tuz

It's good to read some good-old Jobst Brandt

It looks like if you put the caliper in front of the blade, the reaction force will push the wheel in and the QR won't loosen from the load cycling. Otherwise, a through-axle is the way to go. Along with beefy blades or with Frank's ingenious design!

unterhausen

tried drawing a quick free body diagram and just confused myself. Looks like putting the caliper in front does help the immediate wheel launching problem. Probably not a bad idea to add lawyer lips in any event.

calstar 

Has this been done, anyone build or see this design?

thanks, Brian

ksisler

Frank -

I remember the early 70's Proteus Framebuilding book (it had a yellow/tan cover) had a similar design for making a strong fork when heavy duty blades were lacking. That design placed a flexible plate under the top headset bolt with two holes in the outer end of it. Then a pair of seat stays ran from the front dropouts up to the flex plate where a 6mm bolt through the flex plate and into a plug brazed into the head of the stay. Don't remember any mention of effects on brakes (of course this was pre-disk era). Don't remember ever building one and about this time I got access to forged Cinelli full sloping crowns which solved the problem well enough and looked great.

Here is a quick sketch from memory of the Proteus design recommendation;

calstar 

Anyone know of pics of and unraked fork somewhere?

Thanks, Brian


edit: just noticed there was a very similar thread on this subject (Dec 2012, I even posted on it! damn that short term memory), still nice to have further discussion on the subject

BenCooper

I've built a bunch of bikes with the caliper on the front of the right fork leg. It means the braking force is pushing the hub into the dropouts, not pulling it out.

calstar 

Got any pics? What type of tab mount do you use? thanks, Brian

fietsbob

Silk Road fork in #5, keeps the caliper on the left, has dropouts, that open end is the front.
I expect they have them made for themselves.. .

MassiveD

It probably is traditional, but it is associated with tig or Walmart. If they didn't exist they would be a hit at NABHs for the first guy to show them, but as it is...

Those segmented forks seem like a poor place to start building an extra strong fork. There is a good thread on one that failed somewhere, with the whys and wherefores. I would build a plate crown first. Sturdy and all plate fabable.

https://www.velocipedesalon.com/forum...ned-16537.html

I have this CS from my first order from Nova. I got a standard touring CS, a tandem, and then a TANDEM. Holy moly that thing is like a easton baseball bat, but made of steel. Still not recommending it. I think probably the only advantage to the lopsided forks is the modest weight savings, and the look at me I am an engineer factor. There can't be any ride advantage left, and weight wise, one has to pretty much abandon all hope with that kind of set-up. I think just hefty but not crazy all the way is probably better.

MassiveD

That kind of rig is common on choppers though they normally tether to the fork at brake height. I have never built one for a bike, but have suggested it often enough, so far no takers, no how. These days the strut could be amsteel at about zero weight and fab cost. It is also a good method to retro a disc.

MassiveD

The caliper in the front was one of the first things I read about when I was pushing my way through this stuff X years ago. Still, you basically never see them...

BenCooper

Mostly IS mounts, though I'm building a setup for a Brompton at the moment which will have double discs, both post-mount, and the right one will probably go on the front.

tuz

check here. Every failure has it's own circumstances so a picture doesn't tell the whole story. However, basic physics tells me that light blades and disks is a bad idea.

Dave Kirk 

Good morning Gents,

I don't chime in here very often but I do enjoy looking in and see what everyone is up to.

I thought I'd share my findings and thoughts on disc brakes on steel forks. I've done a lot of long term testing and have been using different versions of steel disc forks for coming on two years now. I certainly think that one can make a very good, reliable and safe fork to work with discs if it is done right and I also think that if you just stick some tiny tabs on a lightweight steel fork that bad things can happen. The tough part about talking about this on the web is that a poorly conceived fork looks for the most part like a well conceived fork at first glance.

Cutting to the chase here is what I've found. There are a few key things to keep in mind when designing and building the fork IMO.

First is that diameter and wall thickness are your friend. Tapered blades almost always come much longer than is needed and most of the time folks bend the blades, stuff the tips in and then cut them to length by taking length off the large end. If one cuts the blade to length by cutting off the small end then the part of the blade dealing with most of the braking loads can be considerably larger than if the length is taken off the top. It can make fitting the tips a PITA but it helps a lot. It's also a very good idea to use blades with a heavy wall. This will make them stiffer and more resistant to de-raking.

