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Discussion Starter #1
Being 6'4" I've always been forced to ride ugly bikes with extra-long headtubes and cramped geometry.
I've always admired those M-sized bikes and their perfect lines, which gets screwed up as the sizes goes up and down.
The sizing is not remotely proportional since the CS has always the same lenght and the angles are different.


As example this is the same bike in M and XL size (and that XL is still to small for me)






So now I decided to go custom with my next gravel (just some bikepacking, nothing racey or technical) and I'm asking myself: why not keep the same proportions just going bigger??

Let's say for ease that I'm +10% than the average rider, can I build a frame which is +10% in every single measure (apart from BB drop)?
+10% CS, +10% HT, +10% overall tire diameter, +10% fork ac (already found one), etc.. and keeping the same angles?



Where am I wrong?
 

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I feel like the angles of the two frames you posted are very similar. What's your gripe?

News flash: you're tall. You will need a longer headtube to be comfortable on the bike. And I think a tall headtube is less visually offensive than a ridiculous stack of headset spacers/riser stem/riser bars.

62846395466__F2BD8E10-CB30-4D1C-AF19-73C6B647FDB5.jpg

I'm 6'3" and ride 61cm cross and gravel frames, both with tall headtubes. None of which I find visually offensive. I actually like the fact that it looks like it's my size INSTEAD of looking like a medium that doesn't fit me.

IMG_9721.jpg

IMG_9723.jpg

Embrace it.

Love your big frame because of how it rides and fits, not because of how it looks when you're standing next to it. Function over form.
 

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My problem is, nobody increases the head tube proportionally to the rest of the frame. As you go up in sizes, the seat tube might increase 2" per size, the top tube 20mm or so, and the head tube grows somewhere between not at all and 10mm per size. And on most bikes, the chainstays don't change at all. Madness.
 

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Discussion Starter #6
I'm 6'3" and ride 61cm cross and gravel frames, both with tall headtubes. None of which I find visually offensive. I actually like the fact that it looks like it's my size INSTEAD of looking like a medium that doesn't fit me.

Sorry but I do find your bike visually offensive, and it would be even more if I had to ride the same bike with my nealry 37" inseam and my 220lbs.

But it's not just that: this is the same bike medium-sized: does it look the same ride to you?


you are sit ON TOP of the rear axle, whereas the "regular" size is designed for a more centerward weight distribution. And I'm not even accounting that a XXL rider should be also heavier.
Plus you must be more "bent" than the average rider, since stack and reach (= the distance between your ankles and your wrists) doesn't increase proportionally, so doesn't the wheelbase.


Now, you may be the "just-ride-and-shut-up" kind of guy (and I find it admirable), but since I am going anyway for a custom frame I'm just wondering if it still make sense to abide by the old standards rooted by mass production needs?
 

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Discussion Starter #7
this is how industry deal with sizing, from a 3700$ Ti frame of a well-regarded company:



toptube goes up by ~4% while stack increase at a fixed rate of 10mm (~1,5%).
CS is always the same.
 

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I've often wondered the same thing. Sticking my head in here to see where this thread goes.

Staying with the same driving dimension(s) across a range of bike sizes makes no sense for different size riders.
Different arm, torso, leg, etc lengths will drastically change where the CG is on the bike, and where it can go with rider movement. Not to mention the n+1 other variables that will affect the way the rider will handle the bike.

Telling you to 'embrace' the fact that bikes designed for the masses do not account for tall people, and to suffer with compromised geometry is asinine, when talking about custom built stuff.
 

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Discussion Starter #9
Of course I'm not talking about techincal riding here, I DO understand this logic in designing an Enduro bike, or whenever tight cornering is core business.

Maybe I am plain wrong, I just don't get why.
 

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Nah, it's just a matter of design goals I suppose. I re-read the part where it was mentioned bike-packing and nothing technical. Kinda glazed over that.

