Covert 29 CTD Suspension Tuning
I have been underwhelmed with the lack of bottom-out resistance on the Fox CTD stuff on my Covert 29 (34 140 fork and CTD shock...both from the stock kit). In my quest to better handle my 240 lb riding weight without spanking the bottom of the stroke constantly here's what I've done:
I had been running 200 psi with Trail 1 compression. This gave me about 22% sag in Trail 1 mode where I ride 99% of the time. I would always tag the bottom of the shock multiple (and I mean multiple) times throughout my rides. If I went up in compression it just got too harsh. Descend mode was useless for anything with drops over 2' as the bike bottomed too hard.
The Fox shock has a rubber o-ring on the internal shaft which I assume is the bottom bumper. You hit the bumper with about 1/8" travel remaining. This means that my 2.25" stroke shock uses 2.125" before the bumper and that last 1/8" (0.125") is very hard to use. I have to intentionally slam my weight to the rear on a drop to ever get close to full travel (2.25" in this case). It has been this way on all Fox air shocks I have used. So...I am calling 2.125" the "bottom" on this shock as that is where you tag the bumper and to get that last bit of travel is very hard. I hit the "bottom" all the time...but it is never a hard impact that I feel on the trail. I just look down and the travel o-ring is always sitting at the end of the shaft. I slide it back up and it's at the end again after the next drop over 2'...it doesn't take much.
Long story longer...I bought the volume tuning kit from Fox. Part number 803-00-802 for the CTD shocks. (803-00-612 is the tuning kit for older RP23 shocks and will not work with CTD).
The C29 CTD shock comes stock with the 0.40 spacer. This is #2 out of 5 spacers when going from smallest (1) to largest (5). I just installed the 0.80 spacer (#4 out of 5).
I dropped my air pressure to 195 lb which now gives me 25% sag in Trail 1 and about 30% sag in Descend mode. Initial ride this morning shows that I came just short of hitting the bottom bumper as I didn't quite make 2.125" shock travel on the shaft. I will ride with this setup for now and see what happens. It did feel like it was more resistant to diving into the stroke and felt a bit smoother on the top as I was able to drop the air pressure a bit.
I have been running 105psi in Trail 1 mode. I don't really pay attention to sag on a fork but it isn't much...less than 20%. (Again, not sure). I just try to set the fork to match the "feel" of the rear shock. This is where the "feel" closely matched the rear shock in stock form. I have to be careful landing drops to flat 'cause if I slap the front down it bottoms the fork with a solid "thunk". Much, much more progression needed in the spring curve.
I opened the air chamber and found very little evidence of oil inside (hope I haven't damaged the seal). Fox recommends 5mL Float Fluid and I am guessing that would be less than 1/8" depth on the piston. During this procedure I also changed the oil in the lowers just in case that was low as well.
There is no volume tuning kit for the fork of which I am aware. It would be super easy as there is plenty of volume at bottom-out. Someone just needs to make some plastic spacers that somehow attach to the air-valve top-cap. I have just started adding Float Fluid to decrease air space. I started with 13/16" and am now at 1 1/8" oil depth. I take this measurement after the bike has been sitting upright and stationary for a time to create consistency. (That way all the oil is pooled at the bottom).
With 1 1/8" of Float Fluid I am now running 100psi in Trail 1 compression to match the new volume configuration on my rear shock described above. So far no harsh bottom but I have used full travel. At 90-95psi I still bottomed the fork with a "thunk".
I have no idea if adding fluid to the air chamber is a good or bad idea. I believe the fork uses a coil negative spring so there should be no air negative chamber with which the oil could cause problems upon migration. I will keep tabs on how consistent the oil volume remains. It could make it's way into the lowers, I suppose. When I pulled the lowers to change the splash oil the air side was blueish like it had Float Fluid in it. This may be why I had no fluid in the air chamber which could indicate that the piston seal is leaking. But...I never lose air pressure and after 3-4 rides the increased oil volume in the air chamber is remaining constant.
So far, my quest to increase the progression in my 2013 Fox CTD fork and shock appears hopeful. I only have one ride on the modified shock and 3-4 on the modified fork. The results thus far are positive. I will continue to play with spacers, oil volumes, and air pressures to fine tune the ride. I have yet to try Descend mode on the shock but am hopeful it will now be useful as I like the way it sucks up rapid fire bumps on rough descents.
