seattube angle - how much difference does it really make?

neeb

I was thinking about the supposedly all-important seattube angle and found this on a site advertising a Cervelo TT bike:

But even if you arent a professional cyclist, there are compelling reasons to ride steep on a time trial bike:

1. With lower back flexibility often a limiting factor, a steep seattube angle allows you to attain a flatter back without having to change your bodys internal angles (hip angle, knee angle, etc). Basically, a steeper seattube angle rotates the rider forward around the bottom bracket, thereby leaving the internal angles intact while positioning the back more horizontal.
2. A steeper seattube angle can allow you to even open up your hip angle, thereby reducing the strain on your lower back as well as making the pedal motion less restrictive.
3. For triathletes, several studies have shown that running times after riding a bike decrease if the bike was ridden with a steep seattube angle.

Is there anything in this, or is it mostly b*llox? Surely a steeper seattube doesn't really "rotate the rider around the bottom bracket", because you are going to position your saddle the optimum distance from the BB in any case. Aren't all of these supposed benefits just down to the position of the saddle in relation to the BB? Surely the range of saddle movement combined with the choice of a setback or non-setback seatpost will give more potential variability than the choice between a seattube angle of 72.5 or 74 degrees? I guess a steeper seattube angle will be vertically stiffer given the same materials, but is this of any benefit?

Thinking about it, seattube angle won't even change the position of the rider in relation to the rear wheel, assuming that the same saddle to BB distance is adopted, as this will be determined by chainstay length.

dennisn

Generally speaking, on a road bike, having a steeper seat tube allows the rear wheel
to be closer to the bottom bracket. This means that the two triangles that the bike is
composed of can be smaller and thereby stiffer(somewhat). This extra stiffness can
mean a bit more power goes to the rear wheel instead of into frame flex. It also can
give the bike a feeling of quick handling due to a generally shorter wheelbase as
opposed the the more stretched out "touring" bikes that have a longer wheelbase
because of pannier and foot clearance requirements. You're correct in thinking that
seatpost setback and seat fore-aft position offers a pretty good range of positions
with which you can find that "perfect" spot. I still haven't found mine but I think I'm
getting closer. I have a steel Gios Compact Pro and it's seattube is pretty steep and yes it
is a pretty stiff bike with somewhat touchy handling. The seatpost has 40 MM of setback
and this allows me plenty of saddle movement to the rear, which is sometimes needed
on these steep seatpost frames, in order to get to that "right" spot over the bottom
bracket. Where ever it is???? Time trial bike geometry is another animal alltogther.

Dennis Noward

neeb

Generally speaking, on a road bike, having a steeper seat tube allows the rear wheel
to be closer to the bottom bracket.

Duh, forgot about that... :oops:

So the steeper seattube effectively allows a shorter chainstay. But the chainstay length will still be the true measure of the distance of the rear wheel from the BB (given the same BB height) and the biggest determiner of the size/stiffness of the rear triangle, right? So the steep seattube potentially allows shorter chainstays, but it's the chainstay length that really matters?

neeb

Basically, my nice Ti road bike (73 degree seattube, 407mm chianstay) is much more fun and nippy to ride than my aluminum cross bike, despite the latter being fitted out with decent wheels, skinny tyres and the same setup (i.e. same saddle / BB / handlebar distances). I thought that the cross bike had a shallower seat tube angle, but actually now I check it is also 73. It does have a considerably longer chainstay at 425mm however, and a shallower head tube angle (72 as opposed to 73 on the Ti bike). Just trying to figure out how much of the difference in feel is due to geometry, materials, weight, psychology, or whatever...

dennisn

neeb wrote:

Generally speaking, on a road bike, having a steeper seat tube allows the rear wheel
to be closer to the bottom bracket.

Duh, forgot about that... :oops:

So the steeper seattube effectively allows a shorter chainstay. But the chainstay length will still be the true measure of the distance of the rear wheel from the BB (given the same BB height) and the biggest determiner of the size/stiffness of the rear triangle, right? So the steep seattube potentially allows shorter chainstays, but it's the chainstay length that really matters?

Yes and yes. I think.

aracer

You're right in that it doesn't do anything for rider position which can't be achieved with a forward position seatpost - in fact such seatposts are exactly what triathletes did use before specialist bikes were available. However unlike such an adapted road bike, the rest of the geometry is all adjusted to allow for the more forward saddle position - if you want to use the same rearward saddle position as on a more conventional roadbike then such a frame is pointless.

One of the changes is in fact commonly shorter chainstays, but this is done (along with a longer than normal front-centre) to adjust the weight distribution of the rider on the bike to close to the same as on a conventional bike despite being further forwards relative to the BB, rather than simply because it's technically possible (given typical aero monocoque frames used in TTs, the seat-tube angle is actually irrelevant to how short you can make the chainstays). Otherwise if you take up a TT position you end up with a lot of weight on the front wheel and poor handling (as the pioneers with forward seatposts found).

neeb

Ok, that makes sense.

I guess longer top tube / shorter stem (as opposed to shorter top tube / longer stem) will also tend to shift the center of gravity further back from the front wheel? (as well as increasing the wheelbase).

John.T

The steeper seat tube does rotate the body around the BB. You will have about the same body shape but your hips are further forward and your shoulders are lower. This gives a better aero position without cramping your stomach area. As Aracer says modern TT frames are built to re-set the weight distribution so as not to wreck the handling. So basically it is not bol***ks. (although this is probably what you are sitting on while riding one. More likely hanging off the front of the saddle though).