Essthreetee

So, I am still (what I would consider) new to all this stuff. But I LOVE learning about it...so here is my question:

If I adjust my Saddle according to the Bottom Bracket (I.e. I run a straight line up from the BB and measure back to the saddle so it is the same on all my bikes) does that counter act the seattube angle?

My Torpado is much steeper than my Lemond or my Canopus...but they are all set up this way.

Just curious as to what the seatubet angle should do....

rhm

I don't know why seat tube angles vary so much.

There was a time in the early development of the diamond frame that seat tube angles were very slack (say 68°) but seat posts were 7-shaped so the seat was mounted in front of the post itself. This was kinda clever in that the effective top tube increased if you raised the seat, but it was inherently weak and resulted in a heavy seat post.

By the mid 50's designers had settled on 72° or 73° and that seemed to work. But after the mid 70's head angles got steeper and seat tube angles soon followed. This doesn't make sense to me, since it resulted in seat posts with more and more setback to get the seat into the right position.

Whatever. If you have the seat in the same position relative to the pedals on every bike, you're doing it right. as long as you can do that, the seat tube angle is pretty irrelevant.

repechage

The early diamond frames rhm mentions indeed do seem counter to reason with the "seven" shaped seat posts that just displace the saddle forward. That written, there appears to have been a change in convention over time as to what the optimum placement of the saddle, (really hip and knee joints) in relation to the bottom bracket center. The steeper seat tube angles appear on track bikes first from my referencing, and then there was variation as to type of event, sprinters got steeper, pursuitists far there back. 6 day and omnium bikes all over the place.
These "discoveries" flowed into road bikes. So, you can accept where the bike designer wishes the rider to sit, or adjust to taste. ( I ignored the whole later Triathlon super far forward branch) also note the UCI has pretty tight regs on where a saddle can be, derived to counter those darn French who almost threw all overboard with success against the clock on a recumbent in 1938. That was decided way before modern kinesiology studiesof cycling, and we are left with the rule, that influences heavily still.

I see no problem with setting all your bikes up in a uniform saddle adjustment, assuming it is a good one.

Left out, "spinners" like the saddle forward, but still the saddle nose measurably behind the bottom bracket, I am working from memory, but 50mm comes to mind per the UCI, "mashers" farther back.

noglider 

I have a wild-ass theory that bikes with steep head tubes are designed for quick handling, and moving the rider's weight forward on the bike adds to the effect that the design seeks to achieve. On my racing bikes, I feel as if there's more weight on the front wheel than on my other bikes. But it's just a theory.

repechage

Reasonable conclusion, seat tube angle only plays a part, top tube length and the front center dimension while not tied together exclusively work in concert. The FCI, the Italian cycling federation commissioned a study in regard to road race bike design published in the early 80's, I found a translation once, but it became a bad link.
Anyway, it suggested and foretold the " typical Italian race geometry" that persists to this day. Pinarello and Gios were early adopters of the guidance. Shorter top tubes, longer stems, steeper head angles and reduced fork rake.
What I consider the nervous race bike school of design. Fun for the first two hours, fatiguing late in the race. My personal view it has attributed to the nervousness of the pro peloton. Ever compare images of today's pro peloton to say the footage in LeCourse en tete? To me it looks like today's riders cannot ride a straight line to save themselves!
The public's best hope for better is the popularity of the gravel grinder "adventure" bike. In general, more sensible design.

Salamandrine 

Forget the pro peloton, most amateur riders I see on the road can't ride a straight line to save themselves. When I first joined a bike club, zig zagging was not tolerated. Squirrels were told to go ride the rollers till they could ride a straight line, in no uncertain terms.

Anyway, it's really the saddle position or more exactly your femur length and knee position that matters. The plumb bob off the knee method works pretty well for getting the saddle position right. I know someone has an hair up their rear on the interwebs about this, but it does work pretty well to get you a good starting position. Don't dismiss it.

72º works out pretty well for Brooks saddles. It's my preference these days. For a racing bike and non-leather saddle, I really think the old Masi 73.5º parallel geometry and 'neutral' trail was a perhaps optimal for a go fast racing bike. Very instinctual and quick but not twitchy.

jimmuller 

It has always struck me that ST angle is more important to a builder than to a fitter. Have a bike with steeper ST? Then move the saddle back on the rails, and your saddle-BB position will be the same.

ST angle matters to a builder because it constrains chain stay length. A steeper ST angle places the point of closest approach between wheel and ST lower and allows the BB to be closer to the DOs. When the rider then moves the saddle back to compensate it moves the rider's weight back toward being over the wheel. So you end up with a different weight distribution and a tighter rear triangle, both seemingly significant.

63rickert

The current UCI rule states the nose of the saddle (they largely assume a standard 27cm saddle) must be 5 centimeters behind the center of the bottom bracket. Unfortunately the rule is enforced erratically. It has nothing to do with the older rules that banned recumbents. It is simply a safety rule. Based on the observation that riders sitting far forward can't ride a straight line and tend to fall a lot. Sitting far far forward also greatly increase the chances of going over the handlebars under hard braking. Center of gravity is just all wrong when sitting forward.

