Defining materials - ti and steel

[Kontact]

I was asked by several to start a new thread where I don't make mention of or answer questions about anything but the narrow topic. Hopefully everyone else can do that, too:

Could a frame be constructed out of steel tubing that would very closely emulate the ride of a popular titanium frame?

Sources:
Tom Kellogg, steel builder, titanium road bike luminary and nice guy.
"The Serotta titanium is a good one to look at. Ben Serotta wanted to come up with a titanium frame that had many of the qualities of his well-known and respected steel frames. In general and to his credit, he succeeded pretty well."
http://www.spectrum-cycles.com/62.htm

"In working with Merlin on that first Titanium prototype, it seemed logical that I simply build my ideal steel frame only this time in Titanium. I wanted to compare steel and Titanium with a minimum of confusing variables. At the time, I thought the Titanium prototype would ride similarly to my steel frame since the design of the two frames was identical. Boy was I in for a shock.

After only a few seconds on the Titanium bicycle I swore I must have completely redesigned the frame. It seemed to fit the same. I measured it and it had the same wheelbase. The clearances of my steel and the Titanium prototype were also the same. One thing was quite different though. The titanium bike felt considerably quicker handling. Handling responsiveness seemed to be instantaneous on the Ti prototype compared to my trusty steel frame. I still remember how alive that first ride was."
http://www.spectrum-cycles.com/614.htm

Tom Kellogg opinions that Ti can be made to have nearly identical properties to a steel frame, but appears to be saying that Ti has a "lively" property that steel frames would have a hard time emulating.

So would a steel builder be able to emulate the precise ride characteristics of a Seven/Merlin/Spectrum/Moots/Kish ti frame, or are there important enough material differences that a precise duplicate would be almost impossible?


Disclaimers:
Both materials make excellent frames, so this is not a question of good or bad.

This is not an attack on Kellogg, steel builders, titanium builders or any bikes either make. It is an exercise in comparison.

I am a fan of Spectrum, but also a realist that understands that I am quoting ad copy. My TK quotes are offered as a jumping off point, not proof of anything.



My opinion (form your own): Steel and titanium are like chocolate and vanilla. Both are tasty ice creams flavored with beans, one not "better" than another, but having qualities that one person might prefer over another, even if they like both. They are equally useful, but not interchangeable in their resulting qualities on the finished product, no matter how the product is made. So ti and steel frames might be equally competitive at the same tasks, but complete the tasks in a different manner.


Have fun, boys. :)

[AngryScientist]

So would a steel builder be able to emulate the precise ride characteristics of a Seven/Merlin/Spectrum/Moots/Kish ti frame, or are there important enough material differences that a precise duplicate would be almost impossible?




i'll start:

how do you determine if one bike "emulates the precise ride characteristics of a Seven/Merlin/Spectrum/Moots/Kish ti frame, or are there important enough material differences that a precise duplicate would be almost impossible?

serious question.

for me anyway, i ride a 50cm bike - very small, most steel/ti bikes i've ridden feel very similar, i'm guessing because the frames are so small that they dont flex and move as much as a 60cm frame would.

i am not a material characteristic expert, but i do not see how its possible to answer the question exactly.

if you were to ask me as a test subject, i would say you could absolutely build a custom tubing steel bike to match the qualities of a good Ti frame in my size, no questions asked in my book.

[Kontact]

I imagine the best way to judge such a thing would be to ride the bike.


If I get the gist of your objection, it is that your frame is too small to have any particular ride qualities, or too stiff to tell the difference between them. You are probably not wrong, but that would make me wonder what would happen if you had a bike made for you?


So should we say that the two bikes are both 56cm frames made for the same 165 lbs. experienced rider to test, and the ti frame was made to be reasonably balanced in stiffness vs. smoothness? (Obviously, you can use any material to construct a bike that rides like stone, but the assumption is that this is a nice riding frame.)

[jr59]

I'd like to know the difference between 953 SS and Ti?

I ride a 62 or so. I have ridden a lot of different steel bikes and a few Ti bikes.
I "think" I can tell the difference between most light weight steel bikes and Ti. They feel different.

But now wit the 953 SS, I have no idea.

