Home made Fork Brace

[SteveD]

I would like to see a double bolt clamp, does anyone make them?

Dunno. Local bloke...exie though.

http://www.cafebeemer.com.au/products.htm

[chasbmw]

Steve,

Mr cafe Beemer needs some more work on his engine, he is really only getting fairly stock horsepower despite his modifications. It will be interesting to see how he gets on.
Charles

[ME 109]

Hmmm, not a lot of difference between the two, although CB is stronger around the clamp area which would resist stretching/loosening. CB use a good grade of ally too.
I'd prefer a bolt with a nut rather than the bolt screwing into the ally.

[Ken in Oklahoma]

I too am having a hard time seeing any significant advantage of that top plate. The Modulus of Elasticity (resistance to deflection) of steel is roughly three times that of aluminum. The thickness of the aluminum is, what, two to three times of the stock top plate? It's been a long time, but I seem to remember that resistance to deflection is a function of thickness squared. So there would have to be some added stifness.

But!

Look at the massive airhead bottom triple tree. It is so thick and therefore so resistant to deflection that the added resistance of the subject top plate would surely be insignificant in comparison. Now then, if you could modify a bottom triple clamp to become a top triple clamp, then you would have something, roughly doubling the resistance to deflection.

It would appear to me that the chief function of the BMW designed top plate is primarily to locate the top of the fork tubes relative to the bottom triple tree. In other words it is the lower triple tree's stiffness that determines how well the fork tubes are kept from deflecting up and down relative to each other. Presumably it was BMW's intention, for whatever reason, that the lower triple clamp do most of the work in keeping the fork tubes sufficiently well loacated to each other.

Now then, that CC upper triple clamp might be quite useful for another reason. Duane has talked a lot about how the upper and lower triple trees weren't machined properly, or rather the tolerances were too loose, all of which could and did cause the fork tubes to be stressed and held out of alignment on some forks as delivered from the factory.

Plus, if polised nicdely, that upper triple clamp would add some lovely bling, if bling is your thing.


Ken

[ME 109]

Good post Ken, tis surely an interesting subject.
From my own experience, the fork legs can flex in the triple tree without a great deal of effort. That is, with the top plate and fork brace etc off.
That flex would consist of the fork leg itself, as well as the triple tree.

The benefit of a billet type top plate is to prevent the forks
from moving in a walking type flex, something that the oe plate can't really do even with the high torque recommended for the fork top nuts.

The whole point of a billet type or similar top plate is to clamp the fork leg in a way that it cannot move radially? in the clamp, provided the centre steering head nut on the clamp is also a perfect fit.
That's why I'd like to see a double bolt each side of a billet clamp.

The thickness of the oe top plate would be sufficient as the load is applied to the width of the bracket, not the thickness so to speak. So I think the thicker ally plate would also be ok.

I think it comes down to a rock solid clamping of the fork legs, as well as a zero slop fit of the steering head nut through the clamp.

Apologies for a mess of writing from a madman, and a question mark could be applied to each statement.

....I might add that I 'think' the triple tree is ok with just one bolt each side because there is far less 'radial?' twisting force applied there.

[Roy Gavin]

The CB triple is $291- plus post - I just landed in Oz a SJ triple for my G/S for $140-.

It is a nice bit of gear , 5/16" or 8mm + thick, compared to the 3mm of the OEM steel plates.

It is well machined with all edges neatly rounded and it polished up like chrome.

I have a billet triple on 75/7 which I bought a long time ago from Motobins which is less than 1/4" thick , and the CB triple looks the same thickness.

I dont expect a great improvement as I already have a Cafe Racer tube type lower which made a big difference, but I rIde the G/S back to back with my paralever / Ohlins / HPN insert R100GS, so I get plenty reminding how crap a well worn pair of G/S forks can be.

There must be a little bit flex in the stock plate, as the centre nut will loosen PDQ unless it is up real tight.
I think one man alignment will be easier, as the fork top nuts no longer need to be so tight , and the fork tubes can be clamped rigid before you nip the top nut and the indicator brackets up.
It will be easier to get the axle level too , as one fork tube can be dropped a fraction in the clamp, and and the alignment/ can also be checked after the triples and axle are tightened, as the springs can now be removed without upsetting the top clamp, and of course you dont risk loosing alignment when you change the fork oil or adjust the head races.

There is a lip on the top of the fork tube hole, so you cant drop the tubes through the tripled unless you machine this off, but the possibility is there if you really want too.

And $140-bucks is not a lot for a nice bit of bling!

