savvas

Hi Folks,

I am not a frame builder but am a fairly experienced bike mechanic. Fortunately I have a good friend who is very experienced in the frame-making trade. I've asked him to consider building me a cargo/freight bike similar to Kemper Filibus (load platform at the front over the front wheel with linkage steering). I'd want disk brakes and a frame that is as strong and light as possible.

I have a Pacenti MTB crown designed for 28x19 blades that I'd like to use. The challenge is that the front wheel would be a 20"/406 wheel!

Would I need special fork blades made for such a fork or is it possible to somehow just shorten standard mtb blades?

If I need special 20" fork blades, can the forum recommend any to me?

The design also requires insertion of a threaded fitting on top of the crown for attaching a rod end/rose jointed linkage for the steering. Would it be appropriate to drill and braze such a fitting into the top of the crown on one side? Or should I forget the idea of using the Pacenti crown and just have my friend build up a 'biplane' plate crown (which he could do very easily)?

Comments and advice most welcome,

Thanks, Sam.

CliffordK

If you choose to use your crown, then I'd shorten your standard MTB blades by cutting off the bottom, then make custom dropouts somewhat like one sees on cheap BMX bikes, probably tigging closed the hole, then attach your dropout (or make the plug and dropoout as one piece (bent).

It may also be much easier to simply modify a unicrown fork from a BMX bike.

unterhausen

I'm not sure what a MTB blade is in this context. There are blades of different thicknesses that will fit that crown. It seems to me that bending a fork blade that short is an issue, and that's required with a Pacenti crown. I would consider going with a segmented or unicrown fork.

As far as dropouts go, there is a style of fork that MTB builders use, Garro, Potts, Cunningham, for example. I forget who they usually attribute it to. A road dropout is brazed or welded to the inside of the blade and a scalloped piece of sheet metal is brazed or welded to cover the gap. I would think an experienced builder would be familiar with this style.

photo credit, Steve Garro: coconino cycles custom bicycles 928 774 7747 www.coconinocycles.com

tuz

I have done exactly that for a cargo bike, using the fancy Pacenti crown. I used standard blades; you'll want to trim them from the bottom as much as possible. But I don't know if all fork blades have the same length in their straight and tapered sections. I attached the dropout the normal way (i.e slotted) but the "Potts" way will allow to trim more from the bottom, which is a good thing for disc brakes. About that, disc brakes mounts and curved blades are a bit of a pain to work with. I'm making a new 20" cargo bike fork and it is the biplane type with recycled blades from a MTB unicrown fork: they are very sturdy (1.125" dia @ 1mm wall to 7/8" dia) and have the "dog-leg" bend which plays nice with the disc mount.

You could braze a nut on top of the crown to attach the rod-end bearing. I brazed a plate + bolt on the bottom side of the fork.

unterhausen

Tuz, do you have any pictures? Did you find raking the blades to be an issue? I suspect that you don't have to use disc-specific blades on a fork that short, which should make raking a little easier.


Seems like Potts credits Cunningham for that style dropout. I wouldn't doubt it was invented by someone else though.

tuz

I'll shoot pictures later. If you are trimming from the bottom you need to bend the blades more than usual (with knowledge of the bending radius you can sort of predict by how much), or cut the blade before raking. And yes in principle there is about 3/4 of the lever arm on a 20" wheel vs 26", but on a loaded cargo bike the mass is higher so the torque might be equal. I used a fancy "Willits" mount.



cargo fork

pwyg

You will need to hunt around for some continental oval blades with a long enough fat section to do this. I built a 406 unicycle with the same crown a few years back the bigger issue is going to be the shortness of the blades and width of the crown. The sockets are not angled in for such a short blade. That was more an issue for me as I needed to hit a 73 mm width, not 100 mm

unterhausen

tuz, that's a nice looking fork, thanks for posting that picture