View Full Version : Yet another bike related math problem

07-29-2005, 03:01 PM
Christian - the Thomson did not feel right when I rode it, so I re-checked the measurements - turns out that the Salsa puts the front of the saddle in exactly the same place when the saddle is all the way forward. Go figure. Now on with the general post...

For those of you that are really good at these calculations, I am not, so I would really appreciate your help.

I am down to my last adjustment on my new bike. It is the distance between the front of the saddle to the center of bars.

Old bike - TT: 57.8 cm; ST Angle: 75*; Stem: 110mm, +17*; Seat to bar drop: 3.1cm; Front of saddle to center of bars: 53cm.

New bike - TT: 59cm; ST angle: 73*; Stem: 130mm, -10*; Seat to bar drop: 3.5cm; Front of saddle to center of bars: ???

If the STA and the stem angles and drop were the same on both bikes, I would just add 2cm for the difference in stem length + 1.2cm for the difference in TT length and set the front of the saddle 56.2cm from center of bars. It is the angles that are throwing me. BTW - I am using this mainly to get the saddle set-back dialed in. I realize I will have a longer reach, but I would rather have the saddle in the same spot relative to the BB than move the saddle forward to get the 53cm reach.



Bruce K
07-29-2005, 03:14 PM
That math is kind of beyond me.

Where's Paul Levine and his computer when you need him? :banana:

I think the Bicycle Forest website - www.bikeforest.com -may have the geometry software that will help you figure this out.


Needs Help
07-29-2005, 04:49 PM

I say that half in jest because your question doesn't make much sense to me. Your new bike has a longer top tube and a longer stem, so if you want the same 53cm distance as your old bike, you would have to jam your seat forward. However, you said you realize you will have a longer reach on your new bike, but then what does 53cm have to do with anything?

If you want the same setback on your new bike, then measure your old setback and try to match that on your new bike by using a post that will get you there. Since your new seat tube angle is slacker, you will be farther behind the bottom bracket if you use your old post. If your old post had some offset to the rear, you need a post with 0 offset or some forward offset.


07-29-2005, 05:06 PM
Read my last paragraph again. I am not trying to calculate reach or I would simply set the front of the saddle to be 53cm from center of bar and be done.

I could set it to be 56.2 (53+2+1.2) and hope that gets me the same saddle setback, but I don't know if the difference in STA, stem angle, and seat-to-bar drop make a difference. Thus my question.

As for getting the same setback as on my last bike, I don't have a reliable way to get the bike level and measure that, so this is a workaround.

Needs Help
07-29-2005, 05:09 PM
If it helps, the horizontal length of a +17 degree stem(are you sure that is correct?) that is 110mm long is 91mm, and the horizontal length of a -10 degree stem that is 130mm long is 129mm.

Needs Help
07-29-2005, 05:15 PM
Do you know your old center of bottom bracket to seat top measurement? If you do, you can calculate how much further behind the bottom bracket the new seat tube angle puts you.

As far as I can discern, what happens forward of the bottom bracket has no effect on your setback. For instance, you could take your old bike and weld on 5 cm more of top tube length, change the stem length to 150mm, and change the stem angle to -17 degrees, and your setback would remain unchanged: your seat tube length would be unaffected and your seat tube angle would be the same, so you would be sitting in the same spot relative to the bottom bracket. All that would have changed would be the front of the bike got longer, and the handlebars were in a different position. That wouldn't affect where your butt and knee cap were positioned unless you scooted foward on the saddle to enable you to reach the bars.

07-29-2005, 06:16 PM
What you're doing makes no sense. What you really want is for someone to calulate the difference between your old +17 stem setup and the new, but you didn't provide nearly enough info to do that. An accurate comparison requires the old and new headtube stack height, and the stem clamp heights in addition to the angles you provided. That's way too much trouble to bother with.

If you want the saddle positioned at the same setback as the old bike (4.8cm), then all you need to do is position the right crankarm horizontal, place some masking tape on the arm and mark 4.8cm behind the crank bolt center. Then place the bike on a level surface (level the TT if it's non-sloping) and use a level or plumb bob to verify that the saddle tip lines up with the mark on the crank arm.

I can't figure out why you put a much longer stem on the new frame. The "reach" of these two frames is almost identical. Here's how it's calculated:

Reach is the TT length minus the frame setback. Setback is the cosine of the STA times the c-c frame size.

Unfortunately, you left out the frame size, but from your previous post, here it is:

Old bike: 58 cm ST c-c, 7 cm BB drop, 75* ST angle, 77.7 cm from c of BB to top of saddle measured along the ST, ideal fore/aft saddle position is 4.8 cm from front tip of saddle to plumb line intersecting the center of the BB.

New bike: 59.5 cm ST c-c, 8 cm BB drop, 73* ST angle, 77.7 cm from c of BB to top of saddle measured along the ST, ideal fore/aft saddle position is ??? from front tip of saddle to plumb line intersecting the center of the BB.

Ideally, both frames should be the same size when making this comparison, but this should be close enough. The first frame has a setback of 58cm x cos75 = 15cm. The reach would be 57.8 - 15 = 42.8cm. To improve accuracy, Ill calculate the reach of the new frame at the same point along the seat tube. 58 x cos73 = 17cm. The reach of the second frame would be approximately 59 17 = 43cm. This tells you that there is a measly 2mm difference in the reach of these two frames.

This analysis ignores the (usually) minor variation that occurs due to differences in the HTA.

The bottom line is, once the saddle is set properly, any difference you measure from the old 53cm saddle tip to center of bars is an increase in the reach due to the new stem.