Straight vs Curved Fork

BentChainring · Oct 8, 2009

I should add to my above post that it does not take into account the compliance of the fork, or the structure of its attachment to the frame... this can make a big difference, but for the purposes of this description, I spoke from a purely geometric dynamic point of view.

BentChainring · Oct 8, 2009

sometimerider said:
IMO, a curved fork, given otherwise similar geometry and materials, is less likely to break (the bend point of a straight fork is a high stress point) and provides better suspension.

Other things being equal, a curved fork is better.

This is a case of "it depends"

Lets start by just looking at the crown. Regardless of fork curvature, for a given offset fork, the crown will see the same stress. In other words the crown does not know if the fork is straight or curved, it just knows that there is X amount of offset, and therefore a moment of Y (Force x Lever arm length). Now, if we look at geometry of the crown and steerer, the highest stress is going to be located at the interface of the steerer and the crown. This is true for ANY fork... straight, curved, etc...

Now...again, assuming the same fork offset... if we look at how you transmit the load from the axle centerline to the crown you can either have a straight beam in bending (straight fork) or a curved beam in bending (curved fork). Simple. Now... I had to consult my machinery design textbook for this... but the stress distribution for a curved beam, bent in the plane of bending (ie... how a bike fork is loaded in normal operation) is hyperbolic with the highest stress at the smaller radius (that is, at the front of the fork). Luckily, this point of highest stress is in compression, and will not cause metal fatigue.

ANother thing to consider, is that when a metallic fork blade is formed, it is bent cold. This means there are residual compressive stresses at the back of the fork, and residual tensile stresses in the front. Therefore, when loaded in normal operation, you are trying to relieve the residual stresses that were left in the fork during forming (assuming they persist through brazing). This greatly increases fatigue life of metallic components.

Carbon is another thing. It will simply explode if you don't set your tire pressure to exactly 7.2bar, and have your bike computer set to Kph

:thumbsup:

laffeaux · Oct 8, 2009

BentChainring said:
Two forks on an identical frame/rider/etc.

Fork one - "straight" (ie, with no "rake)
Fork two - some amount of rake (forward offset)

Fork two does not exist on a real bike in reality. Straight forks (that are actually for sale and used on bikes) are available with identical offsets to curved-blade forks. "Straight" forks most often achieve their offset by having the fork angle differ from the steerer tube angle (on some mountain bike forks, the angle is achieved by offsetting the dropout form the fork).

Identical "trail" numbers can be achieved with either type of fork.

BentChainring · Oct 8, 2009

laffeaux said:
Fork two does not exist on a real bike in reality. Straight forks (that are actually for sale and used on bikes) are available with identical offsets to curved-blade forks. "Straight" forks most often achieve their offset by having the fork angle differ from the steerer tube angle (on some mountain bike forks, the angle is achieved by offsetting the dropout form the fork).

Identical "trail" numbers can be achieved with either type of fork.

I think your referring Fork one... the straight -zero offset fork.

They may not exist in production ( I am not aware either way ), but they CAN be produced... and by changing the head tube angle you can produce a bike with almost identical handling.

Trail is the important number, not head tube angle or offset on its own. Many recumbents that I have worked with (high speed fully faired recumbents) typically have negative fork rake. This functions to increase trail MUCH more than would be necessary on an upright bicycle to give the bike more yaw authority.

sometimerider · Oct 8, 2009

BentChainring said:
This is a case of "it depends".

Doesn't it always.

I think what you're saying matches (and in a few spots, exceeds) my understanding of the issues at play (hey, I've had some physics and engineering classes).

With a curved fork, the stresses endured when an impulse tries to compress the fork are distributed over the curved portion. In a straight fork, they are concentrated right at the bend below the head tube. I think the latter is harder to deal with and less likely to be both strong enough and able to behave elastically.

Thanks, Bent.

BentChainring · Oct 8, 2009

sometimerider said:
Doesn't it always.

I think what you're saying matches (and in a few spots, exceeds) my understanding of the issues at play (hey, I've had some physics and engineering classes).

With a curved fork, the stresses endured when an impulse tries to compress the fork are distributed over the curved portion. In a straight fork, they are concentrated right at the bend below the head tube. I think the latter is harder to deal with and less likely to be both strong enough and able to behave elastically.

