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No More Bent Downtubes!!!

Lelandjt

Turbo Monkey
Apr 4, 2008
2,508
821
Breckenridge, CO/Lahaina,HI
This has annoyed me for a while and I must be the only one. MANY bikes now have a downtube that leaves the BB and goes forward several inches before angling up. This makes it much more prone to impacts and of course heavier and weaker than if it went in a straight line from BB to headtube. There are a few suspension designs that need some bend for clearance but look at these bikes and you can see the bend is exaggerated beyond what's necessary and purely a cosmetic trend (Trek). There's even plenty bikes (Spesh Stumpjumper & Enduro) with that bend and no suspension stuff that needs clearing. It's like they saw it on another frame and copied the look. How stupid when you consider the other lengths that are gone to in the name of maximizing strength/weight. Would the bike below really not look as good with a straight downtube and would that affect ANYONE's purchasing decision? Mention carbon frames and someone will bring up rock strikes. However, I'm the only one who questions a frame design that increases the chance of a downtube rock strike.


P.S. The bent/slacked seat tube on that frame is stupid too but I'll save that rant for another thread. I'm sure the engineers who make these frames are aware of both issues but why they ignore practicality in favor of aesthetics is mind boggling when we're talking about an expensive, performance oriented piece of gear.
 

I.van

Monkey
Apr 15, 2007
188
0
Australia
Do you really think they did it solely for aesthetics? I'd say in the case of the Stumpjumper above, they have done it so a water bottle can fit in there.

Also, there are geometry/fit reasons for a bent seat-tube.
 

Huck Banzai

Turbo Monkey
May 8, 2005
2,523
23
Transitory
Im curious too; Is there some good structural reason for the forming? It DOES put the frame/dt in 'harms way' and unless it is to allow space for a lower shock mount/basket, why?

Make a frame vulnerable with unnecessary added material for a water bottle mount? That aint a weenie xc bike, we have camel baks - why screw up a frame for that? Sure the above Spec is a performer, but why the swoopy needless tube? Why the interrupted seat tube? Is it really needed for wheel clearance? It DOESNT look good, it looks clunky, so unless there is some structural purpose, WTF!?!!!

Make fun of the OP all you want, but really? Sheep accept, I aint no steenkin' sheep!
 

sethimus

neu bizutch
Feb 5, 2006
4,881
2,132
not in Whistler anymore :/
Im curious too; Is there some good structural reason for the forming? It DOES put the frame/dt in 'harms way' and unless it is to allow space for a lower shock mount/basket, why?

Make a frame vulnerable with unnecessary added material for a water bottle mount? That aint a weenie xc bike, we have camel baks - why screw up a frame for that? Sure the above Spec is a performer, but why the swoopy needless tube? Why the interrupted seat tube? Is it really needed for wheel clearance? It DOESNT look good, it looks clunky, so unless there is some structural purpose, WTF!?!!!

Make fun of the OP all you want, but really? Sheep accept, I aint no steenkin' sheep!
so how many enduro/am bikes did you have where the downtube was damaged in normal use?
 

Tomasis

Monkey
Feb 26, 2003
681
0
Scotland
sethimus, exactly..

if it was on downhill bike, it'd be different matter and we dont have water bottles on DH rigs :D :D
 

Wa-Aw

Monkey
Jul 30, 2010
354
0
Philippines
I'm pretty sure a benefit of this is BB stiffness. Might be just marketing but I saw it a few years back when this was trending. From a common sense point of view I can understand how the BB area could be stiffer if the junctions of the ST and DT are a wider angle apart, just saying. I'm clueless and guessing here but this thread is destined to be retarded anyway.

For Trek's (I'll use the Session as an example) it's definitely needed in the design to fit a piggyback shock and I don't see how they are exaggerating it for cosmetics.

As per spec AM bikes... Maybe they use the same downtube as current/previous designs and it worked and they don't want to spend money on tooling? Not sure how keen they are on engineering for aesthetics, demo's are ugly as sin.


But I too miss old school looks. I loved the 303WC that came out the other year. The straight tubes looked, for lack of a better term, bad ass. I have a pet peeve for overly curved toptubes just to get that stupid stand over height measurement low even though the place they are measuring from has no room for, or is not where your balls would go. Who the hell rides on their top tube these days anyway? The only thing it makes harder would be posing beside your bike. But I'm sure a lot of riders are going to complain about not being able to do turn downs.
 

