Topic: Front end "Shudder"

[Lumbee]

...77 750F.  I've had this bike about a year, and this has been a problem since I got it.  When riding at say 15 or 20mph if I take my hands off the handle bars, and just let it coast, the handle bars/front wheel will slowly go into a shudder, or speed wobble.  I then must immediately grab the bars, or the bike will surly shake itself to the point of sending me over the front end!   

Its not really a problem as long as my hand are on the bars, but I know something is not quite right.  Any ideas?  I've put the bike up on the center stand and checked everything on the front end out, and nothing is loose.  The font tire still has good tread, and no cracking with the correct rotation direction.  The rear wheel is in line.  This one is got me stumped.  Short of buying a different front tire just to try something different, I'm not sure what to try!     Any comments or suggestions appreciated.

[KB02]

...When riding at say 15 or 20mph if I take my hands off the steering wheel, and just let it coast, the handle bars/front wheel will slowly go into a shudder, or speed wobble.  I then must immediately grab the bars, or the bike will surly shake itself to the point of sending me over the front end!   

...Short of buying a different front tire just to try something different, I'm not sure what to try!     Any comments or suggestions appreciated.

Yeah, Don't take your hands off the bars. 

Seriously, the two things that I would think to check right off are your steering head bearings and wheel truness and balance (Do you have spokes or Comstars?)
For the former - Put the bike up on the center stand and have some push down the bac end so the front is up in the air. Pull on the front wheel to see if there is any slop on the steering head.
As for the Latter - While the front end is in the air, spin the tire and watch for any deviation in the roundness of the wheel. And just let it come to a stop to see if it sits heavy on one side or not (like a pendulum swinging back and forth). If so, might just be out  of balance.

[crazypj]

Its he same effect as with a shopping trolly/cart. he caster is off. you could try adjusting steering head bearings, inflate tyre pressures closer to max (or around 32psi) and check REAR wheel bearings, alignment and swing arm pivot. If he fork springs are weak he front end will be steeper than normal and affect steering.
PJ

[Lumbee]

...I have the comstars.  As far as it being out of balance, there is no wheel hop at interstate speeds, so I kind of think thats not a problem.  Already checked the steering head, and its solid, although I'll most certainly recheck.  I'll put an indicator on the wheel and check for runout this weekend.  Funny thing is, I had these wheels on my previous daily rider (78F), and had no problems on that bike, so that makes me think maybe the problems is in the neck or forks.

[Lumbee]

...how much resistance sould be in the handle bars?  If there is to little resistance should that cause my problem?

[sparty]

I have a simular situation, it began after having a new front tire put on so Ive always blamed it on just not being ballanced perfectly.  For me the font is smooth as can be up till about 35mph then it has the wobble exactly as Lumbee described.  If I were to let go of the bars at that point Id be toast.  Holding the bars with one hand, I feel the vibration but its far from uncontrolable.  This wobble dissapears around 40-45 and its smooth again.  I could take my hands off going 70+mph.  Is this just a ballancing issue?

Sounds like a balancing issue... I have the same issue.  My wobble starts around 35 but ends quickly, then resumes around 50 and ends quickly, My bike is super smooth at 100, so that is where I keep it.

Sparty

[nteek754]

Hey all I been dealing with this for a while now  not so as I cant ride but I blamed it on my front tire and when put a new one on  grrrr the same thing  A buker buddy said balance nothing to do with it  nope at high speed no hop  but my swingarm bushings are wasted again not that I cant ride but  I ve got new bushings allready insalled into another swingarm and new bearing kit going in goose neck   so I will know first trip out this spring  unless I get to it and we gat some nice days like we did last winter I rode 4 times in january up here in Maine  like lumbee said keeping hand on handlebar keeps it from wobble I can justput a finger on it and it keeps it stable but somethin aint right we will see have fun seven fifty four ever Craig in Maine

[TwoTired]

Have you checked to see if the fork tubes are bent?  And, if not, are they perfectly parrallel in the triple trees?

Do your front and rear tires track in the same groove?  Have you tried tweaking the rear wheel alignment?

Are your tire profiles same front and rear?  Are they oversize?  What is the tread pattern?

Are the rear swing arm bearings tight?

Have you changed the ride height front or rear from stock?

What is your weight distribution front to rear?  Is it different from stock?

Any fairings or handle bar length/weight differences from stock?

Describe your mirrors.  Only one?

Are the wheel bearings loose?

Are both front and rear wheels true?

Oh never mind.   I have no clue what's wrong with your bike....
 

[Lumbee]

...good to see u back TT...thats a good list to work from...going to the printer as we speak!   

[medic09]

Okay, one more thing to check.

