Sticking fork tubes

HMBAtrail · Jul 25, 2012

I am having a bit of trouble overcoming some pronounced sticktion on the forks of the Commando. Fork tubes are off the bike. Stanchions are from Forking by Frank, external compression and rebound from Landsdowne, top out sleeves and turcite bushings from JE Engineering.

The upper and lower bushings as well as the aluminum sleeve from JS are installed and just sliding the tubes in and out of the lowers takes a fair amount of effort. They will move but hardly smoothly. How much resistance should I be seeing on this?

comnoz · Jul 25, 2012

They should fall by themselves without springs. Jim

Hobot · Jul 25, 2012

Roll stanchions against each other to check true-ness. Its also possible the bushes got squashed some in handling for out of round-ness. I learned to expect out of round new bronze bushed but no ID if can occur in JS composite ones. There is some variation in OD of stanchions so measure their OD and the bush ID for possible mis-match. About only resistance in normal forks comes by far from the spring in the tubes so assume you ain't got them inside for this testing hang up. May have to ink various surfaces for clues from witness marks.

john robert bould · Jul 25, 2012

Been there more times than i care to remember!
Please note the sliders are getting on ,i have some in the work shop for refurbing...precise steel sleeve pressed in and new"plastic bush's" makes them better than new,
I have a picture or two..i will get posted..via Lab. fingers crossed :wink:

HMBAtrail · Jul 25, 2012

Fork tubes are straight as a pin. Rolled them across the surface plate and all is well there. Turcite bushes slide onto tubes no problem at all so i.d. of bushes and/or o.d. of fork tubes appear to be OK as well. The problem arises when I insert the fork tubes (with lower turcite bush and alu. sleeve) into the fork lower. Yeah, it goes in but with a bit of force and to slide it up and down (without springs or damper installed. Just fork tube inserted into fork lower) takes some force. It doesn't bind anywhere in the travel, it is tight throughout.

Contemplating running a 38mm flex hone brush through the lowers just to clean up the sides and see what happens.

With the forks installed on the bike there is next to no change in rebound as I suspect it is overwhelmed by the amount of stiction. To move the fork through its travel I need to roll the bike forward and apply hard front brake.

P.S. I am using 150cc of 15wt fork oil

john robert bould · Jul 25, 2012

Hi Skim the lower bush on a manrill, avoid trying to enlarge the whole leg!

Hobot · Jul 25, 2012

1+ on taking down the bush not the slider insides. But would it be better to take off the ID or the OD of the slit bush for better slip function?

I would still check stanchions by rolling against Each Other, rather than just on flat glass, which may not reveal the doubled out of true amounts that only bothers when mounted together sliding.

john robert bould · Jul 26, 2012

Up date? please

jseng1 · Jul 26, 2012

If they're tight then I'll send another pair of lowers that are .001" smaller if you return the be tight ones so I can check them. I also need to know the diameter of the steel tube where the lower bushing slips on. I could use a few measurements from several fork tubes to get an average - so Norton owners get out your calipers and chime in.

Jim

Brooking 850 · Jul 26, 2012

Hi Jim, I will send you some measurements from the 2 x sets I have apart at present, 3 x set is on the Roadster and in full use, maybe late next week as I am still away at work.
Recieved my postal notice for my last order thanks, there will be more to come once I get the engine apart.
Regards Mike

john robert bould · Jul 27, 2012

Jim.
As i have already posted...the reduced stanchion lower bush dia can vary 2/3 thou, Which means your lower limit ,say plus 3 will produce a slack bush in some .
The steel original is "way up on size " to produce a "fit" that will suit most, plastic if a "tad" tight will expand when tapped on ,and will expand.
I have made them tight on the lower dia and machined them to suit
But sending the whole fork legs is a bit expensive postal wise.
A guy sent me some from the states, cost him £50 customs duty and £100 post! plus the cost of re-sleeves and stanchions and bush's..it was cheaper to buy new legs!!

comnoz · Jul 27, 2012

What ever happened to the old practice of laying shim stock under the lower bushing ? Jim

john robert bould · Jul 27, 2012

Jim . It went out when loctite arrived! Believe me its good, my ex-loose lathe chuck key bar is still firm after 12 months! BUT its no use on plastic :!:

comnoz · Jul 27, 2012

But I don't think locktite would work under a lower bushing to get a good fit. Jim

john robert bould · Jul 27, 2012

Some as excellent gap filling properties. 2-3 thou is normal.

comnoz · Jul 27, 2012

john robert bould said:
Some as excellent gap filling properties. 2-3 thou is normal.

Sure locktite would fill a gap but the idea of shim stock is to expand a slotted metal or plastic bushing to fit a lower slider. Common practice on lots of motorcycles. It works on a Norton too if the bushing is slotted. Jim

john robert bould · Jul 28, 2012

Jim,
Getting a one thou shim wrapped in place around a lower bush dia and getting the bush on without it tearing is proberly behond most , Every motorcycle builder should have access to a lathe,and make up over/under size bush's when required.
I personaly have had lathes 40 years ...motorcycle tool number 1 :!:

comnoz · Jul 28, 2012

john robert bould said:
Jim,
Getting a one thou shim wrapped in place around a lower bush dia and getting the bush on without it tearing is proberly behond most , Every motorcycle builder should have access to a lathe,and make up over/under size bush's when required.
I personaly have had lathes 40 years ...motorcycle tool number 1 :!:

It's a piece of cake. Install the bushing too far. Up onto the chrome. Then wrap the shim around the recess and slide the bushing back down. It snaps into place on the shim. Jim

lcrken · Jul 28, 2012

An alternative to the stock bushing is a split bushing from a Japanese bike. I think we used to use one from a Suzuki back in the late '70s or early '80s, maybe a GS. It was a metal bushing with, IIRC, a bronze bearing surface impregnated with teflon. The fit was dialed in with brass shim stock behind it. The idea was that it should have less friction than the stock steel bushing. I think I orginally got this info from Jim Schmidt, so he might be able to recall exactly which model the bushing came from. I think I still have a couple of them tucked away somewhere. If I can find them, I'll post the part number. They worked really well. At this point, I don't recall if I had to turn down the diameter of the fork leg to fit them, but I don't think so.

Ken

comnoz · Jul 28, 2012

I built a few Nortons with Jap lower bushings. They worked great. That teflon bronze is some slippery stuff and lasts a long time. I don't think you had to machine the fork tube but I think you had to narrow the bushing a bit.
I have also machined out the stock upper bushing and pushed a thin walled teflon bronze bushing inside of it. I believe it was an old Yamaha bushing but I don't recall for sure.

I always used shim stock under the lower bushings to tighten up the forks on the older bikes. Particularly the European dirt bikes. It made a world of difference. Jim

Sticking fork tubes

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