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Bike size described as "50cm" but none of the measurements are 50cm!

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Bike size described as "50cm" but none of the measurements are 50cm!

Old 03-03-21, 08:38 AM
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wilson_smyth
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Bike size described as "50cm" but none of the measurements are 50cm!

I have this bike (link here).
I have the small, which they say is 50cm and It fits me pretty perfectly IMO.
I cant figure out why they call it a 50cm bike though, none of the measurements on the bike are 50cm. same for all the other sizes.
Can anyone answer why they classify it as "50cm"?




I ask as im considering something with drop bars for my next bike and probably buying online so wont be able to test. I need to know how to relate frame geometries of other bikes to what i have, in order to find something with drops that will fit.
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Old 03-03-21, 08:58 AM
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Sizing is purely a manufacturer's choice on how they show it. To my knowledge there is no Bureau of Bike Standards to tell us how a bike should be sized to a person.

It's easy for us to think that we can come up with a measurement between two points on a bike and then that means that any bike model that has that measurement will fit the same person. But it doesn't. Other geometry and tube lengths and geometry of components like handle bars and cranks make a difference.

It might be that Cube has some other measurement that they use that is not shown because it's not a useful bike geometry measure. Might be they just made it up to keep what they feel is the size bike that the average person looking for a 56 cm will fit properly on. Doubt that's true, but it will be with in the allowed practice for bike sizing.

They could just as well use "A" "B" "C" "D" sizes.

And maybe because others have your same question, it might be why more and more manufacturers are going to XS, S, M, ML, L and XL sizing.

Last edited by Iride01; 03-03-21 at 09:06 AM.
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Old 03-03-21, 09:04 AM
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The two numbers that matter the most are the 542mm stack and the 389mm reach, as far as fit is concerned. That's the poorest drawing I've ever seen. Most brands have a geometry chart with more info. For a race bike, the stack is tall, the reach is long, the seat tube length is very short at 460mm and the chain stays are the longest I've ever seen at 430mm. Chain stay lengths of 405-415 are more common.

Reach can only be compared at the same stack height. If another frame has a shorter stack, add 3mm of reach for each 10mm of stack.

Here's a link to the geometry chart for my frame.

https://www.cinelli-usa.com/supersta...rameset-white/
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Old 03-03-21, 09:08 AM
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note: the bike in question is a flat bar bike. that's why the reach seems long.

Last edited by mack_turtle; 03-03-21 at 12:05 PM.
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Old 03-03-21, 09:08 AM
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And here's a drawing from Bianchi's website. Bike size is Column A1.... But it isn't obvious where the top of A1 is on the diagram. I suspect some trigonometry can tell us what that number is based on but I am not sure which other measurement belongs in the formula. Confusing, isn't it!

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Old 03-03-21, 09:25 AM
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Your bike is probably 50cm if measured using the effective top tube, which is a horizontal measurement and not the actual intersection of a slanted top tube with the seat tube.

Even still there has always been a slight difference between size measurements between center of bottom bracket to center, or top, of top tube.

John
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Old 03-03-21, 09:30 AM
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Frame sizing is generally listed as the seat tube length from the center of the bottom bracket to either the center of the top tube or the top of the top tube. On some bikes with a sloping top tube(like this one) the manufacturer will list the size as the imaginary spot where the top tube would hit the seat tube, if it were horizontal. In this case, the actual seat tube is probably 460mm to the center of the top tube and the imaginary spot where a horizontal top tube would intersect the seat tube is at 500mm.
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Old 03-03-21, 09:48 AM
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There is a pronounced amount of top tube slope. Perhaps 50cm is the effective seat tube length, meaning what a seat tube length would be if the top tube were level. Effective top tube length is a common measurement for the same reason- sloping top tubes muck up traditional geometry measurements.

Regardless of why they call it a 50, buy the bike that fits you well. There is plenty of geometry listed to inform you.

Also, you mention wanting a drop bar, but this is a flat bar. Those are different styles.
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Old 03-03-21, 10:03 AM
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thanks for the info all, very helpful!

Originally Posted by mstateglfr View Post
Also, you mention wanting a drop bar, but this is a flat bar. Those are different styles
Yes, im aware of this, Ive had the flat bar for about a year, but ive found myself adding inner bar ends to make my hands more comfortable, and so thinking about just jumping right in and get a bike with drop bars and hoods.
It will definitely not be as relaxed as the flatbar, but thats good as i find it more and more than im getting up out of my seat to get up to speed on the flatbar and hoping a slightly more aggressive position will help here.
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Old 03-03-21, 10:21 AM
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Originally Posted by wilson_smyth View Post
thanks for the info all, very helpful!


Yes, im aware of this, Ive had the flat bar for about a year, but ive found myself adding inner bar ends to make my hands more comfortable, and so thinking about just jumping right in and get a bike with drop bars and hoods.
It will definitely not be as relaxed as the flatbar, but thats good as i find it more and more than im getting up out of my seat to get up to speed on the flatbar and hoping a slightly more aggressive position will help here.

A drop bar Specialized Diverge in the largest size has 662mm of stack and 392mm of reach.
A flat bar Canyon in the largest size has 635mm of stack and 445mm of reach.

These bikes have the same seat tube angle, almost identical head tube angles, and similar fork rake.
Just mentioning this since not all drop bar bikes are more aggressive. Obviously some of the Canyon's reach is because it has a flat bar instead of drop which reduces overall effective reach, but still-a drop bar bike neednt be more aggressive.
Something to keep in mind as you sort thru options.
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Old 03-03-21, 11:01 AM
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Originally Posted by mstateglfr View Post
A drop bar Specialized Diverge in the largest size has 662mm of stack and 392mm of reach.
A flat bar Canyon in the largest size has 635mm of stack and 445mm of reach.

