Question for the math majors in our forum

[swalburn]

I may be picking up a new frame in the future, and I need some help with the math. My current bike has 56.5 top tube with a stem that is 12 cm. It is a thomson x2 set to a -10 degree slope. I may be purchasing a bike with a shorter head tube. I am going to use a positive 6 degree 12 cm stem or flip my thomson to a positive 10 degrees. Can someone tell me how much this will raise the bars compared to my current setup. My current bike is fierte if that helps. I'm currently doing lots of yoga so I'm probably a lot more flexible than when I was previously fit. But for right now I just want to know how much I have to gain by changing stem variations assuming everything else stayed the same.

[Louis]

Swalburn,

I'm no math major, but I can tell you that in order to figure out how much the vertical component of the +6* and +10* stems will be you'll also have to specify the head tube angle.

As stated one can figure out how much the bars move relative to the old position, but one can't figure out how much of that is vertical motion and how much is horizontal, unless one assumes a number for the HTA.

You may want to consider a shorter stem, since by flipping to a positive angle you'll be shortening the reach.

Louis

[RPS]

If you flip a 12 CM stem over from -10 to +10 degrees, you'll have to affix it to the steerer tube approximately 4 CM (a little over 1-1/2 inches) lower to end up with the bars at the same elevation. That should give you a rough idea of where you'll end up given the shorter head tube and whatever spacers you have to work with.

[FierteTi52]

This Chart may help you:
http://www.habcycles.com/fitting.html

[mflaherty37]

Also, with a 73 deg headtube taking 4CM off the height moves the handlebars 1.16cm further out. Flipping a 10 deg 12cm stem from pointing up to pointing downwards adds 1.21cm as well.

[sbornia]

see this thread (http://forums.thepaceline.net/showthread.php?t=24811)

[Louis]

OK, I put it all in a spreadsheet. Confirming what others have said above:

Assuming a 73 deg HTA and a 12cm stem:

Going from -10 deg to +10 deg raises the bars 4 cm and brings them 1.22 cm closer to you.

Going from -10 deg to + 6 deg raises the bars 3.2 cm and brings them 0.86 cm closer to you.

[Louis]

I just ran the numbers for a 73.5* HTA and the numbers barely change at all, so I think you should now have what you asked for.

LMK if you want to try different stem lengths.

Louis

[RPS]

What I tried to say was that you'd have to slide the stem about 4 CM further down along the axis of the steerer tube, just like adjusting an old "7" quill down by inserting it an additional 1-1/2 inches.

Assuming his combination of shorter head tube and spacers allows for the bars to be set at the same exact elevation relative to the ground or saddle, the reach will be essentially the same. Additionally, a degree of HT angle doesn't make enough difference to worry about IMO.

Flipping a stem over doesn't change the reach when the bars elevation is held constant -- you end up right where you started from. If I'm wrong on this, please let me know.

[swalburn]

If you flip a 12 CM stem over from -10 to +10 degrees, you'll have to affix it to the steerer tube approximately 4 CM (a little over 1-1/2 inches) lower to end up with the bars at the same elevation. That should give you a rough idea of where you'll end up given the shorter head tube and whatever spacers you have to work with.

Thanks, that was exactly what I was looking for.....I should have mentioned that both bikes have 73 degree head tube angles.

[RPS]

swalburn, if it turns out you don’t have 4 CM to work with on the new frame (which is a considerable amount unless you were using a lot of spacers under your stem or your new frame has a much shorter head tube), a 12 CM by 0 degrees will split the difference and only extend your horizontal reach two millimeters (again, calculation assumes your goal is to end up with the bars at a predetermined fixed elevation).

[swalburn]

I'm currently riding a 58 Fierte. I'm about 5'11. I have 2 cm of spacers under my stem and have the stem set up at -10 degrees. You can check the geometry at the serotta website. I have a buddy who is selling a Large look 585 frame. It has a considerable geometry differences from the Fierte. Since purchasing the Fierte, I've lost about 20 pounds and have improved my flexibility doing yoga. The HT on the Look is only 156. The top tube is 56, and it has a head tube angle of 73.75 degrees. I've been crunching some numbers and my rough estimate indicates to me that there is about a 5 cm HT difference between the two bikes. I'm assuming about 3 cm for the for the difference in HT and another 2 cm for the fact that the serotta has a conventional headset. Obviously these are rough numbers. I'm also comparing other differences between the bikes. I'm just trying to see if I could get comfortable on this bike. I think I should probably be refit, but I want to wait because I know I will be even more flexible and thinner during the actual cycling season. At the end of last season I barely rode on the hoods at all on the Fierte. I was in the drops most of the time. I've been trying to get into a slightly racier position on the bike.

I really appreciate everyone's input on this topic.

[vaxn8r]

Swalburn, one thing you have to ask yourself is why would you do it? My thoughts wouldn't be so much making the bike duplicate what you now ride as much as learning to ride the race bike. 15.6 HT is already a generous size. Add a cm or two for spacers and then keep your 80 deg stem the way it is. Another option is a Richey -6 deg stem and then even one less cm of spacer. Either way I'm guessing you'll have about 6-8 cm of saddle to bar drop. That may be more than you're comfortable with now but it's going to ride and feel more like a race bike. I also predict you'll easily accomodate to it once you enjoy how it was meant to be ridden.

You already have a "racy" bike in the Fierte. Try something a little different.

[swalburn]

I agree with everything you say, but there is a point were I'm kidding myself. I had an allez pro size 56 cm, and had a -8 degree stem with almost no spacers, and at that point I was kidding myself. It was my first bike. My lower back would get a little sore on longer rides. I'm in better shape now, and have logged a lot more miles. I just don't want to make a very expensive mistake. I'm considering spliting the difference. I thinking of setting the look up with 3 cm of spacers and a positive 6 degree stem. It will put me in a racier position, and allow for me to make the position more aggressive in the future as my cycling improves.

[RPS]

It will put me in a racier position, and allow for me to make the position more aggressive in the future as my cycling improves.When in doubt, I agree that it’s best to start on the high side since it's easy to lower it later as you adjust.