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engine to frame alignment


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VOC Member
Hi All

My very good friend Mostyn Phelps from the Dorset section VOC has asked me to post the details of a problem he is having related to his twin pulling to the left and tracking down any related defective components. He has started by levelling and accurately checking various alignments, but really want to start from a basic datum, so I quote his message to me.

"Following our discussion last evening I wonder if you would be kind enough to do a bit of "internetting" for me as you are much more familiar with all this. Essentially I would like to know if the crankcase joint face is on the centre line of the machine and therefore if it is then the centre of the front wheel will be on this notional line and so will the back wheel. If this joint face is not on this notional centre line by how much is it offset and to which side when viewed from above"


Comet Rider

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VOC Member
On my Dad's Comet the handling problems when he bought it where traced to a bent bottom link.
Hands off at 40 MPH it pulled badly to the left, and when the bottom link was checked on Bob Culver's fixture, it was found to be approximately 1/8" out of true



The centerline of the machine is shown on page 6 of the riders handbook, so I guess that it should be possible to check where the crank case join line is relative to that. Good Luck, Mostyn. ET43

roy the mechanic

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VOC Member
Sometimes there can be a simple answer (if you are lucky). i would recommend first check that both wheels are in line, using a plank or a piece of string. Haphazard chain adjustment is a likely cause of "steering pull" . If the wheels are in line, next check the drive chain line, if this is wrong then the motor must be out. I reckon this to be unlikely unless the machine has been in a serious accident or has been built by either an amateur or an animal!
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VOC Member
One other thing to check if all else is in line is the vertical alignment . Set the bike up with a spirit level so that the rear wheel and tyre are vertical. Then use the same level or a bob wieght on a string to ensure that the forks and front wheel are also vertical. You will have to do that with the front damper tightened up and the wheels in line.


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VOC Member
I agree with what's said above, but from a different view, why is it pulling to one side?

Has something been adjusted recently? Was it adjusted right? Check adjustment.
Has it always pulled? Check everything above.
Has it had a bump? Check for crash damage and everything above.
Has it got progressively worse? Check for wear and tear in bearings etc.

If the bike hasn't recently been completely rebuilt, I usually find things like this are down to operator error. Silly things like 4 turns on one wheel adjuster instead of 2 turns on each - quite easy when you reach a certain age and get sidetracked with cups of coffee etc.

To get to your original question there's a very recent thread from Monkeypants on this forum where he discusses in great detail where the centre of the engine is relative to the centre line of the machine.



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VOC Member
thank you gentleman, not sure where monkeypanks gets his 9mm offset from and we don't find page 6 of the rider handbook conclusive. Mostyn has provided details of the checking proceedure he has undertaken to date, which may be of interest to you:-

Your query has certainly caused a flutter in the pidgeon loft.
The sequence of events is as follows, the machine is up on a rigid
surface with both wheels removed. 1/2" diameter bright steel bars put
in place of the rear fork pivot, the rear wheel spindle and the front
wheel spindle. That in the rear wheel position is pulled against the
ends of the fork slot and held there (after the chain adjusters were
backed right off)An engineer's level was then use to level the fork
pivot (using screw jacks under the machine)in the belief that this area
was the least likely to suffer any wear or accident damage. The distanc
between the fork spindle and the rear wheel spindle was checked and
found to be constant for both sides. The level was applied to the BMS
bar in the rear wheel spindle position and found to be level, i.e. there
was no twist in the R F M .Distances between the fork pivot and the BMS
bar in the front forks were equalised on both sides as a way of
cetralising the front forks. The level was applied and the reading
reasonably consistant with the other readings.Then a catenary wire (a
length of 0.8 mm TIG welding wire which is pretty much as hard as piano
wire) was set up and aligned with the engine casing joint to establish a
datum, a square was then applied to the bars which had been put in the
place of the front and rear wheel spindles and the position of the
catenery wire transferred to the individual spindles. It was found that
the notional datum line struckthe front spindle on the centre of that
spindle and was about 1/2" offset to the nearside at the rear spindle.
On the advice of our club Chairman Chris Kaye I checked the sprocket
alignment by applying a square and straight edge to transfer the offset
of the outer face of the gearbox sprocket to the rear fork blade this
turned out to be 11/8" the distance from the face of the rear wheel
sprocket to the hub boss was found to be 11/8" (OK there then) I am
drawn to the conclusion that the error is in the front forks/UFM area
but knowing the value of the offset (if any) would be comforting I
anticipate having to examine the fork links and perhaps making up an
arbour to locate in the steering head bearings and drop down to my datum
line. Can't think of anything else at the moment!

Regards Mostyn


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VOC Member
Re: Pulling to the left my Comet does this and I have tried the usual suspects wheel alignment etc with no success so I am very interested in this thread in particular the reply from "Comet Rider" which mentions "Bob Culvers fixture" can anyone tell what this is as I am wanting to check my bottom link for trueness.

Regards Tony


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VOC Member
Ah, if only everyone in the world of Vincents were as thorough as that. If when all other checks have proven that there are no faults then the following might be of interest. Years ago I used to drive a Range Rover which steered perfectly. Just before Xmas one year I had new tyres fitted and as I drove out of the tyre fitting shop it became clear that the car was pulling to the right. This was so bad that if I took my hands off the steering wheel the car would pull itself up over steep road cambers. I took the car back assuming that the fitters had somehow disturbed the tracking angles or something. Setting the car up on a laser jig showed that all was as it should be. A lot of tests and changes were made including changing wheels over etc. Finally the two front tyres were then taken off and turned round. The car now pulled to the left. Taking the tyres off and replacing them with new ones of the same type totally solved the problem. Neither I nor the chaps in the tyre shop could see any asymmetry or other fault in the pattern of the tread or the shape of the tyre. Asking around, a fellow VOC member who worked in the motor trade told me that this was not unknown. If you have a front or rear tyre which can be fitted the other way round then it might be worth while taking off the rear tyre first and fitting it so that it rotates in the opposite sense to the way it is rotating now and if that does not work then do the same with the front tyre. Good luck.


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VOC Member
Perhaps the easiest way to test out Norman's theory would be to swap the sides of the front brake plates and reverse that wheel. Not so easy with the D rear unless a second drum is fitted. Is it possible that the new wheels have been built true and concentric but with the hubs off centre?

I have found my Knight to be sensitive to changes in front tyre wear or pressure, not pulling but wobbling, weaving and feeling most unstable. The rear, in my case tubeless is totally forgiving of pressure faults down to about 4psi although this does make it impossible to deviate from a straight line without grounding the stand.

Another question for Mostyn may be, does the pulling occur regardless of crossfall, application of power or brakes, or could any of these be blamed?

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