Thanks, Bent.

Yep... but with a curved fork, you get the same stress at the bottom of the head tube. The shape of the fork doesn't matter to the crown. All it knows is someone is bending the crap out of it, and if the rake is the same, the moment is the same.

phew... I should go build a bike or something, eh?

sometimerider · Oct 8, 2009

BentChainring said:
Yep... but with a curved fork, you get the same stress at the bottom of the head tube. The shape of the fork doesn't matter to the crown. All it knows is someone is bending the crap out of it, and if the rake is the same, the moment is the same.

I'm not sure that's true if one fork can better act as a spring than another. And I think a curved fork does. If the fork's design can absorb impulses, the crown will see less stress.

A straight (but angled) fork can also act as a spring, but most of the bending stresses are concentrated at one point.

BentChainring · Oct 8, 2009

sometimerider said:
I'm not sure that's true if one fork can better act as a spring than another. And I think a curved fork does. If the fork's design can absorb impulses, the crown will see less stress.

A straight (but angled) fork can also act as a spring, but most of the bending stresses are concentrated at one point.

The 'springy-ness' of the fork doesnt matter since something has to react to the loads of the spring (the crown).

Maybe we are thinking of different affects... I am merely talking about the load from my fat arse sitting on the bike. I think you may be considering hitting something from the front. In that case, you may get higher loads in the straight fork... Ill have to think about that one...

sometimerider · Oct 8, 2009

BentChainring said:
Maybe we are thinking of different affects... I am merely talking about the load from my fat arse sitting on the bike. I think you may be considering hitting something from the front. In that case, you may get higher loads in the straight fork... Ill have to think about that one...

You're right, I haven't been talking about static loads. That shouldn't be a significant design issue. It's the dynamic loads, such as when you hit a pothole, that can break a fork. And how the fork responds to them contributes to ride comfort (a very important issue to me and my arthritic wrists).

HMBAtrail · Oct 8, 2009

BentChainring said:
Two forks on an identical frame/rider/etc.

Fork one - "straight" (ie, with no "rake)
Fork two - some amount of rake (forward offset)

The bike with Fork one will have poorer handling at lower speeds, and may 'feel' a big sluggish at higher speeds... in other words, you will feel like you need to drag the bike into a lean to get it to corner..

The bike with Fork two will have better handling at lower speeds, and should 'feel' more responsive at higher speeds...in other words, you will be able to get the bike to corner with less lean... this is only true up to some point when the bike tends to be overly 'twitchy'..

Not entirely sure what you are going for on the above statement. Straight blade doesn't mean "zero offset". A straight bladed fork can, and often does, have the same rake as a curved blade. A zero offset fork I am sure has application (the acrobatic bikes come to mind) but for practical road or mountain bike purposes they don't exist. In order to get a trail measurement of 60mm with a zero-offset fork you would need an ~80 degree head angle. See what I'm saying?

On lugged forks, for example, a straight bladed fork has the crown offset by 6 or 7 degrees, thus providing the rake of the fork. For curved blades, there is no offset in the crown and the rake is determined by how much curve you put in the blade. Take a curved blade fork and a straight bladed fork with identical axle to crown measurements and the same offset (rake) and there would be nary a whit of difference between how the bike rides.

Technically speaking, you could have a curved blade with zero offset if you did two bends on it. One rearward and the other forward. Or, I suppose you could have a curved blade with negative rake if you simply installed it backward.

What I am getting at is two forks with the same rake will have the same handling characteristics. There is no "sluggish" feeling simply because a straight blade is used. The characteristics you are describing are a result of trail.

As has been said several times already, the difference is largely aesthetics.

Voodoochile · Oct 8, 2009

Okay so why were most of the older bikes built with a curved fork? I honestly don't know if straight bladed forks have been around all along but I always remember seeing curved forks. It seems like it would be easier to make a straight fork (maybe I'm wrong). Why would the builders go to the trouble of bending the blades?

Voodoochile · Oct 8, 2009

Does this answer my question? Hmmm
http://en.wikipedia.org/wiki/Bicycle_fork

HMBAtrail · Oct 8, 2009

Voodoochile said:
Okay so why were most of the older bikes built with a curved fork? I honestly don't know if straight bladed forks have been around all along but I always remember seeing curved forks. It seems like it would be easier to make a straight fork (maybe I'm wrong). Why would the builders go to the trouble of bending the blades?

