Extending a tube

[JW1942]

I have a mountain bike fork that has a steering tube that is short by 20mm. So I plan to lathe up a stepped tube, which I will shrink fit and Loctite into the existing tube. The OD is approx 1.13in and ID is approx 0.96in. The existing material is Aluminum, probably 6061. I plan on using the same.

For a tight shrink fit with this diameter, how much oversize should make my extension tube?

thanks!

[J Tiers]

Duuno, possibly a thou and a half per inch?

I did something similar with a new fork on my Lambert that had a shorter tube than the original bimetal fork. I hate it and need to change it. The extension is just too flexible, and I do not feel it is secure, so I hate riding it.

[Doozer]

I'm just sitting here on a Wednesday night
extending my tube.

-Doozer

[The Metal Butcher]

Doozer you crack me up.

[754]

Better pin it as well, don't be surprised if it is 7075 material..
how thick is it ? Matbe you could thread it internally ? And step, and thread the extension.

[Ian B]

Machine up a complete new one? Then you don't have joints to worry about.

Ian

[Davidhcnc]

If it was my project, I would make it .001" loose and trust the loctite to do the job.

If you want to press it then .0015" would be my choice and lubricated with loctite on the way in.

[Erich]

What are the effects of a joint failure? I have visions of a fork falling off when wheelieing over an obstacle. I like my teeth and my face so I am not a fan of this scenario.

[J Tiers]

What are the effects of a joint failure? I have visions of a fork falling off when wheelieing over an obstacle. I like my teeth and my face so I am not a fan of this scenario.

Possibly in his case... "too short" suggests it does not reach to the top bearing, so his extension actually has direct structural function in holding the fork in place. In mine, a failure would leave me holding a handlebar that is not connected to the wheel anymore... I do not see a good outcome in either case, so a different approach, such as a better fitting fork is a good plan.

The other problem in his scenario is that the handlebar support fits down into the fork tube and is typically held there by a split cotter setup. The support is sized to fit the ID of the tube, but in his case, it will all have to be in the extension portion, since the fork tube ID will have the adapter filling it. Does not leave a lot of grip for the handlebar support, unless the adapter is long.

A better fitting fork looks even better at that point

[old mart]

If you had at least 2" of overlap, then 601 Loctite and a 0.001" loose fit would take several tons to pull it out. As long as the wall thickness is at least 3/16 and the aluminium is one of the strong alloys, I can't see any problems, even if the extension includes the top bearing.

[mattthemuppet]

that's an 1 1/8" steerer tube and by the description it doesn't sound like it's a threaded steerer tube. It'd have to be a pretty old bike to have one of those anyway (25yrs+?). Sounds like the steerer is not quite long enough for the stem to clamp securely onto it, in which case the joint will be inside the stem clamp. Not much to worry about there structurally. 0.001" over on the insert, put it in the freezer and heat the steerer with a hair dryer for a bit and it'll go in nicely. Wouldn't hurt to pin it though, just in case.

Going to be a bit of a fiddly job as it'll need to be a stepped insert - step down on the OD to press/ shrink fit into the steerer and step down on the ID at the other end for the star nut to go in. Not difficult, but concentricity at both ends with the major OD and the step at each end will be important.

[ezduzit]

I have a mountain bike fork that has a steering tube that is short by 20mm...

Sell/trade it and get one the right length or long enough to shorten.

[mattthemuppet]

yeah, a new set of uppers for a decent set of mountain bike forks (no idea what the OP has) are only about $200, bargain.

[754]

Some folks likevto modify their stuff, does not always make complete sense, but some of us here will try to help you bring your idea to a good ending..

[ezduzit]

...some of us here will try to help you bring your idea to a good ending..

Not by using Loctite. I'd like to see him do this safely.