Huge difference in stack but tiny difference in reach between frame sizes?
 

Can someone explain this to me, I see it across all frames from all manufacturers. Why would there be only a 4mm in reach between 56 and 58mm, but 32mm difference in stack height? This is for a carbon Diverge.

At 5'11, 32mm pant inseam, 72" wingspan, even though 56mm is the conventional size for me is there a downside to going to 58mm if I prefer a more upright riding position with such a minimal increase in reach?

Specialized Bicycle Components

http://i64.tinypic.com/n30qh5.png

32mm inseam? 32mm is approx 1.25 inches, so I'm guessing you mean a 32 inch inseam. :p

Have you ridden a 56cm and a 58cm to determine the actual difference in reach?

Ah yep 32". I've ridden a 56cm and it felt fine but I'd appreciate a slightly more upright riding position if it didn't compromise the rest of the fit.

Compare your current 56 in stack and reach. Factor in how many spacers you have under your stem, and stem length. That should tell you which frame will make it easier to put your handlebar where you want it. If the stack on the 58 is greater than your current stack + spacers it's going to be problematic it fit it.

The head tube angle is slacker than the seat tube angle, that plus the increasing head tube length makes the reach increase a lot slower than the stack increases as you go up the geometry chart.

Due to the way it is measured a frame with 32mm higher stack will actually have a top tube length 10mm longer if the reach measured the same. So if it is +4mm reach it is actually +14mm when combined with the +32mm stack.
The Cervelo website explains stack and reach fairly well. Have a look at that and you will see how it works.

The 58 cm frame will be equipped with 175mm cranks, which are rather long for your legs, unless you happen to pedal like Jacques Anquetil or Greg LeMond. My opinion is, rather than buy an oversized bike to raise your back angle, find a model in the correct size with an extended head tube.

Frankly, your long arms should enable you to handle a lower handlebar with ease. Maybe you should be looking at flat-bar bikes.

Dean, if you can explain that better, I am happy to reconsider it, but I don't think you have it right. Stack and reach are absolute indicators and once established are not modifed by other measurements. Talking about what the top tube length or seat tube angle or head tube angle are will not increase or decrease the stack and reach. The top tube will have to be 14 mm longer to produce a 4 mm longer reach on a 32 mm higher stack, yes, but the stack and reach stay what they are. And they are all that matter.

I just noticed this geometry chart is for a Specialized Diverge. The curious thing about Specialized is that head tubes take an enormous disproportionate leap in tallness at 58 cm. The only explanation I can fathom for this is that Mike Sinyard is quite taller than average, he rides a 58 or a 61 Roubaix, and he tends to sit tall in the saddle.

https://sp.yimg.com/xj/th?id=OIP.M1c...=0&w=300&h=300

I'm not sure if Dean is taking about the same thing, but the problem with stack and reach, as they are defined today, is that the reach is not measured at the same height across the frame sizes. To take the Diverge as an example, if you take a size 56 with 35 mm of spacers, its actual reach to the stem will be more than 4 mm shorter than on the size 58 with 5 mm of spacers. The actual difference in reach will be around 13 mm, so a little more than a full stem size. It can be easier to visualise this by using bikegeo.net and changing varous measurements to see how it influences the nominal and the actual stack and reach.

@OP, how high do you need the handlebar to be? The Diverge already has a super upright geometry and you should be able to get the bar close to saddle height on the 56. Sizing up to 58 would increase the stack, but it would also increase the actual reach to the bar by 2 cm (the factory stem is a cm longer on the 58). Yes, that can be fixed with a shorter stem, but you would be left with a frame that has a higher top tube, higher BB and longer wheelbase, and that will make it feel bigger, heavier and less agile. Depending on what you prefer, you might also describe is as "more solid and sure-footed", but you won't know that without test riding both back to back.

As Fiery said due to the reach being measured at the stack height this has to be allowed for when comparing the reach on two different frames with different stack heights. So although the stack/reach idea is an improvement on previous ways of comparing frames they are not independent of each other. Which is basically what you are saying anyway I think?

Op,
Your question can be answered pretty easily, if you provide two bits of data.
1. What is your saddle height from BB center to top of saddle along the seat tube?
2. What is you preference for handlebar height relative to the saddle? Do you like your handlebar level with, below or above the saddle and by how much?

This will direct you to the appropriate frame size as you can ride either 56 or 58 at your size and proportions.

Btw, I own and ride two identical geometries. I am 6'1" and ride a 58 Roubaix and Secteur.

Answer the above 2 questions and we can solve your dilemma.

35 mm increase in head tube length between 56 and 58. That's where the added stack comes from. That should actually bring the bars closer despite the top tube lengthening---which is why reach goes up so little. Answer Part A, should fit fine. Answer part B, follow RonH's advice and test a 58. "Should fit fine" and "Fits fine" are factually unrelated.

vinuneuro
I have no idea about the difference.
Notice you want to move to A more upright riding position .Might want to test ride A endurance type road Bike is very similar to a road race bike, but with geometry, specification and frame materials tuned to make them more comfortable over longer rides and rough roads.Most have longer head tube,chain stay and wheel base.But A shorter top tube.I just started looking into buying one.I hope I got most of this info right.

As I was planing on getting A newer Giant TCR,I have A '04 already.But I'm getting older,been off the bike way too long.And want to try A different geometry and see how it fits.See what your LBS has to say and offer.
Looks like the Specialized Roubaix has same wheel base as the bicycle you are looking at,but different specs.

Going to try A test ride A Giant Defy Advanced,Scott Solace,Cannondale Synapse CARBON,Specialized Roubaix and Jamis Xenith Endura Elite.

Stack and reach can be adjusted independently. The effect of top tube length on reach is not independent of stack (or seat tube angle for that matter). So once you determine how much more stack and reach you want in your next size frame up the ladder, simplistically you add top tube length and head tube length to get it. Of course seat tube angle also often changes as could BB height and front center and make it more complicated. I think what you are trying to say is that the amount of top tube length you add has to be the amount of extra reach you want PLUS the reach lost due to the higher stack height slanting back at the head tube angle. And yes that loss is about 1/3 of the stack height increase.

And this is all just the frame. When the bike is built up and adjusted for the buyer, the final stem height, length and angle all play a role in the finished reach drop to the bars.

First .... test-ride a 58 if you can.

Then, if you can't ... go by the numbers listed. If reach increases a very small amount, you can easily readjust with a slightly shorter stem, which will have minimal impact on operations or aesthetics. You can also buy a stem which angles up sharply, giving you an even more upright rising position. It might look odd to some "purists," but they won't be paying for or riding it.

I guess I would look at it this way, if none of that worked .... How much do you plan to spend on the bike, and how long do you hope to keep it? For me, if I had to spend a weekend day and a few gallons of gas driving to a particular store to test-ride a particular bike in order to make sure that a multi-year, multi-thousand dollar investment wasn't wasted ....

Just a small update and closure for this thread. I was finally able to ride a 56 and 58 back to back at a store 2hrs away. Those who said stack and reach are not independent of frame size were spot on. The 56 fit very well given my dimensions and despite the geometry chart indicating only a 4mm difference in reality it's a lot more of a stretch than that when moving to the 58.

Thanks again for everyone's feedback.