[SuspectDevice]

(disclaimer: I design hardtail and road bike frames for a living)

Blame 29'rs(crown clearance).

Also, bent DT or hydroformed downtubes that look bent are pretty much mandatory to pass the CEN front-impact standards on a light frame with a longer fork. It's fine for people like me who re-hash the 90's but consumers tend to expect more "organic" forming. Gussetts cost a lot of money too. They(formed downtubes) make the front end much stronger and more than just on paper.

I think they're ugly. We use big-ass DT and Headtubes and external lower headset cups instead. :edit I use a larger downtube on the short headtube bikes too. Can't be too safe.

The bigger tubes help move stuff farther from the tube centerlines, creating more clearance by moving the center of the ht away from the mitered edge of the DT.
The 110mm headtube in the picture is as short as you can go before you have a sketchy-looking and time-intensive miter.
It looks pretty sweet but the clearance limitations limit geometry choice somewhat.

Most of the forming and bending on modern MegaBrand bikes is pretty useless but that headtube bend is mandatory if you want to makes bike reliably and inexpensively.

 

[chequebookracer]

Cheers SuspectDevice, from your clear post I read that bending a downtube to be near perpendicular to the headtube means you can fit a bigger diameter downtube on a set headtube length without complicated mitres with the top tube.

So dragontalesdh was right, a bent downtube can give more strength, even if he didn't explain himself with clarity or very politely. Re-reading his original post it makes a lot more sense now, I'd assumed he was speaking rubbish with his use of the term stiffness but actually he is right as long as you always associate bent tubes with bigger tubes.

There you go, I'm an engineer and was wrong, I must work for Kona.

John

 

[jonKranked]

i love this time of year, people getting so antsy waiting for the lifts to start spinning

 

[IH8Rice]

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

ive seen plenty of dented DT's on the Session's even with their "armor"

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.

 

[MinorThreat]

I put a 'Norwesco ridge cap' downtube protector on all my dh bikes.

ftfy.

 

[dilzy]

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?

Stronger or stiffer? Make up your mind.

I ride an sx trail on all sorts of terrain. It's quite amazing how big a rock the front tire can kick up which you then go smashing into.

 

[Udi]

From a practical standpoint, I completely agree with the OP. The thing to note here is that there are a lot of less-calculable stress and load factors (especially impact related) that can affect a mountain bike, especially a downhill bike.

Bouncing around against the downtube on the back of pickup trucks, flicking big rocks up into the downtube, and every now and then, running the downtube into an immovable object.

I've put a hole in a demo8 frame downtube before from punching it into a rock, and have read similar experiences from a few other riders here. No it's not common, but yes it does happen - and the bent downtubes make it far more likely to happen in situations where contact wouldn't occur if the tube was straight.

Fair enough if the design absolutely requires it, but as the OP pointed out there are a number of frames (especially DH frames, where water bottles aren't even used) that bend the DT more than necessary, or do it for no real reason.

 

[iRider]

Fair enough if the design absolutely requires it, but as the OP pointed out there are a number of frames (especially DH frames, where water bottles aren't even used) that bend the DT more than necessary, or do it for no real reason.

You mean looking good in the lift line is not a good reason to have a bend DT on your bike?

 

[Kntr]

ftfy.

It works and for $25 it has saved my down tube on a few bikes. Then I don't have to worry one bit when I'm bashing rocks. Plus, you cant really see it when you stand back. Ya, it does look like gutter material.

 

[Huck Banzai]

It works and for $25 it has saved my down tube on a few bikes. Then I don't have to worry one bit when I'm bashing rocks. Plus, you cant really see it when you stand back. Ya, it does look like gutter material.

I had one of those on my Bullit, but I baked on flat black coats, and attached it with black double sided tape. StealthBling'K

 

[rockofullr]

I really don't understand how having a straight tube will get you any noticeable amount of clearance. Your BB will still be at the same height.

Am I missing something? If we are just arguing for fun that's fine too.

 

[Lelandjt]

^We are just arguing for fun. I felt like having an interesting discussion about frame design. By moving the lower part of the DT forward (4"+ on some bikes) I was saying that it's closer to the front wheel and more likely to get hit with rocks. Mostly it just irked me as an aesthetic, rather than practical design with a minor weight penalty and that offends my engineer's mind (4 semesters bitches!). Also it struck me as a funny meme that all the big companies are copying, someone had to call them out.

 

[amishmatt]

Between these stupid pregnant downtubes, and the equally stupid arched toptubes, they might as well just make the frame a full circle already.

 

[William42]

Between these stupid pregnant downtubes, and the equally stupid arched toptubes, they might as well just make the frame a full circle already.

it seems fitting that this should come from ellsworth

http://www.ellsworthride.com/

 

[jonKranked]

i think this is a little closer