These bikes have the same seat tube angle, almost identical head tube angles, and similar fork rake.
Just mentioning this since not all drop bar bikes are more aggressive. Obviously some of the Canyon's reach is because it has a flat bar instead of drop which reduces overall effective reach, but still-a drop bar bike neednt be more aggressive.
Something to keep in mind as you sort thru options.

very useful information, thank you.
Sways me away from purchasing online, although there's zero stock anywhere in the country.
Even trying to purchase in store, there is no bike store that will let you try a bike for more than 5 mins around the carpark before purchase, and really thats not enough. any bike will feel fine for 5 mins.

Its a lot of money for something that you have no idea will be optimal. or perhaps any bike within reason can be adjusted to fit very well. I need more tries on other bikes i think!

Last edited by wilson_smyth; 03-03-21 at 11:04 AM.
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Old 03-03-21, 11:46 AM
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The 410 mm seat tube measurement is a typographical error. Those measurements wouldn't form a closed figure. Do you really believe that the seat tube is shorter than the chainstay? Do the math.
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Old 03-03-21, 12:48 PM
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Originally Posted by Phil_gretz View Post
The 410 mm seat tube measurement is a typographical error. Those measurements wouldn't form a closed figure. Do you really believe that the seat tube is shorter than the chainstay? Do the math.
I think 410 is the seat stay length, and 460 is the seat tube.
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Old 03-04-21, 02:54 PM
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Ive taken the info from this thread, done research, learned a lot and also have more questions than ever!
One key issue is that i have a flatbar hybrid that cube describe as "xs-50" (reach 389, stack 534) and so Im really unsure what size to get in a drop bar.

Generally, im looking for an endurance frame geometry, but something i can put decently wide tires on if i want so its going to be a gravel or cross bike.
For comparison, cube have gravel bikes also labeled "xs-50" with a reach of 383.9 and a stack of 529.8.

Comparing the two in my head tells me handlebars are closer and lower on the gravel bike, but i really dont know what that means.

Their next size up, labeled "s-53" has a reach 385 of and a stack of 554, a reach very similar to my flatbar, but a higher stack so, my thought process says ill be very upright on the larger frame size.


Can anyone advise the correct way to compare these numbers and sizes, when i cant actually hold or sit on any of the bikes?
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Old 03-04-21, 06:06 PM
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Start with an accurate saddle height. Your height is also relevant. Keep in mind that the reach to the top part of drop bar is similar to a straight bar bike.

Many years ago, I changed a straight bar hybrid bike over to drop bars. It wasn't hard to get a decent fit.
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Old 03-04-21, 06:29 PM
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The diagram in the OP has to be inaccurate, I'm not sure I would trust any of it. Try drawing a 46cm frame and you'll see what I mean.
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Old 03-05-21, 08:23 AM
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Originally Posted by wilson_smyth View Post
Ive taken the info from this thread, done research, learned a lot and also have more questions than ever!
One key issue is that i have a flatbar hybrid that cube describe as "xs-50" (reach 389, stack 534) and so Im really unsure what size to get in a drop bar.

Generally, im looking for an endurance frame geometry, but something i can put decently wide tires on if i want so its going to be a gravel or cross bike.
For comparison, cube have gravel bikes also labeled "xs-50" with a reach of 383.9 and a stack of 529.8.

Comparing the two in my head tells me handlebars are closer and lower on the gravel bike, but i really dont know what that means.

Their next size up, labeled "s-53" has a reach 385 of and a stack of 554, a reach very similar to my flatbar, but a higher stack so, my thought process says ill be very upright on the larger frame size.


Can anyone advise the correct way to compare these numbers and sizes, when i cant actually hold or sit on any of the bikes?
When you are unsure and don't have any experience to tell you otherwise, go with the size the manufacturer recommends for that particular model.
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Old 03-05-21, 09:32 AM
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Originally Posted by 70sSanO View Post
Your bike is probably 50cm if measured using the effective top tube, which is a horizontal measurement and not the actual intersection of a slanted top tube with the seat tube.

Even still there has always been a slight difference between size measurements between center of bottom bracket to center, or top, of top tube.

John
I think his Core diagram shows ETT as 542 mm, so that doesn't fix it.

The uncertainty in "traditional" frames: The TT diameter is 25.4, and the TT is generally horizontal. Center to center and center to top differ by 12.7 mm, or inch with these assumptions. Another measure, used by old Masi and some others is from center of BB to top of the seat tube. The seat tube top is usualy at the top of the ST-TT lug, so it can be several cm longer than c-c. But with a slanted top tube, all those assumptions are useless. Stack (dimension Y in the Bianchi picture and 534 mm in the Core picture) is a much more useful measurement. It correlates to the reality of how low you can position the handlebar.

Distance from sitbone contact to the handlebar contact points (a fitting dimension, not a sizing dimension) is based on saddle rails, saddle design, seatpost setback, seat tube angle, reach (dimension Y on the Bianchi picture), head tube angle, stem extension, stem angle, handlebar reach, and brake lever design (for road bars and brifters/Ergopowers).

The parts of sitbone to grip that are behind the BB are related to knee position relative to BB, not to reach. Reach is from the BB to the top of the head tube. Note I am NOT saying the knee has to be above teh pedal axis. I like it a few cm behind, for my body.
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