OzVMX Forum
Clubroom => Tech Talk => Topic started by: steve234 on February 19, 2018, 10:11:12 am
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G'day, I have a set of Betor forks from a Bultaco MK7 125 in which one of the fork tubes sticks at the lower end of the tubes stroke. It only happens to the non brake arm side slider. I have stripped and cleaned them and replaced the seals and it still does it, just not as bad. Have tired the tube in the other slider and it moves through the full length of the stroke nicely. Any ideas on a fix would be greatly appreciated.
Cheers,
Steve
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the leg will be bent ever so slightly and only binds on full compression. roll it along a flat surface and it should be apparent. otherwise the slider is out of shape at the bottom :o
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G'day, I have a set of Betor forks from a Bultaco MK7 125 in which one of the fork tubes sticks at the lower end of the tubes stroke. It only happens to the non brake arm side slider. I have stripped and cleaned them and replaced the seals and it still does it, just not as bad. Have tired the tube in the other slider and it moves through the full length of the stroke nicely. Any ideas on a fix would be greatly appreciated.
Cheers,
Steve
Probably dinted; you might need to get in the slider with a wheel cylinder hone.
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If the tube works in the other slider that would suggest the other tube will bind at the same spot in your problem slider, to me that means the slider is the issue
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Had the same problem with my bultaco a few weeks ago, eventually used a brake cylinder honing tool( 3 arm type)just a bit at a time with some oil to lube it everything good now.
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Thanks guys should be able to sort it.
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Yep probably/possibly a bent alloy slider. Either that or a dent. If dent i saw a guy push the fork leg down past that area & work away at ithe dent tapping around it till it relieved it & all good. Mind you that was on a YZ80H in 1981 lol & pretty thin sliders. But a mate many years ago mx'd a bsa b50mx i think they're called . Basicaly a trail bike but landing hard all day from a particular big jump bent both lower sliders forward & the forks stuck down. Luckily he was a good engineer & gently blocked & pressed then till they were very close then got a hone down there & got them perfect. Was a motor reconditioner so probably had a good parallel hone if that's the word. Pretty disappointing for it to even happen--bloody cheesy things. With 'yours' revolve the slider on the stanchion & eyeball the center line of the damper bolt hole in the bottom & see if it revolves true to decide if it's curved in the slider or just dented somewhere along the outside. Pretty sure my mate hat betor forks on his wr400 husky in about "74 & were good too. If they're standard on Bullies i used a new Mk8 pursang for a season & loved it. Had realy well damped rear shocks too out of the box if betor too on the rear?.
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How do you straighten the fork tubes?
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Best way to straighten bend fork tubes is with a press, V blocks and a dial gauge. And a pinch of patience!! ?
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as big thumpa says
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can also try hydraulic pipe bender. Using vee blocks can lead to having square fork tubes :)
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Fork tubes--the steel long bits, yes straighten in a hydraulic press but really an engineers job if you've not done it before or can end up with a dogs leg pretty easily, [i've done it]. But if you're talking the lower alloy sliders, or just one sticky slider, then also ask an engineer but won't be easy. Not even sure how i'd do it but as i said above--mate had all the gear in a motor recon shop to hone it till it was free to slide but do think he pressed the alloy slider 'somehow' to improve it before honing it. If i 'had' to do it [& it was my own] i might try running the straight steel stanchion down to the sticking point then set it up in the press supported under the axle end [or lower if leading axle] plus up the steel stanchion the other end then with a block of wood shaped to the alloy slider, press it a little about where the sticking point is, which will be internally supported by the steel tube or the bush on the end if it has one. Or ignore all that as sounds too complicated now i've read it. Take them to a good M/cycle sympathetic engineer & see what he says. Tell him my idea so he can at least say--jeez ya can't do that lol. Ok re-read your post & see it's sticking at the bottom of the stroke, so i dunno--needs honing at that point. if that's possible. Long rod & sandpaper in a drill--bit rude but--. Look & feel around it for a dent from the outside & try to concentrate sanding inside on that area. Just a thought though, the forks are lucky to actually get all the way to touching bottom so is it sticking down when riding?. Sorry about the essay.
Edit--& after all my woffling, the answer might be here in the last post--brake cylinder hone--have a look.
https://forum.ozvmx.com/index.php?topic=42575.0
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Cheers for the input. Bought this bike about a year ago supposedly right to go pass scrutineers no sweat ???? turns out a total piece of shite, another vmx stitch up. So both fork tubes are bent, carb is flogged and the list goes on.
How do you get the damper rod out? Have pulled the circlip out and the bush moves a bit but won’t come out. Any tricks?
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should just pull out from memory but tread carefully. crap can get in there and stick it up, especially if the steel bits have had some corrosion. damper might also go out the top as well?