Topic: Strange bounce,40-50 mph, it's not the tires?

[1976cb750f836]

Ive benn chasing this dragon 4-5 years now. Ive had 2 different wheels on each end, 2 different tires on each end. New neck bearings(adjusted on tight, and loose side), new swing bushings, new hagon shocks, with good bushings, changed fork oil. Happens under power and when coasting, worse under light throttle. Always between 40-50mph, smooth before and after. Under hard acceleration none felt. Only time I feel less is when doubling . I have newish chain and both sprockets, I've checked sprockets for out of round . Ive ran with over loose chain, and overnight chain, maybe a tad worse when overnight? I've ballenced tires .
I have rear wheel aligned using the 8ft fluorescent tube method. I'm out of ideas?
Help?

[pjlogue]

Have you checked your hop and run out on front and back rims?  You said you changed wheels but you could have 2 rims and both be out of true.  Brakes aren't rubbing?  The 40-50 mph is about the speed an out of true wheel will show up.  I had it on my '76 750K after I put on new rims and didn't get the trueing correct on the first try. 

-P.

[PeWe]

Tire pressure OK?
I follow Honda recommendation on label. Too much is not comfy.

[Sigop]

I had similar. After truing wheel still not right, and finally saw the tire was on the rim (Avon Road Rider over a tube) slightly off line.  Saw a sidewall circle around tire that showed it off kilter.  When they over inflate to install then bring it back down that was when it was sliding slightly sideways.  Anyway, that was my fix.

[BenelliSEI]

+1 to Sigop.

[Bodi]

Despite your assurance that the bike's wheels and tires are perfect, there are two prime suspects that can cause bounce when on a smooth flat road: Wheel imbalance and out-of-round wheels/tires.
Mounting modern tubeless tires on our old tube profile rims works, but they don't quite fit properly. Getting the tire bead seated perfectly evenly all around the rim is not easy. Every tire I see has a small line around the bead that should be almost the exactly equal distance from the wheel rim all the way around the wheel. Deflating the tire and unseating the bead, lubing the rim flange and tire bead generously with wheel mounting lube or soapy water, then inflating it a bit at a time while pounding around the tire with a heavy mallet can work. Overinflating (be super careful going above the sidewall maximum pressure, a tire explosion can be deadly even with these small ones) with the bead well lubed can seat the bead evenly sometimes.
Check runout on the wheel itself. Both radial and side-to-side runout can excite chassis oscillations. Loose wheel bearings - even sloppy swingarm or shock bushings - can trigger a natural chassis oscillation. Modified frames can have very weird problems, removing any tubing sections is probably taking away some frame stiffness that Honda added for a very good reason... they were not renowned for wasting steel and fabrication time.

[1976cb750f836]

Mag wheels! They are true. (2 different sets).Tire bead is centered. Ive played with air pressure up and down. Sometimes i feel like its coming from rear? When my 75lb son rides behind me it almost all goes away? (Why i think its rear).
2 things I didn't mention.15 years ago, I T-boned a rear door on a car at bout 35mph, pushed fork tubes back,bent bars,sprained both wrist,(I flipped over car,landed on feet,luckily didn't get hurt), wheel into header. Bought a complete 78 front end and new header. No frame or neck damage  I could find.
2, past owner, frame section cut to remove top end with engine on bike. Each cut is patched with tube coupeling, bolt and set screw. No visible movement, no paint chipping.
I have read post horror stories bout cut frames though?
3, the extra weight of a rider behind be, making it go away , should help with diagnosis,  but I'm stumped?
More ideas please.
Ps, haygons set on softest setting. Tried them 1 click stiffer, bounce got worse?

[jgger]

Just a thought, put it on the center stand and have your 75 lb.. son sit on the back of the seat to raise the front wheel. Grab the front axle and try to wiggle the wheel in a front to back movement. This will tell you if the stem bearings are too loose or if there is slop in the fork sliders. I had a similar deal on a bike, that one had after market tubes that were under sized just enough to make the bushing to tube clearance out of spec.

