I think the most important thing to allow a bike to go down the road straight is having the front and rear wheels in vertical plane and in line. The in line part is easily adjusted, the in vertical plane is, hopefully, built into the frame design.
If you think about it a frame is just a device to hold the wheels fixed in that position. You could deliberately build a frame that ran at some angle to the line of wheels centres and still pilot it down the road OK.
It seems the simple straightforward way to build a bike is to keep the frame centre line on the wheel centre line, so most are done that way, but it isn't necessary. As far as the rear wheel sitting in the swing arms, the Commando swing arm may be symmetrical, but most swing arms are not. Some are even one sided, such as Ducati and Triumph Triples.
So to get the wheels in line just use a long straight edge. If there are differences in tire width front to rear, measure these amounts and allow for them.
Others no doubt have there own preferred method but here is mine
With the bike on the lift on it's wheels and lightly snugged down with support straps, place a heavy short block of 6"x6" or something similar beside the rear wheel and another beside the front wheel. Put some spacer pieces (1"wide works and is easy) on top of the rear block alongside the rear tire fore and aft at the widest part of the tire profile. Bump a long straight edge 7' long or more up against the spacers with light pressure so that the spacers are contacting the rear tire.
I use a wooden 1x3 or 1x4 or anything similar that is available, but it must sight dead straight. If you aren't accustomed to sighting things straight, check the straightedge with a string line.
The straight edge will lay on the blocks and project the line of the rear wheel forward onto the front wheel. Adjust the bars gently while measuring fore and aft spaces between the straight edge and the front wheel. When the front wheel is held parallel to the straightedge, the measurement in between tells you if the wheels are in alignment or not.
For example, if your bike has the same sized tires and rims front and rear, then the measurement of the space between the straightedge at the front wheel should be 1" (the rear spacer width) when the front wheel is held parallel (equal measurement fore and aft) to the straight edge.
If this measurement happens to be only 1/2", then the rear wheel must be adjusted via the chain adjuster on the nonsprocket side (assuming chain tension is already correct) in the appropriate directiuon to swing the rear wheel over such that the 1" space is attained at the front wheel.
A lot of words, but the whole process only takes a few minutes and makes the bike go down the road a whole lot better.
Apologies to those who already routinely do this procedure or some version of it, but it comes to mind that perhaps not everyone is aware of this?
I should add that, with the Commando, once the wheels are adjusted into alignment with the straightedge, they should also be sitting on the frame centreline, although personally I would not get too fussed if the rear wheel was 1/4" one way or the other off the centre line. Much more than that and perhaps things should be altered.
Deets suggested using a plumbob to plumb down to the rear tire from the rear hoop. This will work well as long as the bike is setup perfectly plumb and the rear hoop is not bent. The process of setting the bike up plumb is a bit fiddly and this is already a long post, so I'll leave that for later.
Glen
If you think about it a frame is just a device to hold the wheels fixed in that position. You could deliberately build a frame that ran at some angle to the line of wheels centres and still pilot it down the road OK.
It seems the simple straightforward way to build a bike is to keep the frame centre line on the wheel centre line, so most are done that way, but it isn't necessary. As far as the rear wheel sitting in the swing arms, the Commando swing arm may be symmetrical, but most swing arms are not. Some are even one sided, such as Ducati and Triumph Triples.
So to get the wheels in line just use a long straight edge. If there are differences in tire width front to rear, measure these amounts and allow for them.
Others no doubt have there own preferred method but here is mine
With the bike on the lift on it's wheels and lightly snugged down with support straps, place a heavy short block of 6"x6" or something similar beside the rear wheel and another beside the front wheel. Put some spacer pieces (1"wide works and is easy) on top of the rear block alongside the rear tire fore and aft at the widest part of the tire profile. Bump a long straight edge 7' long or more up against the spacers with light pressure so that the spacers are contacting the rear tire.
I use a wooden 1x3 or 1x4 or anything similar that is available, but it must sight dead straight. If you aren't accustomed to sighting things straight, check the straightedge with a string line.
The straight edge will lay on the blocks and project the line of the rear wheel forward onto the front wheel. Adjust the bars gently while measuring fore and aft spaces between the straight edge and the front wheel. When the front wheel is held parallel to the straightedge, the measurement in between tells you if the wheels are in alignment or not.
For example, if your bike has the same sized tires and rims front and rear, then the measurement of the space between the straightedge at the front wheel should be 1" (the rear spacer width) when the front wheel is held parallel (equal measurement fore and aft) to the straight edge.
If this measurement happens to be only 1/2", then the rear wheel must be adjusted via the chain adjuster on the nonsprocket side (assuming chain tension is already correct) in the appropriate directiuon to swing the rear wheel over such that the 1" space is attained at the front wheel.
A lot of words, but the whole process only takes a few minutes and makes the bike go down the road a whole lot better.
Apologies to those who already routinely do this procedure or some version of it, but it comes to mind that perhaps not everyone is aware of this?
I should add that, with the Commando, once the wheels are adjusted into alignment with the straightedge, they should also be sitting on the frame centreline, although personally I would not get too fussed if the rear wheel was 1/4" one way or the other off the centre line. Much more than that and perhaps things should be altered.
Deets suggested using a plumbob to plumb down to the rear tire from the rear hoop. This will work well as long as the bike is setup perfectly plumb and the rear hoop is not bent. The process of setting the bike up plumb is a bit fiddly and this is already a long post, so I'll leave that for later.
Glen
