Design Question - Relationship between STA and TT

[saab2000]

Could someone who knows more about this than I do explain to me what is the relationship between seat tube angle and top tube and why? I have heard that a shallower seat angle effectively creates a longer top tube. Trying to visualise what is happening here......

I have several different bikes with different STAs and different top tube lengths. Some seem to fit better than other, making it difficult for me to know what I am looking for in a bike.

My two main rides are a Croll (with fork by DWF of Anvil) and a Strong.

The Croll has a 73 STA and the Strong a 72.5. The Strong has a 59 tt and a 13 stem. The Croll a 58 and a 14. They both have identical saddles, but wildly different handlebars. The Strong is good, but the Croll with that long stem seems a touch better, especially in handling. That is not a slam the Strong, which is a sweet ride by almost any definition!

Why is that? What's going on here?

[BdaGhisallo]

Without going into the minutiae of actual numbers and angles think of it this way:

Divide the top tube into two with a plumb line hung from it ( assuming it is horizontal), passing through the bottom bracket. Let's call the section of the tt behind the bb 'x' and the remainder at the front end 'y'. Now we also know that slacker seat tube angles give greater setback 'x'. Now think of keeping a constant tt length. Now imagine a sta of 90deg. In this case 'x' is zero and 'y' is equivalent to the entire top tube length. Now make the sta 60deg. That would be a big slope and would pull a lot of the tt of our given length behind the bb in the form of a very large 'x'. Consequently 'y' will be quite small.

I always think of frame reach as being 'y', and 'x' is what we all know as frame setback. So the slacker is the sta, the less reach forward from the bb there will be, and the shorter the wheelbase will be for otherwise given dimensions, as more of the tt is positioned over the rear wheel and not pushing the front end forward from the bb.

So if you keep the head tube where it is on a bike and stretch the toptube rearward to give a slacker sta then, yes, you have created a longer tt as you had to increase 'x' to get that slacker sta whilst keeping 'y' constant to maintain the head tube in position.

Am I clear enough?

I always prefer to think in terms of a given tt length, that a slacker sta gives less reach, while steepening the sta, thereby lessening the setback 'x', will result in greater reach forward of the bb. And since the bb is the point that bike fit and frame design revolves around we should think of it like that, imo.

Geoff

Draw yourself an outline of a frame on a sheet of paper and put a dotted line vertically through the top tube and the bottom bracket and it will become clearer.

[CNote]

Assuming identical saddle setbacks (and saddles), same saddle to bar drop, same stem angle, etc., your Croll should have a slightly longer reach.

Imagine a bike with a 90 degree STA. Ridiculous, I know, but good for explanation's sake. Imagine another bike with a STA about 10 degrees more slack (80). Both bikes have the same TT length, so the second bike by rule has more top tube behind the bottom bracket (the 90 STA bike has no TT behind the BB). However, if the saddle setback is the same on both bikes (let's say directly over the BB, ridiculous again), the reach on the 90 STA bike is longer because the rider's position relative to the BB is the same. Clear as mud?

[Len J]

but first a word about reach. you cna't just compare TT and Stem length (assuming the same stem angle) and ignore differences in Handlebar reach. For my discussion, I am going to assume four constant variables:

-Handlebar reach
-Stem angle
-Seat tube length
-and head tube angle

For a 57 cm ST length, the rule of thumb is that for every 1 degree less of ST angle, you need 1.2 cm of TT length to get the seat to bar reach the same. Here is why.

Let's say you have a 73 degree STA and a 57 TT length. Let's also say that your seat is jammed all the way back in order to get your KOP where you want it (wherever that is). Now if you try to duplicate that position on a bike with a 72 degree STA, your seat to BB relationship doesn't change (to get to the same KOP position), what happens is now the Seat post further back on the rails (because of the slacker STA. The result is that if you drop a Plumb Bob from the nose of the saddle, it is now (on the 72 degree STA bike) further from the seat tube (because of the slacker angle. To get the same reach, you need a longer TT to compensate for the slacker STA to maintain the same position relative to the BB.

Hope that helps.

Len

[Needs Help]

Top image: top tube length increased for slacker(=smaller) seat tube angle.
Bottom image: top tube length held constant with slacker seat tube angle.
http://www.pbase.com/pbase222/image/43591853.jpg
http://www.pbase.com/pbase222/image/43591851.jpg

[saab2000]

Part of the issue I have is that my bars are different on all my bikes. This is clearly throwing a wrench into the whole thing.

The first of the above diagrams is more or less what happened with my Strong. It has a shallower seat angle, and consequently a longer reach. It works pretty well, but I think that it could work better.

I like to ride on the tops of the handlebars a lot, but not feel as though I am reaching a crazy amount when going onto the hoods or drops. My Cinelli 66 deep drop bars (on my Croll) can be approximated with some Deda deep drop Newtons. My Strong has Modolo Curvissima carbons.

The reach from the tip of the saddle to the center of the bar is identical on both bikes. I get numb hands on the Strong and not on the Croll. I am dumbfounded.

I guess I sit a bit further back on the Strong because it has a slightly shallower seat angle. The frames are both very nearly 59 cm (c-t) on the seat tube.

Perhaps for my next bike I will bring my Croll to a builder and have it duplicated with some improvements like an 8 cm bb drop, modern and stiffer tubes, etc. It seems to be the bike which works best for me of all of them in terms of fit. The Strong is a close second.

When I ordered my Strong I was spending a lot of time on my Anvil, which I felt to be too short horizontally. So I ordered the Strong with a 1 cm longer tt. Then I installed a seatpost with more setback into the Anvil and suddenly it felt just right. By then I had taken delivery of the Strong and the setback feels good, but I feel I like it might be too long, except it feels perfect on the tops.

What I need to do here is just shut up and ride...... :beer:

BTW - Those diagrams are great in aiding in visualising what is going on here.

[Dave]

First, these frames should be the same c-c size for 100% accuracy. I'll assume they are. Without the c-c frame size, the reach of the frames can't be calculated with total accuracy either. Since they are nearly 59cm c-t, lets say they are 57.5cm c-c. Close enough.

The reach of the Croll frame is 58 - (57.5 x cosine 73) = 41.2cm.

The reach of the Strong frame is 59- (57.5 x cosine 72.5) = 41.7cm.

From the above, the Strong frame has a .5cm longer reach, but a 1cm shorter stem, for a total of .5cm less reach.

Since the handlebars are different, that's relatively easy to compare also. Place a 1" wood dowel or broom handle (anyting of that general size) across the brake hoods and push forward to simulate where the crook of your thumb would rest. Measure the distance from the inside of the handle bar to the inside of the dowel on both bikes to compare the handlebar reach. You could also take a measurement from the tip of the saddle to the dowel and compare how much difference there is in the total setup.

This comparisom also assumes that the brake hood height, not the top of the handlebars, it approximately the same. Different bars may be set to the same height, but some bars have more "ramp down" that will lower the brake hood, unless the bar is rotated upward. My suggestion is to pick a favorite bar and use it on both bikes (that's what I do). If you haven't read some of my previous comments about handlebar (actually brake hood) height and stem length, I'll repeat that changes in height are similar to changes in length. Thus, bars set higher, but with a longer reach may offer a similar fit to those set lower, but with a shorter reach.