Using 86+ Lower Legs with 83/4 Fork Tubes

General forum on frames, brakes, suspension, wheels and tires

Moderator: rztom

Post Reply
Message
Author
User avatar
Off Road
-----
-----
Posts: 2070
Joined: Thu Oct 01, 2009 1:31 am
Location: Alberta, Canada

Using 86+ Lower Legs with 83/4 Fork Tubes

#1 Post by Off Road » Sun Feb 22, 2015 2:53 am

IMO, the 2 piston calipers on the 86+ RZ's work much better than the single piston calipers on the 83/4 bikes.
However, they will not bolt onto the 83/4 lower legs, because the distance between their mounting holes, is not the same as the 83/4 calipers.
In order to use 86+ calipers, I needed to use the 86+ lower fork legs.

I didn't want to use the 86+ fork tubes because they are 1.5" longer than the 83/4 tubes, and wont work with the standard bolt on handlebars.
(You would need to use clip-ons).

For my conversion, I used a set of 83 forks, and a set of 90 forks and calipers.

Image


According to the parts fiches that I've seen, the 86-90 RZ forks are all the same.
I "assume" that the 85 forks are also the same, but can't find any info to confirm that they are.

When I got both my sets of forks disassembled, I found 2 major difference between them.

1 - Lower Fork Tube Diameter

The 90 tubes are the same diameter from top to bottom (35 mm)

Image


The Lower part of the 83 tubes have been machined down to something smaller than 35 mm.

Image




Race Tech published an article on the operation of damping rod forks.

http://racetech.com/html_files/DampingRodForks.HTML" onclick="window.open(this.href);return false;


When the forks compress, the fork tube goes further into the lower leg, and forces oil from the leg, into the fork tube.
The amount of oil displaced is determined buy the thickness of the fork tube. (O.D. minus I.D.)
Since the outside diameter of the 83 tube is smaller than the 90 tube, it will displace less oil for an equal amount of travel.

("IF" I understand what I have read, then the 83 forks will have "Less" compression & rebound damping, than the 90 forks)
And if I'm wrong, somebody please let me know.




2 - Damper Rod Holes

The 90 damper rods have 6 holes (3 on each side), and an inside diameter of 9.1 mm.

The 83 damper rods have 8 holes (4 on each side), and an inside diameter of 9.8 mm.

(Ignore the ruler. It's useless due to the perspective of the picture)


Image

The Distances were measured using a moderately accurate ruler.
The Hole Diameters were measured using a Numbered Drill Index. The drill bit diameters were confirmed using a digital caliper.
5.38 mm = #3 drill bit
3.82 mm = #24 drill bit
1.97 mm = #48 drill bit
1.54 mm = My best guess. A #52 drill (1.59mm) was too big, and a #53 drill (1.49mm) was too small.



While researching damper rod information, I found the installation instructions for the Race Tech fork emulators.

http://www.rzrd500.com/phpBB3/viewtopic ... highlight=" onclick="window.open(this.href);return false;

According to Race Tech, the middle hole(s) in the damper are for rebound damping.

Again, "IF" I have understood the info, then the 90 "Variable Damper" forks have 2 levels of damping.

When the forks start to Rebound from a fully (mostly ?) compressed position, then there are 4 holes for the oil to flow through.
This would give you small amount of rebound damping.
As the forks get longer, there will be a point when the lower hole in the damper rod is blocked off.
When this happens, the oil only has 2 holes to flow through.
This would increase the resistance to oil flow and increase the rebound damping.


If the above is correct, then the "Standard" 83 forks have 3 levels of rebound damping.
A small amount of damping when the oil has 6 holes to flow through.
A larger amount of damping when the oil is only flowing through 4 holes.
And an even larger amount of damping when the oil is only flowing through the top 2 holes.



After evaluating the information that I found. I decided to use the 83 damping rods in the 83 fork tubes.

Image




Here's the Part Numbers of the items that I needed for the conversion.

The 83 forks and the 90 forks both use the same Fork Seal. - (oem # 1UA-23145-00).

