If you consider all the tolerances in the position and location of the interfaces, joints, bushes and bearing between the position of the front wheel contact patch and the alignment of the rear wheel, even the most accurate swing axle marks are of no-use in setting up the alignment. In short, the straight bit of wood or metal isn't a bad solution!
To the contrary, the marks on the swing arm are generaly quite accurate, the issue with lining up the rear wheel with the front via a striaght bit of wood is that the wheels are different widths so it is the centres that need to be aligned and not the sides. Reckon a few people have made this mistake which has lead them to belive that the swing arm narks our out of line.
Yes, you can make a 'tool' by adding a block to the end of the wood that is used at the front to account for one half of the difference in width between the two wheels though it ends up being Heath Robinson to say the least.
Furthermore due to the rake of the forks the slightest twist in the front wheel when aligning and all is lost.
Moreover some bikes area actually designed to have out of line wheels, where as the Kawaski ZRX1100 (I have one) is in line the later ZRX1200's rear wheel is off line by IIRC 10mm to allow a wider wheel to be fitted within the same basic layout as the 1100 without effecting the chain run. Seems that 10mm is not noticed by the rider.
No, best to rely on the manufacturer making it properly, on all of the bits lining up and then using the marks on the swing arm for what they are meant for, or get a laser check done.
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"that the swing arm marks are out of line."
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>>To the contrary, the marks on the swing arm are generaly quite accurate, the issue with lining up the rear wheel with the front via a striaght bit of wood is that the wheels are different widths so it is the centres that need to be aligned and not the sides. Reckon a few people have made this mistake which has lead them to belive that the swing arm narks our out of line.
I suspect even the dullest mechanic would check with their straight edge on both sides, to account for varying width wheels. Setting the forks straight isn't usually a problem.
I have tended to use the swing arm marks as a way of putting the wheel back quickly, once I have established the alignment via other means, rather than as the basis of an absolute measurement.
However, I would probably have to think again if faced with a bike which runs with the wheels on different lines like the one you mentioned.
Number_Cruncher
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Thanks for the replies. Some good information here. I never realised that some bikes have the rear wheel slightly offset. That complicates things further.
I still can't quite get my head around why an accurate marking on a swingarm cannot be used. I understand that the swingarm pivot bushes and bearings introduce an element of "slack", but surely no more than on a car where I've seen more than one tyre fitting centre use a laser tool shining on a "gauge" hanging on the rear wheels to set the front wheel alignment. There is surely more rubber, pivot points and other opportunities for play and inaccuracy in the suspension of a car than between rear wheel and frame on a bike. The frame itself should be perfectly straight, so we're only talking about the swingarm pivot and the rear wheel bearings. Perceptible freeplay in either of these is not an acceptable condition and will fail an MOT test.
What I have done before in the absence of said bits of wood is to wind the adjusters on both sides in as far as they go and then count turns out to get both sides equal. For finer adjustments once the chain tension is roughly correct, I count flats on the adjusters. This has always seemed to work. Again, this doesn't take into account any slack between swingarm and frame, but on a straight bike with swingarm bearings and bushes in good condition, this should surely be virtually nothing.
Cheers
DP
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>>I still can't quite get my head around why an accurate marking on a swingarm cannot be used.
Taking a point made by Pete M above, and going a bit further with it, any tiny error in the alignment of the head bearings will affect the position of the front tyre's contact patch. That can't be accounted for by marks on the swing arm unless those marks are made after that particular bike has been assembled - no manufacturer is ever going to do that on a production line!
Once you have good alignment (however you choose to achive it), take a note of the relative offset on the marks on each side of the swing arm (it may be that the marks align, it may not), and from then on, you can achieve a quick accurate alignment by maintaining this relative offset.
