Topic: Chop Hop

[Boomologist]

Hi,
Since there is so much good knowledge here thought I'd toss this out.
My chopper has always had a bounce in the front wheel exactly at 55mph. It almost acts like a tire out of balance but it isn't. I've had both tires balanced many times, changed tires, trued the wheels (spoke type). Even put a different front end and wheel on it and the bounce is still there.
When the bounce happens if I slightly ride the front brake it goes away. If the air pressure in the front tire is low the bounce isn't as great.
Steering head bearings are snug.
I've put up with this for 8 years now and sure would like to fix it. 
Any thoughts are appreciated.

[Tim.]

How are your forks braced?  Could it be the extra length of the tubes and the rake?  Have you checked to see if this is a typical problem with choppers?

[DammitDan]

Maybe it's just a handling problem caused by the altered front end?  Was the different wheel you tried a different diameter from the one you're using now?

[Boomologist]

Yes, the wheel I tried was a little smaller and it was on an "old school" Springer with no brake. I had the balance checked before I installed it.
The bike isn't drastically raked. It's the one that displays with my posts.
The frame is a modified CB350 with CB750 wish bone. Front tubes are Suzuki.
The bounce seems to be in time with the rear wheel but it's been balanced, replaced and trued.

[Bodain]

If the wheel is out of round, you will get the bounce no matter how many time you balance it. It will always be out of round.
Case in point... On my 550 project. When I finally got to a point where it was ready to ride I had bounce in my front wheel. More speed, the bounce got worse. I suspected the wheel was not round... On the centerstand I jacked up the front end such that the tire was maybe 60 thousands from the ground.. I gave the tire a spin. The problem stood out like a sore thumb. Seeing the problem I now removed the tire and remounted the rim on the front forks with a dial indicator on the floor. The wheel was 100 thousands out of round.

Thus began my first experience with truing up a rim...

[DammitDan]

I'm confused... Your rim was warped longwise instead of side to side, and truing the spokes brought it back?  I don't see how adjusting spoke tension would have anything to do to fix a compressed (squashed) rim.

If your wheel is out of true it would cause a wobble, but would it cause a bounce?

[Green550F]

Sounds like it's hitting a harmonic to me. I'd bet on changing the oil weight in the forks. That should change the damping rate and shift the harmonics a bit. (However I'm an electronics engineer not a mechanical engineer so I could be wrong.)

Why wouldn't spokes affect the roundness of a wheel as well as the warp? And why wouldn't one check this while trueing a wheel?

[Boomologist]

I spent many evenings truing the wheels. Even went so far as to build a stand so the wheel could be up on the bench. Has a dial indicator on it. Both wheels are as true as you could possibly get them. There was a little tread wobble on the rear tire but that has been replaced.
It's a mystery.
Oh, both sprockets and chain have been replaced also. I did notice that when you rotate the back wheel and watch the chain it will sag, then tighten up, then sag and tighten up as the wheel turns. I thought this could be a stretched chain but replaced it and it's the same. The sag occurs once with each rotation of the wheel. Using a ruler the amount of sag is about 1/2 to 3/4 inch.
The more I type the more I think the bounce is probably related to the chain sag which appears to be caused by something in the rear wheel or a bad sprocket.

[Bodain]

I'm confused... Your rim was warped longwise instead of side to side, and truing the spokes brought it back?  I don't see how adjusting spoke tension would have anything to do to fix a compressed (squashed) rim.

If your wheel is out of true it would cause a wobble, but would it cause a bounce?

The rim was not squashed. It just was not round, which resulted in front end bounce. Since this was my first experience with truing a wheel I had many frustrating hours on it.Ultimiately I had to loosen every spoke and start at the beginning serveral times. On about the third attempt I got it right and a world of experience.

[DammitDan]

The more I type the more I think the bounce is probably related to the chain sag which appears to be caused by something in the rear wheel or a bad sprocket.

A couple of possibilities here then... Either your chain is at the end of its' life, it's not lubed well enough (causing some links to become stiff and "stick"), or your rear wheel is adjusted so that it's not running true, causing a slight bind in the chain.  I would put it up on the center stand and play with the adjustor nuts on either side for a while and see if you can't get the sag to go away.

I think you're right that it may be possible that a wobble in the back wheel is causing a vibration that affects the front wheel at certain speeds.

I'd bet on changing the oil weight in the forks.

Hit this one suggestion up too!  Can't hurt much 

[dr. destructo]

If he's been dealing with this for 8 years, it seems if its a chain issue, it would have gotten worse over time? maybe not though..

Were you the one dad did the mods to the front end?  If so, did the bike have this problem when it had the stock front steering/suspension on it? or did the problem only arise after "chopping" it?

out of all the things you've tried, would it be worth while to try swapping on a stock front end again and see how that does?

[Boomologist]

The chain has been replaced many times during the past 8 years but not the asprocket's.

Yes, I did all the mods to the bike but I can't say how it did before because it's built from parts from many different bikes. Even the frame is two bikes combined, CB350 & CB750.    Frankencycle 

I did try a Springer on it with same results. Don't think a stock 350 would fit it now. The front would set on the ground.

About the only thing I haven't replaced or tried is the rear wheel.

[joeb]

This may sound alittle crazy but you may not have enough weight on the front end, because you ruled out the front end and the wheel. If you replaced it with the same type. When was the last time the forks were rebuilt and what weight oil is in the forks? May need to go to a lighter fork spring  or a lighter fork oil. Just some thoughts.

