# Framebuilders - Calculating "Reach"

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View Full Version : Calculating "Reach"

GMM
02-19-11, 01:39 PM
Hi Guys,

If possible, could someone please determine the "Reach" of these three frame sizes, size 51, 53 and 55cm, in this Bianchi Vigorelli geometry chart?

http://www.bianchiusa.com/bikes/gran-fondo/vigorelli/

We have contacted Bianchi Europe directly, but they have not responded for what ever reason.

Any info, how-to advice, is greatly appreciated.

Thanks

unterhausen
02-19-11, 02:54 PM
I assume by "reach" you mean the horizontal distance from the centerline of the head tube to the centerline of the bb. Looks to me that you'll have to assume a fork length, which isn't that hard. Use 375mm +10mm headset stack and you'll have everything you need to get an answer within a few mm.

Road Fan
02-24-11, 05:31 AM
Another common definition for "reach" is the horizontal distance from the seat tube centerline to teh center of the handlebar clamp. This would be equal to the effective top tube plus the stem extension in the horizontal direction. To get reach just add the B1 dimension for the frames you're interested in to the stem extension. The site doesn't give extension, I'd use 90 mm as a general guess.

GMM
02-24-11, 07:59 PM
We're looking for the virtual measurent taken from the BB-center to the center of the upper headset bearing. Appears to be about 370mm, size 53cm, with a 368mm fork + 30mm bearing stack. Does that seem right?

Road Fan
02-26-11, 02:18 PM
Ok, thanks for clarifying. What frame size are you interested in? You said 52, but they make 51 and 53. I assume you want a number better than a guestimate? For the 53 at least the effective TT is 535. The seat tube angle is 74 degrees. The setback at the seat tube top is then

SB=530*cos(74) = 146 mm.

The distance from the BB to the front of the effective top tube is

ETT-SB = 535-146 = 389 mm.

If the bearing stack refers ti the distance along the top tube from the center of the TT intersection to the upper end of the headtube, you need to subtract 30*cos (head angle) to compensate. The head angle is 71.5 degrees, so the compensation is about 9.5 mm, for a reach of 379 mm. So about 9 mm longer than your calculation. I don't know exactly what we did differently, but I didn't see a column for a 520 mm frame.

GMM
02-26-11, 07:44 PM
Thanks for catching that - size 53cm is correct.

Is the "reach" measured to the top-tube/head-tube union, or the center of the upper headset bearing? We always thought it was the later, that may explain why then numbers are "short". Does that target vary from maker-to-maker?

unterhausen
02-26-11, 10:49 PM
the article I saw about reach shows the measurement going to the top of the head tube. Since different headsets have different stack heights, this is the way to go. Of course, it's affected by the lower stack height, but there is not as much variation there.

Road Fan
02-27-11, 10:33 AM
Unter's description makes sense, and it'w what I assumed. GMM, you need to measure from the top of the head tube to the point where the TT center and the head tube center intersect. Use that length instead of the 30 mm that I assumed was the correct dimension. But I don't expect taht to make up for a 9 mm difference.

GMM can you lay out how you did your calculation, maybe using my post as a model? it might be just a matter of needing the math teacher to check our work. I assume the 9 mm matters to you or your customer.

GMM
02-27-11, 02:31 PM
Road Fan, Unterhausen
We used a CAD application to build a wire frame using Bianchi’s Geometry specs. The problem we’re having is that the "reach" specs vary drastically, due to the head-tubes 71.5 degree angle, depending on how we measure the frames "reach".

When we pull these three measurements from the CAD’s wireframe we get:

A. Distance from the BB’s center to the top of the upper headset bearing = 373mm
B. Distance from the BB’s center to the top of the headtube = 379mm
C. Distance from the BB’s center to the toptube/headtube intersection point = 388mm

As you can see, each measuring method produces a fair amount of difference.
The deal is; we’re interested in this bike but our LBS does not stock them - no test ride available and Bianchi doesn’t know/hasn't responded, and we can’t just buy a \$1700 bike without knowing if it will fit.

Should we assume that the "B" method of measuring is the industry standard?

unterhausen
02-27-11, 03:18 PM
I have yet to see anyone using reach as a sizing criteria. Are there fit calculators that use it?

repechage
02-27-11, 06:20 PM
I have yet to see anyone using reach as a sizing criteria. Are there fit calculators that use it?

I have run across a few fitters and builders who reference it. I think it is a way to evaluate the bike as how long a stem one will end up with given that the frame is mass produced and one might ignore where the frame is suggesting on where to place the saddle by the seat tube geometry.

