Hi all!
When driving with higher speed my central girder fork spring goes into resonance and vibrates with high frequency. Has somebody observe the same at his machine? Or ist hat simply a normal phenomenon? I wonder what the material is doing after a while. Isd a fatique break possible?
Cheers, Thomas
Have you tried tightening the fork shock absorber, Thomas? It may also be worth checking that the shock absorber is correctly assembled.
Yes, Paul, everything's in order. But I'll check it.
I now have tried tightening the fork shock absorber. No improvement. But I think this is no surprise because it is high frequency vibration whereas the absorber is for low frequencies.
Is any other vibration noticable at speed?
Does the vibration seem in tune with the engine?
Vibrations are not my problem because their are inherent of the machine. As I said, the spring vibration is in resonance at certain high revs. My basic concern is a potential break because of material fatique and I wonder if I am the only one who has that.
but surely resonance means that there is another vibration source making it resonate.
PHYSICS
the reinforcement or prolongation of sound by reflection from a surface or by the synchronous vibration of a neighbouring object.
Yes, as I said, it comes with high revs. The motor delivers the vibration. And the spring is in resonance because it seems to have the same eigenfrequency (=natural frequency). And because this eigenfrequency is much higher than that of the girder fork the damper (for the girder fork) can't stop it. The only remedy could theoretically be an additional mass right at the spring. But that's impossible to install. It seems that I am the only one who observes that and, hence, I am concerned that the spring breaks after some time.
I would check that the head steady is tight..If nothing else works could you get some rubber to insert in the spring coils to try to soak up some of the vibration.
For me everything looks fine. The head bolt is fixed and the symmetric spring is well placed. I also thought about elastic material in the coils but fear this not to be a good solution. After all, something must be wronmg because nobody else report this behavior. I'll try to fiddle around again.
Does the top of the spring look off centre to the top boss?
By "top boss," do you mean the top support, as indicated in the picture? The situation there is symmetric as you can see in the other picture. In the third picture there might be a tendency towards the left. But could that really the origin of my problem?
I;ll take a look at both of my bikes tomorrow to see if the spring bends like yours..Your second lot of pics must be from a better angle as the top of the spring is symetrical with the top mount.
Certainly looks as if not all is right with that spring if it does bend side ways, weak ,worn out or just wrong?
can you post a pic of the links from the side, on the stand and off it with weight onthe forks.
the links should not point up.
Yes there's something not quite right. I notice you have a bolt securing the spring which is unusual its normally a worm stud an nut arrangement. I've arrowed picture below. These are dog leg type forks a bit earlier but the principle is the same.
(https://i.postimg.cc/ZKYVZ5m7/dogleg-forks.jpg) (https://postimg.cc/kDZxv9BK)
fully assembled.
(https://i.postimg.cc/WzyDQ8TP/DSCF6963.jpg) (https://postimg.cc/D4qyGqSC)
Well i checked both of my springs , but they are on the earlier dogleg forks, they are perfectly straight..one is an old spring and the other a new one I had made by Alberta Springs..I had 3 made, the other 2 went to the club shop.
Your bike Thomas lists a different spring that appears to have been used in '36 & 37 only..
Have you tried loosening the top and bottom bolts and turning the spring to a different position to see if it makes any diference to the resonance..John
Hi together!
Very usefull information, indeed! Thank you for that. Attached are some pictures taken under load and when front wheel lifted. Good access is difficult but for now I don't want to dismantle it. It seems that John is right with the 36/37 springs. I have no worm stud. And I didn't try to turn the spring because it seems that this won't be possible at all.
John, I found Alberta Springs in Great Wenham. What Spring did you buy there? Are thy familiar with the old stuff? And could a new spring really deliver remedy?
Best, Thomas
Thomas,, looking at the photos it appears as if the claws on the spring mounts are not gripping the spring in at least 2 positions. Could this be where the vibration is coming from?.
John, where do you see that?
Pah, today I went to Nürburgring with the Sunbeam. About 150 miles over all... and at the end got a puncture 15 miles from home... :o
Picture 4 and picture 2.
