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PostPosted: Fri Mar 18, 2016 3:09 am 
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Its been years since I saw this last post on WW:
viewtopic.php?f=3&t=88590&start=30

i'm wondering if there's such a thing as lightweight, yet stiff, Ti frame? or is that unicorn?


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PostPosted: Fri Mar 18, 2016 4:28 am 
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Location: Tokyo
As I understand, stiff Ti framesets are usually heavier (tapered headtube, oversize chainstays etc). (Legend or Passoni)
And the lighter ones has smaller diameter tubes and won't give you the same stiffness as carbon framesets.


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PostPosted: Fri Mar 18, 2016 8:01 am 
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What's light for you?


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PostPosted: Fri Mar 18, 2016 8:29 am 
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Joined: Sun Oct 05, 2008 8:09 pm
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Location: Aix en Provence
1235g for a 62cm frame. It's perfectly stiff enough.

Image


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PostPosted: Fri Mar 18, 2016 8:49 am 
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^California Love

http://forums.thepaceline.net/showthread.php?t=27202
Originally Posted by David Kirk (on the Serotta Forum)
Hey,

This is a favorite subject of mine.

I'll say up front that I have no idea what bike you might like best. I'll leave that alone.

First I'll say something that you won't hear from many builders.......curving or bending the stays into any configuration other than straight will make the stays flex more. There are very few absolutes in life but this is one of them. Curved stays flex more than the equivalent stay without the bend. They won't flex a lot more......just a little. It depends on the type of bend and the degree of bend. One thing is for sure, the bent stay can never be stiffer due to it's bend. The biggest reason to bend a stay is to allow for better clearances with tire and chainring. This is a bigger deal on mountain bikes than road but true nonetheless.

In any given material, stay diameter is the biggest factor in determining the stiffness. The bigger the diameter, the stiffer the stay. The problem is that if you make the stay bigger in diameter you run into clearance issues with both the chainrings and tire. So..... most stays are about the same in this regard. One can use huge stays (Big Leg Emma) but then you need big, deep dents to allow for the clearance room and the dents go a long way to negating the gains in stiffness made by the larger diameter stay.

There is a current trend in frame design to control BB flex with a big-arse downtube. This may feel like a good thing at first and it will certainly help with front derailer rub but in the end it does not make for a stiffer bottom bracket. It's often overlooked but it's the chainstays that make the drivetrain stiff. A big downtube might be good for other reasons but it doesn't do squat to make for a stiff drivetrain. A Slingshot frame is a great example of this. It's simple when you think of it. The energy goes into the frame at the BB and goes to the rear wheel.........and the only thing between the BB and the wheel is the chainstays. This is one of the big issues in designing a good Ti bike for a big or heavy rider. Titanium is much more flexible than steel. The way to make it as stiff as steel is to make the diameter much larger. But this has it's own clearance issues. So they make the stays oval (making them stiff in the vertical direction and soft laterally.......not ideal) or they put in huge dents which has it's own issues. So most Ti builders compensate for the wimpy stays with a huge downtube. And like I stated above this has little effect on true drivetrain stiffness. This is one of the reasons that many larger folks feel that Ti bikes don't have the snap they are looking for and why it's fallen out of favor with many racers.

When pedaling a bike out of the saddle chainstays are asked to do a number of different things. They undergo compression which is easy for almost any stay design to deal with as the loads are low. They undergo a torsional (twisting) load which most stays deal with fairly well regardless of shape. Larger diameter helps a good bit with the torsion. The other thing that happens to stays is that they see a lateral bending load as the BB is pushed from one side to the other. This is where ovalized stays can get in trouble. An oval tube has a major diameter and a minor diameter. There is a pretty good rule of thumb that addresses how an oval tube will flex compared to a round tube. The oval tube will flex about the same as a round tube that has the same diameter as either the major or minor diameters of the oval. In other words if you have an oval tube that is 30mm by 17mm in cross section it will flex about the same as a 30mm round tube in one direction and about the same as a 17mm tube in the other direction. So if you think of oval chainstays you in effect get the lateral stiffness of a rather small round tube.

