Slow cooking - R100/7 twin-disc conversion at last

[Ken in Oklahoma]

. . . I too would like to know how well they did in the 70's

As I recall from reading Duane's thoughts on the matter, and from a personal conversation, not very well

The "re-alignment" of the poorly aligned forks part of the fix involves (if needed) the hogging out the circle(s) in the top plate so that the top of the fork legs can be moved as required. Presumably Sachs (the maker of the BMW forks) idea was that the lower triple tree would define the parallelism of the forks and, if anything, tightening the top nut on the fork legs would help secure it in place.

One idea that I've never been comfortable with is the notion that, by putting sufficient static pressure on the fork leg(s), the metal, meaning the aluminum bottom triple tree or the fork leg itself, can be "cold flowed" into the proper parallelism. "Cold flowing" in this context refers to applying stress to the metal in the elastic region (in other words, before it bends). This is seen on a stress/strain graph as an upward curve that is linear at first and then curves a bit until the the curve peaks at the "tensile strength" point. That point is where the metal starts becoming permanently deformed. Below that point, when the stress is removed, the metal will return to its original position (the very definition of "elastic"). Above the point the metal starts taking a permanent set, or at least a set different from the original unstressed shape. After the "tensile strength" locus on the graph the curve starts heading downward (indicating bending). The curve ends when the material fractures at the ultimate strength point.

This concept came to me in a "Strength of Materials" class which could be thought of as an introduction to metallurgy. And that was long ago in a galaxy far away. But the implication is that actual yielding of the fork legs must occur before a new permanent set can be attained. And that yield point would require a very heavy clamping arrangement with no good way to know when enough force was enough.

The term cold flow apparently can mean different things. Apparently it also encompasses operations that bend metal, such as swaging or even hammering a rivit. But cold flow in this context simply means bending something, holding it under stress for a while at ambient temperature, and the material will take a new set.

Now I'm not challenging Duane or Randy Glass because they have something I don't, namely empirical experience and a lot of it. Moreover the notion is subscribed to by Stephen Botcher, who used to post on Boxerworks and is an engineer whom I respect. My purpose in making this post is not to stir the pot as much as it is to get some information I can manage to sink my teeth into. I did an admittedly cursory search for "cold flow" on the internet and didn't see anything that I hadn't seen before.

Addendum: At one of the rallies at my place, during a discussion of fork alignment, I did a test of sorts to see if there was any noticeable mis-machining of lower triple trees. I happened to have a couple "spare" lower triple trees and spare fork tubes so I decided to slide two lower triple trees onto two fork tubes. The idea was that if the triple trees weren't machined the same they would bind on the fork legs as they were slid together. Interestingly, they didn't. That would suggest, of course, that the sample of two that I tested were pretty damn good in terms of being machined alike.

Addendum #2: In the foregoing discussion I'm talking about the /5, /6, and /7 forks with the "thin" flat steel upper triple tree.

Ken

[chasbmw]

I'm sure that BMW did quite a lot of work in the /5 to /7 era to make their bikes more stable, as BMW are finding out at the moment it's not a good thing for your bikes to exhibit moments of irrational instability, especially if people lose their lives as a result.

I know that there were revisions to fork damping over that time and I would expect that the machining errors in the top plate that Duane found on the /5s was dealt with. I don't really get the whole Randy Glass thing, forks get put under quite a lot of forces during their working lives and after getting everything set up to be correct to within a gnats whisker, I would guess that it would be out of kilter again after a few miles.

I did find that a Telefix fork Brace worked for me on the 90/6, you do have to be very careful fitting it to make sure that you don't induce stiction, but the bike felt more stable afterwords.

[Major Softie]

I'm sure that BMW did quite a lot of work in the /5 to /7 era to make their bikes more stable, as BMW are finding out at the moment it's not a good thing for your bikes to exhibit moments of irrational instability, especially if people lose their lives as a result.

If you are, as it seems, speaking of some current model issue, I'm not familiar with it.

[barryh]

Rumors about the latest model tank slappers and the Kevin Ash tragedy at the South African launch ?

