Author Topic: Problem with forks  (Read 1566 times)

Offline clive f

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Problem with forks
« on: 29.05. 2011 19:35 »
I have a 1948 BSA star twin and I have just replaced the fork seals but the forks are still causing me a problem. The forks drop when the brake is applied and only go up and stay up again when physically pulled up. This was the original reason I changed the seals (with the appropriate tool) and also because they were leaking.

Any one got any suggestions to solve this or recommendations on where I can get them totally rebuilt?
1948 BSA A7 (Rigid)
1962 BSA C15 Star
1958 BSA A7 (swingarm)
1978 Honda CD175
2004 Triumph Bonneville T100

Online muskrat

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Re: Problem with forks
« Reply #1 on: 29.05. 2011 20:46 »
G'day Clive, welcome to the forum.
                                               Sounds like the fork leg bushes, or the forks are twisted and binding the bushes. Did you completely strip and clean the forks before replacing the seals?
Could the PO have replaced the bushes and not sized them to the legs and sliders? I had this problem once.
Cheers
PS where in the world are you?
'51 A7 plunger, '57 A7SS now A10CR, '76 XT500, '77 AG175 '83 CB1100F, '81 CB900F project.
Australia
Muskys Plunger A7

Offline clive f

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Re: Problem with forks
« Reply #2 on: 29.05. 2011 23:37 »
Hi I am in the UK, Lincolnshire to be exact. I bought new fork seals according to part numbers on the web, I didnt completely strip the forks, just replaced the seals, they seemed to locate ok though.

1948 BSA A7 (Rigid)
1962 BSA C15 Star
1958 BSA A7 (swingarm)
1978 Honda CD175
2004 Triumph Bonneville T100

Offline Jules

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Re: Problem with forks
« Reply #3 on: 30.05. 2011 07:15 »
Hello Clive, seems like forks are a general problem at present! Being in the throes of repairing mine a couple of things strike me...
1. the bleed holes in the bottom of the fork legs that the oil transfers around could be restricted?
2. the springs could be weak/worn, hence not enough rebound force to overcome the oil damping?
3. very thick oil?
3. bent legs causing stiction in the bushes?
Tell me if I'm talking through my backside Muskrat, but it did strike me that the other things could have an effect before replacing bushes/legs.........  *smiley4*, cheers

Online bsa-bill

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Re: Problem with forks
« Reply #4 on: 30.05. 2011 09:10 »
Hi Clive

You didn't by any chance grip the fork leg in a vice or grip the mudguard  bracket in a vice to remove the seal holders?
A few of us ( the honest ones with no shame) will admit to having done this in a forgetful moment, it results in a distorted tube that prevents the stanchion sliding up and down as it should.
All the best - Bill
1961 Flash - stock, reliable, steady, fantastic for shopping
1959 Rocket Gold Flash - blinged and tarted up  would have seizure if taken to  Tesco

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Re: Problem with forks
« Reply #5 on: 30.05. 2011 10:10 »
I wouldn't say that Jules, your points do have merit.
Clive, if they were mine I'd take them back out, clean and inspect all the parts. IMO the forks are one of the most important parts of a motorcycle, a wobble at the back can be handled but at the front is dangerous.
Once you have the wheel and guard off push and pull each leg to see how much play there is in the bushes. You should not have any more than a mm or two at most. Also doing both at once checks the steering head bearings. Also check they are parallel and not twisted in the triple clamps.  Now remove and compress and extend each leg for signs of binding. Dismantle, cleaning and checking as you go. Springs should be 10 3/4" long. If it all checks out OK assemble with 142cc 30wt and try again.
This is a job I do to any new acquisition and then again every two years or 30K miles.
I may be wrong but my money is still on the bushes.
Cheers.
Just saw Bill's post. Yep been there.
'51 A7 plunger, '57 A7SS now A10CR, '76 XT500, '77 AG175 '83 CB1100F, '81 CB900F project.
Australia
Muskys Plunger A7

Offline NickSR

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Re: Problem with forks
« Reply #6 on: 31.05. 2011 23:01 »
Hi Clive
Where abouts are you in Lincolnshire ? I am 5 mins from Newark Show Ground.

Regards
Nick
1962 Super Rocket
1955 BSA C11G
1998 BMW R850R

Offline Topdad

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Re: Problem with forks
« Reply #7 on: 01.06. 2011 10:25 »
Good morning all, just a thought ,have you bounced them with the nuts and bolts  on the bottom yoke and wheel spindle loose ,it's worked on mine in the past,regards Bob.
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Offline RichardL

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Re: Problem with forks
« Reply #8 on: 08.01. 2019 19:21 »
Using this topic for my new fork issue post.

Fact is, I've only fit sliders to stanchions once, and that was about 12 years ago on the A10. Now, On my A7 barn find, I'm trying to get the stanchions with the original bushings to fit in the original sliders. Sliders look great inside. No evident rust. Old bushings look good and are a good fit at the top of the sliders. Problem is, the bottom bushing does not want to go in past a few inches, after which it binds up. I also tried with new a new bottom bushing and same trouble. Having said that the sliders look great inside, I have to add that the new bushing came out with scratches.  Really not sure what's going on. I seem to recall no real trouble separating the stanchions from the sliders when dismantling. I think the only thing that has changed is the sliders got powder coated, but the insides were protected at the time. I assume some abrasive action will be needed. Should I hone inside the sliders or sand the exterior of the bushing. I guess sanding the bushing only affects the readily replaced part.

