Monday, December 10, 2007

In search of the perfect fork blade

When a fork blade comes from the tube manufacturer’s factory, it is straight; the framebuilder bends it to a curve that suits his requirements.

An un-raked road fork blade is oval at the top; the oval section runs parallel for about a third of its length. Then the cross section becomes round and starts to taper gradually to its smallest section at the bottom end.

The fork blade is bent on a curved form that is sometimes made from hard wood. I used one I made myself from two heavy-duty steel fork blades, bent in the desired curve, and brazed together side by side. This made a natural grove between the two blades where the blade would sit as I was bending.

I would slip a short piece of tube over the thin end of the form and the blade I was bending to hold it in place. Then start bending, first by pushing down by hand. The thin end of the blade bends easily, and I would finish off by squeezing it in a vise.

Bicycle tubing is hardened, and it will spring back after bending. Because of this, the form needs to be a greater curve than the finished fork blade will be.

A fork blade is several inches longer than it needs to be. The framebuilder chooses where he will put the bend, and where he will cut to length. For example, if I were making a criterium frame and wanted a very stiff fork, I would cut from the bottom, thin end.

If I were building a touring frame, and wanted a flexible fork for a more comfortable ride, I would cut from the top end and leave the blade thin at the bottom end. The framebuilder creates the perfect fork blade, by selecting the best place to bend the blade, and by choosing how much to cut from either end.

It is rather like a furniture maker choosing where to cut from a piece of wood to achieve the best end product. Once I arrived at the perfect fork blade, it was then an easy matter to repeat the process again and again.

On a John Howard

On a Fuso

And on a Recherche

One exception to this process was the Reynolds 753 fork blades. 753 was heat treated to a degree that the material could not be bent after. These were bent at the factory, then heat treated, and the framebuilder then cut to the required length. You will notice on the 753 Fuso Lux frame (Pictured below.) that the fork bend is a different shape than the ones bent by me. More pictures of this bike can be seen here.

Chainstays and seatstays are also tapered and the same selective cutting to length is employed. In this case, where the cut is made depends a great deal on the size of frame and its end use.

The perfect fork blade is stiff enough to allow precise handling, but with some flex to absorb road shocks. It also looks pleasing to the eye. I have a theory that when something is designed correctly from a functional standpoint, it has a natural aesthetic beauty. This is true of a boat, a bridge, a building, and even a bicycle frame.

The modern trend of building straight forks of course saves the framebuilder a great deal of time and effort. If this look has become acceptable, why should today’s builder go through all the time consuming process I have described here?

The straight blade is angled forward so the same fork rake or offset is achieved and handling would be the same. I can’t comment on the shock absorption qualities because I have never built a frame with a straight fork.

In my view, a great deal is lost aesthetically, so where does that leave my theory about function being linked to aesthetics? On the other hand, is it simply that beauty is in the eye of the beholder?


Anonymous said...

Today the term "natural aesthetic beauty" is usually used by engineers about ugly machines that are the product of a marvellous engineering process. :)

For example the Open 60 class boats.

Unknown said...

That Recherche is one beautiful bike and well equiped for endurance training rides imho. I could just dream away right now and see myself cruising on that over the tarmac bikepaths through the forests and heaths that characterize the (flat) landscape on the border of Belgium and the southern part of The Netherlands (like this )

Really like the drivetrain, frame, and wheel choices on that one.

About aesthetics following from function: when you (Dave) are designing and building a bike, are you sure that there's not something hidden in the back of your mind that makes you to make it look good to you? That is, you might add something to the function (objective/criteria) of the bike (it rides and it has to look good to me).

And aren't those marvellous engineers not making sure that it looks exactly as specified by some other marvellous engineers. Like marketing said that we need to have these crazy-curvy forks that those Italians thought of because their asian supplier bought some overpriced machine that could mold rakes with more than one bend, and double-curves surfaces are every mechanical engineer's wet dream?

It's a pity that most engineering studies are not that broadly oriented. This here engineer's mother fortunately holds a teaching degree in art, hence my fondness of the esthetecyclist

Anonymous said...

Thanks Dave
A perfect explaination as to why I never bothered replacing the fork on my Recherche with Carbon. Even if I would save some weight, the price paid for change in handling could never be worth it. People designing today's bikes, using stock forks are missing out on half the equasion IMO.

Bob J

Ron George said...

A bike, or any design for that matter, should have quality, 'rightness', beauty, proportion, balance, rhythm and scale. Form follows function.

Check out the Europeans. Their bicycle designs, or any design for that matter, is simply awesome. They put a lot of time into it (although they don't have it right all the way)

I believe any engineering design should revolve around the aesthetic and emotional values of the end user in mind. This aesthetic portion should be incorporated into the creative design process. This is what hundreds of schools and colleges across the country are lacking out, I believe.

Hopefully the technology of tomorrow won't leave out the emotional aspects tied in with right hand portion of the brain out.

Anonymous said...

Dave, in my opinion those "straight forks" are just plain ugly. They just don't look "right". They look "broken" to me. I don't have the means to prove it, but I just have the feeling that the traditional curved fork may even be stronger.

The Bates fork shown here recently also looks a little "odd", but not in a bad way. It looks distinctive (but I doubt if it gave a softer ride; I'd guess harder, if anything). At least it wasn't ugly.

Anonymous said...

This has nothing to do with the current blog, although with the way I eat, you would think I'd be interested in anything that has to do with forks. Congrats Dave on making the front page of velonews ( as their site of the day.

Anonymous said...

My best bike ever is an Eddy merckx 7-11 bike w/the 753 tubing. I often wondered about the fork, with all its rake concetrated near the axle. I have never been able to match that particular bend.

Anonymous said...

While I think that a straight blade fork looks good on some bikes (Colnago Master specifically), I agree that a traditional frame looks right with a curved fork. As far as aesthetics go I'm a big believer in form following function. I hate useless fluff such as the curved top tube on the new Specialized frames or those goofy forks and chainstays on the Pinarellos.

Anonymous said...

Amen to that... No offense to the fine designers at Pinarello, but the contorted fork and seatstays on the Prince look downright unnerving (and almost angry).

A bicycle needs to have a certain "qi" to it. (Qi is the Chinese feng shui term, pronounced "chee", meaning energy flow)... Sometimes it may be unseen, and only felt in the ride characteristics... Other times this qi is clearly visible... what I prefer to call "mechanical beauty"...

A well-designed steel fork, with the right curve, matched with the right frame... can have an extraordinary chi. I'm sure you've rolled on a bike that has that indescribable quality...

Nice stuff, Dave.

Anonymous said...

Over the last 15 years, most new "upper-end" bike frames are sold without a fork, you are expected to pick from among the many carbon fiber forks out there and given little advise on how to do so. I recently picked up an older Reynolds 753 steel fork and am considering putting it on my ride. I have been told that 753 is too brittle & flexible for a large rider. Should I be concerned? I find it hard to beleive that it would be any more prone to breakage than an aluminum or carbon fork.
Thanks - Tom

Anonymous said...

Two different ways to do it. They both have their place, crap examples of both abound. Some things find favor w/ older people, some w/ younger.

Anonymous said...

I have two old Paramounts and both have about the same amount of fork trail but one has the fork bend in the middle of the blade and the other has the bend at the end of the blade. What will be the difference in performance and feel ??