Most important fit measurements?
02-03-23, 11:40 AM
#51
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Originally Posted by Kapusta
Well if it works on one person for one bike then it must be reliable 
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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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
There's also my "first guess" formula:
Road frame size (cm) = your height (inches) - 14
Originally Posted by terrymorse
"Picking an appropriate frame size does not require partial differential calculus.
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.
02-03-23, 12:35 PM
#54
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Originally Posted by Kapusta
Which lands me on a frame 6cm too big.
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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.
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.
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.
02-03-23, 01:33 PM
#57
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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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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.
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.
02-03-23, 04:36 PM
#59
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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!
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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.
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02-03-23, 07:17 PM
#61
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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.
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.
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.
02-03-23, 07:35 PM
#62
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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.
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.
02-03-23, 07:43 PM
#63
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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.
02-03-23, 07:47 PM
#64
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Originally Posted by Lombard
You mean like this?
02-03-23, 07:55 PM
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Originally Posted by Darth Lefty
there’s only seven bike sizes.
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.
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.
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.
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.
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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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.”
02-04-23, 10:26 AM
#70
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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.
Using reach, the STA is used to evaluate the seat post setback needed.
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.”
02-05-23, 01:09 AM
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Originally Posted by Kapusta
...Yes, I am clearly an outlier...s.
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02-05-23, 08:41 AM
#73
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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.
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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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.
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