It's just what I guess you could call a pet peeve of mine. Seeing tiny little XC riders on squashed up 29'ers (cause %5 better rollover is worth the compromised geo, right?) and the opposite, big riders on bikes that are just too darn small.
Think about the CG on a bike, it's usually somewhere around the bottom of the rider torso (ish). Now think of where that is on the bike, with different size riders. Someone "ideally" fitted for a size medium, and someone 'ideally' fitted for a size XL will have two completely different riding experiences.

I'm sure there's plenty of flaws with purely proportional scaling of bike sizes, but it's an interesting idea I think about occasionally.

Also, this comes to mind. Shaq with a 36er looks just about right. Haha
external-content.duckduckgo.com.jpeg
 

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To keep everything truly proportional, you'd need bigger wheels, custom fork with more A-C. Without bigger wheels, seems you'll never achieve your asthetics.

Also, agree it's BS that companies don't lengthen CS for larger sizes.
 

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Discussion Starter #12
To keep everything truly proportional, you'd need bigger wheels, custom fork with more A-C. Without bigger wheels, seems you'll never achieve your asthetics.

Also, agree it's BS that companies don't lengthen CS for larger sizes.
Speaking of +10% it's like a 29x2.0 wheel compared to 29x2.2
I think it also make sense given a taller rider is also heavier and needs more cushion. The fork in the 1st picture is 420ac, and yes there are forks in the 465-470 range (=+10%).

If we really want to go fussy a taller tire brings also a higher BB, so you might want to increase the BBdrop as well.....


But I don't want to go too deep into details, I just wonder if there's a reason for same CS and (much) different angles other than cost cutting.
 

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It's an artifact of having to make your size chart look progressive, when you had two constraints: the longest top tube of the original road bike tube sets being 600mm, and the need to make top tubes look short on small bikes by making the seat tube super steep while minimizing toe overlap.

If you go to https://bikeinsights.com/bikes/5a51...bikes-cross-check?version=2018&build=drop-bar and scroll down to the stack and reach chart you can see two things:

- the bikes themselves aren't following a fixed ratio
- the trend line shows that frames aren't scaled - i.e. the get taller faster than they longer.
 

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Speaking of +10% it's like a 29x2.0 wheel compared to 29x2.2
I think it also make sense given a taller rider is also heavier and needs more cushion. The fork in the 1st picture is 420ac, and yes there are forks in the 465-470 range (=+10%).

If we really want to go fussy a taller tire brings also a higher BB, so you might want to increase the BBdrop as well.....


But I don't want to go too deep into details, I just wonder if there's a reason for same CS and (much) different angles other than cost cutting.
that's my point, it doesn't make sense to me
When people get taller, generally, they have longer limbs and shorter torso. This can be described as "Ape Index" or (Wingspan from tip of finger to tip of finger) / (Height). An Ape index of "1" means that your arm span equals your height.

When you examine people at the extreme ends of the spectrum the pattern emerges where short people have an Ape Index of <1 and tall people have an Index of >1.

All to say that as you get taller (or shorter) frame sizes and configurations that are mass produced follow these trends because of sales volumes.

If your body configuration is outside of these parameters then you could be out of luck finding something off the shelf.

Add to this that femurs tend to proportionally longer than the tibia/fibia (same for the Humerus vs Radius/Ulna) and there are all kinds of medical conditions that can lead to growth in different proportions such as Marfans syndrome (Abe Lincoln) or various pituitary tumors.

Regarding frame design- I have built frames for short people and tall people (6'10"). The approach I always take is to begin with the Lemond system of measuring the inseam for obtaining the "Frame Size" and saddle height.

Using that frame size, Ill set my Serotta fit bike to a "square" size ie the TT is the same as the ST or "Frame Size".

This is by no means a Universal system. It is a data point and a starting point to get a person pedaling. Then comes all kinds of adjustments depending on the type of bike, rider dimensions etc etc etc.

The first thing I always do is set the saddle position relative to the BB. Different for every rider, purpose, bicycle type etc.

For you, you may have long femurs. On a road bike, this will put your seat tube at a smaller angle ie ''slacker''. This is because the power stroke radius is longer (your position over the pedal at 90 degrees). That in turn would shorten the "reach" (horizontal BB center to HT center).

