Topic: Front end hop/vibration 37 mph-44mph only, HELP!

[ncstatecamp]

Im having a bit of an issue with the front end of my 72 CB750 having a bit of the shakes, like a addict quitting cold turkey. I only get the movements between 37ish/44ish (gps speedo). Its both a shake (side to side) and a hop (up and down)

New tires - Avon Roadrunners
New Seals/rebuild forks
New progressive fork springs
2 part Progressive rear shocks
New clean/aligned chain
New swing arm bushing
New neck Allballz bearings/seats (Checked for square, no binding)
Has fork brace (OEM front fender, chopped)
New Spokes (Trued to within spec)
Forks are even in tree
Air Pressure Good

My first instinct was lacing was improper but why would I get such a small range of hop
Second Instinct was that I was hitting a resonance in the fork springs or something so I went ahead and got progressive.

Im going nucking futs trying to figure this out, its really irritating b.c its at traffic cruising speed only. Above or below and I glide along smooth.

[Old Scrambler]

Did you ride the bike before the updates..........and all was OK?

Fork Brace may be binding...........fresh spokes could be loose..............tire bead may not be fully seated on the rim...........bushings are binding and the chain is too tight?.................and the most often cause is an improperly aligned front wheel due to poor axle/sppedo-drive/spacer install.

[ncstatecamp]

Did you ride the bike before the updates..........and all was OK?

Fork Brace may be binding...........fresh spokes could be loose..............tire bead may not be fully seated on the rim...........bushings are binding and the chain is too tight?.................and the most often cause is an improperly aligned front wheel due to poor axle/sppedo-drive/spacer install.

I got the bike as pretty much a rolling frame and engine, so no.
Ive tried the thing, I believe hondaman recommended, where you loosen everything up, pump the suspension, tight everything back up.
All spokes are tight and pinging when hit (like a turning fork) Chains got about 1/2" -3/4" slack, wheel is aligned. Wouldnt most of those give me an all time hump/wavy ride though? not just that small window?

[becken]

I had a shake in my 550 and found I was running too low a tire pressure. Here's what Avon recommends:

Make: Honda

Model: CB 750, K0-K6 / F1 / F2 1969 1976


Front Pressure: 2.3 bar (33psi)
Rear Pressure: 2.5 bar (36psi)

This is for a 3.25-19 front and a 4.00-18 rear Roadrider.

[wally550k0]

I don't see where you bothered to balance the front wheel.  Ignoring it will earn you a nasty tank slapper.

[jude0007]

I don't see where you bothered to balance the front wheel.  Ignoring it will earn you a nasty tank slapper.

Yeah, seems to be a balance issue and is hitting a vibration critical at that speed (frequency)

[BobbyR]

You could have a balance issue also. In cars people thought that a tire was out of balance at 60 mph and also 65mph. What was actually happening was one tire was out at 60, and another tire was out at 65. If you rule out the real nasty stuff, you may find it is a matter of balance. Tire pressures with these newer tires sometimes needs some trial and error changes. 

[ncstatecamp]

Alright, now this sounds like it could be it. I'll get this looked at!

[ncstatecamp]

So part of the problem solved, no more wobble (changed my air pressure a bit higher). Still have a bit of a hop, ill be getting it balanced asap.

[BobbyR]

With a hop I would spin the tire and see if it is running eccentric. Could be a rim adjustment or a tire not seated. If you already did this I apologize.

[ncstatecamp]

With a hop I would spin the tire and see if it is running eccentric. Could be a rim adjustment or a tire not seated. If you already did this I apologize.

It looks pretty good, I'm gonna ask em to look closer when I get it balanced.

[Bailgang]

.tire bead may not be fully seated on the rim...........

+1 Had the same kind of hop on my 550 after mounting new tires and that was the problem for me.

[Shane72]

Yesterday I finally conquered a similar problem on my K2.

The problem manifested itself on smooth roads, between 45-60 mph, cycling, rhythmic set of bounces felt through the front end and handlebars.  I rebuilt the whole front end, went through two brand new sets of tires (thinking I had an out-of-round situation), balanced and rebalanced again, seated and reseated again...nothing.

A few weeks ago I was heading south with the setting sun on my right side when I plainly saw the problem in my shadow...the rear wheel was hopping like mad in the exact cyclic cycle of what I was feeling in my hands and the frame.  Time to concentrate on the rear wheel, and maybe the suspension as well.  I really hoped the problem was only the wheel, as my shocks are new Hagons, which I love and did not want to change.

The rear wheel was a 16" harley-style conversion rim that took a tremendous amount of weight to balance (like 3.5 ounces!), but it would balance eventually.  Yesterday, I changed it out to an 18" wheel of appropriate vintage, and put another freshly-balanced new tire on as well.

Boom!  Like butter.  That 16" wheel, though balanced, was absolutely not acting like it on the street.

The only explanation that I can come up with now is that the rim itself must not be round.  I will be putting it on the truing/balancing stand once the tire is off to find out for sure.

FWIW, and YMMV.

All the best,

Shane

[ncstatecamp]

Thanks shane, I'm running the original wheel and the hops definitely from the front, but it could very well be balance so I'm getting that checked out. Thanks again.