Undersize Fork Stanchions

lcrken · Jan 19, 2015

I'm in the process of building a MK3 for my grandson to ride (He rode my MK3 at the rally last year, and really liked it) with me at the rally this year. While working on some fork mods, and looking for a good set of stanchions, I measured several from my parts collection, and found all of them to be undersize. The service manual says the diameter should be 1.3575 ' - 1.3590". The six I measured ranged from 1.3530" to 1.3570". I'm not sure of the provenance of these tubes, but I think one pair was original MK3, and one was a replacement set that I bought several years ago from either Forking by Frank or Old Britts, and the other is of unknown origin.

Just wondering if anyone else has found this to be the case. I only noticed it because the fit to the top bushes seemed really sloppy on the stanchions I started with.

Ken

Hobot · Jan 19, 2015

Factory Trixie's are 1.355" and Peels replacements 1.350", neither set from Forking Frank I'd been alerted to on other older forums. That was then so may be different now. Try JMS composite teal bushes that may work a treat with your narrow tubes. Even full OD stanchions are sloppy fit in factory bushes though not detectable but by vision riding or by hand in shop it don't seem desirable. This is a type quandary I start calling world wide list of vendors for scope of issue and how to avoid.

john robert bould · Jan 20, 2015

45mm PEEK, Lathe and sharp tools= bush's that fit...but it is not easy. The thin wall on the top bush tends the "bend" away from the tool,i make a mandrill to support the plastic ,and feed 20% quicker than steel, otherwise it micro melts

spray of wd40 helps, Once finished the product is second to none!

Onder · Jan 20, 2015

But wot about the stanchions? Are they pretty much all made by the same shop and sold by
the usual vendors? Come to think of it, didnt AN have something to say about this? They
must have theirs made specially with a engraved serial or similar?

Onder · Jan 22, 2015

Poked around further and AN claims to use special steel or "correct" steel and they say nobody
else does. ANIL is their trademark engraved on each leg bottom.
Not saying others are substandard or whatever.

john robert bould · Jan 22, 2015

"Soft" steel will bend on impact , But is that a bad thing :?: cars have weakness built in,,crumple zones ... And if this fork bending saves you stopping sudden and saves the frame :roll: put me down for soft! I don't mean that soft they bend on hard braking :lol:

Hobot · Jan 22, 2015

excuse me to disagree about any pilot protection gained by fork tube bending absorbing impact which definitely saves some stems and yoke by bending 1st but pilot will not be on the bike just before that level of G's spike. There no reason for any compliance in fork tubes which should be rigid as possible and just strike off another reason not to ride cycles if some impact damages them enough to notice. All fork tubes should be robust enough for infinite life short of wearing down. Its mostly the chrome thickness and refined finishing that sets the final OD. 15-20 yrs ago there was a rash of poor stancions so best bet is AN or make do best as can. Personally if the thin tube are otherwise perfect is DIY PEET bushes, oh la la.

comnoz · Jan 22, 2015

The fork tubes I have measured in the past were all over the place. I haven't measured any of the new Andover tubes.

The coefficient of friction for lubricated bronze against steel is .16

pommie john · Jan 23, 2015

When I put my race bike together in 1984 I had exactly the same problem. I thought maybe th equality of parts might have improved in a few decades, but apparently not!

The new ones I bought ( in Norton packaging) were from memory 4 thou under the minium size . That makes it 1.353. The same as the ones you measured.

I made a pair of top bushes to fit them.

Hobot · Jan 23, 2015

Pommie your avatar shows some mean distorting forces skewing the forks and frame, very cool.

Bernhard · Jan 23, 2015

pommie john said:
When I put my race bike together in 1984 I had exactly the same problem. I thought maybe th equality of parts might have improved in a few decades, but apparently not!

The new ones I bought ( in Norton packaging) were from memory 4 thou under the minium size . That makes it 1.353. The same as the ones you measured.

I made a pair of top bushes to fit them.

