[kebarb]

Hi guys!

Posting here because i have noticed there is alot of good knowledge here on the DHX RC4 and the Enduro bike from specialized.

Some time ago i obtained a green Specialized Enduro Carbon 2011 model, and not knowing what to do with it i started rebuilding it from scratch so that i can use it as gravity toy. Its not the EVO model. Process is well on the way, and i am now working on the shock.

In this process i noticed there is Fox Coil options:
Enduro bike with coil shock

So i got myself a new oldstock RC4 and bought new shaft, reduced the length of it and the stroke. I now have a 8,5x2,25 shock that i want to adapt to the Enduro Frame and gravity riding.

Since there seems like lots of people done this before me, i would like to know how to adapt the boost-valve and the shims on the main piston to suit the frame best. Right now it seems to have standard base tune. It looks very similar to this, only i measured slightly different thickness on some shim.

Compression, measured in mm. assumed inch. Similar to the one in another thread here on ridemonkey. This shock came off a KTM Aphex DH bike that was never built up. However the shock has no label for the tune or anything like that.

.900" x 0.15mm (0.006")
.800" x 0.12mm (0.005")
.700" x 0.12mm (0.005")
.600" x 0.12mm (0.005")
.500" x 0.12mm (0.005")
.400" x 0.26mm (0.01")
.400" x 0.26mm (0.01")
.400" x 0.26mm (0.01")
Backing plate

Does anyone here have a good starting point for this shock on this frame? I would be very happy for all input on this subject please. I am doing full rebuild, so a suggestion on good starting point for shimstack would be awesome.

Original RP2 shock has LOW velocity tune and MEDIUM rebound, 250 boost valve.

Also any input on how to evalute boostvalve (position sensitive damping) need/disabeling.

EDIT: Basic shim modelling info:
webpage describing different shim configurations

//K

 

[Happymtb.fr]

You might find some info here

Anyone have stock shim stacks for a Fox RC4?

I have a couple of RC4 shocks that I'm going to lighten up the valving on for lower leverage frames. They're both normal OEM spec (as far as I know). I'll probably have to order some shims, so I'd like to know what's in there first. Any baseline low leverage stack suggestions are also...

ridemonkey.bikemag.com

The bike is not so progressive, almost linear, which might make a coil setup tricky depending on your riding style...

Specialized Enduro 2010

Seguimos con las Novedades de Specialized para el 2010. En esta entrada voy a hablar sobre la Nueva Enduro de 160mm de recorrido. La nue...

linkagedesign.blogspot.com

 

[Redwood Mulch Chucker]

Nice, looks like the big shaft RC4 so that will be good in this case.

I don't remember valving specifics, but like @Happymtb.fr said the biggest issue will be the leverage curve with the kick up at then end of travel. I got an RC4 coil to work great on that frame, except it would bottom out very easily.
To help with this you can have the IFP volume be small (and a high pressure) so it will add some end stroke ramp up. Maybe try and source a larger/stiffer bottom out bumper?
Maybe a progressive coil spring as well? Those weren't really available back when I tried.

For valving I would get a standard piston and ditch the boost valve stuff for sure.

What does the base valve look like (under the reservoir)? With the large diameter shaft, a lot of oil is going to flow through the base valve and it can produce lots of damping.

Be sure to use red loctite on the piston! Due to the clevis design and frame misalignment, I had issues with the piston coming loose while riding. It would un-thread itself.

 

[kebarb]

For valving I would get a standard piston and ditch the boost valve stuff for sure.

What does the base valve look like (under the reservoir)? With the large diameter shaft, a lot of oil is going to flow through the base valve and it can produce lots of damping.

Thanks for reply guys!

I am thinking about this boost valve. If the frame has this leverage change in the end, will that make the boost valve good or bad then? Will it be difficult to reach full stroke with boost valve still present?

I was googling for some info on removal of the boostvalve, but i didnt find any. Is there a easy guide for this? I am assuming its located under piggyback tube. Is there another piston there too as you mention, with shims?

EDIT: Found post with shock guru Udi and he typed:
The other fun thing you can do is lop out the boost valve if the frame has sufficient EOS progression, it generally results in better handling of square edged impacts, particularly high amplitude ones. Just squeeze the circlip shut (mind the reservoir walls) and remove the clip followed by the valve.
removal of boost valve FOX RC4

//K

 

[kebarb]

You might find some info here

Anyone have stock shim stacks for a Fox RC4?

I have a couple of RC4 shocks that I'm going to lighten up the valving on for lower leverage frames. They're both normal OEM spec (as far as I know). I'll probably have to order some shims, so I'd like to know what's in there first. Any baseline low leverage stack suggestions are also...

ridemonkey.bikemag.com

The bike is not so progressive, almost linear, which might make a coil setup tricky depending on your riding style...

