Help calculating saddle position between two bikes with diff

Discuss light weight issues concerning road bikes & parts.
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by elviento

Just a quick tip for you guys:

Generally 1 degree change in seat angle or head angle, gives you roughly 12mm (half an inch) of change in position (whether saddle position, stem position, bar position, etc.). Give or take 1mm.

So half a degree would give you a quarter of an inch.

May not work for the really anal type or the extremely tall/short riders, but for 99% of the population it should work fine.
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by roadytracky

kulivontot- "I don't think you've contributed to this thread except to say that worrying about pedal stack, crank, length, and seat length don't matter, which is simply wrong. But then at the same time you suggest that every bike you have is set up to within a millimeter, which you have done by basically buying the exact same bike every time." I never said anything even close to that. Feel free to cut and paste some quotes. Thanks. I mentioned I currently have 4 bikes: 2 road bikes, a track bike, and a road bike...far from being "the exact same bike." My current favorite method for matching positions between bikes with different geometries (stack, reach, front center, every measurement) as well as varied crank lengths (again, a 1cm. difference in crank length, as you mentioned seems extreme to me, but as you mentioned in your last reply, not "every person on this forum owns nothing but carbon race bikes for the sole purpose of racing them in crits every weekend." I did lose sight of that fact.) is outlined in the Leonard Zinn article I linked to. The Zinn article includes the measurements bricky21 referred to, and adds a few more. Zinn's method compensates for everything.

"I think if your crank is 1cm shorter, then you need to push the saddle back 1cm to maintain the same distance between saddle and pedal at the 3'o clock position." - That was the 1st piece of advice you gave that I as well as Bricky21 objected to. It's just plain incorrect.

The OP said, "Same crank. Same seat post. Same saddle. I had Parlee build a z1 with the same geometry as my z5 but a longer top tube and slacker seat angle to net same reach" so as Bricky21 pointed out, "Your making a mountain out of a mole hill." And I guess by arguing with you, I am too.

The OP got his answer and then some. Rick's computation, "730[cos(73.5)-cos(74)]=6.116 mm." Answered it nicely. I'm moving on, time to go ride.

by Weenie

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by kulivontot

Look, I realize we're going way beyond what the op asked for in the original post, but seeing as the topic involves duplicating the fit between multiple bikes and others have already mentioned measurement methods, I don't see the harm in refining the methods. The original question was answered within 4 posts. The answer is 6mm/0.5 degrees.
Maybe I'm not being clear here. I use Lennard Zinn's method as both you and Bricky advocated. However, you can see very clearly in the article that the measurements being taken only take into account the stack and reach of the frameset centered about the bottom bracket of the frame. My first point was that you should not set up your saddle position based on the tip of the saddle, as the length of saddles differ greatly and really you're concerned with referencing the horizontal distance between where your sit bones hit to the bottom bracket.
My second point was that we really shouldn't be referencing horizontally with the bottom bracket, but rather where the pedal spindle is in the 3 o'clock position. My reasoning is that a difference in crank length will change this dimension and effectively change the angle of your knee in the 3'oclock position. Since this is the point in your stroke where the power is greatest, I feel that this will be where the strain will be greatest on your knee, and therefore duplicating the angle is most critical. Obviously a change in crank will change the vertical distance between the seat and crank in the 6 and 12 o'clock positions, but since there is very little power at this point in the stroke, the difference in knee strain should be minimal.
This seems to be the point that you (and only you, you will notice no one else has commented on this point) disagree with. If my reasoning is incorrect, as you seem to think, please EXPLAIN why, rather just flaming me as being wrong. I'm ok with being wrong, I just don't like to be treated in a condescending and dismissive manner. I'd much rather have an actual discussion and learn something. This seems to be fairly intuitive to me overall that you'd want to match the dimensions of your contact points at the moment of greatest power and having ridden bikes with 1cm different crank lengths, I can confirm that not adjusting for this is noticeable.
My third point was that a difference in stack height of your pedals or shoes will alter the vertical distance between the saddle and the bottom bracket. I think this is fairly self evident and has been discussed already in the Bont shoe thread, as members have stated that lower stack shoes allows them to lower the seatpost and drop stem spacers without affecting the effective saddle-bar drop.
All of the above are not mentioned in the velonews article, as the diagram very clearly shows the dimensions to bottom bracket and handlebar only.
I think the key is to fix the X/Y coordinates of the saddle (sitbones) relative to the pedal spindle in the 3'oclock position. Since measuring the crank exactly parallel to the ground is difficult, you can probably approximate by taking the center of the BB + the rated crank length. From there, you adjust the handlebar reach/drop to match.

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