Fork Bending

[Tony Pappalardo]

I've got a question about fork bending dies. I'm making a die and don't know where I should measure the radius. For a 4.5" radius should the die be cut to the 4.5"radius and have the groove cut into it or should the die be cut larger and the radius be measured to the bottom of the groove? 

I plan on cutting the groove round with a 3/4" cutter 3/8", or so, deep so part 2 of the question is how important is the decision considering the dept of cut and the fact that the blade tapers?

I'm new to the whole bicycle building process, 3 built, and this is my 1st effort at shaping the fork blades, having used pre bent blades on my other bikes. This will be for low trail and I like the look of the tight radius low on the blades.

Thanks for any help\recommendations.

TonyP

EP, Il

[Duane Draper]

Unless there is a reason it has to be exactly 4.5in radius, either way will be fine. There is springs back to consider as well.  

To your question, generally bends are measured along the centerline of the material being bent and is referred to as. Enter Line Radius (CLR). So if you want 0.500” tube bent around 4.5” CLR, cut the groove 0.250” deep.  NOTE that may not be deep enough for your specific material and bend; you may get some flattening.  However, as an example, that’s how you can calculate the die and depth of groove. I am sure Machinery Handbook has something more precise but for bike stuff, I wouldn’t over-think it.  Some dies have a V shaped groove and the work just fine. 

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From: frameb...@googlegroups.com <frameb...@googlegroups.com> on behalf of Tony Pappalardo <ajp...@hotmail.com>
Sent: Saturday, November 1, 2025 5:58:14 PM
To: Framebuilders <frameb...@googlegroups.com>
Subject: [Frame] Fork Bending

 

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[Steven Shand]

Normally you describe the radius as being on the centreline of the tube. That means that for most of the time you'll cut the depth of the groove to half the diameter of the blade, meaning the die can be cut to the radius as that will mostly* be on the centreline of the tube.

You'll need to account for spring back. This might involve some trial and error but you'll either have to bend beyond your desired offset or modify the die radius to take into account the springback.

I'm not sure you need to get the radius really accurate. The main thing is to get the offset correct. If you like a low bend with a tight radius just shoot for that without getting too hung up on the numbers.

If you're bending after you've attached the dropouts then the process might need to change a little in order to achieve the correct offset for the finished fork. I find bending without the dropout gives me more flexibility in fine tuning the offset.

*Ignoring the taper.

Steven

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[Anthony Pappalardo]

Steve, Duane, Thanks.

Centerline of tube, for whatever reason wasn't even in my head, and just reading it turned the light on.

I knew that I could get the offset wanted without having the exact radius wanted, but wanted to have a better understanding of the hows and whys of what I'm looking to accomplish.

Spring back is something that I've been expecting and am planning a stop of some sort to help insure matching bends.

I'll be bending before dropouts.

Again, Thanks

TonyP

EP, Il  

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From: Steven Shand <steve...@gmail.com>
Sent: Saturday, November 1, 2025 8:24 PM
To: Tony Pappalardo <ajp...@hotmail.com>; Framebuilders <frameb...@googlegroups.com>
Subject: Re: [Frame] Fork Bending

 

[michael giannico]

I built fork blade benders for years under the name nicola cycles.We had six inch and nine inch radius, red oak dies. The red oak was two inches thick in its final form.And all I did was route a v groove into it.And installed a top cap to hold the tip. Pics attached

Mike Giannico

Nicola Cycles

Oakland, CA

845-821-8502

To view this discussion visit https://groups.google.com/d/msgid/framebuilders/CH3PR13MB67009134C3913B99E0C9312EACC6A%40CH3PR13MB6700.namprd13.prod.outlook.com.

[Jon Norstog]

I did pretty much the same thing, but with CD-X plywood,  Worked OK, but I should have done a tighter radius.

To view this discussion visit https://groups.google.com/d/msgid/framebuilders/CAFcCy7bodgSRRqZGkqXyizqA0_8cHwS409PG%2BZi96AJ4XcX2kg%40mail.gmail.com.

[Michael F.]

Alex Meade makes some really nice aluminum fork bending mandrels and IMHO they are quite reasonably priced. Especially when you build your own and kink / destroy a few pairs of blades as you discover the ins and outs of raking. It gets worse with trying to use wood for tight radius bends. I speak from unfortunate experience. 

On Nov 2, 2025, at 12:31 PM, Jon Norstog <mutton...@gmail.com> wrote:



I did pretty much the same thing, but with CD-X plywood,  Worked OK, but I should have done a tighter radius.

On Sun, Nov 2, 2025 at 10:59 AM michael giannico <nicola...@gmail.com> wrote:

I built fork blade benders for years under the name nicola cycles.We had six inch and nine inch radius, red oak dies. The red oak was two inches thick in its final form.And all I did was route a v groove into it.And installed a top cap to hold the tip. Pics attached

<5836.jpg>

To view this discussion visit https://groups.google.com/d/msgid/framebuilders/CAEasqY1rH12uMFi7QUDo7kykB5pQdu_buWcESiFstaC7tJQLOg%40mail.gmail.com.

<fork_bender_a.jpg>

<fork_bender_c.jpg>

[Cliff McLeroy]

4.5" is really tight and you could have some buckling depending on the blades. I like an elliptical shape both for the appearance and the increasing radius as you bend up the tapered blade to reduce the buckling tendency. If you're using wood be sure it's really hard. Lastly make sure your anchor is very strong and stiff. There's quite a bit of force here and if it pulls from the die you'll end up with more straight section at the end. I usually find a socket that fits snugly in the end of the blade so I can really clamp it tightly without squishing it.

-Cliff

[Anthony Pappalardo]

Jeez Cliff, now you got me worried about that tight radius. 

I'm going to make it out of aluminum and have been familiarizing myself with the use of the rotary table that I'll be using to make it. I cut a coupla pieces of good 3/4" plywood which are bonded together. It was when I was thinking about cutting the groove that I realized my initial question. I'm going to finish this into a die and use it on some non forkleg thin wall to see what kind of luck I have before committing to the aluminum. If I have problems I can loosen up the radius on the aluminum.

I really like the idea of the plug at the clamp.

Thanks,

TonyP

EP, Il

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From: frameb...@googlegroups.com <frameb...@googlegroups.com> on behalf of Cliff McLeroy <karl...@hotmail.com>
Sent: Sunday, November 2, 2025 3:59 PM
To: Framebuilders <frameb...@googlegroups.com>
Subject: [Frame] Re: Fork Bending

 

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[Cliff McLeroy]

I've only done straight blades the last few years, but when I was bending it was with true temper blades. I'm sure there are many that are better for bending, especially if you need a lot of rake (I was doing up to 50 mm). I've heard the Kaisei blades have a long thin section with the taper higher up. Definitely look for longer blades. Contrary to what I was expecting, the one time I had a buckle, or really more of a slight bump, it was with thicker-walled blades. You could get some 0.5x0.035 or 0.049 chromo tubing for testing. If that buckles then the blades will too.