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Most important fit measurements?

Old 02-03-23, 11:40 AM
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Originally Posted by Kapusta
Well if it works on one person for one bike then it must be reliable
"Fist full of seatpost" method is reliable for the vast majority. That's why it worked for so many years.

If you fit someone on a frame, and the seatpost was slammed, it was time to say "let's try you on the next smaller size". Or if there were two or more fistfuls of seatpost, you tried the next bigger size.

There's also my "first guess" formula:

Road frame size (cm) = your height (inches) - 14

Picking an appropriate frame size does not require partial differential calculus.
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Old 02-03-23, 12:27 PM
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Originally Posted by terrymorse
"
There's also my "first guess" formula:

Road frame size (cm) = your height (inches) - 14
Which lands me on a frame 6cm too big.

Originally Posted by terrymorse
"Picking an appropriate frame size does not require partial differential calculus.
Is stack and reach really that hard?
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Old 02-03-23, 12:32 PM
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Originally Posted by terrymorse
Picking an appropriate frame size does not require partial differential calculus.
No, but ordinary differential equations are a minimum requirement.
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Old 02-03-23, 12:35 PM
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Originally Posted by Kapusta
Which lands me on a frame 6cm too big.
Based on my experience fitting thousands of people, I find that very hard to believe.
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Old 02-03-23, 12:38 PM
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Originally Posted by terrymorse
Based on my experience fitting thousands of people, I find that very hard to believe.
I agree. I bet he's riding too small a frame.
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Old 02-03-23, 01:16 PM
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Originally Posted by terrymorse
Based on my experience fitting thousands of people, I find that very hard to believe.
I am 5'-10". Using stack and reach numbers arrived at over many years of experimentation and an in depth professional dynamic fitting, the frame size I most often end up with is 50cm. I suffered through many years of bikes than never fit right thinking I should be on a 54 or 56cm frame.

Yes, I am clearly an outlier. However, using using stack and reach with some online calculators to mess with stem and spacer configurations nails it for me every time.

Using a "fistfull of seatpost" is really no better than using stand-over to fit a bike.

Heck, in the MTB world, Reach is now the primary number used to size bikes.
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Old 02-03-23, 01:33 PM
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I use effective top tube length. Seatpost length is largely irrelevant in comparison. Of course one needs to know one's preferred ETT!
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Old 02-03-23, 02:09 PM
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Unless you have a real reason to think you are an outlier you probably aren’t.

There are a hundred fit systems and ten thousand internet opinions about them but there’s only seven bike sizes. 2xs-2xl. That’s only on the most popular models and the smaller ones might be pink with a different model name. A few very popular models might have a “M-L” size. Maybe few as three (M L XL) for more boutique brands. Hard to sell a manly man a “small” anything.

Some brands are half a size off. Notably Salsa, for me. Their M is halfway to a Large. A Cannondale medium fits me spot on at 5-8 but a Salsa I need a stem swap. Their recommended height range reflects this just fine.
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Old 02-03-23, 04:36 PM
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Originally Posted by Carbonfiberboy
I use effective top tube length. Seatpost length is largely irrelevant in comparison. Of course one needs to know one's preferred ETT!
ETT length is not as good as reach, because any change to the STA changes the reach. Reach is flawless.
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Old 02-03-23, 05:10 PM
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Originally Posted by DaveSSS
ETT length is not as good as reach, because any change to the STA changes the reach. Reach is flawless.
Huh. I thought reach was the horizontal distance from the BB to center of the HT. That only tells us what's happening in front of the BB. Changes to the STA do not affect reach. Actual reach from saddle to HT seems better measured by the ETT.
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Old 02-03-23, 07:17 PM
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Originally Posted by 79pmooney
Also look at chainstay length and front center. (Front center - distance BB to front hub.) At a given fit, your weight is at some (small) distance in front of the bottom bracket. It is then distributed between the front and rear wheels depending on the distances of that weight center to the respective hubs. A bike that changes this distribution will change that weight balance.

You can find your weight center and weight distribution using just a typical bathroom scale and old fashioned phonebooks. (Boards, bricks ...) Place the scale beside a hallway wall with the same height's worth of phone books your bike's wheelbase away. Set bike on scale and book. Hop on. Using the wall, sit in your usual position and read (or better, have someone else read) the scale, then swap scale and book and repeat. Now you have the weight distribution with simple math and with little more, the distance forward of the BB.

If you are satisfied with your bike's handling, especially how "planted" your wheels feel on turns, these are numbers to remember.
Chainstay length, front-center, and saddle setback from a plumb line vertical through the BB axis and measuring back to where you like to have your sit bones located.

Also, what do you mean by "the distance forward of the BB?" I can think of a bunch of useful meanings of that phrase, but don't quite see where you're at.
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Old 02-03-23, 07:35 PM
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Originally Posted by 79pmooney
Also look at chainstay length and front center. (Front center - distance BB to front hub.) At a given fit, your weight is at some (small) distance in front of the bottom bracket. It is then distributed between the front and rear wheels depending on the distances of that weight center to the respective hubs. A bike that changes this distribution will change that weight balance.

You can find your weight center and weight distribution using just a typical bathroom scale and old fashioned phonebooks. (Boards, bricks ...) Place the scale beside a hallway wall with the same height's worth of phone books your bike's wheelbase away. Set bike on scale and book. Hop on. Using the wall, sit in your usual position and read (or better, have someone else read) the scale, then swap scale and book and repeat. Now you have the weight distribution with simple math and with little more, the distance forward of the BB.

