mm stem might not handle well with a 50 mm stem, and vice versa.

I think the thing that gets lost in the whole way the big companies follow trends here is the Rivendell might be a poor choice for high performance riding but it might fit some riders better.

The issue is you can't necessarily get a bike with angles closer to the Rivendell but performance/functionality more like the Santa Cruz.

That is definitely a bit of an exaggeration as the Rivendell is probably too extreme in one direction, it's just a general idea.

For me this shift makes some of the same bikes I actually owned and fit really well on 15-20 years ago not fit well today.

Who knows, I will probably just get a custom frame next time, as it will allow me to do other things I care about more now like get external hoses.

I do wonder about downsizing cranks but my understanding is really that I would expect to have to move the saddle further back proportional to the amount I shortened the cranks and that wouldn't really help. Though maybe just smaller range of motion would shorten the effective length of my femur and let me get more forward. In any case I tried shorter cranks years ago and it didn't really have much effect.

As is 73.3 STA I need a 2cm setback post and then the saddle is still slid back a bit and then I need a shorter TT or shorter stem, and the shorter stem doesn't help handling. A 72.5 STA like all the custom designs I had or received the saddle is more centered without the need for as much setback on the post and it always just worked.

[Mark McM]

Originally Posted by Wakatel_Luum

I am 179cm tall, I require a TT of at least 56cm CTC and a ST length of approximately 52-53cm CTT with a sloping TT. I use a 120mm stem currently with a drop from the saddle to the top of my handlebars at about 130mm.

I have a long upper body with shorter legs. The reach on the Factor Bikes geometry chart for example does not provide a TT length which I require. A ST/HT angle slight variation is fine. Reach only provides a portion of the space I require to feel comfortable on a bike otherwise I have to compensate with a longer or shorter stem..

Zero credit for not showing the math.

When you say that need a TT of 56 cm, what you are really saying is that you found a frame or two with a 56 cm TT that fits. But that doesn't mean that all frames with a 56 cm TT will fit the same (or even fit at all). Seat tube angle will affect where the handlebar/stem are located for a given TT length. Seat tube angles commonly vary from about 71 deg to 75 deg (sometimes more), so with the same TT length and saddle position, the handlebar/stem may be a few cm closer, or a few cm further away, so you could be cramped on a 56 cm TT frame with shallow STA, and stretched out too far on a 56 cm TT frame with a steep STA.

Normally, we position the saddle to to get the correct balance over the cranks/pedals, and we measure this position as the horizontal location of the saddle with respect to the BB (called the "set back"). We'd also like to set the position of the handlebar/stem to some specific horizontal distance from the saddle for the correct rider reach to the handlebar/stem. We already measure the saddle position with respect to the BB, now if only there was some frame measurement of the handlebar/stem location that was also directly measured with respect to the BB. That way, we with a single dimension we could use find out if a given frame would provide the rider the correct reach to the handlebar/stem ... hmm, how do directly measure the reach for a given frame ... Eureka, that's it! What we need a frame Reach dimension!

[BdaGhisallo]

Originally Posted by prototoast

To take two examples on fairly extreme ends of the spectrum, the Rivendell A. Homer Hilsen and the Santa Cruz Stigmata, these are both broadly speaking all road/gravel bikes, but they are designed for a rider to be in very different positions. The Rivendell bike has a very slack seat tube angle, paired with long chain stays, a short reach, and a short front center. In contrast, the Santa Cruz bike has a steep seat tube angle, short chain stays, with a longer reach and longer front center.

Trying to put the rider in the same position with respect to the bottom bracket on these two bikes would result in a bike that responded very poorly, because the weight distribution between the front and rear wheels would be off. A rider would have difficulty controlling the front wheel on the Santa Cruz with a massive setback seat post, or would have difficulty maintaining traction on the rivendell with far forward position. The bikes just wouldn't handle right that way.

In the case where you're talking about two road bikes that have otherwise identical numbers except for a half a degree of seat tube angle, it's fine to move things around a little bit, but bike geometry is about balancing lots of different dimensions. I think we would all intuitively understand that it would be silly to tell someone to buy a bike based only on the seat tube angle, because you can just control for the reach and stack with different stem lengths and angles. A bike designed around a 120 mm stem might not handle well with a 50 mm stem, and vice versa.

As you say, though, your two examples are at extreme ends of things. Those two bikes are both meant to be ridden on gravel and have two different setups, a road bike and a dutch city bike are both meant to be ridden on tarmac and also have very different setups.

I would expect to sit differently on bikes that different in nature and design. But for a small slice of the bicycle spectrum - spirited riding on tarmac, for instance - I would expect my saddle position to be fairly fixed. Sure, other aspects like reach and drop to the bars might differ slightly but the location and orientation of my backside shouldn't really change.