I had a prob like this on my SR 500 and did all the 'differential diagnoses'.  After a week of clueless, annoying rule outs, I noticed by happenstance that the cotter pin locking my rear axle bolt was missing.  The nut had been working progressively looser, until I started getting wobble.  I kept thinking the problem was up front, but the cause was in the rear.

And no, I did not leave the cotter pin out.  The PO had tightened the chain just before I bought it, and, well, kinda forgot it...

Maybe this should be in that thread we have about goofs...

 

[Bob Wessner]

Having put on a new chain at one point, and doing some tweaking to the chain adjustment at the time, I had that cotter off several times and the nut loosened . On mine, the cotter is in a particularly bad place. I ended up replacing the cotter pin with as large a diameter hitch pin as would fit through the hole in the axle instead.

As for the original issue, thought this might be worth taking a look at.

http://www.motorcycleproject.com/motorcycle/text/shucking.html

[BobbyR]

I had this happen when I went to modern tires. I went up in pressure and it is gone. Do the cheap simple stuff first. Modern tires have softer sidewalls.

[Lumbee]

...thanx for that link Bob W...that sounds exactly like my problem...I'll go through the process they out line and see if it helps...

[TwoTired]

I had this happen when I went to modern tires. I went up in pressure and it is gone. Do the cheap simple stuff first. Modern tires have softer sidewalls.

Modern tires are often designed for wider rims than found on the old SOHC4.  The narrow rims don't support the sidewalls properly and higher pressures are needed to stiffen the wall and maintain the tread position relative the wheel centerline.  The pressures also crown the tread more, leading to high center tire wear(and reduced contact patch).  Anyway, the higher crown reduces tread wander as the instigator for the wobble.  Until the tire wears down, of course, and those chicken strips show up, sometimes with returned hands off wobbling.

Not certain, but I think the rear tire tread design/ crown / footprint issues may instigate the wobble moreso than the front.  It did on my CB550s anyway.  Wobble went away after simply changing the rear tire from a 120 on back to the stock size.  Well, as close as I could get, anyway; 4.00 vs 3.75.  And, I have a windshield mounted on the bars/forks.  Of course, the tread design changed, too.

FYI

Also, another thing to add to the list is tread true.  If the tire bead wasn't seated evenly around the circumference of the rim, the tread wanders as the tire rotates.  This can induce wobble from either the front or rear tire. (Or both)

Cheers,

[HondaMan]

...77 750F.  I've had this bike about a year, and this has been a problem since I got it.  When riding at say 15 or 20mph if I take my hands off the handle bars, and just let it coast, the handle bars/front wheel will slowly go into a shudder, or speed wobble.  I then must immediately grab the bars, or the bike will surly shake itself to the point of sending me over the front end!   

Its not really a problem as long as my hand are on the bars, but I know something is not quite right.  Any ideas?  I've put the bike up on the center stand and checked everything on the front end out, and nothing is loose.  The font tire still has good tread, and no cracking with the correct rotation direction.  The rear wheel is in line.  This one is got me stumped.  Short of buying a different front tire just to try something different, I'm not sure what to try!     Any comments or suggestions appreciated.

Search for my comments about the front end alignment issues on the CB750 frame. After the K0, they ALL did this to some degree. If you have a squared-off (worn) rear tire or an uneven chain (or sprockets), or "notchy" steering head bearings, the problem will be much worse.

If all the above is OK on your bike, the fix is ugly: I'm hoping Dave (who has a K0) can find a good way to make some measurements for me so I can try to come up with an easy fix. The "real" fix is to re-true the front wheel to the K0 geometry, to get the wheels lined up (tricky for most guys). They are NOT in line on a stock CB750 frame. That's where the trouble starts.

It's also not as easy to fix as you might think: changing axle spacers and/or tilting the rear wheel are popular fixes, but both have significant drawbacks.

[TwoTired]

Not to be aurgumentative, Hondaman.  But, did you notice the bike in question here was a 77 F model?
I don't claim to be any kind of frame expert, but simply comparing dimensions and frame specs in the Honda shop manual, seems to support the notion that the F model's frame and set up geometry is unique for that model.

Are the fixes and nuances for the K frames applicable to the F frames?

Just trying to limit Mark's confusion, perhaps. 

Regards,

[bryanj]

Head races that are too tight will give this problem as well as too slack, also definately play with both tyre pressures a bit, both up and down.

