vol

What aspects of bike riding are affected by the seat tube angle? (larger angle = more vertical) Does slightly larger angle makes pedaling easier, or harder?

(I'm trying to find out the possible factors that contributed to one of my bike's easier to ride than the other. Among the differences are their seat tube angles.)

SkyDog75

The angle of the seat tube doesn't simply make pedaling easier or harder. Assuming you don't negate the difference in angle via seatpost setback and saddle adjustment, the seat tube angle would affect your fore/aft position relative to the bottom bracket, which I suppose could have some effect on the perceived effort needed to move the bike. It could cause some handling differences, too, with your weight further forward and a shorter cockpit unless you use a longer stem, which could also affect handling. It's all interconnected, but seat tube angle is just one small piece of a bike's geometry. It can -- but doesn't always -- influence a number of other parameters, depending on the geometry decisions the builder made. For example, a steeper seat tube would allow a builder to user shorter chainstays and place the rear wheel closer to the seat tube, shortening the wheelbase. The builder wouldn't have to use shorter chainstays, though.

What kinds of bikes are you comparing? Are the fit measurements the same? (Bottom bracket to saddle length, saddle setback relative to bottom bracket, saddle to handlebar distance, crankarm length, etc.) Is the gearing the same? Tire type, width, and pressure?

MichaelW

Most riders adjust the fore/aft position relative to the bottom bracket to suit, using alternative seatposts so the angle is less significant than it used to be.
big bikes use slack angles and small bikes use steep angles to compensate for using medium sized cranks for all rider sizes.

Homebrew01

When comparing the ride of 2 different bikes, seat tube angle is a minor factor.

rpenmanparker 

First OP, you have it backwards. Steeper seat tube angle means more vertical, not less. Second from the rider's perspective seat tube angle is just one way of adjusting saddle position relative to the crank. Steeper seat tube placed the rider more directly over the BB and crank. Slacker angle puts the rider more behind the BB and crank. Seat post setback, saddle rail length, saddle position on the post all work with seat tube angle to determine rider position relative to the crank.

Kopsis

Seat tube angle combined with chainstay length also determines clearance between the rear tire and seat tube. Slack STAs may force a longer chainstay if the frame designer wants to allow room for larger tires and/or fenders.

Looigi

What matters is where the saddle is compared to the crank (height above and distance behind). It's these distances (along with position of the bars) that determine the fit and need to be correct to position a rider optimally on the bike. How you get there via a combination of seat tube angle, saddle post setback, and position of the saddle on the seat post is largely immaterial.

rydabent

A seat tube angle more verticle makes the bike more twitchy. The angle of the seat tube affects the placement of the saddle in relation to the BB. There is some belief that there is a prefered placement of the saddle to the BB. Also the pig headed old fools at the UCI DEMAND as certain placement to be legal.

OTOH there are crank forward bikes that have a very relaxed seat tube angle that many people prefer that places the seat well behind the BB.

hueyhoolihan

short answer is, apparently, nobody knows.

pat5319

Slacker angles tend to have smoother rides. Steeper angles tend to have better performance

Sixty Fiver

I could tell you the secret but then I'd have to kill you.

hueyhoolihan

tell me! i can be discreet.

fietsbob

C-of-G of Riders weight is shifted , rearward as the number. off 90 degrees is lower. its one way to get Setback..

a seatpost choice is another way to do this setback adjustment, as is simply having the seat tube pass behind the BB shell,
which is done on most folding bikes with a telescoping seat post as part of the folding sequence.

HillRider

One thing seat tube angle does effect is the perceived top tube length. A more slack seat tube angle requires a longer top tube to have the bars in the same position relative to the bottom bracket. So, when comparing top tube lengths among competing frames, the seat tube angle has to be considered.

vol

Thanks for all the replies. I understand the angle affects the fore/aft of the saddle, but assuming the same fore/aft, it seems the main thing is:

So what's the impact of that on the ride?

This is the clearest answer , but could you explain why? Maybe slacker angle is more comfortable as it's closer to a recumbent?

fietsbob

Its more how it makes You fit on the bike .. steep is on a race bike, down and on all fours , shallow on a Utility-Cruiser sit up bike.

and all the other parts picks are in service of that function-style ..

