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Rockshox Pike: Really low PSI to hit correct Sag

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#41 ·
Huh, interesting. I'll add another step to your procedure to remove another variable - bleed the lowers prior to testing and at full extension. Depending on when the foot nuts seat, you'll get a bit of pressure build from the uppers going into the lowers.

Trying to think what failure mode of the MRP could result in what you're describing. Worst case it should act as all the tokens (port plugged) or no tokens (diaphragm failed). I don't think there's a way for air to leak out of it without going to atmosphere.

Which MRP model/version do you have?
 
Discussion starter · #42 ·
InVeloVeritas said:
Huh, interesting. I'll add another step to your procedure to remove another variable - bleed the lowers prior to testing and at full extension. Depending on when the foot nuts seat, you'll get a bit of pressure build from the uppers going into the lowers.

Trying to think what failure mode of the MRP could result in what you're describing. Worst case it should act as all the tokens (port plugged) or no tokens (diaphragm failed). I don't think there's a way for air to leak out of it without going to atmosphere.

Which MRP model/version do you have?
Click to expand...
That makes sense. and Model A

I just got back from an extended testing session doing laps totaling over 4 miles. I found the perfect spot that is down hill, but not too steep with plenty of medium sized roots, a couple of loose spots, 1 small jump, and 1 small drop. The whole loop is right around 1/4 mile.
The fork stayed plush the whole time. So much so I started playing around with adding some pressure and adjusting the sag setting.
I settled on 58 (analog gauge best guess) for 22% sag this with 1 token.
I fully get the earlier question of why 30% I can see now I was trying to make the fork more plush by aiming for that deeper sag when there was a larger issue.
With the current setting I didn't bottom out on either the drop or jump, but I'm sure a larger hit would so I'll put in a 2nd token next time I take it apart.
Next step is to call MRP ask WTF to try to figure out how this is possible.
Hopefully I can get a longer ride in tomorrow to see if it holds up.
 
#44 ·
Part of what can happen with 30% sag on a fork (particularly a longer travel fork) or more then 35% on a shock... is that you can be smashing into the wall of progression on the air spring. longer travel forks and larger stroke air shocks, are typically more progressive right off the start, simply because there is a larger stroke to compress the starting air volume. That's generally why you need less tokens in a 160mm fork then you do a 120-130mm fork. That issue is compounded as the longer travel products typically have larger stanchions etc.

That at times, can make the suspension product seem harsh and lead people to dropping air pressure, when in fact the right move is more air pressure and less sag. This allows you to absorb small/moderate bumps with a reduced spring rate, and save that wall of progression for really huge events when you're focused on maintaining the sketchy, instead of focused on how plush the bike feels.
 
#45 ·
Not that this solves the MRP issue, but it's interesting to read that RS backed off the baseline compression damping with the release of C1 (to compensate for the reduction in negative volume). Further reinforces that it's not a great 'upgrade' for a fork with the original stiffer tune.
 
Discussion starter · #46 ·
Update: Went on a longer ride today (good thing Covid has made me unemployed so I have all this free time to test suspension) I've now got over 20 miles on the fork, and it felt the same the whole time
:thumbsup:
I bottomed out on a larger drop which I was expecting so will need to add another token, and probably leave it alone until it's next 50 hour service
I had a nice long chat with MRP yesterday afternoon.
They're a small company and the guy that answered the phone is knowledgeable about all aspects of the products.
That is very refreshing.
We talked through the whole process I went through to come to the conclusion I was drawing. We both agreed the way the ramp control works It wouldn't cause my issue. It just doesn't make sense
However
"When you have eliminated the impossible, whatever remains, however improbable, must be the truth" -Sir Arthur Conan Doyle

So I filled out a service request and will send it back for inspection. We'll see if anything is wrong with it or not.

Through this process of research and youtube, and message forums I've learned a lot about suspension, and air suspension concepts. I've got plenty of experience tuning motorcycle suspension for road racing, but I've never taken forks or shocks apart.
How the negative side interacts with the positive side, and how the uppers and lowers move together now all makes sense.

But 1 thing I haven not seen a solid "this is how it's done" is the installation of the air spring. Referring to my picture in post 22 I have installed the air spring in the fully compressed position. This took a lot more PSI to hit desired sag number, and was much closer to the recommended starting PSI.
When installing the air spring in the fully open position which is what I have now I'm running the stated 58 which is really low for 180lbs (likely more)
I saw some videos of people doing fork rebuilds and they install the spring fully open (uncompressed) like I have currently.
:confused:
 
#47 ·
Glad you're on the right path, and seeing positives from what is surely a frustrating situation. The air spring is usually installed fully extended, because it's a pita to get the foot nut on with it compressed. With having the air shaft precompressed, you would be venting the lowers while pushing them on (footnut still removed as you install), effectively creating a second negative spring. It's something not commonly thought about - the bottom of the air shaft seal head acts as a second air piston as the whole assembly slides into the lowers. By sealing them fully compressed, you would end up with a vacuum in there instead of atmospheric pressure. Over time and riding that would decay as air slips in around the stanction wipers.
 
Discussion starter · #48 ·
Mick-e said:
I had a nice long chat with MRP yesterday afternoon.
They're a small company and the guy that answered the phone is knowledgeable about all aspects of the products.
That is very refreshing.
We talked through the whole process I went through to come to the conclusion I was drawing. We both agreed the way the ramp control works It wouldn't cause my issue. It just doesn't make sense
However
"When you have eliminated the impossible, whatever remains, however improbable, must be the truth" -Sir Arthur Conan Doyle
Click to expand...
I wanted to follow up on this thread.
I sent the MRP Ramp control unit back for servie after a couple of back and forth emails describing my problem, and efforts to resolve it and including a link to this thread.
A couple of weeks later I got a brand new ramp control unit in the mail.
Not a rebuilt version of the one I sent in, and I didn't get charged the $30 rebuild fee.
I reached out to MRP to ask what they had discovered, and why I got sent a new unit, ect ... , but I did not get any further communications
Sooooooo........ :p
Something failed inside the unit, and they don't want to talk about it.
Please see quote above.
:)

I have installed my new ramp unit. I am running the exact same air pressure as I was with my stock air head installed for the exact same sag, right around 23%. (And to follow up I am running the B1 air shaft, I don't have a need for extra travel so no C1 for now) The fork feels great and I can add bottom out ramp at will.
I've learned a lot.
Thanks to all who helped along the way
:band:
 
#49 ·
Mick-e said:
I wanted to follow up on this thread.
I sent the MRP Ramp control unit back for servie after a couple of back and forth emails describing my problem, and efforts to resolve it and including a link to this thread.
A couple of weeks later I got a brand new ramp control unit in the mail.
Not a rebuilt version of the one I sent in, and I didn't get charged the $30 rebuild fee.
I reached out to MRP to ask what they had discovered, and why I got sent a new unit, ect ... , but I did not get any further communications
Sooooooo........ :p
Something failed inside the unit, and they don't want to talk about it.
Please see quote above.
:)

I have installed my new ramp unit. I am running the exact same air pressure as I was with my stock air head installed for the exact same sag, right around 23%. (And to follow up I am running the B1 air shaft, I don't have a need for extra travel so no C1 for now) The fork feels great and I can add bottom out ramp at will.
I've learned a lot.
Thanks to all who helped along the way
:band:
Click to expand...
Hi Mick your thread helped me to change my C1 DebonAir to the previous generation. Thank you!
 
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