[eddief]

seems as if part of equation certainly tough to quantify is the material at a molecular level. i mean it. steel has its characteristics and and ti its own. in my googley research in my days of owning ti, i always wanted to know if its molecular structure allowed it to simply dampen more/better than steel? my bet is yes.

here's a quote from some web place:

Another unique property of titanium is it's low elastic modulus. The elastic modulus is a measure of how stiff a material is and is directly related to a material's ability to transmit shock waves. Titanium's low modulus translates into a natural dampening effect on vibrations and shocks which allows titanium frames to have a smooth ride even without additional suspension elements. The combination of titanium's high strength and low modulus make the material very "springy" and in fact aircraft springs were one of the first uses for titanium alloys. Because of this spring like effect, the material allows for good energy transfer and does not sap energy from the rider like a suspension system or weaker frame materials will.

[mister]

many variables that make a good bike
not worth it to get stuck too hard on frame materials

find a good builder
tell him what you want in a bike
get frame built to fit
enjoy the thing

have a strong feeling
a davidson is a davidson
a spectrum is a spectrum
a kirk is a kirk
and a sach is a sachs
so on and so forth

[Kontact]

have a strong feeling
a davidson is a davidson
a spectrum is a spectrum
a kirk is a kirk
and a sach is a sachs
so on and so forth
So you would say that one good steel builder would not be able to duplicate the ride quality of another good steel builder, even if they were using the same tubing/lugs/etc? Therefore a cross material comparison is equally impossible?

[mister]

So you would say that one good steel builder would not be able to duplicate the ride quality of another good steel builder, even if they were using the same tubing/lugs/etc? Therefore a cross material comparison is equally impossible?

a good builder probably could
but i'd feel like an ass asking one to

atmo you don't go to builder x to get a bike built like builder y

[oldpotatoe]

[QUOTE=Kontact]I was asked by several to start a new thread where I don't make mention of or answer questions about anything but the narrow topic. Hopefully everyone else can do that, too:

Could a frame be constructed out of steel tubing that would very closely emulate the ride of a popular titanium frame?

Isn't 'emulate the ride' subjective? Not sure how you would measure this.

2 identical bikes, one steel, one ti, ridden by a bunch of people, who then write down their impressions? Asked to objectively evaluate the ride, w/o making it a loaded test?

[Kontact]

I'd like to know the difference between 953 SS and Ti?

I ride a 62 or so. I have ridden a lot of different steel bikes and a few Ti bikes.
I "think" I can tell the difference between most light weight steel bikes and Ti. They feel different.

But now wit the 953 SS, I have no idea.

To a large extent, steel is steel. The difference between different alloys and heat treats have more to do with the point at which the steel deforms, not how it acts before that point. So I would not expect 953 to ride differently than S3 with the same wall thicknesses, all else being equal.


Oldepotatoe, it is a thought experiment, so you would be the tester, or the steel builder would be the tester. It is subjective, just as "vanilla" is a subjective flavor. But most bike people seem to believe that the two types of frames do "feel" differently, so it shouldn't be a stretch to ask when the two feel the same.

[flydhest]

I don't know if it is intentionally missing the point or not, but what Tom said was that Ben set out to design a Ti bike that would ride like the steel bikes he designed. Then Tom says, I built a Ti bike the same way I built a steel bike and they rode differently.

There is no inconsistency. "It's the tubes, dammit" was a half-joke (half not) thing Ben used to have on socks, the website, catalogs. The way I always took it was that he wanted the company to have its own tubing because the tubing is part of the design. Ben started with a certain ride that he wanted a bike to have and designed bikes to achieve it. The quotation you give of Tom takes a different starting point, a bike designed to be steel, but then make it out of Ti. The Serotta example is to start with the ride and then, conditional on the material, you have to adjust your design to get the ride. The TK example was, take the design, adjust the material, and the output is the ride characteristic.

[oldpotatoe]

To a large extent, steel is steel. The difference between different alloys and heat treats have more to do with the point at which the steel deforms, not how it acts before that point. So I would not expect 953 to ride differently than S3 with the same wall thicknesses, all else being equal.


Oldepotatoe, it is a thought experiment, so you would be the tester, or the steel builder would be the tester. It is subjective, just as "vanilla" is a subjective flavor. But most bike people seem to believe that the two types of frames do "feel" differently, so it shouldn't be a stretch to ask when the two feel the same.

I'm thinking a builder could make a ti frame ride like steel and vice versa..I have a Waterford R-22 and Moots..they ride a lot alike.

