AMF Harley SX/SS 250 - Engine Mounting - Vibration Damping

[Dixiesatellite]

Hi,

I have a 1976 AMF Harley Davidson SX/SS250 which I am restoring. I had one a long time ago (way long), which was in very bad shape because 1/ build quality was really poor and 2/ I did not have the money to fix it. I am building another bike now which I am making a lot of modifications to, to redo what the factory should have done in the first place.

The 2-stroke 250 engine is mounted in the frame without any damping (no rubber, nothing) which results in lots of vibration and stuff cracking / falling off. I plan to mod the frame to reduce vibration, but am not sure which route to go. Can anyone advise what the best setup / recommended setup would be? The frame will be blasted / welded and powder-coated and I would like to make any mods at the same time.

Thanks for your comments,

Ian B.

 

[helio lucas]

that is very "black magic", mod a frame to cut the vibration. the first place is to balance the crank. pressed cranks have a tendency to get out of aligment with time and this produces a lot of vibration. also check the balance shaft if aplicable. if the engine has a heavier (big bore) or lighter piston, the crank might have to be machinedto suit.

i have build a scooter frame some time ago, and the major concern was the vibration from engine to the frame. the engine is the swing arm and originally the dampening was made by rubber bushings on the swing arm\engine pivot wich make poor handling.
first attempt to connect the engine to frame rigedly, we felt so much vibration from that 50cc engine on the handlebars that was difficult to ride more than 2 minutes...
i had to devise some kind of dampening. with a lot of trouble and work that was achieved. it only cuts the up and dow vibration not the lateral, but this was a new frame. not a mod to a original one...

 

[Dixiesatellite]

HI Thanks for your reply. I was thinking about bushing the frame engine lugs to provide better insulation - either using rubber bushings or poly grommets.

Ian B.

 

[helio lucas]

if you are confident, then go for it.
but take you time to study the project and look very closely to be sure everything is right. for me doing drawings work in the way of understanding some things that just by looking i cannot get first. over entusiasm use to kill very good projects... guess how i discover this?
you have to really understand why was the frame built this way... it might have a good reason that is not on front of the eyes. most of the times is like this with me at least...

would the chain pull compress the rubber bushings and make them almost solid?
is the engine engeneered to vibrate at will without breaking some internals?
is the engine part of the frame to stiffen it? :nod:

as the americans say: only my two cents :cool:


ps: i still think that balancing the crank is the first step!!!

 

[Dixiesatellite]

Hi Lucas,

Thanks for your comments. I am taking about 12 months to build this Italian Harley-Davidson bike and I have spent the last 3 weeks stripping it down completely, ready for blasting / frame welding and powder coating. The factory frame welding was very poor (spot-welding in many places instead of seam welding) and there is some corrosion and cracking. The frame is a neat frame, with a twin-down tubes and more than capable of handling the single-cylinder 2-stroke 250cc engine, but does need to have some engine mount mods performed. The ere will be no modifications to the steering head, other than replacing caged ball bearings w/ tapered bearings.

The 74~78 AMF 250cc engine is a torquey dirt bike engine, much like the Yamaha DT250 or Suzuki TS250 of the time and putting out about 20 bhp, but it is bolted directly to the frame via 4 anchor points. The weight of the bike is about 120 kg. It is a chrome bore cylinder w/ piston porting (no reed valves), direct oil injection (Mikuni) and CDI. The engine is very torquey, but a pretty crude engine.

I certainly agree re. balancing the crank - the plan is to rebuild the crank w/ a better rod and stronger bearings as the HD-Aermacchi (and later Cagiva) stuff was just not very good (in my opinion). Engine vibration is a big deal in the frame, so I was planning to weld duplicate brackets opposite the original brackets to hold rubber / poly bushings w/ longer mounting bolts. The location of the engine will not change - but the engine will be isolated from the frame in these 4 ea. locations. I don't think this will affect the operation of the engine - I think the decision not to bush the engine was a decision due to cost ($). Vibration on these bikes causes a lot of problems in terms of not only rider comfort (holding on after 70 mph (110 kph) was almost impossible, as well as being able to see the speedometer), but also blown electrics, parts falling off and stress cracks I am planning a KTM / Wiseco replacement rod / piston due to newer rod design, which ensure the bottom end gets plenty of oil supplied to the needle bearings and cage. The crank will balanced by a specialty shop.

The rear wheel on all the '74~'78 SX / SS 175 / 250 2-strokes is a cush drive quick-release wheel w/ 6 ea. 'donuts'. These are all worn out, so I will need to find new ones, or have new ones made.

Other than that, the plan is to replace all the soft bolts / screws w/ better quality Allen bolts etc, repaint and redo the seat. I consider the vibration of the bike the most important aspect of what is otherwise a pretty fun bike. General factory quality also ruined them, which is why there are so few of them today. An interesting thing also is that many of the small parts needed (cable parts / seals / bearings / Dellorto carb parts such as jets etc.) can be bought from Lambretta scooter dealers for a fraction of prices from specialty suppliers.

Thanks for your comments,

Ian B.

 

[helio lucas]

I am taking about 12 months to build this Italian Harley-Davidson bike and I have spent the last 3 weeks stripping it down completely, ready for blasting / frame welding and powder coating. The factory frame welding was very poor (spot-welding in many places instead of seam welding) and there is some corrosion and cracking.

hi ian...

are you sure they only spot welded the frame due to costs?
or it was because not over stiffening some parts an allow some movement to not stressing other parts.

what is the main vibration? lateral or vertical?

are you in italy?
that look a very sweet bike indeed!!! could you post a pic of the whole bike? :nod:

 

[Dixiesatellite]

Hi Lucas,

I am located in Japan. Here is a pic of the frame - I don't think the poor welding is due to design - just poor welding. The rest of the frame also needs a clean-up (such as the shock mounts). I don't have a good pic of the bike at the moment, but will find something to post. They are pretty cool looking bikes when done right.

Ian B.