These examples are not definitive, I am using them to illustrate how frame designers may approach the design of each frame size. Its not as simple as expanding everything by 10% or whatever.

A lot of this stuff was worked out by the Italians a long time ago, at least for road type frames. I am guessing that a lot of people in the industry just copy off of each other without doing a lot of targeted research themselves.

Some advice that may help you design your frame or at least give you a better understanding of what you might need...

Look up Greg Lemond's frame size formula. It is described in his first book and widely available on the internet. There is a story about his TDF coach that raised his seat and brought him to victory. He describes the formula his coach used to size him and his reaction to that advice.

This info is outdated today but it is a beginning and its simple to understand. From there, get yourself on BikeCad...the free version, and play around with it. there are 100 things that all change when you change one thing. As you work with it you will get better at predicting and understanding what is going on.

I hope that helps. Feel free to PM me with a question or if you are in the Bay Area...I can size you on my Serotta.
 

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Discussion Starter #19
Thanks @Halekai, lot of useful infos!

I get that thing with Ape index but I don't think it's different enough to justify the small difference in bike sizing.
Speaking of which, mine is exactly 1 (wingspan=height).


For you, you may have long femurs. On a road bike, this will put your seat tube at a smaller angle ie ''slacker''. This is because the power stroke radius is longer (your position over the pedal at 90 degrees).
I don't get this.

Being my fremur (and shoes!) longer I'd expect to make up with:
  • a taller saddle height: being the seattube angled a taller one takes the rider further back, but so CS length couldn't be the same without compromising the CG
  • a longer crank arm, but again crank arms nowadays only differs by a ridiculous amount (~±4%, whereas my inseam differs by a good +20% compared to a M-sized rider)

slaking the ST looks to me much like a cheap workaround


Look up Greg Lemond's frame size formula. It is described in his first book and widely available on the internet.
I just did, but it came out way short in the reach. My bike has a longer reach and still I had to push the saddle at end of the rail to make it work.
Anyway, I don't think the pro road params can work for an average in other disciplines.
Maybe a pro needs a shorter chain length for efficency, not much of a subject in recreational cycling

I hope that helps. Feel free to PM me with a question or if you are in the Bay Area...I can size you on my Serotta.
Thanks a lot but I live in Italy ;)
 

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Thanks @Halekai, lot of useful infos!

I get that thing with Ape index but I don't think it's different enough to justify the small difference in bike sizing.
Speaking of which, mine is exactly 1 (wingspan=height).


I don't get this.

Being my fremur (and shoes!) longer I'd expect to make up with:
  • a taller saddle height: being the seattube angled a taller one takes the rider further back, but so CS length couldn't be the same without compromising the CG
  • a longer crank arm, but again crank arms nowadays only differs by a ridiculous amount (~±4%, whereas my inseam differs by a good +20% compared to a M-sized rider)

slaking the ST looks to me much like a cheap workaround


I just did, but it came out way short in the reach. My bike has a longer reach and still I had to push the saddle at end of the rail to make it work.
Anyway, I don't think the pro road params can work for an average in other disciplines.
Maybe a pro needs a shorter chain length for efficency, not much of a subject in recreational cycling


Thanks a lot but I live in Italy ;)
LeMond's formula should get you your ballpark saddle height and frame size.
Take the frame size and make it your top tube length...That is a "square frame".

That is a place to start.

Next you need to decide where your saddle needs to be in relation to your BB. That is going to depend on what type of riding and your level of experience, flexibility and fitness etc. You can use the "plumb bob" method or another.

At this point you have a level top tube. Next, for most people who are not racers or hard core cyclists I rec adding a little height to the head tube...say 3 cm as a place to start. That will keep you from being so hunched over. Now, If you feel cramped...then increase your top tube length.

Most of the time I build in an extra 3 cm into the head tube. Most people end up adding some spacers under their stem. Having a little extra head tube helps integrate the frame and headset.

All of this is merely a place to begin. You have access to your body measurements and proportions so if you have long femurs...bring your seat tube back a degree or two. If you have a long torso...add 2-3 cm to your top tube.

The critical thing is that you are systematic in your approach.
 
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