TZ - Forking

Post by **oldjapanesebikes** » Fri Aug 07, 2009 7:08 pm

Using theforking info TZ has put on his web site as inspiration, I ordered in a set of the cartridge emulators from the XS650 site to try out. The opportunity to install them arrived this week as my Orange GT750 needed new fork seals. however, I wasn't feeling happy about the number of holes the cartridge emulator installation instructions wanted me to drill in the damping rod - I'd forgotten how short they are when compared to the ones in later GS forks - per the photo below.

Some folks (Fred !) would point out that just changing the forks would make more sense than trying to mess with the original ones, but I like to tinker and I'm not convinced that 'stiction' is the main issue with the original fork design. Looking at the GS and GT forks, the biggest single difference is the length and increased diameter of the dampening rod (its 4 mm larger in diameter than the GT dampening, rod and over 2 inches longer).

So I was thinking of just making the stock dampening rod longer, adding new holes as per the installation instructions that came with the cartridge emulators and seeing how that worked. Before doing so, I was wondering whether anyone else has tried doing this ? And a question for TZ - did you drill all the holes the instructions call for ? Do you have a photo of what the end result looked like ?

tz375 · Post by **tz375** » Sat Aug 08, 2009 3:07 pm

Holes? What holes? We don't need no stinkin' holes...

Yes I drilled whatever it called for. All that does is to make enough area so that the compression damping in the damper rods is ineffective. The Emulator takes over that function.

Sorry Ian, no pictures of the bottom of the damper rod.

I did fill one of the top (rebound) damping holes though.

Post by **oldjapanesebikes** » Sat Aug 08, 2009 3:43 pm

Thanks TZ - I'm going to do this in two steps - first off I want to see what if any difference adding a set of Progressive springs makes. I've got those in now and will test ride them this coming week. As I was reaching for my drill, it occurred to me that having a spare set of stock damper rods to go back to might be a smart idea (you know - just in case

), then I'll do the damper rod modification with the emulators.

We'll see how it goes...

tz375 · Post by **tz375** » Sun Aug 09, 2009 4:56 pm

Ian,

Stop being a wuss. Go on. Drill those holes.

Your approach is better in that you can see the effect of the different changes. BUT - The damping needs to be changed. There's way too much high speed damping (hitting bumps) and not enough low speed (braking for corners).

Springs won't help either of those conditions but will help the bike ride better. If the springs are stiffer than stock you'll need more rebound damping to match and that's where blocking one hole comes in to play.

I'd try both together because that's what you want in the real world - better springs and better damping.

Suzukidave · Post by **Suzukidave** » Sun Aug 09, 2009 5:05 pm

I dont know if this info would help ya any but here it is http://www.ozebook.com/compendium/suzi/mags/forks.htm

Suzukidave · Post by **Suzukidave** » Sun Aug 09, 2009 5:06 pm

tz375 wrote:Ian,

Stop being a wuss. Go on. Drill those holes.

Richard you sure have a way with words

Post by **oldjapanesebikes** » Sun Aug 09, 2009 6:38 pm

Suzukidave wrote:I dont know if this info would help ya any but here it is http://www.ozebook.com/compendium/suzi/mags/forks.htm

Yep - have read those through a couple of times, but thanks for providing the link again for reference ! I was curious whether anyone else (other than TZ) on the board had tried doing these mods, as you often find that what seemed like a great idea in the '70's doesn't hold up as new information comes available.

I also just realised that I started this thread in the wrong area

Sorry about that ! Possibly one of the Admins could move it please ?

Suzukidave · Post by **Suzukidave** » Sun Aug 09, 2009 6:55 pm

This reads like a performance mod to me

Post by **oldjapanesebikes** » Thu Sep 03, 2009 6:31 pm

Just an update - I have done a couple of hundred miles now on the new fork set up. This is just the progressive springs in place of the stock ones, and it is a much nicer ride. I hadn't appreciated just how much of a difference it would make. I have also swapped out my rear shocks for a set of new Ikons (Koni clones out of Australia) - the spring rate dampening is probably similar, but the Ikons are the correct 'stock' length being about an inch shorter than the Magnum Progressives I was using - I wanted something more suitable for people with short legs (like me

)

I'll try the damper rod modifications in the spring - too many projects, too little time ...........

