Home made Fork Brace

[Duane Ausherman]

But ME 109, to use a aftermarket top plate and claim how much better it handles is tantamount to claiming just how well one is able to ride in a subtle manner.

The main defect in the top plate is that the holes are in the wrong place for proper alignment. Once hogged out to the right place, the holes are rather large.

Ask Frog about the accuracy of center to center holes of the lower clamps.

To just buy an aftermarket top plate and bolt it on will almost assure one of handling problems. Know what you are doing before spending and bolting on things.

But, but, it is so much easier to spend $$ on bling than to actually go through the procedure to align the forks.

[ME 109]

But ME 109, to use a aftermarket top plate and claim how much better it handles is tantamount to claiming just how well one is able to ride in a subtle manner.

The main defect in the top plate is that the holes are in the wrong place for proper alignment. Once hogged out to the right place, the holes are rather large.

Ask Frog about the accuracy of center to center holes of the lower clamps.

It is all measurable stuff. The hole centres of the top plate must be identical to the triple tree, and the off set to the centre hole and steering head must be identical.
And the fork legs must torque up perfectly in the triple tree using the glass test and the same distance apart at the top and bottom of the fork legs to within .1mm

And of course the fork legs must be perfectly straight when laid side by side on a flat surface and rolled together. With my new/s hand fork legs, I could not fit a .05mm feeler gauge any where between them. Straight.

And torquing the fork brace bolts without affecting the looseness of the axle..........

There is quite a procedure to follow, but it is all there on your website.

How much variation did Frog find?

[Zombie Master]

I was wondering. If you had the forks on the bike, with no wheels, brakes or fender, and were sure of the alignment, couldn't you mount the side plates of the brace on the forks then carefully TIG tack the tubular brace in place? Then remove the brace and finish the welding job? It would be nice to make the brace out of some very light Chrome molly or Unobtainium! I've always felt my brace improved stability.

[Roy Gavin]

Chuey, the billet triples are made that shape to keep the fork tubes at the same level as the flat plate triple- using a bottom triple at the top would require you to raise the tubes by around the amount of the additional thickness of the new top triple.
This may or may not be an improvement, but it will affect stability a little.

The SJ triple has a lip at the top of the tube holes to keep the tubes close to the original position position, so i you want to play around with fork height you have to machine it out.

Absolute accuracy in fork alignment is not usually a requirement on old worn forks, or on new ones for that matter, as the OEM tolerances on the bushings , tubes and sliders are kept loose so that the forks will work in the real world, and will continue to do so if the tubes are slightly misaligned, bent or take a set with age , as most do.

The real world is not zero tolerance!

[Duane Ausherman]

Roy is right, zero isn't in the real world.

BMW used parts for the fork that couldn't even achieve their own too large tolerances. We found it best to get each step in the alignment procedure as close as possible. That means far better than the BMW spec. As it is assembled, the errors mostly just add up and may become large enough to give sticktion. What is wrong with having the fork specs too perfect? I posit, nothing is wrong with it being as parallel as can be measured.

That fork design is great, but the parts were not good at all.

[Chuey]

One thing that hasn't come up on this thread, but should be said is that when the handlebar is properly attached to the top clamp it is a stiffening factor. That said, my Cafe Racer is my best (favorite) handling bike and from the git go it could be ridden "no hands" easily and for as long as you want. It has clip ons, so no stiffening effect of the handlebars. That fork, as all of mine, has been aligned by me using the Randy Glass method.

Credit where credit is due department: Frog is the one who pointed out the stiffening effect of the handlebars.

Chuey

[vanzen]

Chuey, the billet triples are made that shape to keep the fork tubes at the same level as the flat plate triple- using a bottom triple at the top would require you to raise the tubes by around the amount of the additional thickness of the new top triple.

My recollection of the process to convert a lower tree to be used as an upper
includes machining the area of the lower tree where it meets the neck stem –
to reconcile this difference.

However, IMO, raising the tubes a bit (if not that certain amount) might benefit handling characteristics !
Stock geometry is very conservative and results in a fairly lazy machine !

[Duane Ausherman]

Before modifying a bottom clamp for use on top, slide both onto fork tubes to check that the holes have the same center to center distance.

If you have two of them laying around, try it anyway. You might be surprised.

Put your "perfect" fork tubes on V blocks, mount a dial indicator and rotate them. Again, you might be surprised. In an accident, they always bend just below the lower clamp. The ones badly manufactured will show a rather gently curve over the entire length.

In 1973 we had to return nearly 1/2 of all replacement fork tubes due to being out of spec. My shop did a lot of work and that meant a lot of fork tubes. Butler and Smith wasn't happy that we found this error. Once they got them back, they measured them and then shut up about it. However, the parts department just shipped them out without checking them. The administrative cost of the returns was greater than setting up a jig to measure them. The parts guy was "very friendly" to my shop. He told us that we were the only shop to discover the bad tubes, or at least the only shop to return any.

Bottom line, to build up that fork, either stock or modified, one must understand the goal, have the precision tools and know how to use them. Then be willing to modify or reject parts.

I wish that you could just throw the parts together and have it work. For those that doubt the need for fork alignment, just ask those that have gone through the procedure.

All that said, I must admit that some (not many) forks work Ok without being even close to tolerance. Don't risk it.

[vanzen]

Duane's take is spot on.

... but I'll still add one thought:
Modification of the forks system will be completely without merit
unless the point of departure is a stock fork system that is in excellent condition.