The 83 forks and the 90 forks both use the same Metal Slide 2 (installed on the bottom of the fork tube) - (oem # 10V-23135-00)

The 90 forks use Metal Slide 1 (installed at the top of the lower leg) - (oem# 51L-23125-00)

The 90 Forks use Dust Seal - (oem # 1UA-23144-00)

User avatar
JonW
Posts: 10600
Joined: Sat Oct 10, 2009 4:33 am
Location: Sydney
Contact:

Re: Using 86+ Lower Legs with 83/4 Fork Tubes

#2 Post by JonW » Sun Feb 22, 2015 2:57 am

Superb write up (as usual) Steve!

This was one of the things LB and I were discussing just the other day, as Racetec did the emulators for the USA bikes... ie bikes with 83/4 fork legs, and not the 85 on versions with Variable damper. We decided that since racetec's ethos was to remove the OEM damping in the leg, it didnt actually matter. This is because when you fit their emultaor you braze up the OEM holes and drill some large ones to remove the effects it would have, leaving the emulator to do the job. Unless anyone disagrees with that?
80 XT500 Supermoto!
81 RD350LC Resto
82 RD421LC Hybrid
82 RD350LC decapitation project
82 RD250LC JDM '251LC' YPVS
83 RZ350 Resto
84 RZ500 Resto
85 RZ350 F1 Resto
86 RZ350 F2 Resto
86 RZ350 F2 Hybrid

Like Watches? www.PloProf.com & www.DeskDivers.com

User avatar
Off Road
-----
-----
Posts: 2070
Joined: Thu Oct 01, 2009 1:31 am
Location: Alberta, Canada

Re: Using 86+ Lower Legs with 83/4 Fork Tubes

#3 Post by Off Road » Sun Feb 22, 2015 6:11 pm

Hi John.

I didn't install the Racetech emulators, I just put the 83 fork tubes & dampers in the 86+ lower legs.
The links to Racetech were given to show where I got my information.

With the Racetech setup, you drill out the compression damping holes so that they don't restrict the oil flow.
Then all the compression damping is controlled by the emulator.

The rebound damping is still controlled by the weight of the oil and the size of the rebound damping hole.

On the Racetech instructions, they say to braze up the four (4) rebound damping holes,
so it appears that the instructions were written for the 86+ bikes.

What I do find interesting (confusing) is that Racetech says to drill two (2) - 2 mm holes.
These seem to be very small holes compared to the size of the stock holes.

The 90 dampers have 4 holes (2x5.38 mm and 2x1.97 mm). This gives you a total surface area of 29.02 sq. mm.

The 83 dampers have 6 holes (2x5.38 mm and 4x1.54 mm). This gives you a total surface area of 30.37 sq. mm.

The Racetech dampers only have the 2 - 2 mm holes. This gives you a total surface area of only 6.28 sq. mm.

If we look at the Stock forks when they are partially extended, and only the 2 top (5.38mm) holes are controlling the rebound damping.

This still gives a total surface are of 22.92 sq. mm, which is 3.5 times larger than the area of Racetech rebound damping holes

User avatar
JonW
Posts: 10600
Joined: Sat Oct 10, 2009 4:33 am
Location: Sydney
Contact:

Re: Using 86+ Lower Legs with 83/4 Fork Tubes

#4 Post by JonW » Sun Feb 22, 2015 7:43 pm

Interesting stuff Steve, I knew you would have done the maths!

I guess the only things of note are that its not just the volume of the holes, but where they are sited that makes a difference... even tho the emulator is sposed to be doing the work!

This makes me wonder if the 2mms are just replicating the OEM rebound holes?

...and it is strange to me that racetch have such a small amount of 'hole' as well, I would have expected them to be say 50sq mm to be as open as possible as per their pic (below).

Image
80 XT500 Supermoto!
81 RD350LC Resto
82 RD421LC Hybrid
82 RD350LC decapitation project
82 RD250LC JDM '251LC' YPVS
83 RZ350 Resto
84 RZ500 Resto
85 RZ350 F1 Resto
86 RZ350 F2 Resto
86 RZ350 F2 Hybrid

Like Watches? www.PloProf.com & www.DeskDivers.com

Bare
Posts: 1196
Joined: Sun Jan 29, 2006 2:18 am
Location: vancouver

Re: Using 86+ Lower Legs with 83/4 Fork Tubes

#5 Post by Bare » Mon Feb 23, 2015 7:01 pm

Small point:
Racetech wants one to remove those damper rod holes.
YS and Mike's Do not.
They explicitly say not to, only requesting want a massive enlargement of the comp dampening holes
Curious.
Em valves work.. mostly.
Rebound damping (The important bit) seems little affected by the fittment of Valves...even that's arguable.
Also the fine tuning of the rascals is a Genuine PITA.. getting in/at them several times in a day also the results seem as not linear with number of screw turns.
Should be.... but it ain't.
Rebound damping is / remains best altered by Oil choice.
In MY bike, Mobil 1 ATF proved the best at this.
YMMV :-)