SquidPuppet

A slaker angle allows for more flex which acts like suspension when riding on rough surfaces. The more vertical the post, the less flex, less bump absorbtion, harsher ride.

Longer chain stays will have a similar softening effect on ride quality. That comes at the price/benefit of a longer wheelbase. More stable yet less nimble handling.

Straight blade forks vs curved can effect ride comfort too.

Everything effects everything.

Booger1

For those people that can't sleep without knowing......

The seat tube angle of your bike's frame is a crucial factor for positioning and performance. It's important because the angle determines your positioning relative to the bike's bottom bracket (the area with the bearings that your cranks rotate on). This positioning profoundly influences the dynamic of your muscle movement while pedaling. When you switch to a frame with a different seat tube angle, a change of only 3 degrees is analogous to completely changing your swim stroke or your run's stride; it will take time to adapt to, and can have either a positive or negative influence on your performance. Production bike frames available offer a range from 73 degrees for a traditional 'road bike,' to 78 degrees for a 'tri-bike.' This range can make a difference in seat position of about 3 inches forward or back (depending on your leg length) relative to the bike's bottom bracket.
So what effect does your positioning in relation to the bike's bottom bracket have on your pedal stroke and power? Seat tube angles that are shallow (73 to 74 degrees) place you further behind the bottom bracket and favor quadriceps and gluteal strength over hamstring strength. They benefit athletes who've spent significant time 'in the saddle' (bike racers) getting their bodies used to flexibility and power demands on their quads, glutes, and lower back. The shallow seat tube angles also create a better position for climbing—power at lower cadences. Steep seat tube angles (75 to 78 degrees) put you further forward and benefit athletes who have well-developed hamstring muscles (runners). Hamstring muscles along with our gluteus (butt muscles) are the dominant muscles driving us forward when we run. When you watch an experienced cyclist at speed notice they often move forward at high effort/cadence on the flats. Steep seat tubes are more appropriate for flat courses. The steeper seat tube angles also open up (increase) the angle between our torso and legs as we maintain our optimal aerodynamic position. We're able to rotate our hips forward making less of a bend through our lower backs, thus reducing the stress and fatigue to that area.
A steep seat tube angle (forward position) simulates the motion of running progressively more the further forward we place our seat. A forward position can ease the transition between bike and run, which is especially helpful in sprint distance events. But there's a downside to moving forward, a point of diminishing returns.
The quadriceps is our strongest muscle, and it can store much energy in the form of glycogen (carbohydrate in storage form). Our quads apply force during the down stroke of the pedaling motion—working with gravity through our body weight. The hamstring muscle group has less power potential, and in addition must work against gravity while pulling back and up to generate power. Favoring hamstring powered pedaling tends to deplete and fatigue the same muscles during both the bike and run, so we may be able to get away with this in short events, but it will probably catch up with us to some degree during longer events.
Another consideration is variation of power requirements on different parts of a race course. While riding fast on flats, it's natural to slide forward on the saddle as we increase our momentum with greater leg speed (cadence). As we climb a hill or fight headwinds, our cadence slows and power becomes crucial. This is when we need to slide back on the saddle to develop more torque. Riders with ultra-steep 78-degree seat tube angles often complain that they can't climb comfortably, especially when out of the saddle. Clearly both 'forward' and 'traditional' shallow seat tube angles have merit and and every frame is a compromise.
I favor a seat tube angle of 75 to 76 degrees and feel it gives me a balance between top-end speed on the flats and power on climbs. This is the position used by bike racers for their time trial bikes, and a triathlon is nothing but one long time trial unless you race junior or pro ITU draft legal races. Taller riders can move back one-half to one-degree and smaller riders should move forward by the same amount. This seat tube angle adjustment accommodates body size and femur (thighbone) lengths. Most frame manufacturers do adjust seat angles for height on their stock frame sizes.
You may be saying to yourself, "I haven't seen many frames with a 75-76 degree seat tube angle," and you're correct. There are two ways to compensate for a seat tube angle that you feel isn't optimal. The first adjustment option is at the clamping area of your seat post to saddle rails. The clamp that holds the seat's rails allows a range of about one-and-a-half to two inches of forward or backward adjustment. The second option is a special post that's curved or 'bent.' Either of these options can effectively shift you to the position you want with a variety of seat tube angles. Of course moving your seat forward or back also affects the distance to your handlebars and aero bars, but discussing that would go beyond the scope of this article. Suffice to say, keep all of your bike's proportions (especially top tube length) in mind when purchasing a new frame or making changes to your current bike.
The majority of multisport athletes seem to be moving away from the radical forward positions of 5 to 10 years ago. I encourage experimentation in the off-season to determine what works best for you. In setting a new seat position keep this in mind: You will perform most efficiently with the positioning your body has adapted to over time. Changes force your body into a new adaptive 'learning curve' and will not be advantageous in the short-term unless your position was very inefficient prior to the change.

