Topic: Stiffening early CB500 forks? Front fender hitting exhaust?

[BakonBittz]

Finished my first CB500K1 and it rides beautifully! Only one issue I would like to correct: The front forks seem way too soft. Even to the point of the fender hitting the exhaust under heavy braking. Granted I'm not a small guy (~250lbs) but I would think there would be much more clearance in that area.

Unfortunately the bike started off as a basket case and I purchased some "Early CB500 four forks" which contain a different threading on the top nut. The shock shaft threads to the top nut keeping everything form overextending. If it were the standard CB500 forks i could just use the ~1" pvc/steel spacer.

I did however add some spacers under the "jam nut" and it still feels super soft. Is there a better way to add some stiffness into these early forks?

One other concern i just thought of: I fear these forks might be from a smaller bike and not up the task of a slightly heavier 500? and of course I put a bunch of work into them 

I've included a picture of my forks and the bike in general, any insight is appreciated!

[bryanj]

Its hitting because those cross over exhausts stick out to far

[BakonBittz]

Hmm but I kinda love the crossover exhaust, I pieced it together myself

Is there any way to safely limit fork travel then?

[bryanj]

Not without making them increadibly stiff and compromising the handling

[dave500]

try heavier oil,i like atf fluid,i tried 7.5 fork oil recently and found it too stiff for me so back to atf,have you had the forks apart?or did you just buy them and install?

[BakonBittz]

try heavier oil,i like atf fluid,i tried 7.5 fork oil recently and found it too stiff for me so back to atf,have you had the forks apart?or did you just buy them and install?

Forks were fully disassembled and thoroughly cleaned, as well as the stiffening spacers added in the first picture. I’m pretty sure it had the original oil in them based on the fish smell, never had one that bad before. I used 15weight Lucas synthetic fork oil.

Is there any way to tell if these came off of a cb350f/400 instead? I measured right at 28” extended off the bike from the top of the fork tube to the flat of the axle seat.  Or should I just get new (known to be cb500) springs and try again?

[dave500]

if I thought 7.5 was too heavy for me your 15 must be like a rock?drain it and try atf,are the tops of the forks protruding above the clamps?

[HondaMan]

We used to improve the 500/550 forks by adding a 1/2" spacer, which made them quite high. I wonder if those are CB400 forks?

[BakonBittz]

We used to improve the 500/550 forks by adding a 1/2" spacer, which made them quite high. I wonder if those are CB400 forks?

That’s kinda what I’m starting to think. The bike at rest with those spacers still sits about half way through the suspension stroke.  External dimensions seem to be the same? 35mm x ~28”. Surprising they fit so well and hassle free! 

Maybe I’ll pull these springs, actually measure them against the manual  then order new cb500 Springs and see if they will fit. They probably are stiffer, and wouldn’t hurt to freshen up the ~47 year old springs.


Edit: holy cow I didn’t realize the sheer information on spring length, diameter, and rate that is available for all these bikes. Maybe slightly over/underdamped since I don’t know what forks/valving I’m starting with, but should make a huge difference.

[scottly]

350/400s have smaller diameter fork tubes than 500/550 and 750 35mm tubes. I have a set of late 750 springs I can send you for the postage, if you'd like to try them.

[Kelly E]

350/400 forks are 33mm

[fastbroshi]

Does anyone think his weight is probably moving the working spectrum of the forks near their limits?  250lbs already moves the wheel up quite a bit.  I was always around 200lbs on my 550 and I remember thinking how harsh impacts were at my weight.  I thought it definitely could've used a revalve and springs on my forks. 

[web]

So how long are these forks fully extended, for starters?

FWIW my 500 K1 legs measure 79cm bolthead to nuts, inclusive. Interestingly, the 550 K3 measures just 76.

[RAFster122s]

Bump, any more to add or did the holidays hit the pause button like for most of us?

[BakonBittz]

350/400s have smaller diameter fork tubes than 500/550 and 750 35mm tubes. I have a set of late 750 springs I can send you for the postage, if you'd like to try them.

If you're still around the forum that is an incredibly generous offer and thank you, I finally figured out a solution but thank you again, best of luck to you and yours!

