"proper" seat tube angle?

[scrooge]

I'm in the process of ordering a custom tandem and got a call the other day from the designer. He wanted to know if I was satisfied with my current STA on my single(if I was able to get into the position I like etc...). My current angle is 72.25, with 200 mm cranks. Is this wierd? I've never dropped a plumbline from the knee (not clever enough to figure out where the point on the knee is supposed to be, or if KOPS even means anything. I've just fiddled around and am relatively satisfied with where I'm sitting). I've noticed that most of the tandems from this builder (co-motion) have 73 degree STAs. So, is there a "proper" STA? How will this affect the handling of the bike? For you math types--how would the long cranks affect the STA? (The tandem will have them too).

While we're at it, how does one measure STA?

[Dave]

While there is no proof that placing the knee directly over the pedal will guarantee the most efficient pedaling, at least it's a starting point. There are several methods to measure KOP and they do NOT all produce the same result. One method is to drop a plumb line from the boney protrusion below the kneecap to the pedal spindle. Another method, favored by Andy Pruitt, is to use the the front of the knee and drop the plumb line to the front of the crank arm. Not all cranks have the same distance from the pedal spindle to the front, so there can be .5-1.0cm difference, depending on the brand of crank you own.

As for the STA, what you want is an angle that will produce the position you want with the seatpost style you want and center the saddle fairly well on the seatpost. Every degree of STA will move the seatpost about 1.2cm. The exact amount depends on your saddle rail height.

Another thing to keep in mind is that you can't just change the STA without making the appropriate change to the TT length, or the bike won't fit the same. As an example, I've got one frame with a 74.5 STA and 52.5cm TT that fits nearly identical to one with a 72.5 STA and 54cm TT. The only difference, is the one with the 72.5 STA requires a straight-up (no offset) seatpost to position the saddle where I want it (the same place as the other frame). The other frame works best with a 2cm setback to the post.

http://www.coloradocyclist.com/bikefit/

[Marco]

are you sure that you have that right?. I would think that it is 74.5 sta and 54 tt on one bike and 72.5 sta and 52.5 tt on the other.

Also, who can speak to the difference between riding a 74.5 sta vs. 72.5 sta apart from the issue of making sure that you have the top tube (and presumably saddle to bar drop) issues figured out?

[PBWrench]

Beit Hillel or Beit Shammai?

[Marco]

or Langdon Co-op? Sorry to answer a question with a question: it's a 5,000 year old tradition.

[Ti Designs]

Proper seat tube angle would be the angle that puts your seat somewhere in the center of it's adjustments in your riding position, so you have room to tweek in either direction. This should be the goal of any custom...

As for your position and KOPS, I see things in a different light. KOPS is an oversimplification of the fitting process. I don't understand how some people spend endless hours obsessing over little details on their bike, yet they're OK with a generic solution to the saddle to pedal relationship.

You have two large muscle groups which deliver most of the power to the pedals. The glutes which drive the hip down, and the quads which extend the knee. KOPS is based on a 90 degree angle at the 3:00 position, which yeilds the greatest mechanical advantage ( SIN(90)*crank length ). Now take that model at that position and see just what the quads can add - nothing, zero, zip, zilch! ( COS(90)*crank length). To get beyond the oversimplification you need to understand the job of both muscle groups and how position changes that job.

The quads extend the knee, their primary function is moving the crank in the horizontal plane. Moving the saddle forward over the pedal stroke tends to emphasize the use of the quads over the top of the stroke. The glutes move the hip down, but seeing as there are two pivots below it (ankle and knee), the resulting force at the pedal isn't so simple. Moving the saddle back allows the rider to use the glutes sooner in the pedal stroke, so it's a matter of time. Mechanical advantage also plays a role, as the seat moves back the angle at the hips increases and increases the mechanical advantage to a point. There's also range of motion to take into account - you don't have unlimited range. When you start getting beyond your range of motion, you start forcing one leg over the top with the other, both wasting energy and causing pain or injury.

I can't set a rider up in the perfect position when doing a fitting. The best I can do is get them in the ball park and explain that while they are riding they will feel the burn in one or both of those muscle groups. Everybody has a different balance of strength between the two, a good position is one that makes the best use of both. Tuning the poisition is a matter of taking a lot of rides with an allen wrench and the knowledge of what's going on.

