OzVMX Forum
Marque Remarks => Yamaha => Topic started by: TonyB on August 14, 2007, 01:23:25 pm
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Can anyone tell me if there are "modern' style monshocks available for yz465 from someone? By modern I mean very supple for the first half of the travel and then stiffening up for the rest. I realise thats why linkages appeared but I was wondering if anyone has used a hard soft spring setup with different valving etc. My shock is ok for large jumps and bumps but smaller high speed ripples and square edges damn near break me and the bike. The damping and spring action is too hard for smaller bumps. Also the front end sould do with progressive action too. Any help would be appreciated, as i am going though my bike and sprucing it up while the bones heal from the last getoof
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I can't see any reason why you couldn't fit a more modern piston into the shock. They're a fairly generic 50mm outside diameter, and the shaft diameter should be easy to deal with.
Based on the small number of Yamaha shocks I've seen inside, the lower spec and/or older the bike is, the crappier the piston design.
The old shocks also ran a lot of gas pressure - up around 215psi IIRC, whereas 160~185psi is the norm. This all suggests to me that Yamaha was compensating for something by running a lot of pressure.
So... cutting to the chase, I think there's a lot of room for improvement by modifying the standard shock, if you have the knowledge/bravado. I've been kicking around the idea of using the piston from as late a model rear shock as I can find, and then having a fiddle with oil weights, gas pressure, spring rates etc. I haven't jumped into doing so, mostly because I still haven't learned about the subtlties of valving alterations and their effect on things like high vs low speed damping, etc (eg: how do two 0.20mm shims behave compared to one 0.40mm shim? etc)
FWIW, KTM's PDS set up wouldn't be impossible to replicate for a skilled back yard engineer. It's beyond my skills and resources, but not impossible... I have been idly wondering about whether it would be possible to convert a PDS shock guts to fit the back of an old Yammie, but I'm going to experiment with the piston swap before getting too excited about that degree of custom fabrication (walk before you run and all that). The thing that scares me about this idea is that the PDS shock has so much internal tunability that it'd be easy to confuse the shit out of yourself...
Walter(YSS)'s last post in the market research thread is worth a read.
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Ya can Ohlins kit the G monoshock, bets to talk with your ohlins man, plus if you dont mind spending the money he can make a complete shock for ya.
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Wheres the ohlins man? this shock is actually the j unit which was the shock for the first linkage unit but i'm using as the old monoshock. Its quite a bit longer than the old monoshock .The piston rod is much bigger, and the action feels better but is still way to harsh on square lips etc. i wonder if a modern piston will fit?
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Tony,
Are you running the standard spring?
I have an yamaha monocross set up on my bike and have spent years and $$$$$ trying to get it right, even a high dollar white power shock didn't work.
Before running out and spending lots of money on shock components I suggest you get the spring rate right first ie the right spring to carry your 100plus kg frame.
Here is a simple test to see what you have, if it's standard then right away it will be way to soft.
Part 1
Put your bike on a center stand so both wheels are off the ground then measure from the center of your axle to your seat bolt on the frame, record this measurement.
Then get your gear on and hop on your bike (sorry forgot to mention you will need two or three people for this) sit or stand in your natural riding position,bounce up and down a few times then let the bike settle. and again get someone to measure from the axle to the seat bolt. (While your still on the bike)
Then deduct this measurement from the first measurement, it should be between 95 and 105 mm roughly. This will depend on weather you have had 1 lamington or 10, before starting.
If it is less than 95 mm then you need to reduce the spring preload, which is done by loosening the large nut at the top of the spring on the treaded section of the shock body, two threads equate to about 5mm of spring preload. If it's more than 105mm then increase the preload, by tightening the nut.
Once you get to the magic figure with then your RACE sag is set. Part one completed.
Part two.
Put your bike back on the center stand with both wheels off the ground, again get the measurement between the axle and the seat bolt, then take the bike off the stand, bounce the rear end a bit and then let it settle, again take the measurement, and deduct it from the first measurement, it should be between 15 and 20mm, either side of that your spring is either too soft or too hard. This is called you STATIC sag.
If it is less than 15mm then your spring is too SOFT, if it is more than 20mm then you spring is too HARD.
No that's not a missprint and I will explain it all in my next installment of " 7564 steps to monoshock enlightenment"
And if you change the shock spring always get fork springs to match ther is only one thing worse than badly sprung but balanced bike and that is a badly sprung UNbalanced bike.
I will be here till 5pm weekdays and 12noon on staurday. ;)
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Tony as I understand it you have a YZ465G/H and you have put a YZ490J rear shock in? Will I did a very similar thing except I have a YZ125G that I put a YZ125J rear shock into it. I to found that the shock was to harsh so it was pulled down, checked over and the valving on the piston is very basic. But yes evo555 is totally correct in saying you have to get the correctly rated spring. Without doing that you will keep going round in circles. Once you have the correct spring rate then it is a matter of re-valving the shock as will as different grades of oil. This is all trial and error stuff. One thing you need to do is put a normal air valve on the bottom of the reservour so you can let the nitrogen out easily and while doing all the testing you can use air until you think you a pretty close to having the shock how you want it.
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I actually settled on a "j" model shock as the best option, but I had a spacer inserted to reduce the travel as the rear end sat to high, giving me the impression that the rear was too stiff, when it was just transfering too much weight onto the forks, it really improvred the rear action, but it never turned the same after that. Mind you it is carrying an extra 20kgs in the form of a fourstroke motor so I couldn't vouch it's action in a yz/it.
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Sean at SOS Penrith does Ohlins into Monoshocks. I can't vouch for the effectiveness as I haven't put my bum on bike so modified but I recently ask on the behalf of someone else and I think the indicative price was $150-350.
no email only phone, 02 4732 5533.
