arkiemark

Looking at some classic bikes the other day and admiring how all of them have front forks that have a little slope to them, which set me wondering...where, when, and why did this originate? Most modern bikes (steel included) seem to have straight legged forks which are probably cheaper to produce, but just don't look quite as nice. Did the sloped fork have some sort of association with structural stiffness or was it purely aesthetic?

Scooper

There is a school of thought that curved fork blades, especially those with the curve close to the dropouts, absorb road shock better than straight bladed forks because the road shocks act almost perpendicularly on the fork blade, making the blade easier to flex than a gradual bend or a straight-bladed fork.

Jan Heine wrote a great article, Fork Blades Optimized for Comfort and Speed, about this in Bicycle Quarterly a year or so ago.

Here's a figure from the article showing road shock vector transmitted through the fork to the frame. A straight bladed fork transmits road shock to the frame almost on the axis of the fork blades. Fork blades aren't very compressible, so the shock isn't mitigated like it is with curved fork blades that absorb the shock by bending along the curve radius.

arkiemark

WOW! Thank you for the interesting info. This basically reaffirms how well thought out classic and vintage bikes were compared to today's counterparts, not only with forks, but with materials, integrated accessories, and components.

StephenH

There's two different issues there. One is the geometry of the fork, how far the axle is in front of the axis of the fork pivot. The other is the curve itself. I think some of the old bikes that had a more obvious curve also had a lot more rake (I think that's the term), and this still varies among different styles of bike.

You'll also see some of the old forks (on balloon tire bikes) with the big curve in the forks, but then they had truss rods to help support it- which would also pretty much eliminate any extra shock absorbtion.

due ruote 

further reading:
https://en.wikipedia.org/wiki/Bicycle...cycle_geometry

Bianchigirll 

Hello if I can add two cents here myself. if you compare the angle of old v new forks at the crown you will notice the the newer straight blades are angled at the crown. this is where the 'rake' is incorperated intot he fork. also I do not know how exactly to explain it but rake also has something to do with handling. if I remember correctly if there was a straight line from the headset through the hub to the ground the bike would be very dificult to control.

Scooper

Yes; the rake (or offset) in a straight bladed fork is built in at the point where the blades join the crown. Heine, in his article, suggests that this creates stress risers at that point.

I think you're referring to the relationship between rake (or offset), head tube angle, and trail. This relationship is beautifully explained in ex-framebuilder Dave Moulton's blog: Trail, fork rake, and a little bit of history.

As Dave says in his blog, for a road bike with a 73 degree head angle the optimum trail seems to be around 2 to 2 ½ inches (5 to 6.3cm.) for responsive but stable handling. A track bike might have a little less trail for more manueverability, while a touring bike might have a little more trail for stability.

divineAndbright

When Colnago was developing a "ferrari" road bike, some guy from Ferrari asked why the bend in the fork, and Ernesto said the absorption theory, then the Ferrari guy suggested why not make them straight, and showed on a computer it would actually dampen road vibrations better than a curved fork, so colnago tried it out and racers liked it and found the bikes handled better to.

Picchio Special

There are also those - and Jan Heine is perhaps the foremost proponent - who advocate for less trail for touring or other load carrying bikes (randonneuring bikes in specific) where a significant amount of weight will be carried in the front. This makes sense if you consider that high-trail bikes steer more via lean as opposed to handlebar turn, and leaning is not something you want to be doing a lot of when carrying a lot of weight.

Picchio Special

Marketing. Which is Ernesto's real gift anyway.

Scooper

Yep; that's the story from Colnago. A more cynical view is that bending thousands of fork blades to precisely the same matched curvature was very expensive, and Ernesto decided to make the blades straight because it was less labor intensive, incorporating the rake into the blade/crown interface.

I really don't know which story is true, but I've heard both.

Scooper

Point taken.

divineAndbright

I wouldn't be surprised either way! I gotta admit I do like straight blade forks both looks and handling, however they certainly don't really work aesthetically with vintage bicycles!

On the other hand I only like very slight bend which is hard to come across, I basically like track handling on a road bike.

