3speed

I'm looking for a loaded tourer right now and I was surprised to see that the "58cm" Soma Saga had almost identical stand-over height and seat tube length as my 55cm Miyata 914, but a few cm longer top-tube, seemingly throwing off what I thought might be a good fit. I would think you'd want a short top tube to give a more comfortable, upright, riding position for touring. What am I missing here?

MileHighMark

SOMA Sagas are sized by top tube length. I had a 58cm Saga and sold it because it was too big. I would have been fine on a 56cm, though.

andyfloyd

A longer toptube will net you a steeper headtube usually. A Steeper headtube angle / seat angle will quicken turn in and response to any turning input. A touring bike generally has a shorter toptube, and more relaxed angles for the seat/head tube. This makes the touring bike more stable as a more aggressive angle will make the bike feel more twitchy and a touring bike generally tries to feel a little more muted and stable.

Salubrious

Another difference to look for is the distance from the rear tire to the seat post. Under about 1/2" and its more likely to be a racing geometry; 1" or more is apt to be touring.

nlerner

In short, steeper angles and tighter wheelbase.

Cougrrcj

Wheelbase, baby! either longer chainstays, toptube length, more fork rake, headtube angle or any combination thereof... My old '75 Fuji S-10S has a longer wheelbase than most '23"-frame' bikes of the same vintage... makes for a more relaxed (less 'twitchy') ride.

RobbieTunes

Don't worry, once you ride a Saga, you'll know it's not a racer.
It's a nice frame, designed to carry you, your gear, all day, stable and comfortable.

dddd

Top tube length only relates to fit after considering the seat tube angle (how far foreward is the seat cluster "angled" relative to the BB).

You also have to consider the stem length (where a steeper haed tube angle favors a longer stem to preserve steering response).

So, different head tube angles (as may affect chosen stem length) and seat tube angles (as moves the top tube fore and aft) are pertinent to any top tube length decision.

As far as relaxed rider position, a lower pedaling torque might have the rider falling foreward unless the rider's weight is moved rearward on a more layed-back seat position.
A racer on the other hand is pedaling harder, so a more-foreward rider position will counter the pedaling torque so the rider doesn't have to pull themself foreward especially on steep climbs.
The more-forward position allows the rider to be more aero with less need for a sharp bend at the waist (which can limit power).
A more-foreward saddle position also makes for a quicker, more effortless transition from seated to standing, perfect for quick jumps and attacks on rolling hills and switchbacks.


Longer rides mean lower pedaling intensity, so no surprise that Merckx "Century" model frames have a more layed-back seat tube angle as the only change from their standard Corsa Extra.

I do mostly fast road rides between 25 and 80 miles at an average speed of 17-20mph. As such, I prefer an aggressive rider position but with a relatively high bar and a stable steering geometry. Examples of my preferred fit are shown below. The top bike is 72X72 degrees, with a 110 stem, while the bottom bike is barely 71X71 degrees and as such needs a wider bar to control the steering using the same 110mm stem length.
Surprisingly, both bikes are also super-comfortable for all my local "utilitarian" riding, as the foreward rider positioning with higher bars is a great benefit here where it is quite hilly.

I recommend a larger frame size when selecting a frame with a more layed-back seat tube angle, both to extend the top tube and to allow for a comfortable handlebar height.

The saddles shown are level at the front half. The rear portion kicks up but is soft enough that the ramp is quite subtle.
And my arms do get tired riding these on the longest rides, but the forward positioning also spares my legs (which is ultimately what always limits my endurance).

Lastly, a racing bike may have a slightly steeper head tube angle, longer stem and shorter top tube (in combination) to allow the rider to more safely draft the rider in front of him/her with less chance of touching wheels.
The bikes shown below are not built this way, they are touring models set up for fast riding on fairly technical road surfaces, so optimal drafting is of somewhat less importance. The auxiliary "safety" brake levers are important on these bikes to allow periodic "rest riding" with hands at the upper position. I only ever ride the hoods when charging uphill, out of the saddle.

