Why did curved forks disappear?

Question

Forks on historical bikes are often curved forward in a smooth curve, sometimes called a "French bend". This is sometimes claimed to add a small amount of compliance and comfort. It effectively increases the rake and trail without slackening the head angle. Why did the industry stop making forks like that?

Current forks (except for retro designs) are mostly straight, having a sharp "bend" at the crown, i.e. steerer tube and fork legs usually don't form one straight line. The angle is small though, so hard to see on most bikes.

Answer 1

The main reason is the materials used. Basically, on all bikes with bent forks the bent forks are made of steel. Steel has been mostly replaced by carbon and aluminium.

While steel is heavier, it is not prone to fatigue when bending within its fatigue limit, making it possible to use thinner tubes with more flex and allowing the bend in the fork to work like a basic shock absorber.

Aluminium has no fatigue limit, thus it needs to be stiff enough or it will eventually break, making it not suitable for comfortable flexing forks. A big curved bend puts more stress on the material than a small sharp bend at the crown.

Carbon has properties which are very different to metals. Carbon parts are not just defined by the shape of the object, but also by the layup of carbon fibers, making it possible to get shock absorbing characteristics without needing a bend to work as spring, just by engineering the right layup of fibers.

That's the why many aluminium bikes use carbon forks.

There are other big reasons why bent forks are less common on modern bikes. You could say, why use an expensive carbon fork on your aluminium bike, if a cheap steel fork would be as comfortable?

Looks!

A thin tubed round bend steel fork just doesn't look right on a thick edgy carbon or aluminium frame. Also thin steel tubes aren't as aerodynamic as the optimized shapes you can make with carbon or hydroformed aluminium.

Answer 2

Frames and forks were made of tubes that are brazed into lugs. Lugs are cast standard parts. Frames are sized by using tubes of the correct length. The respecive production processes makes lugs stiff and tubes flexible.

The front axle is offset roughly 40 mm from the axis of the steerer tube. This offset is named rake. Rake determines steering characteristics.

Most curved forks are made from steel and consist of a straight steerer tube, a crown lug, and two tapered tubes as fork members.

In the context of above aspects there are a number of good reasons to have curved members:

• by curving the fork members any rake can be set with a straight crown

• curving tubes was easy for metal workers of the old

• crown lugs are standard parts, different rake for different steerer angles would necessitate a large number of crowns; this would drive costs of crowns up considerably

• a stiff part that also joins members by brazing is not a good spot to take torsional moments

• curved forks lived on because of tradition for a while after the technological necessities changed

Why are such forks gone? Simply put. If no external constraints force you to curve something, keep it straight.

• Stress in a straight beam is smaller than in a curved beam (cf. curved beams).

• Better shock damping: a long stretch of material bending a bit is better than a short section bending a lot. (For the same reason it is better to have long leaf springs in a truck's suspension than short ones.)

• Modern manufacturing and quality control of curved parts is more expensive.

• Fashion changed and there is not much demand for curved forks for aesthetic reasons alone.

Answer 3

A big part of the change from curved to straight forks is the engineering requirements for disc brakes.

Curved forks provide structural flexibility, in that forces applied to the tire become transverse (or almost transverse) forces on the fork blade, allowing it to bend and absorb shocks.

Curved forks work best with rim brakes, because the brake forces are applied at the crown or the crown end of the fork blades, and the blades can be engineered for shock absorption.

With disc brakes, the blades need to be able to support a torque load at the end of the fork blade, and so need to be made stiffer, which puts a ceiling on the shock absorption that could be achieved by structural flexibility.

Once structural flexibility is no longer on offer, the various aesthetic and material concerns raised in the other answers start to drive fork design.

Some discussion on this topic: https://www.renehersecycles.com/myth-9-fork-blades-dont-flex/

Answer 4

In addition to what was discussed in other answers, there is one significant detail: flat mount disc brakes. The flat mount brakes that are almost universal on road bikes require a flat mounting point right where the curve used to be. Building the flat mounting surface into a curved fork blade would be more complicated than straight one and not look as clean, and since the curve isn't necessary for function, straight blades won.

As far as I remember, curved carbon forks were more common than straight ones until disc brakes became common. Straight ones existed even before carbon fiber, but they were considered to be exotic and just for the looks.

Answer 5

My 1979 Sekai 4000 has a nice French bend in the fork. I ride several bikes, one with a carbon fork and 35 mm tires, and this bike with its 25 mm tires soaks up the bumps better. I can see it working. On a section of boardwalk that rattles everything, me included, that fork is springing up and down and takes the rattle out better than any of my other bikes. Old tech but it sure works well! Not implying we all go back to the 70s but sometimes as the tech moves forward, the benefits of the old tech are lost.

Answer 6

When I was a kid in the '60s and early seventies we used to cut the front forks off an old bike and slip them onto the end of our bike's forks and make a chopper. It was very cool but very dangerous so around 1980 they started making front forks that couldn't be attached to one another. I think that is the answer you are looking for. Nothing complicated.