Many modern bikes, including many popular ones, simply cannot be set up to conform to UCI rules. Once a rider is emotionally invested in one of these bikes they will claim the UCI rule is simply wrong, with no understanding of the issue at all.

I sure do remember the days that Salamandrine references. Where I was it mostly got done by force of example. We had old sixday riders who sat rock steady in the saddle. Old white haired guys who seemed to barely move at all who were still fast.They had unquestioned authority, did not need to say much. When squirrels showed up they learned fast or simply were not seen again. Now it is all squirrels.

The pro peleton is no longer much of an example for anyone. The old pros raced 200 days a year unless they raced 250. It was a job. The new kids learned from the old hands and they all knew each other well. Now the pros race 70 days a year, or less if on the injury list. They know their own team, not the entire peleton. Each time out they want to make a big impression on their sponsors. So they take chances. And never learn the job.

My 1950 Bates has completely modern geometry. When I had the Gloria Garibaldina (decades before anyone knew it was collectible) it was a 1930s bike with completely modern geometry.

If you try to achieve the identical position on every bike you ride you will spend too much time fussing with hardware and not enough time riding. Ultimately you can't make them all the same. Don't try. They are different. Enjoy them as they are.

John E

Interesting discussion. I think some of the seat tube angle decisions had more to do with aesthetics than anything else, i.e., with a desire to make the head and seat tubes approximately parallel, to make the frame "look right."

My 1960s era road bikes (1959 Capo to 1970 Peugeot) all have 72 degree seat tubes, and the 10-years-later Bianchi has 73, as did my 1971 Nishiki Competition. All have equal head and seat tube angles. The Peugeot is disproportionately long in the top tube by at least 2 cm, and the Nishiki was correspondingly short in the top tube. I can comfortably use a stem of typical/average reach on everything else.

Essthreetee

All of these thoughts and inputs are very interesting. Weight distribution, aesthetics, wheelbase length, handling characteristics....can't wait to read more.

Ex Pres 

Steeper ST angle allows for a shorter chainstay, and therefore a shorter wheelbase - or alternatively a larger tire size, if the builder so chooses.

A ST angle that is too far off from the norm could screw up the relationship between the chainrings and the front derailleur.

A steeper ST angle also "pushes" more of the TT out in front of the saddle, so if you're comparing measured TTs among your bikes, other things being equal, your bikes with steeper ST angles should have shorter measured TTs, or you'll need a shorter stem. There's a rule of thumb for this, but I don't have it here on the tip of my fingers.

busdriver1959

You nailed it. The current fad is short chainstays/short wheelbase. I think the marketing people have found that they can make a bike sound fast with too short chainstays. They tell us that we will corner a lot faster with a short wheelbase. They're just selling numbers. Unfortunately for us, chainstay length/wheelbase does't have a significant effect on cornering ability until you go to extremes like a cargo bike. Cornering ability happens at the front end, primarily with trail. In the real world, the back end just follows the front end and small changes in wheelbase are transparent to most of us. Where short chainstays fail is in out of the saddle climbs or sprints. For some reason, short chainstays allow the back wheel to fishtail more easily than a longer bike when the rider is hammering out of the saddle. I imagine that most of us would agree that scrubbing the back tire sideways across the road does not contribute to getting down the road faster.
So, back to the OP's question. Seattube angle doesn't matter as long as you can get the saddle where you need it. Or, if you are a marketing manager at a big bike company, it helps you sell bikes for all the wrong reasons.

DiabloScott

This would give you the same fitting for your lower-body position on all your bikes, but they might not fit the same for your upper body because of the differences in top tube and stem lengths. I do think it's the right way to set your saddle position once you know which one works best, but just don't expect all your bikes will feel the same.

Barrettscv 

This. Also remember that a lower bb drop requires a steeper seat tube angle or longer chain-stays.

gugie 

ST angles, to a large degree, were determined by available lugs. Or the other way around, lugs were designed around ST angles that worked well. At any rate, if you're building with lugs, you've got a narrow range to work in. Now that many custom builders TIG, the constraints of lugs are gone, yet the standard angles still abound.

catgita

One oddity of seat tube angles is making bikes for tall riders with 72.5 or 72 degree seat tube angles, and for short riders with 74 or 75 degrees. This is not because tall or short riders have different femur proportions, it is because, traditionally, everyone uses crank lengths in a very narrow range of lengths. So to maintain the KOPS (Or there about), a tall rider needs to push the saddle further back, and a shorter rider forward, following the disproportionate crank length.

With my 37" inseam and 200mm cranks (exactly proportional), 73 degrees runs right up the middle of the saddle. I have one at 72.5, which puts the clamp near the back of the rails. When the saddle is that high, the angle adjustment range that the saddle rails provide is not much.

rhm

There is a common complaint on bike forums generally (not so much in c&v, more in commuting, long distance, and touring) to the effect "I can't use a Brooks saddle because the rails aren't long enough to give me the setback I need."
And it's probably true. Brooks rails don't give much fore-aft adjustment. If they did, the whole front of the saddle would have to be wider to accommodate the seat post hardware.
But the curious thing is, the saddle gets the blame. The problem is that the seat tube is too steep.

