dennisgb said:
I've read that article multiple times and keep coming back to the issue of manufacturing tolerances and whether they really can effect handling. The reason is that most people don't have access to the equipment used to straighten the frame like they did. Can we all get perfection? Is it necessary?
With the swing arm axis square (at right angles) to the steering head axis in two planes (plan and front view if you like) at 90 deg to each other then as the swingarm pivots, the rear wheel, viewed from the rear, will move in a vertical plane parallel to the frame centre line plane. The alignment will remain constant as it moves up and down.
When the axes are not square then as the swingarm pivots the degree of alignment changes because the rear wheel is not moving in a vertical plane but an arc, viewed from the rear. Therefore rear wheel to frame centre line alignment is constantly changing as the suspension absorbs bumps and the swing arm moves up and down. This is what causes the weaves on Commando's and any bike where these axes are not square. Commando's are more sensitive to this misalignment than other bikes because of the isolastic assembly but it is NOT the iso's which cause the problem - it's the relationship between the steering and swing arm axis.
I don't know what Norton manufacturing tolerances were, regarding the chassis, but whatever they were they weren't adequate. What I do know is after I'd squared front and rear iso brackets to the steering axis, followed by the swingarm axis and then marked a frame centre line reference on the swingarm, the rear wheel centre line was nearly 1/4" off the frame centre line. I ended up moving the whole iso assembly - engine, gearbox, cradle, front iso etc - approx 1/4" across to the right side of the bike (timing side) to achieve rear wheel centre line to frame centre line alignment. The rear wheel now sits central to the frame seat loop, also there's now clearance between the chainguard and drive side Koni shock.
In this case 'perfection' is 90 degrees and is only necessary if you don't want your bike weaving and wobbling on the straight and around corners.
dennisgb said:
Some measurements can be made and it's prudent to do them when rebuilding a bike to insure that frame is not damaged, but beyond that what can we really do?
The comments on "The Bare Minimum" make sense and then insuring that the iso's are in good shape (new) along with an aftermarket head steady (Dave Taylor) and proper adjustment seems to be the best approach.
I will be doing new iso's and head steady on my MKIII over the winter, and plan to do as much measurement as I can so maybe my opinon will change once I get into it.
The iso's only effect handling if the clearances are too big. All the rubbers do is absorb engine vibrations - they have no affect on handling whatsoever.
Prior to alignment front and rear iso mounts were out of parallel to each other in front and plan views. Obviously neither were square to the steering head axis. Prior to alignment the bike would get out of control through bends when hitting bumps. Post alignment there are no problems. I'm not making this up. All Commando's should handle without problems, but the frames need to be checked and precisely aligned.
Only chassis mods on my bike is an iso head steady and fork top bush and damper tube mod. Getting your bikes frame aligned is essential if you want it to handle. The popular chassis mods e.g Dave Taylor head steady, are addressing the symptoms not the cause.