BTW-My R-32, 953 rode a lot like my R-22.

[Kontact]

I don't know if it is intentionally missing the point or not, but what Tom said was that Ben set out to design a Ti bike that would ride like the steel bikes he designed. Then Tom says, I built a Ti bike the same way I built a steel bike and they rode differently.

There is no inconsistency. "It's the tubes, dammit" was a half-joke (half not) thing Ben used to have on socks, the website, catalogs. The way I always took it was that he wanted the company to have its own tubing because the tubing is part of the design. Ben started with a certain ride that he wanted a bike to have and designed bikes to achieve it. The quotation you give of Tom takes a different starting point, a bike designed to be steel, but then make it out of Ti. The Serotta example is to start with the ride and then, conditional on the material, you have to adjust your design to get the ride. The TK example was, take the design, adjust the material, and the output is the ride characteristic.
I didn't post the two as an inconsistancy. I believe that both are true - that you can build a Ti bike to ride much like steel, but that there might be some property of Ti that would be difficult to get steel to act like. Maybe it is eddief's "low elastic modulous"? I could understand being able to increase the modulous, but not being able to go the other way, for instance.

[flydhest]

To answer the specific question, and to echo oldpotato, yes, a skilled builder could make a bike that mimicked another bike's ride. The way the question was asked, however, is weird. Weird because the example bikes listed have an array of ride characteristics. As a result, "precise" seems impossible. What does "precise" mean here?

In fact, the Tom quotation you put up answered the question for you. He states that Serotta made a Ti bike that rides like one of Serotta's steel bikes. Why is there a question if you are willing to rely on Tom's views as reliable (which, I am. I'd follow Tom anywhere . . . especially if he was cooking his world famous ribs. Damn those are good.)

[Kontact]

To answer the specific question, and to echo oldpotato, yes, a skilled builder could make a bike that mimicked another bike's ride. The way the question was asked, however, is weird. Weird because the example bikes listed have an array of ride characteristics. As a result, "precise" seems impossible. What does "precise" mean here?
"Precise" meaning "below a reasonable threshold".

And maybe steel and ti bikes do already ride nearly the same? That's part of the question, really. If that's already true, then the answer is: Yes you can duplicate the Ti bike ride with steel.

I wasn't implying that the list of Ti bikes were identical, just typical. The goal would be to pick one and duplicate that one. Makers like Kish who work in both materials would be good candidates (except for TK, since he has already stated a given his opinion, in a sense).

[Charles M]

Wait wait wait....


YES


....

[Kontact]

Wait wait wait....


YES


....
Okay. How about aluminum?

[TAW]

I wonder if the "feel" could be duplicated as well as the stiffness. I have had a CSI and a Concours. The CSI was stiff, but not comfortable for me to ride. The Concours was comfortable, but not very stiff. These were comparisons for me with the same wheels/tires and same basic components.

Realizing that body weight and bike weight are factors, do you think a stiff bike in certain materials would naturally have a harsher ride?

[jr59]

I'm thinking a builder could make a ti frame ride like steel and vice versa..I have a Waterford R-22 and Moots..they ride a lot alike.

BTW-My R-32, 953 rode a lot like my R-22.


Which rode like your moots!

You are not helping!! :banana:

[EDS]

I believe great bikes can be made out of steel. titanium, alumnium or carbon.

Anyone who think otherwise are either ignorant or purposefully blind.

Probably one of the best people to ask about materials is Carl Strong, since he has made custom bikes out of all four materials and, by all objective standards, knows what he is doing.

[rpm]

One way to get a handle on the gestalts of different frame materials is to have the same builder make frames of different materials for the same (hypothetical or real) client using the same geometries.

Back in the mid-90's the magazine Road Bike Action had Harry Havnoonian make frames made of titanium, carbon, metal matrix, aluminum, and steel, all using the same construction bonding the tubes into stainless steel lugs. Then a group of riders tested them and offered their opinions. The carbon and titanium frames were best liked but with different ride feels. The ti felt squishier but was regarded as the best sprinter. The carbon was stiffer and won the best overall by a whisker.

More recently Jan Heine had Jeff Lyon build identical frames with different steel tubes to test out his planing idea, not surprisingly confirming his liking for light, flexy tubing.