If anyone has walked a similar path and has anything to offer, the assistance is greatly appreciated.
Edited to drop 5psi from the shock readings as I just found my gauge was sitting 5psi high. Same pressures just a different reading per my recalibrated shock pump
Last edited by titusquasi; 06-13-2013 at 11:35 AM.
Hi there! Good thread...
I think you must have had a fairly early production Covert 29. More recent shocks are shipping with the 0.6" volume spacer pre-installed. In either case, there is an element of rider weight and preference in the tune of the shock. Certainly no problem to increase the volume spacer for a more progressive feel.
At your size I might recommend sending the shock in to Fox to have it re-tuned for more compression damping, which would also help to keep you up in the travel with less air pressure. This is kind of what you are doing by riding the bike in the "trail" mode. I am not an expert on the Fox internals, but I think the shock should react better if you have a firmer shim stack doing the work rather than a closed off trail/climb circuit. Better oil flow, possibly less heat, etc...
I was just thinking I would try dropping the pressure a bit more and running the damping on Trail 2 or 3 (instead of 1). It may not feel any different but I like to tune.
Originally Posted by Transition Bikes
I just ran into a thread in the Suspension forum and found out that PUSH is about to release an aftermarket "volume adjust" for the 34 fork. I will be moving to this once it's released.
If I can't get things clicking to my liking I will probably send both the shock and fork to PUSH (not the first time) to help adjust things to my weight.
Regardless, when I'm feeling good and can push the pace the Covert 29 absolutely RIPS! I love how the thing flys over rough terrain, pumping and popping every feature I can find.
Titus, did you run the rear shock on previous bikes that far under your own weight? I run my rear shock at 185psi, equal to my riding weight with full gear. My normal setting is Trail 2, and it only bottoms lightly on big drops or jumps, but not hard enough to push the o-ring off of the end of the shaft. I have a TALAS 34 RLC up front that I run at 65psi in order to get full travel.
I'm around the same weight geared and have never really had good success with Factory tuned shocks. I typically send the fox stuff off to PUSH for tuning. My latest setup on my Bandit 29 is a CCDB air and Fox 34. I run just over 90 psi on the 34 in Trail mode setting 2. That seems to do mostly well but just doesn't have the high speed performance of the CCDB air. It is somewhat known that the 2013 CTD dampers had some issues where you would have to run higher pressures to keep from blowing through the travel. I called fox and there are going to change out the damper on my fork to a 2014 which is supposed to address some of these issues.
Do you have a link to this thread about the PUSH spacer? I cannot find it via the MTBR search.
Originally Posted by titusquasi
I probably weigh 225 w/gear, I run 200psi in the shock and ride it in the trail 1 setting. I will open it up for chunky sections but otherwise the trail setting feels good. I def use all the travel but have not bottomed it too hard, a few clunks.
titus- I think the trail 2 or 3 setting might just affect the initial travel tho not make it more progessive?, could be wrong about that tho. Of course a Push tune would solve the issue, I have always had great results with them and they make air forks feel coil smooth too.
Originally Posted by trailbrain
Pressure used in a rear shock is highly variable from bike to bike and shock to shock. It depends on the bike's suspension design and leverage ratio and the design of the shock being used.
Originally Posted by Bulldozer27
I would suggest always use sag as a reference. I usually try to run around 25% sag (the shock moves 25% into it's stroke when the rider is seated in the riding position...on a 2" stroke shock that is 1/2"). The Covert 29 uses a 2.25" stroke shock.
The Covert 29 is a lower leverage ratio design which should mean you may use less air pressure than other designs. On my ASR 7 and 575 I use around 260-270 psi.
Regardless of my weight I am running around 25% sag. I am very particular about measuring this and that now requires just 195psi with my current shock setup at my 240lb ride weight. I have the 0.8 volume spacer (larger than stock) in the stock Float CTD shock with the compression set on Trail 2.
My guess is that if you are running body weight on this bike the sag is very minimal...10-15%? That should make for a harsh ride but I am no suspension guru by any means.
Another variable to consider is that shock pumps can vary on what pressure reading they provide at a given actual pressure. I have seen pumps that vary 10-15psi from one pump to the next. So, my pump may read 200psi and your pump may read 215psi even though it's the same actual pressure. That's why going off sag is so convenient.