[Boomologist]

Thought about adding weight to the front to see what happens. I'll try that next.
Forks were done last year and 5w oil was used. One thing I did try was stiffer springs. It made it a lot worse so springs  may be part of it.

[DammitDan]

Wait a minute, you changed the chain without changing the sprockets too?

I thought this was a no-no...

[Boomologist]

Yes, I go through one chain a year and have only changer sprockets once on this bike in 8 years. I could probably get two seasons per chain but I replace the chain every spring as part of my spring tuneup.
I may be wrong but I see no reason to change sprockets if there is no wear showing. I do inspect the sprockets for wear in all the places the chain touches them, especially in the bottom of the teeth.
When times were hard I would run a sprocket until it showed some wear at the base of the teeth. Then I would take it off and turn it around and run on the other side of the teeth.

[Lumbee]

hey Boom, I have quite a bit of hop on my chop.  Mine isn't rythmic, and its more of a studder than a hop...I attibuite it to the fork lenth.

...oh, and start sending me u'r used chains...I'll put them to good use...   

[Boomologist]

Perhaps it's caused by changing the geometry of the suspension??
I even thought it could have been the little bit of play between the strut tubes and the housing they ride in but going to a springer eliminated that thought.
I thought it could have been the front fender catching wind but, again, the springer didn't have a fender. So far I know what isn't causing it. 
When I get it together I'm going to try some weight on the front and try truing that rear sprocket. Other than that I'm out of ideas.   

I do have plans to build another chop.    I have spare CB750's &  CB900 engines, lots of wheels and bolt-on's. Did the 900's ever use chain drive? Is there a belt drive conversion available?
This time I'm doing my own frame instead of hacking up two different frames to make one. A neighbor has a bender and a frame jig that I can use. When I start I'll take lot's of pict's and do a documentary.

[fourplay]

Boom,

This bounce problem has me curious. I have ridden some pretty cheesy chops in my day and I have never experienced anything like what you are talking about.

I am going to go out on a limb here. I think you are experiencing some serious frame flex. A long frame will flex quite a bit. Plus there is not a lot of steel in some of the earlier frames. Well, not as much as you might see in an aftermarket frame.

Once the frame starts flexing it will keep ocilliating up and down. The ocillatioins will get worse or get to a point where it gets only so bad and stops at that frequency.

You have checked so many things that it really makes me think it is your frame.

[Terry in Australia]

There's something seriously wrong with your bike Ron, but I guess you already know that. If you're killing a chain every year but your sprockets are ok, and your chain is continually stretching/slackening, I'm guessing that your rear sprocket is badly out of round, or your sprocket carrier is seriously stuffed.

What are your rear wheel bearings like mate? Your forks aren't that long or raked out that far that it's too light up front, and I don't think that fork oil or damping rates will make any difference at all, nor will strapping on a couple of house bricks, I'm betting that the problem is eminating from the rear of your bike and resonating forward. Fix it fast Ron, before it spits you off! Cheers, Terry.

[Boomologist]

I change my chain every spring even if it looks good. Just part of my maintenance. They are probably good for two or more years but I just change them.
Rear wheel bearings have been replaced.

Not that it couldn't be, but if it was frame flex wouldn't there be cracks showing after 80k miles? The frame tubes appeared to be that thicker old steel that is impossible to bend. It is also double tubed (tube inside a tube) from the steering head down the wishbone to the curve at the bottom. This is done at the factory for added strength I guess.

It should be back together today and I'll play with the sprocket and wheel.

[Lumbee]

if it was frame flex wouldn't there be cracks showing

...not at all, in fact, a good frame should have some flex to it, this takes pressure off the joints.  To rigid a frame is likely to crack at the joints...

[Dusthawk]

I am intrigued by this thread as I have seen several chops who report hopping. I myself have never noticed any with mine and mine looks even longer than other avatars. It is also a springer so I don't know if that is where the difference lies.

To answer a question that wasn't very well addressed about truing, adjusting the spokes affects both side to side and lengthwise trueness. If you loosen one side and tighten the diametric oppisites the same amount, you get lengthwise adjustment. If done wrong, it can result in an out of round rim. The way to correct side to side wobble is to loosen one side set of spokes where the wobble occurs and tightening the other the same amount, repeat until true.

Peace,

Jeff

[Boomologist]

Update!
Think I may have found the problem (well, a problem).
I measured the axle to sprocket bolts on the rear wheel. The sprocket is almost a full mm off center. This is what causes the chain to loosen then tighten with each rotation of the wheel. This could definitely transfer to the front end as a wheel hop. I picked up another rear wheel today and am in the process of lacing my rim to it. I'll let you know if it fixes the problem.

[merc2dogs]

If you've taken the sprocket off, was it oversize holes or some thing else?

 On my bike the replacement starter was not a very snug fit on the drive plate, but the bolts were a snug fit so it centered fine.
  once got stuck miles from home because a replacement starter had mounting holes that were too large for the bolts, mounted up great, but after a few starts the oversize holes would let the starter shift enough that the teeth would just brush the ring gear.
 

ken.

[Boomologist]

If you've taken the sprocket off, was it oversize holes or some thing else?

The sprocket holes are snug fit as is the center hole. It appears the hub itself is not centered and could have been a defect when new but most likely the hub got bumped real hard sometime.
When measured from the axle to each bolt there is a huge difference between two of them.

[Boomologist]

Update:
I replaced the hub in the rear wheel and laced my rim to it.
Chop hop is gone! No more front end bounce!
The old hub was about 1 mm off center which made a huge difference in chain tension and this transfered to the front as wheel hop. Never ceases to amaze me.