It would be cool to have someone commission Serotta to build the same bike geometry in steel, carbon, and ti and then have some knowledgeable foks evaluate the differences in ride. It would prove nothing, but it would be fun and instructive.

[Kontact]

I believe great bikes can be made out of steel. titanium, alumnium or carbon.
So do I.

I don't think anyone on this forum is ignorant or blind.

[EDS]

It would be cool to have someone commission Serotta to build the same bike geometry in steel, carbon, and ti and then have some knowledgeable foks evaluate the differences in ride. It would prove nothing, but it would be fun and instructive.

Don't they already sort-of do that with the "Ready Custom" line - you can get everything but aluminum, including a mixed material bike?

[David Kirk]

I was working at Serotta, making lots of steel bikes, when the company introduced Ti bikes. What we really wanted was to build TI bikes that rode and felt just like the steel bikes we made, but where lighter. It's pretty easy math to go though and figure out how stiff a given steel tube is and then play with the diameter and wall of the Ti tube to replicate that same stiffness. And this is for the most part what we did.

We had the tubes made and built the bikes and rode them and they of course felt not all that similar to the steel bikes. Not a world different but certainly not the same. I took frames, Ti and steel, and measured the stiffness of the assembled units in various ways and the numbers were very close. But the bikes felt different. Not so much better or worse, but just different. In fact most folks that rode the early bikes liked them but felt they were much different from the steel bikes. I can't recall any skilled rider not being able to feel the difference.

So why did they feel different? I wasn't 100% sure to be honest. Either the very small difference between the bikes was something we were able to feel or it was something else. My feeling is that it was both - that a skilled rider who has ridden many bikes can indeed feel very small differences and that there was an X factor that we didn't understand.

At some point there were scrap Ti tubes (poor surface finish as I recall) to play with and I started messing with them and comparing them to their steel brothers in different ways. One very high tech test was taking the different tubes and banging them over the corner of the metal work bench. Like I said - very high tech. Here the difference in feel was huge. The way the two tubes transmitted shock to my hand was markedly different and the sound was also altogether different. The steel tube rings at a very high pitch and has a good bit of sustain to it while the Ti tube was muted in comparison and transmitted less shock to my hand. These two tubes has the same stiffness by design mind you.

Many, many miles and months later I began to understand that the way the two materials transmit road shock to the rider was much the same way. The steel bike had more 'ring' and 'brightness' to it while the Ti bike was more muted and isolated feeling. I was just as fast on either but they were different. I suspected at the time that the difference boiled down to the hardness of the material itself. The harder the material the more 'ring'.

One could pretty easily make a bike out of steel and then make a bike out of wood that has the same stiffness and then ballast them to weigh the same - but of course they will not ride the same at all. One will feel, wait for it........ like it's made of wood, or 'wooden'. Carbon bikes can be the same way (depending on the density of the material and the resin/cloth ratio and the layup schedule) and the two identically stiff steel and carbon bikes will feel different. Not necessarily better or worse, but certainly different. Some prefer one feel over the other and Vive la differance!

So - in my mind it will almost always boil down to personal preference. If you like the feel of a material then that should material should be your first choice. I prefer the feel of steel over anything else. I know when I say this that eyebrows get raised and there is a "of course you say that, you sell steel bikes' but I see it differently. I make steel bikes because that is what I prefer to ride. It feels the best to me. If I preferred another material that is what I would work with and sell. That said if Serotta wants to give me a Mievici or Crumpton wants to send one of his my way I won't poo-poo it. One needs to keep an open mind.

Time for a ride.

dave

[maunahaole]

He said sustain....

http://www.youtube.com/watch?v=UAl3SahqGLU

[PaMtbRider]

My wife has two Spectrums built by Tom. Her first was a Ti super that she absolutely loves. Because of that, when it was time for an S&S travel bike we went back to Tom and said make a steel bike with couplers that fits, rides, handles, like her Ti bike. Given a choice, she will pick the Ti bike to ride because it "feels" better to her.

[Kontact]

I was working at Serotta, making lots of steel bikes, when the company introduced Ti bikes. What we really wanted was to build TI bikes that rode and felt just like the steel bikes we made, but where lighter. It's pretty easy math to go though and figure out how stiff a given steel tube is and then play with the diameter and wall of the Ti tube to replicate that same stiffness. And this is for the most part what we did.