This clearly appears to be a manufacturing problem.
It appears that they are cutting down on the hard chrome that is put on to the Stanchions before they are ground down to size in a centreless grinder, or they are taking too much chrome off.

http://www.pittedforks.co.uk/fork-rechroming.ph
http://www.bing.com/images/search?
q=motorcycle+fork+stanchions+in+centreless+grindeer&FORM=HDRSC2#view=detail&id=E713D6FC4E45CBD5E9B2D6C764F5A74F60601C8F&selectedIndex=0

http://www.sturdygrinding.com/images/pr ... _large.jpg

john robert bould · Jan 23, 2015

Center-less grinding as its problems ,in so much the lower bush dia can be out of concentric with the main tube, this cause's the stanchion to wedge and bind when the lower and upper bush's come together , Regarding chrome , "Hard" chrome ,this is expensive re-chrome is over £100 , the platter grinds off approx 4 thou [2 per side] then applies 5-6 thou this is then ground back to size,and polished. Producing a hard wearing "deep" surface,
All cheap Ebay stanchions are only flash coated...Microns ,just enough to stop the rust. Gaiters are a must ,grit and grime will soon scratch through the thin coating. The basic rule applies "You get what you pay for"

lcrken · Jan 23, 2015

Thanks for all the inputs. I think I'll go with the set of stanchions I have that look like new, but are a bit undersize, and make up some undersize bushings. Because I'm not using the stock damper rods and tubes, I already need to use either longer bushings or spacers to limit the fork extension, so making up a set of longer, undersize bushings would seem the best solution.

That means I'm looking for some input on choice of materials. It looks to me like my choices are Turcite A (a Delrin-like acetal, but with silicone added for slipperiness) like Jim Schmidt uses for his kits, PEEK (a high strength PTFE plastic) like John Bould uses, oil impregnated bronze like the originals, or a bar of Delrin 150 that I already have on hand. I'd really like to use the Delrin, because that is by far the most economical solution, but I don't have enough experience with it to know how well it would work in this application. I'd appreciate any helpful info from those who have already been through this process, like Jim and John, as well as comnoz, who I'm sure also has some expertise in this area, and anyone else with something to add.

For comparison, the prices I found for the different materials, in 2" diameter bar, per foot of length are:

Delrin 150 - $16
Cored oil impregnated bronze - $66
Turcite A - $113
PEEK - $440

You can see why the Delrin looks so attractive to me! It might be possible to find stock size bushings in some of these materials that could be machined to fit, but I haven't explored that, and it doesn't seem real likely to me.

Thanks for any help here.

Ken

john robert bould · Jan 23, 2015

Ken ,
Delrin will be good . You may need a mandrill to finish the od, turn a bar to push the bush onto with a small taper on the last 5mm to give a drive .

lcrken said:
Thanks for all the inputs. I think I'll go with the set of stanchions I have that look like new, but are a bit undersize, and make up some undersize bushings. Because I'm not using the stock damper rods and tubes, I already need to use either longer bushings or spacers to limit the fork extension, so making up a set of longer, undersize bushings would seem the best solution.

That means I'm looking for some input on choice of materials. It looks to me like my choices are Turcite A (a Delrin-like acetal, but with silicone added for slipperiness) like Jim Schmidt uses for his kits, PEEK (a high strength PTFE plastic) like John Bould uses, oil impregnated bronze like the originals, or a bar of Delrin 150 that I already have on hand. I'd really like to use the Delrin, because that is by far the most economical solution, but I don't have enough experience with it to know how well it would work in this application. I'd appreciate any helpful info from those who have already been through this process, like Jim and John, as well as comnoz, who I'm sure also has some expertise in this area, and anyone else with something to add.

For comparison, the prices I found for the different materials, in 2" diameter bar, per foot of length are:

Delrin 150 - $16
Cored oil impregnated bronze - $66
Turcite A - $113
PEEK - $440

You can see why the Delrin looks so attractive to me! It might be possible to find stock size bushings in some of these materials that could be machined to fit, but I haven't explored that, and it doesn't seem real likely to me.

Thanks for any help here.

