Suspension fork collapsing: RS Revelation Race

[Ed-Zulu]

Hi there fellow Hubbers,

 

Hoping that perhaps one of you might be able to assist with the following...if not, I'm the owner of expensive scrap metal.

 

I purchased, from CRC, a SRAM Rock Shox Revelation Race Duel Air 140mm travel suspension fork. After riding it for about 4 months it collapsed during a race in late June 2010.

 

Took it to the LBS and they speedily sent it of to Slaapstad where it was serviced and returned to me. Now working again. I ride very little during July - December 2010, approximately 250km's.

 

Start whacking the bushpaths around town again in Jan 2011 and by end of Jan the bloody thing goes suckety again (about 400km's done)...off to Slaapstad for entire internal workings replacement surgery. Everything is checked and cross checked and the fork is returned with big bill and working again.(also received the replaced parts)

 

You guessed it...it collapsed again the past weekend!

 

I have now done the following: checked air pressure - negative chamber is 10-15psi above positive, so there is a transfer of air between the two chambers. So in my mind it must be that the o-ring between the chambers is faulty or that the casting is enlarged letting air through, specifically when getting hot???

 

I also noted that there is excessive oil at the schrader valve after inflation of the top chamber, is that normal, why isn't gravity sucking in this case?.

 

Looking forward to your wise and valued replies

[Johan Bornman]

.

 

I purchased, from CRC, a SRAM Rock Shox Revelation Race Duel Air 140mm travel suspension fork. After riding it for about 4 months it collapsed during a race in late June 2010.

 

I have now done the following: checked air pressure - negative chamber is 10-15psi above positive, so there is a transfer of air between the two chambers. So in my mind it must be that the o-ring between the chambers is faulty or that the casting is enlarged letting air through, specifically when getting hot???

 

I also noted that there is excessive oil at the schrader valve after inflation of the top chamber, is that normal, why isn't gravity sucking in this case?.

 

Looking forward to your wise and valued replies

 

 

The new-generation of Rox forks are fond of doing that. What happened there is that the piston leaked air from the postive chamber into the negative chamber below it.

 

The stock O-ring is a 20mm (IIRC)x 2.4mm

 

I find fitting a 20 x 2.5mm or 20 x 2.6mm helps prevent it. Also, standing a long time makes the O-rings go flat on the sides where they're compressed, and they leak.

 

The little bit of oil coming out the top is nothing to worry about. On top of the positive piston is a teaspoon of oil to lubricate the O-ring and it always squirts out when deflating. It has nothign to do with your case. Should that oil have squirted out the bottom valve, then you know the oil leaked from the positive to the negative chamber -- it is the smoking gun, since the negative chamber has no oil of its own.

 

Nowhere on that fork is there a seal that works against a casting. It is all smooth machined and anodised aluminium tubes. The problem is not there.

 

If I understand your assessment of the discrepancy in air pressures correctly, you imply that the positive and negative had equal pressure to start off with and now there is a difference. Had this been the case as you describe, the fork would have extended. If it leaks from postive to negative, the fork compresses. If it leaks from negative to positive, the fork elongates. I suspect the difference you see (very difficult to measure properly), has to do with the fact that if you put in say 100 at the one end and 100 in the other, the one you inflated first will now no longer be at 100. This is because that air chamber changed size when you inflated the other side. A different size with the same number of air molecules in there implies a different air pressure.

 

Why doesn't gravity suck? I take it you assume since the fork was upright and the oil now at the far end of the positive chamber, letting air out the top wont have an effect on the oil? Good question that and a wide-awake observation. It is a rather thick oil and it will coat the sides of the chamber for a long time - several hours - after inversion. When you release air, the air doesn't come from the centre of the chamber directly up and into the valve. It flow upwards, yes, but it also flows past the sides and top wall and drags some of the coating oil with it. Build a mental picture of a case where the air shoots up straight without flowing past the sides and one where it moves everywhere with equal speed towards the valve and only accellerates as it hits the valve's bottleneck. In this scenario there is drag between the air and the sides of the chamber and some of the coated oil is dragged outwards. This is quite evident from the air bubbles in the oil. The oil is almost frothty when it shoots out the valve, even though it is perfectly clear in its natural state.

 

I'm not 100% convinced I've solved yoru type of problem yet and I haven't had enough cases to experiment with, but I'd start by looking at fatter O-rings. I'd also like to try harder O-rings - perhaps a 70-shore (standar is about 60 shore and very hard is 90-shore) and even quad rings. But each of these experiments take time and customers are not too enthused by being guinea pigs.

 

My own fork is of the previous generation that's pretty bomb-proof in that department.

Edited May 20, 2011 by Johan Bornman

[Capricorn]

interesting JB. why's yours bombproof? By design or dabbling?

[Johan Bornman]

interesting JB. why's yours bombproof? By design or dabbling?

 

I wish I knew. It just is. The piston seals, stays sealed and never gives hassles. The newer Rox with a redesigned piston is the type that gives this type of trouble. One day when I have two of them open at the same time I'll do some measurements and comparisons.

[Ed-Zulu]

I wish I knew. It just is. The piston seals, stays sealed and never gives hassles. The newer Rox with a redesigned piston is the type that gives this type of trouble. One day when I have two of them open at the same time I'll do some measurements and comparisons.

 

Thanks Johan, I'll ask the LBS to do make the change this time. Will let you know if the problem is solved.