Huck Banzai

Turbo Monkey
May 8, 2005
2,523
23
Transitory
so how many enduro/am bikes did you have where the downtube was damaged in normal use?
All of my bikes have some denting/abrasion in that area, I have a huge gauge in my Santa Cruz BLT2 lower link which is far more recessed than that.

Its pretty common, and why new CF bikes have deflectors/armor there (XC, AM, Trail included); do you ride your bike on level groomed single track? Funny thing is given trek as an example - yeah - they need the space (and have the armor...), but the swoop is much less severe than the Spec where space isnt needed.

the "destined to be retarded" and quick disdain are the 'retarded' feedback.

Nothing to say, say nothing, its a pretty reasonable question; Especially on a forum where marketing is accused wherever there's any attempt at progress, this one gets a pass?

If it were for stiffness, we'd see it on road bikes where vulnerability is irrelevant. Hey - maybe there is a good reason I dont see, but no one is offering anything but counterargument. What IS it for, and why wouldnt everyone use it if it has a benefit other than being new and different?



Some people state "I didnt/dont have an issue" - and also have a big ring, no bash guard and walk a lot.
 
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FlipFantasia

Turbo Monkey
Oct 4, 2001
1,659
492
Sea to Sky BC
H2O, bro.
this

so how many enduro/am bikes did you have where the downtube was damaged in normal use?
and this.....used more traditional and more bentier down tubes, and it really doesn't matter, aesthetically I'd agree that it's look nicer if it was straight, but I like having a water bottle cage on my bike that can fit a large bottle. I actually wish they'd pushed the mount closer to the rear shock on my new Reign, cause it only fits a small bottle, and that sucks for 8-9hr epic days.
 

mtg

Green with Envy
Sep 21, 2009
1,862
1,604
Denver, CO
Like others have mentioned, the downtube bend is most likely for water bottle clearance. And, we've all seen the rage when somebody makes a frame that can't hold a water bottle.

On the second point, I agree on the seat tube angle. A steeper actual seat tube angle with an offset seat post reduces the fore-aft movement of the seat when it is raised/lowered = good.
 

William42

fork ways
Jul 31, 2007
3,908
634
and others just like having a water bottle really really a lot more then a camel back, because riding with a camel back for xc/trail/am sucks balls and makes your back hot and sweaty and throws your balance off.

And for those of us that buy our trail bikes for trail riding (hint, most mountain bikers) and not for "a sweet mini DH bike for taking to whistler to prove how baddass we are that we can handicap ourselves and still hobble our way down the hill" climbing and distance are a fact of life, and so bottle cages are genuinely important.

I also ride with a big ring/no bash, but I sure as sh1t dont walk, either on the ups or downs.

Actually, that's not true, if there is a particularly tech steep climb that I want to get to the top of, I'll frequently walk, but tech climbs are not my strongest point.

edit: that said, I have several dents in my downtube (trancex) from rock strikes, but nothing to write home about or really be bummed about, certainly nothing dangerous.

double edit: but I totally agree slack seat angles are silly, and that if a bicycle doesn't need a bent dt to accommodate a shock or bottle cage, then it is silly to slap on one.
 
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Lelandjt

Turbo Monkey
Apr 4, 2008
2,508
821
Breckenridge, CO/Lahaina,HI
Accomodating a water bottle is a somewhat valid excuse but I still think it's mostly for aesthetics. If it doesn't bother you guys fine. I think it's a silly fashion at the cost of weight and putting the downtube more in harm's way.
It's nice to hear some people agreeing with my seat tube angle opinion as I've been the only one saying that for a couple years.
 

kidwoo

Artisanal Tweet Curator
of course heavier and weaker

Don't be so sure.

Which is stronger and stiffer, a wider angle at a junction or a shallower one? Headtube yo. Look at the headtube.

With the wall manipulation that specialized and trek do you've got to remember that we're not talking about straight welded round tubes here either.

Looking at where those bolts are, a bottle cage is obviously a consideration (and look how LOW it is :)), but don't assume it's at the expense of the frame's integrity.
 