Ken

Hobot · Jan 23, 2015

Loose stanchions on scale of a few 100ths is total non issue Ken and if these stanchions were run prior w/o notice its backs up my non crude conclusions. But if ya must make composite bushes take note that JMS teal bushes come with a full length slit to consider why. Peels extended forks were best acting forks I've ever experienced on or off road on forks compared to upgradced sport bike to MX cycles. Only thing that bothered me was when brave enough to take eyes off zooming in bluff face or railing I was unbable to see lower end clearly from its oscillating bluring just before I snatched forks the wrong way to leap a hi side in time. My only concern was instead of getting an instant of spike in front traction the vibrations of fork and tire howl would combine so front wouldn't spike a jerk around but just slide off hwy turn before rear bite saved us. I was so pleased to see JMS bushes took up all detecable slack w/o any bind.

lcrken · Jan 23, 2015

comnoz said:
The fork tubes I have measured in the past were all over the place. I haven't measured any of the new Andover tubes.

The coefficient of friction for lubricated bronze against steel is .16

file:///C:/Documents%20and%20Settings/me.HP-D9685A91BAAB/My%20Documents/Co-Efficient%20of%20Friction%20_%20Plastic%20Properties%20Tables%20_%20Plastics%20Technical%20Properties%20_%20Dotmar.htm

Hi Jim,

can't read the file you posted (and I'd really like to!). The only way I've found to post document files is to scan them into jpg and use Photobucket. If anyone has a better way, please post it.

Ken

comnoz · Jan 23, 2015

lcrken said:
comnoz said:

The fork tubes I have measured in the past were all over the place. I haven't measured any of the new Andover tubes.

The coefficient of friction for lubricated bronze against steel is .16

Hi Jim,

can't read the file you posted (and I'd really like to!). The only way I've found to post document files is to scan them into jpg and use Photobucket. If anyone has a better way, please post it.

Ken

Click to expand...

I fixed it.

What the plastics mfgs say is if you want something with less stiction than oiled bronze you are about stuck with PTFE. And PTFE will only work if it is a coating on a metal bushing -like the OEMs do it.

You just need to find someone who will PTFE coat the original bushings. That would cure the clearance issue and make them slipperier. Jim

lcrken · Jan 23, 2015

Thanks, Jim. Looks like Delrin (acetal) doesn't come out any near as well as the oil impregnated bronze. Interesting stuff. I'd like to see what the friction coefficient is of Turcite A against steel. What I've read tells me that the added silicone makes it very slippery, and I assume that's a large part of why Jim Schmidt picked it, but I haven't seen any numbers yet. I'll try to get some time to do some more research, but eventually I have to call it done and build something.

Ken

comnoz · Jan 23, 2015

lcrken said:
Thanks, Jim. Looks like Delrin (acetal) doesn't come out any near as well as the oil impregnated bronze. Interesting stuff. I'd like to see what the friction coefficient is of Turcite A against steel. What I've read tells me that the added silicone makes it very slippery, and I assume that's a large part of why Jim Schmidt picked it, but I haven't seen any numbers yet. I'll try to get some time to do some more research, but eventually I have to call it done and build something.

Ken

I believe Turcite A was pretty close to PTFE for friction but with more deflection than a metal backed PTFE bushing. Jim

Hobot · Jan 23, 2015

If going to this effort I doubt anyone has examined tested all the factors Jimmy Schmidt put into his bushes so might see what he settled on in nice teal tint. Still to put things in realistic perspective one would need to ride with old then new bushes and I bet ya can't tell but maybe one set weeps fluid more. It may be possible to feel the jiggles of loose fits while riding straight but mostly would be below the wind buffing on forks and the road texture inputs. In Peel the thickness of the handle bar grips absorbed slackness vibes to undetectable to feel. Peel was so flabbergastingly able to isolate ***everything*** I could feel the wind eddies off fork sliders when front air bone in an angle changing flight arc and found they only bother handling in waves of ground effect lift/drop oscillations so am more a spoiled princess than most as my thoughts able fork slack issues. Wind eddies off forks not felt on straight forward leaps as engine vibes suddenly unopposed and suspenion oscillates about 3 cycle in a second before stopping. Loose forks didn't bother Peel a bit on various angle landings just ground breaking loose did.

Undersize Fork Stanchions

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