We have to keep this discussion on STA and its implication within a small slice of the spectrum, or it's a pointless one. We wouldn't argue that saddle position should be the same on a road bike and a recumbent, would we?

[litcrazy]

I agree that not many people are deciding between and riv and a Santa Cruz. However, I do think that more and more riders may be attracted to having a atb/drop bar mountain bike in their stable as they’re become enamored with the possibilities their gravel bike opened up.
One school of thought strongly advocates for getting one’s bars much higher than a typical gravel set up in order to descend in the drops. Similarly, many argue for a more upright touring position for a rider doing the a multi day event like tour divide as opposed to a single big day like unbound.
In this case, the issue of needing to move one’s saddle backwards as one moves one’s handlebar position up and back in order to keep one’s hips open could be relevant.
Trek’s recent division of its gravel bike into two separated product lines seems a nod in this direction.

Originally Posted by BdaGhisallo

As you say, though, your two examples are at extreme ends of things. Those two bikes are both meant to be ridden on gravel and have two different setups, a road bike and a dutch city bike are both meant to be ridden on tarmac and also have very different setups.

I would expect to sit differently on bikes that different in nature and design. But for a small slice of the bicycle spectrum - spirited riding on tarmac, for instance - I would expect my saddle position to be fairly fixed. Sure, other aspects like reach and drop to the bars might differ slightly but the location and orientation of my backside shouldn't really change.

We have to keep this discussion on STA and its implication within a small slice of the spectrum, or it's a pointless one. We wouldn't argue that saddle position should be the same on a road bike and a recumbent, would we?

[Wakatel_Luum]

Originally Posted by Mark McM

Zero credit for not showing the math.

When you say that need a TT of 56 cm, what you are really saying is that you found a frame or two with a 56 cm TT that fits. But that doesn't mean that all frames with a 56 cm TT will fit the same (or even fit at all). Seat tube angle will affect where the handlebar/stem are located for a given TT length. Seat tube angles commonly vary from about 71 deg to 75 deg (sometimes more), so with the same TT length and saddle position, the handlebar/stem may be a few cm closer, or a few cm further away, so you could be cramped on a 56 cm TT frame with shallow STA, and stretched out too far on a 56 cm TT frame with a steep STA.

Normally, we position the saddle to to get the correct balance over the cranks/pedals, and we measure this position as the horizontal location of the saddle with respect to the BB (called the "set back"). We'd also like to set the position of the handlebar/stem to some specific horizontal distance from the saddle for the correct rider reach to the handlebar/stem. We already measure the saddle position with respect to the BB, now if only there was some frame measurement of the handlebar/stem location that was also directly measured with respect to the BB. That way, we with a single dimension we could use find out if a given frame would provide the rider the correct reach to the handlebar/stem ... hmm, how do directly measure the reach for a given frame ... Eureka, that's it! What we need a frame Reach dimension!

I was going to but I can't be bothered...

[marciero]

If you are not comfortable with the math, as has been noted, there are online calculators for this. Its a wash either way -given stack, reach, and STA you can find your effTT - well as setback - and given STA, effSTL, and effTT you can find stack, reach, and setback. What "is "simpler" might come down to what is either provided by manufacturers or what is easiest to measure. The reluctance to forego TT and ST has to be at least in part a vestige of the square frame standard. Myself included-I dont know my stack and reach numbers offhand.

As far as measuring these, on a square frame it is obviously easy to measure STL and TT. But on a sloping top tube I think you can still get a pretty good estimate of ETT and ESTL by eyeballing the horizontal. And if you know the top tube angle and STA you can get ETT and ESTL precisely from the actual measured TT and ST. (its not right triangle geo but can use law of sines). Again probably online calculator.

[benb]

Originally Posted by Wakatel_Luum

I was going to but I can't be bothered...

You should explain because you definitely don't require a bike with a particular TT & ST measurement unless it's your aesthetic preference or something like that.

A custom builder could build you 5 different bikes with different TT lengths and different STAs. Then they could slope all the TTs differently and they'd all end up with different ST lengths.

Yet all 5 of those bikes could put the contact points in exactly the same place using the exact same stem & handlebars.

And they could put the wheels in exactly the same place too* , the only thing that would be different would be the amount of setback on the seatpost and where the saddle was in the rails.

* = maybe an extreme slack STA would force longer chainstays

[fa63]

It seems some folks on here get picked on at times for "showing their work" in responses to questions related to bike geometry (or aerodynamics, etc.), so I just wanted to say that it makes me happy to see people come to the defense of science and math