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The Vintage Japanese Motorcycle Owners Group: Discussion Forums

Vintage Japanese Motorcycle Owners Group :: View topic - Installing cartridge valve emulators in vintage forks


Installing cartridge valve emulators in vintage forks

 
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spurlock
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Joined: Jul 08, 2014
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Location: Vacaville, CA

PostPosted: Fri Mar 03, 2017 12:45 pm    Post subject: Installing cartridge valve emulators in vintage forks Reply with quote

I just recently became aware of the Race Tech cartridge valve emulators for improving compression damping on old style forks, and after installing them in my NX250 I thought I'd report on the process and results. First, in answer to the question, "What the heck are cartridge valve emulators?," this link explains it all:
Emulators - how they work

But the basics are: Damping in old style damping tube forks is accomplished by forcing oil through small orifices. When riding over a bump at low speed, the damping of the forks' compression stroke will usually be very light, allowing the forks to compress and absorb the bump. But for harder hits at higher speed the forks will begin to compress very fast. But the resistance to oil flow through an orifice increases as the square of the flow speed, so the compression damping effect goes sky high when hitting square edged big bumps. Bottom line, you don't get full benefit of the available fork travel because the forks can virtually hydraulic lock. By using spring loaded valving, the cartridge emulators convert the compression damping system to one that is speed sensitive, so that as the speed of fork movement increases the damping force tapers off, giving you the benefit of maximum suspension travel right when you need it most. This mimics modern cartridge damped forks which use flexible washers or similar to control oil flow. With the emulators, rebound damping remains controlled by the stock orifices which works well enough since rebound speed is fairly constant, rebound being only powered by the fork springs.

Installation:
The first step in installation is to pull the damping tubes out of the forks, drill out the stock compression damping holes and add extras so they no longer do any damping. This way all compression damping will be done by the cartridge valve emulators. On most forks you only need to remove the socket head screw at the bottom of the fork lower, turn the forks upside down and the tubes drop out the top without completely disassembling the forks. In the case of the NX250 the stock holes are two approx. 6mm holes near the bottom end at left in the picture. They do not go through both sides of the tubing, so compression damping is by forcing oil through just these two holes. I drilled them out to 7mm, and drilled all the way through the tubing to create four large holes instead of the stock two smaller holes. I deburred the outside and used a dowel and sandpaper to deburr the insides.



Race tech offers the emulators for a wide range of motorcycle brands and models, and they are adjustable over a wide range. They come with two sets of springs. I used the medium (blue) springs and set the preload at 2-1/2 turns on the adjustment screw.



The emulators are dropped into the forks and sit atop the damping tubes. The fork spring follows and holds them in place. During slow fork compression damping is handled by two small holes in the round brass plate. At higher fork speed the plate is forced upward against the spring, letting oil flow through much larger passages.



After adding the right amount of fork oil I installed the springs and fork caps and slipped them back into the bike.

This was the time to think about another fork problem, "stiction," the static friction resistance to movement. Mike Nixon has a comprehensive article on minimizing this problem in a great article here. I've always just used an abbreviated method shown below, but might try Mike's full procedure on the GB next time.

The object is to minimize any excess friction or binding in the forks so they respond fluidly to each bump instead of sticking. After making sure each fork slider will fall down its tube easily without the springs installed, with the front fender removed and the bike jacked up I install the axle in the left fork, snug it up, and check its alignment with the right fork. Here my axle was about 1-1/2mm lower than the right axle clamp, so I raised the left fork up that amount in the triple clamp and fork crown. Now the axle will not put any binding forces on the forks.



Next is to fit the fender to the forks, making sure the fender does not try to pull the fork lowers together or apart. I use calipers to measure the inside width of both front and back fender mount lugs on the forks.



Then remove the steel fork brace from the plastic fender and bend as needed to get the same dimensions. This way the fender will not be trying to pull the forks together or apart when installed. But for now I leave the fender off until after the wheel in installed.



Next and most important is to make sure the forks are not forced together or apart at the axle end. First I leave a small gap between the speedo drive and the fork lower to eliminate any interference in aligning the right fork side to side on the axle.



Then with the axle lightly greased and the axle cap nuts snugged and then backed off 1/4 turn and the fender not yet installed, I compress the forks a couple of times to let the right fork lower find its happy lateral position on the axle, then tighten the top nuts followed by the bottom. Without this step the forks might end up slightly spread apart or pinched together at the bottom, adding wear and "stiction" to the forks.



For fork oil I used the original Honda recommended ATF which is quite thin, but for my riding style it seems about right. I may go to something slightly heavier if the bike feels squirrely on bumpy high speed corners. The rebound damping is adjusted by changing oil viscosity, and the emulators affect only the compression damping. I'll be doing more testing but so far the ride is hugely improved. I've been riding on some rough, deeply rutted roads and deliberately aiming for the worst abrupt bumps and the forks are definitely much more compliant than stock, soaking up the big hits but also giving a smoother ride over the normal pavement bumps. My forks have the stock springs and due to the emulators' 16mm length the preload increased from stock 5mm to 21mm. But checking my static sag as I sit on the bike (145 lbs. with gear), the front sag is 31% of total fork travel, just about textbook perfect. Now I deliberately hunt for big-ass bumps!

