Help calculating saddle position between two bikes with diff

[Ufgators]

I have to admit I could have calculated this when I was still in school. I have a bike with a seat tube angle of 74 and a new bike with a seat tube angle of 73.5. The reach is exactly the same for the frame. If my saddle height measured from bb to center of seat is 730mm, how many millimeters forward would I have to move the saddle on the 73.5 degree bike to have the same position? It is hard to measure with a plumb bob with this type of accuracy. Thanks.

[fa63]

I can do the math and give you an exact number when I have more time, but it should be around 5 mm.

[bricky21]

Butt the front wheel up to a wall. measure from the wall back to the tip of your saddle, and then from the wall back to the center of the BB. Subtract the first measurement from the second measurement, and remember that #. Do the same thing with THE new bike and adjust until you get the same #. No geometry required

[Ufgators]

Thanks a lot! Definitely some smart people here on this board!

[kulivontot]

Butt the front wheel up to a wall. measure from the wall back to the tip of your saddle, and then from the wall back to the center of the BB. Subtract the first measurement from the second measurement, and remember that #. Do the same thing with THE new bike and adjust until you get the same #. No geometry required

I measure to the point where my sit bones contact the saddle itself, as the length and sit points tend to vary from bike to bike, but ultimately you're interested in where your contact points actually are. Some other points that aren't accounted for in this method are different crank lengths and pedal stack height. 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.

[Ufgators]

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.

[bricky21]

I measure to the point where my sit bones contact the saddle itself, as the length and sit points tend to vary from bike to bike, but ultimately you're interested in where your contact points actually are. Some other points that aren't accounted for in this method are different crank lengths and pedal stack height. 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.

Your making a mountain out of a mole hill He's using all the same stuff just on a new frame. None of that other stuff can be calculated based off of old measurements.

[kulivontot]

Who knows, maybe op will have more than one bike at some point in time? Can't hurt to be thorough.

[roadytracky]

"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.


Your making a mountain out of a mole hill He's using all the same stuff just on a new frame. None of that other stuff can be calculated based off of old measurements."

kulivontot - Nobody around here rides 2 different bikes with crank lengths that vary by "1cm?" If the OP is riding 165mm cranks on 1 bike and 175mm cranks on the other, I'd say he's got bigger problems than worrying about fore/aft saddle positioning. I could see maybe a 2.5mm difference between track and road bikes; that's how I always spec my road and track bikes, but 1cm between 2 road bikes? C'mon. Also, most of us know to ride the same model saddle from bike to bike.

[DaveS]

The answer might be more complicated than you think. Is the reach the same, at the exact same stack height? If the stacks are different, the published reach values can't be compared directly. The difference isn't huge, but people make this mistake all the time, comparing two adjacent frame sizes with 20mm different stack heights. If the listed reaches are the same, then there's really a 6mm difference, once a correction is made to the same stack height for both frames.

If you're swapping all of the parts, you should have taken a measurement from the saddle nose to the center of the bars on the old bike and transferred that to the new one. Of course that only works if the reach on both frames is really the same.

Measuring from a wall only works if both frames have the same front-center dimension.

[roadytracky]

This article explains it all. Very comprehensive (maybe too comprehensive). Out of laziness I still just use a plum line, tape measure, and level, but the method explained in the following article is THE way to get it exact.

http://velonews.competitor.com/2012/05/ ... lly_216035

[kulivontot]

kulivontot - Nobody around here rides 2 different bikes with crank lengths that vary by "1cm?" If the OP is riding 165mm cranks on 1 bike and 175mm cranks on the other, I'd say he's got bigger problems than worrying about fore/aft saddle positioning. I could see maybe a 2.5mm difference between track and road bikes; that's how I always spec my road and track bikes, but 1cm between 2 road bikes? C'mon. Also, most of us know to ride the same model saddle from bike to bike.

Clearly you must not own enough bikes if every bike you own has the exact same crank length and exact same saddle on every bike. I own a variety of different road bikes, some of which have vintage non-ideal cranksets. I'm not about to go scouring ebay for period-correct cranks of the "correct" length, when I can just account for it with the fit. Op is asking us to solve a trigonometric equation of a difference of half a degree; the difference is going to be measured in millimeters, which contrary to what you may believe are indeed noticeable. If you do your bike fit by feel, then by all means ignore small deviations, but if you're going to take the time to make measurements and try to dial things in exactly, you might as well do it right and account for the little things.