My thought is the additional weight on the back allows the front to get funky, same with the stiffer shock setting. I think your issue is in front. Especially with the added info.

Good luck.

[pjlogue]

Have you checked the swing are bushings and shocks for any damage after the impact?   Also, how did you balance the wheels?  On bike or off using a good wheel balance?  Spinning the wheel while mounted on the bike (or using the wheel bearings in the hub) isn't a good way to balance a wheel.  Too much drag from grease and seals. 

-P.

[Don R]

 I've had shops balance my tires and they were way off. I do them myself on a truing stand now. Just a thought.

[1976cb750f836]

No movement in front, tubes tight. When son behind me ,bounce almost goes away(not gets worse). I ballencer my wheels myself (after getting shabby work at shop),on a home made ballencer stand. I use a piece of greased 1/2in threaded rod as a axle, (smaller than bearing so no drag), while spinning I tap with small rubber mallet on axel, this vibrates , so heavy spot always goes to bottom. (You can put wheel on, without spinning, and just tap axel, and Heavy spot will go to bottom) may sound hillbilly ,but it works.
I have a Leakey fork seal, but acts same right after oil change, I even tried it under full, before I added oil. The weight on back, actually puts more weight on both ends, I slide forward with nads almost touching tank(My seat has benn shortened).
I've benn All over this front end, several times. Before I attack the fork seal,(and go over the whole front), I think I'll go to the rear. I haven't disassembled it since I painted the bike bout 15 years ago.
Besides the swingarm ,and shock bushings, is their anything else to look for when I remove the arm?
Could their be something going on in rear hub, that I ant thought bout(I put in new bearings when bike was painted)?
Keep the ideas coming?

[Don R]

 Put it on the center stand or whatever you have to level it. Put an appropriately long straight edge on the front and rear tires with a level or angle finder to see if they are both the same degrees from vertical. I wonder if the neck got a twist? Maybe level the swingarm and compare to the front wheel?

[1976cb750f836]

Put it on the center stand or whatever you have to level it. Put an appropriately long straight edge on the front and rear tires with a level or angle finder to see if they are both the same degrees from vertical. I wonder if the neck got a twist? Maybe level the swingarm and compare to the front wheel?

Already checked this after crash, before I decided to repair it.
Going to completely disassembly both ends of suspension,  and go over everything.

[Marks78]

Keep the ideas coming?

I know the tires/wheels are probably not out of balance but it might be worth trying something like "DYNA beads" or "RIDE-ON" balancing products just to see if they helped.

[1976cb750f836]

Already tried the dana beads, no luck.
I have a 77 goldwing hear that I'm storing for a friend.
Will the front wheel fit, just well enough for a test ride?

[Don R]

 Man, I wish you luck. If you lived closer, I'd loan you a full front end assembly to try. That might narrow it down a bit.

[Don R]

 My second 750 after 25 years off of bikes had a 70 mph wobble, it was a combo of dinged neck bearings and lowering blocks. Not much help to you but sometimes it's a mix of small things.

[pjlogue]

Have you checked your brakes to see that the clearance between rotor and pads is to spec?  If you have a caliper piston that doesn't retract properly when the brake is released and your rotor is even just slightly out of true you can get a pulsing that feels like wheel hop.  It is more prominent in the rear. 

-P.

[1976cb750f836]

Its not bad enough to be dangerous, so I've decided to ride ,until summer end. Maybe I can experiment some and determin for sure which end is actually causing bounce.
Different air pressure, (lower to 10lb or so should soften the bounce?),one end at a time should make some kind of noticeable difference. Or maybe a heaver than my son passenger?
Bike is lowered 1in shorter shock in rear, but I can't see that causing problem. As best as I can remember, this started after crash? No bounce when I had the spoke wheels. But again, I went over bike , looking for bends or cracks or any kind of damage. I did convert to the 77-78 dual disc front during the rebuild. Best description I can give is it feels like an egg shaped wheel?
But with either wheel and tire on ballencer stand, you can measure run out in thousands, not fractions?
As far as last couple post, just went over the front brakes, new fluid, sanded disc, and pads.
Their was some drag on front before, bike harder to push than it should be, had to peck caliper with rubber mallet ,but after cleaning,now bike rolls around no drag.