[Duane Ausherman]

Typing, for me, is so slow and fails to communicate well. I just wish that we could all sit around a campfire and discuss this issue. I need to know more about this subject and since I was in the biz, I suspect that a lot more is known about these forces.

[vanzen]

I've ridden BMW twins for over 35 years, and started with the Earles' fork
models which also used the flat plate at the top. I've used them on the
road, in enduros and raced them. I've done this with the standard fork
arrangement and with machined yokes and noticed no difference at all.
On most of the leading link forks that I built I went for a similar flat
plate idea (although welded to the stantions), subconciously inspired to a
large extent by the BMs. no doubt.
When you consider the loads imposed on that BMW plate it is quite an
appropriate design for the job.
I think you'll find that most opposition to the standard plate stems from
the fact that it's not how others do it, and it certainly doesn't look
"trick".
~Foale

Those are all of the functions of any top clamp. The issue of course is
whether the Beemer item is as good at holding its fair share of that torque
on the tubes, as would be a thicker clamp pinch-bolted to them. It's MHO
that that the BMW top can simply never measure up to the latter in that
respect, as a design method.

That is because no threaded fastener is secure against loads that tend to
rotate it in alternate directions. Even with very long fasteners (that
preserve a lot of preload with some loosening), the above is usually
sufficient to eventually un-load them, and the BMW fork caps aren't long
enough to even have that advantage.
~Hoyt

I side with Hoyt.

... and Tony Foale regarding the BMW main-frame:

The main "weakness" of the BMW series /5 /6 /7 frames is in the area of the
SA pivot mounting. Side loads on the rear wheel will tend to make the
upright tubes holding said pivot move fore and aft.
This can largely be prevented by mounting a bracing tube on each side.
These tubes can be attached to high up on the front down tubes or to the
underside at the front of the mini backbone. At the rear I mount them above
the SA pivot. Some others put the rear mountings below the SA pivot (see
p182 of the Clymer manual 500-1000cc twins 1970-1979, October 1979) but the
under side is better supported by the bottom rails which are tied to the
rigid crankcase anyway.
These tubes obviously need to be detachable for engine removal etc. and so
the detail of the fixing can be very important to the success of the mod.
I don't like the simple drilled flat brackets combined with a flatened tube
end that I've seen. I've used mating bosses with one counter-sunk at 45deg
and drilled and tapped, and the mating one turned with a male 45deg, the
other side counter bored for the head of an allen bolt. These are mounted
at right angles to the bracing tube and so the taper takes the main load and
the bolt simply holds them together, as long as the bolt is tight then there
is no slop, and I've never known the bolts to come loose.

Another though less important frame mod. is to put a short bracing tube from
the rear of the top of the head stock back on to the backbone, this will
reduce some distortion under braking.
Other than that the frame loop needs no other bracing, afterall it's heavy
enough as is.

The above refers only to the main frame loop, and is not meant to indicate
that the other factors mentioned by others, such as suspension units,
correct maintanence etc. are not important. You'll get no worthwhile
results from such mods. unless all the other bits are in good nick.

On machines with earlier engines there is one mod that I've found very
useful to help general handling as well as gear changing, and that is to
reduce the flywheel mass as much as possible, you just can't go too light.
Later engines had a different flywheel which was about as light as they
could go from the factory. Probably the easiest way to go is to use the
later flywheel assembly. It saves a lot of machining, drilling and
balancing work. Those BMW flywheels were made of tough stuff.
~Foale

[Chuey]

Those are all of the functions of any top clamp. The issue of course is
whether the Beemer item is as good at holding its fair share of that torque
on the tubes, as would be a thicker clamp pinch-bolted to them. It's MHO
that that the BMW top can simply never measure up to the latter in that
respect, as a design method.

As for top clamps on twin shock bikes, I like the idea of an upside down bottom yoke used as a top clamp much better than the ones I've seen on the market. I don't like how thin the aftermarket ones get in the middle. It seems the reversed bottom yoke would be much stronger.

Chuey

[ME 109]

While on a ride this afternoon I came to the conclusion that my bike does not need any handling modifications.
My RS has a stock front end which I believe is very well aligned.
It has good condition ikon shocks and a near new rear Mac 50 (last one in Oz I think)
It just doesn't display bad manners under almost all riding situations, even when heavily loaded.
Stopping when heavily loaded is another subject.

Near perfect fork alignment with well working innards is the basis on which all handling assessments should be made*.
Closely followed by the condition of the rear tyre. I've found that any flattening off of the tread is where wobbling is born*.
A flattened off rear tread in conjunction with a misaligned front end would be the cause of most handling complaints*

* Bold statements made by me.