Motorcycle Tech: CB550 Fork Spring Upgrade

I recounted the tales of woe I experienced while riding this 1978 CB 550 to a two-day Kickstart Classic event in Alabama, within the CB550 rear shock upgrade. The suspension largely contributed to the woe in that tale and the ache in my back.

We upgraded the rear suspension by installing a set of Progressive Suspension 12 Series rear shocks and springs. In this issue, we’re installing a Progressive fork spring kit. The kit we’re installing is made specifically for this bike, but Progressive has tons of fork spring kits available for just about any make and model.

When deciding to upgrade the old or stock setup, there are several factors to consider: fork oil weight, progressive or standard springs (see step 8), or even rebuilding the entire fork. The front end on this bike had no oil leaks and moved nicely on the stock slides, so it didn’t require a complete overhaul. Not yet, at least.

Ride quality and perceived expectations from a new set of springs really come down to personal preference. I always refer to the owner’s manual or a well-trusted forum board to find the bike’s factory specs, then build off the stock platform. In this case, the old CB had a bouncy front end and tons of chatter. So, I wanted to slow down the rebound and soften up the forks. I used a new set of springs that were close to the factory spec, but a heavier-weight fork oil.

An important thing to note is that fork springs are under a tremendous amount of pressure, and the energy stored in a compressed spring can really pack a punch. So be aware of this and use caution when removing fork caps. Elevate the front tire and stay out of harm’s way when doing this. Enough talk—let’s get started on part two of the CB suspension upgrade.

10mm ratchet
S12mm ratchet
Adjustable wrench large

Progressive Suspension
Fork spring kit $97/#11-1106

Spectro Performance Oils
Spectro Fork Oil $12.99/SAE 10

Like we did in the rear shock installation (issue 219), prop up the bike, this time keeping the front wheel off the ground.
Remove the handlebar using a 12mm ratchet.
Remove the fork tube cap. We used an adjustable wrench to do this. The cap will be under pressure from the stock fork spring, so remove this with care, and keep your face well off target.
Remove the stock springs. The old coil should be easily accessible, no tools required.
Drain the old fork oil by removing a drain plug located on the bottom of each fork leg with a 10mm ratchet.
With the top fork cap and bottom drain plug removed, spray a fork oil cleaner like Spectro’s Suspension Cleaner in both the top and bottom of the tube, flushing out any built-up oil residue. You can also run an appropriately sized pipe brush through both tubes.
After both fork tubes have been cleaned and fully dried, reinstall the stock drain plug and copper crush washer.
Drop the new Progressive springs into each fork tube. As you can see, these coils get tighter at the top, making them progressive. If you’re installing springs like these, make sure the progressive end is at the top.
Add new fork oil. Check your owner’s manual for the appropriate oil weight and amount to add. We’re using a 10w Spectro fork oil and measuring using the bottle’s built-in graduated marks.
Reinstall the stock fork caps. This requires a bit of pressure. Take care not to cross-thread the caps when doing this; first rotate the cap counterclockwise until you feel it drop onto the threads, then tighten to the appropriate spec.
Reinstalling the handlebars with stock hardware using a 12mm ratchet.
This CB’s re-spring is completed and ready for the road. RC