Next up is the design of the dropout - I use a dropout with a very low angle or even 0° slot. This means that is the rider doesn't use the QR properly that the wheel will not get pulled out of the drops due to braking torque. If the slot angle is proper then the axle gets pushed sideways in the slot instead of out of it. It's also important to make sure you can offset the drop to the inside edge of the blade so that the rotor will not hit the inside of the blade.

Next are the disc tabs - there are two key things to consider here IMO - how far up the blade they go and where they mount to the blade rotationally. The first part of this is pretty obvious to most folks.............by making the tab extend up the blade it spreads the load and changes the leverage on the blade imparted my braking force. Because the blade is tapered you end up transferring a good bit of the load to a larger diameter section of the blade with an extended tab - a good thing. If done right it will also give the upper part of the tab more surface area where is meets the blade to lessen the point loads and any chance of the 'can opener' effect. The other thing to consider is where the disc tabs mount to the blade in a rotational sense (must be a better way to say that). When viewed from above it's important that the tab be mounted so that it meets the blade tangentially and not perpendicularly. By having it mount tangentially the tab does not try to push through the blade and collapse the tube but instead tries to twist it..........and the blade deals with that very well. Getting the tabs to sit on the blade in this fashion requires that you think ahead a good bit and not just make the fork and then see where then tabs land. Since the tabs mount in a fixed relationship to the drops it's the blade that needs to be moved in/out to get this alignment. One can do this with fork crown width and picking how the drops sit in the blades. A bit of a pain but worth it in the long run. The fact that the tab doesn't load the tube straight means that it is less prone to making the tube cross section change shape. I haven't tested this well enough to prove it but I feel strongly that the tab pushing on the blade and wanting to change the shape of the cross section of the blade can contribute to the de-raking effect. Mount the tab low and perpendicularly to the blade and it can flatten the tube which of course will pull the blade backward lessening the rake.

I went through many designs before I found one that performed the way I wanted with no weird twisting or shudder. I then worked on making sure the fork would stay together over the long term. In the end I have a fork I feel good about. I rode and tested the final version for a year before offering anything for sale. I can't tell you how sore my hands got riding down endless washboard dirt roads while using the front brake hard. In the end I have a fork that holds its geometry and alignment, handles and rides well and should stay together for them long term.

I hope that my findings help others make safer forks. A high tide raises all boats.

Have fun.

dave

seeker333

You guys over-think this. Surly sells their Disc Trucker fork in black, for 26 or 700 wheel, for as little as $90 shipped - less than you can make them. Buy one, ride it - it works, looks good too. Crown rack boss is a neat feature.

Let Tange do the hard stuff (maxway makes the frames, fork steerer is branded tange).

https://classic.aebike.com/product-li...5-m27576-qc30/

GrayJay

Plenty of reasons to try something different, there are good reasons we build frames instead of just getting a LHT.

The bend in those LHT forks is all concentrated in the middle of the leg, looks more like a (human) leg with a bent knee than a fork nicely raked along lower 1/3 of the leg.

Dropouts are verticle instead of forward facing as they should be for disk.

Builder might want different A-C, rake, width measurements or just astheticaly might want a differet crown style that fits rest of the bike together.

tuz

Thanks for the input Mr. Kirk!

calstar 

A big +1. And heres a link to a Kirk Frameworks sequential build, very nice.

https://www.kirkframeworks.com/jk_cross1.htm

Brian

awetmore

A friend pointed me to this thread.

I made the dropout on my fork, it is not a Willits dropout. I have a small CNC mill and made the dropout in my basement. I make most of my own front dropouts. It is different from most in that it is designed to take a rear caliper, not a front. Rear calipers sit closer to the axle than front dropouts, and I did that for aesthetic reasons. My dropout is also cut to match the curve of the fork blade, while the Willit's is designed to go against a straight blade fork. They are bent (usually not very nicely) to fit on curved fork blades. If you look closely you can almost always see the distortion from the bend.

Otherwise Dave Kirk already wrote a lot of what I did on this fork. I cut the blades from the bottom, not the top, to increase the diameter of the fork blade at the dropout. I also used a sloping crown (not normally my preference) to get more fork blade to trim. The fork blades were the thickest in my inventory (they are Columbus Thron).

The fork has about 1500-2000 miles on it these days and has had no issues.

There is one issue with it. When choosing to use the rear ISO mount I read the disk mount drawings carefully and have everything aligned for that. However I didn't read the hub standards and neglected to notice that rear hubs put the disk 5mm farther from the dropout than front hubs. This means that the caliper is closer to the rotor than any calipers are designed for. I corrected for this by modifying the ISO to post mount adapter and because the BB7 caliper is very flexible in setup. However it is unlikely to work for something like a Shimano Servowave brake.