Hi I am hoping some chaps out there may have some answers. I want to cut a couple of inches off a pair of 84 FJ fork tubes and have the internal threads re-cut 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?
Thanks Chiz
Quote from: chiz on November 05, 2020, 03:41:41 PMHi I am hoping some chaps out there may have some answers. I want to cut a couple of inches off a pair of 84 FJ fork tubes and have the internal threads re-cut 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?
Thanks Chiz
Chiz,
Man, that's a lot of re-work, and it's a one-way street. Why not just loosen the fork tube clamps, and raise the tubes in the clamps without any cutting? If the hydraulics are in the way, you could use clip-on handlebars, attached near the tops of the tubes, so the hydraulics can stay at the same height, just above the tubes? The handlebars may get a bit higher, depending on the clip-ons that you choose, but that may not be a bad deal, there. When it comes to lowering the front end, a little goes a long way. Just a thought.
.
Yup I follow your reasoning for sure the raised tubes in yokes are the way I have them now. For the reason of not going into a long winded why I don't want to go this rout is why I have chosen to go with cutting the raised tubes.....spoils the look I am going for.
Thanks Chiz
Chiz, going with the tubes raised in the triple clamps would seem to be the most practical way to go, as it allows reversal of the mods. If you go with cutting the tubes shorter, you'll have to re-tap to be able to put the fork caps back in. Then you need shorter springs and just cutting the ones you have down will actually increase their effective spring rate because the rate is affected by the free length, outer diameter, wire diameter & material properties of the spring. You will also be significantly affecting ground clearance, especially when cornering.
May I ask what your purpose for doing all of this is?
George
If you want to actually shorten the forks, there is a MUCH EASIER, TRIED AND PROVEN METHOD!
Internally shortening the forks is very common. You disassemble the forks, add a spacer on top of the top-out spring, and them reassemble the forks. Typically you will reduce the preload spacer by the same amount (or a bit less) of the shortening spacer you've added.
Chris
Adding a link - sure there are others
https://www.youtube.com/watch?v=OoMQtK4M42M (https://www.youtube.com/watch?v=OoMQtK4M42M)
Not sure this will work on a FJ if you lower that much, the cowling will smash into the front fender when braking hard or hitting a bump in high speed.
the bike will also be very under steered and the rear wheel wants to come up on your side when braking and braking into a turn.I think you have to
lower rear end of bike to and then there is very little ground clearance left.Here is a pick of the fork fully compressed
Internally lowering the forks will not change how much the fork compresses. All it does is keep them from extending as far. But, raising the forks through the triple clamps will allow the wheel to come closer to the lower triple clamp or the fairing.
Ok interesting reply's all forks and yokes are going on a different bike which had a much shorter fork but much less stout. I will go over all of this and pick the best option. But the forks need to be shorter so yes seems you guys are giving me some options. Now I got to figure out how I'M going to replace that little fuse box cover on the dash that took a powder somewhere on a nice long ride today
Thanks Chiz
Anyone 3D print the fuse cover yet?
Quote from: FJmonkey on November 06, 2020, 03:09:09 PM
Anyone 3D print the fuse cover yet?
You got bookfake? Monkey
They printed some back a few months
https://www.facebook.com/groups/1414088652227511 (https://www.facebook.com/groups/1414088652227511)
Quote from: gdfj12 on November 06, 2020, 06:37:49 AM
May I ask what your purpose for doing all of this is?
George
I'd like to know Why also :scratch_one-s_head:
On my 1990, I have the RPM valves installed and the Racetec springs. The springs and valves were in when I purchased the bike, with the springs being very over sprung. My guess would be .95kg or 1.0kg springs. In an attempt to soften the front end, I have reduced the spacer from 4 inches to 3 inches. The result was a bit softer, but this also lowered the front end dramatically. Fortunately the stiff springs keep the fairing from contacting the front fender during hard braking. In the following photos, you can see how low the front end is. In the second photo, you can see that my fingers are NOT on the brake lever, collapsing the front end.
Fred
All motorcycles were designed with varying length's of front suspensions which had to "fit" the design of the rest of the package. The package "bike" that the FJ forks will be going on need to be shorter to emulate the length of original components but provide a facility to mount discs etc etc. I have a pair of forks front end.... it's just the shortening bit that is posing a bit of an issue.
The short stroke diving issue is not an issue at all as there is no faring for the fender to hit
Thanks
Chiz - sounds like a great upgrade for another bike - what is it? A cafe project?
I'll emphasize again that damper rod forks have been shortened with an internal spacer for years (decades!). This is one of those times where the easiest, least expensive method is also the safest.
(http://fjowners.com/gallery/12/171_07_11_20_10_29_13.png)
Add a 2" long spacer at the red arrow. Reduce the preload spacer at the blue arrow by up to 2" (I'd start with an inch or so and then trim more as needed). As an alternative - to adjust the spring rate - you can cut coils instead of shortening the preload spacer.
There is no change at all to the internal workings of the fork - the spacer merely restricts the fork from fully extending. It does shorten the total travel by the amount you shorten them - that would be the only draw back. The FJ forks are about 5" of travel - so that would be reduced to about 3". Since it sounds like it is going on a smaller, lighter bike and it sounds intended for more performance oriented use, 3" should be plenty of travel. You may get lucky and the too soft for an FJ fork springs may be perfect for the new bike. If they are still a bit soft you can always trim some of the tighter wound coils off and add to the preload spacer as required.
Cutting a coil spring increases the overall spring rate. On a progressively wound spring - the tighter wound coils are the softer rate and are the ones you'd want to cut.
Thanks....69 R3 All original bits are carefully restored and stored and not thrown away. Chiz
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
Doing all this work to end up with shorter damper rod POS forks....Nope, I don't get it...
They would have to be a huge improvement over the stock forks on that bike.
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
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
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
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.
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
Damm well this has burst my bubble no I was not aware Thanks for this most pertinent revelation. Chiz
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