Another way to get in the ballpark is to use % body weight. At 195psi and a 240lb ride weight that is 81%. Your 185lb ride weight would be 150psi. Then again our shock pumps may be off...my 150 could be 165 on yours. Sag is still the best solution. Most bikes today are designed to run with sag in the 25-30% range for best performance.
In the stock configuration I was bottoming the shock far too often with sag in the 25-30% range. I ended up running around 22% sag and was still tagging the bottom constantly with a harsher ride quality. Thus the quest with the larger volume spacer to reduce the size of the air chamber causing the shock to ramp up faster through its stroke. I am now able to run 25% sag and have better bottoming control yet the ride quality is smoother due to the greater sag percentage.
Hope this helped and I am enjoying the exchange in this thread.
I think I'm finally getting somewhere with the fork's progression. I added more oil and am now at 1 1/2" of oil sitting atop the piston. I am using 100psi and the Trail 2 setting and the thing is finally using less than full travel on most hits. I still get there on the big stuff but it seems much improved.
That's a big reduction in the air chamber volume...what was Fox thinking? I just hope the PUSH solution has the capability to reduce the volume by at least this much.
Are they doing this as a warranty service or are you paying. If so how much? Is it just the damper change or will they change something on the air side to reduce the air chamber volume?
Originally Posted by trailbrain
Any info is greatly appreciated. Thanks.
Are you setting the sag with the rear shock in your normal riding position, Trail 2? If so, that is the problem and it's why you're running such a low pressure. I had a Specialized Epic, and the Brain shock dampens low-speed compression in a similar manner to the CTD shock. If I set the sag with the Brain engaged at my riding weight, the sag measurement would be less than 5%. Because of this, I had to back out the Brain to fully open when setting sag, and then reset to riding position. The sag measurement in Trail position isn't representative because of the damping. Set your 25% sag in the Descend 1 position (more or less fully open) and then reset to Trail 2 for riding. Try it; I'm certain you'll end up with a psi closer to 240 and bottoming out will be greatly reduced.
Originally Posted by titusquasi
Learning to Fly
Perfect thread Titus!
I'm right in the middle of this currently. I'm also a big dude at 6"3" 235lbs but riding a DB Mason FS (a preproduction color way test frame). Seriously loving it! But still working on the suspension too. The rear shock now that you mention it does use all it's travel all the time but I don't feel it bottom harshly and the O ring is almost always just on the shaft, hehe. I had good luck with the volume spacers on my Nomad w/ RP23.
I'm having basically the same issues with my 2013 Float 34 CTD wTrail Adjust. Have gone through all the same steps — more air for a too harsh ride in Descend, then less air for a plusher ride, run in Trail Medium for better platform that I ultimately blow through on bigger impacts that sometime results in getting pitched out the front door. I actually think it's a bit un-safe!
I just added 20cc of fluid to the air chamber and it made a HUGE improvement. I did a quick solo ride at the bike park (Duthie) and immediately felt much more confident, the fork wasn't going to bottom/fold beneath me. I ran in Descend mode with my usual 115psi, it stayed up in its travel and wasn't so quick to dive coming into jumps etc. Can't wait to get it in some bigger terrain to continue dialing it in. The +20cc really gives me hope for getting the fork riding well. Moving forward I think I'll loose 5psi as the progressive quality was good but it could still be a little plusher for more natural trail and continue to adjust the rebound.
Then there is the updated 2014 CTD cartridge.
I called Fox to see what they could do to help me with the issues I was having with my fork. The updated 2014 cartridge service was their only offering saying "I think that should help." It wasn't very confident sounding. The 2014 cartridge adds more platform/damping to the whole range of adjustments, which seems like a good thing, but would that alone deal with the diving issue? Would we still need to add air or take up volume to prevent the bottoming? The press junkets about the 2014 34 also mentions a revised air piston, but I was un-sure if this was for just for the Talas.
If anybody knows about the cartridge tune vs. air volume adjustment I'd love to hear about it. I've also made a call to PUSH to try to get info from them but no call back yet. Maybe I'll try an email instead.
Keep this thread going. I'd love to get the fork running right!