Post by **Suzsmokeyallan** » Thu Sep 03, 2009 8:03 pm

I'm with Fred on this one, those old stock GT forks are really very basic in design, have very limited travel and are lacking in all areas except the weight department.
Stiction is a significant thing with these forks, i have my RE5 ones apart right now and both have some very shiny sections where the lower two inches on the inner tube has been rubbing hard inside the lower leg. That area and also another section where it passes by the seal is also very shiny.
Having slider bushings makes stiction a lot less than without them, also remember these bikes are not light so that does not help it either.
Dont forget adding longer damper rods makes the forks extend out more when the bikes on the centre stand.
Your stock brake hoses will be too short unless you push the stancions up in the triple clamps to offset the extra travel the longer rods give and return the bike to its stock steering angle when unloaded.
Progressive springs will work wonders to the ride as youve found out as will altering the dampening, but in the end theres only so much you can do to those forks.
By the way we have not even touched the oil viscosity issues yet,,hmmmm

Post by **oldjapanesebikes** » Thu Sep 03, 2009 10:32 pm

Yep - heard most of that before Allan, but what I still haven't heard, other than from TZ, is some comment from people who have actually tried the dampener rod modifications outlined at this link:

http://www.ozebook.com/compendium/suzi/mags/forks.htm

And more specifically, whether it made any difference, and if so was it 'good' or 'bad'?

With the exception of my '1978 GT750' custom, what I want with my own bikes is a stock, 'period' look, so I'm happy to make changes to the internals, but I'm not interested in bolting on newer style forks - there are lots of other folks already doing that

tz375 · Post by **tz375** » Thu Sep 03, 2009 10:56 pm

Ian, what springs did you use? I'm looking for new softer springs for mine which is as hard as rocks now. The motor is in, but not complete, so that may change when the other bits are added.

I saw that Racetech now list a short spring for the GT in lighter weights 0.75 -1.0Kg/mm

Didn't lane do those mods to his forks? Where is Lane? I haven't seen him post for a while. maybe he's busy working on that K ...saki thing.

Post by **oldjapanesebikes** » Fri Sep 04, 2009 12:34 am

tz375 wrote:Ian, what springs did you use? I'm looking for new softer springs for mine which is as hard as rocks now. The motor is in, but not complete, so that may change when the other bits are added.

I just used the Progressive Suspension fork spring kit - part number 11-1120 which fits the 73 through 77 models of GT750 forks (plus a batch of HD's as well

) Spring rate is 0.63-0.89 kg/mm (35-50 lb/in) and they include a set of spacers for the GT750. I got them from One Stop Moto in California - they were fast and friendly, and even called me on the phone just to double check the order !! I'd order from them again.

My 'test track' is a concrete paved section of the Deer Foot Trail here in Calgary - frankly it is just about the worst excuse for a roadway I've ever seen. If you watch other sport bikes on this section, you see a lot of space under the tires, as they are in the air most of the time due to the bumps ! Before, with the original springs this section of road was dreadful to ride on the GT750 - now its just 'bad' so the ride quality is much improved .

Post by **Suzsmokeyallan** » Fri Sep 04, 2009 6:16 am

Ian i had a set of those longer floating head damper rods on the 76 Buffalo but to be honest i didnt feel anything superior over a stock set with one hole blocked off.
If you want the period look which i can relate to how are those cartridge emulators working out for you. The progressive springs, the emulators and fiddling with various fork oils should make it the best it can be.
As for the number 2, down south near Peigan trail by Freds theres a concrete section that quite interesting to ride on, its the strangest thing to feel you'd swear you have elliptical rims on your bike.

Post by **Suzsmokeyallan** » Fri Sep 04, 2009 9:07 am

Going back at this again i pulled out some items i had to see the complete scenario. After looking at it carefully im going to try this out and see what the results are, thanks a lot Ian!!!!,,LOL.
Using the floating head damper rods which are better and about one inch longer will give the forks some added range of travel, and since the 76 forks have a flat top cap i can slide them up through the top triple clamps to reduce some 'top out' when the bikes on the centre stand and regain more of a stock unloaded stance.
Then using these progressive springs and spacer tubes i can toy with the possibility of blocking off one of the top holes in the rods.
However i'll be fiddling with some oil viscosity first to see whats the difference they make.

The difference in damper rods, stock is the shorter fixed head ones.

The difference in springs, stock on the top, progressive type with spacer at the bottom.Spring and spacer are the same length as stock.