vintagerando

How the heck do they get an angle like that to work? The frame/fork is by Crust Bikes of Belmar, NJ.

madpogue 

^^^^^^ I'm digging the biohazard chainring.

79pmooney

What matters in fork geometry is where the hub sits relative to the axis of the steerer (and head) tube(s). How the tubing gets from the crown to the hub doesn't matter. Traditionally, fork blades continued down the steerer line, then were bent forward to place the hub the proper distance from the line for the desired handling. But that is just tradition. Look at the Pinarellos of the past 10-15 years, Their trademark was a fork that swooped forward, then back to the hub.

That fork looks like roughly the same geometry as my old Competition, just executed very differently.

Ben

Lascauxcaveman 

It looks weird. As do the recent Pinnys.

Chombi1 

It's a straight legged fork.....that's not???

busdriver1959

JaccoW

Most modern bikes use a straight legged fork like that, though often its because it is carbon. It's still a low trail design so pretty comfortable for longer distances.

3speedslow

JohnDThompson 

It's a «René Herse» crank:


https://www.renehersecycles.com/shop...-double-crank/

vintagerando

Yeah.....just seems like, if you were out of the saddle and pushing down, all your mass would be on that crown/blade area.
This bike was posted on The Radavist. The caption mentions that the rider this bike was made for is a "lighter rider".

2manybikes

The fork legs are straight, but they fasten to the fork crown at an angle. Imagine, or use a straight edge against your screen, and make a line straight through the head tube and extend the line down to the level of the wheel axle height from the ground. You'll see the geometry is normal. The leg is just made differently. It's not new.

wschruba

Hmm. I'd been looking at some of their stuff, had no idea that they were a 30 minute drive from me.

unterhausen

I can't tell if that fork has a crown or if it's segmented construction. The blades still do most of the deflecting, but segmented construction can have issues if the "crown" tubes are underbuilt.

They also have a 1" steerer disc fork.

SJX426 

squirtdad

excuse the poor graphics, but this may help visualize

79pmooney

Looks to me like an ordinary fork blade run into a crown where the casting itself features the blades thrown forward. Structurally identical to a conventional crown. In all, this is completely traditional except except one step is omitted (bending the fork blades) and the bend characteristics and therefore the bumpy road ride will be different. (But there is so much variation in forks; taper, thickness, bend amount, location and radius that this probably just fits somewhere in that vast matrix.)

Ben

T-Mar

As in post #14 , my immediate thought was to Colnago and their Precisa fork, which they introduced circa 1987. Though there were early examples, they popularized the concept for road bicycles. BMX bicycles adapted straight leg forks the motorcycle industry. From the very late 1970s onward, straight blades were the dominant style in BMX.

Steamer

Straight forks are more efficient.

Just like L-shaped cranks are more efficient.

Duh!



(just joking, I am not actually that stupid)

SJX426 

Once could argue that the curve in the blade not only positioned the wheel for steering performance but also provided some suspension. If you think about it, the crown always was subjected to forces associated with suspension as well. Maybe that is part of the reason for dual or triple plate crowns, to provide strength against the moment arm. there were some pretty extreme "bent" forks BITD. I always thought the following was a bit more than most but have seen others more extreme.

2008-06-01 15.25.53, on Flickr

There was a thread a couple of years ago of the Trek fork failures as a result of the moment arm forces, IIRC 1982 420"s?

And yes the crowns will deform (from the same bike):

P9051190, on Flickr

unterhausen

Straight fork blades still bend. IIRC, someone measured it, and straight blade has something like 10% less compliance as a curved fork


I do see something like a crown, and we could probably track it down further. Usually, the crowns made for straight blades have 7 degrees of slant, so that ends up with a little more than 45mm of rake, maybe 50mm on a blade this long. There is some play, so you can get more rake that way. I think Crust has had their own crowns made.

I'm curious how long the finish will last before rust takes over. Unless it's oiled, which is something some builders do on their own bikes.

gugie 

^This

SJX426 

That is interesting! I would have suspected more of less! There is still the moment arm between the crown and the hub axle, so it makes sense there still would be compliance. I guess it would depend on the shape of the tubing. I suppose the crown takes up the 10% or it is overall less compliant by 10%?

Any Mech Engr with lots of dynamic FEA want to opine?

T-Mar

CCM Flyte, 1936-1941.

unterhausen

Apparently someone made an angled crown, because Eric Estlund posted this fork recently https://www.instagram.com/p/BuZIGmil...on_share_sheet


I am a mechanical engineer with a sufficient background in FEA to have an opinion about this, but I'm not going to build a fork model to test this hypothesis. I don't think crowns can contribute much to the compliance, they are a lot thicker section than fork blades. You wouldn't want dynamic FEA anyway. A bent fork will have more compliance down in the bend, but the rest of the blade will probably not bend quite as much. I had a straight bladed fork for quite a while, it didn't seem any different to me than any raked fork I ever rode. It had about 55mm of rake. I have never ridden a bike with more rake than that for very long though. Working on changing that.

T-Mar

When Colnago introduced their straight blade Precisa fork in the very late 1980s, they had Ferrari perform testing on it and their traditional, curved rake fork. Reportedly, the Precisa fork had superior vibration absorption and better stability on uneven surfaces. They claimed this improved headset life expectancy and resulted in greater comfort, control and safety. I would have suspected some weight savings but none was mentioned. Of course, they didn't mention the cost savings afforded by eliminating one process from the blade manufacturing operation. This would initially have been offset by the tooling costs for a new crown but there would have been long term savings.