[brentos]

A couple days ago, I posted a question about offset shock hardware and recommended manufacturers. After checking the clearance, it looks like only one eyelet on the shock has enough clearance to fit offset hardware, for a modest 2mm shortening.

This will come up short of what I'm hoping to achieve in order to lower & slacken the bike for my son, who will run 155 or 160 mm cranks. Which leads me to the next question. What are my options for a shorter shock?

1) Retrofit an air can intended for a 152x32mm Fox shock. Every way I think about it this will work, shortening the shock by 6.XXmm (.25"), effectively turning it into a 158x32mm shock, with the leverage ratio it will reduce travel by 15mm. I'm OK with the travel reduction to optimize geometry. I'd combine with one offset shock hardware (front eyelet), for a total of 8 shorter effective shock length. It looks like the added length vs. the 152mm shock would be in the length of the damper body extended out from the air can. Frame clearance is good all around. Looking at shock cross-sections, seems like there wouldn't be any issues from a shock perspective either.

Is there any reason shortening travel with just an air can won't work?

2) Purchase a 152x32mm shock. These are hard to find, it looks like the only manufacturer that even intends to make one again is Manitou (Mara). Although I'd prefer the additional BB drop associated with this option, there aren't any available for sale, and I can't find any on the used 152mm shocks. Even if the Mara was available, I'd be looking at $350 more than just retrofitting a shorter air can to the existing shock. This would lower the rear about 29mm, so about 20mm at the BB, which would be perfect. Frame/tire clearance is plenty for shorter shock length at full compression.

Does anybody know where to find a decent 152mm x 32mm shock that won't break the bank?

Thanks, looking for answers to the questions above, other options.

 

[seitenryu]

The bearing/shaft sealhead will top-out against the inside of the air sleeve. You need to replace the air sleeve, shaft and damper rod(s) to accomplish what you want. If the sleeve is too long vs the shaft, the equalization port will never pass the air seal.

You can adjust the stroke, within the same overall shock length, but you can't shortcut the opposite. At least not without some fabrication capability. The exception would be mounting an extra long body for clearance due to a unique frame design.

 

[PhillipJ]

You should be able to find a Monarch in 152x31 which is the same inch dimensions rounded differently.

 

[brentos]

Thanks for the reply, I very much appreciate it!

Two questions:

1) Wouldn't the equalization port be moved further into the travel, rather than going into the region of negative travel, and therefore still be accessible in the range of travel? Passing the transfer port doesn't seem like it would be a concern.

2) I can see where the shock would top out against the air sleeve (closing of the green rectangle to zero volume), but wouldn't the air in the negative spring prevent that? I had assumed the bottom out mechanism on modern air shocks was where the negative spring force and positive spring force equaled, but is this not true?

As I interpret your statement, the bearing/shaft against the seal head is the designed top out point. I can see where my assumption would have been wrong, but if that were the case, wouldn't top out be a "thunk" if it were a mechanical hard stop? Either way though, I can see where a different seal head/busing that has a different location of the air can seal would give me an unanticipated result.

Is it possible that these two max stroke limiters are designed to be in nearly the identical point in travel? Then I would be able to manually extend the shock a bit further than it's natural settling point, but I'd be OK with that as it would never extend beyond the 165mm eye to eye hard stop on the bushing/seal head when unleaded. I'll see if I can track down the fox engineering print for when they still made the shorter travel DPS shocks to see if the bushing P/N is different.

Again, I appreciate the response!

 

[brentos]

Looks like the bearing assembly that holds the air seal is the same for each, but the Shaft/Rods are different.

-So if the top out point is equalization of Neg/Pos spring force, then I would likely get what I'm hoping for.

-If it's the damper rod/bearing, then I would only get a shorter air spring with less volume and the air can hitting the bearing.

I could be waay off in how I interpret your response though. Thanks again.

 

[brentos]

You should be able to find a Monarch in 152x31 which is the same inch dimensions rounded differently.

That would be ideal, if I could find one. I am more comfortable doing a custom tune on a Monarch than on a Fox Boost Valve for my lightweight son.

 

[seitenryu]

That would be ideal, if I could find one. I am more comfortable doing a custom tune on a Monarch than on a Fox Boost Valve for my lightweight son.

