Fork stanchions

Alan W · Jul 29, 2011

I have recently replaced my corroded fork stanchions with new shiny ones from a respected UK supplier, along with new seals & bushes top & bottom. I now find that I have a front wheel 'shimmy' above 50 mph which is definitely NOT good! I stripped the forks down again and found that one slider was very, very tight to slide up & down the stanchion by hand (OK, nearly impossible) with oil drained & top nut removed etc, whereas the other was firm but smooth. I switched the sliders over & the problem remained on the same stantion. I have also done same again with the old bushes fitted, but it made no difference. I am assuming of course that the sliders are OK, as I did not have this issue before I replaced the stanchions.

I measured the stanchions' diameters & found the 'stiff' one was 34.50mm dia, whereas the looser one was 34.45 mm dia (about 5 thou in old money), both appear straight & not bent in anyway. Before I contact the supplier to get them replaced my question is, what is the correct diameter for the fork stanchions, and how hard should it be to slide the sliders up & down on re-assembly?

Alan

Nortoniggy · Jul 29, 2011

Looking in the workshop manual.
Main Tude OD - 34.29mm to 34.467mm
Top Bush Fitted ID - 34.53 to 34.48mm

Looks like one of your stanchions is oversize. In engineering terms 5 thou is quite a big difference.
I don't know which 'respected' parts supplier you bought them off, but there is one that never gets anything wrong, so best of luck.

Ian

AntrimMan · Jul 29, 2011

My book is a little different. It is for a MKIII.
Main tube 34.518 - 34.480 1.3589" - 1.3574".
Are the top bushes new? If the corroded tube impregnated the existing bronze upper bush with some orphaned chrome flakes then it could be tight.
Slide the bushes alone up and down the tube in the working area, then with the slider and nothing else in place, no damper tube, no spring, no seal, no nut.
They should barely slide freely down under the the weight of the slider when held verticle. They should not be stiff to slide once sliding is initiated.
BTW .05mm is roughly .002" not .005", only saying.

AntrimMan · Jul 29, 2011

sorry, I see the bushes are new so disregard the flaky chrome statement.

AntrimMan · Jul 29, 2011

another thought that I should share...
It is common in machine building to finish size sintered bronze bushes after fitting into the housing (slider in your case).
They are often not dead on concentric od to id or round. Sintering is like that.
Best sized with a roller burnisher or ball burnisher, ie chipless. Go to a web site for Boston Gear Bost-Bronze and they will have a section making these claims.
All the best, you'll soon be sorted.

Nortoniggy · Jul 29, 2011

Mine was from a 1972 ish manual. Just looked at imperial size in manual & it's 1.3590 to 1.3575 which if you work out the millimeter equivalent, rather than accepting the book value, is 34.480 to 34.518. Obviously whoever wrote the '72 manual wasn't very good with those new fangled millimeter things.

Ian

Alan W · Jul 29, 2011

Slide the bushes alone up and down the tube in the working area, then with the slider and nothing else in place, no damper tube, no spring, no seal, no nut.
They should barely slide freely down under the the weight of the slider when held verticle. They should not be stiff to slide once sliding is initiated.

Will try this this weekend. Thanks for the quick replies.

Alan

PS FYI I got them from Andover Norton direct, but these things happen, so I'm not bitching!

T.C. · Jul 29, 2011

Interesting - my manual (admittedly a fleapay Xerox) is a bit different:

Main tube OD: 1.3589/1.3574 (34.518/34.480mm)

Top bush fitted ID: 1.3595/1.3604 (34.531/34.556mm)

So, we need a third party to chime in with a tie-breaker :? Or maybe a third set of numbers...
For grins & giggles, I just ran out to the garage & checked mine in a no-wear spot: 1.3577 on the left and 1.3582 on the right side. It's been a couple years since I rebuilt the forks, but I think they were pretty stiff when reassembled w/new bushes. The ride was fine, though.

I am not surprised that you're getting shimmy with that large a difference in the tubes, one side is running really free and the other is not so much. The total diameter tolerance is 0.0015", and your stanchions are different by 0.0019"; like Iggy said, that is huge in this application.

john robert bould · Jul 29, 2011

Alan W said:
I have recently replaced my corroded fork stanchions with new shiny ones from a respected UK supplier, along with new seals & bushes top & bottom. I now find that I have a front wheel 'shimmy' above 50 mph which is definitely NOT good! I stripped the forks down again and found that one slider was very, very tight to slide up & down the stanchion by hand (OK, nearly impossible) with oil drained & top nut removed etc, whereas the other was firm but smooth. I switched the sliders over & the problem remained on the same stantion. I have also done same again with the old bushes fitted, but it made no difference. I am assuming of course that the sliders are OK, as I did not have this issue before I replaced the stanchions.

I measured the stanchions' diameters & found the 'stiff' one was 34.50mm dia, whereas the looser one was 34.45 mm dia (about 5 thou in old money), both appear straight & not bent in anyway. Before I contact the supplier to get them replaced my question is, what is the correct diameter for the fork stanchions, and how hard should it be to slide the sliders up & down on re-assembly?

Alan

.05 is two thou in old money..but big enough to cause stiffness!

Hobot · Jul 29, 2011

Just measured Ms Peel's fine working Roadholders with Frank's stanchions used 7000 rough miles to find each one in the seal sliding area are 1.378".

Hobot · Jul 29, 2011

I retained the 40,000 mile orginal top bushes as 3 set I tried were all out of round and when hammer tapped over stanchion to make so round they just fell to bottom they were found to be too loose, ugh.

Alan W · Jul 29, 2011

Guys, am sitting at my desk at work in England with an image of Commando owners all over the world running out to their garages to measure fork stanchions! Its wicked & much appreciated

Alan

Norton-Villier · Jul 29, 2011

Dont ya just love that

Phil

bwolfie · Jul 29, 2011

I would run out and measure mine, but they have not arrived in the mail yet!

john robert bould · Jul 29, 2011

hobot said:
Just measured Ms Peel's fine working Roadholders with Frank's stanchions used 7000 rough miles to find each one in the seal sliding area are 1.378".

1,378? sounds big :?: 1,358 yes

john robert bould · Jul 29, 2011

I have a new pair of Ebay Commando 34.42 and feather bed set 34.48 appears to be pot luck the book states 34.53-34.48 so the featherbed ones from Velocette are right on top limit...the Ebay stuff is .06 down .. which is to slack,if the bush is top limit aswell.
Generaly .002 [.05mm] clearance is a good fit

Hobot · Jul 29, 2011

1,378? sounds big :?: 1,358 yes

Ugh you are right, I just re-did it to see I'd held mic at an angle. This time each
= ~1.365" >> except OD varies from 1.362 to 1.3 72 as I check a-roundness, ugh.
Obviously stanchions come in a scope of tolerances and then wear on top of that.

Trixie is out in hot shed so I didn't rush out with rest of the world but just reached forward from my recliner having am coffee. This time I stood over it to square up with jaw pressure. Yet on this collect Borg Brain note, I swear there have been impossible stuck stuff on pre-Ms Peel that after I cried in public lists the mental focus changed the quantum arrangement of atomic bonds and structures in my favor and it'd about hand twist off or come right out next attempt. Enough times its part of my mechanical resources I still pull on time to time.

Fork stanchions

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