Variety is the spice of life. Some like it one way while others like at another. Ain't life grand?

But, yes it is easier to build a straight bladed fork simply because you don't have to go through the process of bending the blades. But, the additional work is minor.

Voodoochile · Oct 8, 2009

HMBAtrail said:
Variety is the spice of life. Some like it one way while others like at another. Ain't life grand?

But, yes it is easier to build a straight bladed fork simply because you don't have to go through the process of bending the blades. But, the additional work is minor.

So why on earth would you bend it? Looks? Shock absorbsion? I would only guess the later would be a reasonable explanation for the curve. Maybe it's just a learning curve.

HMBAtrail · Oct 8, 2009

Voodoochile said:
So why on earth would you bend it? Looks? Shock absorbsion? I would only guess the later would be a reasonable explanation for the curve. Maybe it's just a learning curve.

The best guess would be tradition. Lugged fork crowns that require bending the blades have been around for years and years. Those that have the rake built into the crown are more recent. (Disclaimer: I am certain there are examples of straight bladed forks going back 100 years but humor me). Hell, try finding a track crown for straight blades. You'll be looking for quite a while.

Did fork crowns with the offset built in come into vogue because people demanded it? Was it coincidental with the advent of investment cast crowns versus pressed steel? Was it to speed the build process by eliminating a step?

I'm not sure on any of these nor do I think it is overly important in the world we live. I think it is great that there are options.

Voodoochile · Oct 8, 2009

Maybe the early builders found it was easier to achieve the offset by bending the blades and attaching the steer tube and crown straight.
I'm a tool and die maker and work with steel all day every day for over 20 years. I've also designed many dies and machines for production and personally the only time I would bend something is for ease of application, unless there was a very good outcome for the reason of the bend. Everything we do is built from an idea. Bending is not easy for me unless I have a machine to make the bend consistently and accurately.
I’m not looking to get flamed. LOL

Voodoochile · Oct 8, 2009

Just thinking realy quick. If I were going to build a fork which I have no experience at all. We build dies mainly to stamp out parts with many forms and bends like a bottle opener for example. If I were to build many I may fasten the steer tube and crown straight to save time cutting and welding angles. I may then build a machine to bend the blades to achieve the offset. How do the builders bend the fork blades on a steel frame? They don't do it by hand do they? Or does the tubing co. do that for them when they mfg. the tubes. This of course leaves out CF and Al.
I know I'm pushing it guys but I'm interested.

Dave Hickey · Oct 8, 2009

Voodoochile said:
Just thinking realy quick. If I were going to build a fork which I have no experience at all. We build dies mainly to stamp out parts with many forms and bends like a bottle opener for example. If I were to build many I may fasten the steer tube and crown straight to save time cutting and welding angles. I may then build a machine to bend the blades to achieve the offset. How do the builders bend the fork blades on a steel frame? They don't do it by hand do they? Or does the tubing co. do that for them when they mfg. the tubes. This of course leaves out CF and Al.
I know I'm pushing it guys but I'm interested.

Most(all?) tubesets come pre-bent...I have a Reynolds 531 tubeset in my garage... It has the fork blades already bent...

HMBAtrail · Oct 9, 2009

Dave Hickey said:
Most(all?) tubesets come pre-bent...I have a Reynolds 531 tubeset in my garage... It has the fork blades already bent...

I have never bought or used a pre-bent fork blade. This is how I do fork blades and seat stays. I have a different bender for top tubes.

Dave Hickey · Oct 9, 2009

HMBAtrail said:
I have never bought or used a pre-bent fork blade. This is how I do fork blades and seat stays. I have a different bender for top tubes.

Cool...I didn't know that..good to know that some builders still bend their forks

Straight vs Curved Fork

BentChainring

BentChainring

laffeaux

BentChainring

sometimerider

BentChainring

sometimerider

BentChainring

sometimerider

HMBAtrail

Voodoochile

Voodoochile

HMBAtrail

Voodoochile

HMBAtrail

Voodoochile

Voodoochile

Dave Hickey

HMBAtrail

Dave Hickey

Top Contributors this Month

Recommended Communities