3speed

MileHighMark - How tall are you and what is your stand-over height? What size do you ride in something with a more aggressive riding style than the Saga, like an 80s "racing" type road bike? I think this frame is selling at a decent price if it's a decent frame(gotta do more research), and I'd like to get it if it would fit, but obviously it would be a waste of money if it doesn't fit...

MileHighMark

I'm 5'11", and have a 73cm saddle height (I don't know my stand-over height).

My "go-fast" bike is a 56cm Black Mountain Cycles. Pretty standard geometry--73/73, and 56cm TT. My commuter is a 57cm Salsa Vaya, but that has a more sloping top tube.

3speed

OK. I have long-ish legs, but I'm only ~5'8". So the 58cm Saga would definitely be too big for me if it's too big for you. Thanks.

Puget Pounder

Keep in mind a couple of things about matching fit... For a bike with EQUAL top tube lengths:

1. Changing the angle by 1 degree, adds/subtracts 1 cm from the TT length. Steepening the STA by 1 degree, effectively makes the TT physically 1 cm shorter. Steepening the HTA by 1 degree, makes the TT physically 1cm longer.

2. Despite the info above, the one with the steeper seat tube angle ends up being being effectively longer if you are trying to achieve the same saddle setback (which is important for fit). The steeper STA puts you further forward, but you need to offset that by moving your saddle backward, or getting a more setback post.

Take a look at the angles between touring bikes and racing bikes. The touring bikes will definitely have more slack head tubes than the race bikes, decreasing the effective length of the TT. Don't go by TT length alone, bikes with different angles and TT lengths will fit more closely than at first glance.

MileHighMark

Also, FWIW, my 58cm Saga's STA was steeper than the claimed 73-deg, thus making the effective TT longer.

dddd

As far as the head tube angle, this would seem to have relatively little effect on the feel of the top tube length and overall "reach". This because of the very short difference in height between the bars and where the top tube length is measured from.
The head tube angle can however effect the range of stem lengths that will give the bike good neutral handling.
The seat tube angle, otoh, is angled from the bb all the way up to where a top tube measurement is taken from, so a 1-degree steeper seat tube will push the top tube foreward about .8 or .9cm (depending on seat tube length).

Also, I kind of disagree with comment #1 in that the frame angles actually do not change the physically measured lengths of the tubes (I'm just struggling with your wording).
But of course a builder will have to match physical frame tube lenghs with chosen angles so that all tube junctions meet with no gap or overlap.

I totally agree with your last comment, after comment #2. You described the situation quite well imo.
I caution against over-generalizing about "touring" and "racing" geometry without looking at the spec's though, since many frames have a surprising choice of angles and because the smallest and largest frames have their angles adjusted almost radically in many cases.
Road bike frames built with multisport "tri" capability can for instance have very steep seat tube angles combined with a touring-style 72 degree head tube angle, while road bike frames built for stage racing or century riding can have somewhat the opposite combination of angles.
Case in point is the Peugeot UO8, UO9 and UO10 frameset, which from 1978 to 1979 (US models) underwent a geometry reversal from "Fast Century" (steep head tube, relaxed seat tube) to "Triathlon/Multisport" (steep seat tube and relaxed head tube).
The mid-range UO9 model took on the SuperSport moniker with that change, fwiw, and took on better tubing, new lugs, shorter stays and brazed (no longer spot-welded) cable stops.

dddd

Good point here, as many times the published size-specific geometry is either flat wrong or deviates grossly with frame size!

Puget Pounder

Sorry mate, you are right on all counts.

Good point on head tube. Lack of understanding on my part.

I used "physical length" because I needed a different word than "effective", as to not confuse anyone.

And yes, I was generalizing even though the point of my post was not to generalize frame sizing in the first place.

Listen to DDDD, he's a smart cookie.