-Bill

_________________
1975 Honda CB125S
1975 Honda XL250K2
1989 Honda NX250
1989 Honda GB500tt
1990 Honda GB500tt
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05c50
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Joined: Oct 26, 2007
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Location: Greensburg Pa

PostPosted: Sat Mar 04, 2017 4:16 pm    Post subject: Reply with quote

Ohhh, now I understand! I've heard of emulators, but I had no idea how they worked. Thanks for the link and your description of how you installed them. I'll be very interested to hear from you after you've put a few more miles on with the new suspension. Thanks

.....Paul

_________________
�77 CB550f
�77 CB750a
�78 GL1000
�80 CB650c
�75 GL1000-finally finished
�79 KZ200
68 CB450K1-taking up all my time
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KirkN
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Location: Orlando, FL

PostPosted: Mon Nov 13, 2017 1:09 pm    Post subject: Reply with quote

So, any updates after all this time and miles?

Now that you've upgraded your CB125's rear shocks (in your Hagon post), are you thinking of using emulators in the 125's fork as well?

Kirk
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spurlock
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PostPosted: Mon Nov 13, 2017 1:30 pm    Post subject: Reply with quote

KirkN wrote:
So, any updates after all this time and miles?

Now that you've upgraded your CB125's rear shocks (in your Hagon post), are you thinking of using emulators in the 125's fork as well?

Kirk


I've been running the emulators in the NX250 and GB500 since earlier this year and am very happy with them. No downside that I can see, and hitting unexpected big square edged pot holes or ruts feels noticeably less violent than before. I have not bothered to experiment with different adjustments on the emulators, so they could possibly be dialed in even better. But I have a system for measuring how far the forks collapse on big hits, and both bikes are using just about full travel now whereas before they would stiffen up well short of full travel on the biggest hits.

The 125 has completely different fork construction, so there is no way to just drop in an aftermarket valve.

-Bill

_________________
1975 Honda CB125S
1975 Honda XL250K2
1989 Honda NX250
1989 Honda GB500tt
1990 Honda GB500tt
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KirkN
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Joined: Feb 23, 2007
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PostPosted: Tue Nov 14, 2017 11:13 am    Post subject: Reply with quote

Hmm.... interesting (and disappointing, for me) about the fork construction.

I have a 1972 SL125, which I was guessing had the same construction as your '75 CB's. Mine has a similar damper tube to what you show for your NX installation. I did a quick search on Race Tech's site, and they imply that the emulators will drop in on top of any damper tube type as long as it has a recess in the top of the damper for the emulator to sit. Mine look very similar to your NX's, with that nylon seal ring at the top. But then, I didn't call Race Tech to discuss.

I've been thinking about upgrading my SL's fork to make it a bit better off-road - nothing too gnarly, mostly graded Forest Service roads and 2-tracks. First I have to get replacement fork tubes from Forking by Frank, because mine are pretty pitted, leading to seal leaks. But after that, I was thinking of the emulators.

Ah well...

*edit* - hmm... just did a quick look at Honda parts fiches, and my SL's fork really is different than your CB's. Huh. Mine is more like your NX's after all. Hmm... maybe time for a quick call to Race Tech for me. Very Happy
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spurlock
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PostPosted: Tue Nov 14, 2017 11:32 am    Post subject: Reply with quote

KirkN wrote:
Hmm.... interesting (and disappointing, for me) about the fork construction.

I have a 1972 SL125, which I was guessing had the same construction as your '75 CB's. Mine has a similar damper tube to what you show for your NX installation. I did a quick search on Race Tech's site, and they imply that the emulators will drop in on top of any damper tube type as long as it has a recess in the top of the damper for the emulator to sit. Mine look very similar to your NX's, with that nylon seal ring at the top. But then, I didn't call Race Tech to discuss.

I've been thinking about upgrading my SL's fork to make it a bit better off-road - nothing too gnarly, mostly graded Forest Service roads and 2-tracks. First I have to get replacement fork tubes from Forking by Frank, because mine are pretty pitted, leading to seal leaks. But after that, I was thinking of the emulators.

Ah well...

*edit* - hmm... just did a quick look at Honda parts fiches, and my SL's fork really is different than your CB's. Huh. Mine is more like your NX's after all. Hmm... maybe time for a quick call to Race Tech for me. Very Happy


Hey Kirk, the K0 SL125 forks did not have the damping tubes, but the K1 and K2 did. The emulators need to fit freely inside the fork tubes (OD of emulators slightly smaller than ID of tubes. Also, there is a spring loaded valve plate on top of the emulator that has to be smaller than the ID of your fork springs by maybe 1/8" or so.

-Bill

_________________
1975 Honda CB125S
1975 Honda XL250K2
1989 Honda NX250
1989 Honda GB500tt
1990 Honda GB500tt
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KirkN
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Joined: Feb 23, 2007
Posts: 1289
Location: Orlando, FL

PostPosted: Tue Nov 14, 2017 12:42 pm    Post subject: Reply with quote

Thanks for the info, Bill.

I'm hoping that since the SL100/125s are/were popular vintage racers, maybe Race Tech has a set designed that are for the later 125 fork. We'll see...

Thanks,

Kirk
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