Friday, November 24, 2023

 Too much friction spoils the party…

In bringing the CB350F back to life, I noticed that the forks were pretty stiff upfront. Hitting small bumps would just push the front end of the bike up like it was a rigid fork machine. The fork seals were leaking anyway, so I pulled the forks off, re-sealed them, and filled them with some synthetic ATF from the auto parts store.


Stiction: an abbreviation of “static friction” - the friction between stationary surfaces at rest, which resists them beginning to slide over one another.

Friction: the resistance that one surface encounters when sliding over another. Generally speaking with suspension this refers to “dynamic friction”, ie the frictional force while already in motion

Nothing was bent or damaged on either fork, but when the whole front end was reassembled, it still rode like an empty 1-ton pickup truck. I had already changed the worn-out rear shocks with some slightly longer aftermarket copies, but that didn’t do a lot to improve the ride quality.

My recollections with re-assembling 1970s Honda street bikes, even after a tire change is that the fender stay mounts are not precisely fitted and it has been my belief and experience that when you force the fender stay into the insides of the fork sliders unnecessary friction/stiction is induced onto the fork suspension system.

With the CB350F, I removed the front axle and nut, tightened them together, and then mounted one end into the bottom of the fork slider, secured with the two nuts on the cap. With both fork tubes set about evenly at the top of the fork crown, there was a gap of about a 16/th of an inch above one end of the axle in the opposite slider. I loosened the fork tube on that side and tapped it down until the slider just rested back on the end of the axle. This keeps both sliders at the same level, preventing binding on the fork bushings. See end photos.

Secondly, when I bolted the fender stay to the right side slider, opposite of the disc brake mounting hardware, a gap was clearly seen between the fender stay and the fork slider/brake hardware mount. I added two 6mm washers on the front bolt and one at the back to keep the fender stay from pinching the fork sliders together, causing fork bushing binding.

The front and rear fender stay bolts were loosened up to allow the ends to center themselves on the bolts. I also loosened up the fender stay mounting fasteners to the fender itself. The goal is to allow the fender to just float in between the fork sliders, moving as a unit up and down with minimum amounts of friction caused by side-loading the sliders against the fork bushings.

There are super slick fork seals, fork seal grease and fork oils that are all designed to lessen the friction and stiction in the front suspension. All of these things can help to allow freeer movement of the front forks during compression and rebound motions.

Honda and most other manufacturers were not fully cognizant of the dynamics of fork action in the 1960s and early-1970s, with most of them having little rebound control. Progressive wound fork springs can be helpful in allowing more initial movement of the front end when encountering small bumps. Changing the whole spring rate is sometimes necessary. Most bikes were designed with the maximum load rating in mind, which is generally too stiff for a solo rider of average weight.

Any or all of these things can cause fork binding that wears the fork bushings and inside of the fork sliders unnecessarily. Take time to carefully look at how the front forks and fender are fitted to the bike to minimize suspension travel problems.

Just refitting the front fender to the forks, as described above, allowed for a noticeable improvement in the ride quality on this particular bike.

Bill Silver aka MrHonda


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