Real Simple......

Shall we discuss how to set handlebars next....

vol

Thank you so much for the elaborate explanations. I myself ride only hybrid, but I'm glad the more savvy cyclists can get something from these replies. I was wondering why I always felt like trying to hard reach the handlebar on the bike with 70° seat tube angle even though the distance between seat and handlebar is almost the same as the other bike with 74°, and I couldn't seem to feel like "standing up" pedaling on the 70° bike as I did on the 74° one even though both handlebars and both saddles are about the same height. Now I realize on the one with 70°, there is always an angle between my torso and legs, while on the one with 74° angle, when the pedal cycle is at the lowest point, my torso and leg are closer to being in a straight line (180°), making me feeling like standing. That's probably also an important reason why I found it easier to go uphill on the one with 70° seat tube angle.

Would be nice

SkyDog75

The seat tube angle could be a contributor to that, but only as a small piece of a big puzzle. You'd have to look at the sum of your contact points to compare the two, not just the seat tube angle. Saddle-to-crank distance and setback. Saddle-to-handlebar distance and drop. Saddle angle. Handlebar width, reach, drop, and angle. Brake hood positioning. Crank arm length and Q factor.

Comparing the feel of two bikes by just the seat tube angle is like comparing two cars strictly by the number of cylinders the engine has. It's not a completely meaningless number, but it's just one of many numbers that add up to the whole.

hueyhoolihan

given what's been said, now i can't imagine how anybody on a recumbent can possibly even motivate the bike at all, although i know they do because i was absolutely smoked by some skinny old man on a very narrow fully faired one just the other day.

Kopsis

The notion that STA affects vertical compliance of the frame doesn't hold water if you actually do a mechanical analysis. The amount of flex in the seattube an any typical angle is negligible because the dual truss structure of a standard bike frame is specifically engineered to prevent it. You can get some flex in the seatPOST since it is anchored at only one end (the ST - TT junction). In that case the STA does affect the seatpost deflection, but it's only one of several factors and may or may not actually be perceptible.

Chainstays and forks may flex appreciably in the vertical (depending on design) and are the source of virtually all of a given frame's shock absorbing qualities. But even these are generally dwarfed by the contribution from the tires.

SquidPuppet

I worded that poorly. I did not mean that the seat tube itself flexed, but that the slacker angle allowed the structure to flex more. Every frame builder I have ever talked to has explained it that way, the more slack the head tube and seat tube angles, the more compliant the ride.

DaveLeeNC

Would I be correct in taking from the well written quote above that what really matters is your saddle position relative to your BB - STA only matters (other than maybe 'the ride/comfort parameter') because it typically affects the BB to saddle parameter. Or did I miss something?

Thx.

dave

Kopsis

Not to knock frame builders, but very few have a mechanical engineering background. They may have years of experience with what works and what doesn't, but if they're telling you that the main triangle flexes more with a slack STA, then they don't really understand the physics involved. If the main structure really did flex that much, it would feel like a wet noodle when putting down power.

On the custom frame I ride, the builder picked the STA so that with the seatpost I like and the saddle I ride, it put my sit bones exactly the right distance behind the bottom bracket to get the fit I want. The overall geometry was all about fit and handling. Ride quality was dialed in through the chainstay, seatstay and fork tubing selection and design.