New Fox 38 suck down issue

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New Fox 38 suck down issue
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Posted: Jul 5, 2022 at 11:04 Quote
Hi All,

Just bought an ibis ripmo, and they only come with 38’s these days, I realize this is probably overkill. I’m a light dude at 155 or so, running about 70 psi, and 30-35% sag, but even on the hardest hits I don’t get very close to bottom out. Maybe only 75% of the travel is used, and meanwhile the bike alone seems to sit into the travel about 10mm. I would like it to be a bit less progressive, but also be fully extended when I’m not sitting on the bike. Might there be an issue with the negative chamber port? I have tried extending and using the equalizer valves to neutralize pressure to no avail.. any advise is much appreciated.

Thanks,
Mattias

Posted: Jul 5, 2022 at 12:08 Quote
Bigtony831 wrote:
Hi All,

Just bought an ibis ripmo, and they only come with 38’s these days, I realize this is probably overkill. I’m a light dude at 155 or so, running about 70 psi, and 30-35% sag, but even on the hardest hits I don’t get very close to bottom out. Maybe only 75% of the travel is used, and meanwhile the bike alone seems to sit into the travel about 10mm. I would like it to be a bit less progressive, but also be fully extended when I’m not sitting on the bike. Might there be an issue with the negative chamber port? I have tried extending and using the equalizer valves to neutralize pressure to no avail.. any advise is much appreciated.

Thanks,
Mattias

Yeah I'll try to help answer your questions based on my personal experience with my fox 38 and fox 40. Im going to assume you have set your compression and rebound to factory setting.

The 10mm sag at bike weight thing can happen. Both my 38 & 40 do it once in awhile. Not 100% sure, but I think its too much air in the neg spring during inflating. Assuming your air spring does not have a ridiculous grease build up then try this....

So, my fix is let the air out of the fork in 20ish psi increments. Cycle the fork to equalize the air each time and repeat. Pull the fork tall (fully extended) before adding air. Then when adding air go about 15psi and cycle the fork to equalize pos/neg chamber. Remember to fully extend fork each time and then add more air. Something about full extending the fork and even holding it extended as I add air seems to help. This works for me and my forks are butter smooth. There could be other techniques as well. Oh, use a bike tire pump to do the majority of the inflating, then a shock pump for the last few psi. Pro tip! lolll

So you're never getting full travel? But running 30-35% sag? You sure that much sag? Sounds like you are riding to deep in the travel and essentially bottoming onto the ramp up tokens. Must feel like crap. This is not a good set up.
I'd say start by running zero tokens and less sag (more psi), maybe around 20% +- sag. If you bottom then add a token. Repeat process. I know you might think more psi will be harsher and bottom less but thats not how the airspring works with less tokens. With less tokens you have more air. The fork will have more mid stroke support, likely feel much plusher, and less end rampup. With too many tokens people end up with too little air, no midstroke support, and riding at too much end ramp up (harsh).

Hope that makes sense a little. Its confusing to type that in as few words as possible.

Posted: Jul 5, 2022 at 12:10 Quote
Just to confirm: Have you measured the amount of exposed stanchion and it's less than the full travel of the fork?

Posted: Jul 5, 2022 at 12:53 Quote
So long as you have equalized the fork during inflation by compressing the fork enough that air can bypass the spring head into the negative chamber, you should be ok with pressure being equal on both sides. I do this about every 10psi increments during inflation. There is no right or wrong increment, just so long as you do it.

Your fork will sit naturally in its travel with no one on the bike. Your bike itself is sprung weight so it makes sense for your fork to sit into its travel slightly. Think of it this way: you have so little friction that the small weight of the bike (in comparison to body weight) can compress the spring. This is good. If you lift the front end up and the fork extends that 10mm, don't worry. If it stays compressed, you may have a vacuum in the negative spring and therefore you need to equalize the fork. I doubt this is the case.

The air bleed buttons on the back of the lowers do nothing to equalize the air chamber inside the fork. Don't confuse this with having any affect on the air pressure you put into the spring. The air bleeds are just for releasing any pressure that builds up in the lowers over time.

To change progressivity, remove volume spacers as suggested. You may need to add air pressure to adjust to the change in volume but wouldn't be much.

I don't recommended using a floor pump to pump up the fork chamber. The volume is so small a shock pump is more than sufficient.

Posted: Jul 5, 2022 at 14:18 Quote
CSdirt wrote:
Bigtony831 wrote:
Hi All,

Just bought an ibis ripmo, and they only come with 38’s these days, I realize this is probably overkill. I’m a light dude at 155 or so, running about 70 psi, and 30-35% sag, but even on the hardest hits I don’t get very close to bottom out. Maybe only 75% of the travel is used, and meanwhile the bike alone seems to sit into the travel about 10mm. I would like it to be a bit less progressive, but also be fully extended when I’m not sitting on the bike. Might there be an issue with the negative chamber port? I have tried extending and using the equalizer valves to neutralize pressure to no avail.. any advise is much appreciated.

Thanks,
Mattias

Yeah I'll try to help answer your questions based on my personal experience with my fox 38 and fox 40. Im going to assume you have set your compression and rebound to factory setting.

The 10mm sag at bike weight thing can happen. Both my 38 & 40 do it once in awhile. Not 100% sure, but I think its too much air in the neg spring during inflating. Assuming your air spring does not have a ridiculous grease build up then try this....