I can't comments on ride quality but workmanship is A+. I'm sure others here will vouch for his work as well.
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Works Performance in the U.S. is now offering NEW shocks for the Yamaha Monos. ;)
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I had my early model 77 shock regased and had the spring reset and set up for my lets say big body It worked fine after that. The spring was reset by my local car spring works so it didnt cost to much I think regasing and spring about $110.00. I did sell the bike to a guy who weighs about half me he did have a little trouble being hard.
Cheers
Fatboy
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As for the forks, don't forget YSS is now making cartridge emulators.
mx250....haahaaahhaaaaa! "going to hell on a Montesa"...just gotta love it.
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The spring i am using is the IT465 spring which is soft, the sag etc is setup properly but i think there is too much damping on the compression stroke. I have the compression damping screw turned right out to the last click before it unwinds freely and still the damping on compression is quite firm. There is also a oil mist/residue around the top of the shock and the frame area, I think its seeping or misting past the compression damping knob. This shock was stripped and rebuiilt by RAD suspension in Brissie.
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One way to soften the dampening up would be to drill bigger oil feed holes and redo the shim stack with more shims. Also lighter wieght oil(2.5-5) and I think the nitrogen (from memory) is about 120psi-140psi. The compression dampening rod can get jamb, so take the knob right off and check that the rod is moving in and out.(dont lose the little pin when you take the knob off).
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Tony,
If you are running a it465 h spring on a yz j shock, then you will be getting a real frankenstien ride.
I would recommend going back to the IT shock and getting the spring rate right first, then look at valving.
You can waste a lot of time and money on the J shock before you can get it right and then it still not be anywhere near what our old bodies demand. The design is nearly 35 years old after all.
I use a guy at capalaba called db race suspension, I have never had any probs with his work, and unlike rad he does set up as well.
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An Ohlins piston will fit the monoshock and can be set up with a shim stack similar to a modern bike and a spring from a modern bike will also fit. Call Brett at Classic Bike Tune on the Gold Coast on 0755935355. He can build you a shock with those mods and the correct spring rate. He races a YZ465 with this set up and has built several shocks for other vintage racers.
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What is in the promax kit?
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(eg: how do two 0.20mm shims behave compared to one 0.40mm shim? etc)
Nathan 2x .20 shimms are much softer/ less dampening than one .40. Why because its .40 but split in the middle.so less resistance.
Similarly with coil springs if you put 2x 5kg springs ened to end and a 10 kg spring of the same length and tested them the 2x 5's would be softer .There is a formula used to calculate this so that if you use dual springs , one soft one hard you can work out the overall rate.
A shimm is also a spring that bends under pressure and returns to its original form when the pressure is gone . ie oil pressure.Same physics pretty much.
Also if you are going to use a modern piston in a non linkage bike the shimm stack will be very different than a modern linkaged bike .
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(eg: how do two 0.20mm shims behave compared to one 0.40mm shim? etc)
Nathan 2x .20 shimms are much softer/ less dampening than one .40. Why because its .40 but split in the middle.so less resistance.
Similarly with coil springs if you put 2x 5kg springs ened to end and a 10 kg spring of the same length and tested them the 2x 5's would be softer .There is a formula used to calculate this so that if you use dual springs , one soft one hard you can work out the overall rate.
A shimm is also a spring that bends under pressure and returns to its original form when the pressure is gone . ie oil pressure.Same physics pretty much.
Also if you are going to use a modern piston in a non linkage bike the shimm stack will be very different than a modern linkaged bike .
Cool, thanks. Seems obvious once you've pointed it out...
Although the relationship of shim stiffness to spring stiffness can't work exactly like you said, as the two 0.20mm shims are in parallel (you have to 'operate' both at once) rather than in series.
The formula for spring stiffness if the springs are stacked on each other is:
1/S1 + 1/S2 = 1/ST.
Where,
S1 is the stiffness of the first spring,
S2 is the stiffness of the second spring,
and ST is the total combined stiffness.
If there's two springs sitting next to each other (like a pair of fork legs), then its just:
S1 + S2 = ST
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(eg: how do two 0.20mm shims behave compared to one 0.40mm shim? etc)
Nathan 2x .20 shimms are much softer/ less dampening than one .40. Why because its .40 but split in the middle.so less resistance.
Similarly with coil springs if you put 2x 5kg springs ened to end and a 10 kg spring of the same length and tested them the 2x 5's would be softer .There is a formula used to calculate this so that if you use dual springs , one soft one hard you can work out the overall rate.
A shimm is also a spring that bends under pressure and returns to its original form when the pressure is gone . ie oil pressure.Same physics pretty much.
Also if you are going to use a modern piston in a non linkage bike the shimm stack will be very different than a modern linkaged bike .
Cool, thanks. Seems obvious once you've pointed it out...
Although the relationship of shim stiffness to spring stiffness can't work exactly like you said, as the two 0.20mm shims are in parallel (you have to 'operate' both at once) rather than in serie
Yes you are quite right it wasn't an exact anology.The shimms are in parallel but you get the idea how 2x .20 shimms have less resistance than 1 x .4 shimm.
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YSS may have one, they said that there were special prices coming up before xmas...
Brett
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hi guys, ive just gotten hold of a yz465 and its also been fitted with the 82 j shock, must have been the done thing at some stage. Am i better of keeping this shock or sourcing and putting a original shock back in the bike. i would probably prefer going with an original, but if the j shock is goin to be better for racing i will stick with it. It also has 82 j forks on it so would be interested in your thoughts on these over stock 81 h forks.