For all the above reason I feel that a round stay is best. You get the most bang for the buck in ever direction. You get good clearances with minimal denting and you get a nice stiff stay and drivetrain.

Wow............that's more writing than I thought it would be. Thanks for sticking with me.

Dave


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PostPosted: Fri Mar 18, 2016 11:37 am 
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addictR1 wrote:
Its been years since I saw this last post on WW:
viewtopic.php?f=3&t=88590&start=30

i'm wondering if there's such a thing as lightweight, yet stiff, Ti frame? or is that unicorn?


I doubt you'll get under about 1100-1200g, so that might influence your judgement of 'light'.

What does 'stiff' mean and what load are you to impart to it?

I'm fairly light (60kg) and pretty modest power (3.5W/kg), and/so the Ti experience works for me: my Ti build is 7kg, excluding pedals and cages etc. (i.e. a sort of 'specification' weight). Carbon fork/steerer and bars.

The ride feels quite creamy but/and is responsive and sharp handling - I've never felt any 'flex' but perhaps I'm not likely to.

I think you'd need to try one.

(Interesting points by Dave above. My tubes are a mix of elliptical and hydroformed 'complex' shapes. The elliptical ones actually 'swap' their major/minor diameters - i.e. what was a horiz ellipse becomes a vertical one at the opposite end. Curiously just like my 180s De Rosa and its Columbus Max tubeset - which is beautifully compliant...)


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PostPosted: Fri Mar 18, 2016 12:04 pm 
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Location: NorthEU
Will not go in to facts as i am no builder. But, the problem may also be this. If you ask a few guys what stiff really is and how stiff does it need to be, i think you'll end up with lots of different views.
To make a stiffer BB, some builders use an additional tube connecting chainstays (this is located just behind the BB).
My ti-frame is certainly not light, wide tube diameter and double wall at some places. It's made this way to be stiff, which it also is.

I asked the builder alot of questions of tubes, thickness and weight. He told me, most customers asking for light often complains in the end because the bike end up lacking in stiffness.
Also, the bike should be durable.

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PostPosted: Fri Mar 18, 2016 12:10 pm 
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Location: Urbana, Illinois
My 2008 Lynskey R320 M/L is plenty stiff for this old 4 watt per kg 72 kg rider. Frame weight is 1240 grams and bike weight complete with pedals and cages 5.76 kg.

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PostPosted: Fri Mar 18, 2016 5:20 pm 
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Marin: around 1100g for frameset, which i found thus far is Litespeed T1SL.
RJKflyer: i'm at 68Kg.. hoping to drop some more weight.
Juanmoretime: holy cow... only 5.76kg including pedals? that's insane!


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PostPosted: Fri Mar 18, 2016 10:54 pm 
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Juanmoretime wrote:
My 2008 Lynskey R320 M/L is plenty stiff for this old 4 watt per kg 72 kg rider. Frame weight is 1240 grams and bike weight complete with pedals and cages 5.76 kg.


What wheels do you run?


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PostPosted: Fri Mar 18, 2016 11:04 pm 
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RJKflyer: my guess is it has to be carbon.. kinda hard to achieve that with alloy wheelset.


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PostPosted: Sat Mar 19, 2016 12:32 am 
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RJKflyer wrote:
Juanmoretime wrote:
My 2008 Lynskey R320 M/L is plenty stiff for this old 4 watt per kg 72 kg rider. Frame weight is 1240 grams and bike weight complete with pedals and cages 5.76 kg.


What wheels do you run?


To ride at that weight I run a set of 24mm Farsports tubular rims with ED hubs and Sapim CX Ray spokes. The wheel set is 960 grams. The tires are Challenge Forte.

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