Engineering advice needed for poor shocks


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virgilmobile
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Engineering advice needed for poor shocks

Postby virgilmobile » Thu Jul 11, 2013 10:37 am



I am not a mechanical engineer or ever played one on TV.
I do have a idea I'd like to run by those that are.....
Every "used" bike I've had lately has had many miles on them....Even after fork/front end/shock service,the front end exhibits some "stiction".
Now before you jump,understand that this isn't my first rodeo and I do understand the mechanics and that even with new slides,seals,oil,alignment that somethings are just worn out.....

I proved my bike does not have a stiction problem.It's a torque angle binding problem.Years of use has just worn the shocks a bit.
By new ones????I don't think so..
If I angle the bike where the forks are perfectly vertical,my compression and un-load return distance is within 1/4"

With the rake of the forks and angled torque they do "bind" and compression/unload/rest is in the 2" range.

My idea is simple..Use small steel bearings in the binding area.
Obviously they would have to be the same diameter as the slide is thick.

This is just preliminary ideas and I do entertain any input.
Thanks



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themainviking
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Re: Engineering advice needed for poor shocks

Postby themainviking » Fri Jul 12, 2013 9:23 am

Another thing that is common to a stiction problem is overtorqued axle pinch bolts, as unlikely as it sounds. The axle pinch bolts should be the last thing torqued up when a front end has been apart, or even when a front wheel has been off, and they should be torqued exactly to manufacturers specs. If you think your torque wrench is tightening even a couple pounds heavy, this could still be the problem.
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virgilmobile
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Re: Engineering advice needed for poor shocks

Postby virgilmobile » Fri Jul 12, 2013 9:53 am

Yea,Been there done that more than once...
Didja ever wonder why,when you set on your bike,only the rear moves.???It takes more than a light load change to get the front to slide....As it wears,the force needed to move the forks increases...
Mine reacts fine to hard bumps in the road but is just stuck over the easy stuff...These light terrain changes shows up as a bouncing front end....
Adjust,clean,align,replace,fiddle with,etc,etc.....It always improves,but NEVER fixes it.
There is simply wear in the slides that only a new set will cure.
Eventually the pre-load vs. compression/relax ends up getting worse as miles pile on....

At first the static test yields an acceptable 3/8" or so....after 5k miles it's back to a 1.75" difference.

It's just dragging the slide in the tube....service it and it lasts for a while but that doesn't address the problem or possible modification.

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Re: Engineering advice needed for poor shocks

Postby eklimek » Sat Jul 13, 2013 8:00 am

I will follow this with interest.

I presume you have resisted simply doing away with the tube fork.

:idea: An alternative approach -
http://www.dotheton.com/forum/index.php?topic=44998.0
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virgilmobile
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Re: Engineering advice needed for poor shocks

Postby virgilmobile » Sat Jul 13, 2013 10:38 am

The looks are fine,I'd just like to improve on the durability of the design.
Even Teflon bushings will drag to some degree when they are not pressurized with lubricant.
Once you get them moving,the oil does suspend the Teflon coated slide bushing from the aluminum tube.Let it rest for a second and the angle pressure squeezes the oil out,sticking the slide again.
The loose bearing pack would allow small movements till the oil could again be circulated behind the slide.

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eklimek
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Re: Engineering advice needed for poor shocks

Postby eklimek » Wed Aug 28, 2013 8:05 pm

Update needed. Did you pursue this further?

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virgilmobile
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Re: Engineering advice needed for poor shocks

Postby virgilmobile » Thu Aug 29, 2013 6:52 am

Update.Just polished the slides with1000 grit on a long shaft and drill.
Replaced upper and lower slide bushings ($45).
New oil and 2 oz. of friction modifier.
The roller bearing will not work.The bearing balls would not have opposing surfaces to roll on being caged.
Initial tests showed that the balls would roll for a few mm. Between the fork and the slide.
And then stop rolling at the end of the cage.the seized ball would then gouge the aluminum.

A complete roller bearing cluster would have to be used.
Not mechanically feasable

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Re: Engineering advice needed for poor shocks

Postby otistool » Thu Aug 29, 2013 11:30 am

This would be a great idea if the balls could ride on a very hard [rc64/65] shafts. There for, the balls in your case will indent the tube making it useless and probably causing the chrome to fracture and flake off. I have worked with this princible with a product produced by Thompson linear bearings.

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virgilmobile
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Re: Engineering advice needed for poor shocks

Postby virgilmobile » Thu Aug 29, 2013 11:53 am

It was worse.The balls impacted and created a ridge in the soft aluminum because they did not roll.
You need 2 opposing moving surfaces to roll a bearing In between them.
It's not able to roll with the full stroke of the shock because the bearings were fixed to one surface.

It isn't possible to modify the shock to fit full roller bearing guides in the tube and the slide.Theres not enough room inside to replace the lower slide with a full roller type bearing or a verticle continuous feed tube.
Or so I believe.
I had to settle with new slide bushings and a bit of friction modifier.

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Re: Engineering advice needed for poor shocks

Postby Dogsled » Sat Aug 31, 2013 1:43 pm

What the heck is 'friction modifier' ??????

And how did you come up with 2oz's, I assume that's per tube.
"Fight until hell freezes over, then fight on the ice"

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virgilmobile
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Re: Engineering advice needed for poor shocks

Postby virgilmobile » Sun Sep 01, 2013 7:23 am

Friction modifier is a additive used in pro racing or off road equipment.
The amount was a best guess.I just like the number... :D

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Re: Engineering advice needed for poor shocks

Postby Dogsled » Sun Sep 01, 2013 10:18 am

Thanks, sounds interesting. when I did my front end the upper slider bushing was really worn, it must get the most action...Teflon? I was wondering what the heck that coating was that was half worn off!


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