The handling of a motorcycle can be greatly affected by its “straightness”. Even a minor “topple-over” in a car-park can be enough to warp the chassis and thus affect the handling. Making sure a bike rides properly is a major concern when buying a second-hand bike.
When a bike is not straight, one of the problems is that the rear wheel is not directly in-line with the front wheel. When this happens, it takes more effort to lean the bike one-way, than the other. Let’s take an example:
In our case, the rear wheel remains centred, but the front wheel is to the left of centre. Leaning to the left is easier than leaning right. When leaning to the right, the bike has to “climb” past the centre of the rear tyre. It’s almost as though you are “leaning uphill”. Leaning left, the bike has already passed the highest point of the tyre and hence falls into the turn.
Just as brick-layers do, it is possible to get a good idea of straightness with a piece of string. Preferably, with the bike on a paddock stand (or somehow held vertical) the idea is to run the piece of string down the sides of the rear tyre and towards the front of the bike.
Starting with the middle of the string, wrap it around the rear tyre two or three times. Carefully loop the end of the string around itself twice, near the edge of the tyre.
If you pull the string too tightly, where you have looped the string will slip back past the edge of the tyre. The problem with this happening is that it then makes it difficult to get both stringlines at the same height. It is not the end of the world, but it does make measuring more difficult.
Pass the lengths of string through to the front of the bike. If you would like, you can attach the ends to a rod – ensuring that the gap between the two sides is equal to the width of the rear tyre. Through a true moment of serendipity, I happened to have an old Ventura gear-sack mounting bar with lugs on it that were exactly the right distance apart.
Ensure the front wheel is pointing straight ahead and stretch the string until it is under a slight tension. If all has gone according to plan, the only place the string will touch the bike again is at the front of the rear tyre.
From the front of the bike, you should be able to ensure the string is travelling straight along the path of the rear wheel. The distance from the string, to the sides of the front wheel should be equal on both sides.
As evident in the photos the RGV is a long way off being straight. I already knew that. It is actually so far out of line, you can see it with the naked eye. What is worse: SOME SPIDER HAS BUILT ITS NEST IN MY FRONT TYRE!
In theory, it would be possible for the chassis to be bent and for the wheels to still be in-line at some point. If the headstock of the bike were off to one side, but twisted so that the forks were angled back toward the centre line, it may be possible to not notice the misalignment with a string test. But, for a quick and fairly accurate approximation of straightness, the string test is a good guide. In the meantime, the RGV is off to the frame-specialist to get itself straightened out.