User avatar
JonW
Posts: 10600
Joined: Sat Oct 10, 2009 4:33 am
Location: Sydney
Contact:

Re: Using 86+ Lower Legs with 83/4 Fork Tubes

#6 Post by JonW » Mon Feb 23, 2015 8:25 pm

Interesting, thanks bare.

My thought was that the comp holes should be enlarged to basically see them as null and void with the emulator doing the work instead, but we still need to account for rebound, which it seems is still an issue as the emulator is great on compression but not rebound. hmm...

Yeah the fine tuning is a pita, its not like you can just twirl a screwdriver like the modern units and you cant even come up with a clever long screw thing as the RWU forks have an adjustible length top of course, unlike USDs. You could make and fit something upside down but the adjuster would be where the axel runs on our bikes. not ideal.

BTW, Racetec suggest you can adjust the Em by opening the top, pulling the spring and fishing the Em up with a hooked piece of wire, adjust and drop back in. Thats easier than removing the legs and rebuilding each time as you can do one at a time on the bike, but still not ideal.

I do wonder if we could rengineer an em valve to work upside down on one side, then that would work for rebound and leave the other one for comp, thats how a lot of WP forks work, ie one leg for each. This is a nice solution as each fork does one job well and not both jobs 'ok'.

I wish we had the same testd stats for mobil 1 ATF that we have for the other oils in the tips section, that would be intersting. sadly few seem to test ATFs :/

Steve, sorry for bombing your thread mate...
80 XT500 Supermoto!
81 RD350LC Resto
82 RD421LC Hybrid
82 RD350LC decapitation project
82 RD250LC JDM '251LC' YPVS
83 RZ350 Resto
84 RZ500 Resto
85 RZ350 F1 Resto
86 RZ350 F2 Resto
86 RZ350 F2 Hybrid

Like Watches? www.PloProf.com & www.DeskDivers.com

User avatar
L.B
Posts: 2008
Joined: Sat Aug 13, 2011 9:59 am
Location: Hunter Valley,Australia
Contact:

Re: Using 86+ Lower Legs with 83/4 Fork Tubes

#7 Post by L.B » Tue Feb 24, 2015 11:27 pm

Lol, my brain is going to explode!Jon,please please pretty please,which way do we go with these........

User avatar
JonW
Posts: 10600
Joined: Sat Oct 10, 2009 4:33 am
Location: Sydney
Contact:

Re: Using 86+ Lower Legs with 83/4 Fork Tubes

#8 Post by JonW » Wed Feb 25, 2015 12:24 am

Jase... we agreed to do what it shows in this thread I thougt?
http://www.rzrd500.com/phpBB3/viewtopic ... 6&start=30" onclick="window.open(this.href);return false;

We still need to buy springs of course...
80 XT500 Supermoto!
81 RD350LC Resto
82 RD421LC Hybrid
82 RD350LC decapitation project
82 RD250LC JDM '251LC' YPVS
83 RZ350 Resto
84 RZ500 Resto
85 RZ350 F1 Resto
86 RZ350 F2 Resto
86 RZ350 F2 Hybrid

Like Watches? www.PloProf.com & www.DeskDivers.com

Bare
Posts: 1196
Joined: Sun Jan 29, 2006 2:18 am
Location: vancouver

Re: Using 86+ Lower Legs with 83/4 Fork Tubes

#9 Post by Bare » Fri Mar 27, 2015 11:55 pm

Yesss! Stock RZ springs are wayyy Too soft. Many would claim they are unusably soft. After fitting Higher rate springs I now 'believe' that claim.
Double the spring rate and you've significantly improved the fork action.. with nothing else done.
Which brings us to the adjustment of the EM valves problem.
Adequately stiff springs are a 4 hands and pinched finger adventure.. trying to get the fork tube plug back on.
Not one of Lifes' finest moments... clearly Not something one willingly wants to do ..several times in one day

Post Reply