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DWF said:
What are you talking about? I can watch to fork blades flex! They bow under braking. They flutter over bumps. I look down the leg and see the movement. That's test enough. I'm sure that there is flex at other parts of the fork, but I know that most of the movement is coming from the blades. I've aligned thousands of forks when I was at Fat City. I know what bends on a fork. The blade. Even on the stiffest of forks. I've seen tons of destructively tested forks. I've watched them get tested. The blades flex like crazy.
 

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pvd said:
What are you talking about? I can watch to fork blades flex! They bow under braking. They flutter over bumps. I look down the leg and see the movement. That's test enough. I'm sure that there is flex at other parts of the fork, but I know that most of the movement is coming from the blades. I've aligned thousands of forks when I was at Fat City. I know what bends on a fork. The blade. Even on the stiffest of forks. I've seen tons of destructively tested forks. I've watched them get tested. The blades flex like crazy.
Yes, fork blades are not infinitely rigid, they do flex, but most of that flex occurs at the crown and steerer tube. How much do you think a crown & steerer tube have to flex to allow a fork tip to "flutter" over bumps?

Think of it another way, where do you see forks fail (not including disk brake induced point loads on improperly designed forks)?
 

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DWF said:
Jan Heine did some fork flex tests a few issues ago in Bicycle Quarterly and, paraphrasing from memory, found that the tips were indeed providing a significant amount of flex.

I have also long been in the "fork flex comes from the crown and steerer" camp, but now I'm beginning to wonder.

This is a 10-minute FEA problem with a simplified mock-up, but I don't have access to Solidworks/Cosmos any more. Darn.
 

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dr.welby said:
Jan Heine did some fork flex tests a few issues ago in Bicycle Quarterly and, paraphrasing from memory, found that the tips were indeed providing a significant amount of flex.

I have also long been in the "fork flex comes from the crown and steerer" camp, but now I'm beginning to wonder.

This is a 10-minute FEA problem with a simplified mock-up, but I don't have access to Solidworks/Cosmos any more. Darn.
Ever since VBQ admitted they couldn't take linear measurements more accurately than +/- 3mm, I've taken it all with a grain of salt.

It is easy to test though. It's also easier to understand when you consider that a steerer tube is not actually restrained by conventional headsets. If you rigidly clamp the steerer tube in place, you will obviously see no flex there and you would be confined to measuring torsional deflection at the crown and that is an order of magnitude harder to measure.

Not everything is as it seems.
 

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DWF said:
Ever since VBQ admitted they couldn't take linear measurements more accurately than +/- 3mm, I've taken it all with a grain of salt.
Yes, though that refers to their ability measure frame dimensions. In the fork test they were measure a short distance between two objects with a set of calipers, so I assume they were able to make that measurement accurately.

DWF said:
Not everything is as it seems.
Right, which is why the VBQ test kept me up at night. It challenged what I had previously considered to be Right, and I couldn't find a fault in their experiment. Now I'm just trying to figure out if one of the the two theories is wrong, or if both are right and there are more subtleties to fork flex than one would expect.
 

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It seems to me that the headset would capture the steer tube (near both extremes of its length) enough to make any flex inconsequential. Being able to quantify this flex presents the problem of differentiating steer tube flex from any bearing play present in the headset, itself.

I don't have the answers but I'd certianly like to see any reference material folks can site on this matter.
 

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Must... stay... out... of... sh!tstorm!...Mustn't... get... dragged... in!

Oh, hell.

there is some flex between the headset bearings. Not a huge amount on 1 1/8 and reasonable travel forks, but go to 1", especially in carbon, and those things can flex quite a bit.

Everything flexes; let's not fight. Why not get back to hatin' on Jones?
 

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It's strange though. We had a road bike of ours tested (Columbus XCR, 1 1/8th Chris King aheadset, Edge composites fork, 38mm downtube) and another bike in the shootout was the Colnago Master-Light (oxymoron)(Columbus whatever, 1" ahead Campy, Steel fork, 31mm downtube) and the deflection at the headtube was the same on both bikes, right down to two decimal places.

I think there's stuff going on in the headset that we don't fully understand (or carbon forks are much flexier than we realise). Or at least I don't, because everything here points to our bike being massively stiff and it just wasn't in comparison.
 

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DWF said:
Yes, fork blades are not infinitely rigid, they do flex, but most of that flex occurs at the crown and steerer tube. How much do you think a crown & steerer tube have to flex to allow a fork tip to "flutter" over bumps?

Think of it another way, where do you see forks fail (not including disk brake induced point loads on improperly designed forks)?
I Know! I Know!. I built a fork last year and it failed--and in the same place my mtb forks failed in the '80s. The steer tube bent forward from vertical loads. Not backward from braking or frontal impact.

There is a reason mtb forks have evolved with larger diameter steerers and/or double crowns.
 

