A forum for owners of Yamaha TTR250 trail and enduro bikes!

impulsive · Member

loss of gas pressure in rear shock

TTRfan · Super Guru

Sorry to hear of the problems.

Are you using a !nitrogen needle" to check and pressurise the bladder or have you fitted a Schrader valve to the reservoir cap?

The amount of air/nitrogen in the shock bladder is tiny and I think only a small amount would have to escape to allow a significant drop in pressure.

The bladder is really quite thick and the one that I saw from an early shock was in as new condition. Unless someone else knows different, I doubt the bladder is the culprit but, if it was, it would only mean the air and oil were mixing but surely that wouldn't cause a pressure drop?

I have only ever rebuilt one TTR shock with Steve so this is all relatively new to me.

I can only suggest that you submerse the whole reservoir to see more easily where any air may be escaping.

If the chrome on your rod is in good shape and there is no evidence of oil leaking then I doubt you are losing pressure there.

Let us know what you find please!

Brian

ttboof · Super Guru

Not sure where you have lost pressure

If you have filled it with air it may have heated up being pumped in once cooled the pressure may have reduced. Air can go through the bladder a bit faster buy no idea how much faster

Nitrogen is less likely to go through rubber and is very stable across varying temps that's why it's used as its more stable than air

If you add a bit if detergent to your water when leak testing you will have a much better chance of seeing any leak

-- Edited by ttboof on Sunday 23rd of February 2020 09:58:07 AM

impulsive · Member

I'm using a needle for filling.

The chrome is in good shape, no nicks or pitting etc and no oil leaks that I can detect.

It was probably around 50F when I first filled it. The temperatures here can swing wildly this time of year: 40F or less at night and 75-80F in the day but I can't imagine a 100psi difference regardless of extremes.

I'll check again with soapy water and see if I can dunk the entire can.

On a positive note I just got the bike plated as fully street legal. Shockingly easy, just said I wanted to register it for full road use as well as off-road, paid the fees and got a plate and stickers and went on my way. No equipment inspection or any questions!

impulsive · Member

Well after 2 weeks the shock only shows a 5psi loss and that can be just from checking it w/the gauge.

Before it was about 5 weeks that showed the 100psi reduction. I filled it to 155psi now and I'll see where it's at in 5 weeks. I think if I put a 90deg fitting in my needle/schrader adapter I might be able to get it into the reservoir without having to remove the shock.

I'm going to reassemble everything and hope for the best.

TTRfan · Super Guru

Sounds OK now. Fingers crossed for you!

ttr steve · Super Guru

I came across an interesting fact whilst following this thread. Now you all know the nitrogen needle kits I sell in the shop? Yep, well I have been testing a rear shock that Brian has supplied the shop (we rebuilt it together) and is for sale, but I wanted to check the pressure, so I used my needle to test it and found there was only 100 PSI when Brian and I rebuilt it we put 120 PSI in it, I then tested it again and was only 80 PSI, then I had a eureka moment........The kit I sell fills with 20 PSI when you put the needle in, thus thinking you have lost pressure, but you haven't.

I hope that makes sense to anyone testing the rear shock with my kits or one's they have made themselves.

Steve

jasparach · Senior Member

Something to take note of.

impulsive · Member

I wanted to check the pressure, so I used my needle to test it and found there was only 100 PSI when Brian and I rebuilt it we put 120 PSI in it, I then tested it again and was only 80 PSI, then I had a eureka moment........The kit I sell fills with 20 PSI when you put the needle in, thus thinking you have lost pressure, but you haven't.

I don't quite understand. Going by that it sounds like you would then lose 20psi from the reservoir every time you insert the needle.

I'm using an identical needle/adapter as pictured in the shop. Looks like the same pump too. I had it filled to 150psi, then it checked 145psi 2 weeks later. Wouldn't I have seen 130psi then? Is there a chance the small volume of air within the adapter pieces equates to false readings at the pump gauge?

Not trying to be argumentative, just want to clearly understand the situation.

Regardless, modifying the cap to install a schrader valve directly is starting to look like a good idea.

impulsive · Member

And one more item of information that may(or may not!) be relevant is that I thread the pump chuck onto the schrader first then insert it.

When finished adding air I do not unthread the chuck, rather just pull the needle out still connected to the pump.

ttr steve · Super Guru

The void inside the needle and schrader kits fill up with the pressurised air already in the shock, so you lose a bit of pressure when you first put the needle in the shock.

Just an observation of my own experience

-- Edited by ttr steve on Monday 9th of March 2020 10:12:17 PM

ttr steve · Super Guru

Does every forum get spam?

TTRfan · Super Guru

ttr steve wrote:
Does every forum get spam?

Yep but I usually get to it, delete the posts and ban the user before anyone sees it wink

I was a bit slow this time as I was busy elsewhere disbelief

Brian