One thing for sure Kevin Ash would have understood the issues we are talking about. He seemed to me to be a fine Engineer as well as a journalist.

[Major Softie]

I find nothing on the web about any "rumors," only that he died on the test ride. Is there actually any evidence that there was something wrong with the bike, or just unfounded speculation?

[barryh]

Rumors that there may have been a problem with the bike but I agree unfounded speculation that this necessarily had anything to do with the accident.

I'm not insensitive to the fact that speculation is in bad taste in these circumstances. Like many others I'd been reading Kevin's blog for some time and there is no doubt that there was tremendous respect for him in the motorcycle community. People naturally think "there but for the grace of God go I" and wonder how it could happen to such an experienced rider.


http://idratherberiding.com/2013/01/24/ ... ss-launch/

A brief extract from the above link which I thought was written with appropriate sensitivity.


"However, the GS did disappoint me during a casual ride down an open dirt road. I was standing up to get cool air through my riding gear, traveling about 45 mph, when I encountered a washboard section. In an instant and with seemingly no provocation, the bike went into a wild tankslapper that threatened to throw me from the bike. It was a very panicked second before the steering regained its composure.

We can’t yet say for certain this is a problem with the bike, but another journalist reported a similar experience when he was riding while standing. This is quite unexpected considering the chassis geometry is essentially unchanged from the previous model, plus there were no stability issues whatsoever during high-speed road riding.

Here is the account from Bike Magazine:

The only unpleasant shock during the whole 280-mile ride is a tankslapper so violent it snaps the lockstops off the frame. BMW chassis engineers seemed shocked to hear of it, claiming they never experienced anything of the sort in five long years and one million kilometers of testing. It does seem that I just happen to have stumbled on exactly the wrong combination of speed and road surface at the wrong time. Assuming it really is just a one-off – which, in all fairness, would appear to be the most likely explanation – then the new R1200GS is a remarkable machine."

[chasbmw]

I think that Bike are being rather nice to the new GS in their review.

Assuming that Kevin suffered a similar problem, then 3 people having tank slappers on a launch with perhaps 50 Journos is not good even though these guys ride their bikes harder than I would be capable of.

Anyway going to have a look at the bike local dealer on Saturday, I really liked the revised design of the bike

[Major Softie]

Thanks, Barry. I hadn't heard any of that stuff, and my search about Kevin's accident didn't turn up any of that.

[Mal S7]

My fascination for the day, has been playing with the forks, while I wait for parts. Bolted up the new top plate, as a practice, sans fork springs. Did the glass plate across the tubes test, maybe ,, maybe a poofteenth of a click detected, but pretty good for having just thrown it together.

This is the forks at full travel, i.e. bottomed out. Remarkable how close they come up to the frame.



And that made me think about the clearance for the gaiters. Here is a new gaiter squeezed tight.



I read somewhere a few years back (Duane perhaps?) that for this reason the gaiters have to be pulled down and clamped at the ridge about 2 inches below the top of the fork. Not many people seem to do this. As always I was a bit skeptical, but now I see. The gaiters will prevent full travel if clamped at the top ridge. Unless maybe they blow outwards like balloons when you hit that monster pothole. We need some high speed photography and a volunteer ...

I'd like to do a similar test with my fork springs now, keep reading complaints that progressive springs have "too large a wire diameter". Is that supposed to mean the coils bind before full travel is reached? I'd like to test the theory hmmm not so easy to get them springs scrunched up though.

later
Mal

[Chuey]

I've done four forks in the Randy Glass method so far. I will continue to do them as I use more forks for whatever reason, replacement or new to me bikes.

Thing is, it seems to me that the fork legs can splay very easily when suspended out in a horizontal plane with the upper and lower triple trees on. I hold the fork by the steerer tube in my vise. I made a wooden carriage for the dial indicator and as I move it up and down along the stantion tubes, they deflect ever so easily. What I'm saying is that I have to be careful to not put any pressure on the tubes that could deflect them in the direction I'm measuring. That makes me think there could be some compliance in the tubes even when they're captive within the lowers. Of course, the axle is the link that ties all this together. It is also the factor that makes it so we can't measure to find what we want to know about the stantions.

Chuey