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Discussion Starter #1
I am interested in lowering my S&S88's RPMs at highway speed in an attempt to reduce vibration. So I have read a bit about overdrive transmissions... from a Jims Fat-5, OD6 to DD7, etc. What have you done, if anything, and why?

Could not the same thing be accomplished using a judicious choice of front sprocket (25t) Tranny driven pulley (34t) and a 55t rear pulley? That should be good for ~0:86:1 final drive. I don't think a 15% shorter gear is going to affect starting off that much - especially if like me, you're into cruising and not into drag racing. Any difference after take-off will simply change the MPH at which at which one shifts up/down. The only down side I can see would be (perhaps) some additional wear on the clutch pack. Is the basket able to handle that a 15% increase in torque? If not, is there a better choice for a wet clutch?

Have any of you tried to adjust the drive ratio in this manner?:If so, what was your success - or failure? If not, and you decided to go the transmission route, why did you choose that path over altering the pulley/sprocket configuration?

Kind regards,
Michael


Chief Green-Silver
 

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Yes, you can. Look over on the Harley forums and most have done sprocket changes instead of paying for the overdrive trans. I will be looking to change out mine as soon as I get it up and ready and then look to see what the best solution would be. The clutch and primary will have no issue whatsoever with the small change up and starting off, you wouldn't be riding the clutch anyways. Back when I had a bike with a chain, I swapped sprockets depending on what riding I was doing that week.
 

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Discussion Starter #4
(whispers: I've been reading some of the HD forums... <grin>)

One approach I like is the 27t compensator sprocket, proposed by (and now defunct?) Illusion Motorsport. V-Twin Manufacturing currently sells one -


Our Indians run a 24t sprocket w/ a 37t clutch basket. That puts their primary drive ration at 1.54:1 (or 1.54 revolutions of the engine to one turn of the basket.) I noticed many of the softails run a 25/36. That puts their primary drive ration at 1.44:1 (compared to our 1.54:1) and gives me cause to wonder... "Why did IMC change the primary drive ratio???")

The benefit of a 25t sprocket would be in keeping the chain length about the same... but I think not so a 27t sprocket. The 27/37 sprocket combinatoin would yield a primary drive ratio of about 1.37:1 for a reduction in RPM of about 12.5% throughout the powerband. This change would require no change to the speedometer calibration (not that that's an issue) or any other pulley changes. HOWEVER - the 27t sprocket would eat up an 1" more chain... so I do believe it would entail a longer (non-standard?) drive chain.

- Michael

 

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You should be able to go to your local independent shop and have them cut the chain to whatever length you need. Most that I know of buy the chain in bulk(rolls).
 

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I replaced my troublesome tranny with the Ultima "Builder's Kit", which comes with new 6 speed internals but uses your original case.
My mechanic had no problem installing it, the unit costs less than 600.00, and I now have a 6 speed Scout that has never run so well as it does now.
It's perfect for the Scout!!
Highway cruising is a breeze, and for a quick pass just kick it back down to 5th for a nice quick burst of speed.
Best thing I have ever done to mine!!
 
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