Bianchi sizing confusion

SallyT

Good morning!

I'd be very grateful for some advice. My head has been turned by a lovely Bianchi but I'm baffled as hell about the sizing. I'm 5'8.5 with a 32.25 inseam. My Wrench Science fit says a 53cm toptube centre to centre and 54 centre to top. I'm happy on a Giant size medium and Specialized 54 (I don't own two bikes by the way. One day....).

The bike I'm looking at is available in 51, 54 and 57cm sizes. According to Evans, a 54 is for riders 171-173cm so by that reckoning I'm a tiny bit taller than that range.

I might be being simple, but the geometry chart I've attached is puzzling me. I'm taking BS as being the length of the top tube (530mm on the 54 model) and 51 as the effective top tube (550mm). As it's a sportive-style sloping top tube, shouldn't the effective top tube length then be shorter? Of course, I have probably got this completely wrong!

Anyway, sorry for rambling. Put simply, am I right to take a gamble on the 54cm or save my cash and look elsewhere?

Thank you!

noodleman

Im exactly the same height and inseam as you. I have a 55cm infinito which i had fitted and a 53 Oltre which i didnt get a fit on. Both of them fit me fine. If anything, the oltre feels better. Id say you would have no probs with a 54.

SallyT

Brilliant! Thank you!

neeb

Effective top tube length is the length that the top tube would be if the bike had a horizontal top tube. While it's true that a sloping top tube would be longer than the effective top tube length if both were measured from the head tube to their intersection with any horizontal line near the back of the bike, if you think about it, a horizontal top tube would extend further back, as it would be meeting the seat tube at a higher point and the seat tube is sloping back at 73 or 74 degrees...

The problem with the geometry table above is that the diagram is misleading, even if the figures are (probably) correct. B1 (effective TT length) won't really extend until it meets a horizontal line drawn through the intersection of the sloping top tube and the seat tube (that point is arbitrarily dependent on the angle of the sloping top tube after all), it will extend to a where the intersection between the seat tube and a horizontal top tube would be, if the bike had such a top tube.

Hope that makes sense...

Short version - ignore actual top tube lengths on bikes with sloping top tubes, it's only the effective top tube length that matters.

neeb

So I would be careful if you think you really need an effective top tube length of 53cm, it appears that the size 54 has an ETT of 55..

What model of Bianchi is it?

SallyT

It's the Vertigo Veloce. Yep, not sure it's the right size.

neeb

Just to illustrate how effective TT length can often be longer than actual TT length:

Basically, it's not a right angled triangle.

55cm seems moderately long for an effective top tube on a 54cm "sportive" geometry bike however. I would ask lots of questions!

neeb

And just to further complicate things, even effective TT length doesn't give you the "reach", as it is influenced by seat tube angle. You need to take seat tube angle into account along with effective top tube length, or else use "reach" instead. Reach is measured from the head tube to a vertical line through the BB. Two bikes with the same reach but different effective (and actual) top tube lengths and different seat tube angles will have the same reach to the bars once the saddle is placed in the same position relative to the bottom bracket.

Short version: it's complicated..

SallyT

That all makes sense. I will continue my hunt. Three weeks worth of bike shopping, yet I buy the first car I see in a garage... at least my priorities are right!

Thanks for your help.

darkhairedlord

neeb wrote:

Reach is measured from the head tube to a horiziontal line through the BB.:

er, no it isn't!

neeb

darkhairedlord wrote:

neeb wrote:

Reach is measured from the head tube to a horiziontal line through the BB.:

er, no it isn't!

Well, vertical line not horizontal line obviously (oops!). Typo corrected.

neeb

To the OP - here is a better geometry chart for the vertigo that includes frame reach (X in the chart). Can you get frame reach from your wrench science results?



Frame reach & stack (Y in that chart) taken together really are the best way to know whether a frame will fit you or not.

They basically give you the exact location of the top of the head tube assuming your saddle is always set in the same position relative to the bottom bracket.

The problem is that not many people know their ideal reach and stack..

SallyT

Here's my vital stats:

WS Recommended Road Sizes
Frame Size center-to-center: 53 cm
Frame Size center-to-top: 54 cm
Overall Reach: 61.78 cm
Saddle Height: 71.77 cm
Handlebar Width: 42 cm

neeb

SallyT wrote:

Here's my vital stats:

WS Recommended Road Sizes
Frame Size center-to-center: 53 cm
Frame Size center-to-top: 54 cm
Overall Reach: 61.78 cm
Saddle Height: 71.77 cm
Handlebar Width: 42 cm

It's difficult to relate that precisely to frame geometry without knowing saddle setback, i.e. how far back the saddle is set from the bottom bracket (how far back from the pedals in effect), as well as what they mean exactly by "overall reach". Saddle setback should be determined first, then you look at getting the reach to the bars correct from there.

That figure for "overall reach" sounds like it could be measured from the tip of the saddle to the brake levers, or maybe from the middle of the saddle or the top of the seatpost to the bars? In any case, it's difficult to do much with it unless it is defined more precisely.

I think those "frame size" figures are for the seat tube rather than the top tube, in which case they can only be used as a very rough guide for sizing (seat tube length in itself isn't that important within certain bounds).

But even if you had exact figures as outputs, I'd be wary of using an online fit calculator to choose a bike frame. Reach in particular is very difficult to get right by just measuring lengths of arms and torso etc.

Are you completely new to road cycling, i.e. you don't have a bike already that fits or which you know is too big or too small?

SallyT

No not completely new. As mentioned above, I know my sizes in a couple of brands.

Yes, it's a complicated process! Obviously the best thing to do is go and ride the bike but when there's online bargains tempting you, it is worth a little look.

neeb

SallyT wrote:

No not completely new. As mentioned above, I know my sizes in a couple of brands.

Sorry, missed that! Which Giant and Specialized models? If you have a bike that you know fits you perfectly already, it's possible to work out exactly if another bike will fit from the geo charts.

cgraley77

I have the Vertigo Veloce 57cm...and I'm 6ft - fits perfectly. Not sure if that will help or not!

SallyT

Giant Defy - medium
Specialized Roubaix - 54
Merida Ride - 52

If that helps?

CGraley77 that doesn't really help, but thank you!