Tool to hold a suspension fork tube - what type of fit?

[JW1942]

I want to make a motorcycle bike suspension fork tube clamp (see pic below). This is to hold the tube tight in a vise while mounting and unmounting the "foot" that holds the axle.

The nominal diameter of the fork tube is 48mm,, the actual diameter is 47.92mm.

What type of fit am I aiming for? I am guessing transitional fit. Shoot for exact size?

thanks

[A.K. Boomer]

If actual diameter is 47.92mm I would go for something around 47.82 mm just for deflection purposes --- if you go the same size or larger you run the risk of deforming the tube and also if your purpose is to clamp the slightly smaller diameter will aid in this,

that pic looks wrong to me --- the vise should have an equal width piece clamped in the other side to even out the forces - not good practice for the poor vice and also not even clamping forces on the mounts,,,

you will have to clamp with more force to get the same results but at least it will be even...

[Bob La Londe]

I always used a couple pieces of plywood cut in a squared off U shape to go over the the center portion of my bench vise. I learned that trick from an old cabinet maker named Lewis Apple. often those pieces of plywood would remain in my vise for months at a time before I needed to remove them to hold something that they weren't suitable for.if I did crunch them getting a good firm grip on something that needed to be held tightly, it was dead easy to just cut another pair and drop them in afterwards. of course if you have one of those rotating vices with pipe Jaws on the bottom and square jaws on the top your plywood blocks could fall out when you turn it over, but I don't find that to be such a big deal.

Interestingly John Apple, Louis Apple's son, gave me the steel workbench that vise was mounted on and the vise when they moved out of town. it was his way of getting rid of a lot of junk. I had to take all the boxes of stuff that were under the workbench in order to take it though. some of it turned out to be good junk, and a few pieces turned out to be stuff I'd given him.

[754]

Squared off wood is ok for solid stuff, I would certainly not use it to hold expensive thin wall components.
I would go size for size on the hole size then slit..

[old mart]

When you do it, a bit of heat would help expand the aluminium forging on the hard chromed leg. I would make the bore size for size when clamped up and then spray a thin coat of primer paint into the bores, and wait til it was dry. A large nut and bolt of the right length would relieve the strain on the vise.

[JW1942]

Excellent advice - thanks all!

[BCRider]

Fork tubes are far from "thin wall". So while I don't thing that a couple of "V" blocks would have enough gripping area I would say that a very close to slightly undersize hole in wood will work like a charm. Or an oversize hole and some manner of friction material like suede leather or that anti skid shelving mat stuff.

For a metal holder I'd again go slightly oversize and use a thin layer of suede leather as a gripping medium. That way you avoid any risk of grit in the metal scratching the chromed surface of the tube.

[754]

I dint know where you source your metal but what I use has no grit in it. Why use wood when you can use aluminum.
don't forget many triple trees are aluminium and hold forks just fine. He probably can't hold these with a triple tree, as its an upside down fork.

[BCRider]

Grit comes from any shop area. We don't work in operating room style conditions after all. So grit becoming embedded in an aluminium block becomes a possible risk. Perhaps not the first time it is used after being freshly made. But what about after sitting around for a few weeks, months or years?

And why use wood? Simply because it offers some compressive ability to start out slightly undersize and compress the surface to form to a good full contact fit around the tube that also has a good degree of grip due to the nature of the friction between the slider and the wood.

Either will work just fine of course. But I'd rather save a nice block of aluminium for a better project and use wood for a fork leg fixture of this sort.

Also over my years tinkering with motorcycles I've found a few things can ding or scratch a fork slider that we would not expect to be an issue. And that's why I'd be using a slip of suede leather or maybe a wrap of paper between a metal jig and the slider tube.

[I make chips]

Just bore some aluminum to the right diameter then hacksaw it in half. Been there done that.
*That's why you always loosen the top nut before you take the forks off the bike!

[BCRider]

That's why you always loosen the top nut before you take the forks off the bike!

Quite true! Leaving the tubes held in the triple trees is how casual mechanics can avoid the issue of making blocks like this is the first place. I've broken down and assembled maybe 4 to 6 sets of forks in my time. I vaguely remember making a wood holder for one set but the others all were primarily broken down and re-assembled using the triple trees themselves as my holder. And every bike comes with the proper sized holder!

I'm not so sure that even the pro guys doing this stuff for a living don't do the same. After all there's a crazy number of fork tube sizes to deal with. A full set of sizes could easily see two dozen or more sets of blocks.

[Bented]

Measure the bore of the triple clamps and make the tool the same size.

[BCRider]

The nominal diameter of the fork tube is 48mm,, the actual diameter is 47.92mm.

What type of fit am I aiming for? I am guessing transitional fit. Shoot for exact size?

Going back to the first question as using your shop to make such a block is a perfectly fine option...

I think I'd aim for a size for size fit at 47.92 or maybe 47.91. But an acceptable tolerance would likely be between 47.90 and 47.96.

A hair tight and the edges of your blocks will flex out a touch just fine and give a nice even grip around a good portion of the circumference. Spot on or just a hair loose and the tube will deform to a slight oval shape as the blocks close and squeeze the tube. But still with good broad large areas of full pressure contact.

Much smaller and the blocks would rub along the edges of the openings pretty hard on spots of the surface. Much bigger and the tube would need to flatten more to obtain nice large contact patches thus needing a lot more pressure to flatten the tube into the saddles for good contact area.

[old mart]

Seems that a lot of the people posting cannot tell the difference between conventional and upside down forks.

[754]

Ditto that last post.
I have built blocks for straightening forks, and I have worked on blocks for holding bicycle fork components, , I think a lot of people are over thinking this, and thinking wrongly. Things get scratched because you didnt clean it, its your job to clean it off., I have cut Down and rethreaded many brand new fork tubes, using a 3 jaw and I am not scratching them, I use padding and check run out.

NOW this talk of holding a tube with a spit clamp pad that is SMALLER than the piece it's CLAMPING to me us a load of crap, if you think its worth trying fine but, I won't recommend it to anyone..