Rigid Paughco Rubbermount Sportster Frame Build

[Dennis]

Keith Larrett took the Stolen Sportster for a spin and says its "fu**ed."  He insisted on looking at the manual.  On the stock sportster, both the swing arm and the engine are connected to the same big rubber bungs.  He was pretty sure that my problem is that vibrating engine in the rubber mounts is not in sync with the rigid rear wheel, and this causes the drive belt to jump.  To test his theory, he disconnected the rear hiem joint stabilizer that I made (photo from page one), to see if it was better or worse.



It didn't take more than a 100m on Tabor to determine that it was way worse with the stabilizer disconnected.  I'm leaning towards Keith's theory.

I hit the WWW for rigid conversions.  I spoke to John and Hardtailchoppers.com who manufacturers them.  He has sold some to a few other Paughco frame builders who had similar problems.  By similar, they had converted to chain drive and the chain slaps around way too much, no matter how properly they're adjusted.

http://hardtailchoppers.com/2004-up-sportster-solid-motor-mounts.html

His machinist expects a new batch to be ready for next week, so the plan is to touch bases then.

Peace & Grease, Dennis

[Rusty Bucket]

Hmm, bold move to consider wholesale rigid mounting - but I agree the engineering in the rubbermount with it's intersecting planes is...er, an intimidating playground to find oneself.  It had not occurred to me to contrast the movement in the engine to the completely unmoving rear wheel - true enough, in hindsight, it seems like it could be a source of problems through the main connection they share - the drive.  I would be interested to hear Paughco's reaction to the idea that the combination was fundamentally incompatible;  if there was a single iteration of rubbermount /rigid wheel that had worked - the idea that it could NOT work would be quashed.

[stevecrout]

I might be way off base but it seems that once the ability of the frame to adjust to forward pull from the engine is diminished (by removal of a swing-arm) and since the drive is now almost inelastic the result could be a jerking forward motion that comes off like an out of round vibration?

I wonder if a simple damper mechanism on the drive belt might absorb a lot of the bladder busting stuff without having to go completely solid mounting?

A simple diagram is attached.

Again, I might be off the mark but it seems there needs to be some give in the drive train at some point.

[Donovan]

I am no engineer but the diagram that Steve sent seems to make sense and should be able to adjust to the various engine conditions vs the rigid rear end.

[Rusty Bucket]

  Some of the rubbermount Sportster's mounts - the front one, I think, at any rate, and maybe the main rear as well - are captured like a spring eye bushing, and traverse the frame side-to-side.  This should resist deflection fore and aft, preventing the cyclical tension/release of the chain/belt varying tension with the engine's pulses.  This would require the captured bushing to be fairly stiff, though, and if the rubber used in the rubbermount was both soft and thick, I suppose it could deflect enough fore and aft to create the exact cycling one wanted to avoid.  If that was the case, just as in cars and trucks getting cyclical vibrations and deflections, substituting polyurethane for the soft rubber might be the engineering 'half-measure' that would find the happy ground between the problems inherent to the soft rubber, and those that would likely appear with the fitting of solid metal bushings and spacers.
  There seem to be successful rubbermount hardtail choppers out there, ones whose riders would vouch for them, but it's not clear to me whether they are strutted swingarm models, weld-on hardtails, or aftermarket hardtail frames - I haven't heard anyone claim they have ridden a Paughco hardtail rubbermount Sportster for 3 years in every condition with nothing but glowing satisfaction... but maybe some would if given the chance.  Whether they are stretched would be another question - there does seem to be a certain amount of tensioner talk.
   

[Dennis]

I think you are onto something.  Vance and Hines makes these ...


http://xr1200.vanceandhines.com/race-kit/35-0260/

However, the hole through the middle is for the swingarm axel, which I don't have.  The little "dimples" are just so they line up in the frame a certain way.  I'm thinking a visit to Jeff may be in order.  Start with just the rear in some hard but non metallic stuff.


Peace & Grease, Dennis

[Dennis]

This is curious (see link).

They claim "Use our Rubber Mount Sportster Rear Mount Bushings when converting your late model, rubber mounted XL to hardtail. Sure, you can hardmount the rear, but why? Do it right, do it with these bushings. Save yourself a bunch of hassle. Retains stock rubber isolators and swingarm bolts."


I can see how it would mount, and I have the stock bolts to mount them, but I can't see how they'd solve the problem unless they just squish the rubber and make it stiffer. 

http://shop.bareknucklechoppers.com/product/rubber-mount-sportster-rear-mount-bushings/

I sent them a detailed email question and left a phone message. 

Peace and Grease, Dennis

[Hans]

can you see evidence on the belt that it's been skipping.  I would think that you would see rounding or scuffing on the leading edge of the belt teeth.

[stevecrout]

Another 2 cents worth -   I recently visited PG Plastics for the material needed to fabricate the plastic 'puck' used in the BMW drive train as a shock absorbing drive plate. Seems to me this sort of plastic, has the strength yet enough give under torque to do the job for you too.

It had the consistency of a hockey puck but you could tool it up nicely in a lathe.

 

[fast1]

    It felt to me like the belt was skipping or at least trying too when I rode it. The belt tension and alignment was perfect so I discounted it and felt like it was in the clutch. Rubber mount engines are new to me but I completely agree with Keith. Makes perfect sense. I think those aftermarket bits you posted a link to are a band-aid and a compromise. Liking Steves idea! CHEERS.