Willy Vanilly

Monkey
Jul 27, 2003
194
0
San Jose
OBVIOUSLY the downtube bend is to lower the center of gravity of the bike so you can corner like a m*f*

But then again, if you wanted to lower the center of gravity while keeping your mass centered during hard cornering, you could just attach some of these to the BB area (innovate or die after all)...
 

tuumbaq

Monkey
Jul 5, 2006
725
0
Squamish BC
good luck putting a dent or even a ding on a Trek downtube that has an 1/2 inch think rubber guard...dumb thread, take a deep breath, relax and go ride your bike
 

no skid marks

Monkey
Jan 15, 2006
2,511
29
ACT Australia
good luck putting a dent or even a ding on a Trek downtube that has an 1/2 inch think rubber guard...dumb thread, take a deep breath, relax and go ride your bike
I'd rather no bent tube, no rubber padding, and no bottle mounts myself. Aren't newer treks renowned for dinging. Rubber won't stop full weighted impacts into say a log.
I'm broken and can't ride:cray:
 

davec113

Monkey
May 24, 2009
419
0
Don't be so sure.

Which is stronger and stiffer, a wider angle at a junction or a shallower one? Headtube yo. Look at the headtube.

With the wall manipulation that specialized and trek do you've got to remember that we're not talking about straight welded round tubes here either.

Looking at where those bolts are, a bottle cage is obviously a consideration (and look how LOW it is :)), but don't assume it's at the expense of the frame's integrity.
Without knowing whether the curve was a practical consideration, like fitting a shock or water bottle, or because it's a better structural design is hard to say. It could be both. I would guess the BB area would be stiffer with the curve, plus it also allows for shock clearance. A curved dt is a better solution than Giant's "glory hole", lol.

I have a Remedy and Session, the Remedy does have a few small dents in the dt area. My Session has a thick carbon fiber dt protector so I don't have to worry about it. The Remedy has a lifetime warranty so I figure I'll get a new one when it cracks anyway.
 
Apr 18, 2011
72
0
Crossville Tennessee
I really do pitty the epidemic of loud mouth ignorance. Im an engineer at East Tennessee State university. Allow me to fill you in on your ponderings and misleadings because frankly threads like this piss me off. (someone exploding about a bikes design thinking he/she knows more about physics than a liscensed team of engineers) Granted. i would like to say that Engineers are not ALWAYS right and do make mistakes. Take the entire line of Kona bikes for example. if you dont believe me look up broken bikes on youtube and you will see that most of the bikes in the video are konas.

Now. A bent downtube was created for many reasons. While it is not inherently stiffer if force was applied straight from a the head set towards the BB. this force is rarely if ever applied to a bicycle frame in any form of "major" force. i.e. hard wrecks and big drops. The curve of the tube however addresses the more prevalent forces that are much more likely to occur to the average rider. the way this works is because the bend in the fram allows for a shallower angle when attaching the downtube to the headtube it has allowed for a larger weld resulting in more surfface area for the joints. AKA STIFFER AT THE HEADTUBE. .Many companies have worked their piping, (Giant, Trek, Norco, And Specialized) all who use the bent downtube, to attached the headtube mainly to the downtube alone and the top tube is welded mostly to the downtube if it touches the headtube at all. this being said. on an average day of mountainbiking the most common force ever applied in mass is frontal. whether it be from frunning into objects like trees and rocks or rolling over rough terrain or simply braking. From the ground up to the top of your head tube there is a lot of leverage you are not taking into account. with the bent down tube and the top tube acting as a triangulator to distribute forces. the many things that would have enicially bent an ordinarily straight downtube will have no effect and your frame will last longer. that being said there are advantages to both designes. one person mentiond road biking. it is true. a bent down tube will be heavier so a straight tube makes for a lighter bike. road bikes make up for this by having a very long head tube for more surface area. most of them are over 5". and with the advancement of carbon fiber frames are allowed to put more carbon on the inside that you cant see but it is your best friend. that in mind you still do see many road frams broken just past the head tube on the very rare occasion that a roady runs directly into something like a wall or the side of a vehicle. Mountain bikes will handle that type of force much more efficiently. in the case of the water bottle clearance, yes it does help having a bent frame for that and some designes have bent frames solely for this. in the case of the stumpy. Specialized did not make this a full XC bike. it is only a step down from their aggressive trail Enduro. it is still a very adept mountain machine, making the bent tube a bit more effective and look better all in one package. in many cases (more often than not) Full on XC race bikes will adopt the straight down tube for the simple fact that the type of riding is much less intensive on the frame. So now that iv had MY rant. I hope you followed. to the original poster. if you just dont like the look of a bent down tube thats fine. plenty of companies make straight tubed bikes that kill on every terrain. (Santa Cruz, i ride one myself) but before you go blowing smoke in the face of the people who make your bikes. consider the fact that the person who built that thing you ride probably knows just a little bit more than you do about frame design. CHEERS :thumb:
 