63rickert

Small frames have steep seat tubes for a simple reason. It is not because anyone imagines that small riders pedal differently. It is about keeping feet out of the front wheel. Steep seat tube brings the bottom bracket rearwards. Shallow seat tube sends the bottom bracket forwards.

There are only three things a designer can do to move the front wheel away from a riders feet. One, longer top tube. Obviously there is a limit on that for smaller riders. Two, increase fork rake. Building different size frames with different fork rakes is relatively expensive. Manufacturers don't do expensive. Using enough rake to make a difference is beyond the imagination of most. Few if any have any understanding of rake, trail, handling. Number three is adjusting the spread between seat and head angle. If the seat is steep and the head shallow small frames can have toe clearance. Even on big frames if you stray far from norms there will be lack of clearance. The limiting factors are the bike handles poorly with very shallow head angle (especially so if the rake is not adjusted), and the bike can't be pedalled by most normal humans if the seat tube is too steep.

The spread of plausible seat angles for road bikes is very narrow. 72 to 74 degrees is about it. 75 degrees for small frames. Fairly straightforward. There are jokers in the deck. Time triallists and triathletes (not normal people at all) use different angles and then create a fashion thing that gets applied to normal bikes for normal people. Nut jobs like yours truly are perfectly happy riding DL-1s with 66 degree seat angle and the seat clamp in 'normal' position. DL-1 is a good data point for what happens with shallow angles. The wheelbase is 47". This has some purpose, the bike is wildly stable. But most do not want a bike that long and they don't want a long bike with a 68 or 70 degree seat angle either.

rhm

Or, Four, use proportionately smaller wheels?

Yes, that's a good point. Not a good thing.

Yes, I'm with you there. The seat tube on my Fothergill is 68 degrees (as close as I can measure). The head tube is a fairly normal 73 degrees, though. The top tube is abnormally long, but with the seatpost clamp on the front of the seatpost, the riding position is pretty close to normal and it suits me very well. It's a very comfortable bike, and only heavy because I have so many steel accessories attached.

palincss

Actually, the material the saddle is made of is not the issue, it's the length of the rails. Brooks saddles have short rails compared to most "non-leather" saddles, and slamming the saddle back will only get you so far - not far enough to compensate for a steep seat tube angle. You probably also will need a seat post with a lot of setback, such as the Nitto S84. Or, you can use a leather saddle with long rails, such as the Selle AnAtomica.

rhm

No, the issue is not the length of the rails; it's the width of the front part of the saddle. If Brooks made the clamping area of the rails extend farther forward, the skirts of the saddle would bulge out where the rails (and the seat post clamp) interfere with the leather.

These photos, of someone's catastrophically butchered Brooks Pro, illustrates this pretty well. They cannot make the clamping area of the rails longer without making the front of the saddle wider.



Many people find the rather extreme Y-shape of the Brooks saddle to be unnecessary, and prefer something more like a V-shape; and that is certainly a legitimate complaint about the Brooks design. If Brooks was willing to make the saddle more like a V-shape, the clamping area could be longer; but it would be a very different saddle. Better? Maybe; I haven't made up my mind about this.

Trakhak

That's a possible minor contributing factor, but the more obvious objective is to enable the use of longer top tubes for riders with longer torsos and shorter top tubes for riders with shorter torsos while achieving the target headtube angle and wheelbase length.

63rickert

@ rhm

I have handled a lot of Brooks saddles over a lot of years. I noted what you said about saddle rails and nose width the first time. Should have immediately gone and looked at a saddle. That photo demonstrates your point phenomenally well. Always something more to learn. Especially from good teachers. Thanks.

catgita

That would imply that a taller rider should sit further behind the pedals, and a shorter rider would sit forward of the KOPS position, simply because of torso length, not pedaling effectiveness. One of the basics of bike fit is to move everything else relative to your correct saddle to pedal position, and not try to adjust reach by moving your saddle. The correlation with crank length discrepancy has been discussed in industry and among custom builders for decades. These days, at least shorter cranks are available, which makes this a little less common on smaller frames.

79pmooney

I am a believer that seat placement should be in relation to the BB and have noting to do with the set tube angle; except the seat tube angle is often closely ties to the purpose of the bike and hence my best position on it. For overall position, I think of a "magic" triangle of BB, seat and handlebar. I then (sometimes) rotate this triangle to, for example make for a more efficient upwind machine (my fix gears; everything rotated forward) or a cruise (everything rotated back). The bike with a laid back seat tube angle often cries for that second position of the triangle.

Now, all that said, there is still no guarantee a "normal" seatpost will land within the rails of my seat at its correct position. Good thing is there is no restriction on seatpost setback. A framebuilder can build a post of any setback you want. My builder has made for me two bikes with 74 and 74 seat tubes (to get the rear wheel forward for good handling and allow decent size tires. 160mm setback posts put the clamps right in the centers of the rails. I also have an MKS MTB post of years ago with the sliding QR setback that goes from moderate to ~170mm setback. (Quote me and I'll dig up its name. They are still around and built like tanks so I would have qualms riding a very well used one.)

Ben