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Verifying oil volume in shock

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Posted by: NO HAND---------------------

Hi guys,
How is it possible to verify if we put too much oil in a shock or not enough?

I have used one guideline in the past but I wonder if it is reliable . What I have been using is: once everything is finished and assembled, but BEFORE filling with nitrogen, I depress the bladder and release the 50 lbs of air I had put in previously and see if the bladder cap lowers.
-If it lower between 1/16" to 1/8" I guess I'm fine.
-If it lowers more than 1/8", there must not be enough oil.
-And, if it doesn't lower at all, I'm afraid I got too much oil and the bladder might be collapsed.

Is that the right way to check if I've put enough oil?


Posted by: ShawnMc---------------------

I dont know anything about shocks, but Ive got a question:

What would be the danger of overfilling the shock by 10-15cc? Seems like the bladder would just compensate for the difference, Since IM postive that is wrong, can anyone tell me why?


Posted by: NO HAND---------------------

Well, the volume inside the bladder has to be within certain parameters because it acts as an air spring. The smaller the volume and the more abrupt the bladder damping rate will be, I mean the faster the air pressure will rise with a given reduction in volume. I don't know to what extent this can be felt from the operating shock in real life. I have seen shocks with too litle air space in the bladder that didn't leave enough space for the shock shaft volume. This led to some damage to the shock body where the large circlips go. Also I have seen shocks with too little oil that had the bladder go so high it went through the opening of the compression adjuster and wore a hole through the rubber. I know the bladder has to be close to the volume it has when it is unassembled in your hands. But, how much it can vary from that is still a mystery to me.


Posted by: ShawnMc---------------------

Ive heard of places assembling the shock submerged in shock oil so they dont have any air in the damping circuits. I dont know if its true or not. If it is true, It doesnt sound like they worry about the measurement of shock oil during assembly.

Ive also heard of guys driling and tapping the shock body to change the oil without opening the shock. Again in that instance they were just filling to the point of max capactiy through the hole. But Id think there would be a spec somewhere saying shock X should carry X amount of oil prior to charging.


Posted by: NO HAND---------------------

Quote:
Originally Posted by ShawnMc
...It doesnt sound like they worry about the measurement of shock oil during assembly...
I think it would be next to impossible to try and put a precise volume of oil, because of the spills and everything... I think it is easier to confirm that you put the right amount of oil by using the the bladder, I guess.


Posted by: marcusgunby---------------------

I always vent the shock(after the bleed) with zero pressure in the bladder-usually by opening up the comp adjuster-loosen it till the o ring shows then press the n2 valve again-alot of extra oil can come out.


Posted by: NO HAND---------------------

Marcus, is this procedure done after the seal head is installed? I would think so. It really sounds like a good idea. I wonder if say for example, a shock that had the bladder all folded up because of massive overfill, would the rubber bladder be strong enough to push all the excess oil through the compression adjuster? I was thinking of trying to find a way of rebuilding shocks without removing the compression adjusters. But, now I'm starting to think it is inevitable if I want to do a real good job. The more I'm thinking about this, doing this procedure as the last step would also insure even the last bubbles have a chance to escape too. Way cool . Also, my shock has a top out spring, so I guess I have to install the spring to ensure the shock shaft is fully extended when "venting" the shock.


Posted by: marcusgunby---------------------

Yes after the seal head is installed, i to didnt want to have to remove the adjuster but you have to really, unless its a KX kyb with the bleed screw-even easier.
Mine has the top out spring, but i do it the same way.I dont pull it so the spring is collapsed.

If you vent the bladder it will expand to its right size-thats why all the oil comes out.


Posted by: NO HAND---------------------

Thanks, these are real tricks of the trade and not everybody is ready to share that kind of info.


Posted by: bridge414---------------------

Thanks for the info. I'm just wondering what happens during the venting process if there isn't enough oil.


Posted by: NO HAND---------------------

You will pull air in as you push the valve.