Bump, any more to add or did the holidays hit the pause button like for most of us?

The holidays and a couple other things lol

I ended up being able to compress the spring more and insert a longer spacer. Perhaps not totally correct( but without buying new or custom springs) it works for my weight and coil binds/bottoms out just before the fender hits the exhaust. Thank you everyone for the ideas and feedback!

[BenelliSEI]

Congrats! Did you measure the spring “free length” to compare to limits set in your manual? If the spring has collapsed, no amount of packers will help. You could also try a smaller, f’glass fender like the one on my Rickman?

[Don R]

 Air forks? I have a set for a 400F, wouldn't help you but someone may have some for a 500.

[RAFster122s]

If you drill your caps you can put a valve in a hose that joins both sides so you have balanced air pressure. Just need good Oring seal on your caps for it to work.

[Alan F.]

3 to 11 psi seems to be the common spec right?

[RAFster122s]

Yes Alan...between this and adjusting your oil weight you can improve the forks somewhat. Not going to be like Gold Emulator Valves and new springs are probably a good idea given how original springs and many replacements would fatigue and go beyond minimum length specs. Modern metallurgy is much improved and fork springs last much longer now.

[Alan F.]

What do you think of adding a 3rd hole to the damper rods?

[BakonBittz]

Congrats! Did you measure the spring “free length” to compare to limits set in your manual? If the spring has collapsed, no amount of packers will help. You could also try a smaller, f’glass fender like the one on my Rickman?

I didn't ,making excuses here: I was trying to finish up a ton of different projects and was being a bit lazy. I agree with RAF that modern (or at least not 50 year old springs) would make a world of difference. Awesome Rickman btw! Its crazy how much different these bike look compared to a stock ones

If you drill your caps you can put a valve in a hose that joins both sides so you have balanced air pressure. Just need good Oring seal on your caps for it to work.

As a bigger guy, I  always liked the idea of air forks and that is a great idea to balance them. Unfortunately I'm not sure if these early fork designs can have the cap drilled as easily as the later design. I suppose once I get some more free time I should get another set of forks, make them perfectly the I want them and then swap them out.

What do you think of adding a 3rd hole to the damper rods?

Now i'm in way over my head lol, kinda. i love the idea but being honest if I'm gonna take apart another set of forks it will probably have emulators and be set up for my weight or any weight id be carrying. Small aside: I've been trying to concentrate on being productive vs just being busy, I would love to tear apart the bike and fix a few things, but it rides great now and I wanna put some miles on it before I revert to my usual scope creep and find myself rebuilding a motor.....just cause

[RAFster122s]

If you wanted to get fancy you could buy an accurate small liquid filled air pressure gauge and then put your fill port for standard shrader valve along with the gauge. Or just check it with a good quality Bourdain tube air pressure gauge. Low pressure variety or your pressure range is going to be likely to have errors due to the range of readings. Car tire pressure gauges of no more than 60 psi for car tires and for big trucks that go in the 90-110 psi range you would use a 150psi gauge.
For this application I would prefer to use a 20 or 30psi range gauge.
A bar of aluminum can be drilled and tapped for the hose connections and the valve.
I know you can get fluid filled fuel pressure gauges for cars and most should handle air as easily as fluids as corrosive as gasoline or any oil...

You drill your cap and tap it through the top center or not much offset. Center connector block needs hose fittings that are compression style to screw into the drilled threaded compression hose fittings. That way it can be disassembled for servicing the forks.
Pneumatic hoses that are compatible with fork oils can be custom made by a commercial hose fabricator in your city or larger nearby city. Or, if doing it online or by phone orders you may need to send photos or drawings with dimensions of what you are doing.
Better to do it in person if possible.
If you have a drill press it is very simple to fabricate, vice and drill can work if you are careful but I wouldn't want to do it that way.
Taps for the threads you cut into the caps and the aluminum block.

[Kelly E]

You can buy adjustable preload fork caps. I replaced the air caps on my 84' Sabre 700 with a set and they work great. They are 37mm but they also sell 35mm caps.