dilzy

Monkey
Sep 7, 2008
567
1
I really do pitty the epidemic of loud mouth ignorance. Im an engineer at East Tennessee State university. Allow me to fill you in on your ponderings and misleadings because frankly threads like this piss me off. (someone exploding about a bikes design thinking he/she knows more about physics than a liscensed team of engineers) Granted. i would like to say that Engineers are not ALWAYS right and do make mistakes. Take the entire line of Kona bikes for example. if you dont believe me look up broken bikes on youtube and you will see that most of the bikes in the video are konas.

Now. A bent downtube was created for many reasons. While it is not inherently stiffer if force was applied straight from a the head set towards the BB. this force is rarely if ever applied to a bicycle frame in any form of "major" force. i.e. hard wrecks and big drops. The curve of the tube however addresses the more prevalent forces that are much more likely to occur to the average rider. the way this works is because the bend in the fram allows for a shallower angle when attaching the downtube to the headtube it has allowed for a larger weld resulting in more surfface area for the joints. AKA STIFFER AT THE HEADTUBE. .Many companies have worked their piping, (Giant, Trek, Norco, And Specialized) all who use the bent downtube, to attached the headtube mainly to the downtube alone and the top tube is welded mostly to the downtube if it touches the headtube at all. this being said. on an average day of mountainbiking the most common force ever applied in mass is frontal. whether it be from frunning into objects like trees and rocks or rolling over rough terrain or simply braking. From the ground up to the top of your head tube there is a lot of leverage you are not taking into account. with the bent down tube and the top tube acting as a triangulator to distribute forces. the many things that would have enicially bent an ordinarily straight downtube will have no effect and your frame will last longer. that being said there are advantages to both designes. one person mentiond road biking. it is true. a bent down tube will be heavier so a straight tube makes for a lighter bike. road bikes make up for this by having a very long head tube for more surface area. most of them are over 5". and with the advancement of carbon fiber frames are allowed to put more carbon on the inside that you cant see but it is your best friend. that in mind you still do see many road frams broken just past the head tube on the very rare occasion that a roady runs directly into something like a wall or the side of a vehicle. Mountain bikes will handle that type of force much more efficiently. in the case of the water bottle clearance, yes it does help having a bent frame for that and some designes have bent frames solely for this. in the case of the stumpy. Specialized did not make this a full XC bike. it is only a step down from their aggressive trail Enduro. it is still a very adept mountain machine, making the bent tube a bit more effective and look better all in one package. in many cases (more often than not) Full on XC race bikes will adopt the straight down tube for the simple fact that the type of riding is much less intensive on the frame. So now that iv had MY rant. I hope you followed. to the original poster. if you just dont like the look of a bent down tube thats fine. plenty of companies make straight tubed bikes that kill on every terrain. (Santa Cruz, i ride one myself) but before you go blowing smoke in the face of the people who make your bikes. consider the fact that the person who built that thing you ride probably knows just a little bit more than you do about frame design. CHEERS :thumb:
No one who does Engineering is this bad at composing a paragraph. I'm almost positive your full of it, but I can't in good faith devote my life to reading your blob of text.

PS, I think it looks stupid, but until you build one with a straight down tube and one with a bent down tube in FEA or IRL that are the same weight then test the stiffness, you'll never know.

PpS, it looks stupid and hangs low enough to get smacked all! the time, so I'd take the slightly less stiff option if that's the compromise.
 
Apr 18, 2011
72
0
Crossville Tennessee
On a second and completely unrelated note. What kinda bike are you riding and what kinda terrain are you riding? Throwing rocks from the front tire is one thing but your obviously doing something wrong if your tube is physically getting hung on stuff.
 

Huck Banzai

Turbo Monkey
May 8, 2005
2,523
23
Transitory
Ok i will put in a way you can understand. A bent down tube makes for stronger joints that are under high stress. Bottom Bracket and Head Tube for example. Dumb enough for ye?:banghead:

Stronger joints or tubes? Because the joint is the weld, and who's pulling welds apart? and if they did - its a far more likely a bad build, not design.

