Moss Engineering

33 Kings Lane
South Croxton
Leics. LE7 3RE
UK

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Vibration

A few years ago, an owner from Australia wrote to me to ask if I could give him any advice to help alleviate the serious vibration of his Scott.  I suggested checks to be sure that the engine itself was in acceptable condition.  Some time later, he responded that he had carried out all these checks and the engine seemed to be in good condition, but he still had the problem.  My suggestions and reasons were this.

The original Scott design was the concept of the inspired creative mind of Alfred Scott.  It featured a heavy flywheel and an engine that was securely bolted to the frame.  A Scott engine is not balanced as most other engines, but the energy that causes vibration is absorbed or damped by the rotating mass of the flywheel plus the weight of the engine unit.  On modern bikes they often put small weights in the end of handlebars, as by increasing the weight, they can quench vibration.  We must remember that there are a very few really gifted men born each century and that no amount of teaching will produce a painter or composer to match the legendary names of the past.  It was inevitable that the sincere, well trained and experienced engineers who became responsible for design and development following Alfred’s early departure from the company, were of lesser stature.  Some changes they made in good faith, brought problems, but some, like vibration, were inclined to become apparent only after some time in use.  In the late 1920’s Scotts were faced with the problem of keeping parity with the increasing performance of some of the other competing motorcycle manufacturers.  They responded by introducing the Long Stroke Engine, where the duty on the cranks in relation to available metal, reduced their operating life.  They also reduced the weight of the flywheel and the crankcase had been changed from the original Two Speed pattern to a new design with different fixings.   The new flywheel of reduced weight, does not have enough mass to absorb the vibration forces generated by the engine.  If the engine components become worn or are not in correct alignment, then the engine will generate appreciably more vibration, which makes the situation worse.

In order that the vibration is absorbed, more mass is required, and by fixing the crankcase securely to the frame, the weight of the frame becomes added to the weight and flywheel effect of the engine to give a much increased damping effect.  Unfortunately, the design of the fixings that secure the crankcase to the frame are not completely effective and this can allow the engine to dance up and down in the frame.  This produces heavy uncomfortable vibration.

Let us consider each mounting point in turn.

The rear upper engine mount is the best of the three as whether it is used in a single down tube frame or a duplex type, it is provided with a central strut section to clamp up firmly against. The crankcase is open at this point and totally able to bend inwards the small amount necessary in order to clamp firmly against the central solid element.  Please note that you rarely see cracking or broken lugs in this position.

The bottom rear engine mounting comprises two lugs which are machined to achieve an internal gap of 5.001”.  The forward section of the gearbox undertray, which will act as the strut element to which the rear lugs of the crankcase will be clamped, is machined to have a width of 5.000”  Unfortunately, the designer did not realise the significance of the bottom deck of the crankcase being continued right up to the lugs.  Unlike the top rear, which has some flexibility to clamp up securely, the bottom lugs are prevented from flexing inwards slightly to allow secure clamping to the undertray by the presence of the bottom deck of the case.  The crankcase often is able to dance up and down here and wears the intended abutment faces so that the gap becomes bigger.  Take a good look at these rear lugs.  The left hand inside face is usually the most prone to wear. 

                                                                                                         

When the fine pitch 5/8” nuts are done up, they try to break off the lugs.  Look at all the Scotts you see and count the ones with broken and re welded rear lugs or those with cracks around. 

                                

The solution is to allow the lugs to flex inwards a little by sawing two slots alongside the lugs in the underside deck of the case.  If you have wear on the inside of these lugs, machine or file them flat carefully and fit close fitting washers.  As most wear takes place on the rear lower left side, you might find that it is only necessary to fit a washer to this side. These pictures show machined slots and a washer ready to fit.


                                                                   
We are left now with the front engine mounting.  In this case, the frame is quite flexible enough to move under clamping to be secured against the crankcase. Unfortunately, however, whereas both rear mountings have a solid spacer between them to clamp against, the front does not.  The bosses look substantial, but inside, where you can not readily see, there is only an outer wall.  The result is that if you try to do up this fixing securely, the case will warp inwards under the unsupported force and will alter the crankshaft end float setting.  Look again at the Scotts you see and note that a significant number have had pieces of the front bosses broken off and re welded.  Without a solid strut inside, this fixing can not be tightened enough to make it secure, so the engine can often dance up and down here too.

                                                                                       

The best solution that can be implemented providing that the previous vibration has not worn the holes in the bosses too big, is to tap out both front bosses to accept M16 helicoil inserts. 

                                                                             

      Get a length of M16 steel threaded studding and cut off two pieces 10cm long.  With the engine in the frame, wind in the stud section so that you are left with enough length protruding to fit a heavy duty washer and an M16 nut each side. 

                                                                  

     This will provide a solid and secure fixing for the front of the crankcase that will not allow the engine to vibrate in the frame and will neither break the bosses nor distort the case and alter the crank end float figure.  If you have had vibration and your engine is in reasonable condition, you can make a dramatic improvement by following these procedures.  As regards other options, we like to put a steel ring under the RH rim of the flywheel to increase its total weight.  This also makes the engine smoother, makes tickover and town driving more controlled and enhances the handling stability of the bike.  To those who treasure originality above all else, warts and all, I hope you enjoy your warts!  For me, I think Alfred would never have committed these engineering oversights, so consider these discrete compensations will help in some very modest way to make the bike that bears his name, a little more worthy of bearing it.

PS.  The Australian owner to whom I suggested some of these solutions years ago, responded to tell that his bike was transformed!  

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email roger@mossengineering.co.uk or richard@mossengineering.co.uk