Posted by: russ17---------------------

Have you ever pulled the bladder out and it is in a colapsed state, this is do to a over filling of the shock. With the entrance of the sealhead you have created an unequal pressure on the bladder( being there is more pressure on the outside of the bladder compared to the inside of the badder) By venting the bladder as Marcus has said, will create equal pressure as a starting point.(setting the bladder to it original state) Thus creating the right oil volume.
Also the overfilling of the shock creates a difference in the rising rate. This is where I feel that the rezzy cans with the separator piston could be to an advantage IMO, as you can manipulate the rising rates.
Bridge414, If there is not enough oil in there you will definetly have an air in the shock.

Russ


Posted by: motoprof---------------------

I have written some excellent shock and fork how-to articles that are posted at www.motocross.com/motoprof. It is no secret and shocks require very little tools.

When I rebuild shocks, I fill the shock body first (without the bladder installed. I then overfill the bladder resevoir, then install the bladder and clip. Then, I top off the shock with oil where the seal head is, install the shaft assembly and bleed really good. Put in about 15-20 PSI of air in the bladder, top off the shock body one more time, slide the seal head down until you cannot push it down any further. Release the air from the bladder while you are pushing the seal head down. Install the circlip, fill shock with nitrogen, and test how well the shock shaft comes back out (rebounds). It should be smooth.

As far as how much oil goes into a shock, KYB and Showa does not specify it. Neither do any of the big four bike service manuals.

Again, more detailed instructions can be found at my website.


Posted by: NO HAND---------------------

Motoprof, glad to hear from you. I want to say my hat's off to you for all the details and the well explained article on suspension rebuilding. I believe you have been the first to offer that kind of info availlable online and still have have the advantage of clear pictures over the others. I have found only one area where I had problems in shock rebuilding with this method, and it is related to oil volume in shocks. I found using this method gave good results with Kayaba shocks but I had collapsed bladder with showa ones. I suspect this is because the showa seal head is bigger and displaces more oil volume. I found the idea from Marcus Gunby; of venting the shock as the last step, very useful to ensure the bladder is at its natural position and allows to have 100% consistency in the bladder damping curve. I would be very interrested to hear your toughts on this subject.
regards
J-S


Posted by: motoprof---------------------

No Hand,

Thanks for the kind words about my site. I first started writing the suspension articles for the web sometimes around 1996 or so. I wanted others to know what I did. Eric Gorr was my "poster child". He always shared his mechanical ideas and skills. I learned a lot from him through talking to him and reading his now defunct magazine. I just cannot see hiding the "secrets" that I learned and do not respect those who do. I am a teacher of motorcycle and small engine repair. What if I hid my "secrets" from them?

As far as my thoughts on Showa oil volume goes, I do not think that this is a problem. The seal heads are not really that much bigger. I have done hundreds of shocks over the years and have never had any problems. Then again, I never did any type of experiment to prove myself right or wrong. I learned Shock rebuilding from Eric Gorr and Race Tech and I do it their way. I always figured that anyone who has done as many shocks as they have, would have found this problem if there was one.

Your way does seem to have some merit. I used to bleed the shock through the screw that goes in the top of the shock. I would have a oil fillied bottle hanging above the shock as I worked it up and down making sure the bleed hole was the highest point. I guess this would end up doing the same thing you do. The hardest shock I ever did was from the 87 Showa that had the resevoir mounted directly over the top shock mount. That was a hard one to bleed.

Just my thoughts. :thumb:


Posted by: DEANSFASTWAY---------------------

Sometimes if the oil level is just right the minute you let out your 10 psi bladder pressure the seal head will sort of ease itself into the shock body , espescially if the seal head is not drilled . I think its most important on barrier shocks . I saw a Custom Axis/Penske shock blow the reservoir cap across the track at a quad race one day after the rider hit a G out really fast, it probably had too much oil .Those older 87 type Showa shocks are alot like the TRX 400Ex quad shock . There easiest to bleed through the comp adjuster laying on their side. It seems they have a giant bladder as compared to the shock body size.



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