We had the tubes made and built the bikes and rode them and they of course felt not all that similar to the steel bikes. Not a world different but certainly not the same. I took frames, Ti and steel, and measured the stiffness of the assembled units in various ways and the numbers were very close. But the bikes felt different. Not so much better or worse, but just different. In fact most folks that rode the early bikes liked them but felt they were much different from the steel bikes. I can't recall any skilled rider not being able to feel the difference.

So why did they feel different? I wasn't 100% sure to be honest. Either the very small difference between the bikes was something we were able to feel or it was something else. My feeling is that it was both - that a skilled rider who has ridden many bikes can indeed feel very small differences and that there was an X factor that we didn't understand.

At some point there were scrap Ti tubes (poor surface finish as I recall) to play with and I started messing with them and comparing them to their steel brothers in different ways. One very high tech test was taking the different tubes and banging them over the corner of the metal work bench. Like I said - very high tech. Here the difference in feel was huge. The way the two tubes transmitted shock to my hand was markedly different and the sound was also altogether different. The steel tube rings at a very high pitch and has a good bit of sustain to it while the Ti tube was muted in comparison and transmitted less shock to my hand. These two tubes has the same stiffness by design mind you.

Many, many miles and months later I began to understand that the way the two materials transmit road shock to the rider was much the same way. The steel bike had more 'ring' and 'brightness' to it while the Ti bike was more muted and isolated feeling. I was just as fast on either but they were different. I suspected at the time that the difference boiled down to the hardness of the material itself. The harder the material the more 'ring'.

One could pretty easily make a bike out of steel and then make a bike out of wood that has the same stiffness and then ballast them to weigh the same - but of course they will not ride the same at all. One will feel, wait for it........ like it's made of wood, or 'wooden'. Carbon bikes can be the same way (depending on the density of the material and the resin/cloth ratio and the layup schedule) and the two identically stiff steel and carbon bikes will feel different. Not necessarily better or worse, but certainly different. Some prefer one feel over the other and Vive la differance!

So - in my mind it will almost always boil down to personal preference. If you like the feel of a material then that should material should be your first choice. I prefer the feel of steel over anything else. I know when I say this that eyebrows get raised and there is a "of course you say that, you sell steel bikes' but I see it differently. I make steel bikes because that is what I prefer to ride. It feels the best to me. If I preferred another material that is what I would work with and sell. That said if Serotta wants to give me a Mievici or Crumpton wants to send one of his my way I won't poo-poo it. One needs to keep an open mind.

Time for a ride.

dave
What a fantastic story! Thanks so much for sharing that, Dave.


Do you have any adjectives that you'd apply to contrast the ride quality difference between steel and ti? TK like "resilliant" for ti, but I've never been able to mentally translate that one.

[Lifelover]

I was working at Serotta, making lots of steel bikes, when the company introduced Ti bikes. What we really wanted was to build TI bikes that rode and felt just like the steel bikes we made, but where lighter. It's pretty easy math to go though and figure out how stiff a given steel tube is and then play with the diameter and wall of the Ti tube to replicate that same stiffness. And this is for the most part what we did.

We had the tubes made and built the bikes and rode them and they of course felt not all that similar to the steel bikes. Not a world different but certainly not the same. I took frames, Ti and steel, and measured the stiffness of the assembled units in various ways and the numbers were very close. But the bikes felt different. Not so much better or worse, but just different. In fact most folks that rode the early bikes liked them but felt they were much different from the steel bikes. I can't recall any skilled rider not being able to feel the difference.

So why did they feel different? I wasn't 100% sure to be honest. Either the very small difference between the bikes was something we were able to feel or it was something else. My feeling is that it was both - that a skilled rider who has ridden many bikes can indeed feel very small differences and that there was an X factor that we didn't understand.

At some point there were scrap Ti tubes (poor surface finish as I recall) to play with and I started messing with them and comparing them to their steel brothers in different ways. One very high tech test was taking the different tubes and banging them over the corner of the metal work bench. Like I said - very high tech. Here the difference in feel was huge. The way the two tubes transmitted shock to my hand was markedly different and the sound was also altogether different. The steel tube rings at a very high pitch and has a good bit of sustain to it while the Ti tube was muted in comparison and transmitted less shock to my hand. These two tubes has the same stiffness by design mind you.