Your advice, as always, is appreciated.

Richard L. 
Plan on signing up for the world-wide 2017 DISTINGUISHED GENTLEMAN'S RIDE on September 30, 2018. Watch website at https://www.gentlemansride.com for details.

Online chaterlea25

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Re: Problem with forks
« Reply #9 on: 08.01. 2019 21:23 »
Hi Richard,

Have you some way to measure the insides of the fork sliders for any ovality or distortion?

In a previous topic on forks I posted a picture of a honing tool I made up, Its problematic to hone down past the tapered plug though  *sad2*

https://www.a7a10.net/forum/index.php?topic=10811.msg81276#msg81276

John

1961 Super Rocket
1963 RGS (ongoing)

Offline RichardL

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Re: Problem with forks
« Reply #10 on: 08.01. 2019 22:22 »
John,

I do have a bore gauge for small bores, whcih should work with the sliders. I'll have to test it when I get home. The tool you made looks great but requires tools I don't own (lathe and a mill) to make it. I was thinking of making a sandpaper flap wheel with 600 grit wet/dry paper. Won't go past the prong at the bottom, but I am thinking that that portion is the least likely to be deformed (being strengthened by the nearby circumferential weld). Would you agree that if I find no ovality I should remove the material from the bushes (with a hedge trimmer?)? 

Richard L.
Plan on signing up for the world-wide 2017 DISTINGUISHED GENTLEMAN'S RIDE on September 30, 2018. Watch website at https://www.gentlemansride.com for details.

Offline RichardL

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Re: Problem with forks
« Reply #11 on: 09.01. 2019 00:07 »
OK, my bore gauge doesn't really reach far enough into the sliders for very good analysis, but enough to tell they are somewhat oval. I got one stanchion with an old bush to the bottom without much forcing, but the bush returned with a full pattern of scoring. I think it must be from rust I couldn't see. As for the ovality, the only thing I can think of is the slider deformed through normal use in years before I owned the bike. Then, the bush became keyed to the shape of the slider.

Does any of that make sense?

Richard L.
Plan on signing up for the world-wide 2017 DISTINGUISHED GENTLEMAN'S RIDE on September 30, 2018. Watch website at https://www.gentlemansride.com for details.

Offline Tomcat

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Re: Problem with forks
« Reply #12 on: 09.01. 2019 08:01 »
G'day Richard
Here's a tool I made up to take dents out of my sliders. The rod goes into the vice and the mandrel goes into the slider, then some gentle panel beating next to the damage and dents come out. Not sure how this would work on oval shapes but I'd certainly give it a go. You are most welcome to use my tool but shipping x 2 would be a killer  *eek*
Cheers TC
'48 A7 '59 SR '74 850 Commando TDM900

Online Rex

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Re: Problem with forks
« Reply #13 on: 09.01. 2019 10:37 »
Another easy solution which sometimes works is to loosen the lower yoke pinch bolts and the stanchion top nuts (crack the taper too) and rotate the stanchions. Pure trial and error, but there's often a position where the stanchions are at the optimum slider-free condition.
Not always easy due to the headlamp mounts or shrouds, but worth a go before major strip-downs.

Offline Swarfcut

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Re: Problem with forks
« Reply #14 on: 09.01. 2019 10:55 »
Hi Clive

  Judging from the list of bikes you have, I am sorry if if some of this is a bit too basic, it is always difficult to know how good folks are with the greasy aspects of ownership.
  Bends and bows in the fork legs can be found by rolling the bare legs on a flat surface.
 Drain off the oil and see how well each leg moves by itself.  If you find a tight spot its worth dismantling the forks completely, washing out the sliders and making sure the lower bushes slide freely.

 When reassembling the forks, oil the bushes and lubricate the oil seal lips and exposed leg with a smear of grease so that the seal does not run dry. Each leg should move in and out smoothly, easier to check if they are drained off.

 Any pitting in the fork leg will compromise the effectiveness and longevity of the seal....short term fix is epoxy filler to fill the pitting and smoothed off to match the fork leg.  They can be ground and hard chromed back to size, but the cost may be prohibitive compared to a new pattern leg.

    With the mudguard bridge unbolted or removed, reassemble all loosely and then tighten as directed in the manuals....start with the top nuts, pump the forks to settle them, align the front wheel, pump again, tighten lower yoke pinchbolts, pump again, then wheel spindle, finally top yoke pinchbolt. You should now have forks that compress and rebound. If they bind with the mudguard bridge reassembled, that's an easy fix. If set up correctly and the return is poor but with no evidence of binding, and filled to spec. with the correct oil, the  cause is weak springs or blocked oil bleed holes as mentioned above. The mudguard bridge is important for the stability of the sliders, but must not apply any sideways force to them, otherwise the bushes will bind.

  Swarfy