84 fork shortning

CutterBill · November 07, 2020, 04:58:23 PM

Quote from: chiz on November 05, 2020, 03:41:41 PM
I was asked by the folks that are going to do the work if the tube was specially hardened?
Also the springs will have to be cut and can not think of how this would effect fork action or even if the same amount of fork oil should be loaded?
Also I had not planed to shorten damper rods, I know that shortened rods essentially have the same overall shortening effect as tube cutting, but I have not an iota of knowledge as to how any of this will effect actual damping?

Since no one actually answered your question, I'll give it a shot...
1. No, the fork tubes are not hardened. The chrome is of course very hard but the machine shop already knows this.
2. Shortening a coil spring will increase the spring rate (make the spring stiffer.)
3. I'm guessing that you will need to use less oil in the shortened fork. If the stock amount was used, it might fill up the entire tube and leave no air gap. Oil doesn't compress well so you might well end up with a solid fork. You will need to experiment with this.
4. Shortening the damper rods should have no effect on damping. But if you don't shorten them, they may prevent the fork tubes from retract. Again, you will need to experiment.
Bill

Pat Conlon · November 07, 2020, 11:27:13 PM

Doing all this work to end up with shorter damper rod POS forks....Nope, I don't get it...

T Legg · November 08, 2020, 01:12:58 AM

They would have to be a huge improvement over the stock forks on that bike.

CutterBill · November 08, 2020, 05:59:04 AM

Quote from: Pat Conlon on November 07, 2020, 11:27:13 PMDoing all this work to end up with shorter damper rod POS forks....Nope, I don't get it...

I don't get it either, but that's not the point. He didn't ask for our opinion on shortening forks; he had some very specific questions which, near as I could tell, no one else answered. Who knows, maybe he wants to put them on some farm equipment, or maybe he just wants to do something really odd.

But let's FIRST answer his questions, THEN we can ask... "Say, what are you building?"
Bill

ribbert · November 08, 2020, 06:23:20 AM

Quote from: Pat Conlon on November 07, 2020, 11:27:13 PM
Doing all this work to end up with shorter damper rod POS forks....Nope, I don't get it...

If you'd ever ridden one you'd understand.

Noel

chiz · November 08, 2020, 10:48:04 AM

They did fly around Daytona in 72 with the likes of Mann, Nixon ,Romero etc my guess is though not with stock forks and brakes.
Thanks

fj1289 · November 08, 2020, 11:03:39 AM

Quote from: CutterBill on November 08, 2020, 05:59:04 AM
Quote from: Pat Conlon on November 07, 2020, 11:27:13 PMDoing all this work to end up with shorter damper rod POS forks....Nope, I don't get it...
I don't get it either, but that's not the point. He didn't ask for our opinion on shortening forks; he had some very specific questions which, near as I could tell, no one else answered. Who knows, maybe he wants to put them on some farm equipment, or maybe he just wants to do something really odd.

But let's FIRST answer his questions, THEN we can ask... "Say, what are you building?"
Bill

He's already responded with what they are going on. Out of my own ignorance on how the metal of the tubes is finished (heat treatments or not, etc) I did not speculate on cutting and retreading the fork tubes. But, he did get very pertinent information on shortening the forks - just in a much more commonly accepted way of shortening the forks that is easier, well proven, and safe.

If someone came on here and wanted to know the alloy of the stock Yamaha pistons so they could cast some larger pistons, I'd do the same thing. I wouldn't answer the exact question asked since I don't know. But, since I do have knowledge and experience with what they are attempting to accomplish, I'd offer up an alternative solution that in the end will be cheaper and easier, and potentially safer. And take interest enough to ask about the project.

Maybe I'm being a bit reactionary - but I do think due diligence to help a brother out has occurred - and no harm has been done.

And I'd like to see pics of the old BSA project - sounds cool! And, depending how far down this rabbit hole Chiz plans to go, he can add the emulators and really tweak the valving rates and really dial in the spring rate, etc. I personally don't know what it takes to tweak the valving on a cartridge fork (is it easier than adjusting the emulators?) - but this is something a guy could do on his own with a bit of time, testing, recording, and adjusting until it's damn near perfect.

racerrad8 · November 08, 2020, 07:00:14 PM

Chiz, are you aware of the reduced outer diameter (OD) of the upper fork tubes between the upper and lower triple tree mounts?

Will that be an issue of you shorten them?

Sorry, replying from home so I dont have a photo of the OD reduction.

Randy - RPM

chiz · November 10, 2020, 06:54:15 AM

Damm well this has burst my bubble no I was not aware Thanks for this most pertinent revelation. Chiz

racerrad8 · December 13, 2020, 11:58:13 AM

Chiz,

I rebuilt a set of 84 forks this weekend. I wanted to offer some photos and dimensions of the reduced diameter between the triple trees.

The reduced diameter is 39mm and it varies from the top of the tube the two I had this weekend was around 73mm from the top. The reduction appears to be done by sanding, not machining so it is inconsistent in the depth from the top of the fork.

I am not sure when they stopped doing this reduction, but I do know the 89+ do not have this and are the same length.

HTH, Randy - RPM

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84 fork shortning