Specialized Enduro 2010

Seguimos con las Novedades de Specialized para el 2010. En esta entrada voy a hablar sobre la Nueva Enduro de 160mm de recorrido. La nue...

linkagedesign.blogspot.com

Thanks for reply!

I am old nowdays, and dont ride so much. I dont have a ridestyle any more =)

I like these old retro projects to build bikes that was "the shit" when i was still active rider.

However, i will ride tracks you probably are familiar with, like Shimano, Kälkspåret, Flinbanan, Motown, Lilla Blå and these kind of machine made tracks. So i guess i prefer alot of support mid stroke, and low speed for landing the jumps but i dont need that big hit and rock eating stability for the black DH tracks. However i always enjoy a setup that eats the small chatter and brake bumps.

//K

 

[kebarb]

Updata after some research:

I have avalible also a shimstack from Session 88.
And when comparing the leverage ratios of these 3 bikes / shocks they are the most comparable.

The KTM Aphex seems very different from the Enduro.

Would the Session 88 shimstack be usable on the Enduro setup?

 

[Jm_]

That does not look like a good candidate for a coil shock, much too flat, basically the same ending LR (very, very slight change).

I don't think you'd be getting anywhere near the amount of ramp-up you need. Shoot for 25-30% as a good starting point for coil. 20% is not optimal, but can be made to work with more aggressive bottom-out bumpers, progressive springs, etc. Even the bumper doesn't necessarily make it function better, specially shaped ones from Push and Avalanche create some ramp up in addition to just basically bottoming protection. Hydraulic bottom-out systems are also just that, it's going to be like running into a wall if the bike doesn't have a rising rate leading into it. I don't think it's worth it at your LC.

 

[kebarb]

That does not look like a good candidate for a coil shock, much too flat, basically the same ending LR (very, very slight change).

I don't think you'd be getting anywhere near the amount of ramp-up you need. Shoot for 25-30% as a good starting point for coil. 20% is not optimal, but can be made to work with more aggressive bottom-out bumpers, progressive springs, etc. Even the bumper doesn't necessarily make it function better, specially shaped ones from Push and Avalanche create some ramp up in addition to just basically bottoming protection. Hydraulic bottom-out systems are also just that, it's going to be like running into a wall if the bike doesn't have a rising rate leading into it. I don't think it's worth it at your LC.

Sorry i cant follow you here. 25-30% of what?

And i dont understand the short LC?

Thanks for the post!

 

[Happymtb.fr]

The boostvalve will help resist bottom out if I remember well so I would keep it in your case.

The shimstack from the session is not a good starting point since the frame has a longer shock stroke than yours and is more progressive.
A progressive spring might help if the progression starts early enough.

Avalanche bottom out bumper was harsh when I tried it on a not so progressive frame... I would not recommend it. FWIW

 

[kebarb]

This is the shimstack from the 2011 Trek Session 88.
There is quite big difference between this and the stock stack from the Aphex shock shimstack.

Noticable is that this stack has only 7 shims. But total stack height is 1.35, comparable to the 1.41 from the Aphex.



.900" x 0.15mm (0.006")
.800" x 0.12mm (0.005")
.700" x 0.12mm (0.005")
.400" x 0.24mm (0.01")
.400" x 0.24mm (0.01")
.400" x 0.24mm (0.01")
.400" x 0.24mm (0.01")

Would i be correct to assume that the Session stack has much less compression in the shimstack compression side compared to the Aphex, and that is because the leverage is much lower on the Session? I cant really wrap my head around he result here.

If to choose one of these stacks for a Specialized Enduro, what stack would you guys pick?

//K

 

[kebarb]

The boostvalve will help resist bottom out if I remember well so I would keep it in your case.

The shimstack from the session is not a good starting point since the frame has a longer shock stroke than yours and is more progressive.
A progressive spring might help if the progression starts early enough.

Avalanche bottom out bumper was harsh when I tried it on a not so progressive frame... I would not recommend it. FWIW

Yeah the session has longer shock/stroke, but also longer travel. The leverage (session and enduro) looks pretty similar for a part of the path doesn't it?

Difference is in the end, and i agree with you that maybe the boostervalve should be left intact if it will counter the risk for bottom out. The DHX 5.0 had something like that, but it works way different, and also the Manitou SPV shocks had something similar that again was completly different design. They all try to do the same. As i didnt completly pick apart the RC4 yet i didnt study how the boostervalve of the RC4 works. But i can understand that it has a rising flow resistance with the rising internal pressure of the shock?



//K

 

[kebarb]

I found out that Ironhorse Sunday has a very similar looking profile of the leverage ratio.