There is also a good method, where you tie a loop of string round both front and back wheels, as high as you can without fouling the brake discs and the chain. By looking at how the string runs between the tyre edges, you can quickly set the front steering straight, and align the back wheel. I've also used a similar method (although going round all 4 wheels) to get an initial set-up of a car after major suspension work, to enable me to drive the car elsewhere to do a more detailed alignment (you can't set front wheels to toe in with string!)
Number_Cruncher
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On all the bikes I ever owned I relied soely on the swingarm marks to align the rear wheel, and never suffered uneven tyre wear or handling problems in doing so. Although the alignment may not be micrometer accurate using this method, I reckon it is as close as makes no difference.
Whenever I have had a rear tyre changed I have never seen a mechanic use any other measuring method, and on those where I haven't watched the job being done the bike has always come back with the wheel lined up by the swingarm graduations, so I would gather that the those in the trade are ok with it too.
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I agree with RR, the manufacturers know what they are doing, I beleive that they get the s/a marks pretty much spot on, any home made tool may simply help to confirm as much though if relied upon is likely to be less accurate than the marks.
RE DP's comment, I would not rely on the number of turns / flats on the adjusters because the position of the thread in the s/a may well be much less accurate than the marks on the s/a because it is the latter that the manufacturer has calibrated.
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Eccentric swing-arm pivot or rear spindle is the best way ... but expensive.
Or shaft-drive, of course :-D
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I imagine the marks are put on when the ends of the swingarm are cast, then they are built up and welded into a unit, which introduces extra tolerances. The little laser alignment tool that attaches to the sprocket seem a good way of aligning the chain run, which presumably lines the wheel up.
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>>I still can't quite get my head around why an accurate marking on a swingarm cannot be used. I understand that the swingarm pivot bushes and bearings introduce an element of "slack",
Yes, play and slack can be a problem, but that isn't what I was really getting at by talking about tolerancing. What I meant was that every dimension and alignment on a bike's (or car's) structure is given a tolerance. So, an angle between two parts may be specified as nominally 90 degrees, but, anywhere between 89 and 91 may be acceptable. The position of the holes for the swing axle bearings in the frame may each have, say, a 0.5mm position tolerance, which means that taken together, they could be misaligned by, at worst, 1mm. As you go from the rear wheel to the front, there may be a stack up of tolerances, which mean that marks on the swing arm aren't actually as accurate as they may appear to be.
This idea is what makes me laugh at those who rabbit on about accurate laser alignment rigs - the tolerances they are setting to are usually barn door wide, and can easily be acheived with care, really basic trig, and lengths of string, straight edges, spirit levels, shims, trammels, and other cheap and cheerful pieces of kit. The laser kit just allows you to do it quickly.
Number_Cruncher
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My quick check is to drive though the puddle at the end of my drive before putting the bike in the garage.
The wet marks from the front and rear wheel give an excellent indication of anything being out of line.
(I kid you not). Other than that, sighting the alignment of the chain sprockets is easiest, as is any wear
on the sides of the sprockets a warning.
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as i run several spoked wheeled vintage british motorcycles , setting the chain up can be a problem ,
but i just look down from the ft of the engine , watch the frame rear tube going back , and make a good judgemend "guestimte" , i am usually about right , you see after 50 yrs , the back mudguards are not tied to be quare to the frame
some vof my bikes , and views
www.ras675.co.uk/
alan
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if it starts to cause sleepless nights get your self a trike!
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Thank you all for the informative replies. Nice idea on the puddle, Martin. Must remember that one!!
Number_Cruncher, big respect and thanks as always.
Cheers
DP
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>>Nice idea on the puddle, Martin.
I was really impressed by the simplicity of the method too - I'm fairly sure it would never have occured to me to check alignment in that way!
Number_Cruncher
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>>Nice idea on the puddle, Martin. I was really impressed by the simplicity of the method too - I'm fairly sure it would never have occured to me to check alignment in that way! Number_Cruncher
It offers a guide though you cannot be sure that the bike is travelling plumb straight. I tell you, the swing arm marks do the job.
Regards.
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