1980 CB750K - Forks "hopping" over only small bumps

SenseAmidMadness

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Hey folks of the CB750 forums. Longtime lurker, first time poster here.

I've got a 1980 CB750K with a very annoying ride quality problem. The bike has been serviced in pretty much every aspect; it was taken down to the frame and back up, had a ton of parts replaced including the engine head, upgraded front brake, forks rebuilt, new Ikon shocks, new front wheel bearings, etc. It runs great and rides alright...for the most part.

My issue is that the ride is uncomfortable. It feels like the forks are "hopping" over smaller bumps, and not compressing at all, rather than floating over them. This results in the whole front half of the bike bopping up and down at pretty much every speed from 10-55mph. Faster than that and the bumps are less noticeable. This is even visible in the headlight pattern at night, which jumps up and down all the time.
This isn't like an imbalanced wheel, as it is totally irregular and doesn't follow any kind of rotational or speed pattern. It'll hop a few times, stop, hop again, etc. I can see while riding that the forks are not compressing most of the time, but do compress when a more substantial bump hits them. I can get the forks to compress with just a bit of front brake, and they feel like they slide smoothly testing them while stopped with the front brake on.
The steering head bearings aren't loose and the bike tracks perfectly straight.

The forks were taken apart when the bike was all apart and got new seals and wipers, Progressive dual-rate springs, and 15 weight fork oil. One of the original forks was totally unserviceable -- the damper rod bolt was completely rusted in place and I could not get it out. A used right fork from Ebay (from another 1980 CB750K) was sourced to replace it. Both chrome tubes were polished by hand. Internally I didn't notice any broken parts or severe wear, and at the time I didn't have the measurement equipment to check dimensions. All the internal parts were very dirty with ancient gross fork oil and got cleaned thoroughly before reassembling. The bike then wasn't ridden for about a year due to engine problems.

Since I'm now riding this bike regularly, I've been trying to chase this down and cannot figure it out. I went through a loosening and tightening procedure to remove "stiction" from the forks and lubricated the fork seals with quality non-petroleum grease -- no change (though I didn't loosen and tighten every single part I was supposed to). Played around with the preload spacers for the Progressive forks, including removing them totally at one point -- didn't fix it. Today I even changed out the 15 weight oil for proper Honda 10 weight fork oil, filling both legs with 7.5 oz (which is about 6 inches of oil level), and that did nothing.

The only alteration that's been made to the stock fork geometry is a custom spacer and bracket mounted in the stack of the front axle/wheel bearings -- we changed to a digital Koso speedometer, and I replaced the speedometer drive gear with a custom-machined cone part and bracket for mounting the new speedo sensor. I tried my best to make sure that these two parts perfectly matched the width of the speedo drive gear, but it's possible that the axle when tightened is ending up slightly wider or narrower than it was with the drive gear installed.

I'm out of ideas. What could be wrong? Is something busted inside the forks, or do I have a bad fork misalignment problem?
 

BAHNST

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Squared off or old tire??...

FWIW..Ill add I had a F2 sohc with a similar problem..even went air caps on the forks...
Funny thing was couldnt get anyone else to experience it...have u had others ride the bike??
Might be revs/speed your weight etc that are causing the sensation...?
 
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SenseAmidMadness

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Tires are Avon Roadriders from 2017 in fantastic condition, always stored indoors. I believe they have less than 1,000 miles on them.

Like I mentioned in the post, it's not a regular sort of "hop" like you'd get with an imbalanced tire. It happens, then takes breaks, then comes back.
 

BAHNST

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So if it happens... and you maintain speed/gear does it go away on its own ?...or do you slow down/speed up to get rid of it?

Edit:Tires have a life of bout 5 years...
Ive seen some deform just by sitting unrotated....especially the front
 
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SenseAmidMadness

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I'm going by the manufacturer's recommendation of 5 years from date of first use and 7 years from date of manufacture. They'll be replaced within the next year, but I assure you they're in excellent condition and still have plenty enough grip for how I ride on this bike.

I can't get rid of the hopping by speeding up or slowing down, unless I go slower than about 10mph or faster than about 60 (which really doesn't remove it so much as make it less noticeable). It doesn't go away unless the road smooths out.

I'm very sure it isn't the tire as the frequency of the front-end hopping does not change with speed.
 

BAHNST

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You could try realigning the forks again....
Loosening off the bolts, pumping the forks and then retorquing

With progressive springs id expect u to see some initial compression under most road use
 

gggGary

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My major suspicion is a bent fork tube and or tweaked triple tree (almost always the lower triple)
Due to bend or twist there is static sideways pressure, keeping the forks from sliding easily, only overcome by a significant force (bump).

Find a wall or block, attempt to compress the fork by bumping into it. How does it feel/move? Now loosen the axle bolts and fender if equipped, so the axle can slide sideways in one lower, retry the bump against a wall. Does the action change?
This can help narrow down where the problem is;
 

jpdevol

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If Gary's method shows straight tubes and triple tree (great method and video BTW), I would be suspicious of the speedo spacer causing a binding of the forks. Easy enough to check; put the stock drive back in for testing.

Your good description indicates a serious problem that needs resolved....
 

SenseAmidMadness

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I totally forgot about the method Gary mentioned and still need to try that; however, I did poke at it some more yesterday. If these forks or triple tree are bent, it's so subtle that I can't see it by eye, and didn't notice it while rebuilding these forks a couple years ago. The bike certainly hasn't been crashed or dropped in the 3+ years I've owned it.
I should also mentioned that this "hopping" phenomenon didn't occur when I rode the bike home after purchase, or last year when it was ridden for maybe 15 miles. I think it's something I did recently to cause it and it was probably an assembly error on my part.