If you are satisfied with your bike's handling, especially how "planted" your wheels feel on turns, these are numbers to remember.
Never mind, I got what your saying. I agree!
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Old 02-03-23, 07:43 PM
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Originally Posted by genejockey
Stack will tell you whether you can get the bars at the same height without a crapload of spacers. Reach will tell you whether you can put the bars the same distance away without resorting to an 80mm or a140mm stem. These days, it seems like getting your saddle the right height and setback is just a question of getting a seatpost that allows you to put the saddle in the right place. Head tube angle affects handling more than fit, and it seems like seat tube angle these days primarily determines whether you need a seatpost with setback or not.
I agree as far as stack goes. Reach is another story. What we need to match are the stack for height, saddle setback and height from BB center (I measure from my sit bone contact to the BB center) for same pedaling motion, and then reach BB t top center of head tube to match the reach to bars INCLUDING the saddle setback. Saddle setback from BB plus reach contains the same information as does the full reach and does top tube length. There's a lot of room for cockpit adjustment.
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Old 02-03-23, 07:47 PM
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Originally Posted by Lombard
You mean like this?
In my case it's not the shorts.
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Old 02-03-23, 07:55 PM
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Originally Posted by Darth Lefty
there’s only seven bike sizes.
Today. BITD you could find road bikes made in 17 sizes or more.
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Old 02-04-23, 08:24 AM
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I think there are two different questions that are sometimes being conflated in this thread:

1- How do you determine what fit / bike geometry you need. This is based on your specific needs. There are many different ways that people figure this out: online fit calculators, professional fittings, rules of thumb based off height… whatever.

2- (what the OP asked): Once you know what fit you need, or have a bike that fits you, what are the most important frame measurements that will tell you what bikes will fit. I would say Stack and Reach. Others argue that seat-tube length (i.e., exposed seatpost) or standover height are good enough.
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Old 02-04-23, 09:07 AM
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Originally Posted by Carbonfiberboy
Huh. I thought reach was the horizontal distance from the BB to center of the HT. That only tells us what's happening in front of the BB. Changes to the STA do not affect reach. Actual reach from saddle to HT seems better measured by the ETT.
Changes to the STA will affect the ideal fore/aft position of the saddle. Given a constant ETT measurement, a different STA will change reach.
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Old 02-04-23, 09:14 AM
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Originally Posted by Kapusta
I think there are two different questions that are sometimes being conflated in this thread:

1- How do you determine what fit / bike geometry you need. This is based on your specific needs. There are many different ways that people figure this out: online fit calculators, professional fittings, rules of thumb based off height… whatever.

2- (what the OP asked): Once you know what fit you need, or have a bike that fits you, what are the most important frame measurements that will tell you what bikes will fit. I would say Stack and Reach. Others argue that seat-tube length (i.e., exposed seatpost) or standover height are good enough.
While I would say stack and reach are most important, that doesn't mean other measurements aren't also important. Standover height needs to be low enough so you don't get neutered when dismounting your bike. Seat tube length needs to be short enough that you don't bottom out the post before getting it low enough and long enough that you can get the post high enough while it's still safely secured.
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Old 02-04-23, 09:24 AM
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And I would say more than a few people here mix up frame “sizing” and “geometry.”
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Old 02-04-23, 10:26 AM
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Originally Posted by Carbonfiberboy
Huh. I thought reach was the horizontal distance from the BB to center of the HT. That only tells us what's happening in front of the BB. Changes to the STA do not affect reach. Actual reach from saddle to HT seems better measured by the ETT.
If a TT is a certain length and 16cm of it is behind the BB center, while another bike has the same TT length, but only 14cm is behind the BB, then the reach is not the same. Looking at ETT must be accompanied by a comparison of the STA.

Using reach, the STA is used to evaluate the seat post setback needed.
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Old 02-04-23, 12:36 PM
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Originally Posted by smd4
And I would say more than a few people here mix up frame “sizing” and “geometry.”
That may be true, though geometry can clearly affect your fit.
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Old 02-05-23, 01:09 AM
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Originally Posted by Kapusta
...Yes, I am clearly an outlier...s.
terrymorse said "vast majority", which allows for outliers like you and me. I, however, can not fit by stack and reach alone.
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Old 02-05-23, 08:41 AM
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Stack and reach should never be used alone. STA should be considered, in case it's too steep or slack and requires a seat post setback that's not available. Most frames in my size have a 74 or 74.5 degree STA and a common 25mm setback works. If the STA is 75, then I want a 32mm setback. I've seen brands with a 74 degree STA in all sizes and proprietary seat posts with only a 15mm setback. That's likely to cause problems for someone, but it must make the frame cheaper to build. If a round seatpost is used, some other brand of post could be used.

I've also noticed TREK models with their mast style frames having a limited saddle height range, even with two lengths of seat post to choose from. They list the maximum saddle rail height for the two post lengths.
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Old 02-11-23, 02:26 AM
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For me, it's effective top tube length (adjusted for seat tube angle) and head tube length.
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Old 02-11-23, 03:26 AM
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Originally Posted by DaveSSS
STA should be considered, in case it's too steep or slack and requires a seat post setback that's not available.
That's me for sure. Anything steeper than 73.5 and I seem to need more than 25mm setback, which is hard to come by (especially with a proprietary seatpost shape).
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