Ok...went through setup again to confirm my preferences. I am now running 200 psi in Trail 1 compression. This gives me just under 25% sag in Trail 1 and a bit less than 30% sag in Descend. I feel this is perfectly acceptable for the bike's intended performance. If I go any higher on the pressure the ride just becomes too harsh.
Originally Posted by Bulldozer27
At these settings with the larger volume spacer things are feeling pretty nice. The shock isn't just blowing through its travel on any drop over 18". It doles out the travel on an "as needed" basis. I still hit the bumper on bigger hits but never "feel" the bottom.
By going up 5 psi (from 195 to 200) the pedaling response feels a bit better and the ride quality is still acceptable.
Another variable I have noticed is the rebound setting. I don't know if this makes any sense but the higher the rebound is set, the more resistance to compression there seems to be. It seems the rebound is somehow tied to the compression circuit but I have no idea if this is true. I have the rebound set just a couple clicks from max to handle my weight. This seems to be adding compression resistance on its own. Maybe I am making this up.
Oh yeah...I'm liking the current modifications.
240-245 lb ride weight
34 Float 29: 1 1/2" oil height on top of the air piston. 100 psi. Trail 1 or 2 compression.
Stock Float CTD shock: 0.8 volume spacer (stock is 0.6 per Transition guys). 200 psi. Trail 1 compression.
I'm really liking how the suspension bits are acting at this point and will probably ride with this setup for awhile to gather a longer term impression.
I am still keeping an eye out for the PUSH volume reduction kit of which I have heard mention. I would like to accomplish some or all the air chamber volume reduction with something other than Float fluid.
That's a lot of rebound damping, could be causing things to feel harsh. For reference, I'm not quite as heavy but still run 200psi in the shock and have rebound set at halfway
Originally Posted by titusquasi
I usually try to run the rebound so it doesn't buck me off the saddle after a fast hit. I try to run it as light as I can but don't like getting tossed around on fast hard hits.
Originally Posted by rroeder
I will play with a lighter rebound setting and see how far I can tolerate the "bucking".
Thanks to all for the suggestions. I am taking every one and giving them a try or using them as an opportunity to reevaluate my set-up.
Ok...Rroeder and Bulldozer have me thinking again on my rear shock settings.
I may drop the volume spacer down a size (0.8 to 0.6 which is stock), run pressure a bit higher (shooting for 25% sag in Descend mode) and lighten the rebound setting a bit.
Good stuff here...thanks again everyone.
Yeah I hear ya on the bucking thing, if you run too much rebound than the susp rides too far into the travel at speed and feels harsh, too little and you get bucked on drops or steep rollers. That's the beauty of a shock like the Cane creek double barrel with serarate hi/low adj's, run the low setting slow and hi setting fast
Nice call on backing off the rebound on the rear shock. I took it out 4-5 clicks (forgot to count) and then backed the fork off until it matched the "feel". Harshness is improved and I still feel there is enough control to keep me in the saddle after a hard hit.
I reevaluated my sag to confirm desired pressure. At 240lb ride weight with 200psi in the rear shock I am at 27% sag in Descend mode and 25% sag in Trail 1 compression. With the larger volume spacer I get acceptable bottom control and am able to run sag in a range where harshness is minimized.
I only use Descend when I am going down steep rough terrain where I want the rear to sit down a bit and track the ground well. I am in Trail mode (1 or 2) 99% of the time so this is where I wanted my sag optimized around 25%.
With the optimized suspension bits and lighter wheels this bike is really coming into its own. I'm already looking forward to the next ride and I just stowed the helmet from today's outing!
I assume most people know, but it sounds like PUSH Industries is about to release their tuning options for the 34 forks. Part of this will be a user adjustable volume reduction kit that can be bought separately. I am checking their site daily as I'm tired of oil migrating out of the air chamber and changing my spring progression.
By Joe-it-all in forum Shocks and Suspension
Last Post: 03-26-2012, 05:17 PM
By macming in forum Shocks and Suspension
Last Post: 02-04-2012, 04:18 AM
By DeadlyStryker in forum Downhill - Freeride
Last Post: 10-23-2011, 02:42 PM
By ruralrider528 in forum Intense
Last Post: 07-04-2011, 12:00 AM
By magarnigel in forum Trek
Last Post: 02-06-2011, 03:37 PM