Based on the design of the front axle, it looks like my speedo spacer couldn't cause stiction. The right fork uses the axle nut as its locating feature, with a pair of notches that go on either side of the fork and the cap and hold it laterally between them when tightened. The left side of the axle has no such locating features, just a plain cylindrical surface, and the left fork lower and its cap are also smooth. It looks like this is designed to accommodate for a reasonable range of manufacturing tolerance in the width of the axle stackup. A few of the parts involved like the wheel hub and the speedo drive housing are not entirely precision parts so this makes sense.

I did measure the speedo drive, and it came out to .12mm narrower than my axle spacer and sensor bracket combined, so I removed almost exactly that much from the spacer with sandpaper just to be thorough. I then tightened the axle down. This didn't seem to do anything with regards to how the axle fit in the forks; I pushed the left leg back and forth a little bit to find the place where it would naturally sit on the axle without lateral tension, and also bounced the forks up and down before tightening the fork caps. It looks like there's more space between the left fork leg and my custom speedo bracket now but I can't be sure.

Made a few other tweaks though -- first, and most stupid, was that I had not previously tightened the axle caps the right way, front nut first. I hadn't even tightened them both the same incorrect way. D'oh! So both forks weren't aligned in the same place relative to the axle. Not the most boneheaded mistake I've made on this bike but it goes on the pile.
Second, I loosened the four front fender bolts, bounced the forks up and down a few times, and then snugged them back down much less tight than before. That may have been contributing a bit as this bike has a chromed steel fender.

The tweaks seem to have made at least some difference. I wasn't able to test ride the bike more than 30-ish MPH, but the "hopping" is definitely less severe. I do know that I also overfilled the forks with oil when doing the change a few days ago and that may be making them stiffer. I put 7.5 ounces in each leg. I'm going to drain them again and refill with about 5 ounces each instead, and see what that does.

Anybody got any ideas for how to make sure tightening the fender can't cause fork problems?

Gary, I'll give those checks a try tomorrow when I'm poking at the bike again.

If draining and filling them doesn't fix it, and I don't find any bends, I'm going to attempt an involved "stiction tuning" process, taking the wheel and fork caps off and tweaking/loosening/turning/tightening all the front end parts and bolts and nuts and the axle until I find a position that allows the forks to move the smoothest up and down.
 

gggGary

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Two more "mebbys"?
Only one line in your description mentioned the steering bearings. I don't see a mention of a recheck for looseness.
Second, and this happened to me on an 83 Honda is REAR wheel bearings. Swore I had a FRONT end issue, but eventually the rear bearing got bad enough that the light bulb came on. Everything in the rear suspension should at least be checked, rechecked.
 

SenseAmidMadness

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Two more "mebbys"?
Only one line in your description mentioned the steering bearings. I don't see a mention of a recheck for looseness.
Second, and this happened to me on an 83 Honda is REAR wheel bearings. Swore I had a FRONT end issue, but eventually the rear bearing got bad enough that the light bulb came on. Everything in the rear suspension should at least be checked, rechecked.
I did double-check the steering bearings and they've got no play in them that I can feel. They were re-greased and tightened when the bike went down to the frame.
That's also a good idea. I installed new front wheel bearings, but not rears. I'll give those a check today.
 

SenseAmidMadness

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Find a wall or block, attempt to compress the fork by bumping into it. How does it feel/move? Now loosen the axle bolts and fender if equipped, so the axle can slide sideways in one lower, retry the bump against a wall. Does the action change?
This can help narrow down where the problem is
Tried this out against a wall a bit ago. The forks moved easily when pushed into the wall. When I loosened the axle and fender, it got even easier. I probably didn't have them in the right spots, so I bounced the forks against the wall a few times then retightened the axle and fender with no noticeable reduction in fork friction afterwards.

Double-checked the rear wheel bearing -- no lateral play when pushing on the rim, and smooth rotation with no notches.

Then I drained the forks and refilled them with 150mL of oil each.

Was about to take a test ride to see if it's all fixed, but then a huge cloud of smoke rolled into the neighborhood and my asthma really didn't like that. It'll have to wait.

Fingers crossed, but I think I had just reassembled something wrong and today's work might have fixed it.
 

SenseAmidMadness

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Didn't fix it. Felt like it might be gone for the first minute but it came back! That was a rather annoying 25-mile test ride.
Now I'm wondering if it's got something to do with the new Ikon rear shocks, because I looked down while riding and saw the swingarm moving in perfect time with the "hopping" sensation. It may not be a front end problem at all and I'm open to suggestions as to how I can isolate which end is doing the bouncing.
The Ikons came out of the box with a bit of rust on them. Could uneven shock performance between the two sides cause this?
My current plan is to swap on another pair of shocks, either the OEM ones or a pair I got cheap from a 1980 CB750F, and see if that changes something.
 

BAHNST

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Fwiw...You could also go to one preload and damping setting on the shocks to the other extreme and see if that effects the front end at all....

Fwiw i dont think its the rear of the bike....
 

SenseAmidMadness

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Fwiw...You could also go to one preload and damping setting on the shocks to the other extreme and see if that effects the front end at all....

Fwiw i dont think its the rear of the bike....
Unfortunately these shocks are preload-adjustable only, with three settings. The next step from Ikon costs 50% more and I didn't want to go that serious with suspension for this bike.
I can try that, though, going to the highest preload setting on the shocks and see if that changes it up.
 

SenseAmidMadness

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I did get this figured out, finally -- when my dad ordered the Ikon shocks, he specified that they be set up for a 210-pound rider. Both the springs and the damper valving are set up wrong for my weight.
When I ride with a passenger, or with a box of stuff on the back seat, the suspension feels perfect.
 
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