Quick and easy way to determine if your frame is straight

[John Kat]

I've been concerned about making sure the frames on my bikes are straight or not?
After trying the string method and finding it not only difficult to perform but unsatisfactory, I decided to try something different

The basic idea is to create a reference plane that's (almost) parallel to the plane that cuts the bike into two lengthwise and then take measurements from this plane.
This reference plane is simply the plane of the laser beam that's positionned on the RH side of the engine case after removing the contact breaker cover.
I'm using the GS 1000/1100 bikes as an example here but it must be possible on other bikes.
The laser beam I'm using is the Profi CAT laser chain alignment tool.

The process is the following:
1) you shine the laser beam to the rear of the bike and measure the distance between the beam and the rear disk surface.
2) you measure the distance between the outside surface of the disk and the wheel center to compute the distance between the laser beam ( i.e. the reference plane) and the rear wheel center.
3) you shine the laser beam to the front of the bike and you measure the distance between the front disk and the beam.
To get an accurate measurement you must measure the distances at two points on the disk: one ahead of the fork stanchions and the other behind the fork stanchions.
You try to equalize the two readings by turning the bike's handlebar left or right.
4) you measure the distance between the outside of the disk and the wheel center to compute the total distance between the reference plane ( the laser beam) and the front wheel center.

You are almost there except that it's not garanteed that the measurement plane is parallel to the bike's longitudinal plane of reference.
To lift that uncertainty you need to measure the distance of the reference plane to the center of the engine.

We now have three points of which we know the coordinates:
Rear wheel center, Engine center and Front wheel center

By computing the line that goes from the rear wheel center through the engine center, we can determine if it crosses the front wheel center.

In the case of my GS 1000 skunk I came up with the following:
Rear wheel center taken as the origin of the X and Y axis
Engine center: Y1=a*X1
with Y1= 5 mm and X1 = 860 mm ( the distance between the rear wheel spindle and the engine crankshaft)
The gives a=0.0058 rad or .33? of angle between the bike's center plane and the reference plane.
By projecting this line out to the front wheel the distance ought to be :
Y2=0.0058*(860+660)= 8.23 mm
660 beeing the distance from the crankshaft to the front wheel spindle.

On my bike I measured 8 mm.

In other words my frame is perfectly straight

Here are a few pictures:

 

[John Kat]

I performed another set of measurements on another GS 1000 today ( my GS 1000 ST with GSXR 1st gen wheels).
With my laser beam that is offset by 241 mm from the center of the engine, I found 241 mm offset for the rear wheel's center and 239 mm for the front wheel.
So in practice and ignoring the measurement uncertainties my rear wheel is perfectly aligned and the front wheel is 2 mm out of line.
Good news for sure!

 

[eil]

Good stuff! This should be on BassCliff's site.

 

[posplayr]

Wheel alignments have been discussed at length. In this thread you can read Tone's opinion which is from a practiced professional on how to determine wheel alignment and also how to check if the frame is straight.


http://www.thegsresources.com/_forum/showthread.php?t=147705&highlight=string+method


The debate about laser and string will probably never end, I had a laser and a string and tried to build a 8 ft straight edge (Tone's suggested method) but could not get any straighter than my string. The laser is actually straighter than a string(unless the string is aligned vertically), but determining where it is is harder than determining where a string is.

So my conclusion based on a error analysis of angular accuracy pitting the straightness of a laser with diffuse beam against the sagging but more precise positioning of the string is that the sting is more accurate. Of course regardless of whether you are using string or laser for measuring wheel alignment, the establishment of reference points and offsets if of the same criticality. For me this was all much easier to check and recheck with the strings as it did nor require mounting and remounting and re-re-mounting to takes a set of measurements and recheck.

Frame alignment checks are probably beyond the grasp of most here as it appeared quite difficult to determine without some good reference planes for measurements and these are not easily established with a fully assembled bike.

 

[John Kat]

The parts I don't like about the string method are:
1) the string is attached to the tire i.e. not a solid reference point in my view
2) the setup is lengthy
3) most of the time the centerstand interferes with the string.
4) you need to ensure the rear wheel is straight first which leads to multiple reiteration of the measurement process
5) you will get no indication whatsoever if the rear wheel is offset in the swingarm

With the laser method you can even perform the measurements with the bike on the sidestand in less than 5 minutes flat and if the rear wheel is not perfectly aligned it will not interfere with the measurement.
In fact the method I propose does not simply propose to replace the string by a laser beam but to create a reference surface outside of the bike.
This method is similar to the one used by the pro's when they physically lie the frame horizontally on a jig to check all distances with respect to the reference table.