Ok. It looks a bit suspicious, indeed, but I don't believe that this is the origin. Even if the girder is lifted the spring is under certain load and I can't twist it. This is really strange. There is no clue about the vibration origin and there's no possibility to adjust anything... :-[
BTW, I just found out that my tire valve has been torn off. The air pressures indicated in the manual seem to be too low.
Modern tyres need more pressure than the originals..i run mine on 30lbs/square inch..just over 2 bar.
looks like some contact between the spring and the forks by the look of the paint scar, thats not right either.
Quotelooks like some contact between the spring and the forks by the look of the paint scar, thats not right either.
I believe you mean this here, right? Note that this picture is taken after I completely lifted the wheel and the fork is not under load. That never happens when riding unless I'm going down a ski jump... 8) This has certainly no influence on the vibration.
Cheers, Thomas
no ski jumps here to do that, but our potholes do the same thing :(
That's right, indeed, but there's nothing we can do. Neither with the potholes nor with the spring/girder design... :o The issue remains a mystery to me.
Is a diagram of the girder fork available anywhere? It would be useful to check that the parts have been assembled in the correct order.
Me again John!
I guess you missed my questions: I found Alberta Springs in Great Wenham. What Spring did you buy there? Are thy familiar with the old stuff? And could a new spring really deliver remedy? More information is highly appreciated!
Best, Thomas
I got the centre spring for my Dogleg forks..he had the specification in his book..whether he has the information on yours i cannot be sure..Your spring could be the same as the AJS ones.As to whether replacing the spring will cure the noise,,I don't know..Tracing the source of the vibration would be my main concern, but what is causing the distortion? if the spring were removed from the bike you could at least then inspect it..
Thank you! I will call them up and then I will know. Tracing the source... well, I am almost inclined to open the primarycase, although I wouldn't locate the problem there. Maybe an unevenly worn sprocket due to firing stroke or a slack cush drive...? I have no clue... :-[
Removing the spring would be a major task. But maybe it's the golden bullet.
Cheers, Thomas
I just got a response from Alberta Springs which might be interesting as a future source for girder springs.
QuoteHi Thomas.
Strange problem.
The only thoughts would be that either the end of the spring is a loose fit on the scrolls or there is a crack on the material somewhere under the paint.
We do manufacture the springs at a cost of £50.00 (pounds sterling ) plus the postage cost.
I have looked on my records & think I have correct dimensions, but would need yours measuring up to confirm they are correct.
Hoping this helps.
Best Wishes & stay safe.
Graham Jennings.
Alberta Springs.
It seems a singular problem, indeed, and that I have to dismantle the fork sooner or later...
I am glad you have made contact with Graham..He has a wealth of knowledge on springs..I have been to his workshop a few times on my own and with Vic Youell to discus springs that can be put into the Club Shop..He is always willing to talk, afternoons being better than mornings for him.
I have also referred another member to him to get a rare valve lifter spring made to a broken pattern,which he did without any problems..You are in good hands with him..John
Hi John, I have asked him how realistic is the risk of fatigue fracture and just got his answer:
QuoteI think the risk would be very low.
Usually the springs snap or break mainly due to age & material pitting.
That gives me some relaxation and time to check it out in more detail. I am still trying to identify other possible sources for this behavior (I just tested for evenly slack in the primary chain - it's perfect). As soon as I decide to dismantle the girder fork any advice and helb will be appreciated. I am concerned that I do something wrong, especially with the sensitive bronze bushes.
Cheers, Thomas
I'm just back from a test ride to exclude possible reasons for the vibrating centre spring:
1) Increasing the revs in neutral gear position delivers the same vibration at roughly the same rev intervals with the same type of vibrations.
2) Bike on centre stand at 2nd gear: Same as 1).
3) Same as 2 but applying the rear brake: Same as 1). But the enveloping overall shape of the spring changes dynamically. That could be a sign that the spring isn't centered.
4) Changing the advance/retard/choke at high revs: Same as 1) There's no obvious mis-fire.
5) Point gap of spark plug is 0.7mm. Brown electrode. No mis-fire.
The clutch is ok because I already scrutinized it in detail including disassembling. I didn't check the cush drive, though.