While the air system is part of top-out, ultimately it's a mechanical+hydraulic limit. At full shaft extension, the damper shaft rebound orifice passes into the bearing, generating progressive rebound damping to cushion the mechanical impact. If it can't extend, that effect is limited if not disabled. It won't sit at full extension after equalizing air pressure, but you won't have the extra protection either.

You could install a -200 series 70D o-ring(pick a size that fits snug) just below the sealhead before installing the air sleeve as insurance. It may very well work out, since the air pressure for a kid will generally be low.

What I don't know is whether ifp pressure, unsprung weight, and air pressure together can cause a hard top-out against the sleeve in this condition. Maybe someone can confirm that.

All in all, it might be fine as a bodge, but it's not the pro way to do it.

The parts to convert to that size are available, if your shock is relatively new, but it'll cost you a bit to get it rebuilt.

 

[brentos]

Great insight, I’m glad I asked. I didn’t realize the damper had a hydraulic top out function.

It looks like it would be a $70 experiment that might not give me the result I want at the slight risk of damaging the shock. I might still try just to see.

But it’s clear that a 6.0x1.25 shock is the best option. Onward with the search for one, it kind of seem like an OEM only part though.

 

[earlan357]

I'm wrapping up my son's full suspension trail bike project, a 2010 Yukon FX. I dropped the fork travel from 100mm to 80mm and wanted to do the same with the rear shock to keep the geo right and drop the BB. It has a 10 year old (like new) Giant branded 165x38 air shock which does not have an equalization port or negative chamber. I machined a 7mm nylon spacer and put it on the stanchion above the topout bumper to effectively make a 158x31mm shock. Combined with a 3mm offset bushing, this reduced the rear travel by 20mm and BB height by 28mm. He's only 65 lbs so we get 20% sag at 70 psi. At such a low pressure, the lack of negative volume isn't noticeable and the rebound is effective enough to control harsh topout. We're just riding single track but as he grows I plan to just replace the shock to restore travel, so this seems like a good stopgap in the meantime.

Here's the shock I'm using. Occasionally I see them pop up online for like $80.

Another option might be a Rockshox Monarch with high volume Debonair can upgrade. The EQ port is higher, at about 15% stroke vs 9% on the standard can. People report that the Debonair can has excessive sag on bikes with high leverage ratios and feels too linear. I took advantage of this when I swapped the 184x44 for a 190x51 in my old Stance. The old aircan would sag at 25% at 200psi. The Debonair liked to sag around 30% from 240-270psi. So despite the higher static BB, the height was the same when pedaling. I did need to pack the positive chamber with air volume bands to get regain the progressiveness the bike wanted though.

 

[brentos]

Thanks for all the ideas.

UPDATE:

I found a NOS 5.5x1.0 shock that I wanted to try on the bike, my fear was that the BB would be too low and the seat tube too slack though.

Before I mounted that shock, I tried the air can from the 5.5x1.0 shock on the 6.5x1.5 shock. The result was almost exactly as I anticipated. Adding the air can from the 1" shorter shock cut 1/2" from the overall eye-to-eye length and resulted in a 1/2" reduction in stroke. Makes sense, the air can is.... .5" shorter.

There is no hard topout noticeable, the negative air chamber seems to be preventing that. When I try to extend it beyond it's natural resting state by holding the bike wheel down and pulling up on the saddle it extends only a 1/8" further without any mechanical top out noticeable.

It also feels very linear, great for my son's light weight. I did leave out a .2cc spacer that came on the 5.5x1.0 shock. According to the fox tables, the compression ratio should have gone from 3.0 to 2.3.

I'm missing out on .25" of shock stroke that I would get from a 6.0x1.25" shock, but this is a good interim solution until one shows up. When it does, that 5.5x1.0 might go on my BOB trailer.

I'll also try that 5.5" shock on there to see how the bike rides for my son with the super short shock on there, but 6" eye-to-eye looks about perfect. With that and the shortened crank, it looked like a very natural fit for my 4'8" son.

Now I have to tear into the fork (2011 FIT RLC w/coil negative) to see what I can do there.