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# Seat tube angle and reach

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# Seat tube angle and reach

04-14-10, 05:17 PM
#1
lukasz
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Seat tube angle and reach

I thought that this forum might be the most appropriate place to ask this question:

I'm in the process of buying parts for a Lemond with a 578mm effective top tube, and 72.5 degree ST and HT angles. Assuming everything else (that can be the same) is the same, how much would reach on this frame differ from my current frame (specs listed below)?

Current frame:

eff. Top Tube: 580
STA: 73.5
HTA: 74

Basically the 1 degree difference in head tube angle is what I am wondering about. I don't know if this measurement helps, but my saddle height along the seat tube, from the bb, is about 80cm.

I've spent the better part of 2 hours trying to figure this out. I'm not very mathematically inclined. If only BikeCAD Pro were free!
04-14-10, 09:39 PM
#2
unterhausen

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what measurement do you call reach? To maintain a constant relationship between your hips and your pedals, you are going to have to move the saddle forward on the new frame. Using your 80cm measurement, this means that the saddle center point is 80cm*cos(73.5)=22.7cm behind the bb on the current frame. On the new frame,
80cm*cos(72.5)=24cm. Therefore you have to move the seat forward 1.3cm to get the same bb/hip distance. Thus you have a 1.5 cm shorter tt on the new frame. There is a similar calculation for the stem, but then it's the much smaller vertical distance from the seat to the handlebars that is multiplied by the cos(angle) term. Probably insignificant.
04-14-10, 09:39 PM
#3
unterhausen

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dupe
04-15-10, 07:01 AM
#4
lukasz
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Thanks. That is sort of what I figured from reading about it but upon playing with trig calculations I wasn't getting numbers that made sense. I guess using google as a calculator isn't ideal. By reach I meant the saddle to bar distance, but the numbers you've given me make more sense.

That is what I get for being a humanities student! I am thinking that I will get a slightly longer stem and leave my seat a bit further back than on the current frame.

One more question: Do setback posts affect this calculation in a static way? What I mean by that is, say I am using the same amount of setback on both frames--the same setback can be added to either calculation and the TT remains 1.5cm shorter?

Last edited by lukasz; 04-15-10 at 07:09 AM.
04-15-10, 09:22 AM
#5
unterhausen

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it really only matters where your seat is relative to your bb. With the difference in seat angle you may want to get a non setback seatpost on the new frame.
04-16-10, 08:59 AM
#6
lukasz
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Thanks for the suggestions. I knew posting in this forum would yield very good advice!

I will have to play around with knee positioning and see what I like. Looks like this frame will allow me to get a slightly longer stem though. I have a 110 on the bike I referenced, so I think that a 120mm stem and a 16mm setback post rather than a 20mm post I am using now would give me a very adequate amount of adjustment possibilities, considering the fit on my other frame. I may have to use a some spacers under the stem, but somehow I'll pull through
04-16-10, 09:11 AM
#7
Bianchigirll
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am I reading this right? 8mm is the difference in the toptube? a measurement that small can be changed or affected by your gloves or handlebar tape too
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04-16-10, 01:29 PM
#8
merlin55
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If you are not good in math, just make a scale or full scale drawing of the 2 frames on a large peice of paper...with the bottom bracket a common point.

However unterhausen did the math correctly...
04-16-10, 02:30 PM
#9
lukasz
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I would have drawn it out but I lack a protractor, or at least could not find one around! I knew the formulas were pretty simple from having read about it, though.

Originally Posted by Bianchigirll
am I reading this right? 8mm is the difference in the toptube? a measurement that small can be changed or affected by your gloves or handlebar tape too
Well the difference is 2mm, but then the seat tube angle and seat post extension come into play. Since I'm pretty tall this probably matters more than on a smaller frame as well. I'm not so much worried about the head tube angle because as unterhausen said, it is pretty inconsequential.
04-16-10, 08:38 PM
#10
JohnDThompson
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Originally Posted by lukasz
I've spent the better part of 2 hours trying to figure this out. I'm not very mathematically inclined. If only BikeCAD Pro were free!
04-30-10, 11:37 AM
#11
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Originally Posted by lukasz
Thanks for the suggestions. I knew posting in this forum would yield very good advice!

I will have to play around with knee positioning and see what I like. Looks like this frame will allow me to get a slightly longer stem though. I have a 110 on the bike I referenced, so I think that a 120mm stem and a 16mm setback post rather than a 20mm post I am using now would give me a very adequate amount of adjustment possibilities, considering the fit on my other frame. I may have to use a some spacers under the stem, but somehow I'll pull through
To select a new seatpost, you'll need to look at how your saddle rails fit the seatpost and whether you can slide teh saddle forward the 1.3 cm. If not you need a seatpost with less setback. But fine-tuning seatpost purchase is not commonly done. I haven't seen more than a few seatposts that tell you what the setback is, much less having standardized measurements. And if the measurement is not standardized, you won't know if that 20mm post really has more setback than the 16 mm post. Best thing is to sort it out with the actual frame and saddle, seeing what you really need, then buying the right seatpost.

To make your hip to handlebar distance the same between the two bikes, after you get your hip to BB distance matched as Unterhausen said, you can choose a stem ideally 1.3 cm longer. Then you'd be matched in hip position and in reach.

When comparing bikes based on top tube, people often overlook the effect of changing seat tube angle on the seat position relative to the BB.

EDIT: I think the analysis in this post is incomplete. I think the one I'm giving in post #12 is better.

Last edited by Road Fan; 04-30-10 at 11:55 AM.
04-30-10, 11:53 AM
#12
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Originally Posted by Bianchigirll
am I reading this right? 8mm is the difference in the toptube? a measurement that small can be changed or affected by your gloves or handlebar tape too
I don't really understand how you understand it, but I suspect you aren't reading it right. But yes, gloves and tape will affect how everything will feel.

The difference in the effective top tubes is 580-578 = 2 mm. The difference between the two seat tube angles is 74 - 72.5 = 1.5 degrees. This difference placed the saddle 1.3 mm farther back from the BB on the new bike than on the old bike. The result is that the end of the top tube is 13 mm +2 mm farther back (i.e. closer to the BB) on the new bike than on the old bike.

To make the hip to BB position equal on both bikes, the saddle must be placed with 13 mm less setback on the new bike. The distance from the hip to the head tube axis is now 28 mm shorter on the new bike than on the old bike. To restore the handlebar to the same position it had on the old bike, the stem extension should be 28 mm (call it 3 centimeters) longer than on the old bike.
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