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No sure where to post this.

Until recently I was on a bike with:
386 Reach
537 TT
120 Head Tube

New bike is
376 Reach
530 TT
138 HT

I was using a 100mm stem. I want approximately the same reach. I was thinking 110 would each up the 7mm TT & 10mm Reach difference, but the higher/longer head tube has me scratching my head and not sure how to account for it.

Thanks for any help you can offer.
 

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Google 'bike stem calculator' or similar. There are a few out there that'll tell you everything.
There's more to stems than length. You need to know and consider angles.
 

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Went from 74˚ to 73.5˚ so I guess I would change slightly, but my position over the pedals would be made approximately the same for my knees
I think reasoning in terms of stack and reach of the frame alone, is getting you confused. What you need to know are the positions of the contact points relative to the bottom bracket, then you're good to go.

For the saddle, you'll have to measure set-back and height; set-back is easy, using a level or a plumb line project down from the forward end of the saddle nose and measure the distance horizontally from the level or plumb line to the center of the bottom bracket

There are a few variations on the saddle height measuring technique, what I do is pick the mid point of the saddle where you ordinarily sit (this is easier on a curved saddle, it's going to be the lowest point) and measure from that to the center of the bottom bracket.

As for the handlebars, you need to measure height, which, again, is easy: with the bike level, either in a support or trainer (more emphasis on level in the latter case), you measure up from the front wheel's contact point (the ground if support, the block, in a trainer).

As for reach, you measure from the front end of the saddle to the levers.

IME stem calculators are only useful to evaluate changes on the same bike, if comparing bikes with different geometries, you'll need a full geometry tool, such as bikegeo.net, and the full geometries for both bikes.

Make sure you measure in the same way every time or it's going to be fun. Also keep in mind that, if the bikes have different saddles, bars or levers, adjustments may be necessary.

If you don't have the previous bike anymore, then you're going to have to find out the hard way.

What missing from your set of parameters is the stack.
The new bike will have a taller stack inevitably, however, stack and reach are only useful to compare frames; to replicate a position from a bike to another, they're, at the same time, too little information and not needed.
 

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No sure where to post this.

Until recently I was on a bike with:
386 Reach
537 TT
120 Head Tube

New bike is
376 Reach
530 TT
138 HT

I was using a 100mm stem. I want approximately the same reach. I was thinking 110 would each up the 7mm TT & 10mm Reach difference, but the higher/longer head tube has me scratching my head and not sure how to account for it.

Thanks for any help you can offer.
Went from 74˚ to 73.5˚ so I guess I would change slightly, but my position over the pedals would be made approximately the same for my knees
If you keep the saddle position over the pedals the same, the 73.5 degree seat tube will effectively shorten the top tube close to another 1/2 cm.

Regardless, you can do all the calculating you want, but really, if you like the fit on your current bike, simply take measurements and set up the new bike to duplicate the old one. If there's a fit problem on the old one that you think will be solved by the new one, adjust the measurements accordingly.

First set up the saddle to give the same position over the pedals.

Then measure the reach and handlebar drop on your old bike. Simply set up the new bike bike the same way. The stem length should be fairly straight forward based on the measurement of your old bike, but if you're uncertain about stem lengths, buy a couple of cheap ones and just try them out - see how they feel, and also see how the measurements compare to your current bike.

As far as the new head tube compared to the old: just measure the difference between the seat height (above the floor) and the handle bar height (above the floor) - that will give you handlebar drop. Duplicate that on the new bike by changing spacers and/or flipping the stem.

Don't over-analyze it, it's not that difficult to duplicate your position especially if you have the old and new bikes to take measurements off of. But be sure to set up the saddle on the new bike to give your knee the same position as on the old bike -then go from there and tweak after riding a bit.
 

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the 73.5 degree seat tube will effectively shorten the top tube close to another 1/2 cm.
Lengthen.

BTW it's easier to measure the handlebar height directly, than to derive it from saddle to handlebar drop, which is, in turn, easier to calculate as difference between handlebar and saddle height, than measured directly.
 

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For future reference, Park Tool publishes a very convenient chart for recording all your critical "fit" parameters:
Road Positioning Chart | Park Tool

Note that most smartphones can have an app installed that will measure angles or level very accurately -- iphone already includes one.
 

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Trying to Moderate etween Jay and oct3

Reach is what matters most.

If reach is allowed to VARY, then with a GIVEN eTT length:
- a steeper STA will produce MORE frame reach.
- a slacker STA will produce LESS frame reach.

But when dealing with two frames with the SAME reach:
- a steeper STA will be coupled with a SHORTER eTT
- a slacker STA will be coupled with a LONGER eTT

I ride one bike with a very slack STA (72.5) and one with a steeper STA (74). The slack STA bike has a much longer eTT (580mm). The steep STA bike has a shorter eTT (565mm). Both frames have very similar reach.

When oct3 says "all else being equal," he means if the reach of the frame stays the same but the STA gets more slack, then the eTT must get longer.

When camilo says that the steeper STA will shorten the eTT, he is not quite right. It will shorten the reach, but changing the STA does not in and of itself alter the actual length of the top tube. But it DOES move more of the top tube behind the BB and that shortens the reach.

So you NEED a longer top tube on a bike with a slacker STA, IF you want to keep the reach the same.
 
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