Topic: CB550K Air-assist forks - keep or ditch?

[the nick]

Hi y’all!

I have a 1978 CB550K. The previous owner had installed air-assist fork tube caps, the type with an equalizer tube running between each cap. In my shakedown runs I have been rolling with about 6-8PSI in the forks (via bike pump) as recommended in other threads. My understanding was that with NO pressure they should act as stock forks. In my case, the forks are almost acceptable with air, BUT with no pressure, the forks bottom out on the slightest bump.

That is not normal/expected behavior, is it?

I’m set to do my fork seals and replace the fork oil this week. I imagine a lot of discovery will happen in this process. I suspect this problem is due to low fork oil and/or blown or leaky seals. Does that track?

Should I move forward with the air caps or just replace with OEM style and call it a day?

Thanks!

Nick

[scunny]

Replace your fork seals and oil, run the air assist caps and then decide.

[jonda500]

with no pressure, the forks bottom out on the slightest bump.

That is not normal/expected behavior, is it?

Unless you are extremely heavy, no. Fork oil will help, but only somewhat..
John

[the nick]

That’s what I figured. I’m about 200lbs, so nothing too crazy. Going with 10W fork oil this time around and “EZ” fork seals. I’ll try with the existing caps on, but I fear their seals may also be shot.

Thanks y’all!

Now just to find a new rear wheel 

[seanbarney41]

check your springs when you do the seals.  The manual specifies a free length and they were kinda ssoft even when new.  If you are bottomimg out frequently, that is a spring rate problem.

[the nick]

check your springs when you do the seals.  The manual specifies a free length and they were kinda ssoft even when new.  If you are bottomimg out frequently, that is a spring rate problem.

Thanks for the tip, Sean. Will definitely do so. Got ‘em apart right now and so far everything looks ok so far. The air caps are a bit brittle under closer inspection, I may just ditch ‘em. 

[HondaMan]

Most of these bikes have sacked-out front springs, too. The best way to check this is: bike on centerstand, lift the front wheel off the ground via the handlebars, let it settle back down (at the 'top' of their current height) and watch how far down they compress with the empty bike. If it is more than 3/4" then the springs need to be shimmed. Then, up under prop the engine so the forks are just off the ground, remove the fork caps, and see where the tops of the springs are sitting: if the springs are below the threads inside the fork tubes (they probably are) then shim back to between 1/3 and 1/2 the thread length using either short pieces of cut off pipe or stacks of large washers, or even cut off PVC pipe (I think 1/2" pipe fits?) pieces. When you install the caps, you should just be able to catch the first 2 threads when starting in the fork caps.

Oftne the aftermarket air fork cap kits applied NO shim length, so as to fit many forks. This meant that simply installing them made the forks worse off than they were with the solid caps. That's why I recommend drilling and tapping the existing caps for some threaded Schraeder valves, which will show up in my next book.

[the nick]

Most of these bikes have sacked-out front springs, too. The best way to check this is: bike on centerstand, lift the front wheel off the ground via the handlebars, let it settle back down (at the 'top' of their current height) and watch how far down they compress with the empty bike. If it is more than 3/4" then the springs need to be shimmed. Then, up under prop the engine so the forks are just off the ground, remove the fork caps, and see where the tops of the springs are sitting: if the springs are below the threads inside the fork tubes (they probably are) then shim back to between 1/3 and 1/2 the thread length using either short pieces of cut off pipe or stacks of large washers, or even cut off PVC pipe (I think 1/2" pipe fits?) pieces. When you install the caps, you should just be able to catch the first 2 threads when starting in the fork caps.


Oftne the aftermarket air fork cap kits applied NO shim length, so as to fit many forks. This meant that simply installing them made the forks worse off than they were with the solid caps. That's why I recommend drilling and tapping the existing caps for some threaded Schraeder valves, which will show up in my next book.

THANK YOU sir!