Picchio Special

The cynical one, to be sure. There's no evidence I've seen that a straight fork is a superior design. Otherwise, everyone would have adopted it. Pinarello uses that wavy deal instead, for example. Makes them distinctive. Ernesto adopted straight forks for the same reason he adopted chrome ones - they save time and money.

Picchio Special

The issue with "track handling" is more about trail, AFAIK. Less fork rake actually increases trail (i.e. makes the bike handle less "track-like") unless compensated for by a steeper head tube angle. Which track bikes generally have.

Scooper

A while back, I found Damon Rinard's fork deflection test and was surprised that a Colnago straight blade fork (material not specified), a Merckx curvedfork, and a Waterford Paramount steel curved fork all had essentially the same lateral and longitudinal deflection, while Dimension, Time, and Look carbon fiber forks had significantly greater deflection in both planes.

Here's a Waterford steel fork. Note that it has a more gradual curve.

Scooper

+1

Picchio Special

I'll never get back the hours I spent figuring this stuff out.

ozneddy

I always thought that "Rake" was the angle of the headtube off the perpendicular ! am I wrong ?

Picchio Special

No, rake is the distance between the wheel axle and the steering axis:

https://www.velonews.com/media/Block40.pdf

repechage

The "fly fishing rod" theory of fork bending under load is over rated, for two reasons.

1. modern bikes have head angles in the 72 to 74 degree range, with a shallower head angle and generous rake- Maybe.

2. Do not forget that the steel at the end where the blades get the most bend is thicker walled, by a large margin 2 to 3 times the upper wall thickness due to the drawing process, and after bending its work hardened.

There are differences in fork design that can benefit or encumber ride quality that I truly believe make more of a difference. I would place more of any ride variance on crown design, mass, and alternate designs like a plated crown. Not all "box" crowns are equal either.

If one selects a different material, say titanium, things really get surprising. On my old Teledyne Titan, when one braked moderately, the wheelbase shrank at least 15 mm, it was scary at first to watch, the deflection was along the straight section of the blade just below the crown, the curved material was work hardened and stiffer, the upper region where the most welding was done was very noodle like. Those forks also had steerer problems later in life at the crown race seat, so there was movement there too.

repechage

I would not forget Styling, and Marketing, New Better, Improved, Race Car Engineered.

Cheaper? easier? Those are trade secrets.

Picchio Special

Ernesto I'm sure wishes Enzo had adopted him.

ozneddy

Thanx PS for clearing that up for me !

Longfemur

Maybe someone else can check this out. I used to have a Guru road bike with straight fork. At the same time, I had temporary overlap with another bike, a Peugeot with a curved fork (more like straight, bend, and then a short section of straight to the dropout). Then I got my current Bertrand road bike. It has a beautiful, low curve fork with a smooth, constant bend to the drop out. For curved fork classic bike aficionados, it's gorgeous. Anyway, for a short time, I had all 3 bikes. All were steel, and both the straight fork Guru and the Bertrand had a fork made of Columbus steel. The Peugeot was Cromolite chrome-moly. Wheels were similar, but different makes.

I really hated that straight fork. There was no spring in it at all, and it felt like every ripple in the road surface went right up into my hands. Both curved fork bikes had a pretty obvious springiness that might have been subjective when riding, but was pretty obvious when dropping the bike at the front wheel (at least in terms of feel -- some of you may know what I'm talking about, it's a steel thing). Now, I suppose it could have had something to do with some other factor. The Guru and the Peugeot had a unicrown fork, the Bertrand has a Cinelli semi-sloping crown. But those Cinelli type crowns are known for their stiffness, so I don't think that would account for it.

I don't know for sure that one is better than the other, but I do know that I much prefer a curved fork, and among curved forks, I love the look of a fork that curves low and all the way to the dropout.

My reading on why forks have traditionally been curved is simply aesthetics and perhaps an artistic rather than engineering belief that the curve provided a slight amount of suspension when going over bumps. I don't know about that, but it's very much like the feel some of us like from our quality steel frames.