And difference from start to end is same, about 0.3.

So i guess that same problem as there was fitting a shock to the Synday, will apply on the Enduro.

 

[Redwood Mulch Chucker]

*edit, I would keep the boost valve for your use.
I forgot that it is quite different in design between coil and air shock dampers, it will be beneficial for your use.

It is basically a valve that increases damping with increases in IFP pressure. It limits the flow through the piston and provides more damping the farther you get into the travel (as the IFP volume gets smaller and pressure increases), it is held open with a spring at top out for minimal effect initially. It is essentially position sensitive damping.

I would put the shock together as is, find a spring that gives you good sag with minimal preload, and then try and tune the bottom out with IFP pressure and volume changes. Small changes of +-10 psi will have an effect.

Too much pressure and the BV will kick in too early and make it harsh.

 

[StiHacka]

So, quick question regarding fixing subotimal leverage curve with boost valves / big bumpers / hydraulic lockouts etc. - what's the impact of these on the rebound when recovering from deep travel? Sluggish feeling, more likely to pack up, staying low in travel?

 

[englertracing]

Shimrestackor

 

[kebarb]

So, quick question regarding fixing subotimal leverage curve with boost valves / big bumpers / hydraulic lockouts etc. - what's the impact of these on the rebound when recovering from deep travel? Sluggish feeling, more likely to pack up, staying low in travel?

I am curious about that too. Rebound is another thing i cant wrap my head around, because in my imagination the spring will be the main issue when tuning the rebound. And also the weight of the rear wheel and the swingarm. However i cant understand how the rebound should be tuned according to the frame, other then from that. What factors are important?

//K

 

[kebarb]

Shimrestackor

Do you know how to use that app/script?

Anyone here has access to it?

//K

 

[kebarb]

The boostvalve will help resist bottom out if I remember well so I would keep it in your case.

The shimstack from the session is not a good starting point since the frame has a longer shock stroke than yours and is more progressive.
A progressive spring might help if the progression starts early enough.

Avalanche bottom out bumper was harsh when I tried it on a not so progressive frame... I would not recommend it. FWIW

Hi!

Choosing between the Session88 and the Aphex shock as a starting point, it looks to me as if the Session88 is more similar to the Enduro then Aphex frame, but maybe i am wrong looking at the profile rather than looking at the leverage numbers alone?

Seems that FOX teamed up with Trek to make a custom tune for the ABP linkage:
FOX and Trek team up with RC4 build.

I dont know if they have altered anything else in the shock, or if it just the shimstack configuration?

I am very curious about if there is only one model boost valve, and one model of the compression adjuster unit, or if these are tunable too? (apart from just removing the boost valve ofcourse).

To me the Session 88 looks like a good candidate, just i need to add some MOAR SHIMS to stiffen it up a bit?

However, as mentioned, i dont know if there is other things they changed in this shock.. apart from just the shimstack.

Also i dont know what part of the damping is taking place on the shimstack in this shock. Can i assume that most of the low speed damping is taking place between body and piggyback? And maybe the main piston is rebound damping and high speed damping?

Anyone with insight on this can feel free to correct me.

Thanks

//K

 

[englertracing]

Do you know how to use that app/script?

Anyone here has access to it?

//K

yes, but its not magic.
you basiclaly want to ride what you have, and decide what you think it needs.
something like
"the low speed is good but it doesnt have enough HSC"
then you can try to create a shim configuration that has the same LSC, but 20% more HSC and try again.

 

[Jm_]

My RFX was only about 16% progressive, with a tuned RC4, including shaped bottom-out bumper. The problem was that while it wouldn't bottom, it would blow through the travel pretty easily on bigger drops, so you still had a pretty hard end-event. It feels a lot better to drop on a +20% progressive frame, but the damping obviously has to match (rather than just a progressive spring).

 

[kebarb]

I checked to original shock today. it says:

Original RP2 shock has LOW velocity tune and MEDIUM rebound, 250 boost valve.

So it seems the compression for that shock is on the lower side, but maybe that it ramps up with pretty high boost valve and air shock?

What does the "boost valve" mean on the markings of the shock, is that pressure in chamber of the shock?

//GF

 

[kebarb]

I found a compression shimstack that according to poster was configured way to stiff for Banshee Rune in another thread, he didnt mention what bike i came off, but sure looks stiff:

.900" x .25mm (0.010")
.900" x .15mm (0.006")
.900" x .15mm (0.006")
.900" x .15mm (0.006")
.700" x .15mm (0.006")
.500" x .15mm (0.006")
.400" x .15mm (0.006")

Another guy claims this:
Most of the "low" tunes I have seen are roughly 1/2 of the "medium" tune. So either remove half of the shims or say if the shims are 0.15mm thick then replace every shim with about 2x 0.1mm shims. (1 x 0.15 shim = stiffness of ~3.4 0.10mm shims)

So maybe what i see is that the Session 88 is a Low tune, and that it is about 50% of the other one that is M tune.