I just finished the fork refurb today. Both springs measured 469mm, did not reveal any threads, and required firm pressure to seat the fork tube bolts and thread them in. According to my Clymer manual, this measurement is far above even the stock 450mm length and far above the 425mm “replace” bottom limit. I’m pretty surprised, as the previous owner had commuted daily 75mi on the bike and had to be at least 300lbs. Perhaps the air forks saved the springs but trashed the seals?

Unsurprisingly, I pulled 150cc of fork oil from both forks combined. And each fork drained a different color of dirty oil!

The air caps are indeed the good kind - they appear as old as the bike and appear to be OEM bolts drilled and tapped for Schrader valves with an equalizer tube between them.

I may still try PVC pre-load shims, depending on how the bike rides. I’m still waiting on my wheels from the shop, so it’ll be a wait and see situation. Looking forward to experimenting!

Thanks!

Nick

[754]

10 W oil seems light.. to me anyway.  We used to use ATF a lot ..

[robvangulik]

+1 on ATF, but ATF and power steering is 10W too and so is most hydraulic oil.

[the nick]

10 W oil seems light.. to me anyway.  We used to use ATF a lot ..

I used Amsoil Shock Therapy “Medium #10”. Amsoil says it is closest to 10W, but not exactly 10. Not gonna lie, it’s what was available at the local shop - mostly Harley baggers up here in Northern MI, and that’s what the shops cater to.

The Amsoil website lists equivalents as follows:

Spectro SK400
Maxima Fork Oil 10
Ohlins #15
Spectro Fork Oil 10W
Redline Medium


If I’ve erred too light, I’m game to change it out. Still got 5 days til the wheels go on anyway!

[Erny]

Most of these bikes have sacked-out front springs, too. The best way to check this is: bike on centerstand, lift the front wheel off the ground via the handlebars, let it settle back down (at the 'top' of their current height) and watch how far down they compress with the empty bike. If it is more than 3/4" then the springs need to be shimmed. Then, up under prop the engine so the forks are just off the ground, remove the fork caps, and see where the tops of the springs are sitting: if the springs are below the threads inside the fork tubes (they probably are) then shim back to between 1/3 and 1/2 the thread length using either short pieces of cut off pipe or stacks of large washers, or even cut off PVC pipe (I think 1/2" pipe fits?) pieces. When you install the caps, you should just be able to catch the first 2 threads when starting in the fork caps.


Oftne the aftermarket air fork cap kits applied NO shim length, so as to fit many forks. This meant that simply installing them made the forks worse off than they were with the solid caps. That's why I recommend drilling and tapping the existing caps for some threaded Schraeder valves, which will show up in my next book.

THANK YOU sir!

I just finished the fork refurb today. Both springs measured 469mm, did not reveal any threads, and required firm pressure to seat the fork tube bolts and thread them in. According to my Clymer manual, this measurement is far above even the stock 450mm length and far above the 425mm “replace” bottom limit. I’m pretty surprised, as the previous owner had commuted daily 75mi on the bike and had to be at least 300lbs. Perhaps the air forks saved the springs but trashed the seals?

Unsurprisingly, I pulled 150cc of fork oil from both forks combined. And each fork drained a different color of dirty oil!

The air caps are indeed the good kind - they appear as old as the bike and appear to be OEM bolts drilled and tapped for Schrader valves with an equalizer tube between them.

I may still try PVC pre-load shims, depending on how the bike rides. I’m still waiting on my wheels from the shop, so it’ll be a wait and see situation. Looking forward to experimenting!

Thanks!

Nick

About spring length - do not 100% trust Clymer! Only trustable source is official Honda service manual, bur even this have some typo errors.

There were several types of forks for 500/550 from K0 to K3. I had the same pb with spring length with my 550K1.
Even Honda service manual did not provide correct info. Finally I found it by chance combining manuals from 500/550K four and 500T.

If I rmemeber well there is some thread about it here on forum