[roadytracky]

"Clearly you must not own enough bikes if every bike you own has the exact same crank length and exact same saddle on every bike. I own a variety of different road bikes, some of which have vintage non-ideal cranksets. I'm not about to go scouring ebay for period-correct cranks of the "correct" length, when I can just account for it with the fit. Op is asking us to solve a trigonometric equation of a difference of half a degree; the difference is going to be measured in millimeters, which contrary to what you may believe are indeed noticeable. If you do your bike fit by feel, then by all means ignore small deviations, but if you're going to take the time to make measurements and try to dial things in exactly, you might as well do it right and account for the little things."

Kulikonot - There is just so much bad information in there, I don't even know where to start. I currently have 4 working bikes: My road bike, spare road bike (I ride 5 - 7 days a week so, yes, I need a spare), a mountain bike, & a track bike (track bike for the velodrome, not a fixie). I do have a new road bike on order, but I'll be selling one of my other road bikes wen the new one arrives so...Anyway, that's all I need. More bikes would be nice, but I don't have a need for them (or space). I'm into riding bikes not hoarding them.

OP - Are you into riding bikes or hoarding them? I'm guessing riding.

Kulikonot - I have the EXACT same fit from bike to bike. The exception being the track bike which I always set-up 2.5mm shorter than the road bike and the mountain bike, of course. Yes, I try to have the exact same bars, saddle, pedals, and crank length on all my bikes to aid in attaining the exact same fit & feel from bike to bike. Again, these bikes are for riding not hoarding so maximizing performance and minimizing injuries is paramount.

My method gets me within a millimeter or so from bike to bike. The other method which I also listed (do you even read the complete posts you're responding to because your level of comprehension seems to be lacking.) from Leonard Zinn at VeloNews will potentially get you even closer.

How are you compensating through fit for cranks that are 1cm different in length? Just adjusting saddle height isn't going to compensate for that? Do you have a separate set of shoes for each bike? One with 1cm of shims?

Kulikonot - Everyone, who has replied to the OP has given helpful and CORRECT info except for you. If the fit is off performance will be lost, but more importantly the OP runs the risk of injuring himself. Fit is not something to take lightly.

OP - Check out the Leonard Zinn article when you have a chance. That's your best bet. Good luck

[bricky21]

Measuring from a wall only works if both frames have the same front-center dimension.

Thats the reason for measuring to the bottom bracket as well. As long as you subtract the distance of saddle to wall from BB to wall different front centers wont make any difference. The same thing can be done to get the bars in the same place, or at least as close as possible.

[kulivontot]

Roady, what is your method exactly? 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. The Lennard Zinn article is exactly the same as what Bricky originally recommended, and identical to what I'm proposing, except with the addition of deviations I mentioned, which are not accounted for in anyway by measuring the horizontal/vertical coordinates of the BB, handlebars, and tip of the saddle.
Do some reading and you'll find that the crank length has pretty much no correlation to performance on the bike, but you will definitely experience knee discomfort if you don't get the horizontal distance from saddle to pedal correct. Even the most basic "knee over pedal spindle" test that neophytes use will account for this difference. Obviously, you will have to adjust the reach by changing the stem length as well. As for the stack height discussion, yes, I do wear different shoes on different bikes. You will find that when you cycle in situations beyond your weekly group rides and actually use your bike to go places, it is extremely inconvenient to wear carbon fiber clipless shoes on your commuter/touring bike. Therefore your seat height/stem height will have to be adjusted accordingly. Am I a crazy person to suggest that different bikes will still have different fit beyond the stack/reach of the frameset?
As for claiming that I'm "hoarding bikes," why would I even bother trying to match the fit on them if I didn't ride them? If you think that every person on this forum owns nothing but carbon race bikes for the sole purpose of racing them in crits every weekend you are mistaken. On this forum, you will find a number of vintage bikes and franken-bikes made from parts that were lying around. There's no reason that you can't account for this in the bike fit.
If you're the bike-fit "master," then please enlighten me on how to correctly compensate for differences in these parts without recommending that I throw away all components that don't match?