Help me understand ST angles

[Nomadmax]

Originally Posted by Wakatel_Luum

"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."

But if you set the saddles to the same height/set back on both frames, won't it be the same?

What I mean to say is that on a frame with slacker STA you'll have to move the saddle forward to get the same setback relative to the BB.

[BdaGhisallo]

Originally Posted by Nomadmax

But if you set the saddles to the same height/set back on both frames, won't it be the same?

What I mean to say is that on a frame with slacker STA you'll have to move the saddle forward to get the same setback relative to the BB.

You are correct.

The amount of top tube forward of the BB used to be called the Effective Top Tube back in the 80s. I recall reading an article in the old Bicycle Guide about this very thing. It was written by Grant Peterson of Rivendell I believe. He was apparently the guy who first called the crank width dimension the Q Factor, and he proposed calling the effective top tube the Z Measure.

Frame Reach has supplanted this idea but the principle is the same. Now Effective Top Tube has come to refer to the length of the top tube on sloped frames if they had a traditional level top tube.

[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.

[Alistair]

Originally Posted by Wakatel_Luum

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

That doesn't make any sense. Or rather, you're using two different definitions of reach in the same sentence.

A frame's reach is the horizontal distance from the BB center to the top center of the HT. STA only impacts this measurement if you fix the ETT length, but then you are changing the frame's reach so they aren't the same any more. By definition.

I think what you meant to say is:
"If two frames have the same ETT but one has a slacker seat tube angle [putting the seat further back], that creates additional reach." Which isn't a flaw in using reach - that's the whole point of the measurement.

[mstateglfr]

Originally Posted by Wakatel_Luum

"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."

Just slide the saddle forward a bit on the bike with a slacker seat tube angle. Boom- done, your concern is solved.

[BdaGhisallo]

Originally Posted by Alistair

That doesn't make any sense. Or rather, you're using two different definitions of reach in the same sentence.

A frame's reach is the horizontal distance from the BB center to the top center of the HT. STA only impacts this measurement if you fix the ETT length, but then you are changing the frame's reach so they aren't the same any more. By definition.

I think what you meant to say is:
"If two frames have the same ETT but one has a slacker seat tube angle [putting the seat further back], that creates additional reach." Which isn't a flaw in using reach - that's the whole point of the measurement.

Given a fixed ETT length, the frame with the slacker STA will have less Reach.

[benb]

Originally Posted by BdaGhisallo

Given a fixed ETT length, the frame with the slacker STA will have less Reach.

There is no requirement to keep the ETT the same length though.

If they kept the ETT the same while they steepened the frame it's just a different sized frame now. It's not going to fit the same riders.

Say they did this between two "generations" of the same frame if you fit really well on the old one and got the same size frame in the new generation you would need to shorten the stem and increase the setback at the saddle/post to duplicate your fit.

But in all likelihood because they kept the ETT the same they probably increased the front center and/or changed the rake/trail to compensate for the increase in front center and the bike is not going to feel the same to you as you won't have the same amount of weight on the front tire.

Or you could be in a situation where the reach on that frame is now so much longer you can't even make it work with a shorter stem. So now you need to downsize and then start wondering about whether or not you can get enough stack. Hence we are starting to see things like riser drop bars showing up. (Giant has one of those as a problem solver IIRC, and maybe Specialized too?)

Steeper STA does let them place the rear wheel differently... that seems be about:
- Keeping wheelbase short (might compensate somehow for the increase in front center? )
- Aero benefits
- Previously mentioned cost saving

If you look at the endurance bikes endurance bikes in order to make it more upright with the steep STA applied to all sizes what they had to do was shorten the top tubes a bunch to get the fit right. That brought the wheel in, probably too close in the big sizes, so then they had to slacken the HTA and monkey around with offset + curved fork (in the originals before disc) to get the front wheel back where it needed to be. I still think mine doesn't really handle quite as pleasantly as a more traditional bike that in my size would have had a slightly slacker STA and a more traditional head angle. It's very very subtle but it's not really the same.

Basically they are always playing with as many different #s as possible to get the desired result, changing one # always creates a need to change something else. And for stock bikes that get made in a bunch of sizes they really must agonize over this.

[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).

[Mark McM]

nm

[BdaGhisallo]

Originally Posted by benb

There is no requirement to keep the ETT the same length though.

I don't disagree, but if you're trying to understand the effect on reach of altering the STA, it helps to keep the other variables fixed.

Once someone has it straight in their mind on the effect of one variable, they can turn their attention to other variables.

[Mark McM]

Originally Posted by BdaGhisallo

I don't disagree, but if you're trying to understand the effect on reach of altering the STA, it helps to keep the other variables fixed.

But that's just the thing - STA has no affect on reach, if we assume the saddle will be adjusted as necessary for the same setback. So inserting the ETT in consideration just adds unnecessary confusion.

The primary significance of STA is in the range of saddle setback that can be achieved - but because saddles can be adjusted on their rails and seat posts are available in in different offsets, the same setback can be achieved with a fairly large range of STAs.

[BdaGhisallo]

Originally Posted by Mark McM

But that's just the thing - STA has no affect on reach, if we assume the saddle will be adjusted as necessary for the same setback. So inserting the ETT in consideration just adds unnecessary confusion.

The primary significance of STA is in the range of saddle setback that can be achieved - but because saddles can be adjusted on their rails and seat posts are available in in different offsets, the same setback can be achieved with a fairly large range of STAs.

You're assuming that everyone understands that the top tube length must be adjusted to compensate for changes in the STA in order that the reach remains unaffected, though.

Reading this thread, it's not clear that everyone does.

[Mark McM]

Originally Posted by BdaGhisallo

You're assuming that everyone understands that the top tube length must be adjusted to compensate for changes in the STA in order that the reach remains unaffected, though.

Only the builder of the frame (who has to cut the tubes to length) needs to worry about the length of the top tube. The people who have to use the frame (or to fit the frame to the user) only need to be concerned with REACH (horizontal distance between BB and head tube) and STA.