[Dennis]

I followed up Rusty's idea quite abit; polyurethane instead of rubber.  While the aftermarket does make these for Big Twins, they apparently don't for the Sportster ... despite forum discussions all over the WWW suggesting otherwise.   Numerous forum description about Drag Specialties' versions suggest they are stiffer than stock, not lots stiffer.  Most report a change in vibration after switch to the DS versions.

I got an email response directly from Drag Specialties.  They say they are the same.  They say they're made of rubber.  They should not be stiffer than stock rubber ones.  However, a bunch of subjective impressions should count for something.

They're cheap.  I am thinking of ordering a set.  However the first (and also cheap) part would be the aluminum bushings I referred to above.  As of this post, I still have not heard from the manufacturer on exactly how they are suppose to work, but I came across a forum build where some guy made his own to, "I limited the travel on the rear mounts slightly by taking up the air space between the motor mount and rubber bushing with some tubing and washers like this. ... Prior to the bushing...









Before I blow $375 USD plus shipping etc and commit to solid, I'd like to explore these options.  The easiest first step are the aluminum bushings, which do what the builder in the above photos did.  They would be a very easy install.

Peace & Grease, Dennis

[Rusty Bucket]

Since the problem would seem to be deflection between those two points - drive and driven belt pulleys - anything that would limit deflection in that plane (actually an arc around with the rear pulley at it's centre) would be incrementally helpful, with more compression/deflection-resistant material replacing rubber until the symptoms subside.  Hopefully there would be a point the problem (which I have not seen) would be manageable before the mounts were entirely solid metal.  Start wherever it is easiest to start.  Jeff can cut the metal bushings, and any polyurethane pieces needed.   I was also wondering if a rod-ended longitudinal brace , say from the engine cases under the drive sprocket, to a frame point as far forward as possible directly in front of the pulley (to allow the arm to be as long as possible to restrict angularity)might prevent the deflection.  It would basically be a strap, and would no doubt transmit considerable vibration itself to the frame - but it might prevent deflection between the front and rear pulleys.  If the heim joints were changed to eye bushings, vibration would be reduced, and deflection problems would begin again, but at least polyurethane bushings are FOR SURE available (and can be made) for an arm like that.  The bosses would have to be so strong to support a true zero deflection brace, it's probably just an mental engineering exercise, and not worth pursuing.
  I think a plan to substitute alloy sections into overly large, and overly soft, rubber bushings should be workable, but success might be relative and subjective - there might always be a  point at which the 45 hp engine might overcome the 35 hp mounts.  When Norton isolated their engine from the frame, they left the swingarm attached to the engine, and now we see why.  Is the stock rubbermount swingarm attached directly to the engine cases?  (I imagine it must be) 

[Dennis]

Here is a diagram from a parts manual.



RE: "Is the stock rubbermount swingarm attached directly to the engine cases?  (I imagine it must be)" 

The stock swing arm attaches to the pivot shaft (#7 in parts manual) which runs between the two rubber isolators (#12).  The swing arm is not rigidly mounted to the engine, but separated from the engine by the thickness of the rubber between the pivot shaft and the outside of the rubber isolators.

Rusty did not mention that he got to watch the pivot shaft move aft when I put the bike in gear and let out the clutch with the brake on.  The latest consensus (Rusty, his neighbour Rene, and myself), is that in the stock set up, both the swing arm and the engine should move in the same direction, subject only to the small amount compression in the isolator.  With the rear wheel being rigidly mounted, the engine pulls aft (you can see it), resulting in a looser belt.  At certain rpm, the vibration causes the belt to vibrate up and down a lot.  That rpm seems to correlate with the skipping.  I think that under acceleration, the engine is being pulled backwards, and at the crucial rpm when the belt really starts vibrating up and down, its doing its jump.

Having said all of the above, the bushing inserts in the rubber isolators is going to be my first attempt. 

The bike is actually very smooth.  The frequently reported Harley phenomenon such as vibrating mirrors you can't see with, or foot pegs that make your feet go numb are non existent.  I'm liking the rubber mounted engine, and plan to only stiffen the rear isolator as much as it takes to allow decent, daily use type acceleration.  I don't plan to drag race this thing.

Peace & Grease, Dennis

[fj1200]

Rusty's comment makes my head hurt. Not sure about  what the problem actually is, but this thought occurred to me.  If the problem is the belt getting to loose and vibrating too much, would a tensioner like one used on serpentine belts work? Then it wouldn't matter what piece was moving in which direction. The belt would always be tensioned. Or did I miss the point completely?

[Dennis]

Re: tensioner solution.

The only reason I am not jumping on that option is I don't want to modify either the look of the belt drive as is, or have to fabricate/fit/weld on my beautiful powder coated frame.  The  builder whose photos I copied above had both a slide type tensioner on the bottom of his chain and a skate board roller on the top. 

Re: Chain instead of belt.

No one has suggested that yet, but every hard tailed rubber mounted sportster out there on the WWW seems to have made the conversion to chain.  Even Bare Knuckle Choppers show their bushings in a bike with chain drive.  A chain drive would not skip as easily.  However, I really want to try to keep to my theme of using as much of the donor bike as possible, and I want all of the smoothness of the belt and rubber mounts.  We'll see.

I ordered the bushings this morning.

Peace & Grease, Dennis