//K

 

[Udi]

@kebarb responding to your pm -

Udi and he typed:
The other fun thing you can do is lop out the boost valve if the frame has sufficient EOS progression, it generally results in better handling of square edged impacts, particularly high amplitude ones. Just squeeze the circlip shut (mind the reservoir walls) and remove the clip followed by the valve.

EOS = end of stroke, so in your application I'd leave the boost valve in (at least initially) as others suggested.
I'd just try the shock as-is with whatever base tune is present before deciding what to change (or pick a base tune from a similar LR frame), I'd also start with minimum chamber volume (IFP depth set as standard, then B/O adjuster fully closed) and set minimum chamber pressure.

I wouldn't stress over bottoming out, coil shocks have the benefit of higher sensitivity and flatter spring curves, which together mean that you can use a higher spring rate if needed to control bottom out while still maintaining some benefits. A taller bottom out bumper is also a valid option like Jm suggested. Just pick the lowest spring rate you can get away with that doesn't smash bottom on the biggest landings you personally encounter. The way you describe your riding (similar to mine these days) will work in your favor here too.

So, quick question regarding fixing subotimal leverage curve with boost valves / big bumpers / hydraulic lockouts etc. - what's the impact of these on the rebound when recovering from deep travel? Sluggish feeling, more likely to pack up, staying low in travel?

Leverage curve progression means the progression applies to both spring and damper, whereas boost valves and HBO affect damping only (bumpers have a minor spring component). Biasing this ratio towards damping means more energy dissipation and less energy return, so everything you said results - in my experience the most noticeable one is less pop off jumps / lips, especially small to medium ones. Can actually have some benefit if you're trying to go as fast as possible, but can also be a killjoy if just trying to have fun.

Worth noting that all listed items (BV / HBO / bumpers) function and behave quite differently from each other in terms of that spring/damper ratio (particularly in position-sensitive aspect).

 

[kebarb]

Thanks for reply guys!

Can i ask then, what would you suggest between using a slightly stiffened Session 88 stack vs the standard stack?

Session has alot of similarities during first 80mm of travel, but in the end they separate their paths.

//K

 

[kebarb]

Just as a note, i rode the Enduro this weekend with the standard RP23.

I filled it up according to weight, and SAG was correct. I was amazed about how well it handle the light DH riding.

Then after a few runs i noticed the climbswitch was active, and if turning it off i would bottom this shock very easily. This is a MEDIUM tune RP23. I can understand that this shock tune works Ok for trail riding. however didnt work out that well for DH / FR =)

Lets see how this translates into the RC4 build.

//K

 

[Happymtb.fr]

@kebarb try to measure the sag again on the RP23 without the climb switch. If you bottom easily without it it means that the spring curve is wrong. According to your comments I would go for less sag and/or add volume spacer to the positive chamber of the shock. How much sag did you measure?
Not sure if your RP23 has an Evol air can, but that could help as well

 

[slimshady]

@kebarb try to measure the sag again on the RP23 without the climb switch. If you bottom easily without it it means that the spring curve is wrong. According to your comments I would go for less sag and/or add volume spacer to the positive chamber of the shock. How much sag did you measure?
Not sure if your RP23 has an Evol air can, but that could help has well

I'd also say the Enduros (at least the ones with the X in the front triangle, pre Demo rears) didn't like anything above 22-25 % in terms of sag. And as @Happymtb.fr pointed out, if you set sag with the pedal assist on, you're probably on 30%+ sag after disengaging it.

 

[kebarb]

Thanks for mentioning about the climb switch! I didn´t use air-shock for maybe 10 years, and really i have no clue. I just tried it because i still didn´t yet put together the RC4 shock for this bike.

I will have another look at the RP23, but also maybe this shock is not intended for DH riding? I think i would enjoy it with climb switch off on trail riding. But in the bike park it kind of blows through the travel.

Anyhow, i have to admit, that the RP23 was not such a bad shock for that stuff anyways. And it fits pretty good with the TALAS 180mm up front. Maybe it will serve its purpouse as alternative item for lighter build with carbon wheels. Right now i think the bike is around 13.5kg with deemax and minions.

I am thinking to go FLOAT on the TALAS but still didnt get my hands on the top-cap. (2013 - not avalible in shop). Talas performed Ok. Not much to complain about.

Riding the V10 after the Enduro was still like riding a soft livingroom couch.

//K