Many, many miles and months later I began to understand that the way the two materials transmit road shock to the rider was much the same way. The steel bike had more 'ring' and 'brightness' to it while the Ti bike was more muted and isolated feeling. I was just as fast on either but they were different. I suspected at the time that the difference boiled down to the hardness of the material itself. The harder the material the more 'ring'.

One could pretty easily make a bike out of steel and then make a bike out of wood that has the same stiffness and then ballast them to weigh the same - but of course they will not ride the same at all. One will feel, wait for it........ like it's made of wood, or 'wooden'. Carbon bikes can be the same way (depending on the density of the material and the resin/cloth ratio and the layup schedule) and the two identically stiff steel and carbon bikes will feel different. Not necessarily better or worse, but certainly different. Some prefer one feel over the other and Vive la differance!

So - in my mind it will almost always boil down to personal preference. If you like the feel of a material then that should material should be your first choice. I prefer the feel of steel over anything else. I know when I say this that eyebrows get raised and there is a "of course you say that, you sell steel bikes' but I see it differently. I make steel bikes because that is what I prefer to ride. It feels the best to me. If I preferred another material that is what I would work with and sell. That said if Serotta wants to give me a Mievici or Crumpton wants to send one of his my way I won't poo-poo it. One needs to keep an open mind.

Time for a ride.

dave

Here endeth the lesson.

[David Kirk]

What a fantastic story! Thanks so much for sharing that, Dave.


Do you have any adjectives that you'd apply to contrast the ride quality difference between steel and ti? TK like "resilliant" for ti, but I've never been able to mentally translate that one.

I like the word 'damp'. But what it means to me might differ from what it means to you.

Dave

[Kontact]

Maybe like, steel feels like dry oak, and ti like wet bamboo?

[beercan]

why debate have both!!! :cool:

[Scott Shire]

I'm reminded of Craig Gaulzetti's statement that all of his (wonderful, wonderful) bicycles, ride the same (or at least to his very narrowly focused ideal) regardless of material. Since he's done al, cf, and steel, I think it might be useful to run across the hall and pose the aluminum question to him. More so because he has an obsession with high-end aluminum.

FWIW, I love the ride of great aluminum, and some of my wishlist bikes are aluminum (or scandium/magnesium/etc) ...
Gaulzetti Corsa
Merckx Premium Classic
Cannondale Track
Pinarello Dogma Magnesium
Coppi KCN Star

[e-RICHIE]

I'm reminded of Craig Gaulzetti's statement that all of his (wonderful, wonderful) bicycles, ride the same (or at least to his very narrowly focused ideal) regardless of material. Since he's done al, cf, and steel, I think it might be useful to run across the hall and pose the aluminum question to him. More so because he has an obsession with high-end aluminum.

FWIW, I love the ride of great aluminum, and some of my wishlist bikes are aluminum (or scandium/magnesium/etc) ...
Gaulzetti Corsa
Merckx Premium Classic
Cannondale Track
Pinarello Dogma Magnesium
Coppi KCN Star

my road bicycle is an rs designed (specs) aluminum gaulzetti and it has many features not included on my own frames; welds, huge AL pipes, an ISP, a straight fork (carbon fiber, at that), 31.7mm 'bars and stem, and i cannot feel a single difference between it and the identically made rs road bicycles i have had over the years, be they made of OS steel, classic steel, or any set i used all along (of which i have had at least 2 dozen iterations) atmo.

ps

arrange disorder

:) :) :)
:) :) :)
:) :rolleyes: :p

[dekindy]

I wonder what percentage of cyclists that like the material they rode initially. Like my generation likes steel the best and the current generation likes carbon the best because it is what they grew up with. Any guess as to the percentage? Probably not 100% but maybe the 80/20 rule applies like it does to many other things.

Interestingly enough, I tried to get my buddy to purchase a Bianchi steel road bike when he started riding 5 years ago. They had all-Ultegra Bianchi Vigorelli's for $1,400 and I would have purchased one in a heartbeat if I had not already purchased a Fierte. Instead, he purchased a Specialized Roubaix which is fine for the type of riding that he does. Ironically, he said the other day that he wished he had purchased a steel bike. I am not sure what he is basing that on since he has not ridden a modern steel bike, just inexpensive steel bikes from his youth.