"you still do see many road frames broken just past the head tube" - but not at the welds. Do you see the elementary fault in the correlation in this case?

:tinfoil:
 
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captainspauldin

intrigued by a pole
May 14, 2007
1,263
177
Jersey Shore
Seemes like to fit a water bottle(mine is a XL for reference):
downtube.JPG

That being said, only issue I've had with it, I couldn't fit an old LG-1 with a bashguard, as the bashguard hits the downtube.. I have more of an issue with the cable routing on the downtube, seemes to be more in harms way than necessary, and I had to get new/longer brake lines to run them this way.
 

Sandwich

Pig my fish!
Staff member
May 23, 2002
21,031
5,918
borcester rhymes
in my opinion, the only tube that should be hyrdoformed for shape/crookedness is the top tube like so:



Gives you added stand over and nutbag clearance. The seat tube is straight (hallelujah) and the down tube COULD BE straight, but I'll take it anyways.

I still wish he built that bike and just dropped the CVT.....
 

William42

fork ways
Jul 31, 2007
3,908
634
Ok i will put in a way you can understand. A bent down tube makes for stronger joints that are under high stress. Bottom Bracket and Head Tube for example. Dumb enough for ye?:banghead:
You do realize there are other, actual engineers responding in this thread, not students, right?

Also, I don't think anybody was asking you to "dumb it down." It already achieved that quite nicely. People on this forum generally aren't too anal about grammar and spelling, but you'll get sh1t when what you write is totally illegible. Anyway, if you use some paragraphs in there, form your thoughts into something cohesive, you'll probably get a nicer response!
 

SkullCrack

Monkey
Sep 3, 2004
705
127
PNW
You lost me after this brilliant deduction:

Take the entire line of Kona bikes for example. if you dont believe me look up broken bikes on youtube and you will see that most of the bikes in the video are konas.
To say the videos on YouTube are an accurate statistical sample for determining the structural quality of a company's entire line of bikes is laughable. Correlation does not imply causation.

Along a similar line of thinking to yours, here's some more research you may find fascinating:

With a decrease in the number of pirates, there has been an increase in global warming over the same period.
Therefore, global warming is caused by a lack of pirates.
 
Feb 14, 2011
18
0
Ok i will put in a way you can understand. A bent down tube makes for stronger joints that are under high stress. Bottom Bracket and Head Tube for example. Dumb enough for ye?:banghead:
Hello, I'm an engineer with about ten years experience in various industries, I've been a stress guy for about 5 years, mainly FEA and the odd hand calc. The bent tube is longer than the straight tube so assuming the same material and tube profile, the longer tube will always be less stiff, i.e. it will have a greater displacement for a given load. In the old days (60s/70s) some road bikes had curvy chainstays for precisely this reason; to make them more comfy by the longer stays moving more for a given load.

As for the weld contact area surely it would be greater for a bigger angle, compare slicing a tube at 90 degrees or 45 degrees?

I think really they're only pushbikes and structural design is pretty simple given the historic information (i.e. most manufacturers make frames out of the same thickness tubes as the last bike that didn't break), might as well make them look a bit different and create some brand identity. Otherwise you end up with 'looks like a trek.'

Anyway always interesting to have a bit of a chat about bikes, maybe worth checking the tone of your posts if you want people to take them seriously.

John
 
Feb 14, 2011
18
0
Actually how about this for a reason for curved downtubes, the smaller contact area of the bent tube allows for a shorter headtube that fits in better with long travel bikes, see the picture of the fsr above?

John
 

rockofullr

confused
Jun 11, 2009
7,342
924
East Bay, Cali
I really do pitty the epidemic of loud mouth ignorance. Im an engineer at East Tennessee State university. Allow me to fill you in on your ponderings and misleadings because frankly threads like this piss me off. (someone exploding about a bikes design thinking he/she knows more about physics than a liscensed team of engineers) Granted. i would like to say that Engineers are not ALWAYS right and do make mistakes. Take the entire line of Kona bikes for example. if you dont believe me look up broken bikes on youtube and you will see that most of the bikes in the video are konas.
I disagree with your assessment. I am an engineer and I am always right.