So, my fix is let the air out of the fork in 20ish psi increments. Cycle the fork to equalize the air each time and repeat. Pull the fork tall (fully extended) before adding air. Then when adding air go about 15psi and cycle the fork to equalize pos/neg chamber. Remember to fully extend fork each time and then add more air. Something about full extending the fork and even holding it extended as I add air seems to help. This works for me and my forks are butter smooth. There could be other techniques as well. Oh, use a bike tire pump to do the majority of the inflating, then a shock pump for the last few psi. Pro tip! lolll

So you're never getting full travel? But running 30-35% sag? You sure that much sag? Sounds like you are riding to deep in the travel and essentially bottoming onto the ramp up tokens. Must feel like crap. This is not a good set up.
I'd say start by running zero tokens and less sag (more psi), maybe around 20% +- sag. If you bottom then add a token. Repeat process. I know you might think more psi will be harsher and bottom less but thats not how the airspring works with less tokens. With less tokens you have more air. The fork will have more mid stroke support, likely feel much plusher, and less end rampup. With too many tokens people end up with too little air, no midstroke support, and riding at too much end ramp up (harsh).

Hope that makes sense a little. Its confusing to type that in as few words as possible.

Thanks so much for the thoughtful answer! I did not cycle it and inflate using increments! I’ll give it a go and see what happens. Out of the country for a couple weeks, but yeah I’ll probably also try removing any tokens also to see it that make it more linear.

Posted: Jul 5, 2022 at 14:20 Quote
mitch1992 wrote:
So long as you have equalized the fork during inflation by compressing the fork enough that air can bypass the spring head into the negative chamber, you should be ok with pressure being equal on both sides. I do this about every 10psi increments during inflation. There is no right or wrong increment, just so long as you do it.

Your fork will sit naturally in its travel with no one on the bike. Your bike itself is sprung weight so it makes sense for your fork to sit into its travel slightly. Think of it this way: you have so little friction that the small weight of the bike (in comparison to body weight) can compress the spring. This is good. If you lift the front end up and the fork extends that 10mm, don't worry. If it stays compressed, you may have a vacuum in the negative spring and therefore you need to equalize the fork. I doubt this is the case.

The air bleed buttons on the back of the lowers do nothing to equalize the air chamber inside the fork. Don't confuse this with having any affect on the air pressure you put into the spring. The air bleeds are just for releasing any pressure that builds up in the lowers over time.

To change progressivity, remove volume spacers as suggested. You may need to add air pressure to adjust to the change in volume but wouldn't be much.

I don't recommended using a floor pump to pump up the fork chamber. The volume is so small a shock pump is more than sufficient.

Thanks! Makes sense. All good info. I’ll keep y’all posted on The results In a couple weeks!

Posted: Jul 5, 2022 at 14:20 Quote
barp wrote:
Just to confirm: Have you measured the amount of exposed stanchion and it's less than the full travel of the fork?

Yeah measures 6” and change, should be 6.29

Posted: Jul 5, 2022 at 14:28 Quote
mitch1992 wrote:
So long as you have equalized the fork during inflation by compressing the fork enough that air can bypass the spring head into the negative chamber, you should be ok with pressure being equal on both sides. I do this about every 10psi increments during inflation. There is no right or wrong increment, just so long as you do it.

Your fork will sit naturally in its travel with no one on the bike. Your bike itself is sprung weight so it makes sense for your fork to sit into its travel slightly. Think of it this way: you have so little friction that the small weight of the bike (in comparison to body weight) can compress the spring. This is good. If you lift the front end up and the fork extends that 10mm, don't worry. If it stays compressed, you may have a vacuum in the negative spring and therefore you need to equalize the fork. I doubt this is the case.


I don't recommended using a floor pump to pump up the fork chamber. The volume is so small a shock pump is more than sufficient.

MTB air forks in stock form, should not sit 10mm into travel under bike weight. A tiny amount perhaps (1-2mm), but 10mm is not tiny or acceptable, and shows that the chambers are not equalized properly. Some people have tried to set up this way on purpose, but this guy isn't.

Also, floor pumps are 100% fine to use and insanely easier on a fork. You prefer to hammer away at a shock pump from zero to 80psi cool lolll!! Have at it! But work smarter not harder rule applies here. Wanna know where I learned this trick, from Fox.

Posted: Jul 10, 2022 at 4:23 Quote
Both the 38 and 40 have pretty lengthy break in times as well. As it breaks you'll get more use of the full travel. That said using all of the travel is not and should not be the goal. The goal is to get a good ride feel that let's your wheel stay in contact with the ground and reduce your fatigue. If you use full travel on your average ride you are not giving yourself any buffer if you @#$@ something up. I like saving the last 8-10% for truly disastrous cases or nose heavy landings etc.
Also, remember that sag is a static measurement and is really only useful as a starting point for spring rate. If you are unhappy with the feel of the shock then change something. You can shape the spring curve to get steeper at the end by removing compressible space (tokens, oil, neopos, etc.) to increase ramp up. You can do the opposite as well by increasing that space.
If I had to guess though, you're running too much sag and too many tokens. This keeps you at the foot of the steep part of the curve. Try adding some spring (5%ish) and removing a token.

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