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Thylacine said:
It's strange though. We had a road bike of ours tested (Columbus XCR, 1 1/8th Chris King aheadset, Edge composites fork, 38mm downtube) and another bike in the shootout was the Colnago Master-Light (oxymoron)(Columbus whatever, 1" ahead Campy, Steel fork, 31mm downtube) and the deflection at the headtube was the same on both bikes, right down to two decimal places.

I think there's stuff going on in the headset that we don't fully understand (or carbon forks are much flexier than we realise). Or at least I don't, because everything here points to our bike being massively stiff and it just wasn't in comparison.
I do not understand what the "deflection at the head tube" means or how the fork would/should affect it.
 

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Well, they bolt down the frame/forks at the dropouts, stick 40kg on one 'crankarm' and measure at various points on the frame how far out of plane the thing is.

Yes yes I know it's flawed methodology, but it's not my test.
 

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Thylacine said:
Well, they bolt down the frame/forks at the dropouts, stick 40kg on one 'crankarm' and measure at various points on the frame how far out of plane the thing is.

Yes yes I know it's flawed methodology, but it's not my test.
Are they measuring lateral deflection of the head tube? Seems the fork blades/steer tube would make little difference unless they were REALLY wimpy. Says nothing about how the fork reacts under non-pedaling riding conditions, either.
 

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brant said:
So spaceframe fork is rigid under braking, but the same arrangement in the rear triangle (join the dots), gives vertical compliance?

I will nail my beliefs to the flagpole here - I think "ride quality" comes from "splay" - ie: the wheels spreading apart under load, which is most affected by TOP TUBE section.
"Splay" is what a Slingshot frame is all about. It is a different ride feel than any rigid or suspension frame I have ever ridden. I also like the ride. I like the ride Brant puts into his frames, too.

hmmmm...I just had a idea for a frame that will probably make many of you cringe...
 

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OK, fair warning here, I learned the basics to this FEA program (LISA) in 15 minutes and my FEA experience is confined to college 15 years ago, so take this with a healthy dash of salt. I did this because, as I mentioned upthread, Jan Heine's fork deflection tests in Vintage Bicycle Quarterly Vol. 6, Number 3 were keeping me up at night and now this thread is contributing to my insomnia.

So here's some really raw, basic FEA on a straight and curved fork.

What's going on: Both have the same dimensions top and bottom as well as rake. They have approximately fork-like dimensions. I have modeled them as simple planar meshes in 2D. The top of the blade is constrained, a vertical load is applied at the tip of the blade. The same loads, constraints, and material properties were used on both simulations.

What's not going on: I am not modeling flex in steerer tubes, crowns, etc. I am only looking at vertical loading, not brakes loads or anything more complicated. I am not bringing a wealth of experience or knowledge to this exercise.

So that said - here's the interesting part - the curved fork deflected vertically .88 units. The straight fork deflected .59 units. Generally I would assume that the top of the blades and the unmodeled crown and steerer would deflect the same on either design, so the extra flex must be coming from bend in the lower portion. So I don't think it's as simple as saying "fork flex comes from the crown". Or maybe I'm screwing up. It's possible.

I need to play with this some more so if anyone has any insights I'll try to add them to my further tests.
 

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shiggy said:
I do not understand what the "deflection at the head tube" means or how the fork would/should affect it.
Most of the braking load will be exerted at the longest moment, which is the head tube. My experience/observation with my black sheep ti truss fork is that there is a small deflection of the whole fork at the head tube AND more along the blades.
 

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Thylacine said:
I think there's stuff going on in the headset that we don't fully understand (or carbon forks are much flexier than we realise). Or at least I don't, because everything here points to our bike being massively stiff and it just wasn't in comparison.
An interesting thing I discovered a few years back. I was working at a high-end road shop in Vancouver, B.C. and had a maybe a dozen guys come in 2 years with really strange wear spots on the front of their carbon steerers. It was pretty obvious that the steerers were flexing and rubbing against the upper headset cup.

The first few we just showed to the customers and replaced the fork. I talked to a bunch of different fork and bike makers and no one could explain it. I started asking the folks with the problem forks about their riding habits and such. Each and everyone was a big descender and loved screaming down places like Mt. Seymour (The north shore's not just for MTB riding.) and they all controlled their speed with the front brake.

So my conclusion was the hard braking through corners was flexing the steerer enough so that it was touching the headset cup. In theory the headset should have gotten tighter, but none of the riders asked noticed that during their descent. This is where I think your first sentience in the quote is dead on.

Those of us that have been cyclists for a long time have seen a ton of anecdotal stories that break from the scientific beliefs or predictions of engineers. Similar to DWF's statement about the flex being most in the steerer and crown. I know the test he's talking about and will see if I can find it in my archives.
 

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brontotx said:
Most of the braking load will be exerted at the longest moment, which is the head tube. My experience/observation with my black sheep ti truss fork is that there is a small deflection of the whole fork at the head tube AND more along the blades.
I was asking about the testing Thylacine referred to, not how it works when ridden.
 
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