[Lumbee]

...I had about an hour after work yesterday to play around with the bike.  One thing I noticed is there seems to be a "spring" in the handle bars.  What I mean is with the bike on the center stand if I slap the end of either side of the handle bars there is a noticable "Boying!!!" effect.  Like the trip. trees are to far up on the forks.  I think I'm going to slide them down just a taste and see if that makes a difference.  I also have a fork brace from my chopper I may also install and see if that helps.  I havn't mentioned it before, but I do have a windjammer faring on this bike.  The faring mounts to the frame, and other than adding a small about of weight I would not think this would cause my problem, although the more I think about it, may be it could.   

[HondaMan]

Not to be aurgumentative, Hondaman.  But, did you notice the bike in question here was a 77 F model?
I don't claim to be any kind of frame expert, but simply comparing dimensions and frame specs in the Honda shop manual, seems to support the notion that the F model's frame and set up geometry is unique for that model.

Are the fixes and nuances for the K frames applicable to the F frames?

Just trying to limit Mark's confusion, perhaps. 

Regards,

You could be right, TT: I have not measured the differences between them. This makes me curious: what is listed as being different between the "K" and the "F"?

The heart of the problem on "K" frames is the misalignment of the front tire, relative to the rest of the frame, on K1-K6. I never had the chance to be "intimate" with the "F" frames like I am with the "K" frames.

[Lumbee]

...well, in the vain of "easy stuff first" I lowered the trees down about 1/2" on the tubes just to see if that helps.  Havn't had a chance to test drive it yet, but will report back..

[TwoTired]

You could be right, TT: I have not measured the differences between them. This makes me curious: what is listed as being different between the "K" and the "F"?

Here is the very limited info I have gleened from the Shop manual.  I've no idea if it has any bearing on corrective action you propose.  I'm just doin' the bookworm thing.

Cheers,

[eurban]

Here's a picture that I found on this website http://www.mginproducts.com/Vintage%20Honda%20Motorcycle%20Parts%20CB750%201977-78.html
that shows that the 77/78 Ks and presumably Fs brought their forks 10mm closer (than the pre 77 750s) to steering pivot.  Pic is of their cusutom top clamp but it mimics the stock pieces.  Undoubtedly Honda made a number of changes that may affect handling and or shudder.  On my 78K  (which has a GL1000 front end) I observed that the front wheel (when installed corretly) is not exactly centered between the front forks.  I believe that it is shifted to the left by about 1/16th".  I never measured my front setup when stock but from what I have read here there is also a slight front wheel offset in at least some years of the 750s.  In addition, I found that it was quite easy to install the front forks with a twist in them, this should be checked for when reassembling the front end.  In all likelyhood, It will not magically go together straight n true!  I also found that the alignment marks on my swingarm where a good bit off.  I spent quite a few hours with string lines running front to back to get my rear both perfectly aligned with the front and with proper chain tension.  I recall that the alignement marks were out by about two of the small indicator lines.  Certainly this may apply to all, some or none of other peoples bikes but I would certainly be wary of trusting these factory marks.  Not sure if this helps with the original question but I hope so!

[Lumbee]

...well no luck today.  As I mentioned before I lowered the tubes in the trees down about 1/2", and I still get the shudder if I take me hands off the bars.  I also checked everything about on the wheels, and all seems to be OK.  I left my indicator in shop class, but if I had to guess it looks like the rims may be off my 0.010", which I think is within tolerance.  Both the front and the back rims (comstarts) has dings in them, but as I said, nothing major that I would think would cause this.  I'm thinking now maybe its just the tires.   

[HondaMan]

...well no luck today.  As I mentioned before I lowered the tubes in the trees down about 1/2", and I still get the shudder if I take me hands off the bars.  I also checked everything about on the wheels, and all seems to be OK.  I left my indicator in shop class, but if I had to guess it looks like the rims may be off my 0.010", which I think is within tolerance.  Both the front and the back rims (comstarts) has dings in them, but as I said, nothing major that I would think would cause this.  I'm thinking now maybe its just the tires.   

Can you post a pix of the tires?

[HondaMan]

You could be right, TT: I have not measured the differences between them. This makes me curious: what is listed as being different between the "K" and the "F"?

Here is the very limited info I have gleened from the Shop manual.  I've no idea if it has any bearing on corrective action you propose.  I'm just doin' the bookworm thing.

Cheers,

TT: that's excellent detective work. Here's what I see from that: the steeper caster angle plus the longer trail is exactly what we tried to do in roadracing to "straighten them out" (K models): sometimes with smaller rear tires or larger fronts, longer forks or shorter shocks (or some combination of these). Honda must have lifted the front of the frame sections, either by lengthening the swingarm or shortening the shocks, or they changed the triple trees and lengthened the forks one way or another.

Do you have comparable numbers for the swingarm length and the fork length? If they are the same as for the "K", then the triple trees have to be different, sort of what like Eurban shows above....