[przjohn]

Even Mag Wheels can be out of round. I had an RZ500 and chased this same Gremiln until I found the front wheel sure enough had a slight bobble on a truing stand. Bought another wheel off Ebay and solved the problem.

[pjlogue]

A wheel balancing stand is the way to go for balancing and truing a wheel.  I got one from Harbor Freight but their rod and cones that take the place of the axle were junk.  I bought 12mm hardened ground rod and machined my own cones.  This made all the difference and balancing and truing were easy after that and I have my wheels to withing 0.002" run out/hop.  and within 1/8 oz on balance.

You mentioned the bike was hard to push because of the brakes.  To find if you have a sticking caliper you need to have the wheels off the ground and rotate the wheel by hand to check for any stiff spots.  You won't have enough feel if you roll the bike.  It doesn't take much hang up of a caliper to manifest it's self as bounce at highway speed and it will get worse as things warm up with friction.  You should easily be able to retract the caliper piston by hand by pushing on the caliper.  If you can't retract the piston that way you need to clean the caliper and especially the square O ring grove and O ring. 

-P.

[1976cb750f836]

More things to check, thanks. I'll recheck those brakes.
As far as the extra weight on rear slowing or almost stopping the bounce,
Is their anything in rear hub I'm missing?
I'm super stumped about weight on rear curing the bounce?

[pjlogue]

It could be that the added weight changes the harmonic frequency so you no longer notice it at the same road speed you did without the passenger.  I would check the run out of the wheels.  You can do this by getting a dial gauge and mounting it on the bike and rotate the wheel.  You can pick up dial gauges pretty cheap on eBay.  As for balancing a wheel, I feel the really only accurate way is to remove the wheel and axle and use a balancing stand.  You mentioned hitting the wheel with a mallet to overcome friction.  I have never tried that but would think it would be less accurate than a truing stand.  The mallet may overcome some friction and get the wheel less out of balance but I would be willing to bet the wheel isn't actually balanced. 

There can be many causes of what you are experiencing.  The best way to solve the problem is to rule out the most likely>least likely.  Wheel trueing and wheel balance are the most likely causes.  To rule these out you need to check them with an absolute method, not by eyeball or using a mallet on the wheel to try and determine balance. 

-P.

[1976cb750f836]

I'll recheck run out on both ends before I try other things.
U miss understood my ballencer?
I set 2 Jack stands with a piece of 1/2 threaded rod as a axle, (wheel off bike)
Greased rod, smaller than bearing ID, extends 6in out past Jack stands.
Wheel spins like its floating on air! Pecking on thread rod lightly, vibrates
Heavy to bottom. As stated earlier, don't even have to spin wheel,
Just tap rod and Heavy goes to bottom. If I spin wheel, just let it stop on its on,
Then tap with hammer, it always backs up a bit.(friction on axel)
But tapping while spinning, it stops on heavy spot on bottom.
Try it, I bet most shops have Jack stands and threaded rod ?

[Don R]

 Let us know how it all shakes out. L0L! See what I did there? Seriously, we all learn when one of these tough ones gets fixed. But only if we hear what fixed it.

[Dunk]

Gotta be either out of true or out of balance. I need to true my K1 wheels again, I have a similar slight bounce or shake at about the same speed. Had it once before and truing the wheels fixed it. Seemed like this started after hitting a monster pothole, I tightened some loose spokes and did the best quick and dirty I could on the bike, which improved it, but I need to put the wheels on the stand and get them right.

[Deltarider]

Here's a routine I can advise to all of you. Whenever you are to renew a tyre, have a close inspection of the rim's inside. It's not unusual to see rust build up on the inner sides of the rim, where the tyre's rim sit. This can lead to a tyre not properly seated. So always inspect the rim's inside (edges) before putting on a new tyre and remove eventual rust. You can use a (sharp) screwdriver and a wire brush.