Sorry to burst any bubbles. Should we talk about that rule of where you see the bars in relation to the front hub next???

[e-RICHIE]

Proper seat tube angle would be the angle that puts your seat somewhere in the center of it's adjustments in your riding position, so you have room to tweek in either direction.


agreed.

the angle is a means to an end, not an end in and of itself. it's not
some cult of personality, pop-type information that is magically assigned
to a bicycle. think of your frame as not having a seat tube and see
your saddle as it sits in space. there is both a vertical and horizontal
measurement at work here. measure these with a tape measure and
get it to the mm.
why speak in vagueries (sp?) about parts of degrees that no one can
really measure effectively atmo?

[Dave]

are you sure that you have that right?. I would think that it is 74.5 sta and 54 tt on one bike and 72.5 sta and 52.5 tt on the other.

Also, who can speak to the difference between riding a 74.5 sta vs. 72.5 sta apart from the issue of making sure that you have the top tube (and presumably saddle to bar drop) issues figured out?


Many folks find this confusing. A proper frame comparison is done with the saddle in the SAME position relative to the BB. A frame with a slack STA will require the saddle to be moved forward, thus reducing the reach. My comparison is correct and yours is backwards.

Steep STAs just require a post with more setback. The slack STA may require a post with no setback and reduces the clearance between the tire and seat tube. With the adjustable dropouts on the 381, I can just about get the rear tire to hit the ST. I prefer a steeper angle.

[Marco]

I stand corrected and understand. Thanks for the clarification.

I am still looking for more clarification regarding the STA question and the difference between a steeper and more slack angle. I hear the following voices that I paraphrase (inadequetely and without justice):

e-richie: there is more than one way to achieve the contact points....
dbrk: fit is an evolving process......
Ti-Designs: Here is the scientific explanation of what is happening......

Here is my laymens question: when two people who have the exact same body proportions get into the same car, they may each find a different way to adjust the seat (and lets leave it at just the amount of recline and the corresponding fore-aft adjustment for reach to the pedal) in order to achieve comfort and their individual notion of the ideal fit. Is the STA question any different than that?? By the way- I understand that there are implications for the amount of power that can be produced as Ti-Designs so eloquently discribes.

[jerk]

proper setback or the "right seat tube angle" is one which balances the weight of the rider properly between the two wheels. it should have more to do with body type (big hips, small hips, broad upper body, thin upper body) than getting the knee in line with the ball of the foot going throough the middle of the pedal spindle....although that usually gets things in the ballpark for a cyclist of average size and proportions.

jerk

[Ti Designs]

Here is my laymens question: when two people who have the exact same body proportions get into the same car, they may each find a different way to adjust the seat...

Two points to be made here:

First, body proportions alone is not enough data. There are dozens of fit systems out there that try to fit people base on a number of measurments. They aren't worth the paper they get printed on. Range of flexability and strength of muscle groups have to be factored in.

Second, fit is not an absolute. You can get fitted by a dozen different people and wind up in a dozen different position, all with reasons on why you are where you are. Much like the car seat anaolgy, you can perform all the functions, your job is to find out which position suits you best.

It's funny you bring up car seat position. I autocross a Honda CRX. The Japanese have a more reclined and lower position while the German cars have much more upright positions. I can't get the hang of the weight balance of the VWs, but I switched to a Konig racing seat to get their position...

Lastly, the Jerk has it right - you need to look at the big picture. Things go wrong when you don't. If you set up saddle to pedal relationship based only on leg proportions without looking at upper body, you're not dealing with the true balace of the bike. It's like truing a wheel, you can't deal with one spot, you have to true the whole thing.

[Too Tall]

Scrooge, Dwan can guide you he has tons of experience. If you asked me I'd say to stay with what is working for you on your single. We always setup tandems as if the fr. and rear were mirrors of folks single bikes with zero comprimise. You measure STA easiest with the bike on level ground...use a big @ss framing square to bisect the bb crank bolt and measure the resulting angle. The easy way is to use a digital level.