Mk3 Head Steady Adjustment

Since I am in the process of replacing the iso rubbers anyway, I did some measurements on the spring on my Mk3.
The bike has a head iso to which I made the spring retainer/adjuster to suit.
The primary and its contents are removed, iso bolts are loose, bike sits on the frame.
The conditon of the iso rubbers is not known yet, but they have clearly sagged.

1. Spring rate:
Used an old weighbeam (?) and the vice for this.
It takes 49 kg (108 lbs) to extend from unextended 24 mm (8 windings of 3 mm) to 38.1 mm (1.5 inch)



2. Spring orientation:
The spring sits at almost exactly 45 degrees to the vertical.
So 0.707 of the spring force acts equally in the vertical and horizontal direction.
The effect on the rear iso is that it seems it gets more or less levered with the front iso as a center of rotation.



Spring at rest:



Spring extended to 1.5 inch:



3. Effect on the cradle.
I measured the movement of the cradle at the front (vertical) and rear iso (perpendicular to the line between the two isos centerlines)
when extending the spring to 1.5 inch.
Front: about 6 mm
Rear: about 7 mm

If I get around to it, I will check again with iso internals removed.

Graeme and Mike,

I thought about the headsteady spring a bit more last night. As I said the first time I used elevator shoes, I stand corrected. IF the front and rear isos are properly adjusted and free to move vertically by a few thou, then the spring adjustment can be made without bothering to loosen the isos. During reassembly, or when the iso system is ready for an adjustment, I still believe that it makes more sense to set the spring first.

Slim,

Your bike looks suspiciously like a Mk2, with the old plate headsteady. On my Mk3, with the box head steady, the spring angle is much closer to vertical when the isos are in place. I did the same experiment on the spring last year (this forum is full of riders with a powerful physics persuasion). I tested each side of the double spring separately and obtained a deflection on each side of 0.36" using 50 lbs of force, to obtain 139 lbs/inch. As the springs are in parallel, the total spring constant is 278 lbs/in, which produces just under 140 lbs of total force with the specified 0.5 inch spring extension. Sorry, my bike is fully assembled at the moment, so I can't supply photos of the headsteady/spring, but I jotted down in my workshop log at the time that the spring followed the jug angle. If I follow your deflection numbers correctly, then the spring is pulling the cradle upwards a few mm, which I believe means that the total spring force is comparable to the loaded cradle weight. Did I get this right?


Rick

slimslowslider said:

Since I am in the process of replacing the iso rubbers anyway, I did some measurements on the spring on my Mk3...

Click to expand...

What is that top steady? Looks interesting..

The spring, as shown in the photos, has been fitted the opposite way to how it is drawn in the manual. As the upper spring loop is offset from the spring coils, it alters the angle of the spring if it is fitted the opposite way around.

slimslowslider said:

Click to expand...

http://britmoto.com/manuals/Manuals/850_man.pdf Section, C1.

rick in seattle said:

Slim,

Your bike looks suspiciously like a Mk2, with the old plate headsteady. On my Mk3, with the box head steady, the spring angle is much closer to vertical when the isos are in place. I did the same experiment on the spring last year (this forum is full of riders with a powerful physics persuasion). I tested each side of the double spring separately and obtained a deflection on each side of 0.36" using 50 lbs of force, to obtain 139 lbs/inch. As the springs are in parallel, the total spring constant is 278 lbs/in, which produces just under 140 lbs of total force with the specified 0.5 inch spring extension. Sorry, my bike is fully assembled at the moment, so I can't supply photos of the headsteady/spring, but I jotted down in my workshop log at the time that the spring followed the jug angle. If I follow your deflection numbers correctly, then the spring is pulling the cradle upwards a few mm, which I believe means that the total spring force is comparable to the loaded cradle weight. Did I get this right?

Click to expand...

Rick, I guess I can only tell what the outcome is after I have the iso rubbers out.
However, I for sure do not expect the centres to line up with the holes in the frame lugs.
If only for manaufacturing factors (frame tolerances), design compromises, and of course it is not an original head steady set-up.
PS this figure of 140 lbs, where does that stem from?

Grandpaul,
I think it is a Norvil one. Was on the bike when I got it.

L.A.B. said:

The spring, as shown in the photos, has been fitted the opposite way to how it is drawn in the manual. As the upper spring loop is offset from the spring coils, it alters the angle of the spring if it is fitted the opposite way around.

slimslowslider said:

Click to expand...

Click to expand...

LAB,
you are correct about the incorrect spring orientation.
However, if I mounted the spring per manual, I could not get to the 1.5 inch as the bottom of spring would butt against the steel angle on the bottom.
When you look at the photo closely, you will see that there is only little room left, at least not enough to cover for the extra 1.5 mm that the spring moves closer. Might have to do with my adapted spring retainer/adapter so the orientation may differ all together from the original.
As for the effect of the spring mounted one way or another in my set-up, the "attaching" points of the spring remain the same, so change is negligible I guess.

slimslowslider said:

Might have to do with my adapted spring retainer/adapter so the orientation may differ all together from the original.

Click to expand...

Yes, it looks approximately 1/4" higher than it would be on the standard MkIII box steady.

Hello all, and the best for 2011.

I finally had a chance to try the new spring length today and I think it cuts out some low rev shake, 2000 to 3500 rpm, which wasn't a noticable vibe anyway.
But the vibe at 3800 to 4200 is still anoying through the bars. A high pitched buzz. That's right at legal 100kms (60mph)
Above 4200 it smooths out and stays good to 5000. I don't know after that.
I think that the combination of 18" wheels and a rear tyre that is worn are lowering the speed per rev too much so next time I need sprockets I'll try a larger front, and put up with the narrow rpm vibe and let the excellent torque do it's thing.
(this could also be a balance issue) But it only does this in 4th gear????

The tighter spring has definatly improved the ride at other revs.

graeme

The tightness or looseness of the iso adjuster/shims does not apply any pressure to the rubber donuts at all. So changing the iso adjustment clearance has no impact on the spring tension of the head steady. The isos donuts are free to move vertically regardless of whether there is a lot of iso clearance or none at all.

The Dave Taylor Head Steady I put on my bike treated the spring as an extra that I paid a few dollars more for.

Since the DT unit has rod ends that don't offer any vertical support of the engine all of its weight w/o the spring would be carried by the isolastics. No idea how that would feel and putting all the vertical load on the isolastics is counter to the original design.

I put the spring on and am happy with the results.

Bob

Just a long shot, have you checked wheel balance and if the tyres are evenly fit onto the rims? Oh! and try lubricating the ISOs with some silicone release agent.

Harmonics can be a real bugger.

Best of luck

Cash