Yamaha 1980 Monoshock Upgrades/Mods

[walkahz]

Hi All
Ive got both a 125 and 465 G model Yamaha with the original non linkage yamaha monoshock.

After A few rides on each bike like many older machines the suspension is......lacking. Especially when carrying my 95kg behind.

Forks are relatively straight forward and I will likely throw some cartridge emulators and suitable sized springs which should bring them to an acceptable level.

Rear shock is more difficult.
For those unfamilar with these setups the shock is mounted up the spine of the bike under the tank and there is a remote res mounted in the airflow under the tank.
Im currently pulling apart a spare 125 shock to have a look at and see what im in for. A lot of the period parts for these shocks really focused on finned reservoirs to keep the oil temps down. I believe these run a piston for nitrogen separation.

What I am thinking is to make up a custom res that is finned and possibly for bonus points add a compression adjuster from a more late model bike to get some sort of adjustment there as currently only rebound is adjustable. (not sure if this is worth the extra stuff around though). Rubber res hose to be replaced with braided line. I have a lathe and mill at home as well as contacts with CNC gear so really doing this as more of an interest project more than anything.

Rear springs are progressive stock but genuine stiff springs are near impossible to get so i will likely get a twin spring setup to suit my weight (465 already runs a twin spring that is just too soft so that might make its way to the 125 and the 465 will get a beefier unit.)

Has anyone had much experience with these early yamaha units and will to share any tips?

[dwb79]

Great project.

I've got a few points to add. Hopefully these will help you a little.

- The alloy shock bodies were not hard anodised from new. So most of these old shocks have a large amount of wear inside the body which causes blow-by and therefore a lack of damping, especially around the ride height position.

- These early shocks have a three piece seal head. They have the usual circlip below the seal head, but they have a second one above. This makes dismantling the shock difficult as you cant press the seal head in very far to expose the lower circlip.

- We always replace the three piece seal heads with a more modern one piece version.

- The separator piston in the reservoir can be really sticky. So remove it and grease the oring. There can also be wear inside the reservoir.

- Many of these older shocks have the piston crimped onto the shock shaft. This means it isn't removable without machining.

- Many of these older shocks have a non-revalvable piston/valving arrangement. So if you want to improve the damping curve, you'll need to fit a new piston.

- Because of the crimped shock shaft, there isn't any threads to screw a nut on to secure your new piston and valving. So if you want to improve the damping curve, you'll need to fit a new shaft and piston. Or you'll have to get creative and figure out a way to hold the piston/valving onto the shaft.

- The finned reservoir is mostly a wank. The small volume of oil that the 14mm shock shaft displaces means there is only a small amount of oil moving out of the shock and into the reservoir as the shock compresses. With the extremely long length of hose, I think you'll find that the oil from the shock body never reaches the reservoir and instead just moves backwards and forth inside the hose. You could do some measurements and calculations to confirm this.

- If you want to get the most out of a finned reservoir, you'll need a much shorter hose to allow the oil from the body to enter the reservoir and hopefully circulate to aid cooling. Obviously this is going to require you to re-position the reservoir.

- RaceTech uses a double spring arrangement. Teknik has some 400mm long progressive springs.


Good luck and keep us updated.

[walkahz]

Awesome Thanks for the advice!

Got stuck in last night a bit after i first had to make a set of soft jaws to suit.

Pulled the spring off and let out the nitrogen and removed the res.





There was no bumper left and fell out of the plastic shield in pieces!

I was aware about the anodizing issue so i guess i will find out once i open them up. I think the 465 shock will have this issue as at the moment it bounces like a pogo stick with no damping. I suppose there is no real solution to this? cut the tube body off and weld on a new one potentially? or find a less worn body to start with.
I can check the wear with a dial bore gauge.

I will look into getting them hard anodized depending on the price i get, i know of a few places here in Melbourne who could do this.

Do you have any insight onto grafting on a late model compression adjuster to the res? I know it will be possible making a new res from scratch but unsure if it is worth the effort and extra complexity, these bikes will not be ridden hard with me on them.

Point taken on the fins! Quick calc shows 23mL displacement on a 150mm travel with the 14mm shaft. Might still do it for bling factor but will see when it comes to machining if its worth it.

[dwb79]

Nice shaft clamps. Well made.

Originally Posted by walkahz

I suppose there is no real solution to this? cut the tube body off and weld on a new one potentially? or find a less worn body to start with.

I've never done it myself, due to price constraints, but yes. Cut the body off, machine a new body including circlip groove in the end, weld to shock head and then send the whole lot off for hard anodising.

Originally Posted by walkahz

Do you have any insight onto grafting on a late model compression adjuster to the res?

I've done this a few times. Most of the time my customers just supply me a second shock (from a wrecker or similar) and I've cut and shut with a new hose. Almost all of these OEM shocks with a comp adjuster on the reservoir will only have a low speed adjuster. If you want to get fancy, RaceTech will probably sell you a complete reservoir with low and high speed.

Originally Posted by walkahz

Point taken on the fins! Quick calc shows 23mL displacement on a 150mm travel with the 14mm shaft. Might still do it for bling factor but will see when it comes to machining if its worth it.

You're all over it

[walkahz]

Been working on stripping these down some more. Decided to pull apart the 465 shock at the same time as it is like riding a pogo stick.

Seal head removal was covered by making some spacers to fit on each side of the shaft. The whole lot was put in the press and pressed down squishing the rubber gland in the middle enough to get the circlip out with some wrestling.

This was the state of the oil that came out manky and black no good!



The internals of the shock are also very different to a modern bike.

Only about 3 shims on the compression side and a deflection limiter and none on the rebound, Rebound seems to be taken care of by the holes on the step on the shaft with restriction provided by the pushrod runs through shaft to the adjuster on the end.




If any one has tips for doing some basic valving on these old bangers let me know.

The body on the 125 shock and 465 shock are the same. Which is good because the body is shagged on the 465. There is a lip big enough to feel and see which will no doubt leak oil past the piston. The 465 shock also had no gas charge and hardly any oil left which is why it was such a pogostick. (probably also why its so worn but this bike has had a very hard life in the past)

The 125 body is in pretty good nick. Measured with a bore gauge and total change in dia is within 0.1mm and runout also 0.1mm which i think is pretty good for a 39 year old part.





So plan at this stage is to rebuild 1 good shock for the 465 out of the parts of the 2. The worn out body I am looking at turning up a new sleeve and welding onto the head but need to get my head around how to bore a 46mm hole 300mm odd deep and keeping it straight round and smooth (need to make up a beefy boring bar!)

Initially will keep the body raw but once ive got a bit of a batch of them i might get all 3 Hard anodized at once (price for 1 was at minimum charge so more economical to get a few done)

[walkahz]

Long time since a post.
Been doing some maths in the background to try to figure out a spring rate. Doing this on a povo budget at the moment so tried to use components i already had. Best matching rear spring i had was off a 90 kdx 200.

Yz465 will use a twin spring setup with the kdx spring.
Yz125 will use the original 465 spring.

The rebuilt shock is almost done rebuilt with a new seal 5w oil and freshly zinc plated parts.
Pictures to follow once i get on a pc.