Fork steerer tube repair

[Osiris]

Last week I took one of my recumbents to a local bike shop where they were supposed to trim off a bit of the fork steerer tube. Somehow they mis-measured and cut off too much, so now it isn't long enough for the handlebar clamp to attach securely. Below is a fix recommended to me by a custom bike builder. I did a quick computer model illustrating the basic idea, which involves milling a piece of aluminum so that it will plug into the steerer tube.

The fork's geometry isn't exactly as shown in the model, but it does correctly represent it's construction. The inner sleeve of the steerer tube is aluminum with carbon fiber wrapped around it. The aluminum "plug" will be attached with JB weld or some other kind of metal glue.

Does this look like it would work?

In case anyone asks, no it's not possible for them to simply buy a new fork. This fork was custom built for my M1 Lowracer, and it's not something you can order a replacement for.

 

[ed72]

I'd prolly put a lit duck tape over the aluminum plug just in case the JB weld gives way.

You're not going to get a top cap onto that Alu plug with out some fancy CBC recess dug into it, I assume there cap bolt will use a washer over the Alu plug? It should work but it won't be pretty.

My wife wonders why I refuse to let a bike shop touch my toys.

 

[Osiris]

You're not going to get a top cap onto that Alu plug with out some fancy CBC recess dug into it, I assume there cap bolt will use a washer over the Alu plug?

This plug will sit under the adjustable tiller's clamp, so there's no need for a recess or washer.

 

[LarryOz]

I would probably run a recessed "pin" through the old fork into the new sleeve. That should keep it where it needs to be. You surely don't want to loose any steering ability when you are going 30mph on this guy!
Also:
It just seems so wrong to have an "M5" seat cushion no an "M1"
You are handy - make yourself a "branding" iron with "M1" on it, and sear it into a new cushion. Then it would be one-of-a-kind

 

[Osiris]

I would probably run a recessed "pin" through the old fork into the new sleeve. That should keep it where it needs to be. You surely don't want to loose any steering ability when you are going 30mph on this guy!

The pin is something I had suggested as well, although I'm not sure it's necessary given the strength of the bonding agent used to combine the two. I've already sent inquiries to a couple of local machinists to see what they have to say about it.

Also:
It just seems so wrong to have an "M5" seat cushion no an "M1"

I agree, but I only had that M5 seat cover on for one test ride and have never used it again.

 

[RAR]

How deep is the inner aluminum tube?
How much of the steerer tube clamp will actually cover the original carbon tube?

 

[ed72]

Pinned and appropriate adhesive should work even if you do pop wheelies Sting-Ray style on it.

I don't think you are taking a lot of risk but then again, I did Paris Brest Paris with a broken seat stay (thought it was merely my BB complaining, the airlines busted it). George Hincapie didn't get hurt too bad when his fork broke in 2006 during Paris Roubaix. Fixing your current fork is probably less risky than an Alibaba special for $56.27 including shipment from China.

 

[Osiris]

How deep is the inner aluminum tube?
How much of the steerer tube clamp will actually cover the original carbon tube?

I don't have the bike here, but from what I can recall the clamp covers about 1" of the steerer tube. I've found a local machine shop willing to do the work, so I'll let them determine what the exact dimensions of the aluminum "plug" needs to be.

 

[Osiris]

Pinned and appropriate adhesive should work even if you do pop wheelies Sting-Ray style on it.

I don't think you are taking a lot of risk but then again, I did Paris Brest Paris with a broken seat stay (thought it was merely my BB complaining, the airlines busted it). George Hincapie didn't get hurt too bad when his fork broke in 2006 during Paris Roubaix. Fixing your current fork is probably less risky than an Alibaba special for $56.27 including shipment from China.

I'm actually not at all concerned about the fork breaking. In fact, if the chemical bond between the aluminum steerer tube and "plug" is secure, the assembly should be quite a bit stronger than the original steerer tube was.

 

[RAR]

If it were me I would make the plug quite long so as to transfer the load down the tube away from the top.

 

[Osiris]

If it were me I would make the plug quite long so as to transfer the load down the tube away from the top.

I'm taking the fork and tiller assembly with me to the machine shop and let them decide how long to make the plug. Keep in mind that there is a star nut inside the fork steerer tube, so there's a limit to how deep the plug can go.

 

[Rod Butler]

nice bit of modeling!

 

[rx7mark]

The bonded joint might work OK, if prepped properly. It really will depend on the fit of the plug into the steerer tube and the prep of the two surfaces. You will want to roughen both surfaces to give the adheasive something to grip, and then chemically clean both surfaces with solvent prior to bonding.

Seriously most bonded failures in metal are caused by poor surface prep, too little surface area for the load, or not enough surface texture for the adheasive to grip.

So making the plug longer adds surface area, make sure its a snug fit, and putting radial and vertical scratches or micro grooves in both surfaces will allow a good grip for the adheasive, then drill and pin as suggested as a failsafe, just in case. Thats how I would do this. GOOD LUCK.

For what its worth, I'm an aeronautical structural design engineer with 34 years experience in metals, bonding, and composites.

Mark

 

[Osiris]

Sorry I didn't have time to take a couple of photographs before the M1 was reassembled, but the repair was made just as illustrated in the 3-D model above. The star nut had to be pushed down about 1/4" to make room for the plug, but that was really the only complication. The machine shop where the repair was made didn't feel it necessary to pin the plug to the the steerer tube. They said the bond is so strong that the steerer tube would break before the bond would fail. I've test ridden the M1 several times now, and I'm happy to report that it's back to it's normal self. I've even collected a couple of KOM's along the way.

 

[Osiris]

For what its worth, I'm an aeronautical structural design engineer with 34 years experience in metals, bonding, and composites.

Mark

I'm going to remember that. Nice to have someone like you here as a resource.

I have a question about something else I've been toying with for a different application. I have a set of conventional carbon fiber road bike handlebars. If I wanted to shorten them by cutting a section out of the center, what would be the best way to re-join the two halves?

 

[rx7mark]

I'm going to remember that. Nice to have someone like you here as a resource.

I have a question about something else I've been toying with for a different application. I have a set of conventional carbon fiber road bike handlebars. If I wanted to shorten them by cutting a section out of the center, what would be the best way to re-join the two halves?

Contact me privately or lets start a new thread on your project. I would be happy to help.

Mark