Decades ago I called it effective top tube length and measured to the TT /HT centerline intersections from a plumb line through the BB axle. I was going on the assumption that I wanted to place my saddle with respect to the BB the same, no matter the ST angle. The distance forward of the BB to the steering axis gave me a good indication on the length of stem I was going to require. Remember this is from a time of level top tubes and a stand-over height where the top tube did not change one's voice, nor did it feel as you were riding a girls bike.

The frame that drove this investigation was a track bike I had, it had a long top tube, like 22.75" but a slack seat tube angle of 71.5°... (! what were they thinking?) Anyway, I was set up on the bike when I was just learning frame dimensions... A photo by a friend of me on the bike in a race, showed I was cramped up over the handlebars. I went from a 95mm stem to a 135mm, and looked much better on the bike, but too much weight forward. It was time for a bike that fit. My only guess was the frame was really intended for a big gear pursuiter who wanted to be way back and push into the gear, I was on a gear limit, 79", it was hummingbird legs time, the bike was all wrong for that.

Some builders still use "set back" to place the seat tube angle, it is another way of thinking and just as dictatorial as providing a ST angle as the leg, femur and foot length is known and will drive the numbers along with the rider's pedal style and typical cadence. One either takes the what the builder is suggesting or works around it. I think it is better to know what works first, and find a bike that meets those needs in this S/M/L molded stock frame world. Or, go custom and get what you need.

GMM
02-27-11, 07:59 PM
We always size a frame according to its "reach measurement" too, simply because we know we prefer running 100-110mm stems. Knowing the frames reach makes proper fitting way-easy.

Look at it this way:
If you built three 54.5cm / 150mm headtube frames - one with a 73-degree seat-tube, another with a 73.5 and the third, with a 74-degree tube; each frames reach will differ quite a bit even though each is considered a size 54.5cm, perhaps forcing you to run an "other" length stem.

That's why many fitters still prefer the frame "reach", rather then "ETT" alone. "ETT's" still a neccessary factor, though.

It would be great if every "off-the-shelf" bike company would include it in their geometry specs - so much easier.

repechage
02-27-11, 08:45 PM
We always size a frame according to its "reach measurement" too, simply because we know we prefer running 100-110mm stems. Knowing the frames reach makes proper fitting way-easy.

Look at it this way:
If you built three 54.5cm / 150mm headtube frames - one with a 73-degree seat-tube, another with a 73.5 and the third, with a 74-degree tube; each frames reach will differ quite a bit even though each is considered a size 54.5cm, perhaps forcing you to run an "other" length stem.

That's why many fitters still prefer the frame "reach", rather then "ETT" alone. "ETT's" still a neccessary factor, though.

It would be great if every "off-the-shelf" bike company would include it in their geometry specs - so much easier.

If I were building a frame today with new stuff, I would have to take into account a bit more than yesteryear. Bars are wider, integrated brake levers to me look pretty different from the various mfgs., at least as to where my hand would land and fit, and handlebar reach from the mount sure do appear as they can be all over the place as to throw forward.

Road Fan
02-28-11, 05:15 AM
So GMM, you are trying to determine up front if a 53 mm Bianchi will fit someone without a stem adjustment, if I get it (finally).

I think you and Repechage (and I for that matter) are on the same page as far as setback, seat angle, effective top tube, and reach. I just recently used this strategy to transfer the well-fitting setup of my Woodrup to my other bikes that have shallower seat tubes or more-restricted saddle fore/aft travel.

Your 379 mm does not agree with my 389 mm. We were targeting the same point on the head tube.

GMM
02-28-11, 02:41 PM
That's exactly right, Road Fan. Trying to transfer a fit that works, to a bike with an unknown reach measuremnt.

You're definatly coming up with 389mm, then, for the size 53cm? Not 379mm?

Road Fan
03-01-11, 11:32 PM
GMM, I'm willing to re0run my numbers, but please let's agree on an assumption first. What value would you like to use for the distance from the top of the head tube to the center of the intersection between the actual top tube and the head tube? I assumed 30 mm, but it's only an assumption. Let's at least use the same number, since you don't have the frame and can't measure it directly.

Really, another very reasonable question is, how much accuracy do you need? Is the potential use of a 10.5 cm stem versus 10 cm really a big issue? Can't the difference be made up or at least mitigated by selecting a handlebar with less reach?

GMM
03-02-11, 02:21 PM
Thanks for the offer Road Fan, but, I'm confident in your results. And, like you said, 5-10mm in stem length change won't really matter all that much. Thanks again.

Road Fan
03-04-11, 05:59 AM
You're welcome! Please let me know how it works out.